JP7689799B2 - Wireless communication device and adaptive modulation method - Google Patents

Description

This invention relates to a wireless communication device and an adaptive modulation method, and to a technology suitable for use in satellite communications that utilizes an adaptive modulation method that switches modulation methods based on the propagation environment.

In wireless communication systems, adaptive modulation methods are used that switch modulation methods based on the propagation environment to improve frequency utilization efficiency. In adaptive modulation methods, a receiving station (e.g., a wireless terminal) generally calculates line quality based on the radio waves it receives, and feeds back the line quality calculation results to a transmitting station (e.g., an artificial satellite or base station), which then selects a modulation method based on the line quality calculation results that have been fed back. In this way, the modulation method is changed to one that is suitable for the propagation environment.

A communication device that uses an adaptive modulation method is known that includes a line quality change direction detection means that detects the direction of change in line quality based on a received signal, and a modulation method determination means that, when the direction of change in line quality detected by the line quality change direction detection means is improving, determines the modulation method to be used for signal transmission based on the line quality before the change (see Patent Document 1).

JP 2005-012684 A

In the conventional adaptive modulation method, the calculation result of the line quality is fed back from the receiving station to the transmitting station, but the calculation result of the line quality that is fed back is the state of the line quality a certain time ago, and there is a delay time between the time when the receiving station receives the signal (in other words, the time when the communication state in the line/propagation path was as the calculation result of the line quality) and the time when the transmitting station actually switches the modulation method. There are various possible causes of the delay time. For example, in the case of a satellite communication system, communication is performed via a satellite, so the transmission delay of the information signal used to select the modulation method is large. In addition, the return link for the receiving station to transmit the information used to select the modulation method to the transmitting station is often a TDM (short for Time Division Multiplexing) method, so the delay time of the return link varies and is not stable. Due to these reasons, a non-constant delay time occurs in the information that is the criterion for determining the modulation method. Due to the delay time (also called "adaptive modulation control loop delay") between the time when the receiving station receives the signal (in other words, the time when the communication conditions on the line/propagation path are as calculated for the line quality) and the time when the transmitting station actually switches the modulation method, a difference occurs between the calculated line quality, which is the basis for determining the modulation method, and the actual line quality when the modulation method is actually switched. This causes a problem that it is not possible to select an appropriate modulation method, especially when a large change occurs in the propagation environment, and ultimately reduces transmission efficiency.

In addition, in a method such as that described in Patent Document 1, the frequency with which information on line quality, which is the criterion for determining the modulation method, is acquired must be sufficiently frequent relative to the frequency with which changes occur in the propagation environment. In other words, the information must be acquired frequently, and the receiving station must transmit information on line quality more frequently. This causes a problem in that, particularly when the receiving station is a mobile station, the battery is consumed and the operating time is shortened.

The present invention aims to provide a wireless communication device and an adaptive modulation method that can select an appropriate modulation method without increasing the frequency with which the receiving station transmits information about line quality.

In order to solve the above problem, a wireless communication device according to the present invention is a wireless communication device that performs wireless communication with a communication partner device using an adaptive modulation method, and has a modulation method control unit that selects a modulation method to be used when transmitting to the communication partner device based on combination data of a reception time and line quality in the communication partner device transmitted from the communication partner device, and the modulation method control unit is characterized in that it includes a line quality calculation unit that estimates line quality at a processing time in the wireless communication device after the reception time in the communication partner device when selecting the modulation method to actually perform communication, using a transition probability matrix that is a matrix whose components are actual values of the probability of occurrence of each combination of a fluctuation slope of line quality in a time period before a certain reception time in the communication partner device and a fluctuation slope of line quality in a time period after the certain reception time in the communication partner device, in past communication performed between the wireless communication device and the communication partner device, and a fluctuation slope of line quality in a time period after the reception time in the communication partner device when selecting the modulation method to actually perform communication, which is predicted based on the fluctuation slope of line quality in a time period before the reception time in the communication partner device when selecting the modulation method to actually perform communication.

The wireless communication device of the present invention may be configured such that the modulation scheme control unit includes a first line quality calculation unit that estimates line quality at a processing time in the wireless communication device after the reception time in the device of the communication partner when selecting the modulation scheme for actually performing communication, using a fluctuation slope of line quality in a time period before the reception time in the device of the communication partner when selecting the modulation scheme for actually performing communication, and a modulation scheme selection unit that selects the modulation scheme based on the line quality estimated by the first line quality calculation unit and the line quality estimated by the line quality calculation unit.

The wireless communication device according to the present invention may be configured such that the modulation method control unit includes a reception stagnation response unit that selects the lowest modulation method from among a plurality of modulation methods that are prepared in advance when a predetermined time has elapsed since the wireless communication device last received the combination data transmitted from the communication partner device.

The wireless communication device according to the present invention may change the transmission period of the combined data based on the absolute value of the line quality fluctuation slope in the time period prior to the reception time in the communication partner device when selecting the modulation method for actual communication.

Further, an adaptive modulation method according to the present invention is an adaptive modulation method in wireless communication performed by utilizing an adaptive modulation scheme between a wireless communication device and a communication partner device, characterized in that the wireless communication device selects a modulation scheme to be used when transmitting to the communication partner device based on combination data of a reception time and line quality in the communication partner device transmitted from the communication partner device, and the wireless communication device estimates line quality at a processing time in the wireless communication device after the reception time in the communication partner device when selecting the modulation scheme to actually perform communication, using a fluctuation slope of line quality in a time period after the reception time in the communication partner device when selecting the modulation scheme to actually perform communication, which is predicted based on a transition probability matrix, which is a matrix having actual values of the probability of occurrence of each combination of a fluctuation slope of line quality in a time period before a certain reception time in the communication partner device and a fluctuation slope of line quality in a time period after the certain reception time in the communication partner device, in past communication performed between the wireless communication device and the communication partner device, and a fluctuation slope of line quality in a time period before the reception time in the communication partner device when selecting the modulation scheme to actually perform communication.

According to the wireless communication device and adaptive modulation method of the present invention, the modulation method to be used when transmitting to the communication partner device is selected based on the combined data of the reception time and line quality at the communication partner device transmitted from the communication partner device. Therefore, the modulation method is determined taking into account the line quality as well as the time when the line quality was acquired (i.e., the reception time at the communication partner device). This makes it possible to perform control assuming two factors: the delay of the adaptive modulation control loop and the fluctuation slope of the line quality, and to select an appropriate modulation method. For example, it is possible to select a modulation method with higher transmission efficiency than when a modulation method is selected by uniformly assuming an estimation error of the value of an index representing the line quality. According to the wireless communication device 1 and adaptive modulation method of the embodiment, it is also possible to select an appropriate modulation method without increasing the frequency of transmission of line quality information by the receiving station.

According to the wireless communication device of the present invention, if the lowest modulation method is selected when a predetermined time has elapsed since the wireless communication device last received combination data transmitted from the communication partner device, it is possible to avoid a situation in which the data used to estimate the communication conditions in the line/propagation path is old and the difference between the calculated line quality, which is the criterion for determining the modulation method, and the actual line quality when the modulation method is actually switched becomes large, and it is possible to avoid a situation in which an inappropriate modulation method is selected.

In the wireless communication device according to the present invention, when the transmission period of the combined data is changed based on the absolute value of the line quality fluctuation slope, it becomes possible to accurately track the changes in the propagation environment, which fluctuate greatly in a short period of time, and to select an appropriate modulation method, and when the propagation environment changes slowly, it reduces traffic and the computational load of the device.

1 is a functional block diagram showing a schematic configuration of a wireless communication device according to an embodiment of the present invention. 2 is a diagram showing an example of operation timing relating to wireless communication between the wireless communication device of FIG. 1 and a communication terminal; 10 is a diagram showing another example of operation timing relating to wireless communication between the wireless communication device and a communication terminal of FIG. 1. 2 is a diagram showing an example of a transition probability matrix used in the wireless communication device of FIG. 1; 2 is a diagram showing an example of a correspondence table between specific propagation path estimation values and modulation methods used in the wireless communication device of FIG. 1;

The present invention will be described below based on the illustrated embodiment.

Figure 1 is a functional block diagram showing a schematic configuration of a wireless communication device 1 according to an embodiment of the present invention. Figure 2 is a diagram showing an example of operation timing related to wireless communication between the wireless communication device 1 according to the embodiment and a communication terminal 9 that is a communication partner.

The wireless communication device 1 performs digital wireless communication with the communication terminal 9 via the satellite 8 using an adaptive modulation method, and a wireless communication system is formed that includes the wireless communication device 1, the satellite 8, and the communication terminal 9. Note that the wireless communication device 1 and the communication terminal 9 can use mutually synchronized times, for example, by using time information obtained by the clock function of the GPS (Global Positioning System).

In the description of this invention, "modulation method" refers to at least one of the (narrowly defined) modulation method/modulation multi-level number such as BPSK, QPSK, 16QAM, and 64QAM, and parameters related to encoding (e.g., encoding rate, code length) (Note that BPSK is an abbreviation for Binary Phase Shift Keying, QPSK is an abbreviation for Quadrature Phase Shift Keying, and QAM is an abbreviation for Quadrature Amplitude Modulation).

The communication terminal 9 performs predetermined processing such as amplification, frequency conversion, and demodulation on the radio signal wirelessly transmitted from the wireless communication device 1 and received via the antenna to obtain the communication data transmitted from the wireless communication device 1 (i.e., data used for the original communication between the wireless communication device 1 and the communication terminal 9), and calculates the line quality for each frame based on the received symbols.

The index used as the line quality is not limited to a specific type, and any information that can indicate the degree of goodness/badness of the communication conditions on the line/propagation path may be used. Examples of indexes used as the line quality include the reception level [dB], the carrier to noise ratio (i.e., C/N ratio) [dB], and RSSI (short for Received Signal Strength Indicator).

The communication terminal 9 generates combined data of the line quality calculated for each frame, for example, and the reception time of the wireless signal used to calculate the line quality. The index value representing the line quality (in other words, the degree of goodness/badness of the communication conditions in the line/propagation path) calculated based on the wireless signal received at the reception time Tr_n is represented as "line quality Ctr_n". The subscript n is an identifier for distinguishing multiple consecutive time points from each other in chronological order, and n = 1, 2, 3, ... as consecutive values in which the value of the earlier time point in the time series is smaller.

Time Tt_n is the transmission time at which the communication terminal 9 generates combined data of the reception time Tr_n and the line quality Ctr_n at the communication terminal 9 and transmits it to the wireless communication device 1. The length of time (unit: seconds) from the transmission time Tt_n-1 to the next transmission time Tt_n is called the "period time Δt" (i.e., for example, Δt = Tt_n+1 - Tt_n = Tt_n - Tt_n-1).

The communication terminal 9 generates a transmission frame based on the combination of the reception time Tr_n and the line quality Ctr_n together with the communication data to be transmitted to the wireless communication device 1 (i.e., data used for the original communication between the wireless communication device 1 and the communication terminal 9), and transmits the transmission frame via an antenna after carrying out predetermined processing such as modulating the transmission frame and converting it into a wireless signal.

In other words, the communication terminal 9 generates and transmits combination data of the reception time Tr_n and the line quality Ctr_n at the communication terminal 9 at periodic time intervals Δt.

The wireless communication device 1 is a mechanism for performing digital wireless communication using an adaptive modulation method with a communication terminal 9 via a satellite 8, and mainly has a receiver 2, a demodulator 3, a modulation method controller 4, a modulator 5, and a transmitter 6.

The wireless communication device 1 may further include, as necessary, a control unit configured with a central processing unit (CPU: abbreviation for Central Processing Unit) and the like that has the function of controlling the operation of each component that configures the wireless communication device 1, and a storage unit configured with a ROM (abbreviation for Read Only Memory) or a RAM (abbreviation for Random Access Memory) and the like that has the function of serving as a working area for temporarily storing data and information generated when the central processing unit performs arithmetic processing related to the selection/determination of the modulation method, and as a storage area for storing and storing various information, programs, and data.

The wireless communication device 1 and adaptive modulation method according to the embodiment select the modulation method to be used when transmitting to the communication terminal 9 based on the combined data of the reception time and line quality at the communication terminal 9 transmitted from the communication terminal 9.

In the following description, the time at which the wireless communication device 1 starts processing related to selecting/determining the modulation method using the line quality Ctr_n calculated based on the wireless signal received at the communication terminal 9 at the reception time Tr_n and transmitted from the communication terminal 9 is represented as "processing time Tp_n." The processing time Tp_n may also be the time at which the wireless communication device 1 receives the wireless signal transmitted from the communication terminal 9.

The time difference between the reception time Tr_n at the communication terminal 9 and the processing time Tp_n at the wireless communication device 1 (i.e., Tp_n-Tr_n; specifically, the delay time) is represented as the "time lag Tlag_n" (see FIG. 2). Note that the reception time Tr_n at the communication terminal 9 is before the transmission time Tt_n from the communication terminal 9, and therefore the starting point of the time lag Tlag_n is before the transmission time Tt_n from the communication terminal 9 (although not explicitly shifted in the figure).

The receiver 2 amplifies and converts the frequency of the radio signal that is wirelessly transmitted from the communication terminal 9 and received via the antenna, and outputs the converted signal.

The demodulation unit 3 receives the converted signal output from the receiving unit 2, performs predetermined processing such as demodulation on the signal, extracts communication data transmitted from the communication terminal 9 (i.e., data used for the original communication between the wireless communication device 1 and the communication terminal 9) and outputs it as received data, and also extracts and outputs combined data of the reception time Tr_n and line quality Ctr_n at the communication terminal 9 transmitted from the communication terminal 9.

In other words, in principle (in other words, if the communication conditions on the line/propagation path are good), the wireless communication device 1 receives combined data of the reception time Tr_n and line quality Ctr_n at the communication terminal 9, which is transmitted from the communication terminal 9 at periodic time intervals Δt. However, because the transmission delay of the wireless signal from the communication terminal 9 to the wireless communication device 1 is not constant, the timing/time interval at which the wireless communication device 1 receives the wireless signal is not constant, and therefore the time lag Tlag_n is not constant.

The modulation method control unit 4 is a mechanism for selecting a modulation method to be applied/used when transmitting to the communication terminal 9 based on the combined data of the reception time Tr_n and the line quality Ctr_n at the communication terminal 9 transmitted from the communication terminal 9, and for outputting a signal for controlling switching to the selected modulation method, and includes a first line quality calculation unit 41, a second line quality calculation unit 42, a modulation method selection unit 43, and a reception stagnation response unit 44.

The first line quality calculation unit 41 estimates the value of an index representing the line quality (in other words, the degree of goodness/badness of the communication conditions in the line/propagation path) using the line quality fluctuation slope and time lag. The value estimated by the first line quality calculation unit 41 is called the "first propagation path estimate value."

More specifically, the first channel quality calculation unit 41 estimates the first channel estimation value C1tp_n in accordance with the following equation 1.
(Math. 1) C1tp_n = Ctr_n+(Ctr_n-Ctr_n-1)/(Tr_n-Tr_n-1)×Tlag_n
Here,
C1tp_n: first propagation path estimation value at processing time Tp_n in the wireless communication device 1 Ctr_n: channel quality at reception time Tr_n in the communication terminal 9 Ctr_n-1: channel quality at reception time Tr_n-1 in the communication terminal 9 Tlag_n: relationship between reception time Tr_n in the communication terminal 9 and processing time in the wireless communication device 1
Time lag between Tp_n (Tlag_n = Tp_n - Tr_n)

The "(Ctr_n-Ctr_n-1)/(Tr_n-Tr_n-1)" in the above formula 1, which corresponds to the line quality fluctuation slope, may be calculated by the communication terminal 9 and transmitted from the communication terminal 9 to the wireless communication device 1 together with the combined data of the reception time Tr_n and the line quality Ctr_n at the communication terminal 9.

In other words, the first line quality calculation unit 41 assumes that the fluctuation slope of the line quality in the time period before the reception time Tr_n at the communication terminal 9 continues in the same manner in the time period after the reception time Tr_n, and estimates the line quality (specifically, the first propagation path estimation value C1tp_n) at the time when the time lag Tlag_n has elapsed from the reception time Tr_n (i.e., the processing time Tp_n at the wireless communication device 1).

The first channel quality calculation unit 41 outputs the estimated first channel estimation value C1tp_n.

Here, the first line quality calculation unit 41 performs a process of estimating the first propagation path estimation value C1tp_n when new combination data is received before a predetermined upper stagnation time limit has elapsed since the wireless communication device 1 last received combination data transmitted from the communication terminal 9.

On the other hand, if new combination data is not received before the stagnation upper limit time has elapsed since the wireless communication device 1 last received combination data transmitted from the communication terminal 9, the first line quality calculation unit 41 does not perform the process of estimating the first propagation path estimate C1tp_n, and outputs a signal to the reception stagnation response unit 44 to notify that the stagnation upper limit time has elapsed. In this case, the process by the second line quality calculation unit 42 and the process by the modulation method selection unit 43 described below are also not performed.

The upper stagnation time limit is not limited to a specific time length (unit: seconds), but is set to an appropriate time length, taking into consideration that, for example, if the data used to estimate the communication conditions on the line/propagation path is old, there is a risk that the difference between the calculated line quality that serves as the criterion for determining the modulation method and the actual line quality when the modulation method is actually switched may become large, and as a result, there is a risk that an appropriate modulation method may not be selected. The upper stagnation time limit is set to a time length within a range of, for example, approximately 1.5 to 4 times the cycle time Δt.

As shown in FIG. 3, when the stagnation upper limit time is set to a time length longer than twice the period time Δt, and new combination data is received after a time period twice the period time Δt has elapsed, starting from the reception time Tr_n-2 at the communication terminal 9 among the combination data last (in other words, immediately before) received by the wireless communication device 1 (in the example shown in the figure, the combination data of the reception time Tr_n-1 at the communication terminal 9 and the line quality Ctr_n-1 transmitted from the communication terminal 9 at the transmission time Tt_n-1 is lost), the first line quality calculation unit 41 performs a process of estimating the first propagation path estimation value (in the example shown in the figure, C1tp_n) using the combination data last (in other words, immediately before) received by the wireless communication device 1 (in the example shown in the figure, the combination data of the reception time Tr_n-2 at the communication terminal 9 and the line quality Ctr_n-2).

In the example shown in FIG. 3, the first channel quality calculation unit 41 specifically estimates the first channel estimation value C1tp_n in accordance with Equation 2 below.
(Math. 2) C1tp_n = Ctr_n+(Ctr_n-Ctr_n-2)/(Tr_n-Tr_n-2)×Tlag_n
Here,
C1tp_n: first propagation path estimation value at processing time Tp_n in the wireless communication device 1 Ctr_n: channel quality at reception time Tr_n in the communication terminal 9 Ctr_n-2: channel quality at reception time Tr_n-2 in the communication terminal 9 Tlag_n: relationship between reception time Tr_n in the communication terminal 9 and processing time in the wireless communication device 1
Time lag between Tp_n (Tlag_n = Tp_n - Tr_n)

In the example shown in FIG. 3, the stagnation upper limit time is set to a time length slightly longer than twice the cycle time Δt, and if the combined data of the reception time Tr_n and the line quality Ctr_n transmitted from the communication terminal 9 at the transmission time Tt_n following the combined data of the reception time Tr_n-1 and the line quality Ctr_n transmitted from the communication terminal 9 at the transmission time Tt_n-1 is also lost, new combined data will not be received before the stagnation upper limit time has elapsed since the wireless communication device 1 last received the combined data transmitted from the communication terminal 9, and the first line quality calculation unit 41 will not perform the process of estimating the first propagation path estimate value C1tp_n, and will output a signal notifying the reception stagnation response unit 44 that the stagnation upper limit time has elapsed. Then, the process by the second line quality calculation unit 42 and the process by the modulation method selection unit 43 described below will not be performed.

The second line quality calculation unit 42 calculates the line quality fluctuation slope in the previous time period and then estimates the line quality fluctuation slope after the time lag using a Markov process to estimate the value of an index representing the line quality (in other words, the degree of good/bad communication conditions in the line/propagation path). The value estimated by the second line quality calculation unit 42 is called the "second propagation path estimated value."

More specifically, the second channel quality calculation unit 42 estimates the second channel estimation value C2tp_n in accordance with the following Equation 3.
(Math. 3) C2tp_n = Ctr_n+ΔCtr_n×Tlag_n
Here,
C2tp_n: second propagation path estimation value at processing time Tp_n in the wireless communication device 1 Ctr_n: channel quality at reception time Tr_n in the communication terminal 9 ΔCtr_n: time from reception time Tr_n in the communication terminal 9 to processing time in the wireless communication device 1
Tlag_n: Estimated value of the line quality fluctuation slope over Tp_n Tlag_n: The difference between the reception time Tr_n at the communication terminal 9 and the processing time Tr_n at the wireless communication device 1
Time lag between Tp_n (Tlag_n = Tp_n - Tr_n)

In the above formula 3, the estimated line quality fluctuation slope ΔCtr_n is calculated using the transition probability matrix T.

As shown in FIG. 4, the transition probability matrix T is a matrix whose (i, j) components are the probability values p ij of the occurrence of each combination of a pre-time fluctuation slope ΔCbef_n (see Equation 4 below) of line quality from reception time Tr_n-1 to reception time Tr_n (in other words, in the previous periodic time Δt) at the communication terminal 9 and a post-time fluctuation slope ΔCaft_n (see Equation 5 below) of line quality from reception time Tr_n to reception time _{Tr_n} +1 (in other words, in the later periodic time Δt) at the communication terminal 9.
(Math. 4) ΔCbef_n = (Ctr_n−Ctr_n-1)/(Tr_n−Tr_n-1)
(Math. 5) ΔCaft_n = (Ctr_n+1−Ctr_n)/(Tr_n+1−Tr_n)

ΔC ₁ , ΔC ₂ , and ΔC ₃ in FIG. 4 are values determined as the line quality fluctuation gradient ΔCbef before a point in time and the line quality fluctuation gradient ΔCaft after a point in time, and are negative numbers, 0, or positive numbers, and are actually concrete numerical values.

The transition probability matrix T is created based on the results of past communications between the wireless communication device 1 and the communication terminal 9. Specifically, the transition probability matrix T is created by creating a set of combined data of a pre-time fluctuation slope ΔCbef calculated from the line quality at a certain reception time in the communication terminal 9 and the line quality at a reception time a periodic time Δt before the certain reception time, and a post-time fluctuation slope ΔCaft calculated from the line quality at the certain reception time in the communication terminal 9 and the line quality at a reception time a periodic time Δt after the certain reception time, and determining the probability distribution of each combination of the pre-time fluctuation slope ΔCbef and the post-time fluctuation slope ΔCaft in the set.

The transition probability matrix T is stored, for example, in a storage unit and is read and used as appropriate by the second line quality calculation unit 42. The transition probability matrix T may be updated as necessary.

Specifically, for example, when the transition probability matrix T is expressed as shown in FIG. 4 and the pre-time fluctuation slope ΔCbef_n of the line quality from the reception time Tr_n-1 to the reception time Tr_n at the communication terminal 9 (in other words, in the previous periodic time Δt) is less than _ΔC1 , the estimated value ΔCtr_n of the fluctuation slope of the line quality is calculated according to the following formula 6 using the transition probability matrix T.
(Math. 6) ΔCtr_n = p ₁₁ ×{ΔC ₁ -(ΔC ₂ -ΔC ₁ )/2}
+p ₁₂ ×(ΔC ₁ +ΔC ₂ )/2
+p ₁₃ ×(ΔC ₂ +ΔC ₃ )/2
+p ₁₄ ×{ΔC ₃ +(ΔC ₃ -ΔC ₂ )/2}

Alternatively, the estimated value ΔCtr_n of the line quality fluctuation slope may be determined based on the value of the post-time fluctuation slope ΔCaft having the largest absolute value among combinations of the pre-time fluctuation slope ΔCbef and the post-time fluctuation slope ΔCaft in which the probability value p _ij , which is the (i, j) component of the transition probability matrix T, is equal to or greater than a predetermined transition probability threshold.

Specifically, for example, when the transition probability matrix T is expressed as shown in Fig. 4 and the transition probability threshold is previously set to Pthr (specifically, any numerical value within a range of, for example, about 0.1 to 0.3), it is assumed that the pre-point fluctuation slope ΔCbef_n of the line quality from reception time Tr_n-1 to reception time Tr_n at the communication terminal 9 (in other words, in the previous periodic time Δt) is less than ΔC _1. In this case, if p ₁₄ < p ₁₃ < Pthr < p ₁₂ < p ₁₁ and |ΔC ₁ - (ΔC ₂ - ΔC ₁ )/2| > |(ΔC ₁ + ΔC ₂ )/2|, the estimated value ΔCtr_n of the fluctuation slope of the line quality may be set to "ΔC ₁ - (ΔC ₂ - ΔC ₁ )/2".

In other words, the second line quality calculation unit 42 predicts the line quality fluctuation slope (i.e., ΔCbef_n) in the time period before the reception time Tr_n at the communication terminal 9 based on the transition probability matrix T and the line quality fluctuation slope (i.e., ΔCtr_n) in the time period after the reception time Tr_n, and then estimates the line quality (specifically, the second propagation path estimation value C2tp_n) at the time when the time lag Tlag_n has elapsed from the reception time Tr_n (i.e., the processing time Tp_n at the wireless communication device 1).

The second channel quality calculation unit 42 outputs the estimated second channel estimation value C2tp_n.

Here, as shown in FIG. 3, in the case where the stagnation upper limit time is set to a time length longer than twice the period time Δt, and new combination data is received after a time twice the period time Δt has elapsed, starting from the reception time Tr_n-2 at the communication terminal 9 among the combination data last (in other words, immediately before) received by the wireless communication device 1 (in the example shown in the figure, the combination data of the reception time Tr_n-1 at the communication terminal 9 and the line quality Ctr_n-1 transmitted from the communication terminal 9 at the transmission time Tt_n-1 is lost), the second line quality calculation unit 42 performs a process of estimating the second propagation path estimation value (in the example shown in the figure), using the combination data last (in other words, immediately before) received by the wireless communication device 1 (in the example shown in the figure, the combination data of the reception time Tr_n-2 at the communication terminal 9 and the line quality Ctr_n-2).

In the example shown in FIG. 3 , the estimated value ΔCtr_n of the fluctuation slope of the channel quality in the above-mentioned Equation 3 is calculated by performing a matrix calculation of the square of the transition probability matrix T, and then using the transition probability matrix ^T2 in a procedure similar to that of the above-mentioned Equation 6, or by a procedure similar to that of the method using the above-mentioned transition probability threshold value for the transition probability matrix ^T2 .

The modulation scheme selection unit 43 selects a modulation scheme based on the first propagation path estimation value C1tp_n and the second propagation path estimation value C2tp_n.

Specifically, the modulation method selection unit 43 first receives an input of the first propagation path estimation value C1tp_n output from the first line quality calculation unit 41, and also receives an input of the second propagation path estimation value C2tp_n output from the second line quality calculation unit 42, and identifies the one with the worse line quality between the first propagation path estimation value C1tp_n and the second propagation path estimation value C2tp_n. The propagation path estimation value identified by the modulation method selection unit 43 is called a "specific propagation path estimation value."

The modulation scheme selection unit 43 then selects a modulation scheme based on the specific propagation path estimate. In other words, the modulation scheme selection unit 43 selects a modulation scheme based on the specific propagation path estimate that has the worse line quality out of the first propagation path estimate C1tp_n and the second propagation path estimate C2tp_n, thereby selecting a modulation scheme so as to avoid the risk of selecting the wrong modulation scheme due to an estimation error in the value of the index representing the line quality (specifically, the first propagation path estimate C1tp_n, the second propagation path estimate C2tp_n).

Specifically, for example, a correspondence table between the specific propagation path estimation value CS and the modulation method is created in advance, which includes multiple combinations of the specific propagation path estimation value CS and the modulation method so that the modulation method is specified according to the magnitude of the specific propagation path estimation value CS, and the modulation method selection unit 43 selects the modulation method according to the correspondence table. The correspondence table between the specific propagation path estimation value CS and the modulation method is stored, for example, in a storage unit, and is read and used as appropriate by the modulation method selection unit 43.

An example of a correspondence table between specific propagation path estimates CS and modulation methods is shown in Fig. 5. The example shown in Fig. 5 is based on the premise that the larger the specific propagation path estimates CS, the better the communication conditions in the line/propagation path (note that _CS1 , _CS2 , _CS3 , and _CS4 in Fig. 5 are values determined according to the type of index representing the quality of the line, and are actually concrete numerical values). The example shown in Fig. 5 also selects a modulation method with higher transmission efficiency from among multiple modulation methods as the communication conditions in the line/propagation path become better. Note that a modulation method with lower transmission efficiency is ranked lower, and a modulation method with higher transmission efficiency is ranked higher.

The example shown in FIG. 5 is an example of a correspondence table in which a modulation method/modulation multi-level number is selected according to a specific propagation path estimate value CS. However, the correspondence table between the specific propagation path estimate value CS and the modulation method is not limited to the example shown in FIG. 5. A correspondence table in which parameters related to encoding (e.g., coding rate, code length) are selected according to the specific propagation path estimate value CS may be created and used, or a correspondence table in which a combination of a modulation method/modulation multi-level number and parameters related to encoding (e.g., coding rate, code length) is selected according to the specific propagation path estimate value CS may be created and used.

Then, the modulation method selection unit 43 outputs a signal (called a "modulation method control signal") to control switching to the selected modulation method.

The reception stagnation response unit 44 selects a predetermined modulation method when it receives a signal output from the first line quality calculation unit 41 notifying that an upper stagnation time limit has elapsed since the wireless communication device 1 last received combination data transmitted from the communication terminal 9.

When the upper stagnation time limit has elapsed since the wireless communication device 1 last received the combined data transmitted from the communication terminal 9, that is, the wireless communication device 1 is considered to be in a situation where it cannot receive the combined data of the reception time Tr_n and the line quality Ctr_n at the communication terminal 9 for a predetermined time due to poor communication conditions in the line/propagation path. Therefore, the reception stagnation response unit 44 selects the lowest modulation method (specifically, the multiple modulation methods included in the correspondence table) that are prepared in advance, in other words, the modulation method that is expected to maximize the index value representing the line quality (i.e., the line quality Ctr_n) (BPSK in the example of the correspondence table shown in FIG. 5).

The reception stagnation response unit 44 outputs a signal (i.e., a modulation method control signal) to control switching to the selected modulation method.

The modulation unit 5 receives input of transmission data, which is communication data to be transmitted to the communication terminal 9 (i.e., data used for the original communication between the wireless communication device 1 and the communication terminal 9), and also receives input of a modulation method control signal output from the modulation method control unit 4. The modulation unit 5 performs modulation processing on the transmission data while switching the modulation method in accordance with the modulation method control signal to generate a modulated signal, and outputs the modulated signal.

The transmitter 6 receives the modulated signal output from the modulator 5, performs predetermined processing such as frequency conversion and amplification on the modulated signal, and then wirelessly transmits the processed signal via the antenna.

According to the wireless communication device 1 and adaptive modulation method of the embodiment, the modulation method to be used when transmitting to the communication terminal 9 is selected based on the combined data of the reception time Tr_n at the communication terminal 9 and the line quality Ctr_n transmitted from the communication terminal 9. Therefore, the modulation method is determined taking into consideration the line quality Ctr_n as well as the time when the line quality Ctr_n was acquired (i.e., the reception time Tr_n at the communication terminal 9). This makes it possible to perform control assuming two factors: the delay of the adaptive modulation control loop and the fluctuation slope of the line quality, and to select an appropriate modulation method. For example, it is possible to select a modulation method with higher transmission efficiency than when a modulation method is selected by uniformly assuming an estimation error of the value of an index representing the line quality. According to the wireless communication device 1 and adaptive modulation method of the embodiment, it is also possible to select an appropriate modulation method without increasing the frequency of transmission of line quality information by the receiving station.

According to the wireless communication device 1 and adaptive modulation method of the embodiment, the lowest modulation method is selected when the upper stagnation time has elapsed since the wireless communication device 1 last received combination data transmitted from the communication terminal 9. This makes it possible to avoid a situation in which the difference between the calculation result of the line quality, which is the criterion for determining the modulation method, and the actual line quality when the modulation method is actually switched becomes large because the data used to estimate the communication status in the line/propagation path is old, and it becomes possible to avoid a situation in which an inappropriate modulation method is selected.

The above describes an embodiment of the present invention, but the specific configuration is not limited to the above embodiment, and even if there are design changes within the scope of the invention that do not deviate from the gist of the invention, they are still included in the invention.

Specifically, in the above embodiment, the communication terminal 9 transmits the reception time Tr_n at the communication terminal 9 to the wireless communication device 1 using the time synchronized between the wireless communication device 1 and the communication terminal 9, and the wireless communication device 1 calculates the time lag Tlag_n (=Tp_n-Tr_n), but the wireless communication device 1 may calculate an average delay time and determine the time lag Tlag based on the average delay time. Specifically, for example, when the transmission unit 6 of the wireless communication device 1 wirelessly transmits a signal as transmission data, the signal is transmitted with time information (specifically, a timestamp of the transmission time from the wireless communication device 1), and the line quality Ctr_n and the time information when the communication terminal 9 receives the signal including the time information together with the communication data are transmitted to the wireless communication device 1 (the reception time Tr_n at the communication terminal 9 is not transmitted). The wireless communication device 1 may calculate the time difference between the time information (i.e., the timestamp of the transmission time from the wireless communication device 1) that is transmitted to the communication terminal 9 and returned and the reception time, and may calculate the average value of the time difference over a predetermined period in the past, and may set half of the average value of the time difference as Tlag.

In addition, in the above embodiment, the modulation scheme control unit 4 includes both the first line quality calculation unit 41 and the second line quality calculation unit 42, but the modulation scheme control unit 4 may include only one of the first line quality calculation unit 41 and the second line quality calculation unit 42. In this case, the modulation scheme selection unit 43 is not provided, and the modulation scheme is selected based on the first propagation path estimation value C1tp_n estimated by the first line quality calculation unit 41, or the modulation scheme is selected based on the second propagation path estimation value C2tp_n estimated by the second line quality calculation unit 42.

In addition, in the above embodiment, the modulation format control unit 4 is provided with the reception stagnation response unit 44, but the modulation format control unit 4 may not be provided with the reception stagnation response unit 44. In this case, the stagnation upper limit time is not set, and the first line quality calculation unit 41 performs processing to estimate the first propagation path estimation value C1tp_n, and the second line quality calculation unit 42 performs processing to estimate the second propagation path estimation value C2tp_n, regardless of the elapsed time since the wireless communication device 1 last received the combination data transmitted from the communication terminal 9.

In addition, in the above embodiment, the periodic time Δt is constant, but the periodic time Δt may be changed. For example, when the absolute value of the line quality fluctuation slope expressed as (Ctr_n-Ctr_n-1)/(Tr_n-Tr_n-1) exceeds a predetermined threshold due to a large change in the propagation environment, the periodic time Δt may be shortened to shorten the transmission period of the line quality information from the communication terminal 9 (specifically, the combined data of the reception time Tr_n and the line quality Ctr_n at the communication terminal 9). Alternatively, the periodic time Δt may be changed (specifically, shortened or lengthened) depending on the magnitude of the absolute value of the line quality fluctuation slope expressed as (Ctr_n-Ctr_n-1)/(Tr_n-Tr_n-1) to change the transmission period of the line quality information from the communication terminal 9. In addition, when the absolute value of the line quality fluctuation slope exceeds a predetermined threshold, the communication terminal 9 may give the highest priority to the transmission process of the line quality Ctr_n and the reception time Tr_n at the communication terminal 9. This makes it possible to select an appropriate modulation method by accurately tracking the changes in the propagation environment, which fluctuate greatly in a short period of time, and to reduce traffic and the calculation load of the device when the propagation environment changes slowly.

REFERENCE SIGNS LIST 1 Wireless communication device 2 Receiving unit 3 Demodulating unit 4 Modulation method control unit 41 First line quality calculation unit 42 Second line quality calculation unit 43 Modulation method selection unit 44 Reception stagnation response unit 5 Modulating unit 6 Transmitting unit 8 Satellite 9 Communication terminal

Claims (5)

A wireless communication device that performs wireless communication with a communication partner device using an adaptive modulation method,
a modulation method control unit that selects a modulation method to be used when transmitting to the communication partner device based on combined data of a reception time and a line quality at the communication partner device transmitted from the communication partner device ;
the modulation scheme control unit includes a line quality calculation unit that estimates line quality at a processing time in the wireless communication device after the reception time in the communication partner device when selecting the modulation scheme to actually perform communication, using a line quality fluctuation slope in a time period after the reception time in the communication partner device when selecting the modulation scheme to actually perform communication, which is predicted based on a transition probability matrix, which is a matrix whose components are actual values of the probability of occurrence of each combination of a line quality fluctuation slope in a time period before a certain reception time in the communication partner device and a line quality fluctuation slope in a time period after the certain reception time in the communication partner device when selecting the modulation scheme to actually perform communication, and a line quality fluctuation slope in a time period after the reception time in the communication partner device when selecting the modulation scheme to actually perform communication.
23. A wireless communication device comprising: The modulation format control unit
a first line quality calculation unit that estimates line quality at a processing time in the wireless communication device after a reception time in the device of the communication counterpart when selecting the modulation method for actually performing communication, using a fluctuation slope of line quality in a time period before a reception time in the device of the communication counterpart when selecting the modulation method for actually performing communication ;
a modulation scheme selection unit that selects the modulation scheme based on the line quality estimated by the first line quality calculation unit and the line quality estimated by the line quality calculation unit;
2. The wireless communication device according to claim 1 . the modulation scheme control unit includes a reception stagnation response unit that selects the lowest modulation scheme from among a plurality of modulation schemes prepared in advance when a predetermined time has elapsed since the wireless communication device last received the combination data transmitted from the communication partner device,
3. A wireless communication device according to claim 1 or 2 . changing a transmission period of the combination data based on an absolute value of a fluctuation slope of line quality in a time period prior to a reception time in the communication partner device when the modulation method is selected for actual communication;
4. A wireless communication device according to claim 1, wherein the first and second inputs are connected to the first and second inputs. An adaptive modulation method in wireless communication using an adaptive modulation scheme between a wireless communication device and a communication partner device,
selecting a modulation scheme to be used by the wireless communication device when transmitting to the communication partner device, based on combined data of a reception time and line quality at the communication partner device transmitted from the communication partner device;
the wireless communication device estimates line quality at a processing time in the wireless communication device after the reception time in the communication partner device when selecting the modulation scheme to actually perform communication, using a line quality fluctuation slope in a time period after the reception time in the communication partner device when selecting the modulation scheme to actually perform communication, the line quality fluctuation slope being predicted based on a transition probability matrix, which is a matrix whose components are actual values of the probability of occurrence of each combination of a line quality fluctuation slope in a time period before a certain reception time in the communication partner device and a line quality fluctuation slope in a time period after the certain reception time in the communication partner device when selecting the modulation scheme to actually perform communication,
16. An adaptive modulation method comprising: