JP3750076B2 - Optical receiver, optical receiving method, and optical communication system - Google Patents

Description

【００１８】
上記式（１）において、λsは、所望ユーザの信号強度を示す。この光ハードリミッタとしては公知のものを用いることができる（A.A.Sawchuk and T.C.Strand,"Digital optical computing,"Proc.IEEE,vol.72,pp.758-779,July 1984. J.L.Jewell,M.C.Rushford,and H.M.Hoad,"Use of a single nonlinear Fabry-Perot etalon as optical logic gates,"Appl.Phys.Lett.,vol.44,pp.172-174,Aug.1984.）。
上記式（１）のように、光ハードリミッタ２１によって制限された光信号を、スプリッタ２２によって重みＷの数に分岐し、相関手段としての遅延線で構成される相関器２３で拡散符号の重み位置における光信号と相関をとる。
【００１９】
この相関をとった光信号を、フォトダイオードから成る光検出器２４へ入力する。受信信号は光ハードリミッタ２１によって上記式（１）のようにその光強度が制限されているので、光検出器２４からの各出力信号は２値のいずれかをとることになる。この２値に基づいて、尤度判定手段としての結合尤度判定器２５において尤度判定を行ない、送信された信号を復調する。
たとえば、拡散符号として、符号長Ｌ＝１３、重みＷ＝３、オフピーク自己相関及び相互相関が１（オンピーク自己相関は３＝Ｗ）である光直交符号（ＯＯＣ：Optical Orthogonal Code）を用いた場合を例に、２人の同時ユーザがいた場合について、上記光受信機Ｒで光信号が受信されてからどのように正しく信号判定を行なえるかを説明する。
【００２０】
図３は、光ハードリミッタ２１の動作を説明する図である。今、所望ユーザ（ユーザ１）は、拡散符号Ｃ1＝[1011000000000]、他ユーザ（ユーザ２）は拡散符号Ｃ2＝[0010000100010]を用いて、所望ユーザの受信機で光信号を受信したとする。このとき、光ハードリミッタ２１によって、拡散符号Ｃ1と拡散符号Ｃ2とがチップ位置３（２番目の重み位置）において干渉している。このとき、所望ユーザの光強度に光ハードリミッタ２１のカットオフレベルを設定しておく。よって、所望ユーザが光信号を送信したとき（図３の上段）は、光ハードリミッタ２１の出力は、すべて所望ユーザの信号レベルとなる。
【００２１】
一方、所望ユーザが光パルスを送信しないとき（図３の下段）は、ユーザ２と相関するパルス位置において光パルスが受信されることになる。
この後、拡散符号Ｃ1の重み位置（符号系列における３個の「１」の位置）の光パルスを同時に受信するように、遅延線によって構成される相関器２３によってそれぞれの光検出器２４に入力する。
第ｉ番目の重み位置における光検出器２４の出力に対するＯＮ／ＯＦＦの尤度比は、次式のように与えられる。
【００２２】
【数６】

【００２３】
ｚi：第ｉ番目の重み位置における光検出器２４の出力
Ｈ1：所望ユーザが光パルスを送信している場合
Ｈ0：所望ユーザが光パルスを送信していない場合
Ｐr｛ｚi｜Ｈb｝ ｂ∈｛１,０｝：第ｉ番目の重み位置における光検出器２４出力の尤度
なお、演算等の簡単化のため、上記式（２）における尤度比は、底をｅとして自然対数で表している。
これら拡散符号の重み位置において得られる尤度を用いて、結合尤度判定器２５において次式で表される結合尤度判定を行なうことで、信号判定を行なう。
【００２４】
【数７】

【００２５】
Ｗ：拡散符号の重み
なお、上記式（３）は、Ｔ≧０であれば、送信された情報は「１」であり、Ｔ＜０であれば、送信された情報は「０」であると判定することを意味している。
【００２６】
本実施の形態によれば、光ハードリミッタ２１によって、受信光は光信号制限されるので、光検出器２４からの各出力信号は２値のいずれかをとることになる。多元接続干渉による干渉を受けた重み位置の尤度比は、上記式（２）によって０になる（尤度比そのものの値は１であるが、尤度比の自然対数は０になるという意味）。よって、干渉を受けた重み位置の信号は、信号判定に影響しない。よって、他ユーザに関する情報を知らなくても、多元接続干渉の影響を抑制することができる。
また、光ハードリミッタの後段に、新たなブランチを設けて、加算器としての光カプラによって拡散符号の重み位置の光信号を加算した光信号に対する第２の光信号制限手段としての第２光ハードリミッタを設けた本発明の一実施の形態について図４を用いて説明する。
【００２７】
図４は、本発明の第２実施の形態による光受信機の構成を示す図である。受信信号は第１光ハードリミッタ３１及びスプリッタ３２を介して２つに分岐される。ここで、第１光ハードリミッタ３１は図２に示した光ハードリミッタ２１と同じ働きをする。上側のブランチにおいては、図２に示したスプリッタ２２及び相関器２３と同じ働きをするスプリッタ３３及び相関器３４によって相関をとり、カプラ３５によって光結合して、第２光ハードリミッタ３６に入力する。この第２光ハードリミッタ３６は、次式で与えられるように動作する。
【００２８】
【数８】

【００２９】
ここで、本実施の形態においては、３ｄＢのスプリッタ３２を用いたので、第２光ハードリミッタ３６のカットオフレベルは、第１光ハードリミッタ３１のカットオフレベルの１／２になっている。
この実施の形態において、第２光ハードリミッタ３６で信号制限されることによって、このブランチからに対する尤度比が次式のように求まる。
【００３０】
【数９】

【００３１】
ここで、ｚ0は、図４に示されるように、上側のブランチにおける光検出器３７の出力である。
一方、図４に示される下側のブランチについては、第１実施の形態と同様にして、所望ユーザと相関する重み位置を相関器４０によって、同時に受信できるように構成する。上側のブランチと同様に３ｄＢカプラによって分岐されたことから、各重み位置における最大尤度は、光信号強度が１／２におけるものとなる。上記式（３）及び式（５）を乗算した結合尤度比が０以上であるか否か（ここでは、結合尤度比とは対数表現で定義しているので、加算したものが０以上となるか否か）を判定することで、論理判定器３８において信号判定を行なう。
【００３２】
図５は、図４における上側のブランチの光信号を説明する図である。上側のブランチにおいては、拡散符号の重み位置において相関した光信号を最終重み位置（図５における３番目のチップ位置）に加算した後、第２光ハードリミッタ３６において光信号制限するので、全ての重み位置が多元接続干渉によって干渉を受けないかぎり、光検出器３７の出力は干渉の影響を抑制することができる。
第２実施の形態によれば、上側のブランチの光検出器３７において、Ｗ（重み）倍された光強度で検出されるので、より信頼できる信号判定を行うことができる。よって、さらに通信品質のよい信号受信ができる。
【００３３】
図６は、本発明の効果を確認するためのシミュレーション（数値解析）結果を示すグラフであり、横軸受信信号強度に対する縦軸ビット誤り率を示す。このグラフは第２実施の形態によるシミュレーション結果を示し、暗電流Λd＝０.１、符号長Ｌ＝３４１、重みＷ＝５、ユーザ数Ｋ＝５とする。また、「従来」はビット誤り率を最小にするスレッショルドを用いる場合である。「本発明（上限）」は全干渉ユーザ（所望ユーザを除く）が「１」すなわちパルスを送信している場合であって、多元接続干渉が最悪の状態である。「本発明」は干渉ユーザが確率１／２で「１」か「０」を送信している場合である。「単一ユーザ接続」は一ユーザのみが通信をしている単一ユーザ（Ｋ＝１）の場合であって多元接続干渉がない場合である。
【００３４】
グラフから、多元接続干渉がない「単一ユーザ接続」の場合に対して「従来（光ハードリミッタを有する場合と有しない場合）」はビット誤り率が高いのに対し、従来例の項で述べた「マルチユーザ受信機」の場合でも相当改善されるものの、「本発明」では、多元接続干渉による劣化が少ない、すなわち、多元接続干渉の影響を抑制していることが理解される。
【００３５】
図７は、本発明の効果を確認するためのシミュレーション結果を示すグラフであり、横軸ユーザ数に対する縦軸ビット誤り率を示す。このグラフは第２実施の形態によるシミュレーション結果を示し、受信信号強度Λs＝４０、暗電流Λd＝０.１、符号長Ｌ＝３４１、重みＷ＝５とする。
グラフから、図６と同様の改善が見られるとともに、「本発明」はユーザ数Ｋ≦５において顕著な効果を奏することが理解される。これは、重みＷ＝５であることから、Ｋ＞Ｗにおいては所望ユーザの重み位置で干渉を受けてしまうためと考えられる。
【００３６】
図８は、本発明の効果を確認するためのシミュレーション結果を示すグラフであり、横軸重みに対する縦軸ビット誤り率を示す。このグラフは第２実施の形態によるシミュレーション結果を示し、暗電流Λd＝０.１、ユーザ数Ｋ＝５とする。
グラフから、図６と同様の改善が見られるとともに、最適の重みＷが受信信号強度Λsの増加とともに増加すること、及び、「本発明」の最適重みＷが「マルチユーザ受信機」の最適重みＷよりも小さいことが理解される。ここで、上記実施の形態で用いたＯＯＣ（光直交符号）において生成できる符号数は、
【００３７】
【数６】

【００３８】
と表される。よって、重みＷが小さいときに最小の誤り率を示すということは、符号長Ｌ一定の下で、生成できるＯＯＣの符号数を増大することができるという効果を奏することを意味する。
なお、本発明は上記実施の形態に限定されるものではない。
【００３９】
上記実施の形態においては、光検出器としてフォトダイオードを用いたが、受信感度が優れるアバランシェフォトダイオード（ＡＰＤ）を用いてもよい。
結合尤度判定器２５又は論理判定器３８の尤度判定には、光検出器２４又は４１の出力を加算して判定してもよい。
光ハードリミッタ２１等の動作で、光信号の強度が大きいときは所望ユーザの信号強度λsに制限しているが、１つの所定値であればよく、所望ユーザの信号強度λsに限られない。
【００４０】
【発明の効果】
以上のように、本発明によれば、他ユーザに関する情報を知らなくても、多元接続干渉の影響を抑制することができるので、その信号判定が容易であり、構成が簡素化され、信号処理が高速化され、かつ、優れた通信品質を実現することができ、光通信の利点である高速大容量である特徴を生かすことができる。
【図面の簡単な説明】
【図１】本発明の実施の形態による光受信機を適用するのに好適な光通信システムの構成を示す図である。
【図２】本発明の第１実施の形態による光受信機の構成を示す図である。
【図３】光ハードリミッタの動作を説明する図である。
【図４】本発明の第２実施の形態による光受信機の構成を示す図である。
【図５】図４における上側のブランチの光信号を説明する図である。
【図６】本発明の効果を確認するためのシミュレーション結果を示すグラフ（その１）である。
【図７】本発明の効果を確認するためのシミュレーション結果を示すグラフ（その２）である。
【図８】本発明の効果を確認するためのシミュレーション結果を示すグラフ（その３）である。
【符号の説明】
１１ 光ファイバ
１２ スターカプラ
１３ 光ファイバ
２１ 光ハードリミッタ
２２，３２，３３，３９ スプリッタ
２３，３４，４０ 相関器
２４，３７，４１ 光検出器
２５ 結合尤度判定器
３１ 第１光ハードリミッタ
３５ カプラ
３６ 第２光ハードリミッタ
３８ 論理判定器
Ｔ1,Ｔ2,...,ＴN 光送信機
Ｒ 光受信機[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical receiver, an optical reception method, and an optical communication system in which an optical code division multiple access technique in a high-speed local area network (LAN) or the like is used, and more specifically, multiple access in an optical code division multiple access. The present invention relates to an optical receiver capable of suppressing interference, an optical receiving method, and an optical communication system.
[0002]
[Prior art]
An optical code division multiple access (CDMA) (hereinafter referred to as “optical CDMA”) system encodes each user's optical signal using a spreading code, thereby allowing one optical transmission line to be transmitted. This is an optical communication system for sharing and performing multiple access. Each user firstly modulates information to be transmitted into an optical signal on the transmitter side. The optical signal subjected to the primary modulation is subjected to secondary modulation (hereinafter referred to as “encoding”) in accordance with the assigned spreading code. Then, it is transmitted on the transmission line in this encoded form.
On the other hand, on the receiver side, the optical signal encoded on the transmitter side is secondarily demodulated (hereinafter referred to as “decoding”) by a correlator corresponding to the spreading code. The decoded optical signal is primarily demodulated in the subsequent optical detection unit, and the transmitted information is determined. Then, the determination result is received as a transmitted signal.
[0003]
As described above, in optical CDMA, an optical signal transmitted using a spread code is encoded on the transmission side, and an optical signal received on the reception side is decoded by a correlator corresponding to the spread code to perform optical communication. Do. Therefore, for other multiple access systems that attempt multiple access by assigning time or wavelength to each user,
・ No need to synchronize system ・ No need to control accurate wavelength (optical frequency) ・ Excellent secrecy in principle ・ Self-routing (self-routed transmission) function using spreading code It has advantages such as that.
[0004]
[Problems to be solved by the invention]
However, since the signal of each user is identified by the correlator on the receiving side, multiple access interference occurs due to cross-correlation between spreading codes used by other users. Therefore, there is a problem that this multiple access interference also increases as the number of users communicating simultaneously increases. Therefore, in order to ensure a certain communication quality, the number of users who can communicate at the same time is limited, and the communication capacity is limited.
Conventionally, single-user reception in which decoding is performed using a correlation receiver that correlates with a spread code of a desired user has been widely studied as an optical CDMA reception method. In the case of this type of reception method, as the number of simultaneous users increases, the communication quality deteriorates due to the influence of multiple access interference.
[0005]
It is also known to use an optical hard limiter with a correlator to mitigate this degradation in communication quality (JASalehi, and CABrackett, "Code division multiple-access techniques in optical fiber networks-Part II: systems performance analysis , "IEEE Trans.Commun., Vol.37, no.8, pp.834-842, Aug. 1989.). However, even in this method, since threshold detection is performed, the optimum threshold for maintaining good communication quality cannot be maintained on the receiving side. That is, since the optical CDMA system is an interference-dependent system whose communication quality depends on multiple access interference, signal determination cannot always be performed with an optimum threshold, and good communication quality cannot be maintained.
[0006]
On the other hand, as a receiving method of the optical CDMA system, the received user can determine the signal of the received optical signal by using information related to other users' signals, for example, a spread code, a signal delay amount, or a signal strength. Multi-user reception, which improves the communication quality, attracts attention. In multi-user reception, signal determination of a desired user is performed using signal information of other users as compared to single-user reception, so that the influence of multiple access interference on communication quality on the receiving side is reduced. it can.
[0007]
However, in multi-user reception, excellent communication quality is obtained, but complexity associated with signal determination has been a problem. As the number of users increases, the reception method or the configuration of the receiver becomes complicated, and the amount of calculation required for the signal determination increases exponentially. In order to alleviate this complexity, in one example, signal determination is performed at multiple stages, and the amount of interference is estimated by receiving information about signals of other users at the previous stage, and the estimated amount of interference Is used to determine the signal at the next stage. By repeating this, the communication quality is improved (M. Brandt-Pearce, and B. Aazhang, “Performance analysis of single-user and multiuser detectors for optical code division multiple access communication systems,” IEEE Trans.Commun. , vol. 43, no 2/3/4, pp. 435-444, Feb./Mar./Apr. 1995.).
[0008]
In another example, by utilizing the knowledge of the spread codes of other users and their delay amounts in advance, the optical signal receiving the interference is not used for signal determination, thereby suppressing multiple access interference. (LBNelson, and HVPoor, “performance of multiuser detection for optical CDMA—Part I: error probabilities,” IEEE Trans.Commun., Vol.43, no.11, pp.2803-2811, Nov. 1995.).
In another example, the communication quality is improved by combining both reception methods (JTKTang, and KBLetaief, "A new multiuser detector for optical code division multiple access communications systems," Proc. IEEE International Conference on Communications (ICC'97), vol. 1/3, pp126-130, Montreal, Quebec Canada, June 1997.).
[0009]
However, in any reception method, it is necessary for the receiver to know in advance information related to the spreading code, signal delay amount, or signal strength of another user when performing signal determination. That is, in order to maintain good communication quality, the complexity of signal determination has been sacrificed. In addition, since these signal determinations are performed in the electrical stage, there is a disadvantage that the communication speed is limited by complicated signal processing in the electrical stage even if broadband optical transmission is used. .
The present invention has been made in view of the above-described problems, and an object of the present invention is to provide an optical CDMA optical receiver and an optical receiver that can improve communication quality without knowing information related to other users. A method and an optical communication system are provided.
[0010]
[Means for Solving the Problems]
The optical receiver of the present invention sets the intensity of the received light to one predetermined value for the received light having a magnitude greater than or equal to the signal intensity of the desired user, and receives the received light that is smaller than the signal intensity of the desired user. First optical signal limiting means for limiting the light intensity to substantially zero, first correlation means for correlating the output from the first optical signal limiting means with the spreading code of the desired user, and the first correlation And likelihood determination means for performing likelihood determination using the likelihood of output by the means.
[0011]
A second correlator for correlating the output from the first optical signal limiting unit with the spreading code of the desired user; and a magnitude greater than the signal intensity of the desired user with respect to the output light from the second correlator. Second optical signal limiting means for setting the intensity of the output light to a predetermined value for the output light of the output light and limiting the intensity of the output light to substantially zero for the output light smaller than the signal intensity of the desired user The likelihood determining means uses the output from the second optical signal limiting means for determination, so that all the weight positions are not subject to interference by multiple access interference, the output of the photodetector is Since the output is not affected by interference and the output is detected with the light intensity multiplied by W (weight), more reliable signal determination can be performed.
[0012]
In the optical receiving method of the present invention, the intensity of the received light is set to one predetermined value for received light having a magnitude greater than or equal to the signal intensity of the desired user, and the received light is smaller than the signal intensity of the desired user. A first optical signal limiting step for limiting the intensity of the received light to substantially zero; a first correlation step for correlating the output of the first optical signal limiting step with the spreading code of the desired user; A likelihood determination step for performing likelihood determination using the likelihood of output in one correlation step.
[0013]
Also, the optical receiving method of the present invention provides a second correlation step for correlating the output from the first optical signal limiting step with the spreading code of the desired user, and the desired output light for the second correlation step. The output light intensity is set to one predetermined value for output light having a magnitude greater than or equal to the user signal intensity, and the output light intensity is substantially zero for output light smaller than the desired user signal intensity. A second optical signal limiting step for limiting, and the likelihood determining step is a method of using an output from the second optical signal limiting step for determination.
An optical communication system of the present invention has the above optical receiver.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a diagram showing a configuration of an optical communication system suitable for application of an optical receiver according to an embodiment of the present invention, in which N optical transmitters and one optical device that are multiple-connected by optical CDMA are shown. 1 shows an optical communication system composed of a receiver. Here, an optical receiver used in a direct modulation direct detection optical code division multiple access (CDMA) system will be described. Direct modulation direct diffusion is an optical communication system that modulates and detects an optical signal by paying attention to the intensity of light.
[0015]
On the transmission side of this optical communication system, each of the optical transmitters T1, T2,..., TN has an optical fiber 11 and a star coupler in a form in which an information-modulated optical signal is encoded by an assigned spreading code. 12 to the network. On the receiving side, the optical receiver R receives the optical signals encoded and transmitted from the optical transmitters T1, T2,.
In this optical receiver R, an optical signal encoded with a spreading code of a desired user is decoded from optical signals encoded and transmitted from the optical transmitters T1, T2,.
At this time, interference, that is, multiple access interference is caused due to cross-correlation between spreading codes. As the number of simultaneous users who communicate with multiple access at the same time increases, this multiple access interference has the property that the amount of interference increases.
[0016]
FIG. 2 is a diagram showing a configuration of the optical receiver R according to the first embodiment of the present invention. As an input signal of the optical receiver R, optical signals from the optical transmitters T1, T2,... TN are received from the star coupler 12.
First, the received signal is input to the optical hard limiter 21 as an optical signal limiting unit. At this time, the operation of the optical hard limiter 21 is defined as the following equation, where x is the input signal light to the optical hard limiter 21.
[0017]
[Equation 5]

[0018]
In the above equation (1), λs indicates the signal strength of the desired user. Known optical hard limiters can be used (AASawchuk and TCStrand, "Digital optical computing," Proc. IEEE, vol. 72, pp. 758-779, July 1984. JLJewell, MCRushford, and HMHoad, " Use of a single nonlinear Fabry-Perot etalon as optical logic gates, "Appl. Phys. Lett., Vol. 44, pp. 172-174, Aug. 1984.).
As shown in the above equation (1), the optical signal limited by the optical hard limiter 21 is branched into the number of weights W by the splitter 22, and the weight of the spread code is obtained by the correlator 23 constituted by a delay line as a correlation means. Correlate with optical signal at position.
[0019]
The correlated optical signal is input to the photodetector 24 composed of a photodiode. Since the light intensity of the received signal is limited by the optical hard limiter 21 as in the above equation (1), each output signal from the photodetector 24 takes one of two values. Based on these binary values, likelihood determination is performed by a combined likelihood determiner 25 as likelihood determining means, and the transmitted signal is demodulated.
For example, when a code length L = 13, weight W = 3, off-peak autocorrelation and cross-correlation are 1 (on-peak autocorrelation is 3 = W), an optical orthogonal code (OOC) is used as a spreading code As an example, in the case where there are two simultaneous users, how to correctly perform signal determination after the optical signal is received by the optical receiver R will be described.
[0020]
FIG. 3 is a diagram for explaining the operation of the optical hard limiter 21. Now, it is assumed that the desired user (user 1) uses the spreading code C1 = [1011000000000] and the other user (user 2) receives the optical signal at the receiver of the desired user using the spreading code C2 = [0010000100010]. At this time, the spread code C1 and the spread code C2 interfere with each other at the chip position 3 (second weight position) by the optical hard limiter 21. At this time, the cut-off level of the optical hard limiter 21 is set to the light intensity of the desired user. Therefore, when the desired user transmits an optical signal (the upper part of FIG. 3), the outputs of the optical hard limiter 21 are all at the signal level of the desired user.
[0021]
On the other hand, when the desired user does not transmit an optical pulse (lower part in FIG. 3), the optical pulse is received at a pulse position correlated with the user 2.
Thereafter, the optical pulses at the weighted position of the spread code C1 (three “1” positions in the code sequence) are simultaneously received by the correlators 23 constituted by delay lines to the respective photodetectors 24. To do.
The likelihood ratio of ON / OFF with respect to the output of the photodetector 24 at the i-th weight position is given by the following equation.
[0022]
[Formula 6]

[0023]
zi: output of the light detector 24 at the i-th weight position H1: when the desired user is transmitting an optical pulse H0: when the desired user is not transmitting an optical pulse Pr {zi | Hb} bε { 1,0}: Likelihood of the output of the photodetector 24 at the i-th weight position For simplicity of calculation, the likelihood ratio in the above equation (2) is expressed in natural logarithm with e as the base. Yes.
Using the likelihood obtained at the weight position of these spreading codes, the joint likelihood determination unit 25 performs the joint likelihood determination represented by the following equation to perform signal determination.
[0024]
[Expression 7]

[0025]
W: Weight of spreading code In the above equation (3), if T ≧ 0, the transmitted information is “1”, and if T <0, the transmitted information is “0”. It means to judge.
[0026]
According to the present embodiment, the received light is limited by the optical hard limiter 21, so that each output signal from the photodetector 24 takes one of two values. The likelihood ratio of the weighted position that has received interference due to multiple access interference is 0 by the above equation (2) (meaning that the likelihood ratio itself is 1, but the natural logarithm of the likelihood ratio is 0) ). Therefore, the signal at the weighted position subjected to the interference does not affect the signal determination. Therefore, the influence of multiple access interference can be suppressed without knowing information related to other users.
Also, a second branch is provided as a second optical signal limiting means for the optical signal obtained by adding a new branch after the optical hard limiter and adding the optical signal at the weight position of the spread code by the optical coupler as the adder. An embodiment of the present invention provided with a limiter will be described with reference to FIG.
[0027]
FIG. 4 is a diagram showing a configuration of an optical receiver according to the second embodiment of the present invention. The received signal is branched into two via the first optical hard limiter 31 and the splitter 32. Here, the first optical hard limiter 31 functions in the same manner as the optical hard limiter 21 shown in FIG. In the upper branch, correlation is performed by the splitter 33 and the correlator 34 that function in the same manner as the splitter 22 and the correlator 23 shown in FIG. 2, optically coupled by the coupler 35, and input to the second optical hard limiter 36. . The second optical hard limiter 36 operates as given by the following equation.
[0028]
[Equation 8]

[0029]
In this embodiment, since the 3 dB splitter 32 is used, the cutoff level of the second optical hard limiter 36 is ½ of the cutoff level of the first optical hard limiter 31.
In this embodiment, the signal is limited by the second optical hard limiter 36, whereby the likelihood ratio with respect to this branch is obtained as follows.
[0030]
[Equation 9]

[0031]
Here, z0 is the output of the photodetector 37 in the upper branch, as shown in FIG.
On the other hand, the lower branch shown in FIG. 4 is configured so that the correlator 40 can simultaneously receive the weight position correlated with the desired user, as in the first embodiment. Since it is branched by a 3 dB coupler in the same manner as the upper branch, the maximum likelihood at each weight position is that at which the optical signal intensity is ½. Whether or not the combined likelihood ratio multiplied by the above equations (3) and (5) is 0 or more (here, since the combined likelihood ratio is defined by logarithmic expression, the sum is 0 or more) The logical decision unit 38 makes a signal decision.
[0032]
FIG. 5 is a diagram for explaining the optical signal of the upper branch in FIG. In the upper branch, after adding the optical signal correlated at the weight position of the spreading code to the final weight position (third chip position in FIG. 5), the second optical hard limiter 36 limits the optical signal, As long as the weight position is not affected by the multiple access interference, the output of the photodetector 37 can suppress the influence of the interference.
According to the second embodiment, the photodetector 37 in the upper branch is detected with the light intensity multiplied by W (weight), so that more reliable signal determination can be performed. Therefore, signal reception with better communication quality can be performed.
[0033]
FIG. 6 is a graph showing the result of simulation (numerical analysis) for confirming the effect of the present invention, and shows the vertical axis bit error rate with respect to the horizontal axis received signal strength. This graph shows the simulation result according to the second embodiment, and it is assumed that the dark current Λd = 0.1, the code length L = 341, the weight W = 5, and the number of users K = 5. “Conventional” is a case where a threshold that minimizes the bit error rate is used. “Invention (upper limit)” is a case where all interference users (excluding a desired user) transmit “1”, that is, a pulse, and the multiple access interference is in the worst state. “Invention” is a case where an interfering user transmits “1” or “0” with probability 1/2. “Single user connection” is a case of a single user (K = 1) in which only one user is communicating and there is no multiple access interference.
[0034]
From the graph, the bit rate is high in the case of “conventional (with and without optical hard limiter)” compared to the case of “single user connection” without multiple access interference. In the case of the “multi-user receiver”, the present invention is considerably improved, but it is understood that the “present invention” has little deterioration due to multiple access interference, that is, the influence of multiple access interference is suppressed.
[0035]
FIG. 7 is a graph showing a simulation result for confirming the effect of the present invention, and shows the bit error rate on the vertical axis with respect to the number of users on the horizontal axis. This graph shows the simulation result according to the second embodiment, where the received signal strength Λs = 40, the dark current Λd = 0.1, the code length L = 341, and the weight W = 5.
From the graph, it is understood that the same improvement as in FIG. 6 is seen, and that the “present invention” has a remarkable effect when the number of users K ≦ 5. This is presumably because the weight W = 5, so that interference is received at the weight position of the desired user when K> W.
[0036]
FIG. 8 is a graph showing a simulation result for confirming the effect of the present invention, and shows the vertical axis bit error rate with respect to the horizontal axis weight. This graph shows the simulation result according to the second embodiment, and the dark current Λd = 0.1 and the number of users K = 5.
From the graph, the same improvement as in FIG. 6 is observed, the optimum weight W increases with the increase of the received signal strength Λs, and the optimum weight W of the “present invention” is the optimum weight of the “multiuser receiver”. It is understood that it is smaller than W. Here, the number of codes that can be generated in the OOC (optical orthogonal code) used in the above embodiment is:
[0037]
[Formula 6]

[0038]
It is expressed. Therefore, showing the minimum error rate when the weight W is small means that the number of OOC codes that can be generated can be increased under a constant code length L.
The present invention is not limited to the above embodiment.
[0039]
In the above embodiment, a photodiode is used as the photodetector, but an avalanche photodiode (APD) having excellent reception sensitivity may be used.
The likelihood of the combined likelihood determiner 25 or the logic determiner 38 may be determined by adding the output of the photodetector 24 or 41.
When the intensity of the optical signal is large by the operation of the optical hard limiter 21 or the like, it is limited to the signal intensity λs of the desired user, but it may be one predetermined value and is not limited to the signal intensity λs of the desired user.
[0040]
【The invention's effect】
As described above, according to the present invention, the influence of multiple access interference can be suppressed without knowing information about other users, so that signal determination is easy, the configuration is simplified, and signal processing is performed. Therefore, it is possible to realize high-speed and high-quality communication, and to take advantage of the high-speed and large-capacity features that are advantages of optical communication.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of an optical communication system suitable for applying an optical receiver according to an embodiment of the present invention.
FIG. 2 is a diagram showing a configuration of an optical receiver according to the first embodiment of the present invention.
FIG. 3 is a diagram for explaining the operation of an optical hard limiter.
FIG. 4 is a diagram illustrating a configuration of an optical receiver according to a second embodiment of the present invention.
5 is a diagram for explaining an optical signal of an upper branch in FIG. 4. FIG.
FIG. 6 is a graph (part 1) showing a simulation result for confirming the effect of the present invention.
FIG. 7 is a graph (part 2) showing a simulation result for confirming the effect of the present invention.
FIG. 8 is a graph (part 3) showing a simulation result for confirming the effect of the present invention;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 Optical fiber 12 Star coupler 13 Optical fiber 21 Optical hard limiter 22, 32, 33, 39 Splitter 23, 34, 40 Correlator 24, 37, 41 Photo detector 25 Combination likelihood determination device 31 1st optical hard limiter 35 Coupler 36 Second optical hard limiter 38 Logic judgment units T1, T2,... TN Optical transmitter R Optical receiver

Claims (5)

An optical receiver used in an optical code division multiple access system of an optical communication system,
With respect to the first threshold value set as the light intensity of one transmitter among the plurality of transmitters, received light having a magnitude greater than or equal to the first threshold value is the same as the first threshold value. A first hard limiter as a first optical signal limiting means for limiting the light intensity of received light smaller than the first threshold value to substantially zero with a light intensity of a value;
First correlation means for branching the output light from the first hard limiter into the number of weights and correlating each weight position ;
First light detection means for converting an output from the first correlation means into an electrical signal;
Receiving the output of said first light detecting means are the result T of adding all of the weighted positions of the log likelihood ratio at each weight position is greater or equal to "0" as "1" was transmitted And a likelihood determination means that “0” is transmitted if it is smaller than “0” ,
The likelihood determination means performs likelihood determination using the following equation (1).
Here, T represented by the following formula (1) is the sum of log likelihood ratios at all weight positions,
W is the weight of the spreading code,
z i is an output of the first light detection means at the i-th weight position,
H ₁ is the case where the one transmitter is transmitting an optical pulse;
H ₀ is when the one transmitter is not transmitting an optical pulse;
P r {z i | H b } bε {1, 0} is the likelihood of the output of the first light detection means at the i-th weight position.

An optical receiver used in an optical code division multiple access system of an optical communication system,
With respect to the first threshold value set as the light intensity of one transmitter among a plurality of transmitters, received light having a magnitude greater than or equal to the first threshold value is the same as the first threshold value. A first hard limiter as first optical signal limiting means for limiting the light intensity of the received light smaller than the first threshold value to substantially zero with a light intensity of a value;
A first splitter for branching a part of output light from the first hard limiter;
By a second splitter for further branching a part of the light branched by the first splitter, a first correlation means for correlating each weight position of the light branched by the second splitter, and an optically coupled coupler A second hard limiter having an output as an input, wherein the output light from the first correlator is set according to the number of branches by the first splitter with respect to the light intensity of the one transmitter With respect to the second threshold value, the received light having a magnitude greater than or equal to the second threshold value is set to the same light intensity as the second threshold value and smaller than the second threshold value. A second hard limiter as a second optical signal limiting means for limiting the light intensity of the received light to substantially zero;
First light detection means for converting an output from the second hard limiter into an electrical signal;
Second correlation means for branching the other output light branched by the first splitter into the number of weights and correlating each weight position;
Second light detection means for converting an output at each of the weight positions by the second correlation means into an electric signal, respectively.
Log likelihood ratio T of output by the first light detection means ₂ And the result of adding the log-likelihood ratio at each weight position output from the second light detection means for all weight positions. A certain T ₂ Likelihood determination that (Expression 2) + T (Expression 1) is equal to or greater than “0”, that “1” is transmitted, and that “0” is transmitted if smaller than “0” And an optical receiver.
Here, T is obtained from Equation (1) and is the sum of log likelihood ratios at the respective weight positions,
W is the weight of the spreading code,
z i Is the output of the second photodetection means at the i-th weight position,
H ₁ Is when the one transmitter is transmitting light pulses,
H ₀ Is when the one transmitter is not transmitting light pulses,
P r {Z i ｜ H b } Bε {1, 0} is the likelihood of the output of the second photodetection means at the i-th weight position.
T ₂ Is obtained from equation (2) and is the log likelihood ratio of the output of the first photodetection means,
z ₀ Is the output of the first light detection means for detecting the output of the second hard limiter,
P r {Z ₀ ｜ H b } B∈ {1 , 0} is the likelihood of the output of the light detection means for converting the output from the second hard limiter into an electrical signal.

An optical receiving method in an optical code division multiple access system of an optical communication system,
With respect to the first threshold value set as the light intensity of one transmitter among the plurality of transmitters, received light having a magnitude greater than or equal to the first threshold value is the same as the first threshold value. a first hard limiter step as a first optical signal limiting means for limiting a substantially zero light intensity of less than the threshold receiving light of the first as well as the light intensity value,
A first correlation step for branching the output light from the first hard limiter step into the number of weights and correlating each weight position ;
A first light detection step for converting the output of the first correlation step into an electrical signal;
Receiving the output of said first light detecting step, what the result T of adding all of the weighted positions of the log likelihood ratio at each weight position is greater or equal to "0" as "1" was transmitted And a likelihood determination step in which “0” is transmitted if it is smaller than “0” ,
The likelihood determination step performs likelihood determination using an expression represented by the following expression (1).
Here, T represented by the following formula (1) is the sum of log likelihood ratios at all weight positions,
W is the weight of the spreading code,
z i is the output of the first light detection step at the i-th weight position,
H ₁ is the case where the one transmitter is transmitting an optical pulse;
H ₀ is when the one transmitter is not transmitting an optical pulse;
P r {z i | H b } bε {1, 0} is the likelihood of the output of the first light detection step at the i-th weight position.

An optical receiving method in an optical code division multiple access system of an optical communication system ,
With respect to the first threshold value set as the light intensity of one transmitter among a plurality of transmitters, received light having a magnitude greater than or equal to the first threshold value is the same as the first threshold value. A first hard limiter step as a first optical signal limiting means for limiting the light intensity of received light smaller than the first threshold value to substantially zero while setting the light intensity to a value;
A first splitter step for branching a part of the output light by the first hard limiter step;
A second splitter step for further branching a part of the light split by the first splitter step, a first correlation step for correlating each weight position of the light split by the second splitter step, and optical coupling; A second hard limiter step that takes an output from the coupler as an input, and the output light from the first correlation step is set to the number of branches by the first splitter step with respect to the light intensity of the one transmitter. With respect to the second threshold value set accordingly, the received light having a magnitude greater than or equal to the second threshold value is set to the same light intensity as the second threshold value, and the second threshold value is set. A second hard limiter step as second optical signal limiting means for limiting the light intensity of the received light smaller than the threshold value to substantially zero;
A first light detection step for converting the output of the second hard limiter step into an electrical signal;
A second correlation step of branching the other output light branched by the first splitter step into the number of weights and taking a correlation with each weight position;
A second photodetection step for converting the output of the second correlation step into an electrical signal;
The sum of the result of addition for all the weighted positions log likelihood ratios in output said each weight position the first log likelihood ratio of the output by the light detecting step T ₂ from the second light detecting step When a certain T ₂ (Expression 2) + T (Expression 1) is equal to or larger than “0”, it is assumed that “1” is transmitted, and when it is smaller than “0”, it is assumed that “0” is transmitted. Likelihood determination step and
An optical receiving method comprising:
Here, T is obtained by Equation (1), and is the sum of log likelihood ratios at the respective weight positions,
W is the weight of the spreading code,
z i is the output of the second light detection step at the i-th weight position,
H ₁ is the case where the one transmitter is transmitting an optical pulse;
H ₀ is when the one transmitter is not transmitting an optical pulse;
P r {z i | H b } bε {1, 0} is the likelihood of the output of the second light detection step at the i-th weight position.
T ₂ is obtained from Equation (2), and is a log likelihood ratio of the output of the first photodetection means,
z ₀ is the output of the first light detection step for detecting the output of the second hard limiter;
P r {z ₀ | H b } bε {1 , 0} is the likelihood of the output of the light detection step for converting the output from the second hard limiter into an electrical signal.

  An optical communication system comprising the optical receiver according to claim 1.

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