JP5872875B2 - Mobile terminal, radio communication system, and radio communication method - Google Patents

Description

  The present invention relates to a mobile terminal that performs transmission output control, a radio communication system, and a radio communication method.

(Adaptive antenna array)
An adaptive antenna array having a plurality of antenna elements is typically implemented in a base station. When the antenna array is mounted on the base station, it is possible to suppress the interference wave included in the uplink reception wave and estimate the arrival path of the desired wave by deriving the antenna weight by reception.

  In addition, the adaptive antenna array dramatically increases link capacity while ensuring communication quality by setting the transmit antenna weight so that the SINR is maximized for the wireless terminal to be estimated and the signal is suppressed otherwise. Because it can be increased, research has become active recently. As an example of actual operation, there is an iBurst system that conforms to “High Capacity-Spatial Division Multiple Access (HC-SDMA) WTSC-2005-032 (ATIS / ANSI)”.

(TDMA-TDD and antenna array)
In particular, the TDMA-TDD scheme uses the same frequency for the uplink channel from the radio terminal to the base station and the downlink channel from the base station to the radio terminal. Can be theoretically used for transmission, and the antenna array system can easily be improved in performance. That is, the TDMA-TDD antenna array assumes that the transmission path itself and the propagation characteristics are continuous in the propagation path between an arbitrary point and any other point in the same frequency band in a short period of time. It is assumed that the antenna superposition coefficient matching the transmission path obtained by uplink reception can be used for downlink transmission. In general, this assumption is sufficiently practical when the interval between the reception signal and the transmission signal used for estimation is short and the moving speed of the wireless terminal is low.

(How to get antenna weight)
A maximum ratio combining (MRC) method is known as a simple method for deriving a weight for each antenna from reception signals of a plurality of antennas. The maximum ratio combining (MRC) method aligns the phases of signals received at each branch of the antenna, derives a weight for each antenna according to the level of each received signal, and combines each received signal according to the weight for each antenna. To do. It is frequently used because it does not require complicated calculations and can be expected to improve the S / N ratio to some extent.

  Further, various methods have been devised as methods for improving the S / N ratio in the received signal. For example, as a method for obtaining the antenna weight more precisely, as a method using a Winner solution, particularly as a method for obtaining a Winner solution, a mean square error (MSE: Mean Square) between a training signal (reference signal) and a received signal from a receiving antenna array is used. A sequential update method using an adaptive algorithm (MMSE method) that minimizes (Error) is known. As this adaptive algorithm, an LMS (Least Mean Square) algorithm is often used. The MMSE method can obtain only a small diversity gain when the number of receiving antennas is small, but is often used because the amount of calculation processing can be reduced.

  As methods having better throughput characteristics than the MMSE method, a serial canceller (SIC: Successive Interface Canceller) method, a maximum likelihood detection (MLD) method, and the like have been devised and actually put into practical use. However, these methods tend to have a larger calculation amount than the MMSE method, and a high-speed arithmetic device is required.

(Adaptive antenna array in terminal)
The method of acquiring the antenna weights described above and combining the received signals for each antenna to improve the S / N ratio can also be applied to the mobile terminal. In other words, when a mobile terminal uses a plurality of antennas for reception, the antenna weights are obtained from the received signals for each antenna, and the received signals are combined, the signal-to-noise ratio is better than when receiving with only one antenna. It can be expected to obtain a received signal.

  In order to derive a weight for each antenna using an LMS algorithm or the like, a method of adding a known training signal to a received signal is known. This is often done by using a known training signal pattern shared between the base station and the wireless terminal at the beginning or end of the transmission signal, or both on the receiving side, and using the known pattern as the training signal on the receiving side. It is done.

(Transmission output control of base station based on feedback information from terminal)
In general, the signal level received by the receiver depends on the signal transmission level transmitted by the transmitter, the nature of the propagation path, the distance from the transmitter, and the like. If the other conditions are the same, and the transmission output is stronger under the condition that the received signal is not saturated, the signal S / N ratio of the received signal is improved. On the other hand, since a strong signal affects adjacent frequency bands, it is necessary to consider that the transmission signal does not become unnecessarily strong. In particular, when the OFDM method is employed, since the subcarrier is at a close frequency, fine output control is required. (See Patent Document 1 for transmission output control)

  As one method of transmission output control, there is known a method of controlling to a sufficient transmission output necessary for the communication counterpart by feedback of reception quality information of the counterpart communication apparatus. In general, transmission output control in a base station is performed as part of scheduling for assigning channels to a plurality of mobile terminals according to a set of frequency and time, and a plurality of mobile terminals considering mutual influences with adjacent frequency channels. The scheduling method for can be of “maximum carrier-to-interference ratio” type or “proportional fairness” type.

JP 2011-135473 A

  By the way, the mobile terminal A uses a maximum ratio combining method, the mobile terminal B uses a receiver using the maximum likelihood detection method, and the mobile terminal A, the mobile terminal B, and the base station communicate by performing transmission output control. Suppose. In this case, it is assumed that the mobile terminal B can obtain a combined received signal having a higher S / N of 3 dB reception than the mobile terminal A.

  Both the mobile terminal A and the mobile terminal B feed back S / N ratio information as reception quality information to the base station, and the base station compares the reception quality information from each of the mobile terminal A and the mobile terminal B. However, since the base station does not determine the difference in reception performance between the two mobile terminals, it determines that the transmission signal to the mobile terminal B is unnecessarily higher than the transmission signal to the mobile terminal A. As a result, the base station performs control to reduce the transmission output to the mobile terminal B.

  As a result, the S / N of the received signal synthesized by the mobile terminal B deteriorates to the same level as the S / N of the received signal of the mobile terminal A. In this way, the mobile terminal B that has improved the reception performance at a cost can obtain only the same reception performance as the mobile terminal A that has compromised the reception performance without incurring a cost.

  In addition, since the tolerance to fading of the receiver is generally more stable when the signal level at the antenna end is higher, the mobile terminal B receives weaker reception when moving in a multipath environment. Since it is controlled by the base station to be a radio wave, a case that is disadvantageous than the mobile terminal A occurs.

  Even if the mobile terminal adapts the advanced adaptive antenna array reception scheme, the base station reduces the transmission output of the mobile terminal by transmission output control. As a result, the S / N ratio improved by the adaptive antenna array reception scheme can be obtained. Only the reception quality of the base station may be lowered.

  The present invention has been made in view of the above-described circumstances, and an object thereof is to provide a mobile terminal, a wireless communication system, and a wireless communication method that perform appropriate transmission output control.

  The mobile terminal of the present invention is a mobile terminal that transmits a feedback signal when performing transmission output control with a base station, and has obtained a plurality of antennas and received signals from the respective antennas by an antenna array reception method. A calculation unit that obtains reception quality based on a combined signal and a known signal, and a feedback information determination unit that determines feedback information based on the reception quality and an offset value, the feedback signal based on the feedback information being It has the structure which transmits to a base station.

  In addition, the mobile terminal of the present invention has a configuration including an offset value determination unit that determines the offset value according to the reception quality.

  Moreover, the mobile terminal of the present invention has a configuration including an offset value determination unit that determines the offset value for each multi-antenna reception method.

  The wireless communication system of the present invention is a wireless communication system that transmits a feedback signal when performing transmission output control between a mobile terminal and a base station. The mobile terminal includes a plurality of antennas and reception from each antenna. A calculation unit for obtaining reception quality based on a combined signal obtained by an antenna array reception method and a known signal, and a feedback information determination unit for determining feedback information based on the reception quality and the offset value, It has the structure which transmits the feedback signal based on the said feedback information to the said base station.

  According to the radio communication method of the present invention, a mobile terminal that transmits a feedback signal when performing transmission output control with a base station is known to have received signals from a plurality of antennas as a combined signal obtained by an antenna array reception method. Obtaining reception quality based on the signal, determining feedback information based on the reception quality and the offset value, and transmitting a feedback signal based on the feedback information to the base station.

  The mobile terminal, radio communication system, and radio communication method of the present invention can perform appropriate transmission output control.

It is a block diagram of the radio | wireless communications system which concerns on embodiment of this invention. It is a block diagram of the communication frame of a TDD-OFDMA system. It is a block diagram of the base station which concerns on embodiment of this invention. It is a block diagram of the mobile terminal (3 antennas) which concerns on embodiment of this invention. It is a block diagram of the mobile terminal (1 antenna) which concerns on embodiment of this invention. It is the figure which showed a part of operation | movement of the radio | wireless communications system which concerns on embodiment of this invention.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a configuration diagram of a wireless communication system according to an embodiment of the present invention. As shown in FIG. 1, the wireless communication system includes mobile terminals 1 to 4 and a base station 5. In order to simplify the description, the base station 5 has one antenna. The mobile terminal 1, the mobile terminal 2, and the mobile terminal 4 share one antenna for both transmission and reception. The mobile terminal 3 uses three antennas for reception and transmits and receives one antenna for transmission. It shall be used for both.

  This wireless communication system is divided into an uplink period and a downlink period, and wireless communication is performed using the TDD-OFDMA system, which is a time division multiplexing system. In addition, it is assumed that the communication method used for the downlink between the base station 5 and the mobile terminals 1 to 4 has four subcarriers.

  FIG. 2 is a configuration diagram of a communication frame of the TDD-OFDMA scheme.

  The downlink is divided into 20 subframes. In addition, the reference signal symbol is located in the subframe located at the beginning of the downlink period, and the signal sequence of this symbol is known on the receiving side. Symbols that are not reference signal symbols are information symbols, and information symbols are portable with arbitrary information.

  The communication method used for the uplink has four time slots. The reference signal symbol is positioned at the head of each slot, and the signal sequence of this symbol is known on the receiving side. Symbols that are not reference signal symbols are information symbols, and information symbols are portable with arbitrary information.

  In order to simplify the description, it is assumed that each subcarrier and slot is used for communication between a mobile terminal and a base station that are determined in advance. 2 / slot 2 is assigned to mobile terminal 2, subcarrier 3 / slot 3 is assigned to mobile terminal 3, and subcarrier 4 / slot 4 is assigned to mobile terminal 4.

  When transmitting from the base station 5 to an arbitrary mobile terminal, a downlink reference signal shared with the mobile terminal is placed in the first subframe, and arbitrary information is placed in the subsequent subframe, Create a transmission signal for each carrier. This is performed for the number of subcarriers. After the transmission signals of all subcarriers are obtained, this is subjected to IFFT conversion, RF modulation, and transmission from the antenna.

  FIG. 3 is a configuration diagram of a base station according to the embodiment of the present invention.

  Each downlink reference signal providing unit 502 is input with an input signal 501 corresponding to a subcarrier (for each mobile terminal) to each downlink reference signal providing unit 502 and is placed in the first subframe. A downlink reference signal is assigned.

  Each channel encoding section 503 encodes each subcarrier signal, and each transmission signal amplification section amplifies each subcarrier signal in accordance with the transmission output determined by transmission output determination section 514.

  Multiplexer 505 multiplexes each amplified subcarrier signal, IFFT 506 performs inverse Fourier transform on the multiplexed transmission signal, and RF modulator 507 performs RF modulation on the transmission signal subjected to inverse Fourier transform. And transmit via the antenna.

  The RF demodulator 508 performs RF demodulation on the received signal, the timing separator 509 extracts a signal obtained by separating the demodulated received signal at the timing for each slot, and each feedback signal demodulator 511 receives each feedback signal from each slot. Is demodulated. Each channel decoding unit 512 decodes the signal of each slot as an output signal 513.

  FIG. 4 is a configuration diagram of a mobile terminal (three antennas) according to the embodiment of the present invention. The mobile terminal 3 uses a beamforming multi-antenna reception method using the MMSE method.

  Each RF demodulating section 101 performs RF demodulation on the received signal received by each antenna, each FFT section 102 performs FFT conversion on each received signal that has been RF demodulated, and each subcarrier separating section 103 includes subcarriers. The received signal is separated.

  The antenna weight calculation unit 104 calculates the antenna weight for each antenna using, for example, the MMSE method based on the received signal of the subcarrier of each subcarrier separation unit 103. Based on the reception signal of the subcarrier of each subcarrier separation unit 103 and the antenna weight for each antenna, the combined signal generation unit 105 generates a combined signal from the reception signal for each subcarrier.

  The reference signal demodulator 106 demodulates the downlink reference signal from the synthesized signal, and the information symbol demodulator 107 demodulates the information symbol from the synthesized signal. The demodulated information is received information 108.

  The CNR calculation unit 109 compares a downlink reference signal with a known signal and obtains a CNR (Carrier to Noise ratio) as a value of the S / N ratio. The downlink reference signal is also used for frame synchronization between the base station 5 and the mobile terminal.

  Feedback information determination section 110 determines reception quality information to be carried on the feedback signal as an offset value corresponding to the CNR. Specifically, a value lower than the CNR value by the offset value is used as reception quality information. The offset value determination unit 120 will be described later.

  Uplink reference signal giving section 112 gives a known signal as a reference signal to transmission information 111, and feedback signal giving section 113 gives the CNR obtained by the previous reception as reception quality information to be put on the feedback signal. To do.

  The channel encoding unit 114 encodes the feedback signal and the information symbol, and the RF modulation unit 115 RF-modulates the encoded transmission signal and transmits it at the timing of a specified time slot via the antenna.

  FIG. 5 is a configuration diagram of a mobile terminal (one antenna) according to the embodiment of the present invention.

  The mobile terminal 1 in FIG. 5 is the same as the mobile terminal 2 and the mobile terminal 4. The difference from the mobile terminal 3 is the number of antennas. Since the mobile terminal 1, the mobile terminal 2, and the mobile terminal 4 have only one antenna, the feedback information determination unit 110 sets the offset value to zero. It matches the CNR value of the signal.

  FIG. 6 is a diagram illustrating a part of the operation of the wireless communication system according to the embodiment of the present invention.

  The mobile terminal 3 receives the signal (downlink) transmitted from the base station 5, and obtains the reception quality of the reception signal from the antenna based on the combined signal obtained by the antenna array reception method and the known signal (S1). ).

  Since the mobile terminal 3 has three antennas, the feedback information determination unit 110 of the mobile terminal 3 determines a gain predicted to be obtained by the MMSE method as an offset value (S2).

  The feedback information determination unit 110 outputs a value lower than the calculated CNR value by the offset value as reception quality information (feedback information) to be put on the feedback signal (S3).

  The transmission output determination unit 514 of the base station 5 compares the reception quality information included in the four feedback signals corresponding to each time slot, and determines the transmission signal level for each next subcarrier. At that time, the transmission output determining unit 514 performs control to reduce the transmission level when the reception quality is better than others, and to increase the reception level when the reception quality is poor, and adjusts the overall balance. To do.

  As described above, since the base station 5 does not keep the transmission output low as much as the feedback signal from the mobile terminal 3 is matched with the improvement of the reception performance, the mobile terminal 3 exhibits the performance according to the reception performance. It becomes possible to do.

  By the way, in the mobile terminal 3, when the CNR value is high to some extent, reporting the reception quality information fed back to the base station 5 as low as the reception performance improvement may cause the transmission output of the base station 5 to be unexpectedly lowered. Therefore, it is possible to secure an improvement in reception performance as a margin. However, as the CNR value decreases, it is possible to report the reception quality information fed back to the base station 5 as a reduction in the reception performance improvement. The station 5 determines that communication is impossible and cannot contribute to an increase in coverage.

  Therefore, the offset value determination unit 120 prepares offset values according to the CNR value in multiple stages, so that the offset value is set higher when the CNR value is high, and the offset value is lowered when the CNR value is low. When the CNR value is low, the offset value is set to zero. When the CNR is high, the reception performance improvement is distributed to the reception margin.

  When the CNR value is low, the reception quality information according to the reception performance is fed back to the base station 5, so that the coverage can be increased by the improvement of the reception performance even if the distance between the base station 5 and the mobile terminal is long. .

  In the above example, the mobile terminal 1, the mobile terminal 2, and the mobile terminal 4 have a single antenna configuration, and the mobile terminal 3 uses a beamforming multi-antenna reception method using the MMSE method. The present invention can be applied even when the mobile terminal 4 uses a beamforming multi-antenna reception method and adopts different beamforming methods such as the maximum ratio combining method, the MMSE method, the SIC method, and the MLD method. is there.

  That is, the offset value determination unit 120 for each CNR changes the offset value according to the expected degree of improvement in reception performance for each beamforming multi-antenna reception method, so that the reception value is improved in accordance with the improvement in reception performance. Margin and coverage can be secured.

1 to 4 mobile terminals 5 base stations 101 RF demodulation units 102 FFT units 103 Subcarrier separation units 104 Antenna weight calculation units 105 Composite signal generation units 106 Reference signal demodulation units 107 Information symbol demodulation units 108 Reception information 109 CNR calculation Unit 110 feedback information determination unit 111 transmission information 112 uplink reference signal addition unit 113 feedback signal addition unit 114 channel coding unit 115 RF modulation unit 120 offset value determination unit 501 input signal 502 downlink reference signal addition unit 502 each downlink reference Signal giving section 503 Each channel coding section 505 Multiplexing section 506 IFFT
507 RF modulation unit 508 RF demodulation unit 509 timing separation unit 511 each feedback signal demodulation unit 512 each channel decoding unit 513 output signal 514 transmission output determination unit

Claims (3)

In a mobile terminal that transmits a feedback signal when performing transmission output control with a base station,
Multiple antennas,
A calculation unit that obtains reception quality based on a combined signal obtained from an antenna array reception method and a known signal from each antenna,
An offset value determination unit that determines an offset value for each reception signal combining algorithm used in the antenna array reception method;
A feedback information determination unit that determines feedback information based on the reception quality and the determined offset value;
A mobile terminal that transmits a feedback signal based on the feedback information to the base station. In a wireless communication system that transmits a feedback signal when performing transmission output control between a mobile terminal and a base station,
The mobile terminal
Multiple antennas,
A calculation unit that obtains reception quality based on a combined signal obtained from an antenna array reception method and a known signal from each antenna,
An offset value determination unit that determines an offset value for each reception signal combining algorithm used in the antenna array reception method;
A feedback information determination unit that determines feedback information based on the reception quality and the determined offset value;
A wireless communication system, wherein a feedback signal based on the feedback information is transmitted to the base station. A mobile terminal that transmits a feedback signal when performing transmission output control with a base station,
Receiving reception signals from a plurality of antennas based on a combined signal obtained by an antenna array reception method and a known signal, and obtaining reception quality;
Determining an offset value for each received signal synthesis algorithm used in the antenna array reception method;
Determining feedback information based on the received quality and the determined offset value;
And a step of transmitting a feedback signal based on the feedback information to the base station.

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