The present invention relates to a base station and MIMO-OFDM communication method thereof, and more particularly to a base station and MIMO-OFDM communication method thereof that transmits pilot signals together with data to a mobile station using Multiple Input Multiple Output (MIMO) communication from a plurality of antennas using a plurality of OFDM transmitters.
In many digital mobile communication systems, in order to compensate for distortion of a data signal in a fading channel, pilot signals are multiplexed and transmitted with a data signal from the transmission side. On the receiving side, the pilot signals that were transmitted from the transmission side are received, and the received pilot signals are compared with a known pilot signal to estimate channel distortion (channel estimation), then channel compensation is performed for the received data signal based on the channel estimation value. There are various methods for multiplexing the data signal and pilot signals, however, the case of using the OFDM (Orthogonal Frequency Division Multiplex) method is explained below.
The processing described above is for the case in which the pilot signals are densely arranged in the frequency direction, however, from the aspect of transmission efficiency of the data signal or the capability to follow channel fluctuation, scattered arrangement is known in which pilot signals are sparsely arranged in the time or frequency direction.
The receiver multiplies a received pilot signal after FFT by the complex conjugate of a known pilot signal to calculate the channel estimation value of the subcarrier in which the received pilot signal is embedded. Next, noise and an interference component are somewhat suppressed by averaging this channel estimation value over time among a plurality of adjacent symbols. After that, the channel estimation value of the subcarrier in which that pilot signal is embedded is used to find the channel estimation value of a subcarrier in which the pilot signal is not embedded by performing interpolation or extrapolation in the frequency direction. Similarly, for an OFDM symbol in which a pilot signal is not embedded, the channel estimation value of an OFDM symbol in which a pilot signal is embedded is used to interpolate and find that channel estimation value in the time direction. In the interpolation process mentioned above, linear interpolation is performed between subcarriers or OFDM symbols in which the pilot signal used for interpolation is embedded with the assumption that channel distortion fluctuates linearly. However, to be more precise, actual channel distortion does not fluctuate linearly, but rather has complex fluctuation due to high-speed movement of a mobile station in the time direction or due to delay spread of a multipath channel in the frequency direction, so error occurs in the channel estimation value after interpolation.
Therefore, in the example of scattered arrangement, the transmission efficiency of the data signal improves the wider the insertion interval of the pilot signals is, however, it becomes difficult for channel estimation to follow sudden channel fluctuation that occurs due to high-speed movement of a mobile station, and thus reception characteristics deteriorate. On the other hand, the transmission efficiency of the data signal decreases the narrower the insertion interval of the pilot signals is, and, since it becomes easy for channel estimation to follow sudden channel fluctuation, it becomes difficult for the reception characteristics to deteriorate. Therefore, when designing the arrangement of pilot signals, the geographical environment where the digital mobile communication system will be used, and the estimated speed of movement of the mobile station must be taken into consideration.
Here the case is considered in which in a multi-cell environment as shown in
In the prior system, these interference signals are suppressed as follows. That is, the transmitter uses a spreading code (channelization code) for spreading transmit data, or uses repetition to copy data for one symbol to a plurality of symbols, after which it multiplies the result by cell unique scramble code and transmits the signal. The receiver multiplies the received signal by the same scramble code that the transmitter used, after which by performing de-spreading using spreading code, or in-phase addition to demodulate the signal, suppresses the interference signals from the adjacent cells to a certain extent.
However, pilot signals are transmitted at higher power than data signals, so even though the processing described is performed, the effect from the adjacent cells remains large. Particularly, as shown in
As a countermeasure for avoiding the problem described above, the pilot signals can be arranged in each cell so that they do not overlap. In the example shown in
In the case of MIMO (multiple-Input Multiple-Output) multiplex transmission, it is necessary to obtain a channel estimation value for each transmission antenna, so orthogonal pilot signals are transmitted from each transmission antenna. As in the case of single antenna transmission, the arrangement of these pilot signals should be such that they do not overlap in each cell.
However, in the example of pilot arrangement shown in
When transmitting OFDM signals from a plurality of antennas, there is related art (refer to Japanese patent application 2003-304216) that deters interference in the propagation paths of the pilot signals. In this related art a pilot signal is transmitted from one antenna by a specified subcarrier, and no pilot carriers are output from other antennas, and subcarriers having the same frequency as the pilot sub carrier of the one antenna is taken to be a null signal. However, this related art is not intended for preventing the interference of pilot signals from other cells or other sectors. Moreover, this related art does not enable channel estimation with high precision while preventing interference of pilot signals from other cells or other sectors.
Taking into consideration the aforementioned problems, the object of the present invention is to enable channel estimation with high precision while preventing interference of pilot signals from other cells or other sectors that are not in communication with the mobile station.
Another object of the present invention is to sufficiently minimize interference between pilot signals and to enable channel estimation that is capable of following sudden channel fluctuation when the mobile station is at a cell boundary or sector boundary.
First MIMO-OFDM Communication Method
The present invention is a MIMO-OFDM communication method that uses a plurality of OFDM transmitters to transmit data and pilot signals to a mobile station from a plurality of antennas by multiple input multiple output (MIMO) communication, comprising steps of: transmitting data and pilot signals by MIMO communication, which is multiple data stream transmission, or by single input multiple output (SIMO) communication, which is single data stream transmission, based on the communication environment of the mobile station; and arranging the pilot signals such that pilot signals that are used in channel estimation during single data stream transmission in a specified cell, and pilot signals that are used in channel estimation during single data stream transmission in adjacent cells do not overlap each other in the frequency direction and/or time axis direction.
The first MIMO-OFDM communication method of the present invention further comprises a step of making the power of pilot signals that are transmitted from a specified antenna and used for channel estimation during single data stream transmission greater than the power of pilot signals that are transmitted from other antennas.
The first MIMO-OFDM communication method of the present invention further comprises steps of receiving pilot signals that are transmitted from the base station, measuring the communication environment based upon the received pilot signals in the mobile station and feeding back data to the base station that indicates the communication environment; and deciding in the base station whether to perform multiple data stream transmission or single data stream transmission based on the data indicating the communication environment.
The first MIMO-OFDM communication method of the present invention further comprises steps of receiving pilot signals that are transmitted from the base station, measuring the communication environment based upon the received pilot signals in the mobile station and feeding back data to the base station that indicates the communication environment; and controlling the number of data streams by the base station during multiple data stream transmission based on the data indicating the communication environment.
Second MIMO-OFDM Communication Method
The present invention is a MIMO-OFDM communication method for a base station that divides a cell into sectors and uses a plurality of OFDM transmitters in each sector to transmit data and pilot signals to a mobile station from a plurality of antennas by multiple input multiple output (MIMO) communication, comprising steps of: transmitting data and pilot signals by MIMO communication, which is multiple data stream transmission, or by single input multiple output (SIMO) communication, which is single data stream transmission, based on the communication environment of the mobile station in a sector; and arranging the pilot signals such that pilot signals that are used in channel estimation during single data stream transmission in a specified sector, and pilot signals that are used in channel estimation during single data stream transmission in adjacent sectors do not overlap each other in the frequency direction and/or time axis direction.
The second MIMO-OFDM communication method of the present invention further comprises a step of making the power of pilot signals that are transmitted from a specified antenna and used for channel estimation during single data stream transmission greater than the power of pilot signals that are transmitted from other antennas.
The second MIMO-OFDM communication method of the present invention further comprises a step of measuring in the mobile station the reception power of the sector antennas of each of the sectors, and deciding in the base station whether to perform multiple data stream transmission or single data stream transmission based on the difference in the reception power among adjacent sectors.
The second MIMO-OFDM communication method of the present invention further comprises a step of controlling the number of data streams used in said multiple data stream transmission based on the communication environment of the mobile station in each sector in the case of performing multiple data stream transmission.
Base Station
The base station of the present invention comprises construction for executing the first and second MIMO-OFDM communication methods described above. That is, a first base station of the present invention comprises: a communication method decision unit configured to decide whether to transmit data and pilot signals by MIMO communication, which is multiple data stream transmission, or by single input multiple output (SIMO) communication, which is single data stream transmission, based on the communication environment of the mobile station; and a pilot position control unit configured to control the position of the pilot signals such that pilot signals that are used in channel estimation during single data stream transmission in a specified cell, and pilot signals that are used in channel estimation during single data stream transmission in adjacent cells do not overlap each other in the frequency direction and/or time axis direction.
A second base station of the present invention comprises construction for executing the second MIMO-OFDM communication method described above. In other words, the second base station comprises: a communication method decision unit configured to decide whether to transmit data and pilot signals by MIMO communication, which is multiple data stream transmission, or by single input multiple output (SIMO) communication, which is single data stream transmission, based on the communication environment of the mobile station in a sector; and a pilot position control unit that controls the position of the pilot signals such that pilot signals that are used in channel estimation during single data stream transmission in a specified sector, and pilot signals that are used in channel estimation during single data stream transmission in adjacent sectors do not overlap each other in the frequency direction and/or time axis direction.
Cell Configuration and Pilot Arrangement
Switching Control Between Multiple Data Stream Transmission and Single Data Stream Transmission
In the present invention, in the event that the communication environment with the mobile station becomes poor, the base station switches from MIMO communication, which is multiple data stream transmission, to single input multiple output (SIMO) communication, which is single data stream transmission. For example, when the mobile station MS moves while communicating with the base station BSA, the base station BSA transmits data and pilot signals to the mobile station MS from the four antennas 1 to 4 using MIMO-OFDM communication (multiple stream data transmission) as long as that mobile station MS is in an area AR1 that is near the base station BSA and the communication environment is such that the SIR (signal to interference ratio) is large.
When the mobile station MS moves to a cell boundary area and it receives interference from an adjacent cell and the SIR is small, the BER (bit error rate) increases even though MIMO-OFDM communication is performed. Therefore, in that case, the base station BSA switches to single input multiple output (SIMO) communication (single data stream transmission) and transmits data and pilot signals from only one antenna.
When performing single data stream transmission, the base stations BSA, BSB and BSC of each of the cells transmit data and pilot signals to the mobile station from the first antenna 1, however, as can be clearly seen from (B) of
As a result, during single data stream transmission there is no interference between the pilot signals P1 for channel estimation of one cell and the pilot signals P1 of an adjacent cell, so even at the cell boundary, the SIR of the pilot signals is improved, and accurate channel estimation is possible. Moreover, in the example shown in (B) of
During single data stream transmission, the pilot signals P1 for channel estimation of a cell over the pilots P2 to P4 of adjacent cells in the frequency direction, however there is little effect due to interference. Particularly, in the example of arrangement shown in (B) of
Controlling the Number of Multiple Data Streams
Above, switching control was performed to switch transmission between multiple data stream transmission and single data stream transmission based on the communication environment of the mobile station, for example the SIR value. In the case of this switching control, only the power of the pilot signals P1 need be made greater than the power of the other pilot signals P2, P3, P4, and there is no need to make the power between the other pilot signals different. However, in the case of controlling the number of multiple data streams according to the communication environment, for example the SIR value, the power of each of the pilot signals should be set differently such as P1>P2>P3>P4.
In MIMO multiplex transmission, the signals from each transmission antenna arrive at the receiver while overlapping in the radio channel, so the distance between signal constellation points in the received signal becomes shorter the larger the number of data streams is, and the required reception SIR for achieving a fixed bit error rate BER increases. In other words, the smaller the number of data streams is, it is possible to achieve a fixed bit error rate even when the reception SIR is small. Therefore, in the present invention, the number of multiple data streams is controlled according to the communication environment, for example the SIR value. For example, as shown in
By controlling the number of multiple data streams according to the communication environment as described above, it is possible to take advantage of the merits of MIMO-OFDM communication as much as possible.
Example of Another Pilot Arrangement
In the example of the pilot arrangement shown in (B) of
As described above, with the present invention, pilot signals that are used for channel estimation by a mobile station that performs single data stream transmission can be arranged so that they do not overlap each other in adjacent cells in the time and/or frequency direction, and the mobile station that performs single data stream transmission, can receive a desired signal without receiving the effects of interfering signals from adjacent cells even at the cell boundaries.
In the base station BS, the same number of data streams D0 to DM-1 as the number of transmission antennas M pass through specified processing by respective OFDM transmitters 110 to 11M-1, and are transmitted from transmission antennas 120 to 12M-1. The processing performed by the OFDM transmitters 110 to 11M-1 include error correction encoding, data modulation, data/control signal/pilot multiplexing, scrambling, IFFT transformation, and GI insertion.
Signals, which are transmitted from the antennas 120 to 12M-1 and are arranged so that they are not correlated with each other, pass through independent fading channels hnm (m=0 to M−1, n=0 to N−1), and after being multiplexed in space, are received by N number of receiving antennas 310 to 31N-1. The signals that are received by the receiving antennas pass through processing by OFDM receivers 320 to 32N-1 such as frequency down conversion, AD conversion, FFT timing detection, GI removal, FFT transformation, descrambling, and data/control signal/pilot separation, to generate y0 to yN-1 reception data streams. Each of the reception data streams, are formed by M number of multiplexed transmission data streams, so a data processing unit 33 performs signal processing of all of the reception data streams to separate out and reproduce the transmission data streams D0 to DM-1.
Various methods are proposed for the algorithm for signal processing of separating out the transmission data streams D0 to DM-1 from the reception signals, however, here the MLD (Maximum Likelihood Decoding) algorithm will be explained. When the transmission data streams are expressed by a M-dimensional complex matrix, and the reception data streams are expressed by a N-dimensional complex matrix, they are related by the following equations.
The MLD algorithm estimates the transmission data stream (transmission vector) D by the following equation.
{circumflex over (D)}=argmin∥Y−H·D∥2
Here, by taking the number of arranged signal constellation points of modulated data that is input to the M number of antennas to be Q, then QM number of transmission vector combinations exist. In QPSK modulation, Q=4. In the MLD algorithm, QM number of transmission vector candidates (replicas) are generated, and calculation is performed according to the equations above, and the replica with the smallest result is estimated to be the input data.
A data stream number decision unit 13 decides the number of data streams based on the communication environment with the mobile station MS as will be described later, and inputs the result to a control information mapping unit 14 and data stream dividing unit 15. The control information mapping unit 14 maps the number of input data streams at a specified location as control information, an error correction encoding unit 16 performs error correction encoding processing the control information, and a data modulation unit 17 performs data modulation of the control data.
The data stream dividing unit 15 performs serial-to-parallel conversion of the data signal according to the number of data streams, and inputs the result to OFDM transmitters 110 to 113. The OFDM transmitters 110 to 113 generate transmission signals for each of the transmission antennas 120 to 123 from the respective data streams D0 to D3, and transmits those signals from the transmission antennas 120 to 123. By taking the antenna numbers of the transmission antennas 120 to 123 to be 1 to 4, the number of data streams and the transmission antenna numbers used are as shown in
In the OFDM transmitter 110, the error correction encoding unit 21 performs error correction encoding processing of the data of the first data stream D0 that is input, and the data modulation unit 22 performs data modulation of the input data. A pilot signal generation unit 23a generates pilot signals P1, and a gain adjustment unit 23b adjusts the amplitude of the pilot signals P1. The method for adjusting the amplitude of the pilot signals will be described later.
Next, a data/pilot/control signal multiplexing unit 24 multiplexes the data signal, control signal and pilot signal and performs parallel output of subcarrier components of N samples. As shown in (B) of
At the same time as the above, a reception RF unit 18 receives a signal from the reception antenna 19 that is transmitted from the mobile station MS, after which it performs frequency conversion to convert the frequency from a radio frequency to a baseband frequency, then performs AD conversion and inputs the signal to a control signal demodulation unit 20. The control signal demodulation unit 20 performs demodulation to demodulate the control signal that was fed back from the mobile station, then extracts the downlink reception SIR information that is contained in that control signal. The downlink reception SIR indicates the communication environment of the mobile station MS, and is a reception SIR that is measured using the transmitted pilot signals that the mobile station MS receives from the base station BS. The data stream number decision unit 13 decides the number of data streams to transmit according to the table shown in
The fundamental characteristic of MIMO multiplex transmission is considered. In MIMO multiplex transmission, signals that are transmitted from each transmission antenna arrive at the mobile station while overlapping in a radio channel. Therefore, the larger the number of data streams is, the shorter the distance between signal constellation points of the received signals becomes, and the required reception SIR for achieving a constant bit error rate BER increases. In other words, the smaller the number of data streams is, the more possible it is to achieve a constant bit error rate BER even when the reception SIR is small.
Moreover, the reception SIR generally decreases as the distance between the base station and mobile station increases. Therefore, in order to maximize the throughput of data signals, by measuring the reception SIR and comparing that reception SIR with a specified threshold value, control can be performed so that when the reception SIR is high (when the mobile station is near the base station) the number of data streams is increased, and when the reception SIR is low (when the mobile station is near the cell boundary) the number of data streams is decreased.
In order to do this, the data stream number decision unit 13 decides the number of data streams according to the value of the reception SIR. For example, the reception SIR (communication environment) is divided into four levels as shown in
Next, the mapping position of the pilot signals by the data/pilot/control signal multiplexing unit 24, and amplitude adjustment of the pilot signals will be explained.
When the mobile station is near a cell boundary, the reception SIR of the data signal is relatively low, so the data stream number decision unit 13 decides the number of data streams so that single data stream transmission is performed. Accordingly, from the standpoint of the mobile station that is at the cell boundary, it is sufficient that only the channel estimation value for single data stream transmission have a certain level of quality. Therefore, the power of the pilot signals P1 that are transmitted from the transmission antenna 120 that are used for single data stream transmission is set sufficiently high. That is, the gain adjustment unit 23b of the OFDM transmitter 110 makes the amplitude of the pilot signals P1 large before the signals are transmitted from the transmission antenna 120, and the power of the pilot signals P2, P3, P4 that are transmitted from the other transmission antennas 121, 122, 123 is lower than the power of the pilot signals P1. As shown in
Even though a difference in power is set in this way, from the standpoint of the mobile station that is near the base station, the amount of damping of the pilot signal in the radio channel is relatively small, so there is little effect on the accuracy of channel estimation.
(B) of
During single data stream transmission, the base stations BSA, BSB, BSC of each of the cells transmit data and pilot signals P1 to the mobile station from the first antenna 120, however, in this example of pilot arrangement, the pilots P1 from each cell A, B, C do not overlap in the frequency direction. That is, pilot signals P1 in a cell that is used for channel estimation during single data stream transmission, and the pilot signals P1 of adjacent cells that are used for channel estimation during single data stream transmission are arranged so that they do not overlap each other in the frequency direction.
For a mobile station MS at the cell boundary, only the power of pilot signals P1 from other cells is large and dominant as interference. Therefore, it is necessary that there be no interference from other cells due to the pilot signals P1. Since transmission to the mobile station MS at the cell boundary is single data stream transmission, the pilot signals P1 that are transmitted by single data stream transmission should not receive interference from pilot signals P1 from other cells. In order to accomplish this, the positions of the pilot signals P1 of each of the cells A, B, C are such that they are different from each other, and even though the pilot signals P1 are arranged in the same positions as any of the pilot signals P2 to P4 from the transmission antennas 121 to 123 of other cells, there is no interference.
With the pilot arrangement shown in (B) of
The signals received by each of the receiving antennas 310 to 313 are input to OFDM receivers 320 to 323. All of the OFDM receivers 320 to 323 have the same construction, and a reception RF unit converts the frequency of the received signals from a radio frequency to a baseband frequency, then performs A/D conversion and inputs the result to a FFT timing detection unit 42 and GI removal unit 43. The FFT timing detection unit 42 detects the FFT timing, and the GI removal unit 43 removes the GI according to the FFT timing, and after making the data N sampling parallel data, inputs the data to a FFT unit 44. The FFT unit 44 converts the N sample signals in the time domain to N number of subcarrier signal components in the frequency domain, and a descrambling unit 45 multiplies the N number of subcarrier signal components by the same code as descramble code used by the base station, and extracts the signal received from the base station. A data/pilot/control signal separation unit 46 separates the data signal, pilot signal and control signal that are mapped at specified locations, and inputs the data signal to a MIMO demodulation unit 51 of a data processing unit 33, inputs the pilot signal to a channel estimation unit 52, and inputs the control signal to a control signal demodulation unit 53.
The channel estimation unit 52 first obtains a channel estimation value for the subcarrier of the OFDM symbol in which the pilot signals are embedded. In other words, the channel estimation unit 52 performs a correlation operation to correlate the pilot signals received from the receiving antennas 310 to 313 with a known transmission pilot signal, and performs in-phase addition of the obtained correlation value among 4 symbols, to obtain a channel estimation value for which interference signals from adjacent cells have been somewhat suppressed. Next, by performing interpolation or extrapolation of the channel estimation values for each of the antennas in the time or frequency direction, the channel estimation unit 52 obtains channel estimation values for subcarriers of OFDM symbols in which pilot signals are not embedded. After that, for each antenna, the channel estimation unit 52 inputs the channel estimation values for each of the subcarriers of each of the OFDM symbols to the MIMO demodulation unit 51 and control signal demodulation unit 53.
The control signal demodulation unit 53 uses the channel estimation values to perform channel compensation of the control signals that were transmitted from the one transmission antenna 120 and received by the four receiving antennas 310 to 313, then by performing diversity combining among the receiving antennas, demodulates the control information, extracts out the information about the number of data streams that were mapped at specified locations in the control signal, and inputs the result to the MIMO demodulation unit 51.
The MIMO demodulation unit 51 uses the data signals received from each of the receiving antennas and the channel estimation values to perform a well-known MIMO channel separation process based on the number of data streams, and by performing P/S conversion of each of the data streams obtained from that MIMO channel separation operation and outputting the result, restores the data signal.
Moreover, a SIR estimation unit 34 uses the channel estimation values that are estimated by the channel estimation unit 52 to estimate the reception SIR. More specifically, the desired signal power S is regarded as the sum of the squares of both the real portion and imaginary portion of the channel estimation value (complex value) between a specified transmission antenna and receiving antenna, the interference signal power I is regarded as the variance of a plurality of symbols, and the estimated value of the reception SIR is computed by the ratio of S and I. A control information mapping unit 35 maps the SIR estimation value at a specified location as control information, and a control signal modulation unit 36 generates a control signal by performing a process such as error correction encoding and data modulation, then multiplexes the control signal, user data and pilot signals and transmits that multiplexed signal to the base station BS from a transmission antenna 37.
The pilot signal arrangement shown in (B) of
In the case of an inter-base station synchronization system, it is also possible to employ pilot signal arrangement in which the pilots P1 of each cell do not overlap only in the time direction.
In the first embodiment of the present invention, the number of data streams was controlled from 1 to 4 based on the communication environment (reception SIR) of the mobile station, however, configuration is also possible in which data and pilot signals are transmitted by switching between either MIMO communication, which is multiple data stream transmission, or single input multiple output (SIMO) communication, which is single data stream transmission, based on the communication environment (reception SIR) of the mobile station.
In the first embodiment, a method for reducing the effect of interference signals received by a mobile station at a cell boundary from adjacent cells was explained. Using a similar method, it is also possible to reduce the effect of interference signals received by a mobile station at a sector boundary from adjacent sectors in the case where a cell comprises a plurality of sectors.
Normally, these interference signals are suppressed as described below. That is, at the base station BS, after spreading using spreading code or copying the information of one symbol to a plurality of symbols by repetition, the resulting signal is multiplied by a sector-unique scramble code and transmitted. At the mobile station MS, the signal is multiplied by the same scramble code as used by the base station BS, and then de-spreading using the spreading code, or in-phase addition is performed, thereby the interference signals from the adjacent sectors are suppressed somewhat. However, in regards to the pilot signals, the pilot signals are transmitted at relatively high power, so even though the processing described above is performed, they continue to interfere with adjacent sectors, and have a large effect on the adjacent sectors. Particularly, when the arrangement of pilot signals of each sector is common, the pilot signals of each sector interfere with each other, and the accuracy of channel estimation deteriorates.
In a second embodiment of the invention, as in the first embodiment, the pilot signals P1 from the transmission antennas A1, B1 and C1 that are used in single stream transmission are transmitted at a sufficiently high power, and the pilot signals P2, P3, P4 from the transmission antennas A2 to A4, B2 to B4 and C2 to C4 are transmitted at a lower power than the pilot signals from the transmission antennas A1, B1 and C1. In other words, the transmission power of the pilot signals P1, P2, P3 and P4 is set so that P1>P2>P3>P4. Moreover, as shown in
By doing this, as long as a mobile station at a sector boundary performs single stream transmission, the effect on channel estimation of pilot signals from other sectors becomes minor, so stable reception can be performed. Therefore, by switching the number of data streams transmitted when a mobile station is at a sector boundary to a single data stream, stable reception can always be performed regardless of the position of the mobile station within a sector.
A control signal demodulation unit 20 demodulates control information that is sent from a mobile station MS, and inputs downlink reception SIR information and a sector boundary judgment bit to a data stream number decision unit 13. A sector boundary judgment bit of ‘0’ indicates that the mobile station MS is in the center of the sector, and a sector boundary judgment bit of ‘1’ indicates that the mobile station MS is at the sector boundary. The data stream number decision unit 13 decides the number of data streams based on the reception SIR information and the sector boundary judgment bit. More specifically, first, when the sector boundary judgment bit is ‘1’, the data stream number decision unit 13 decides the number of data streams to ‘1’. However, when the sector boundary judgment bit is ‘0’, the data stream number decision unit 13 decides the number of data streams by comparing the reception SIR information with a specified threshold value as in the first embodiment.
On the other hand, the second sector processing unit 33b multiplies the N number of subcarrier signal components that are output from the FFT units 44 of the OFDM reception units 320 to 323 by the same codes as the scramble code of sector B, then extracts and outputs the signals received from sector B. Next, the second sector processing unit 33b separates the pilot signals that are mapped at specified locations and estimates the channel of sector B as in the first embodiment. Similarly, the third sector processing unit 33c performs channel estimation of sector C.
The SIR estimation unit 34 uses the channel estimation value that was estimated by the channel estimation unit 52 to estimate the downlink reception SIR from sector A by the same method as in the first embodiment. A sector boundary judgment unit 38 uses the channel estimation values from the first to third signal processing units 33a to 33c to generate a sector boundary judgment bit. More specifically, taking the sum of the squares of the real portion and imaginary portion of the channel estimation values (complex values) to be the desired signal power, when the difference between the desired signal power of the first and second sector signal processing units 33a, 33b, or the difference between the desired signal power of the first and third sector signal processing units 33a, 33c is less than a threshold value, the sector boundary judgment unit 38 determines that the mobile station is near the sector boundary and sets the sector boundary judgment bit to ‘1’. For all other cases, the sector boundary judgment unit 38 sets the sector boundary judgment bit to ‘0’. A control information mapping unit 35 maps the SIR estimation value and the sector boundary judgment bit as control information, and a control signal modulation unit 36 generates a control signal by performing processing such as error correction encoding and data modulation, after which it multiplexes the control signal, user data and pilot signals and transmits the resulting signal from the transmission antenna 37.
With the present invention, pilot signals that are used for channel estimation during single data stream transmission in a specified cell, and pilot signals that are used for channel estimation during single data stream transmission in adjacent cells do not overlap each other in the frequency direction and/or time axis direction, so interference from pilot signals of other cells with which a mobile station is not communicating can be prevented, and thus channel estimation can be performed with high precision.
Moreover, with the present invention, pilot signals that is used for channel estimation during single data stream transmission in a specified cell, and pilot signals that are used for channel estimation during single data stream transmission in adjacent cells do not over each other in the frequency direction and/or time axis direction, and the power of the pilot signals that are used in channel estimation and that are transmitted from a specified antenna during single data stream transmission is greater than the power of pilot signals that are transmitted from other antennas, so when a mobile station is at the cell boundary, the inference from pilot signals is sufficiently small, and thus it is possible to perform channel estimation that is capable of following sudden channel fluctuation. The same effects are obtained even when the cell is divided into sectors.
Furthermore, with the present invention, the interval between pilot insertion subcarriers from a specified antenna can be shortened, so during MIMO-OFDM communication, it is possible to perform channel estimation that is capable of following even when sudden channel fluctuation occurs.
This application is a continuation of U.S. application Ser. No. 12/212,481, filed Sep. 17, 2008, now pending, which is a continuation of International Patent Application No. PCT/JP2006/305488 filed on Mar. 20, 2006, the contents of each are entirely incorporated by reference.
Number | Date | Country | |
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Parent | 12212481 | Sep 2008 | US |
Child | 13901021 | US | |
Parent | PCT/JP2006/305488 | Mar 2006 | US |
Child | 12212481 | US |