JP4014770B2 - Optical transmission system and optical transmission method - Google Patents

Description

このmtが常に１００％（あるいは、１）以上の所定値になるよう、正弦波信号５４の光変調度mcを、情報送信信号の強度mi1，mi2…ｍcに合わせて制御する。
【００６１】
図７（ａ）は受信信号５２のチャネル数が多い場合、図７（ｂ）はチャネル数が少ない場合、図７（ｃ）は受信信号５２の信号強度が小さい場合の周波数スペクトルを示している。
【００６２】
図６の光伝送システムは、図７（ｂ）のように受信信号５２のチャネル数が少ない場合や、図７（ｃ）のように受信信号５２の信号強度が小さい場合には、全体の光変調度mtが所定値となるように、増幅器１７の増幅率を可変制御して正弦波信号５４の強度を調整する。
【００６３】
なお、正弦波信号５４のパワー制御として、増幅器１７の増幅率を変化させる代わりに、電圧可変減衰器等を適用してもよい。
【００６４】
第２の実施形態では、情報送信信号とダミー信号の両者でレーザの過変調を行っている。光変調度の割合としては、情報送信信号の振幅値を最大になるように制御し、ダミー信号で光変調度を安定化させることが望ましい。親局２における受信信号において、過変調による振幅制限が存在しても、子局における光変調度が大きいほうが、ＣＮＲ（キャリア対雑音比）が大きくとれ、受信感度もよくなる。
【００６５】
このように、第２の実施形態では、正弦波信号５４の増幅率を調整可能な増幅器１７を設けるため、各子局の信号線路系にＡＧＣ機能が存在していても、光信号の光変調度を常に１以上に維持することができる。このため、安定性がよく信頼性の高い光伝送システムを構築できる。
【００６６】
（第３の実施形態）
第１〜第２の実施形態では、無線信号５１の有無及ぶ信号強度に対して、光信号１００の光変調度を安定化させる制御方法を行っていた。これに対して、以下に説明する第３の実施形態では、無線信号５１の有無により、光信号１００をon/off制御するものである。
【００６７】
図８は本発明による光伝送システムの第３の実施形態のブロック図である。子局１及び親局２の主な構成は、図１と同じであり、同じ構成部に対しては、同じ番号を付している。
【００６８】
子局１ａは、加入者５ａから伝送されてきた無線信号５１ａをアンテナ６で受信し、受信信号５２を得る。子局１ａは、信号検出器７を備えており、受信信号５２の有無を監視している。信号検出器７は、受信信号５２の有無により、電流源１２から出力されるＤＣバイアス電流５６を制御する。受信信号５２が存在しない場合は、ＤＣバイアス電流５６をゼロとし、受信信号５２が存在する場合は、ＤＣバイアス電流５６を所定の電流値に戻す。つまり、子局１で受信する無線信号５１が無い場合は、レーザ１３に注入される変調信号５７はゼロであり、無線信号５１が有る場合にのみ、レーザ１３から光信号１００が出力される。その際に出力される光信号１００のＯＭＩは、100％以上に設定されている。
【００６９】
受信信号５２が存在しないときに光信号をoffする方法は、幾つか考えられる。簡易な方法として、例えば、電気スイッチあるいは光スイッチを用いる方法がある。
【００７０】
図９は光信号のon/off制御に電気スイッチを用いた例を示す図であり、バイアスティ１１とレーザ１３との間にスイッチ（出力切換手段）１８を挿入している。信号検出器７で受信信号５２の有無を確認し、受信信号５２がない場合は、スイッチ１８を遮断する。受信信号５２が存在する場合は、スイッチ１８をオンして、レーザ１３を駆動して、変調信号５７に応じた光信号１００を出力させる。
【００７１】
一方、図１０は光信号のon/off制御に光スイッチを用いた例を示す図であり、レーザ１３の後段に光スイッチ１９を挿入している。図１０では、第４の実施形態と同様に、信号検出器７で受信信号５２の有無を確認する。信号検出器７は、受信信号５２が存在しないければ光スイッチ１９をオフし、受信信号５２が存在すれば光スイッチ１９をオンして、光信号１００をon/off制御する。
【００７２】
このように、第３の実施形態では、各子局が情報送信信号を受信した場合のみ、それに対応する光信号を各子局から送信するようにしたため、干渉性の高い光信号が親局２に送信されなくなり、ビート雑音の影響を受けにくくなる。このため、安定性がよく信頼性の高い光伝送システムが得られる。
【００７３】
（第４の実施形態）
第４の実施形態は、電気スイッチや光スイッチを用いることなく、光信号のon/off制御を行うものである。
【００７４】
図１１は本発明に係る光伝送システムの第４の実施形態を説明する図であり、子局の内部構成を示すブロック図である。
【００７５】
通常、レーザ１３は周辺温度による光信号１００の平均出力パワーの変動を抑圧するために、ＡＰＣ(Automatic Power Control)を行っている。ＡＰＣループが動作している状態で、光信号１００をon/offすることは、光パワーを安定化させようとする制御と相反する。
【００７６】
図１１はＡＰＣを行う場合の子局のブロック構成を示す図である。光信号１００の出力パワーは、レーザモジュール２０に内蔵されているフォトディテクタ（ＰＤ）２１でモニターする。そのモニター値を設定電圧（基準レベル調整手段）２２と比較して、差動増幅器２３で差成分情報６０を得る。その差成分情報６０は、発振等の誤動作が生じないように、ループフィルタ２４により帯域を制限して、加算器２５を介してバイアス電流値５６にフィードバックする。このような一連の制御により、レーザ１３から出力される光信号１００の平均パワーを安定化させている。
【００７７】
ＡＰＣループ（ＡＰＣ手段）が適用されたレーザ１３において、安定して光信号をoffするためには、例えば設定電圧値２２に制御を加えるとよい。つまり、受信信号５２が無い状態では、設定電圧値２２をゼロとして、ＡＰＣループ動作により、光信号１００の出力を抑える。そして、受信信号５２が存在する状態では、設定電圧値２２を所定の設定値に戻し、ＡＰＣループ動作により、光信号１００を出力する。
【００７８】
以上のような方法により、加入者５の接続状態に依存せず、干渉性の強いＯＭＩ≦100％の光信号１００が各子局から親局２に送信されるおそれがなくなる。このため、親局２では、ビート雑音の影響を受けなくなり、受信感度が向上する。
【００７９】
次に、図１１において、光信号をon/offするタイミングを説明する。携帯電話あるいはＷＬＬ等の無線通信は、TDMA (Time Division Multiplexing Access)が広く用いられている。
【００８０】
情報送信信号は、データを構成しているビットで、搬送波を位相、周波数、および振幅変調しており、情報送信信号のビット構成は、図１２のように表される。図１２は、ある子局が、加入者Ａ，Ｂ，ＣとTDMA接続している場合の情報送信信号のタイミングチャートである。情報送信信号は、ヘッダ、音声及びデータ等を含む情報ビットの両側にランプビットを配置した構成であり、隣接するランプビット間にはガードタイムが設けられている。ランプビットは、主に帯域の広がりを抑圧するために挿入されており、情報は含まれていない。
【００８１】
光信号のon/offのタイミングは、図１２に示すように、ランプビットが送信されている時間内で行う。この時間内で行うことで、ガードタイムである情報送信信号がない場合は、光信号は必ずoffになる。また、情報ビットが送信されている時間は、必ず光信号がonになり、情報ビットの損失は起こらない。
【００８２】
一方、図１３は、無線通信の他アクセス方法である、CDMA (Code Division Multiplexing Access)における情報送信信号の構成を示している。図示のように、CDMAの情報送信信号は、図１２のTDMAの情報送信信号と構成が異なっている。
【００８３】
図１３はある子局が加入者Ａ，ＢとCDMA接続している場合のタイミング図である。CDMAの情報送信信号には、ランプビットは存在しない。但し、スペクトル拡散されているため、情報送信信号は、データビットより伝送速度の高いチップビットで構成されている。
【００８４】
CDMAでは、ある程度のチップビット損失に対しては、スペクトル逆拡散を行うことにより、情報ビットを復元することは可能である。光信号のon/offのタイミングは、CDMAである情報送信信号の入出力の状態に合わせればよい。on/offスイッチの遅延等が生じて、若干のチップビットが損失しても、上記理由により、大きな問題にはならない。
【００８５】
このように、第４の実施形態では、ＡＰＣループ内の差動増幅器２３に入力される設定電圧２２の電圧レベルを受信信号５２の有無により切り換えるようにしたため、受信信号５２が存在しない場合には、光信号の送出を抑制することができる。また、各子局内にＡＰＣループを設けるため、光信号の出力レベルを安定化させることができる。
【００８６】
（第５の実施形態）
第５の実施形態は、子局がアンテナで受信した無線信号に二次変調を施して親局２に伝送し、無線信号が存在しない場合には、無変調信号を生成するものである。
【００８７】
二次変調には、代表的なものとしては、二次周波数変調、二次振幅変調がある。また、スペクトル拡散も、無線信号を変調した情報送信信号に拡散符号を乗じるため、二次変調の一種と考えられる。第５の実施形態では、二次変調に周波数変調を用いる例について説明する。
【００８８】
図１４は本発明による光伝送システムの第５の実施形態のブロック図である。図１４では、図１と同じ構成部分には同じ番号を付している。
【００８９】
子局１ａ〜１ｎはそれぞれ、アンテナ６と、周波数変換器１６と、二次変調器（二次変調手段）３０と、バイアスティ１１と、電流源１２と、レーザ（光信号生成手段）１３とを有する。親局２は、光受信器（ＰＤ）１４と、抽出器（抽出手段）３１と、復調器１５とを有する。
【００９０】
子局１ａは、アンテナ６で受信した受信信号５２ａを周波数変換器１６でダウンコンバートしてＩＦ信号５３ａを生成する。二次変調器３０は、ＩＦ信号５３ａを二次変調して送信信号６１ａを生成する。この送信信号６１ａは、バイアスティ１１にて、電流源１２からのＤＣバイアス電流と加算された後、レーザ１３にて、光変調度100％以上の光信号１００ａに変換されて親局２に伝送される。
【００９１】
各子局１ａ〜１ｎの送信信号６１ａ〜６１ｎの周波数帯域は、各子局ごとに設定されており、光合成器４により副搬送波多重される。親局は光多重された光信号を受信し、各子局の送信信号が異なる周波数帯域に配置されているため、副搬送波多重された受信信号を得ることができる。各子局１ａ〜１ｎからの光信号１００ａ〜１００ｎは、光変調度100％以上に設定されているため、ビート雑音は拡散されており、受信感度を劣化させるような大きな影響は現れない。
【００９２】
親局２で受信された受信信号５８は、抽出器３１にて、送信信号６１ａ〜１ｎに分離抽出されて、復調器１５に入力される。加入者５が接続されておらず、受信信号５２がない場合も、二次変調器３０の出力である送信信号６２は、所定のパワーを持っており、光変調度は常に100％以上を保ち、光信号１００の干渉性を抑圧することができる。
【００９３】
図１５は子局１ａ〜１ｎ内の二次変調器３０の内部構成および特性を示す図であり、図１５（ａ）は二次変調器３０を電圧制御発振器（ＶＣＯ）３２で構成した場合のブロック図、図１５（ｂ）は受信信号が存在しない場合の電圧制御発振器の出力信号の周波数と信号強度との関係を示す図、図１５（ｃ）は受信信号が存在する場合の電圧制御発振器の出力信号の周波数と信号強度との関係を示す図である。
【００９４】
図１５（ａ）の電圧制御発振器３２は、各子局がそれぞれ異なる中心周波数ｆmを持つように周波数の設定を行う。電圧制御発振器３２は、受信信号５２、つまりＩＦ信号５３が存在しない場合は、図１５（ｂ）に示すように、周波数fmの正弦波を送信信号６２として出力する。また、ＩＦ信号５３が存在する場合には、ＶＣＯ３２の発振周波数が、ＩＦ信号５３の振幅に応じて周波数変調され、図１５（ｃ）のような周波数スペクトルをもった送信信号６２を出力する。
【００９５】
このように、受信信号（ＩＦ信号）の有無に依存せずに、常に送信信号６２が得られ、必ずレーザを過変調させることができる。
【００９６】
図１６は子局１ａ〜１ｎが周波数変調した二次変調信号を伝送する場合の親局２内の抽出器３１の内部構成を示すブロック図である。図１６の抽出器３１は、分配器（分配手段）３３と、バンドパスフィルタ３４ａ〜３４ｎと、周波数弁別器（周波数弁別手段）３５ａ〜３５ｎと、バンドパスフィルタ３６ａ〜３６ｎとを有する。
【００９７】
抽出器３１に入力された受信信号５８は、まず分配器３３で、各子局１ａ〜１ｎに相当する数に分配される。各子局１ａ〜１ｎからの送信信号６２ａ〜６２ｎは、それぞれ異なる周波数帯域に配置されており、バンドパスフィルタ３４ａ〜３４ｎはそれぞれ所望の帯域を抽出する。その後、周波数弁別器３５ａ〜ｎは周波数―振幅変換を行い、再度バンドパスフィルタ３６ａ〜３６ｎは、各子局１ａ〜１ｎからの送信信号５３ａ〜ｎを抽出する。
【００９８】
このように、第５の実施形態では、各子局内で、アンテナで受信した無線信号に二次変調を施すため、無線信号が存在しない場合には無変調信号が得られる。このため、干渉性の強い光信号が子局から送信されるおそれはなく、親局２がビート雑音の影響を受けなくなる。また、第５の実施形態では、二次変調として周波数変調を行うため、電圧制御発振器を用いて比較的簡易な構成で二次変調信号を生成できる。
【００９９】
（第６の実施形態）
第６の実施形態は、二次変調に振幅変調を用いるものである。第６の実施形態の光伝送システムは、図１４と同様に構成されており、子局１ａ〜１ｎ内の二次変調器の内部構成と親局２内の抽出器の内部構成が第５の実施形態と異なる。
【０１００】
図１７は子局１ａ〜１ｎ内の二次変調器３０の内部構成および特性を示す図であり、図１７（ａ）は二次変調器３０の内部構成を示すブロック図、図１７（ｂ）は受信信号が存在しない場合の二次変調器３０の出力信号の周波数と信号強度との関係を示す図、図１７（ｃ）は受信信号が存在する場合の二次変調器３０の出力信号の周波数と信号強度との関係を示す図である。
【０１０１】
第６の実施形態の二次変調器３０は、図１７（ａ）に示すように、基準信号である正弦波信号６３を出力する局部発振器（第１の局部発振器）３７と、振幅変調器（振幅変調手段）３８とで構成される。
【０１０２】
正弦波信号６３の周波数ｆｍは、子局１ａ〜１ｎごとに異なるように設定される。受信信号５２、つまりＩＦ信号５３が存在しない場合は、図１７（ｂ）の周波数スペクトルに示されるように、周波数fmの正弦波信号６３がそのまま送信信号６２として出力される。また、ＩＦ信号５３が存在する場合には、ＩＦ信号５３の振幅に応じて、正弦波信号６３の振幅が変調され、図１７（ｂ）のような周波数スペクトルをもった送信信号６２が得られる。つまり、ＩＦ信号の有無に依存せずに、常に送信信号６２が得られるため、必ずレーザを過変調させることが可能である。
【０１０３】
図１８は子局１ａ〜１ｎが振幅変調した二次変調信号を伝送する場合の親局２内の抽出器３１の内部構成を示すブロック図である。図１８の抽出器３１は、分配器３３と、バンドパスフィルタ３４ａ〜３４ｎと、局部発振器３９ａ〜３９ｎと、バンドパスフィルタ３６ａ〜３６ｎとを有する。
【０１０４】
抽出器３１に入力された子局１ａ〜１ｎからの受信信号は、まず分配器（分配手段）３３で、各子局１ａ〜１ｎに相当する数に分配される。各子局１ａ〜１ｎからの受信信号６２ａ〜６２ｎは、異なる周波数帯に配置されており、バンドパスフィルタ３４ａ〜３４ｎは、それぞれ所望の帯域を抽出する。それぞれ抽出された信号は、局部発振器（第２の局部発振器）３９ａ〜３９ｎからの正弦波信号と、乗算器（乗算手段）４０ａ〜４０ｎで乗算される。
【０１０５】
局部発振器３９ａ〜３９ｎは、子局１ａ〜１ｎで用いられた正弦波信号６３ａ〜ｎと等しい周波数をもつ正弦波信号を出力する。乗算器４０ａ〜４０ｎの出力は、バンドパスフィルタ３６ａ〜３６ｎを用いて、各子局１ａ〜１ｎからの送信信号５３ａ〜ｎを得る。
【０１０６】
このように、第６の実施形態では、二次変調として振幅変調を行うため、簡易な構成で二次変調信号を得ることができ、各子局の構成を簡略化できる。
【０１０７】
（第７の実施形態）
第７の実施形態は、二次変調の一種として考えられるスペクトル拡散を利用して子局から親局２に情報伝送を行うものである。第８の実施形態が、第１〜第７の実施形態と大きく異なる点は、各子局からの光信号を副搬送波多重する代わりに、各子局からの光信号をスペクトル拡散して同一周波数帯で多重することである。
【０１０８】
図１９は本発明による光伝送システムの第７の実施形態のブロック図である。子局１ａ〜１ｎと親局２との主な構成は図１と同じであり、同じ構成部分には同じ番号を付している。
【０１０９】
第１〜第７の実施形態では、各子局間の受信信号５２を周波数変換器１６にて、それぞれ異なる周波数に配置していたが、本実施形態では、各受信信号を同一周波数帯に周波数変換する。
【０１１０】
例えば、子局１ａでは、アンテナでの受信信号５２ａを周波数変換器によりダウンコンバートしてＩＦ信号５３ａを生成する。拡散器（スペクトル拡散手段）２６は、ＩＦ信号５３ａと拡散符号とを乗算してスペクトル拡散を行う。スペクトル拡散された送信信号５９ａは、レーザ１３により、光変調度が100％以上の光信号１００ａに変換されて親局２に伝送される。
【０１１１】
図２０は親局２での受信信号６０の周波数スペクトル図である。図示のように、各子局１ａ〜１ｎからのスペクトル拡散された送信信号５９ａ〜５９ｎが、同一周波数帯に重畳されている。各子局１ａ〜１ｎからの光信号１００ａ〜１００ｎは、光変調度が100％以上に設定されているため、ビート雑音は拡散されており、受信感度を劣化させるような大きな影響は現れない。
【０１１２】
親局２で受信された受信信号６０は、逆拡散器（逆拡散手段）２７において、各子局１ａ〜１ｎがスペクトル拡散に用いた拡散符号と同一の拡散符号で逆拡散されて、各子局１ａ〜１ｎからのＩＦ信号５３ａ〜ｎにそれぞれ分離され、復調器１５に入力される。
【０１１３】
なお、情報送信信号をスペクトル拡散する方式は、直接拡散する方式 (DS : Direct Sequence)でも、周波数ホッピング方式 (FH : Frequency hopping)でも良い。
【０１１４】
図２１（ａ）、図２１（ｂ）は拡散符号を用いた直接拡散方式の拡散器と逆拡散器のブロック構成を示す図である。図２１（ａ）において、ＩＦ信号５３は、拡散符号発生器４１から出力された拡散符号６４と乗算された後、バンドパスフィルタ３６で不要波が抑圧される。これにより、図２１（ａ）に示すように、スペクトル拡散された送信信号５９が得られる。拡散符号発生器４１から出力される拡散符号は、各子局ごとに異なっており、Walsh符号に代表される直交符号でもＰＮ符号（Pseudo Noise）でも良い。
【０１１５】
基本的には、子局の情報送信信号をスペクトル拡散多重する場合、親局において、各子局からの送信されてきたスペクトル拡散信号のビットは同期されている必要がある。各子局間での同期をとらない構成とするためには、簡単には、拡散符号のビット数を大きくすればよい。拡散符号のビット数が６４、１２８等と大きくなると、逆拡散をした際の干渉信号抑圧度が大きくなり、所望波に対する伝送品質を確保できる。また、拡散符号のビット数が小さい状態でも、所望波に対する伝送品質を高く保てる方法もある。その方法を以下に示す。８ビット構成の直交Walsh符号を各子局に対して以下のように割り当てる(ここでは、１と−１の符号構成とした)。
【０１１６】
子局１ａ [１ １ １ １ １ １ 1 1]
子局１ｂ [1 −１ １ −１ １ −１ １ −１]
子局１ｃ [1 １ −１ −１ １ １ −１ −１]
子局１ｄ [１ １ １ １ −１ −１ −１ −１]
ここでは、４つの子局の光多重を想定する。上記の符号は、各子局１ａ〜ｄにおいて、ビット同期をとらなくても、各拡散符号間の直交性が保たれている。このように同期に依存せずに直交性を保てる拡散符号を選択して、各子局間に割り当てると、各子局間で同期をとる必要がなくなり、構成がより簡易になる利点が得られる。８ビットの直交Walsh符号では、子局４局まで同期を必要としないで光多重できる。以下に４ビット、１６ビットの場合の同期を必要としない拡散符号の割り当てを示す。
【０１１７】
<４ビット>
[ 1 1 1 1]
[ 1 1 -1 -1]
[ 1 -1 1 -1]
<１６ビット>
[ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1]
[ 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1]
[ 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1]
[ 1 1 1 1 -1 -1 -1 -1 1 1 1 1 -1 -1 -1 -1]
[ 1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1]
以上の組み合せから、位相同期を必要とせずに、４ビットでは３局、１６ビットでは、５局の光多重が可能である。位相同期をとる構成であれば、４ビットでは４局、１６ビットでは、１６局の光多重が可能であるが、子局の構成は同期手段を備えるため複雑となる。
【０１１８】
親局２内の逆拡散器２７は、図２１（ｂ）に示すように、子局１ａ〜１ｎからの光多重された受信信号６０と、拡散符号発生器４１から出力された各子局に固有の拡散符号６４とを、乗算器４０にて乗算する。乗算された信号は、バンドパスフィルタ３６にて不要波が抑圧された後、所望の受信信号６３が得られる。
【０１１９】
このように、第７の実施形態は、アンテナ６で受信した無線信号を各子局内でスペクトル拡散してから、光信号に変換して、親局２に送信する。スペクトル拡散された情報送信信号においても過変調を施すことは可能であり、光ビート雑音は低減することができ、親局２での受信感度は高い品質に保てる。また各子局の情報送信信号の周波数帯は同一で良いため、周波数変換部の構成を全て同一にすることができ、低コスト化に適している。
【０１２０】
第７の実施例では、受信信号５１ａが１チャネルという想定で説明したが、複数チャネルでも構わない。複数チャネルに対するスペクトル拡散方式は他の方式もあるが、その方式例については、第８の実施例で述べる。
【０１２１】
また、第７の実施例において、各子局１に備えたレーザ１３の波長が重ならないように設定してあれば、各子局１では過変調をしなくてもよい。
【０１２２】
さらに、第７の実施例であるスペクトル拡散多重方式と、第１〜第６の実施例に示した情報送信信号が存在しない場合に干渉性の強い光信号を送信させない方式は、組み合わせることが可能である。これら２つ、スペクトル拡散多重方式と干渉性の強い光信号を送信させない方式を組み合わせた光伝送システムでは、親局において、光ビート雑音の発生を低減することができ、安定した高い品質の伝送特性を提供できる。
【０１２３】
（第８の実施形態）
第８の実施形態は、各子局１において、受信信号５３が複数チャネルの場合におけるスペクトル拡散方法を示したものである。
【０１２４】
第８の実施形態は、各子局内の拡散器２６の内部構成と親局２内の逆拡散器２７の内部構成が異なる他は第７の実施形態と同様に構成されている。
【０１２５】
各子局１からの受信信号を同時に取り出すためには、親局２内に子局数分の逆拡散器２７を設け、受信信号６０を各逆拡散器２７に分配して入力すればよい。第８の実施形態では、ＩＦ信号５３が周波数ｆａ、ｆｂ…ｆｎに配置されているマルチキャリアで構成されている場合の拡散器２６と逆拡散器２７の構成を説明する。
【０１２６】
図２２（ａ）は拡散器２６の内部構成を示すブロック図、図２２（ｂ）は逆拡散器２７の内部構成を示すブロック図である。拡散器２６は、図２２（ａ）に示すように、バンドパスフィルタ３４、拡散符号発生器４１、乗算器４０、周波数変換器１６、およびバンドパスフィルタ３６からなる拡散部を複数組有する。
【０１２７】
通常，ＩＦ信号５３のマルチキャリアの周波数間隔よりも、スペクトル拡散用の拡散符号６４ａ〜６４ｎの周波数帯域のほうが広い。そのため、マルチキャリアであるＩＦ信号５３を分配器３３で分配し、それぞれのキャリアをバンドパスフィルタ３４で抽出する必要がある。１波ごとに分配されたＩＦ信号５３は、拡散符号発生器４１からの拡散符号６４ａ〜６４ｎと乗算されて、スペクトル拡散される。このとき、拡散符号６４ａ〜６４ｎは、異なる符号であり、また他子局１で使用される拡散符号６４とも異なるようにする。
【０１２８】
スペクトル拡散されたＩＦ信号は、それぞれ周波数変換器１６で、同一の周波数帯に変換され、不要波をバンドパスフィルタ３６で除去した後、加算器４２で合波されて送信信号５９となる。
【０１２９】
ＩＦ信号５３と送信信号５９の周波数スペクトルの様子は、図２２（ａ）のようになる。ここでは、各キャリア成分を再び同一の周波数帯に多重する例を示したが、異なる周波数帯でも構わない。
【０１３０】
一方、親局２内の逆拡散器２７は、図２２（ｂ）に示すように、拡散符号発生器４１、乗算器４０、およびバンドパスフィルタ３６からなる逆拡散部を複数組有する。
【０１３１】
逆拡散器２７は、他子局１からの光信号と光多重されて受信した受信信号６０を、分配器３３により各子局１におけるキャリア数分に分配する。乗算器４０は、分配された受信信号６０と、子局において所望のキャリアに用いた拡散符号６４ａ〜６４ｎと同一の拡散符号６４ａ〜６４ｎを乗算する。乗算器４０からの出力信号は、不要波抑圧用のバンドパスフィルタ３６を経て、所望波である受信信号６３ａ〜６３ｎを得る。各子局１からの受信信号を同時に取り出すためには、親局２内に子局数分の逆拡散器２７を設け、受信信号６０を各逆拡散器２７に分配して入力すればよい。
【０１３２】
各子局１ａ〜１ｎからの光信号１００ａ〜１００ｎをスペクトル拡散して多重する場合、親局２１における受信信号５８を構成する各送信信号５９ａ〜５９ｎのパワーが等しいことが望まれる。各送信信号５９ａ〜５９ｎのパワーが等しければ、スペクトル拡散された全ての送信信号５９ａ〜５９ｎに対して、逆拡散後の抽出信号の雑音特性が最も良好になるためである。
【０１３３】
このように、第８の実施形態では、各子局内の拡散器２６に複数の拡散部を設け、親局２内に複数の逆拡散部を設けるため、ＩＦ信号がマルチキャリアで構成されている場合でも、子局から親局２に光伝送でき、通信効率が向上する。
【０１３４】
（第９の実施形態）
親局２内の受信器１４の受信信号５９ａ〜５９ｎのパワーは、光信号１００ａ〜１００ｎにおける、親局２の受信器１４への入力パワーと光変調度で決まる。この２つのパラメータに対して、受信信号５９ａ〜５９ｎのパワーの制御方法は幾つか考えられる。第９の実施形態は、光伝送リンクの伝送品質を高めるために、各子局１ａ〜１ｎ間で受信信号５９ａ〜５９ｎのパワーを制御する機能を備えたものである。
【０１３５】
図２３は本発明による光伝送システムの第９の実施形態のブロック図である。図２３の光伝送システムは、子局１ａ〜１ｎと光合成器４との間の光伝送路３ａ〜３ｎに光アッテネータ２８ａ〜２８ｎを挿入した点に特徴があり、それ以外は図１９とほぼ同様に構成されている。
【０１３６】
各子局１ａ〜１ｎにおける光信号１００ａ〜１００ｎの光変調度は等しく設定され、かつ、光アッテネータ２８ａ〜２８ｎの損失量は、各子局１ａ〜１ｎの光信号１００ａ〜１００ｎの受信器１４への入力パワーが均一になるように設定される。
【０１３７】
光変調度と受信器１４への光受信パワーを等しくすることで、受信信号５８を構成する各送信信号５９ａ〜５９ｎのパワーを等しく設定すことが可能となる。
【０１３８】
このように、第９の実施形態では、各子局１ａ〜１ｎと光合成器４との間の光伝送路中に光アッテネータ２８ａ〜２８ｎを設けるため、各子局１ａ〜１ｎからの光信号の光変調度を略等しく設定できるとともに、光受信器１４への入力パワーのばらつきを抑制できる。
【０１３９】
（第１０の実施形態）
第１０の実施形態は、第９の実施形態とは異なる方法で受信信号のパワー制御を行うものである。
【０１４０】
第１０の実施形態は、子局の構成が異なる他は、第９の実施形態と同様に構成されている。
【０１４１】
図２４は子局１ａ〜１ｎの内部構成を示すブロック図である。図２４の子局１ａ〜１ｎは、パワー制御器２９と、周波数変換器１６と、拡散器２６と、増幅器１７ａと、局部発振器８と、増幅器１７ｂと、加算器１０と、電流源１２と、バイアスティ１１と、レーザ１３とを有する。
【０１４２】
図２４において、加入者５から送信された無線信号５１がアンテナ６で受信されると、必要に応じて、周波数変換器１６でダウンコンバートされ、ＩＦ信号５３が生成される。
【０１４３】
ＩＦ信号５３は、拡散器２６によりスペクトル拡散され、ゲイン可変の増幅器１７ａを介して、送信信号５９が生成される。送信信号５９は、局部発振器８からの正弦波信号５４と加算器１０で加算され、送信信号６１が生成される。ここで、正弦波信号５４の周波数帯は、送信信号５９とは異なる周波数帯とし、過変調による歪みが送信信号５９の帯域内に現れないようにする。
【０１４４】
パワー制御器２９は、親局２で受信した際の送信信号５９のパワーが所定値pになるように、送信信号５９のパワー強度を、増幅器１７のゲインを変化させて制御する。そして、送信信号６１の光変調度が所定値になるように、正弦波信号５４の強度を、増幅器１７ｂのゲインにより変化させる。
【０１４５】
図２５（ａ）（ｂ）は、親局２との光伝送距離が異なる２つの子局１ａ，１ｂでの送信信号の周波数スペクトル図であり、図２５（ａ）は親局２までの光伝搬路損失が小さい子局１ａの送信信号６１ａの周波数スペクトル図、図２５（ｂ）は親局２までの光伝搬路損失が大きい子局１ａの送信信号６１ａの周波数スペクトル図である。
【０１４６】
受信器１４への光信号１００ｂの入力パワーが小さいため、送信信号５９ｂのパワーPbは大きめに設定し、正弦波信号５４ｂのパワーは小さく設定する。送信信号６１ｂに対する光信号１００ｂの光変調度は、子局１ａの光変調度と同様に所定値に設定されている。子局１ａ，１ｂにおいて、送信信号５９ａ，５９ｂのパワーには依存せず、光変調度は一定であるため、安定して光信号１００ａ，１００ｂの干渉性を抑圧することができる。
【０１４７】
図２５（ｃ）は親局２内の受信器１４の受信信号５８の周波数スペクトル図である。受信信号５８は、送信信号５９ａ，５９ｂ、正弦波信号５４ａ，５４ｂで構成されており、過変調により発生する歪成分は無視する。前述の送信信号５９ａ，５９ｂと正弦波信号５４ａ，５４ｂのパワー制御により、親局２で受信した際の送信信号５９ａ，５９ｂのパワーは、どちらも設定値であるｐにすることができる。図２５では子局１ａ、１ｂに対して説明したが、各子局１ａ〜１ｎに対しても同様のパワー制御方法で、受信信号５８を逆拡散して抽出した各子局１ａ〜１ｎからの送信信号の雑音特性を良好に保てる。
【０１４８】
このように、第１０の実施形態では、各子局内に増幅器１７ａ，１７ｂを設けて受信信号のパワー制御を行うため、第９の実施形態と同様の効果が得られる。
【０１４９】
ところで、マルチポイント-to-ポイントアクセスの光伝送系でスペクトル拡散して多重化する場合、ＬＥＤによる光空間伝送を行うのが一般的である。その理由は、光ファイバ伝送では、複数の光信号が多重されると、ビート雑音が発生するために、受信感度が著しく劣化するからである。
【０１５０】
しかしながら、本発明の場合、過変調を行い、かつ情報送信信号の有無にかかわらずビート雑音を抑制できるため、光ファイバを用いて光信号を伝送することができる。
【０１５１】
子局１間のスペクトル拡散多重は、ＳＣＭ多重と比較して、各子局１ａ〜１ｎにおける送信信号５９ａ〜５９ｎが同一の周波数帯を使用するため、周波数変換器１６のハードウェアを同一の構成にすることができる。
【０１５２】
各子局１ａ〜１ｎでは、直交性をもつ異なる拡散符号をそれぞれ備える必要があるが、拡散符号はメモリー等に記憶させておけばよく、ハードウェアの構成は同じである。
【０１５３】
本第１〜第９の実施形態では、ＷＬＬ、ＩＴＳ等の無線通信の基地局に対して説明をした。しかしながら、本発明は、ＩＴＶ (Industrial TV：産業テレビ)や、デジタル信号等のあらゆる情報送信信号を、送信信号として取り扱う光ネットワークにも適用することが可能である。
【０１５４】
【発明の効果】
以上詳細に説明したように、本発明によれば、親局への情報送信信号が存在しなくても、光変調度が１よりも大きい光信号を各子局から送信するようにしたため、干渉性の強い光信号が親局２で受信されるおそれがなくなり、親局はビート雑音の影響を受けなくなる。このため、受信感度が向上し、安定性かつ信頼性に優れた光伝送システムを構築できる。
【０１５５】
また、情報送信信号が存在する場合に限り、光信号を各子局から親局に送信するようにしたため、情報送信信号が存在しない場合に、干渉性の強い光信号が親局２に送信されるおそれがなくなる。
【０１５６】
また、子局内のレーザにおいては、通常、ＡＰＣ(Automatic Power Control)を使用して、光平均出力を安定化させる機能が備えられる。このＡＰＣの光出力の基準レベルを、情報送信信号の有無により調整することで、光及び電気スイッチ等のアクティブな構成要素を必要とせずに、情報送信信号が存在しない場合においても、干渉性の強い光信号を子局から親局に送信することはなくなる。
【０１５７】
また、情報送信信号を二次変調した後に、光信号に変換して親局に送信する場合においても、情報送信信号が存在しない場合には、無変調波でレーザを駆動するため、過変調を引き続き施すことができ、親局において、光ビート雑音の発生を低減させ続けることができる。
【図面の簡単な説明】
【図１】光伝送システムの第１の実施形態の構成を示すブロック図。
【図２】光変調度を説明する図。
【図３】ファブリーぺロー型半導体レーザを使用した場合の光信号１００の光スペクトル図。
【図４】第１の実施形態における受信信号の周波数スペクトル図。
【図５】従来の受信信号の周波数スペクトル図。
【図６】本発明による光伝送システムの第２の実施形態のブロック図。
【図７】受信信号と、この受信信号を周波数変換したＩＦ信号との周波数スペクトル図。
【図８】本発明による光伝送システムの第３の実施形態のブロック図。
【図９】光信号のon/off制御に電気スイッチを用いた例を示す図。
【図１０】光信号のon/off制御に光スイッチを用いた例を示す図。
【図１１】本発明に係る光伝送システムの第４の実施形態を説明する図。
【図１２】 TDMAにおける情報送信信号のビット構成を示す図。
【図１３】 CDMAにおける情報送信信号のビット構成を示す図。
【図１４】本発明による光伝送システムの第５の実施形態のブロック図。
【図１５】（ａ），（ｂ），（ｃ）は各子局内の二次変調器の内部構成および特性を示す図。
【図１６】子局が周波数変調した二次変調信号を伝送する場合の親局内の抽出器の内部構成を示すブロック図。
【図１７】（ａ），（ｂ），（ｃ）は子局内の二次変調器の内部構成および特性を示す図。
【図１８】子局が振幅変調した二次変調信号を伝送する場合の親局内の抽出器の内部構成を示すブロック図。
【図１９】本発明による光伝送システムの第７の実施形態のブロック図。
【図２０】親局での受信信号の周波数スペクトル図。
【図２１】（ａ）、（ｂ）は拡散符号を用いた直接拡散方式の拡散器と逆拡散器のブロック構成を示す図。
【図２２】（ａ）は拡散器の内部構成を示すブロック図、（ｂ）は逆拡散器の内部構成を示すブロック図。
【図２３】本発明による光伝送システムの第９の実施形態のブロック図。
【図２４】子局の内部構成を示すブロック図。
【図２５】（ａ）（ｂ）は、親局との光伝送距離が異なる２つの子局での送信信号の周波数スペクトル図。
【図２６】従来の光伝送システムの概略構成を示すブロック図。
【符号の説明】
１ａ〜１ｎ 子局
２ 親局
３、３ａ〜３ｎ 光ファイバ
４ 光合成器
５ａ〜５ｎ 加入者
６ アンテナ
７ 信号検出器
８ 局部発振器
９ スイッチ
１０ 加算器
１１ バイアスティ
１２ 電流源
１３ レーザ
１４ フォトディテクタ
１５ 復調器
１６ 周波数変換器
１８ 電気スイッチ
１９ 光スイッチ
２０ レーザモジュール
２１ フォトディテクタ
２２ 設定電圧
２３ 差動増幅器
２４ ループフィルタ
２５ 加算器
２６ 拡散器
２７ 逆拡散器
３０ 二次変調器
３１ 抽出器
３２ 電圧制御発振器
３３ 分配器
３４ａ〜３４ｎ バンドパスフィルタ
３５ａ〜３５ｎ 周波数弁別器
３６ａ〜３６ｎ バンドパスフィルタ
４０ 乗算器
４１ 拡散符号発生器[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical transmission system and an optical transmission method for optically multiplexing an analog signal such as a radio signal from a plurality of slave stations to a master station via an optical fiber.
[0002]
[Prior art]
In recent years, mobile base stations such as mobile communication, WLL (Wireless Local Loop), and ITS (Intelligent Transport System) have been placed at various locations to transmit information transmission signals (radio waves) to the master station using optical fibers. Optical networks are known. In particular, the optical SCMA (Sub-Carrier Multiplex Access) network enables optical signals modulated with different subcarrier signals for each slave station to be multiplexed in the optical transmission path and received at the master station in a batch. To do. An optical SCMA (Sub-Carrier Multiplex) network has a simple configuration and can receive and process a large number of electric signals at the same time.
[0003]
One of the major problems of the optical SCMA technology is beat noise generated by interference of optical signals from a plurality of slave stations. Beat noise is a noise component generated in the frequency band of the wavelength difference Δλ when optical signals A and B having a wavelength difference of Δλ from a plurality of slave stations are received together.
[0004]
Several solutions have been proposed for this beat noise. Among them, there is a method of making the degree of optical modulation greater than 1 when directly modulating a semiconductor laser with an information transmission signal, which is called overmodulation or clipping modulation. According to this method, since the coherence of the optical signal can be suppressed, the wavelengths of the slave stations may match. Therefore, each slave station does not require control such as stabilizing the laser wavelength, and the configuration of the optical transmission apparatus can be simplified.
[0005]
FIG. 26 is a block diagram showing a schematic configuration of a conventional optical transmission system. The optical transmission system of FIG. 26 has a configuration in which a plurality of slave stations 1 a to 1 n and a master station 2 are connected via optical fibers 3 and 3 a to 3 n and an optical combiner 4.
[0006]
In the slave stations 1a to 1n, there are an antenna 6, a frequency converter 16 that converts a signal received by the antenna 6 into an IF signal, a local oscillator 12 that outputs a local oscillation signal, an IF signal and a local oscillation signal. A bias tee 11 that generates a modulation signal based on the signal and a laser 13 that outputs an optical signal corresponding to the modulation signal are provided.
[0007]
Inside the master station 2, there are provided a photodetector 14 for converting optical signals from the slave stations 1a to 1n into electric signals, and a demodulator 15 for performing demodulation processing and restoring a received signal at the antenna 6. .
[0008]
In the system of FIG. 26, a modulation signal obtained by superimposing an information transmission signal on a DC bias signal is injected into a laser, and direct modulation is performed. When the optical signal is generated by the laser 13, the amplitude of the modulation signal from the bias tee 11 is increased so as to be transferred to the laser oscillation threshold value or less, thereby increasing the optical modulation degree.
[0009]
[Problems to be solved by the invention]
As described above, the optical transmission link that suppresses beat noise using the overmodulation technique has a simple optical transmission system configuration, and is expected to be applied to radio base stations such as mobile communication, WLL, and ITS. .
[0010]
In wireless communication such as mobile communication, WLL, and ITS, the presence or absence of an information transmission signal depends on the communication connection of the subscriber. That is, the information transmission signal is bursty and has a time zone that does not exist. In the slave station where no information transmission signal exists, the laser is driven only by the DC bias signal. At this time, since the optical signal output from the laser is not over-modulated, the optical spectrum is not diffused and the coherence becomes high.
[0011]
In the master station, when the optical signal wavelength from another slave station approaches the optical signal wavelength having strong coherence, beat noise appears in the information transmission signal band. Due to the beat noise caused by such one slave station, the reception sensitivity of the information transmission signals of all the slave stations is deteriorated, and in the worst case, there is a problem that reception is impossible at all.
[0012]
The present invention has been made in view of these points, and an object of the present invention is to provide an optical transmission system and an optical transmission method that are stable and reliable without being affected by beat noise. It is in.
[0013]
[Means for Solving the Problems]
According to one aspect of the present invention, a plurality of slave stations that output an optical signal according to an information transmission signal, and an optical combiner that generates an optical multiplexed signal by multiplexing the optical signals output from each of the plurality of slave stations And a master station that separates and extracts information transmission signals corresponding to each of the plurality of slave stations based on a signal correlated with the optical multiplexed signal. Each of them superimposes a predetermined dummy signal on the information transmission signal so that an optical signal having a degree of optical modulation greater than 1 is transmitted to the optical combining means even if there is no information transmission signal to the master station A superimposing unit that generates an optical signal based on an output signal of the superimposing unit, and the superimposing unit detects whether or not an information transmission signal to the master station exists; Output the dummy signal of frequency Switching means for passing the dummy signal if the information transmission signal does not exist, and blocking the dummy signal if the information transmission signal exists based on the detection result of the signal detection means; And an adding means for adding the output of the switching means and the information transmission signal, and each of the slave stations generates an optical signal based on the output signal of the adding means. A transmission system is provided.
[0015]
According to one aspect of the present invention, a plurality of slave stations that output an optical signal according to an information transmission signal, and an optical combiner that generates an optical multiplexed signal by multiplexing the optical signals output from each of the plurality of slave stations And a master station that separates and extracts information transmission signals corresponding to each of the plurality of slave stations based on a signal correlated with the optical multiplexed signal. Each has output switching means for outputting an optical signal corresponding to the information transmission signal only when there is an information transmission signal to the master station, and the optical modulation degree of the optical signal is made larger than 1. The output switching means includes an APC (Automatic Power Control) means for performing feedback control so that the signal intensity of the optical signal matches a reference level, and a signal for detecting whether or not an information transmission signal to the master station exists. Detecting means; and A reference level adjusting means for setting the reference level to zero if the information transmission signal is not present, and adjusting the reference level to a predetermined value if the information transmission signal is present, based on a detection result of the signal detection means; An optical transmission system is provided.
[0016]
According to one aspect of the present invention, a plurality of slave stations that output an optical signal according to an information transmission signal, and an optical combiner that generates an optical multiplexed signal by multiplexing the optical signals output from each of the plurality of slave stations And an optical transmission method for performing signal transmission between the means and a master station that separates and extracts an information transmission signal corresponding to each of the plurality of slave stations based on a signal correlated with the optical multiplexed signal. Each of the slave stations has a predetermined information transmission signal so that an optical signal having a degree of optical modulation greater than 1 is transmitted to the optical combining means even if there is no information transmission signal to the master station. In the process of generating a signal in which a dummy signal is superimposed, generating the optical signal based on the superimposed signal, and generating the superimposed signal, it is detected whether an information transmission signal to the master station exists. And the dummy signal of a predetermined frequency If the information transmission signal is not present, the dummy signal is allowed to pass, and if the information transmission signal is present, the dummy signal is blocked based on the detection result of whether or not the information transmission signal is present. A switching means is provided for adding the output of the switching means and the information transmission signal, and each of the slave stations is configured to add the optical signal based on an output signal obtained by adding the output of the switching means and the information transmission signal. An optical transmission method characterized by generating is provided.
[0018]
According to one aspect of the present invention, a plurality of slave stations that output an optical signal corresponding to an information transmission signal, and an optical combiner that generates an optical multiplexed signal by multiplexing the optical signals output from each of the plurality of slave stations And an optical transmission method for performing signal transmission between the means and a master station that separates and extracts an information transmission signal corresponding to each of the plurality of slave stations based on a signal correlated with the optical multiplexed signal. Each of the slave stations includes an APC step for performing feedback control so that an average signal intensity of the optical signal matches a reference level, and a detection step for detecting whether or not an information transmission signal to the master station exists. An adjustment step for adjusting the reference level so that an optical signal is not output when the information transmission signal does not exist based on the detection result of the detection step, and when the optical signal is output The optical transmission method, wherein the optical modulation index of the optical signal is greater than 1 is provided.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an optical transmission system according to the present invention will be specifically described with reference to the drawings.
[0030]
(First embodiment)
FIG. 1 is a block diagram showing the configuration of the first embodiment of the optical transmission system. The optical transmission system of FIG. 1 includes a plurality of slave stations 1a, 1b to 1n, a master station 2, and an optical combiner (light combiner) 4 including a star coupler that optically multiplexes optical signals from the slave stations 1a to 1n. And have. Each of the slave stations 1a to 1n transmits an optical signal with the optical combiner 4 via the optical fibers 3a to 3n, and the master station 2 transmits an optical signal with the optical combiner 4 via the optical fiber 3.
[0031]
Each of the slave stations 1a to 1n includes an antenna 6, a signal detector (signal detection means) 7, a frequency converter 16, a local oscillator (dummy signal generation means) 8, a switch (switching means) 9, and an adder. (Adding means) 10, a bias tee 11, a current source 12, and a laser 13 are included. Among these, the signal detector 7, the local oscillator 8, the switch 9, and the adder 10 correspond to the superimposing means.
[0032]
The optical combiner 4 sub-carrier-multiplexes the optical signals from the slave stations 1a to 1n and transmits them to the master station 2 via the optical fiber.
[0033]
The master station 2 includes a photodetector (PD) 14 that converts the optical signal from the optical combiner 4 into an electrical signal, and a demodulator 15 that restores the electrical signal to a signal before modulation.
[0034]
Next, the internal configuration of the slave stations 1a to 1n will be described. The slave station 1a will be described as a representative, but the slave stations 1b to 1n are similarly configured.
[0035]
The slave station 1a receives the radio signal 51a transmitted from the subscriber 5a by the antenna 6 and obtains the received signal 52a. The received signal 52a is frequency-converted to a desired band by the frequency converter 16, and becomes an IF (Intermediate frequency) signal 53a.
[0036]
A unique frequency band is assigned to each slave station so that the IF signals 53a to 53n of the slave stations 1a to 1n can be subcarrier multiplexed with each other.
[0037]
The slave station 1a includes a signal detector 7 and monitors the presence or absence of the received signal 52a. The signal detector 7 performs on / off control of the sine wave signal 54 a output from the local oscillator 8 by the switch 9 according to the presence or absence of the reception signal 52 a.
[0038]
When the received signal 52a is not present, the switch 9 is turned on to pass the sine wave signal 54a, and when the received signal 52a is present, the switch 9 is turned off to block the sine wave signal 54a. The sine wave signal 54a is set to a frequency band that does not affect the bands assigned to the other slave stations 1b to 1n. Simply, it may be the same as the band of the IF signal 53a.
[0039]
The adder 10 adds the IF signal 53a and the sine wave signal 54a to generate a transmission signal 55a. The transmission signal 55a is either the IF signal 53a or the sine wave signal 54a according to the detection result of the signal detector 7, and the amplitude of the transmission signal 55a never becomes zero.
[0040]
The transmission signal 55 a is added to the DC bias current 56 a from the current source 12 by the bias tee 11 to become a modulation signal 57 a and is injected into the laser 13. The laser 13 outputs an optical signal 100 a having an intensity corresponding to the modulation signal 57 and transmits it to the master station 2.
[0041]
The master station 2 receives the optical signals 100 a to 100 n transmitted from the slave stations 1 a to 1 n by the optical receiver 14 such as a photo detector, and obtains a received signal 58. The received signal 58 is input to the demodulator 15 and demodulated into information from the subscribers 5a to 5n.
[0042]
Here, the optical modulation index OMI (Optical Modulation Index) of the optical signal 100 is 100% or more. The definition of the optical modulation index OMI (Optical Modulation Index) is shown below. When the slave station modulates the laser with a sine wave of frequency fs, the transmitted optical signal P (t) is expressed by the following equation (1).
[0043]
P (t) = a + b · cos (2pfst) (1)
At this time, the light modulation degree OMI is
OMI = a / b (2)
It is. Here, a is the DC bias current value, and b is the half-value amplitude of the signal current value.
[0044]
As shown in FIG. 2, even when the laser is overmodulated, the half-value amplitude b is used to define the degree of light modulation in the same manner as the equation (1).
[0045]
If the laser is overmodulated, OMI> 100% (or 1). For example, FIG. 3 is an optical spectrum diagram of the optical signal 100 when a Fabry-Perot semiconductor laser is used as the laser 13.
[0046]
As shown in the figure, when OMI> 100%, the optical spectrum band is widened and coherency is suppressed. In order to stabilize the optical modulation degree OMI to a predetermined value of 100% or more, for example, an amplifier having an AGC (Automatic Gain Control) function may be inserted on the output side of the frequency converter 16.
[0047]
FIG. 4 is a frequency spectrum diagram of the received signal 58. When the laser is overmodulated in each of the slave stations 2a to 2n, the beat noise is spread over a wide band while the peak value is suppressed. Therefore, as shown in FIG. 4, even if the wavelengths of the optical signals 100a to 100n from the slave stations 1a to 1n match, the amount of beat noise in the bands of the signals 58a to 58n is small, and the reception sensitivity is improved. The impact is negligible.
[0048]
Even when the wireless communication between the arbitrary slave station 1m (m is an integer of 1 ≦ m ≦ n), the master station 2, and the subscriber 5m is finished and the received signal 52 no longer exists, the sine wave The optical modulation degree OMI of the laser 13 is kept at 100% or more by the signal 54. Therefore, the optical signal 100m output from the laser 13 can maintain a state in which coherency is suppressed, and the influence of beat noise does not appear in the received signal 58 at the master station 2.
[0049]
In the conventional optical transmission link shown in FIG. 26, when wireless communication is completed at a certain slave station 2m, the transmission signal 55m disappears, so the optical modulation degree OMI becomes 0%. In this case, the frequency spectrum of the received signal 58 at the master station is as shown in FIG. Since the optical signal 100m output from the laser 13 has strong coherence, the beat noise contained in the received signal 58 of the master station 2 is not spread over a wide band but concentrated in a narrow band. When the wavelength difference between the highly coherent optical signal 100 m and the other optical signal 100 ′ m approaches the frequency band of the subcarrier signal transmitted from the other slave station 2, the reception sensitivity of the received signal 58 is degraded, and the worst In this case, reception is impossible.
[0050]
When the subscriber 5 is fixed like WLL, the power of the radio signal 51 received by the slave station 1 is controlled to a substantially constant value. Therefore, as described above, it is possible to relatively easily maintain the optical modulation degree at a predetermined value of 100% or more by controlling the switch 9 with the presence or absence of the wireless signal 51 and including the amplifier with the AGC function.
[0051]
As described above, in the first embodiment, by providing the switch 9 for turning on / off the sine wave signal 54, each optical signal having an optical modulation degree of 1 or more can be transmitted even if the antenna 6 does not receive the radio signal 51. Since transmission is performed from the slave station, there is no possibility that an optical signal having high coherence is transmitted to the master station 2, and beat noise in the master station 2 can be suppressed.
[0052]
(Second Embodiment)
In the first embodiment, when the power control of the radio signal 51 between the subscriber 5 and the slave station 1 is performed, if the AGC function is present in the signal path, the incident power of the received signal at the slave station 1 is the master station 2. There is a problem that you don't understand.
[0053]
When the AGC function does not exist in the signal line system, it is difficult to control the optical modulation degree of the optical signal 100 to a predetermined value using only the IF signal 53. This is because if the subscriber 5 is a terminal such as an ITS (Intelligent Transport System) or a mobile phone, the direct wave may be cut off between the slave station 1 and the subscriber 5, and the power of the received radio signal 51 is low. This is because it is not constant. In addition, when the received signal 52 has a plurality of channels, the power of the received signal 52 changes depending on whether all channels are used or not used.
[0054]
Therefore, the sine wave signal 54 output from the local oscillator 12 in FIG. 1 is not only controlled on / off by the switch 9, but also the signal intensity is controlled to set the OMI of the optical signal 100 to a predetermined value of 100% or more. It is necessary to keep.
[0055]
FIG. 6 is a block diagram of the second embodiment of the optical transmission system according to the present invention, which is characterized in that the signal intensity of the sine wave signal 54 is controlled instead of the on / off control of the sine wave signal 54. FIG. 6 shows the configuration of only the slave station 1, and the same reference numerals are given to the same components.
[0056]
The optical transmission system of FIG. 6 includes an amplifier (gain adjusting means) 17 that controls the signal intensity of the sine wave signal output from the local oscillator 8, and the amplification factor of the amplifier 17 can be controlled. Here, the frequency band of the sine wave signal 54 is a frequency band different from that of the IF signal 53.
[0057]
The signal detector 7 monitors the power of the received signal 52 and controls the amplification factor of the amplifier 17 so that the optical modulation degree of the optical signal 100 always exceeds 100%.
[0058]
FIG. 7 is a frequency spectrum diagram of the received signal 52 and an IF signal 53 obtained by frequency-converting the received signal 52. Here, an example having reception signals 52 for a plurality of channels will be described.
[0059]
For example, when the received signal 52 is a plurality of channels, the frequency arrangement of the received signal 52 and the sine wave signal 54 is set at unequal intervals so as to avoid IM3 (3rd interference modulation) generated by overmodulation. Let the optical modulation degree of the IF signal 53 be mi and the optical modulation degree of the sine wave signal 54 be mc. The optical modulation degree mt of the entire modulation signal is expressed by the following equation (3).
[0060]
[Expression 1]

The optical modulation degree mc of the sine wave signal 54 is controlled in accordance with the intensity mi1, mi2,... Mc of the information transmission signal so that this mt always becomes a predetermined value of 100% (or 1) or more.
[0061]
7A shows the frequency spectrum when the number of channels of the received signal 52 is large, FIG. 7B shows the frequency spectrum when the number of channels is small, and FIG. 7C shows the frequency spectrum when the signal strength of the received signal 52 is small. .
[0062]
In the optical transmission system of FIG. 6, when the number of channels of the received signal 52 is small as shown in FIG. 7B or when the signal strength of the received signal 52 is low as shown in FIG. The intensity of the sine wave signal 54 is adjusted by variably controlling the amplification factor of the amplifier 17 so that the modulation degree mt becomes a predetermined value.
[0063]
As a power control of the sine wave signal 54, a variable voltage attenuator or the like may be applied instead of changing the amplification factor of the amplifier 17.
[0064]
In the second embodiment, laser overmodulation is performed using both the information transmission signal and the dummy signal. As a ratio of the optical modulation degree, it is desirable to control the amplitude value of the information transmission signal to be maximized and stabilize the optical modulation degree with a dummy signal. Even if there is an amplitude limitation due to overmodulation in the received signal at the master station 2, the greater the degree of optical modulation at the slave station, the greater the CNR (carrier to noise ratio) and the better the receiving sensitivity.
[0065]
Thus, in the second embodiment, since the amplifier 17 capable of adjusting the amplification factor of the sine wave signal 54 is provided, even if the AGC function exists in the signal line system of each slave station, the optical modulation of the optical signal is performed. The degree can always be maintained at 1 or higher. For this reason, a stable and highly reliable optical transmission system can be constructed.
[0066]
(Third embodiment)
In the first and second embodiments, the control method for stabilizing the optical modulation degree of the optical signal 100 with respect to the presence / absence of the wireless signal 51 and the signal intensity is performed. On the other hand, in the third embodiment described below, on / off control of the optical signal 100 is performed based on the presence or absence of the wireless signal 51.
[0067]
FIG. 8 is a block diagram of a third embodiment of the optical transmission system according to the present invention. The main configurations of the slave station 1 and the master station 2 are the same as those in FIG. 1, and the same components are denoted by the same reference numerals.
[0068]
The slave station 1 a receives the radio signal 51 a transmitted from the subscriber 5 a by the antenna 6 and obtains the received signal 52. The slave station 1 a includes a signal detector 7 and monitors the presence or absence of the reception signal 52. The signal detector 7 controls the DC bias current 56 output from the current source 12 according to the presence / absence of the reception signal 52. When the reception signal 52 is not present, the DC bias current 56 is set to zero, and when the reception signal 52 is present, the DC bias current 56 is returned to a predetermined current value. That is, when there is no radio signal 51 received by the slave station 1, the modulation signal 57 injected into the laser 13 is zero, and the optical signal 100 is output from the laser 13 only when the radio signal 51 is present. The OMI of the optical signal 100 output at that time is set to 100% or more.
[0069]
There are several possible methods for turning off the optical signal when the received signal 52 is not present. As a simple method, for example, there is a method using an electric switch or an optical switch.
[0070]
FIG. 9 is a diagram showing an example in which an electrical switch is used for on / off control of an optical signal. A switch (output switching means) 18 is inserted between the bias tee 11 and the laser 13. The presence or absence of the received signal 52 is confirmed by the signal detector 7, and when there is no received signal 52, the switch 18 is shut off. When the reception signal 52 exists, the switch 18 is turned on to drive the laser 13 and output the optical signal 100 corresponding to the modulation signal 57.
[0071]
On the other hand, FIG. 10 is a diagram showing an example in which an optical switch is used for on / off control of an optical signal, and an optical switch 19 is inserted after the laser 13. In FIG. 10, as in the fourth embodiment, the presence or absence of the received signal 52 is confirmed by the signal detector 7. The signal detector 7 turns off the optical switch 19 if the received signal 52 is not present, and turns on the optical switch 19 if the received signal 52 is present, thereby controlling the optical signal 100 on / off.
[0072]
As described above, in the third embodiment, only when each slave station receives the information transmission signal, the corresponding optical signal is transmitted from each slave station. Will not be transmitted to and will be less susceptible to beat noise. For this reason, a stable and highly reliable optical transmission system can be obtained.
[0073]
(Fourth embodiment)
In the fourth embodiment, on / off control of an optical signal is performed without using an electrical switch or an optical switch.
[0074]
FIG. 11 is a diagram for explaining a fourth embodiment of the optical transmission system according to the present invention, and is a block diagram showing an internal configuration of a slave station.
[0075]
Usually, the laser 13 performs APC (Automatic Power Control) in order to suppress the fluctuation of the average output power of the optical signal 100 due to the ambient temperature. Turning on / off the optical signal 100 while the APC loop is operating is contrary to the control for stabilizing the optical power.
[0076]
FIG. 11 is a diagram showing a block configuration of a slave station when APC is performed. The output power of the optical signal 100 is monitored by a photodetector (PD) 21 built in the laser module 20. The monitor value is compared with the set voltage (reference level adjusting means) 22, and difference component information 60 is obtained by the differential amplifier 23. The difference component information 60 is fed back to the bias current value 56 via the adder 25 by limiting the band by the loop filter 24 so that malfunction such as oscillation does not occur. By such a series of controls, the average power of the optical signal 100 output from the laser 13 is stabilized.
[0077]
In order to stably turn off the optical signal in the laser 13 to which the APC loop (APC means) is applied, for example, control may be applied to the set voltage value 22. That is, when there is no received signal 52, the set voltage value 22 is set to zero, and the output of the optical signal 100 is suppressed by the APC loop operation. In a state where the reception signal 52 exists, the set voltage value 22 is returned to a predetermined set value, and the optical signal 100 is output by the APC loop operation.
[0078]
By the method as described above, there is no possibility that the optical signal 100 with strong OMI ≦ 100% is transmitted from each slave station to the master station 2 without depending on the connection state of the subscriber 5. For this reason, the master station 2 is not affected by beat noise, and reception sensitivity is improved.
[0079]
Next, referring to FIG. 11, the timing for turning on / off the optical signal will be described. Time division multiplexing access (TDMA) is widely used for wireless communication such as mobile phones or WLLs.
[0080]
The information transmission signal is bits constituting data, and the carrier wave is phase, frequency, and amplitude modulated. The bit configuration of the information transmission signal is expressed as shown in FIG. FIG. 12 is a timing chart of an information transmission signal when a certain slave station is connected to subscribers A, B, and C by TDMA. The information transmission signal has a configuration in which ramp bits are arranged on both sides of information bits including a header, voice, data, and the like, and a guard time is provided between adjacent ramp bits. The ramp bit is inserted mainly to suppress the spread of the band and does not include information.
[0081]
The on / off timing of the optical signal is performed within the time when the ramp bit is transmitted, as shown in FIG. By performing within this time, when there is no information transmission signal that is a guard time, the optical signal is always turned off. In addition, during the time when the information bits are transmitted, the optical signal is always turned on, and no information bit is lost.
[0082]
On the other hand, FIG. 13 shows the configuration of an information transmission signal in CDMA (Code Division Multiplexing Access), which is another access method for wireless communication. As shown in the figure, the CDMA information transmission signal is different in configuration from the TDMA information transmission signal of FIG.
[0083]
FIG. 13 is a timing chart when a certain slave station is connected to subscribers A and B by CDMA. Ramp bits do not exist in CDMA information transmission signals. However, since the spectrum is spread, the information transmission signal is composed of chip bits having a transmission rate higher than that of the data bits.
[0084]
In CDMA, it is possible to recover information bits by performing spectrum despreading for a certain amount of chip bit loss. The on / off timing of the optical signal may be matched with the input / output state of the information transmission signal which is CDMA. Even if a delay of the on / off switch or the like occurs and some chip bits are lost, there is no big problem for the above reason.
[0085]
As described above, in the fourth embodiment, the voltage level of the setting voltage 22 input to the differential amplifier 23 in the APC loop is switched depending on the presence or absence of the reception signal 52. The transmission of the optical signal can be suppressed. Further, since an APC loop is provided in each slave station, the output level of the optical signal can be stabilized.
[0086]
(Fifth embodiment)
In the fifth embodiment, the secondary signal is subjected to secondary modulation on the radio signal received by the slave station via the antenna and transmitted to the master station 2, and when there is no radio signal, an unmodulated signal is generated.
[0087]
Typical secondary modulation includes secondary frequency modulation and secondary amplitude modulation. Spread spectrum is also considered as a kind of secondary modulation because an information transmission signal obtained by modulating a radio signal is multiplied by a spread code. In the fifth embodiment, an example in which frequency modulation is used for secondary modulation will be described.
[0088]
FIG. 14 is a block diagram of a fifth embodiment of an optical transmission system according to the present invention. In FIG. 14, the same components as those in FIG.
[0089]
Each of the slave stations 1a to 1n includes an antenna 6, a frequency converter 16, a secondary modulator (secondary modulation means) 30, a bias tee 11, a current source 12, and a laser (optical signal generation means) 13. Have The master station 2 includes an optical receiver (PD) 14, an extractor (extraction means) 31, and a demodulator 15.
[0090]
The slave station 1a generates the IF signal 53a by down-converting the reception signal 52a received by the antenna 6 by the frequency converter 16. The secondary modulator 30 secondarily modulates the IF signal 53a to generate a transmission signal 61a. This transmission signal 61 a is added to the DC bias current from the current source 12 at the bias tee 11, and then converted into an optical signal 100 a with an optical modulation degree of 100% or more by the laser 13 and transmitted to the master station 2. Is done.
[0091]
The frequency bands of the transmission signals 61a to 61n of the slave stations 1a to 1n are set for each slave station and are subcarrier multiplexed by the optical combiner 4. The master station receives the optically multiplexed optical signal, and since the transmission signals of the respective slave stations are arranged in different frequency bands, it is possible to obtain a reception signal that is subcarrier multiplexed. Since the optical signals 100a to 100n from the respective slave stations 1a to 1n are set to an optical modulation degree of 100% or more, the beat noise is diffused, and a large influence that deteriorates the reception sensitivity does not appear.
[0092]
The reception signal 58 received by the master station 2 is separated and extracted into transmission signals 61 a to 1 n by the extractor 31 and input to the demodulator 15. Even when the subscriber 5 is not connected and there is no received signal 52, the transmission signal 62, which is the output of the secondary modulator 30, has a predetermined power, and the degree of optical modulation always remains 100% or more. The coherence of the optical signal 100 can be suppressed.
[0093]
FIG. 15 is a diagram showing the internal configuration and characteristics of the secondary modulator 30 in the slave stations 1a to 1n. FIG. 15 (a) shows the case where the secondary modulator 30 is configured by a voltage controlled oscillator (VCO) 32. FIG. 15B is a block diagram, FIG. 15B is a diagram showing the relationship between the frequency and the signal strength of the output signal of the voltage controlled oscillator when there is no received signal, and FIG. 15C is the voltage controlled oscillator when there is a received signal. It is a figure which shows the relationship between the frequency and signal strength of an output signal.
[0094]
The voltage controlled oscillator 32 in FIG. 15A sets the frequency so that each slave station has a different center frequency fm. When the reception signal 52, that is, the IF signal 53 does not exist, the voltage control oscillator 32 outputs a sine wave having a frequency fm as the transmission signal 62 as shown in FIG. If the IF signal 53 exists, the oscillation frequency of the VCO 32 is frequency-modulated according to the amplitude of the IF signal 53, and a transmission signal 62 having a frequency spectrum as shown in FIG.
[0095]
Thus, the transmission signal 62 is always obtained without depending on the presence or absence of the reception signal (IF signal), and the laser can always be overmodulated.
[0096]
FIG. 16 is a block diagram showing an internal configuration of the extractor 31 in the master station 2 when the slave stations 1a to 1n transmit a frequency-modulated secondary modulated signal. The extractor 31 of FIG. 16 includes a distributor (distribution means) 33, bandpass filters 34a to 34n, frequency discriminators (frequency discrimination means) 35a to 35n, and bandpass filters 36a to 36n.
[0097]
The received signal 58 input to the extractor 31 is first distributed by the distributor 33 into a number corresponding to each of the slave stations 1a to 1n. The transmission signals 62a to 62n from the respective slave stations 1a to 1n are arranged in different frequency bands, and the band pass filters 34a to 34n respectively extract desired bands. Thereafter, the frequency discriminators 35a-n perform frequency-amplitude conversion, and the band-pass filters 36a-36n again extract the transmission signals 53a-n from the respective slave stations 1a-1n.
[0098]
As described above, in the fifth embodiment, the secondary modulation is performed on the radio signal received by the antenna in each slave station. Therefore, when there is no radio signal, an unmodulated signal is obtained. For this reason, there is no possibility that a highly coherent optical signal is transmitted from the slave station, and the master station 2 is not affected by beat noise. In the fifth embodiment, since frequency modulation is performed as secondary modulation, a secondary modulation signal can be generated with a relatively simple configuration using a voltage-controlled oscillator.
[0099]
(Sixth embodiment)
In the sixth embodiment, amplitude modulation is used for secondary modulation. The optical transmission system of the sixth embodiment is configured in the same manner as in FIG. 14, and the internal configuration of the secondary modulators in the slave stations 1 a to 1 n and the internal configuration of the extractor in the master station 2 are the fifth. Different from the embodiment.
[0100]
17 is a diagram showing the internal configuration and characteristics of the secondary modulator 30 in the slave stations 1a to 1n. FIG. 17A is a block diagram showing the internal configuration of the secondary modulator 30, and FIG. FIG. 17 is a diagram showing the relationship between the frequency and signal strength of the output signal of the secondary modulator 30 when there is no received signal, and FIG. 17C shows the output signal of the secondary modulator 30 when there is a received signal. It is a figure which shows the relationship between a frequency and signal strength.
[0101]
As shown in FIG. 17A, the secondary modulator 30 of the sixth embodiment includes a local oscillator (first local oscillator) 37 that outputs a sine wave signal 63 that is a reference signal, and an amplitude modulator ( (Amplitude modulation means) 38.
[0102]
The frequency fm of the sine wave signal 63 is set to be different for each of the slave stations 1a to 1n. When the reception signal 52, that is, the IF signal 53 does not exist, the sine wave signal 63 of the frequency fm is output as it is as the transmission signal 62 as shown in the frequency spectrum of FIG. When the IF signal 53 exists, the amplitude of the sine wave signal 63 is modulated according to the amplitude of the IF signal 53, and a transmission signal 62 having a frequency spectrum as shown in FIG. 17B is obtained. . That is, since the transmission signal 62 is always obtained without depending on the presence or absence of the IF signal, the laser can always be overmodulated.
[0103]
FIG. 18 is a block diagram showing an internal configuration of the extractor 31 in the master station 2 when the slave stations 1a to 1n transmit secondary modulated signals subjected to amplitude modulation. 18 includes a distributor 33, bandpass filters 34a to 34n, local oscillators 39a to 39n, and bandpass filters 36a to 36n.
[0104]
The received signals from the slave stations 1a to 1n input to the extractor 31 are first distributed by a distributor (distribution means) 33 to a number corresponding to each of the slave stations 1a to 1n. The reception signals 62a to 62n from the respective slave stations 1a to 1n are arranged in different frequency bands, and the band pass filters 34a to 34n respectively extract desired bands. The extracted signals are multiplied by sine wave signals from local oscillators (second local oscillators) 39a to 39n and multipliers (multiplication means) 40a to 40n.
[0105]
The local oscillators 39a to 39n output sine wave signals having the same frequency as the sine wave signals 63a to 63n used in the slave stations 1a to 1n. The outputs of the multipliers 40a to 40n are obtained by using the band pass filters 36a to 36n to obtain the transmission signals 53a to 53n from the respective slave stations 1a to 1n.
[0106]
Thus, in the sixth embodiment, since amplitude modulation is performed as secondary modulation, a secondary modulation signal can be obtained with a simple configuration, and the configuration of each slave station can be simplified.
[0107]
(Seventh embodiment)
In the seventh embodiment, information transmission is performed from the slave station to the master station 2 using spread spectrum which is considered as a kind of secondary modulation. The eighth embodiment is greatly different from the first to seventh embodiments in that the optical signal from each slave station is spread with the same frequency instead of subcarrier multiplexing the optical signal from each slave station. Multiplexing with bands.
[0108]
FIG. 19 is a block diagram of a seventh embodiment of an optical transmission system according to the present invention. The main configurations of the slave stations 1a to 1n and the master station 2 are the same as those in FIG. 1, and the same reference numerals are given to the same components.
[0109]
In the first to seventh embodiments, the reception signals 52 between the slave stations are arranged at different frequencies by the frequency converter 16, but in the present embodiment, the reception signals are arranged in the same frequency band. Convert.
[0110]
For example, the slave station 1a generates an IF signal 53a by down-converting the reception signal 52a at the antenna by a frequency converter. The spreader (spread spectrum means) 26 performs spread spectrum by multiplying the IF signal 53a and the spread code. The spectrum-spread transmission signal 59a is converted by the laser 13 into an optical signal 100a having an optical modulation degree of 100% or more and transmitted to the master station 2.
[0111]
FIG. 20 is a frequency spectrum diagram of the received signal 60 at the master station 2. As illustrated, the spectrum-spread transmission signals 59a to 59n from the slave stations 1a to 1n are superimposed on the same frequency band. Since the optical signals 100a to 100n from the slave stations 1a to 1n have an optical modulation degree set to 100% or more, the beat noise is diffused, and no great influence that deteriorates the reception sensitivity appears.
[0112]
The received signal 60 received by the master station 2 is despread by a despreader (despreading means) 27 with the same spread code as the spread code used by each of the slave stations 1a to 1n for spread spectrum. The signals are separated into IF signals 53 a to 53 n from the stations 1 a to 1 n and input to the demodulator 15.
[0113]
Note that the method of spread spectrum of the information transmission signal may be a direct spread method (DS: Direct Sequence) or a frequency hopping method (FH: Frequency hopping).
[0114]
FIG. 21A and FIG. 21B are diagrams showing block configurations of a direct spreader and a despreader using spreading codes. In FIG. 21A, the IF signal 53 is multiplied by the spread code 64 output from the spread code generator 41, and then unnecessary waves are suppressed by the band pass filter. As a result, a spread spectrum transmission signal 59 is obtained as shown in FIG. The spread code output from the spread code generator 41 is different for each slave station, and may be an orthogonal code represented by a Walsh code or a PN code (Pseudo Noise).
[0115]
Basically, when the information transmission signal of the slave station is spread spectrum multiplexed, the bits of the spread spectrum signal transmitted from each slave station must be synchronized in the master station. In order to achieve a configuration that does not synchronize between the slave stations, the number of bits of the spread code may be simply increased. When the number of bits of the spread code increases to 64, 128, etc., the degree of suppression of interference signals when despreading is increased, and transmission quality for the desired wave can be ensured. There is also a method that can maintain high transmission quality for a desired wave even when the number of bits of the spread code is small. The method is shown below. An 8-bit orthogonal Walsh code is assigned to each slave station as follows (here, a code structure of 1 and -1 is used).
[0116]
Slave station 1a [1 1 1 1 1 1 1 1]
Slave station 1b [1 -1 1 -1 1 -1 1 -1]
Slave station 1c [1 1 -1 -1 1 1 -1 -1]
Slave station 1d [1 1 1 1 -1 -1 -1 -1]
Here, an optical multiplexing of four slave stations is assumed. In the above-described codes, the orthogonality between the spreading codes is maintained in each of the slave stations 1a to 1d without taking bit synchronization. In this way, if a spreading code that can maintain orthogonality without depending on synchronization is selected and allocated between the slave stations, there is no need to synchronize between the slave stations, and there is an advantage that the configuration becomes simpler. . With an 8-bit orthogonal Walsh code, up to four slave stations can be optically multiplexed without requiring synchronization. The assignment of spreading codes that do not require synchronization in the case of 4 bits and 16 bits is shown below.
[0117]
<4 bits>
[1 1 1 1]
[1 1 -1 -1]
[1 -1 1 -1]
<16 bits>
[1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1]
[1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1]
[1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1]
[1 1 1 1 -1 -1 -1 -1 1 1 1 1 1 -1 -1 -1 -1]
[1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1]
From the above combination, optical multiplexing of 3 stations with 4 bits and 5 stations with 16 bits is possible without requiring phase synchronization. In the case of a configuration that achieves phase synchronization, optical multiplexing of 4 stations with 4 bits and 16 stations with 16 bits is possible, but the configuration of the slave stations is complicated because it includes synchronization means.
[0118]
As shown in FIG. 21B, the despreader 27 in the master station 2 applies the optically multiplexed received signal 60 from the slave stations 1a to 1n to each slave station output from the spread code generator 41. The multiplier 40 multiplies the unique spreading code 64 by the multiplier 40. In the multiplied signal, an unnecessary wave is suppressed by the band pass filter 36, and then a desired received signal 63 is obtained.
[0119]
As described above, in the seventh embodiment, the radio signal received by the antenna 6 is spectrum-spread in each slave station, converted into an optical signal, and transmitted to the master station 2. It is possible to perform overmodulation on the spread spectrum information transmission signal, optical beat noise can be reduced, and the reception sensitivity at the master station 2 can be maintained at high quality. Moreover, since the frequency band of the information transmission signal of each slave station may be the same, all the configurations of the frequency converters can be made the same, which is suitable for cost reduction.
[0120]
In the seventh embodiment, the description has been made on the assumption that the received signal 51a is one channel, but a plurality of channels may be used. There are other spread spectrum systems for a plurality of channels, but an example of the system will be described in the eighth embodiment.
[0121]
Further, in the seventh embodiment, each slave station 1 does not need to overmodulate as long as the wavelengths of the lasers 13 provided in each slave station 1 are set so as not to overlap.
[0122]
Furthermore, the spread spectrum multiplexing system which is the seventh embodiment and the system which does not transmit an optical signal having strong coherence when there is no information transmission signal shown in the first to sixth embodiments can be combined. It is. In an optical transmission system that combines these two, spread spectrum multiplexing and a system that does not transmit a highly coherent optical signal, the generation of optical beat noise can be reduced in the master station, and stable and high-quality transmission characteristics can be achieved. Can provide.
[0123]
(Eighth embodiment)
The eighth embodiment shows a spread spectrum method in the case where each slave station 1 has a plurality of received signals 53.
[0124]
The eighth embodiment is configured in the same manner as the seventh embodiment except that the internal configuration of the spreader 26 in each slave station and the internal configuration of the despreader 27 in the master station 2 are different.
[0125]
In order to simultaneously extract the reception signals from each slave station 1, it is only necessary to provide as many despreaders 27 as the number of slave stations in the master station 2, and distribute and input the reception signals 60 to the respective despreaders 27. In the eighth embodiment, the configuration of the spreader 26 and the despreader 27 when the IF signal 53 is configured by multicarriers arranged at frequencies fa, fb... Fn will be described.
[0126]
FIG. 22A is a block diagram showing the internal configuration of the spreader 26, and FIG. 22B is a block diagram showing the internal configuration of the despreader 27. As shown in FIG. 22A, the spreader 26 includes a plurality of spreading units each including a bandpass filter 34, a spread code generator 41, a multiplier 40, a frequency converter 16, and a bandpass filter 36.
[0127]
Usually, the frequency band of the spread codes 64a to 64n for spread spectrum is wider than the frequency interval of the multicarrier of the IF signal 53. Therefore, it is necessary to distribute the IF signal 53 which is a multicarrier by the distributor 33 and extract each carrier by the band pass filter 34. The IF signal 53 distributed for each wave is multiplied by the spread codes 64a to 64n from the spread code generator 41 to be spread spectrum. At this time, the spread codes 64a to 64n are different codes and are different from the spread codes 64 used in the other slave stations 1.
[0128]
The spread spectrum IF signals are respectively converted into the same frequency band by the frequency converter 16, unnecessary waves are removed by the band pass filter 36, and then combined by the adder 42 to become a transmission signal 59.
[0129]
The state of the frequency spectrum of the IF signal 53 and the transmission signal 59 is as shown in FIG. Here, an example is shown in which each carrier component is multiplexed again in the same frequency band, but different frequency bands may be used.
[0130]
On the other hand, the despreader 27 in the master station 2 has a plurality of despreading units each including a spread code generator 41, a multiplier 40, and a bandpass filter 36, as shown in FIG.
[0131]
The despreader 27 distributes the received signal 60 optically multiplexed with the optical signal from the other slave station 1 to the number of carriers in each slave station 1 by the distributor 33. The multiplier 40 multiplies the distributed received signal 60 by the same spread codes 64a to 64n as the spread codes 64a to 64n used for a desired carrier in the slave station. An output signal from the multiplier 40 passes through a bandpass filter 36 for suppressing unnecessary waves, and obtains reception signals 63a to 63n which are desired waves. In order to simultaneously extract the reception signals from each slave station 1, it is only necessary to provide as many despreaders 27 as the number of slave stations in the master station 2, and distribute and input the reception signals 60 to the respective despreaders 27.
[0132]
When the optical signals 100a to 100n from the respective slave stations 1a to 1n are multiplexed by spectrum spread, it is desirable that the powers of the transmission signals 59a to 59n constituting the reception signal 58 in the master station 21 are equal. This is because, if the powers of the transmission signals 59a to 59n are equal, the noise characteristics of the extracted signal after despreading are the best for all the spectrum-spread transmission signals 59a to 59n.
[0133]
Thus, in the eighth embodiment, since a plurality of spreading sections are provided in the spreader 26 in each slave station and a plurality of despreading sections are provided in the master station 2, the IF signal is composed of multicarriers. Even in this case, optical transmission can be performed from the slave station to the master station 2, thereby improving communication efficiency.
[0134]
(Ninth embodiment)
The powers of the reception signals 59a to 59n of the receiver 14 in the master station 2 are determined by the input power to the receiver 14 of the master station 2 and the degree of optical modulation in the optical signals 100a to 100n. Several methods of controlling the power of the received signals 59a to 59n can be considered for these two parameters. The ninth embodiment has a function of controlling the power of the received signals 59a to 59n between the slave stations 1a to 1n in order to improve the transmission quality of the optical transmission link.
[0135]
FIG. 23 is a block diagram of a ninth embodiment of an optical transmission system according to the present invention. The optical transmission system of FIG. 23 is characterized in that optical attenuators 28a to 28n are inserted into the optical transmission paths 3a to 3n between the slave stations 1a to 1n and the optical combiner 4, and is otherwise substantially the same as FIG. It is configured.
[0136]
The optical modulation degrees of the optical signals 100a to 100n in the slave stations 1a to 1n are set to be equal, and the loss amounts of the optical attenuators 28a to 28n are sent to the receivers 14 of the optical signals 100a to 100n of the slave stations 1a to 1n. The input power is set to be uniform.
[0137]
By equalizing the optical modulation degree and the optical reception power to the receiver 14, the powers of the transmission signals 59 a to 59 n constituting the reception signal 58 can be set to be equal.
[0138]
As described above, in the ninth embodiment, since the optical attenuators 28a to 28n are provided in the optical transmission path between the slave stations 1a to 1n and the optical combiner 4, the optical signals from the slave stations 1a to 1n are transmitted. The degree of optical modulation can be set substantially equal, and variations in input power to the optical receiver 14 can be suppressed.
[0139]
(Tenth embodiment)
The tenth embodiment performs received signal power control by a method different from that of the ninth embodiment.
[0140]
The tenth embodiment is configured in the same manner as the ninth embodiment except that the configuration of the slave stations is different.
[0141]
FIG. 24 is a block diagram showing the internal configuration of the slave stations 1a to 1n. The slave stations 1a to 1n in FIG. 24 include a power controller 29, a frequency converter 16, a spreader 26, an amplifier 17a, a local oscillator 8, an amplifier 17b, an adder 10, a current source 12, It has a bias tee 11 and a laser 13.
[0142]
In FIG. 24, when the radio signal 51 transmitted from the subscriber 5 is received by the antenna 6, it is down-converted by the frequency converter 16 as necessary to generate an IF signal 53.
[0143]
The IF signal 53 is spectrum-spread by the spreader 26, and a transmission signal 59 is generated via the variable gain amplifier 17a. The transmission signal 59 is added to the sine wave signal 54 from the local oscillator 8 by the adder 10 to generate a transmission signal 61. Here, the frequency band of the sine wave signal 54 is set to a frequency band different from that of the transmission signal 59 so that distortion due to overmodulation does not appear in the band of the transmission signal 59.
[0144]
The power controller 29 controls the power intensity of the transmission signal 59 by changing the gain of the amplifier 17 so that the power of the transmission signal 59 when received by the master station 2 becomes a predetermined value p. Then, the intensity of the sine wave signal 54 is changed by the gain of the amplifier 17b so that the optical modulation degree of the transmission signal 61 becomes a predetermined value.
[0145]
FIGS. 25A and 25B are frequency spectrum diagrams of transmission signals at two slave stations 1a and 1b having different optical transmission distances from the master station 2, and FIG. FIG. 25B is a frequency spectrum diagram of the transmission signal 61a of the slave station 1a having a large optical propagation path loss up to the master station 2. FIG. 25B is a frequency spectrum diagram of the transmission signal 61a of the slave station 1a having a small propagation path loss.
[0146]
Since the input power of the optical signal 100b to the receiver 14 is small, the power Pb of the transmission signal 59b is set to be large, and the power of the sine wave signal 54b is set to be small. The optical modulation degree of the optical signal 100b with respect to the transmission signal 61b is set to a predetermined value in the same manner as the optical modulation degree of the slave station 1a. The slave stations 1a and 1b do not depend on the power of the transmission signals 59a and 59b, and the degree of optical modulation is constant, so that the coherence of the optical signals 100a and 100b can be stably suppressed.
[0147]
FIG. 25C is a frequency spectrum diagram of the received signal 58 of the receiver 14 in the master station 2. The reception signal 58 includes transmission signals 59a and 59b and sine wave signals 54a and 54b, and ignores distortion components generated by overmodulation. By controlling the power of the transmission signals 59a and 59b and the sine wave signals 54a and 54b, the power of the transmission signals 59a and 59b when received by the master station 2 can be set to p which is a set value. In FIG. 25, the slave stations 1a and 1b have been described. However, the same power control method is applied to each of the slave stations 1a to 1n. The noise characteristics of the transmitted signal can be kept good.
[0148]
As described above, in the tenth embodiment, the amplifiers 17a and 17b are provided in each of the slave stations to control the power of the received signal, so that the same effect as in the ninth embodiment can be obtained.
[0149]
By the way, in the case of spectrum spreading and multiplexing in a multipoint-to-point access optical transmission system, it is common to perform optical space transmission using LEDs. The reason for this is that in optical fiber transmission, when a plurality of optical signals are multiplexed, beat noise is generated, so that reception sensitivity is significantly deteriorated.
[0150]
However, in the case of the present invention, since overmodulation is performed and beat noise can be suppressed regardless of the presence or absence of an information transmission signal, an optical signal can be transmitted using an optical fiber.
[0151]
In the spread spectrum multiplexing between the slave stations 1, since the transmission signals 59a to 59n in the respective slave stations 1a to 1n use the same frequency band as compared with the SCM multiplexing, the hardware of the frequency converter 16 has the same configuration. Can be.
[0152]
Each of the slave stations 1a to 1n needs to be provided with different spreading codes having orthogonality, but the spreading codes may be stored in a memory or the like, and the hardware configuration is the same.
[0153]
In the first to ninth embodiments, base stations for wireless communication such as WLL and ITS have been described. However, the present invention can also be applied to ITV (Industrial TV) and optical networks that handle all information transmission signals such as digital signals as transmission signals.
[0154]
【The invention's effect】
As described above in detail, according to the present invention, an optical signal having a degree of optical modulation greater than 1 is transmitted from each slave station even when there is no information transmission signal to the master station. A strong optical signal is not received by the master station 2, and the master station is not affected by beat noise. For this reason, the receiving sensitivity is improved, and an optical transmission system having excellent stability and reliability can be constructed.
[0155]
In addition, since an optical signal is transmitted from each slave station to the master station only when an information transmission signal exists, an optical signal with strong coherence is transmitted to the master station 2 when there is no information transmission signal. There is no risk of being lost.
[0156]
The laser in the slave station is usually provided with a function of stabilizing the optical average output using APC (Automatic Power Control). By adjusting the reference level of the optical output of this APC according to the presence / absence of an information transmission signal, active components such as optical and electrical switches are not required, and even in the absence of an information transmission signal, coherency A strong optical signal is not transmitted from the slave station to the master station.
[0157]
In addition, when the information transmission signal is secondarily modulated and then converted to an optical signal and transmitted to the master station, if there is no information transmission signal, the laser is driven with an unmodulated wave. Therefore, the generation of optical beat noise can be continuously reduced in the master station.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a first embodiment of an optical transmission system.
FIG. 2 is a diagram for explaining a degree of light modulation.
FIG. 3 is an optical spectrum diagram of an optical signal 100 when a Fabry-Perot semiconductor laser is used.
FIG. 4 is a frequency spectrum diagram of a received signal in the first embodiment.
FIG. 5 is a frequency spectrum diagram of a conventional received signal.
FIG. 6 is a block diagram of a second embodiment of an optical transmission system according to the present invention.
FIG. 7 is a frequency spectrum diagram of a received signal and an IF signal obtained by frequency-converting the received signal.
FIG. 8 is a block diagram of a third embodiment of an optical transmission system according to the present invention.
FIG. 9 is a diagram illustrating an example in which an electrical switch is used for on / off control of an optical signal.
FIG. 10 is a diagram showing an example in which an optical switch is used for on / off control of an optical signal.
FIG. 11 is a diagram for explaining a fourth embodiment of an optical transmission system according to the invention.
FIG. 12 is a diagram showing a bit configuration of an information transmission signal in TDMA.
FIG. 13 is a diagram showing a bit configuration of an information transmission signal in CDMA.
FIG. 14 is a block diagram of a fifth embodiment of an optical transmission system according to the present invention.
FIGS. 15A, 15B, and 15C are diagrams showing an internal configuration and characteristics of a secondary modulator in each slave station.
FIG. 16 is a block diagram showing an internal configuration of an extractor in the master station when the slave station transmits a frequency-modulated secondary modulated signal.
FIGS. 17A, 17B, and 17C are diagrams showing the internal configuration and characteristics of a secondary modulator in a slave station.
FIG. 18 is a block diagram showing an internal configuration of an extractor in the master station when the slave station transmits a secondary modulated signal subjected to amplitude modulation.
FIG. 19 is a block diagram of a seventh embodiment of an optical transmission system according to the present invention.
FIG. 20 is a frequency spectrum diagram of a received signal at the master station.
FIGS. 21A and 21B are diagrams showing block configurations of a direct spreader and a despreader using a spread code. FIG.
22A is a block diagram showing an internal configuration of a spreader, and FIG. 22B is a block diagram showing an internal configuration of a despreader.
FIG. 23 is a block diagram of a ninth embodiment of an optical transmission system according to the present invention;
FIG. 24 is a block diagram showing an internal configuration of a slave station.
FIGS. 25A and 25B are frequency spectrum diagrams of transmission signals at two slave stations with different optical transmission distances from the master station. FIGS.
FIG. 26 is a block diagram showing a schematic configuration of a conventional optical transmission system.
[Explanation of symbols]
1a-1n Slave station
2 Master station
3, 3a-3n optical fiber
4 Photosynthesizer
5a-5n subscribers
6 Antenna
7 Signal detector
8 Local oscillator
9 switch
10 Adder
11 Biasty
12 Current source
13 Laser
14 Photodetector
15 Demodulator
16 Frequency converter
18 Electric switch
19 Optical switch
20 Laser module
21 Photodetector
22 Setting voltage
23 Differential amplifier
24 Loop filter
25 Adder
26 Diffuser
27 Despreader
30 Secondary modulator
31 Extractor
32 Voltage controlled oscillator
33 Distributor
34a to 34n Band pass filter
35a-35n frequency discriminator
36a to 36n Band pass filter
40 multiplier
41 Spreading code generator

Claims (5)

A plurality of slave stations that output optical signals according to information transmission signals;
Optical combining means for generating an optical multiplexed signal by multiplexing optical signals output from each of the plurality of slave stations;
In an optical transmission system comprising: a master station that separates and extracts an information transmission signal corresponding to each of the plurality of slave stations based on a signal correlated with the optical multiplexed signal;
Each of the plurality of slave stations is
Superimposing means for superimposing a predetermined dummy signal on the information transmission signal so that an optical signal having a degree of optical modulation greater than 1 is transmitted to the optical combining means even if there is no information transmission signal to the master station Have
An optical signal is generated based on the output signal of the superimposing means,
The superimposing means includes
Signal detection means for detecting whether or not an information transmission signal to the master station exists;
Dummy signal generating means for outputting the dummy signal of a predetermined frequency;
Based on the detection result of the signal detection means, switching means for allowing the dummy signal to pass if the information transmission signal is not present, and blocking the dummy signal if the information transmission signal is present;
Adding means for adding the output of the switching means and the information transmission signal,
Each of the slave stations generates an optical signal based on an output signal of the adding means. A plurality of slave stations that output optical signals according to information transmission signals;
Optical combining means for generating an optical multiplexed signal by multiplexing optical signals output from each of the plurality of slave stations;
In an optical transmission system comprising: a master station that separates and extracts an information transmission signal corresponding to each of the plurality of slave stations based on a signal correlated with the optical multiplexed signal;
Each of the plurality of slave stations is
Only when there is an information transmission signal to the master station, it has an output switching means for outputting an optical signal corresponding to the information transmission signal, the optical modulation degree of the optical signal is greater than 1,
The output switching means is
APC (Automatic Power Control) means for performing feedback control so that the signal intensity of the optical signal matches the reference level;
Signal detection means for detecting whether or not an information transmission signal to the master station exists;
Based on the detection result of the signal detection means, if the information transmission signal is not present, the reference level is set to zero, and if the information transmission signal is present, a reference level adjustment means for adjusting the reference level to a predetermined value; An optical transmission system comprising: Each of the plurality of slave stations has a spread spectrum unit that spreads the information transmission signal using a spread code that is different for each slave station, and converts the output signal of the spread spectrum unit into an optical signal for output. And
The master station has despreading means for performing spectrum despreading on the signal correlated with the optical multiplexed signal using the spreading code assigned to each slave station, and based on the output signal of the despreading means, The optical transmission system according to claim 1 or 2 , wherein an information transmission signal for each slave station is separated and extracted. A plurality of slave stations that output optical signals according to information transmission signals;
Optical combining means for generating an optical multiplexed signal by multiplexing optical signals output from each of the plurality of slave stations;
In an optical transmission method for performing signal transmission with a master station that separates and extracts an information transmission signal corresponding to each of the plurality of slave stations based on a signal correlated with the optical multiplexed signal,
Each of the plurality of slave stations is
Even if there is no information transmission signal to the master station, a signal in which a predetermined dummy signal is superimposed on the information transmission signal so that an optical signal having a degree of optical modulation greater than 1 is transmitted to the optical combining means. Generating the optical signal based on the superimposed signal,
In the process of generating the superimposed signal,
Detect whether there is an information transmission signal to the master station,
Outputting the dummy signal of a predetermined frequency,
Based on the detection result of whether or not the information transmission signal exists, switching means for allowing the dummy signal to pass if the information transmission signal does not exist and blocking the dummy signal if the information transmission signal exists. Provided,
Add the output of the switching means and the information transmission signal,
Each of the slave stations generates the optical signal based on an output signal obtained by adding the output of the switching means and the information transmission signal. A plurality of slave stations that output optical signals according to information transmission signals;
Optical combining means for generating an optical multiplexed signal by multiplexing optical signals output from each of the plurality of slave stations;
In an optical transmission method for performing signal transmission with a master station that separates and extracts an information transmission signal corresponding to each of the plurality of slave stations based on a signal correlated with the optical multiplexed signal,
Each of the plurality of slave stations is
An APC step for performing feedback control so that the average signal intensity of the optical signal matches the reference level;
A detection step of detecting whether an information transmission signal to the master station exists;
An adjustment step of adjusting the reference level so that an optical signal is not output when the information transmission signal does not exist based on the detection result of the detection step;
An optical transmission method characterized in that when an optical signal is output, the optical modulation degree of the optical signal is larger than 1.

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