JP2012156798A - Mobile communication system and base station control apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve throughput of a cell end in a MIMO/OFDM cellular system.SOLUTION: When there is a mobile station MS#2 which belongs to a base station BS#2 to which a frequency band is allocated same as the frequency band which base station BS#1 allocates to a mobile station MS#1 positioned at around a boundary between a cell of the base station BS#1 and a cell of the base station BS#2, the base station BS#1 and the base station BS#2 perform cooperative scheduling so that a sum total of transmitting rank from each base station becomes not more than the number of receiving antennas of the mobile station MS#1. The mobile stations MS#1 and MS#2 extract a desired signal using linearity interference cancellation. Allocation of frequency resources is controlled so that each mobile stations BS#1 and BS#2 in which the transmitting rank is controlled cooperatively become a mobile station positioned in each other's cell end.

Description

  The present invention relates to a mobile communication system and a base station controller using a MIMO / OFDM scheme.

A MIMO (Multiple-Input Multiple-Output) system having a plurality of antennas for the transmitter and the receiver has attracted attention. There are several transmission methods in a MIMO system, that is, a multi-antenna transmission method. The signal-to-noise ratio is improved by the diversity effect obtained by transmitting a single stream by space-time coding or beamforming, MIMO diversity multiplexing (MIMO-SDM: MIMO-Space Division Multiplexing), which focuses on improving the transmission rate by transmitting multiple streams by spatial multiplexing and improving transmission reliability ).
MIMO-SDM can apply a plurality of antennas for both transmission and reception and improve the peak throughput by simultaneous transmission of a plurality of substreams. However, in general, the characteristics deteriorate when the total number of transmission antennas> the number of reception antennas. Further, since the required reception SINR increases, the characteristics at the cell edge where the reception SINR is low are not good.

  In recent years, various mobile radio communication systems such as 3GPP-LTE and IEEE 802.16e Mobile WiMAX have been developed. These systems employ radio transmission technologies such as OFDM (Orthogonal Frequency Division Multiplexing) and MIMO. In particular, the combination of OFDM and MIMO is called MIMO / OFDM and greatly contributes to improving the frequency utilization efficiency of the mobile radio communication system. However, the amount of traffic in mobile communications is still increasing rapidly, and further wireless access sophistication is required. 3GPP LTE-Advanced, IEEE 802.16m (WiMAX 2) is being studied with the aim of further improving frequency utilization efficiency.

In the MIMO / OFDM cellular system using single frequency repetition, inter-cell interference (ICI) occurs. In a high traffic environment, the cell edge mobile station receives a large ICI, and the cell edge throughput is greatly reduced.
As a promising technique for improving the cell edge throughput and the system throughput at the same time, an inter-cell interference cancellation method has been studied.
As shown in FIG. 7, consider a cellular system comprising a cell of base station # 1 and a cell of base station # 2. In the third generation, OFDM-based system, users are assigned so that frequencies do not overlap. However, when the number of users increases, the frequency overlaps between the cells, causing a situation where interference occurs. For example, as illustrated, resource blocks illustrated by user B and user C are allocated to base station # 1, and communication is performed, and user a, user b, and user c are illustrated at base station # 2, respectively. Assume that a resource block is allocated and communication is in progress. In such a situation, only the frequency assigned to the user b can be assigned to the new user A located near the boundary with the cell (adjacent cell) of the base station # 2. In this state, the user A has a situation in which the received power of the desired signal from the base station # 1 and the received power of the interference signal from the base station # 2 are substantially equal, and no throughput can be obtained. That is, it is difficult to assign a frequency to the new user A.
In this state, if the receiver of the new user A has a function of removing inter-cell interference, and the interference signal from the user b can be removed, the reception quality of the user A at the cell edge is improved. This frequency can be assigned to the new user A. This technique is called an inter-cell interference canceller.

  Therefore, the present inventors have proposed a downlink transmission method using nonlinear ICI cancellation processing based on joint estimation of a desired signal and an interference signal for the MIMO / OFDM cellular system (Patent Documents 1 and 2).

JP 2009-100116 A JP 2010-206457 A

According to the above-described method proposed by the present inventors, cell edge throughput and system throughput can be improved at the same time.
However, the ICI cancellation scheme based on this joint estimation requires that the mobile station receiver obtains information related to transmission schemes such as modulation schemes and precoding schemes not only for desired signals but also for interference signals. Therefore, there is a problem that the processing of the mobile station is complicated. In addition, ML (maximum likelihood: Maximum Likelihood) is used to achieve highly accurate inter-cell interference cancellation while the number of spatial multiplexing (simultaneous transmission substreams) in interference station transmission exceeds the degree of freedom of the array antenna on the mobile station side. Non-linear signal processing accompanied by mounting difficulties such as detection and MAP (Maximum A posteriori Probability) detection is required. It is not preferable to implement such nonlinear signal processing in a portable mobile station apparatus from the viewpoint of power consumption and the like.

  Therefore, the present invention can improve transmission characteristics of cell edge users by combining transmission rank (number of spatially transmitted substreams simultaneously transmitted) control based on inter-base station cooperative control and linear interference canceling on the mobile station side. An object of the present invention is to provide a mobile communication system and a base station controller.

In order to solve the above problems, a mobile communication system according to the present invention is a mobile communication system in which a plurality of base stations having a plurality of antennas and a mobile station having a plurality of antennas communicate with each other by OFDM, and downlink transmission When a common frequency band is used in a plurality of base stations, the transmission timing of all the base stations that interfere with each other is different from that of the reception timing including the multipath of the signal from each base station in the mobile station. The base station is controlled so as to be within a guard interval and has a base station controller connected to the plurality of base stations. The base station has a known synchronization signal on the mobile station side and a known pilot on the mobile station side. The signal and at least control information necessary for communication with the mobile station are multiplexed and transmitted on a data channel for transmitting user data. Based on the synchronization signal from the ground station, the optimum reception timing is estimated, the pilot signal is received and the channel response between the own cell base station and the own station is estimated, and the channel response estimation result Based on the control information, a data channel is obtained so as to improve a desired signal to interference signal power ratio by using means for acquiring transmission scheme information including spatial multiplexing number and modulation scheme information from received signals of the plurality of antennas. The base station controller allocates the same frequency band as the mobile station located at the cell edge in a neighboring cell when the mobile station is located at the cell edge of the own cell base station. The sum of the transmission ranks of the neighboring cell base stations for the mobile station being received and the transmission ranks of the own cell base station for the mobile station located at the cell edge is the reception antenna of the mobile station. Such that the number below the are intended to control the neighboring cell base stations and the own cell base station.
In addition, as a data channel demodulation algorithm in the mobile station, a Zero Forcing method or a least square error method is used.
Furthermore, the allocation of frequency resources in the own cell base station and the neighboring cell base stations is scheduled so that the mobile stations whose transmission ranks are controlled become the cell edge users of each other.
Furthermore, the mobile station has means for reporting information including an identification number of the own cell and an identification number of a neighboring cell to the own cell base station when the own station is located at a cell edge, The station has means for notifying the base station controller of information including an identification number of the own cell and an identification number of a neighboring cell reported from a mobile station located at a cell edge of the own cell; The control device, for the base station of the own cell and the base station of the neighboring cell, based on information including the identification number of the own cell and the identification number of the neighboring cell reported from the mobile station located at the cell edge The transmission rank assigned to the mobile station located at the cell edge by the own cell base station, and the same frequency band as the frequency band assigned to the mobile station located at the cell edge by the base station of the neighboring cell. Assign to assigned mobile station The total transmission rank there are, and controls to be equal to or less than the number of reception antennas of a mobile station located in the cell edge.

  Furthermore, the base station controller of the present invention is a base station connected to the plurality of base stations in a mobile communication system in which a plurality of base stations having a plurality of antennas and a mobile station having a plurality of antennas communicate by OFDM. A control device for receiving, from the base station, information including an identification number of the own cell and an identification number of a neighboring cell reported from the mobile station when the own station is located at a cell edge; Based on the information including the cell identification number and the neighboring cell identification number, the own cell base station is assigned to the mobile station located at the cell edge for the base station of the own cell and the base station of the neighboring cell. And the transmission rank assigned to the mobile station to which the base station of the neighboring cell is assigned the same frequency band as the frequency band assigned to the mobile station located at the cell edge. Meter is one having a means for controlling such that the following number of reception antennas of a mobile station located in the cell edge.

According to the present invention, scheduling for performing coordinated control of transmission ranks between base stations is performed, so that the throughput characteristics of cell edge users can be improved.
In addition, since the effective interference wave number is reduced, a large inter-cell interference cancellation effect can be obtained even with simple linear received signal processing such as a minimum mean square error (MMSE) method on the mobile station side. .
Furthermore, the frequency utilization efficiency of the entire cell can be improved by introducing frequency scheduling that assigns the same frequency block to users located at cell boundaries.

It is a figure which shows the system model for demonstrating the basic concept of this invention. It is a block diagram which shows an example of a structure of the base station side apparatus in the mobile communication system of this invention. It is a block diagram which shows an example of the structure by the side of the mobile station in the mobile communication system of this invention. It is a figure which shows the simulation item for performing the characteristic evaluation of the mobile communication system of this invention. It is a figure which shows the block error rate characteristic which is a simulation result of the mobile communication system of this invention. It is a figure which shows the throughput characteristic which is a simulation result of the mobile communication system of this invention. It is a figure for demonstrating the intercell interference canceller.

The basic concept of the mobile communication system of the present invention will be described using the system model shown in FIG.
The present invention is directed to a MIMO / OFDM cellular system such as LTE / LTE-Advanced. Also, in the MIMO transmission according to the present invention, rank adaptation is applied in which the number of simultaneous transmission substreams is controlled according to the state of the radio propagation channel in order to improve transmission characteristics in the MIMO system. Here, in MIMO spatial multiplexing, the smaller value of the number of MIMO transmission / reception antennas is the maximum value of the number of spatial multiplexing, and transmission at this maximum value is called full rank transmission. In rank adaptation, the spatial multiplexing number is adaptively switched from 1 (rank 1 transmission, corresponding to transmission diversity) to full rank transmission in accordance with the state of the MIMO channel matrix to maximize throughput. The optimum number of transmission ranks in MIMO is determined by feeding back information related to the channel estimation result to the transmitting side on the receiving side. For example, in LTE downlink standardized as Release 8 in 3GPP (3rd Generation Partnership Project), the measurement value corresponding to the received SINR information is the Channel Quality Indicator (CQI), and the number of transmission ranks that the mobile station considers optimal is the Rank Indicator. As (RI), those values are fed back, and the base station side determines the transmission rank based on the fed back CQI and RI information. In LTE, it is not always necessary to match the RI value fed back and the transmission rank value, and the final transmission rank can be determined by the scheduler of the base station.
In the present invention, a certain cell edge mobile station (a mobile station located at the end of a cell) receives the largest interference from an adjacent cell, and the interference signal other than the interference signal having the maximum received power is received on the mobile station side. The sum is considered equivalent to Gaussian noise. Therefore, here, a two-cell model in which there are two cells, that is, the own cell (Serving-Cell) and an interference cell (Interfering-Cell) adjacent to the own cell as shown in FIG.
Focusing on the mobile station MS # 1, the base station BS # 1 and the base station BS # 2 can be regarded as a desired base station (desired BS) and an interference base station (interfering BS), respectively. On the other hand, when focusing on the mobile station MS # 2, the base station BS # 1 and the base station BS # 2 can be regarded as an interference base station (interfering BS) and a desired base station (desired BS), respectively.

  The mobile station receiver has an inter-cell interference cancellation function that spatially separates a desired signal and an interference signal in order to improve signal detection accuracy at the cell edge. Further, the desired base station and the interference base station are synchronized with each other using a system such as GPS so that the received frame timing offset including the multipath delay does not exceed the OFDM GI (guard interval). In the cell edge mobile station, the pilot symbols for channel estimation are orthogonal between the desired base station and the interfering base station so that downlink channel information can be obtained not only for the desired base station but also for the interfering base station. It shall be inserted as follows.

  The inter-cell interference cancellation method of the MIMO / OFDM cellular system of the present invention is a combination of (a) multi-cell cooperative scheduling on the base station side and (b) inter-cell interference cancellation signal detection on the mobile station side, Multi-cell coordinated scheduling allows each base station to apply a simple linear interference canceller at the mobile station, instructing each cell edge mobile station to perform low rank transmission, reducing the effective interference wave number, and moving On the station side, inter-cell interference is suppressed by interference cancellation processing based on linear processing.

(A) Multi-cell cooperative scheduling in base station In the present invention, a desired base station (desired BS) cooperates with an interfering base station (interfering BS) that transmits the strongest inter-cell interference signal. The desired base station and the interference base station cooperatively control (a) frequency domain user scheduling and (b) transmission rank in order to maximize the inter-cell interference cancellation effect in the cell edge mobile station.

(A) Coordinated frequency domain user scheduling The present invention requires coordinated low-rank transmission scheduling in a plurality of base stations described below. However, since the low rank transmission scheduling forcibly selects a lower transmission rank than the full rank transmission, the frequency utilization efficiency is occasionally deteriorated. In order to solve this problem, the present invention uses frequency domain user scheduling in which a plurality of base stations cooperate to perform subcarrier allocation.
As shown in FIG. 1, a coordinated base station, that is, a desired base station (desired BS) and an interfering base station (interfering BS) have the same frequency for mobile stations located at the cell edge between these base stations. Schedule to allocate more resources. In general, for users with low reception SINR, rank transmission lower than full rank is suitable. This multi-cell cooperative frequency domain user scheduling can reduce the degradation of frequency utilization efficiency by forcing low rank transmission.

(B) Coordinated low rank transmission scheduling Cell edge mobile stations are often not suitable for full rank MIMO transmission due to low reception SINR environment. In the present invention, the coordinated base station selects a transmission method with a rank lower than the full rank. On the base station side, the number of effective interference signals can be reduced by selecting transmission with a lower rank.

(A) Inter-cell interference cancellation signal detection in the mobile station As described above, the effective number of interference signals in the cell-edge mobile station is reduced by avoiding MIMO transmission at the full rank. As a result, since the degree of freedom of the array antenna on the mobile station side remains, it is possible to remove large inter-cell interference (ICI) using linear signal processing. Here, a minimum square error (MMSE) method or a ZF (Zero Forcing) method is used as the linear inter-cell interference cancellation algorithm.

Specific means for realizing the present invention as described above are as follows.
(1) Each base station has a plurality of antennas in order to support MIMO transmission and communicates with the mobile station by adaptive modulation / coding in a mobile radio environment. A pilot signal and a control signal known on the mobile station side are multiplexed and transmitted on a data channel for transmitting user data.
(2) In order to remove an OFDM-modulated interference wave signal with high accuracy, it becomes an own cell and an interference source so that a reception timing shift including a multipath delay of a signal from each base station falls within the OFDM GI. Synchronize transmission timing between base stations.
(3) The mobile station has a plurality of antennas in order to support MIMO transmission, estimates the optimum reception timing based on the synchronization signal of the own cell base station (Serving-Cell), and knows a known pilot signal (reference signal) , Reference Signal) is received, and the channel response from the own cell base station is estimated.
(4) Based on the channel response estimation result, the mobile station obtains transmission scheme information including spatial multiplexing number and modulation scheme information from the control signal inserted in the base station transmission signal.
(5) The own cell base station and the neighboring cell base station control downlink interference with the cell edge mobile station communicating with the own cell base station. The cooperative control is performed so that the total transmission rank is equal to or less than the number of receiving antennas on the mobile station side.
(6) The cell edge mobile station demodulates the data channel using the received signals of a plurality of antennas so that the desired signal to interference signal power ratio is improved.
(7) A ZF method or an MMSE (least square error) method is applied as a data channel demodulation algorithm of the cell edge mobile station.
(8) Furthermore, in order to increase the frequency utilization efficiency of the entire cell, frequency resource allocation is scheduled so that users who coordinately control transmission ranks become mutual cell edge users.

Next, the configuration of the base station side device and the configuration of the mobile station in the mobile communication system of the present invention will be described. In addition, although the case where there are two base stations is shown here, the same configuration can be made when there are three or more base stations.
FIG. 2 is a block diagram showing an example of the configuration on the base station side in the mobile communication system of the present invention. The mobile communication system of the present invention basically assumes a system in which a plurality of mobile stations can communicate with one base station, but FIG. 2 shows elements necessary for communication with one mobile station. Only shows. Since the present invention relates to downlink transmission (base station → mobile station), blocks related to uplink transmission (mobile station → base station) are omitted.

In FIG. 2, 10 is a first base station, 30 is a second base station, and 50 is a base station controller. The first base station 10 and the second base station 30 are connected to each other via a network and are connected to the base station control device 50. The first base station 10 and the second base station 30 have the same configuration. Here, each of the base stations 10 and 30 is provided with a plurality of N _t0 transmission / reception antennas, but the number of transmission / reception antennas provided in each base station does not have to be the same. Can be a number. In the figure, in the first base station 10 and the second base station 30, blocks indicated by broken lines are blocks used in the case of multiple rank transmission.

In the first base station 10, user data to be transmitted is input to the rank adaptation unit 12 via the buffer 11. The rank adaptation unit 12 divides the user data from the buffer 11 into a number of substreams corresponding to the rank indicated by the scheduler 17 and inputs the substreams to the channel encoder 13 provided for the substreams. I do. The data error-correction-encoded by the channel encoder 13 is interleaved by the interleaver 14, then converted to complex symbols by the I / Q mapping unit 15, multiplied by the precoding matrix by the precoder 16, and adapted to the transmission antenna. Input to the provided multiplexers 18-1 to 18 -N _t0 . In the multiplexers 18-1 to 18 -N _t0 , the pilot signal and the control signal are multiplexed, converted into parallel signals by the serial-parallel converters (S / P) 19-1 to 19 -N _t0 , and then inverse fast Fourier transformed. Inverse Fourier transform is performed by the conversion (IFFT) units 20-1 to 20-N _t0 and is converted into serial signals by parallel-serial converters (P / S) 21-1 to 21-N _t0 . The output signals of the parallel-serial converters 21-1 to 21-N _t0 are not shown after a cyclic prefix corresponding to a guard interval is added by a CP (Cyclic Prefix) adding unit 22-1 to 22-N _t0. The frequency is converted to a carrier frequency by a mixer, and is amplified by a power amplifier (not shown) and transmitted from antennas 23-1 to 23-N _t0 .

In many MIMO / OFDM based systems, rank adaptation is applied. For example, in 2 × 2 open-loop MIMO standardized by LTE, transmission diversity using SBC (space-frequency block coding) corresponding to two modes of single rank transmission and two rank transmission, and SDM (space Two modes called division multiplexing are adaptively switched. Transmit diversity usually has better throughput characteristics than spatial multiplexing in a low SINR environment like the cell edge.
In this case, the rank adaptation unit 12 performs SFBC encoding on the user data when the scheduler 17 is instructed to perform single rank transmission. The data is divided into streams and input to the channel encoder 13 and the channel encoder indicated by a broken line for error correction coding.

The second base station 30 also includes a buffer 31, a rank adaptation unit 32, a channel encoder 33, an interleaver 34, an I / Q mapping unit 35, a precoder 36, a scheduler 37, multiplexers 38-1 to 38-N _t0 , serial-parallel conversion. (S / P) 39-1 to 39-N _t0 , inverse fast Fourier transform (IFFT) units 40-1 to 40-N _t0 , parallel / serial converters (P / S) 41-1 to 41-N _t0 , CP addition units 42-1 to 42-N _t0 and antennas 42-1 to 42-N _t0 are provided, and are configured in the same manner as the first base station 10.

The base station control device 50 controls synchronization and coordinated scheduling between a plurality of base stations, and when there is a cell edge mobile station, the base station to which the cell edge mobile station belongs and a peripheral that becomes an interference source Information indicating the number of ranks is transmitted to the scheduler of the cell base station so that the total transmission rank is equal to or less than the number of receiving antennas of the mobile station. That is, when there is a mobile station to which the same frequency band as that assigned to the cell edge mobile station is assigned in the peripheral cell, the base station controller 50 determines the rank number for the cell edge mobile station and the neighboring cell. The scheduler provided in the base station is controlled so that the total number of ranks for mobile stations to which the same frequency band is assigned is equal to or less than the number of receiving antennas of the mobile station. For example, assuming that the number of antennas of the mobile station is 2, a single rank transmission is instructed to the base station of the cell edge mobile station and the neighboring cell base stations.
Note that the base station control device 50 may be arranged on a network to which the plurality of base stations are connected, or may be arranged inside any one of the base stations.

  In LTE, each mobile station can transmit a measurement report to a base station in the area. For example, when the difference between the RSRP (Reference Signal Received Power: radio quality) of the own cell measured by the mobile station and the RSRP of the adjacent cell is equal to or less than a predetermined value, the mobile station determines the cell ID and RSRP of the own cell The base station control device is configured to transmit a measurement report including the cell ID and RSRP of the adjacent cell to the own cell base station and transmit the measurement report received by each base station to the base station control device 50. 50 can acquire information on the existence of a cell edge mobile station, its own cell base station to which the cell edge mobile station belongs, and information on neighboring cell base stations serving as interference sources. Based on the information obtained in this way, the base station control device 50 cooperatively controls the transmission ranks of the own cell base station and the neighboring cell base stations serving as interference sources.

  Furthermore, in order to increase the frequency utilization efficiency of the entire cell, the schedulers 17 and 37 of each base station are controlled so as to control the allocation of frequency resources so that the users who coordinately control the transmission ranks become the cell edge users of each other. You may make it indicate. That is, in the above case, a mobile station to which the same frequency band as that of the cell edge mobile station is assigned in the neighboring cell is not necessarily located at the cell edge of the neighboring cell. Therefore, although a higher rank transmission is possible for the mobile station, a lower rank transmission is performed. Therefore, the same frequency band as that of the cell edge mobile station of the own cell is assigned to the cell edge mobile station in the neighboring cells. That is, the same frequency band is assigned to the mobile station MS # 1 at the cell edge of the base station BS # 1 and the mobile station MS # 2 at the cell edge of the base station BS # 2 in FIG. By doing so, it is possible to prevent a low rank from being assigned to a mobile station close to the base station, and to improve the frequency utilization efficiency of the entire cell.

FIG. 3 is a block diagram showing an example of the configuration of the mobile station 70 in the mobile communication system of the present invention. Since the present invention relates to downlink transmission (base station → mobile station), blocks related to uplink transmission (mobile station → base station) are omitted.
As shown in the figure, the mobile station 70 has a plurality of N _r reception antennas 71-1 to 71-N _r . Signals received by the receiving antennas 71-1 to 71-N _r is converted into a digital signal after being down-converted to baseband, a cyclic prefix provided corresponding to each antenna (CP) removing unit 7-1 to 73 -N _r and also input to the FFT timing detection unit 72.
The FFT timing detection unit 72 estimates the optimal reception timing based on the synchronization signal included in the reception signal from the own cell base station. Based on this timing, the received signal is demodulated.

CP removing section 73-1 to 73-N _r removes the cyclic prefix that is added to the received signal, the serial-parallel converter 75-1 to 75-N corresponding signals which the cyclic prefix has been removed and outputs to _r, and outputs the channel estimation and the control signal demodulator 74 is provided in common to all the received signal.
The channel estimation and control signal demodulation unit 74 estimates the downlink channel response from each transmission antenna of the own cell base station to each reception antenna based on the state of the received pilot signal (reference signal), and Based on the response estimation result, the control signal inserted in the base station transmission signal is demodulated to obtain transmission scheme information including information on the spatial multiplexing number and modulation scheme. The channel response estimation result and the acquired transmission scheme information are supplied to inter-cell interference (ICI) cancellation signal detectors 77-1 to 77-N _SC (N _SC is the number of subcarriers) described later. The information on the transmission method is supplied to channel decoders 80-1 to 80-i described later.

The parallel converter 75-1 to 75-N _r are the respective output signals from a corresponding CP removal unit 73-1 to 73-N _r is converted into a parallel signal, fast Fourier transform provided corresponding The data is output to (FFT) sections 76-1 to 76 -N _r .
FFT sections 76-1 to 76 -N _r convert each input signal into a signal for each subcarrier, and inter-cell interference cancellation signal detection sections 77-1 to 77 -N provided corresponding to each subcarrier. Supply to _SC .

In inter-cell interference cancellation signal detector 77-1~77-N _SC, the channel estimation and a control signal estimated channel information (channel matrix) by the demodulator 74 and on the basis of the demodulated control signals, for each sub-carrier The desired signal from the own cell base station and the interference signal from the neighboring cell base station are separated from the received signal, and only the desired signal is extracted from these separated signals, so that the desired signal can be accurately obtained simultaneously with the interference signal removal. It is taken out. Although described later for the contents of the processing in the inter-cell interference cancellation signal detector 77-1~77-N _SC, as a demodulation algorithm, ZF method or MMSE: Simple such (Minimum Mean Square Error minimum mean square error) method A linear received signal processing method is used. Then, a substream signal corresponding to the transmitted rank is extracted. When precoding such as SFBC is performed on the transmission side, decoding corresponding to the precoding scheme is performed.

When the mobile station is located at the cell edge and is scheduled to transmit a single rank, transmission diversity is applied, and therefore, the broken line portion in FIG. 3 is not used.
When the transmission rank is i, the reliability information (for example, bit unit) of the signals separated into i substreams of the respective subcarriers output from the inter-cell interference cancellation signal detection units 77-1 to 77-N _SC The log likelihood ratio) is supplied to parallel / serial converters (P / S) 78-1 to 78-i provided corresponding to the substreams, converted into serial signals, and the corresponding deinterfacing unit is provided. The signals are deinterleaved by the Lleavers 79-1 to 79-i, input to the corresponding channel decoders 80-1 to 80-i, demodulated by the channel decoders 80-1 to 80-i, and parallel-serial converted. The output of each of the channel decoders 80-1 to 80-i is combined with a serial signal by the device 81 and output as a received signal.

ただし、Ｎ_SCはサブキャリア数を表す。Ｈ^(s)(k)及びＨ^(u)(k)は第ｋサブキャリアにおけるＮ_r×Ｎ_t0次元の希望基地局〜移動局間及び干渉基地局〜移動局間のチャネル応答行列をそれぞれ表す。ｓ(k)及びｕ(k)は、第ｋサブキャリアにおける希望基地局及び干渉基地局のＮ_t0×１次元の送信信号ベクトルをそれぞれ表す。ｎ(k)はＮ_r×１次元の雑音ベクトルであり、各要素独立な複素ガウス分布に従う。 Next, as an example of the case where SFBC is applied as a precoding scheme at the time of low rank transmission (rank 1 transmission) in the precoders 16 and 36, the inter-cell interference cancellation signal detection units 77-1 to 77- A specific signal processing algorithm for MMSE-based decoding and signal detection processing executed in the N _SC will be described.
As described above, in the present invention, the timing offset between the desired base station (desired BS) and the interference base station (interfering BS) in the received signal does not exceed the OFDM guard interval (GI). Assuming that the frequency offset between the desired base station (desired BS) and the interfering base station (interfering BS) is negligibly small, the N _r × 1-dimensional received signal vector x (k) in the k-th subcarrier at the mobile station Is expressed by the following equation.

N _SC represents the number of subcarriers. H ^(s) (k) and H ^(u) (k) represent channel response matrices of the desired base station-mobile station and interfering base station-mobile station in the N _r × N _t0 dimension in the k-th subcarrier, respectively. . s (k) and u (k) represent N _t0 × 1-dimensional transmission signal vectors of the desired base station and the interference base station in the k-th subcarrier, respectively. n (k) is an N _r × 1-dimensional noise vector and follows a complex Gaussian distribution independent of each element.

ただし、Ｅ{・}及びＩ_Mは、短区間のアンサンブル平均及びＭ×Ｍ次元の単位行列をそれぞれ表す。上付き文字Ｈは共役転置を表す。Ｐ_noiseをサブキャリアあたり受信アンテナあたりの雑音電力とすると、雑音ベクトルｎ(k)は次式を満たす。

When the transmission power per subcarrier of the desired base station and the interference base station is P _s and P _u , the transmission signal vectors s (k) and u (k) satisfy the following expression.

Here, E {·} and I _M represent an ensemble average of a short interval and an M × M-dimensional unit matrix, respectively. Superscript H represents conjugate transpose. If P _noise is the noise power per receiving antenna per subcarrier, the noise vector n (k) satisfies the following equation.

When SFBC encoded signals based on the Alamouti code are transmitted simultaneously from the desired base station and the interfering base station, the received signal is equivalently rewritten by the following equation.

However, the following expressions (5) to (8) are satisfied.

である。また、希望信号は、最大比合成（ＭＲＣ）規範に基づき、等価受信信号ベクトルｘ^〜(l)に等価ＭＩＭＯチャネル行列の複素共役Ｈ^〜(s)(l)^Hを乗算することによって得られる。この仮定は、大きな周波数選択性を有する大きな遅延スプレッドがある環境では適用することができない。このような環境ではＳＦＢＣの直交性が維持されなくなるためシンボル間干渉（ＩＳＩ：inter-symbol interference）が生ずる。セル間干渉（ＩＣＩ）と同時にシンボル間干渉（ＩＳＩ）を除去するため、本発明ではＭＭＳＥ基準のＳＦＢＣ復号及び信号検出を行う。 In a basic SFBC coded system, it is assumed that the difference in channel response between adjacent subcarriers is negligibly small. That is,

It is. Further, the desired signal, based Maximal Ratio Combining (MRC) criterion, is obtained by multiplying the complex conjugate ^{H ~ (s) (l)} H equivalent received signal vector x ^{~ (l)} into an equivalent MIMO channel matrix. This assumption is not applicable in an environment where there is a large delay spread with large frequency selectivity. In such an environment, the SFBC orthogonality is not maintained, so that inter-symbol interference (ISI) occurs. In order to remove inter-symbol interference (ISI) simultaneously with inter-cell interference (ICI), the present invention performs MMSE-based SFBC decoding and signal detection.

ただし、Ｗ_MMSE(l)は、ＭＭＳＥに基づくセル間干渉キャンセラつきＳＦＢＣ復号行列を表す。Ｗ_MMSE(l)は次式のように計算できる。

ただし、α＝１／Ｐ_sは正規化ファクタである。なお、Ｐ_sは希望局の送信電力情報である。一般に基地局は送信電力の情報を報知しており、移動局は報知信号を復号することで基地局の送信電力情報を得ることができる。 SFBC decoding with inter-cell interference cancellation based on MMSE is expressed by the following equation.

Here, W _MMSE (l) represents an SFBC decoding matrix with an inter-cell interference canceller based on MMSE. W _MMSE (l) can be calculated as:

Where α = 1 / P _s is a normalization factor. Note that P _s is transmission power information of a desired station. In general, a base station broadcasts transmission power information, and a mobile station can obtain transmission power information of the base station by decoding a broadcast signal.

ただし、ｚ^〜 _n(l)（ｎ＝１，２）は、ｚ^〜(l)のｎ番目の要素を表す。 If s _rep ⁽ⁱ⁾ is the i-th transmission symbol candidate, the most likely SFBC-coded block transmission symbol can be estimated by the following equation.

_{^{However, z ~ n (l) (}} n = 1,2) represents the n th element of ^{z ~} (l).

Next, the result of evaluating the inter-cell interference cancellation method according to the present invention using computer simulation will be described.
FIG. 4 shows an outline of simulation parameters.
Here, a two-cell model (that is, the number of base stations N _B = 2) was assumed. The number of transmitting antennas N _t0 = 2 for each base station and the number of receiving antennas N _r = 2 of the mobile station were set. The 3GPP GSM 6-path Typical Urban model is used as the path model. Each path follows quasi-static Rayleigh fading, and the fading correlation between paths and antennas is uncorrelated. OFDM subcarrier number N _sub = 72, FFT point number N _FFT = 128, subcarrier interval f ₀ = 15 kHz, effective OFDM symbol length T _s = 1 / f ₀ , guard interval length T _g = T _s /4=16.67 μs (¼ OFDM symbol length) and frame length T = 12 (T _s + T _g ) (12 OFDM symbol length). A turbo code having a constraint length K = 4 is used as an error correction code, and a sub-block interleaver is used as a channel interleaver. For QPSK, coding rates R = 1/3, 1/2, 2/3, 3/4, and for 16QAM, coding rates R = 1/2, 7/12, / 2/3, 3/4 A simulation was performed on combinations of four modulation schemes and coding rates. The precoding scheme is Alamouti SFBC coding when the rank number is 1, and spatial division multiplexing (SDM) when the rank number is 2. Channel estimation and control signal demodulation were ideally performed. The desired signal and the interference signal on the receiver side are separated by MMSE detection accompanied by SFBC decoding, and the desired signal and the interference signal are decoded by the Max Log-MAP algorithm with the number of repetitions = 8.

FIG. 5 is a diagram showing average block error rate (BLER) characteristics, where the horizontal axis represents the average received energy to noise power density ratio (received Es / N0) per symbol, and the vertical axis represents the average block error rate. It is. However, the coding rate of the modulation scheme QPSK is used as a parameter. In FIG. 5, the average CIR (Carrier-to-Interference-power-Ratio) is 0 dB, that is, the average received power of the desired wave and the interference wave is the same. In this figure, the solid line indicates the case where the present invention is applied (“Proposed”), and the dotted line indicates the case where the cooperative control is not performed and the inter-cell interference canceller is not applied (“w / o ICIC”).
From this figure, it can be confirmed that an error floor appears due to inter-cell interference when cooperative control is not performed and the inter-cell interference canceller is not applied. On the other hand, when the present invention is applied, since the influence of inter-cell interference is removed, the number of reception antennas (N _r = 2) is the sum of the number of transmission antennas of the desired base station and the interference base station (2N _t0 = 4) Despite being less, no such floor is observed.
FIG. 5 shows that the block error rate characteristics can be greatly improved in the present invention in a low CIR environment. For example, in the present invention, when the coding rate is 1/2, the required average Es / N0 in the block error rate can be improved by 2 dB when the average CIR is 0 dB compared to the case where the inter-cell interference canceller is not applied. .

FIG. 6 shows a case where the inter-cell interference cancellation method of the present invention is applied to different coding rates in QPSK and 16QAM, where the horizontal axis is the average received energy to noise power density ratio per symbol (received Es / N0). It is a figure which compares and shows a throughput characteristic and the characteristic when not applying the inter-cell interference cancellation method. A solid line and a broken line in the figure respectively show a case where the present invention is applied and a case where the present invention is not applied. However, the average CIR value is set to 1 (CIR = 0 dB).
In the cellular system, the total interference signal power to desired received power ratio excluding the strongest inter-cell interference is considered to be between 0 dB and 10 dB at the cell edge. From FIG. 6, it can be seen that the intercell interference cancellation method of the present invention can improve the average throughput at average reception Es / N0 = 5 dB and 10 dB by 10% and 64%, respectively, compared with the case where the intercell interference canceller is not applied. From these results, it can be confirmed that the inter-cell interference cancellation method of the present invention can greatly improve the cell edge downlink throughput in the MIMO / OFDM cellular system.

10, 30: Base station, 11, 31: Buffer, 12, 32: Rank adaptation unit, 13, 33: Channel encoder, 14, 34: Interleaver, 15, 35: I / Q mapping unit, 16, 36: Precoder 17, 37: Scheduler, 18-1 to 18-N _t0 , 38-1 to 38-N _t0 : Multiplexer, 19-1 to 19-N _t0 , 39-1 to 39-N _t0 : Series-parallel converter, 20-1 to 20-N _t0 , 40-1 to 40-N _t0 : Inverse fast Fourier transform unit, 21-1 to 21-N _t0 , 41-1 to 41-N _t0 : Parallel-serial converter, 22-1 ˜22-N _t0 , 42-1 to 42-N _t0 : CP adding unit, 23-1 to 23-N _t0 , 43-1 to 43-N _t0 : antenna, 50: base station controller, 70: mobile station , 71-1~71-N _r: antenna, 72: FFT data Timing detection unit, 73-1 to 73-N _r: CP removal unit, 74: channel estimation and a control signal demodulator, 75-1 to 75-N _r: serial-to-parallel converter, 76-1~76-N _r: Fast Fourier transform unit, 77-1 to 77-N _SC : Inter-cell interference cancellation signal detection unit, 78-1 to 78-i: Parallel-serial converter, 79-1 to 79-i: Deinterleaver, 80-1 -80-i: channel decoder, 81: parallel-serial converter

Claims (5)

A mobile communication system in which a plurality of base stations having a plurality of antennas and a mobile station having a plurality of antennas communicate by OFDM,
When a common frequency band is used by a plurality of base stations in downlink transmission, the transmission timing of all the base stations that interfere with each other is the reception timing including the multipath of the signal from each base station in the mobile station. The deviation is controlled to be within the OFDM guard interval,
A base station controller connected to the plurality of base stations;
The base station
A synchronization signal known on the mobile station side, a pilot signal known on the mobile station side, and at least control information necessary for communication with the mobile station are multiplexed and transmitted on a data channel for transmitting user data,
The mobile station
Means for estimating an optimum reception timing based on the synchronization signal from the own cell base station, receiving the pilot signal, and estimating a channel response between the own cell base station and the own station;
Means for acquiring transmission scheme information including spatial multiplexing number and modulation scheme information from the control information based on the estimation result of the channel response;
Means for demodulating a data channel so as to improve a desired signal-to-interference signal power ratio using received signals of the plurality of antennas;
The base station controller is
When the mobile station is located at the cell edge of its own cell base station, the transmission rank of the neighboring cell base station for the mobile station to which the same frequency band as the mobile station located at the cell edge is assigned in the neighboring cell, The own cell base station and the neighboring cell base stations are controlled so that the total transmission rank of the own cell base station with respect to the mobile station located at the cell edge is equal to or less than the number of receiving antennas of the mobile station. A mobile communication system.   The mobile communication system according to claim 1, wherein a zero forcing method or a least square error method is used as a data channel demodulation algorithm in the mobile station.   The mobile communication system according to claim 1, wherein scheduling of frequency resources in the own cell base station and the neighboring cell base stations is scheduled so that mobile stations whose transmission ranks are controlled become cell edge users of each other. . The mobile station has means for reporting information including an identification number of the own cell and an identification number of a neighboring cell to the own cell base station when the own station is located at a cell edge,
The base station has means for notifying the base station controller of information including an identification number of the own cell and an identification number of a neighboring cell reported from a mobile station located at a cell edge of the own cell;
The base station control device, based on the information including the identification number of the own cell and the identification number of the neighboring cell reported from the mobile station located at the cell edge, the base station of the own cell and the base of the neighboring cell For the station, the transmission rank assigned to the mobile station located at the cell edge by the own cell base station and the frequency band assigned to the mobile station located at the cell edge by the base station of the neighboring cell 2. The mobile communication system according to claim 1, wherein the total number of transmission ranks assigned to the mobile stations to which the frequency band is assigned is controlled to be equal to or less than the number of receiving antennas of the mobile station located at the cell edge. . A base station controller connected to the plurality of base stations in a mobile communication system in which a plurality of base stations having a plurality of antennas and a mobile station having a plurality of antennas communicate by OFDM,
Means for receiving from the base station information including an identification number of the own cell and an identification number of a neighboring cell reported when the own station is located at a cell edge from the mobile station;
Based on the information including the identification number of the own cell and the identification number of the neighboring cell, the base station of the own cell and the base station of the neighboring cell, the mobile station located at the cell edge The sum of the transmission rank assigned and the transmission rank assigned to the mobile station to which the base station of the neighboring cell is assigned the same frequency band as the frequency band assigned to the mobile station located at the cell edge is A base station control apparatus comprising means for controlling so as to be equal to or less than the number of receiving antennas of a mobile station located at a cell edge.

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