JP6173974B2 - OFDM communication system and subcarrier allocation method - Google Patents

Description

  The present invention relates to an OFDM communication system and a subcarrier allocation method for performing MIMO transmission by allocating each receiving station to each subcarrier of an OFDM (Orthogonal Frequency Division Multiplexing) signal between a plurality of transmitting stations and a plurality of receiving stations. .

FIG. 4 shows a configuration example of a conventional OFDM communication system.
In FIG. 4, the OFDM communication system has a configuration in which a plurality of transmitting stations A21 and B22 perform multistream transmission (cooperative MIMO transmission) to a plurality of receiving stations 31 to 37 under the control of the control station 50. (Non-Patent Documents 1 to 4). Here, in OFDM transmission, when receiving stations 31 to 37 are assigned to subcarriers 1 to 7, it is assumed that subcarriers 1 to 7 are symbol-synchronized. However, since the propagation path lengths between the transmitting and receiving stations are different from each other, the reception timings of all the receiving stations cannot be synchronized.

  Therefore, the transmitting station A21 and the transmitting station B22 set a CP (Cyclic Prefix) as a guard interval in the transmission signal, and the symbol synchronization is performed so that the delay time difference at each receiving station falls within the CP length. However, for example, when each transmitting station transmits simultaneously, symbol synchronization is possible if the delay time difference does not exceed the CP length as in the receiving stations 34 and 35 that are substantially equidistant from each transmitting station. When the delay time difference exceeds the CP length as in 37, symbol synchronization cannot be achieved. In this case, the CP length is increased to cope with it. However, as the delay time difference increases, the required CP length increases, the time band of the control frame becomes longer, and the frequency utilization efficiency decreases.

Yamashita, Fumihiro, et al. "Broadband multiple satellite MIMO system." Vehicular Technology Conference, 2005. VTC-2005-Fall. 2005 IEEE 62nd. Vol. 4. IEEE, 2005. Liolis, Konstantinos P., Athanasios D. Panagopoulos, and Panayotis G. Cottis. "Multi-satellite MIMO communications at ku-band and above: investigations on spatial multiplexing for capacity improvement and selection diversity for interference mitigation." EURASIP Journal on Wireless Communications and Networking 2007.2 (2007): 16-16. Zhang, Hongyuan, and Huaiyu Dai. "Cochannel interference mitigation and cooperative processing in downlink multicell multiuser MIMO networks." EURASIP Journal on Wireless Communications and Networking 2004.2 (2004): 222-235. Irmer, Ralf, et al. "Coordinated multipoint: Concepts, performance, and field trial results." Communications Magazine, IEEE 49.2 (2011): 102-111. Ishihara, Koichi, et al. "Overlap Frequency-Domain Multiuser Detection for Asynchronous Uplink Multiuser MIMO-OFDM Systems." IEICE transactions on communications 92.5 (2009): 1582-1588.

  The multi-satellite MIMO technology of Non-Patent Documents 1 and 2 is MIMO transmission in a line-of-sight environment, and in satellite communication in which the correlation between antennas tends to be high, different signals are transmitted at the same frequency in cooperation with a plurality of satellite repeaters. Thus, MIMO transmission with reduced correlation is enabled. However, in this document, no consideration is given to demodulation that are asynchronous with symbols.

  The inter-base station cooperation techniques of Non-Patent Documents 3 and 4 perform signal transmission in cooperation with a plurality of base stations in order to improve the cell edge throughput in the cellular system. This is because when the transmission distance from each transmitting station is different, a reception timing error occurs, which is caused by intersymbol interference. This document describes that intersymbol interference does not occur if the shift is within a guard interval length (CP length) used for multipath tolerance in OFDM.

  However, when performing multiple transmission station MIMO in a wide service area such as satellite communication, as shown in FIG. 4, the difference in delay time at each reception station increases due to the difference in propagation path length between the transmission and reception stations. It is sufficient that the CP length is equal to or greater than the delay time difference at each receiving station. However, as the delay time difference increases, the required CP length increases, the time band of the control frame becomes longer, and the frequency utilization efficiency decreases. However, Non-Patent Documents 3 and 4 do not consider the case where the frequency utilization efficiency is reduced and the delay time difference is so large that the delay time difference exceeds the symbol length.

  Non-Patent Document 5 discusses a demodulation method in the case where a delay exceeding the CP length occurs due to time-asynchronous transmission to the base station in uplink MIMO-OFDM by a plurality of receiving stations, but the amount of calculation increases. There is a problem. Since the downlink is assumed this time, it is not desirable to increase the circuit scale of the receiving station.

  The present invention provides an OFDM communication system and a sub that can suppress an increase in the CP length of an OFDM signal and can cope with a case where a delay time difference exceeds a symbol length even when the propagation path length between each transmitting station and receiving station is greatly different. An object is to provide a carrier allocation method.

  The first invention is an OFDM communication system that performs MIMO transmission by assigning each receiving station to each subcarrier of an OFDM signal between a plurality of transmitting stations and a plurality of receiving stations. The delay time difference when the signal is received by a plurality of receiving stations is detected for each receiving station, and the OFDM signal subcarrier is segmented for each receiving station where the delay time difference does not exceed the preset maximum CP length. A plurality of transmitting stations are configured to control each transmission timing so that the delay time difference does not exceed the maximum CP length for each segment.

  In the OFDM communication system of the first invention, the plurality of transmitting stations are configured to set the maximum value of the delay time difference when the transmission timing is controlled for each segment as the required CP length in the segment.

  In the OFDM communication system of the first invention, the plurality of transmitting stations are configured to control the transmission timing for each segment and to separate the frequency band of the OFDM signal of the segment.

  According to a second aspect of the present invention, there is provided a subcarrier allocation method for an OFDM communication system in which MIMO transmission is performed by allocating each receiving station to each subcarrier of an OFDM signal between a plurality of transmitting stations and a plurality of receiving stations. A first step of detecting, for each receiving station, a delay time difference when a signal transmitted from the station is received by a plurality of receiving stations; and for each receiving station where the delay time difference does not exceed a preset maximum CP length, A second step of allocating the signal subcarriers in segments and a third step of controlling the transmission timing so that the delay time difference does not exceed the maximum CP length for each segment are executed.

  In the subcarrier allocation method of the second invention, the plurality of transmitting stations set the maximum value of the delay time difference when the transmission timing is controlled for each segment as the required CP length in the segment.

  In the subcarrier allocation method of the second invention, the plurality of transmitting stations control the transmission timing for each segment and separate the frequency band of the OFDM signal of the segment.

  According to the present invention, when cooperative MIMO transmission by a plurality of transmitting stations with different locations is performed using OFDM, even when the propagation path length between each transmitting station and receiving station is greatly different, the delay time difference is the maximum CP length. By performing segmentation for each receiving station that does not exceed 1 and performing subcarrier allocation and transmission timing control, it is possible to suppress deterioration in frequency utilization efficiency due to an increase in the CP length (guard interval length) of the OFDM signal.

It is a figure which shows the structural example of the OFDM communication system of this invention. It is a figure which shows the structural example of the control station 10 of the OFDM communication system of this invention. It is a flowchart which shows the process sequence of the control station 10 of the OFDM communication system of this invention. It is a figure which shows the structural example of the conventional OFDM communication system.

FIG. 1 shows a configuration example of an OFDM communication system according to the present invention.
In FIG. 1, in the OFDM communication system, a plurality of transmitting stations A21 and B22 cooperate to perform coordinated MIMO transmission to a plurality of receiving stations 31 to 37 under the control of the control station 10.

  The feature of the present invention is that the propagation path length between the transmitting station and the receiving station is obtained when the receiving stations 31 to 37 receive the OFDM signals transmitted at a predetermined timing from the plurality of transmitting stations A21 and B22, respectively. The control station 10 segments the OFDM signal subcarriers 1 to 7 so as to perform OFDM transmission only to a receiving station in a range in which the delay time difference due to the difference does not exceed a preset maximum CP length, The transmission timing of the transmitting station A21 and the transmitting station B22 is controlled every time. As a result, the CP length of the OFDM signal of each segment should be less than the maximum CP length and satisfy the maximum delay time difference (required CP length) within each segment, while minimizing the CP length of the OFDM signal of each segment. OFDM transmission is possible between the transmitting station A21, the transmitting station B22, and the receiving stations 31 to 37. However, the segments are frequency separated, and symbol synchronization is not necessary.

FIG. 2 shows a configuration example of the control station 10 of the OFDM communication system of the present invention.
In FIG. 2, the control station 10 includes a delay time difference detection unit 11, a maximum CP length comparison unit 12, and a segment assignment / transmission station control unit 13.

  Hereinafter, a description will be given with reference to FIGS. The delay time difference detector 11 acquires information obtained from the receiving stations 31 to 37 by the transmitting station A21 and the transmitting station B22, and detects a delay time difference at each receiving station. For example, when the transmission station A21 and the transmission station B22 transmit pilot signals simultaneously, the delay time difference between the reception signals at the reception stations 31 to 37 is the reception time t1 of the signal from the transmission station A21 and the reception of the signal from the transmission station B22. As a difference t1-t2 of time t2, the delay time difference shown in Table 1 is obtained. Note that the transmitting station A21 and the transmitting station B22 may acquire position information by GPS or the like from the receiving stations 31 to 37, and calculate the delay time difference from the propagation path length difference.

  Here, when the maximum CP length is set to 10 μs and the transmitting station A21 and the transmitting station B22 assign the receiving stations 31 to 37 to the subcarriers 1 to 7 of the OFDM signal and transmit simultaneously, the delay of the receiving stations 34 and 35 The time difference becomes less than the maximum CP length, and the subcarriers 4 and 5 of the OFDM signal can be symbol-synchronized. On the other hand, since the delay time difference between the receiving stations 31 to 33 and 36 to 37 exceeds the maximum CP length, symbol synchronization cannot be performed unless the CP length is increased.

  The present invention performs segmentation so that the receiving stations 31 to 33 receive the subcarriers 1 to 3 of the OFDM signal while keeping the maximum CP length at 10 μs (segment 1). The transmission timing of the transmitting station A21 and the transmitting station B22 is controlled so that becomes zero. Specifically, the transmission timing of the transmission station A21 is delayed by 19 μs with reference to the transmission station B22. In this case, the delay time difference between the receiving stations 31 to 33 is 0 to 5 μs, which is the relative delay time difference shown in Table 2, and is equal to or shorter than the maximum CP length, so that symbol synchronization of the subcarriers 1 to 3 of the OFDM signal is possible. The required CP length of the segment 1 OFDM signal is 5 μs, which is the maximum relative delay time difference. Note that transmission timing control in which the delay time difference between the receiving station 32 and the receiving station 33 is zero may be performed.

  Similarly, the receiving stations 36 to 37 perform segmentation in order to receive the OFDM signal subcarriers 6 to 7 while keeping the maximum CP length at 10 μs (segment 3). The transmission timings of the transmitting station A21 and the transmitting station B22 are controlled so as to be zero. Specifically, the transmission timing of the transmission station B22 is delayed by 21 μs with reference to the transmission station A21. In this case, the delay time difference between the receiving stations 36 to 37 is the adjustment delay time difference 0 to 4 μs shown in Table 2 and is equal to or shorter than the maximum CP length, so that symbol synchronization of the subcarriers 6 to 7 of the OFDM signal is possible. The required CP length of the OFDM signal of segment 3 is 4 μs, which is the maximum value of the relative delay time difference. Note that transmission timing control may be performed in which the delay time difference of the receiving station 37 is zero.

  The maximum CP length comparison unit 12 and the segment allocation / transmission station control unit 13 of the control station 10 thus compare the delay time difference at each receiving station with the maximum CP length so that the delay time difference is less than or equal to the maximum CP length. Each subcarrier is segmented, segment 1 (subcarriers 1 to 3) is assigned to receiving stations 31 to 33, segment 2 (subcarriers 4 to 5) is assigned to receiving stations 34 to 35, segment 3 (subcarrier 6) To 7) are assigned to the receiving stations 36 to 37, and the transmission timings of the transmitting station A21 and the transmitting station B22 are controlled for each segment. That is, the transmission timing of the transmission station B22 with reference to the transmission station A21 is 19 μs earlier for the segment 1, simultaneously for the segment 2, and 21 μs later for the segment 3.

  At this time, the subcarriers in each segment are synchronously transmitted from the transmitting station A21 and the transmitting station B22 at a predetermined timing. On the other hand, the segments are transmitted with frequency separation, and symbol synchronization is unnecessary.

  For segment 2 (subcarriers 4 to 5), even if the transmission timings of the transmitting station A21 and the transmitting station B22 are the same, the delay time difference between the receiving stations 34 and 35 is not more than the maximum CP length. Similarly to the segments 1 and 3 shown in Table 2, for example, the transmission timing may be controlled so that the delay time difference of the OFDM signal at the receiving station 34 becomes zero. In this case, the relative delay time difference between the receiving stations 34 and 35 is 0 μs and 2 μs, and the required CP length can be shortened from 3 μs shown in Table 2 to 2 μs.

The processing procedure of the control station 10 for efficiently performing the above processing is shown in the flowchart of FIG.
As described as the operation of the delay time difference detection unit 11 in FIG. 2, the control station 10 acquires the delay time difference of the signal from each transmitting station for each receiving station (S1). Next, a list of receiving stations with the delay time difference ascending order is created (S2). The list shown in Table 3 shows the delay time difference of each receiving station in ascending order.

  Next, the transmission timings of the transmitting station A21 and the transmitting station B22 are controlled so that the minimum value of the delay time difference (here, −19 μs of the receiving station 31) becomes 0, and the delay time difference is changed to the relative delay time difference (S3). ). As shown in Table 3, when the receiving station 31 reaches 0 μs, the relative delay time difference is 2 μs for the receiving station 32, 5 μs for the receiving station 33, 20 μs for the receiving station 34, and so on. Here, the relative delay time difference is compared with the maximum CP length (for example, 10 μs), and for the receiving stations 31 to 33 whose relative delay time difference does not exceed the maximum CP length, subcarriers 1 to 3 are set as segment 1 in ascending order of the relative delay time difference. 3 are sequentially assigned (S4: Yes, S5, S6: No).

  On the other hand, the segment after the receiving station 34 where the relative delay time difference exceeds the maximum CP length is changed (S4: No, S7), and the process returns to step S3. That is, the transmission timing of the transmitting station A21 and the transmitting station B22 is controlled so that the minimum value of the delay time difference (here, 20 μs of the receiving station 34) becomes 0, and the delay time difference is changed to the relative delay time difference (S3). As shown in Table 3, when the receiving station 34 becomes 0 μs, the relative delay time difference is 2 μs for the receiving station 35, 20 μs for the receiving station 36, and 24 μs for the receiving station 37. Here, the relative delay time difference is compared with the maximum CP length (for example, 10 μs). For the receiving stations 34 to 35 whose relative delay time difference does not exceed the maximum CP length, subcarriers 4 to 35 are set as segment 2 in ascending order of the relative delay time difference. 5 are sequentially assigned (S4: Yes, S5, S6: No).

  Similarly, the segment is changed for the receiving station 36 and beyond after the relative delay time difference exceeds the maximum CP length (S4: No, S7), and the process returns to step S3. That is, the transmission timing of the transmitting station A21 and the transmitting station B22 is controlled so that the minimum value of the delay time difference (here, 20 μs of the receiving station 36) becomes 0, and the delay time difference is changed to the relative delay time difference (S3). As shown in Table 3, the relative delay time difference is 4 μs for the receiving station 37 when the receiving station 36 becomes 0 μs. Here, the relative delay time difference is compared with the maximum CP length (for example, 10 μs). For the receiving stations 36 to 37 whose relative delay time difference does not exceed the maximum CP length, the subcarriers 6 to 37 are segment 3 ascending order of the relative delay time difference. 7 are sequentially performed (S4: Yes, S5), and the process ends when all the subcarriers have been allocated.

  Thus, it can be seen that the OFDM communication system of the present invention is broken down into three OFDM signal segments, and the required CP length is 5 μs for segment 1, 2 μs for segment 2, and 4 μs for segment 3. On the other hand, as shown in FIG. 4, when OFDM transmission is performed with the receiving stations 31 to 37 as one segment, the required CP length is 44 μs, which is the maximum value of the relative delay time difference. In the OFDM communication system of the present invention, it can be seen that an increase in the CP length is significantly suppressed as compared with this.

10 Control Station 11 Delay Time Difference Detection Unit 12 Maximum CP Length Comparison Unit 13 Segment Allocation / Transmission Station Control Unit 21 Transmitting Station A
22 Transmitting station B
31-37 receiving station

Claims (6)

In an OFDM communication system that performs MIMO transmission by assigning each receiving station to each subcarrier of an OFDM signal between a plurality of transmitting stations and a plurality of receiving stations,
A delay time difference when signals transmitted from the plurality of transmitting stations are received by the plurality of receiving stations is detected for each receiving station, and for each receiving station where the delay time difference does not exceed a preset maximum CP length. Means for segmenting and assigning subcarriers of the OFDM signal,
The OFDM communication system, wherein the plurality of transmitting stations are configured to control transmission timing so that the delay time difference does not exceed the maximum CP length for each segment. The OFDM communication system according to claim 1,
The OFDM communication system, wherein the plurality of transmitting stations are configured to set a maximum value of the delay time difference when the transmission timing is controlled for each segment as a required CP length in the segment. The OFDM communication system according to claim 1,
The OFDM communication system, wherein the plurality of transmission stations are configured to control the transmission timing for each of the segments and to separate the frequency band of the OFDM signal of the segment. In an OFDM communication system that performs MIMO transmission by assigning each receiving station to each subcarrier of an OFDM signal between a plurality of transmitting stations and a plurality of receiving stations,
A first step of detecting, for each of the receiving stations, a delay time difference when signals transmitted from the plurality of transmitting stations are received by the plurality of receiving stations;
A second step of segmenting and assigning subcarriers of the OFDM signal for each receiving station in which the delay time difference does not exceed a preset maximum CP length;
And a third step of controlling each transmission timing so that the delay time difference does not exceed the maximum CP length for each segment. In the subcarrier allocation method according to claim 4,
The plurality of transmitting stations set the maximum value of the delay time difference when the transmission timing is controlled for each segment as a required CP length in the segment. In the subcarrier allocation method according to claim 4,
The plurality of transmitting stations control the transmission timing for each segment and separate the frequency band of the OFDM signal of the segment.

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