JP7094244B2 - A wireless access system and base station in which a base station and a mobile communication device communicate with each other via a relay station. - Google Patents

Description

The present invention relates to a wireless access system in which a base station communicates with a mobile communication device via a relay station.

A non-orthogonal multiple access (NOMA) technique has been proposed in which a base station (BS) transmits and receives superposed signals on a power axis to and from a plurality of mobile communication devices (UEs). Non-Patent Document 1 discloses a wireless access system in which a base station (BS) communicates with a UE via a half-duplex relay station (RS) using the NOMA technique. The half-duplex relay station means a relay station that cannot simultaneously transmit and receive in each of the upstream and downstream directions.

Hereinafter, the uplink communication procedure of the configuration disclosed in Non-Patent Document 1 will be described with reference to FIG. In the first time slot, UE # 1 and UE # 2 transmit a signal addressed to the BS. Hereinafter, the signal transmitted by UE # 1 is referred to as a signal S1, and the signal transmitted by UE # 2 is referred to as a signal S2. RS # 1 to RS # 5 each receive a multiplex signal in which the signal S1 and the signal S2 are superimposed. RS # 1 to RS # 5 take out the signal S1 and the signal S2 from this multiplex signal, respectively. It should be noted that extracting the signal S1 from the multiplex signal is also expressed as decoding the signal S1 from the multiplex signal, and extracting the signal S2 from the multiplex signal is also expressed as decoding the signal S2 from the multiplex signal.

Subsequently, among RS # 1 to RS # 5, one RS is selected based on a predetermined criterion from the RS that can decode both the signal S1 and the signal S2. Here, it is assumed that RS # 3 is selected. In this case, RS # 3 transmits a multiplexed signal of the signal S1 and the signal S2 to the BS in the second time slot following the first time slot, and the BS receives and decodes the multiplexed signal. Since RS # 3 cannot receive the signal during the second time slot, UE # 1 and UE # 2 do not transmit the signal in the second time slot. Then, UE # 1 and UE # 2 transmit a signal in the third time slot following the second time slot.

Zhiguo Ding, et al. , "Relay selection for cooperative NOMA", IEEE Wireless Communications Letters, vol. 5, no. 4, pp. 416-419, August 2016

FIG. 2 is an explanatory diagram of transmission / reception timing of an uplink signal in the configuration of Non-Patent Document 1. First, in the time slot (TS) # 1, UE # 1 and UE # 2 each transmit an uplink signal, and RS # 1 to # 5 receive the uplink signal. Subsequently, in TS # 2, the selected RS, in this case RS # 3, transmits a multiplex signal to the BS. At this time, UE # 1 and UE # 2 and RS # 1, RS # 2, RS # 4 and RS # 5 are in a standby state. Subsequently, in TS # 3, UE # 1 and UE # 2 each transmit an uplink signal, and RS # 1 to # 5 receive this uplink signal. Subsequently, in TS # 4, the selected RS, here RS # 1, transmits a multiplex signal. At this time, UE # 1, UE # 2, and RS # 2 to # 5 are in a standby state. As described above, in the configuration of Non-Patent Document 1, the UE and RS cannot transmit signals at the same time, and the transmission efficiency deteriorates.

INDUSTRIAL APPLICABILITY The present invention provides a technique for improving transmission efficiency in a wireless access system in which a base station communicates with a mobile communication device via a relay station in accordance with the NOMA technique.

According to one aspect of the present invention, the plurality of mobile communication devices include a base station, a plurality of relay stations, and a plurality of mobile communication devices, and the plurality of mobile communication devices are any one of the plurality of relay stations according to a non-orthogonal multiple access technique. A wireless access system that transmits an uplink signal to the base station via one relay station, wherein the wireless access system is the first when the plurality of mobile communication devices transmit the uplink signal in the first time slot. In the first mode in which the plurality of mobile communication devices do not transmit the uplink signal in the second time slot next to the time slot, and the plurality of mobile communication devices in both the first time slot and the second time slot. In, the plurality of relay stations operate in any of the operation modes of the second mode for transmitting the uplink signal, and the plurality of relay stations determine whether the wireless access system operates in the first mode or the second mode. It is characterized by comprising a determination means for determining based on the signal-to-noise ratio of the signal transmitted to the base station.

According to the NOMA technique, according to the present invention, transmission efficiency can be improved in a wireless access system in which a base station communicates with a mobile communication device via a relay station.

The block diagram of the wireless access system by one Embodiment. Explanatory drawing of the first mode. Explanatory drawing of the 2nd mode. Sequence diagram of RS selection processing. The figure which shows the relationship between the SNR of the transmission signal of an RS transmitter and the probability that decoding by BS fails. Sequence diagram of operation mode selection process.

Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. It should be noted that the following embodiments do not limit the invention according to the claims, and not all combinations of features described in the embodiments are essential to the invention. Two or more of the plurality of features described in the embodiments may be arbitrarily combined. In addition, the same or similar configuration will be given the same reference number, and duplicated explanations will be omitted. Further, in each of the following figures, components that are not necessary for the description of the embodiment will be omitted from the drawings.

FIG. 1 shows the configuration of a wireless access system according to the present embodiment. According to FIG. 1, the wireless access system includes one BS, five half-duplex RSs # 1 to RS # 5, and two UEs # 1 and UE # 2. The wireless access system according to the present embodiment operates in either the first mode or the second mode. As described with reference to FIG. 2, the first mode is a mode in which transmission from the UE to the RS and transmission from the RS to the BS are switched for each TS. That is, in the first mode, when UE # 1 and UE # 2 transmit an uplink signal in the first time slot, UE # 1 and UE # 2 transmit an uplink signal in the second time slot following the first time slot. It is also a mode that does not perform. On the other hand, the second mode is a mode in which UE # 1 and UE # 2 can transmit an uplink signal in both the first time slot and the next second time slot. The wireless access system transmits / receives signals according to the NOMA technique, and therefore, the uplink signals transmitted by UE # 1 and UE # 2 are signals in the same frequency band.

FIG. 3 is an explanatory diagram of transmission / reception timing in the second mode. In the second mode, UE # 1 and UE # 2 transmit signals on all TSs except the last TS. In TS # 1, RS # 1 to RS # 5 receive uplink signals from UE # 1 and UE # 2, and decode signals S1 and S2. Here, in FIG. 3, the RS that succeeded in decoding the signal S1 and the signal S2 is indicated by ◯, and the RS that failed to be decoded is indicated by ×. The RS that succeeds in decoding the uplink signal in TS # 1 will be referred to as a candidate RS in TS # 1 below. Similarly, an RS that has succeeded in decoding an uplink signal in TS # n will be referred to as a candidate RS in TS # n below.

From the candidate RS in TS # 1, the RS that transmits the multiplex signal to the BS in TS # 2 is selected. The selection method will be described later. In FIG. 3, RS # 2 is selected from the candidate RSs in TS # 1, and RS # 2 in TS # 2 transmits a multiplex signal to the BS. Since RS # 2 transmits multiple signals in TS # 2, RS # 2 cannot receive the signals transmitted by UE # 1 and UE # 2 in TS # 2. The other RS # 1, RS # 3, RS # 4 and RS # 5 receive signals from UE # 1 and UE # 2. Here, since RS # 1 and RS # 5 are candidate RSs in TS # 1, the multiplex signal transmitted by RS # 2 in TS # 2 is known to RS # 1 and RS # 5. Therefore, RS # 1 and RS # 5 can remove the influence of the multiplex signal transmitted by RS # 2 in TS # 2 and receive the signals from UE # 1 and UE # 2 with high accuracy. It should be noted that RS # 3 and RS # 4 are not candidate RSs in TS # 1, and therefore, for RS # 3 and RS # 4, the multiplex signal transmitted by RS # 2 in TS # 2 is unknown. However, depending on the channel gains of RS # 2 and RS # 3 and RS # 4, RS # 3 and RS # 4 also send signals S1 and S2 from UE # 1 and UE # 2 in TS # 2. It can be decrypted. In the example of FIG. 3, in TS # 2, RS # 1, RS # 4 and RS # 5 succeed in decoding the signals from UE # 1 and UE # 2, and thus become candidate RSs in TS # 2. ing.

Hereinafter, similarly, the RS that transmits the multiplex signal in TS # (n + 1) is selected from the candidate RS in TS # n. Then, in TS # (n + 1), RSs other than the RS transmitting the multiplex signal may receive the uplink signals from UE # 1 and UE # 2, and among them, the signal S1 and the signal S2 may be taken out. The result is a candidate RS in TS # (n + 1).

Next, how to select the RS for transmitting the multiplex signal in TS # (n + 1) from the candidate RS in TS # n will be described. As shown in FIG. 4, RS # 1 to RS # 5, respectively, transmit information indicating whether or not the signals S1 and S2 have been successfully decoded in TS # n to the BS. As a result, the BS determines the candidate RS. For example, the BS selects the RS having the highest channel gain with the BS from the candidate RSs, and in S11, broadcasts a signal indicating the selected RS or unicasts the signal indicating the selected RS to the selected RS. The selected RS transmits a multiplex signal to the BS at TS # (n + 1). In the case of unicast, the RS that does not receive this unicast signal receives the signals from UE # 1 and UE # 2 in TS # (n + 1).

Although the end of the time slot is not provided in FIG. 3, for example, N time slots can be set as one cycle. In this case, UE # 1 and UE # 2 transmit an uplink signal in the TS # 1 to TS # (N-1) th time slots, and do not transmit an uplink signal at the end, that is, in TS # N. Further, any RS transmits a multiplex signal from TS # 2 to TS # N.

Subsequently, in the present invention, how to select the first mode and the second mode will be described. As a premise, it is assumed that the transmission powers of the transmitters of RS # 1 to RS # 5 are equal. Further, it is assumed that the noise levels in the transmitters of RS # 1 to RS # 5 are equal. That is, it is assumed that the SNRs of the signals transmitted by the transmitters of RS # 1 to RS # 5 are equal.

According to the research of the present inventor, in the first mode and the second mode, the theoretical probability that the BS fails to decode the multiplex signal from the RS is the number of RSs, the number of antennas of the BS, the position of the RS, and the target data rate. , It was found that it depends on SNR and the like in the transmitter of RS. Here, the number of RSs, the number of BS antennas, the arrangement position of RSs, the target data rate, and the like are fixed values in the wireless access system. Therefore, the theoretical probability that the BS will fail to decode will vary primarily with the SNR in the RS transmitter. FIG. 5 shows the relationship between this SNR and the theoretical probability that the BS will fail to decode. Reference numerals 60 and 70 indicate the case of the second mode, and reference numerals 61 and 71 indicate the case of the first mode. The difference between the pair of reference numerals 60 and 61 and the pair of reference numerals 70 and 71 is that the number of RSs is 4 and the number of BS antennas is 3 in the pair of reference numerals 60 and 61. In the set of reference numerals 70 and 71, only that the number of RSs is 6 and the number of antennas of BS is 2, and the other parameters are the same.

From FIG. 5, as the SNR increases, the probability that the BS fails in decoding (hereinafter referred to as the failure probability) decreases. However, the magnitude relationship between the failure probability of the first mode and the failure probability of the second mode is reversed at a certain SNR. For example, when the number of RSs is 4 and the number of BS antennas is 3, which are indicated by reference numerals 60 and 61, the failure probability is lower in the second mode (reference numeral 60) until the SNR is about 29 dB. When the SNR exceeds 29 dB, the failure probability is lower in the first mode (reference numeral 61). Further, when the number of RSs is 6 and the number of BS antennas is 2, which are indicated by reference numerals 70 and 71, this boundary is approximately 36 dB.

The BS of the wireless access system holds information about the SNR at this boundary where the failure probability is reversed in the wireless access system as a threshold value. Then, for example, when setting or changing the transmission power of RS # 1 to RS # 5, it is determined whether to set the first mode or the second mode based on the SNR, and the determination result is RS. Notify # 1 to RS # 5 and UE # 1 and UE # 2. It is assumed that the BS also holds information on the noise level of the transmitters of RS # 1 to RS # 5.

FIG. 6 shows a sequence of operation mode selection processing according to the present embodiment. In S20, the BS receives, for example, an instruction from the operator of the wireless access system to change the transmission power of RS # 1 to RS # 5 to the BS. This instruction contains information to identify the transmitted power after the change. In S21, the BS determines whether to use the first mode or the second mode based on the SNR of the changed transmission power. Specifically, when the SNR is equal to or less than the threshold value, the second mode is set, and when the SNR is larger than the threshold value, the first mode is set. In S22, the BS gives RS # 1 to RS # 5 a transmission power setting instruction and, if necessary, a mode change instruction. The transmission power setting instruction includes information for identifying the transmission power to be set. The mode change instruction is transmitted only when changing from the first mode to the second mode or from the second mode to the first mode. Upon receiving the transmission power setting instruction, each RS sets the notified transmission power to the transmitter that transmits a signal to the BS and changes the transmission power. Further, when each RS receives the mode change instruction, it notifies each UE of the mode change instruction in S23. Upon receiving the mode change instruction, each RS and each UE transmits and receives according to the changed mode.

The trigger for changing the transmission power is not limited to the command input by the operator of the wireless access system, and the BS or RS may be configured to monitor some state and change the transmission power. For example, when an RS requests a change in transmission power, the RS notifies the BS of the change in transmission power. This notification contains information to identify the transmitted power after the change. Upon receiving the notification of the transmission power change, the BS performs processing from S21.

Further, the BS, RS, and UE according to the present invention can be realized by a program that operates the computer as the BS, RS, and UE. These computer programs are stored in a computer-readable storage medium or can be distributed over a network.

The invention is not limited to the above embodiment, and various modifications and changes can be made within the scope of the gist of the invention.

Claims (7)

The base station, a plurality of relay stations, and a plurality of mobile communication devices are included, and the plurality of mobile communication devices include the base station via any one relay station of the plurality of relay stations according to the non-orthogonal multiple access technique. A wireless access system that sends an uplink signal to a station.
In the wireless access system, when the plurality of mobile communication devices transmit the uplink signal in the first time slot, the plurality of mobile communication devices transmit the uplink signal in the second time slot next to the first time slot. The operation mode is one of the first mode in which the above-mentioned is not performed and the second mode in which the plurality of mobile communication devices transmit the uplink signal in both the first time slot and the second time slot.
The wireless access system comprises means for determining whether to operate in the first mode or the second mode based on the signal-to-noise ratio of the signal transmitted by the plurality of relay stations to the base station. A wireless access system characterized by being. The determining means determines that when the signal-to-noise ratio is higher than the threshold value, it operates in the first mode, and when the signal-to-noise ratio is lower than the threshold value, it determines that it operates in the second mode. The wireless access system according to claim 1. The wireless access system according to claim 2, wherein the determination means is provided in the base station. The base station operates when the determining means determines to operate in the second mode when the wireless access system is operating in the first mode, and when the wireless access system operates in the second mode. If it is determined that the determination means operates in the first mode, the plurality of relay stations are notified of the change in the operation mode.
The wireless access according to claim 3, wherein the plurality of relay stations notify the plurality of mobile communication devices of the change in the operation mode when the base station notifies the change in the operation mode. system. The base station, a plurality of relay stations, and a plurality of mobile communication devices are included, and the plurality of mobile communication devices include the base station via any one of the plurality of relay stations according to the non-orthogonal multiple access technique. The base station of a wireless access system that transmits an uplink signal to a station.
In the wireless access system, when the plurality of mobile communication devices transmit the uplink signal in the first time slot, the plurality of mobile communication devices transmit the uplink signal in the second time slot next to the first time slot. The operation mode is one of the first mode in which the above-mentioned is not performed and the second mode in which the plurality of mobile communication devices transmit the uplink signal in both the first time slot and the second time slot.
The base station is
It is characterized by comprising a determining means for determining whether to operate in the first mode or the second mode based on the signal-to-noise ratio of the signal transmitted by the plurality of relay stations to the base station. Base station to do. The base station is
Further, the control means for controlling the power of the signal transmitted by the plurality of relay stations to the base station is provided.
The determination means holds the noise levels of the plurality of relay stations, and the signal transmitted by the plurality of relay stations to the base station based on the power determined by the control means and the noise level. The base station according to claim 5, wherein the signal-to-noise ratio is determined. The base station is
The determination means determines to operate in the second mode when the wireless access system is operating in the first mode, and the wireless access system is operating in the second mode. The base station according to claim 5 or 6, wherein when the determination means is determined to operate in the first mode, the plurality of relay stations are further provided with a notification means for notifying the change of the operation mode. ..

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