BASE STATION APPARATUS, WIRELESS COMMUNICATION SYSTEM, AND COMMUNICATION METHOD

Abstract

A base station apparatus includes at least one remote unit apparatus including at least one antenna; and a central unit apparatus connected to each remote unit via a transmission path. The remote antenna receives a wireless transmission signal from at least one wireless terminal including at least one antenna. The remote unit includes a channel estimation unit functioning between the terminal antenna and the remote antenna, using a reception signal received by the remote unit antenna; a reception signal processing unit that detects a reception signal corresponding to the transmission signal, using the channel information estimated by the channel estimation unit; a decoding unit that decodes each signal detected by the reception signal processing unit; and an inter-unit transmission unit that transmits each signal decoded by the decoding unit to the central unit. The central unit is provided with an inter-unit receiving unit and a signal selection unit.

Claims

1. A base station apparatus comprising: at least one remote unit apparatus comprising at least one antenna; and a central unit apparatus connected to the remote unit apparatus via a transmission path, wherein the antenna provided in the remote unit apparatus receives a transmission signal wirelessly transmitted from at least one wireless terminal each comprising at least one antenna, and the remote unit apparatus comprises: a channel estimation unit that estimates channel information between the antenna of the wireless terminal and the antenna of the remote unit apparatus, using a reception signal received by the antenna provided in the remote unit apparatus; a reception signal processing unit that detects a reception signal corresponding to the transmission signal by performing maximum likelihood decision on the reception signal, using the channel information estimated by the channel estimation unit; a decoding unit that decodes each signal detected by the reception signal processing unit; and an inter-unit transmission unit that transmits each signal decoded by the decoding unit to the central unit apparatus, and the central unit apparatus comprises: an inter-unit receiving unit that receives each signal transmitted from the inter-unit transmission unit; and a signal selection unit that acquires the channel information from the channel estimation unit, and selects a signal corresponding to each transmission signal transmitted from the wireless terminal, from among signals received by the inter-unit receiving unit, based on the acquired channel information.

2. The base station apparatus according to claim 1, wherein in the maximum likelihood decision, the reception signal processing unit detects the reception signal using a hard decision output and outputs a code word corresponding to each transmission signal, and the decoding unit decodes each code word output from the reception signal processing unit into a bit sequence.

3. The base station apparatus according to claim 1, wherein in the maximum likelihood decision, the reception signal processing unit detects the reception signal using a soft decision output and outputs log likelihood ratios of bits corresponding to each transmission signal, and the decoding unit decodes the log likelihood ratios output from the reception signal processing unit into a bit sequence.

4. The base station apparatus according to claim 1, wherein the signal selection unit obtains each propagation loss between each antenna of the wireless terminal and each antenna of the base station apparatus based on the acquired channel information, and selects a transmission signal with the minimum propagation loss from among common transmission signals as a reception signal that corresponds to the common transmission signal.

5. A wireless communication system comprising: the base station apparatus according to claim 1; and the wireless terminal.

6. A communication method in a base station apparatus comprising: at least one remote unit apparatus comprising at least one antenna; and a central unit apparatus connected to each remote unit apparatus via a transmission path, the communication method comprising: a channel estimation step in which, when each antenna of the remote unit apparatus receives each transmission signal that is wirelessly transmitted from at least one wireless terminal comprising at least one antenna, the remote unit apparatus uses a reception signal received by each antenna of the remote unit apparatus to estimate channel information between the antenna of the wireless terminal and the antenna of the base station apparatus; a reception signal processing step in which the remote unit apparatus detects a reception signal corresponding to the transmission signal by performing maximum likelihood decision on the reception signal using the estimated channel information, and decodes each detected signal; and a signal selection step in which the central unit apparatus selects, from detected signals, a signal that corresponds to each transmission signal transmitted from the wireless terminal, using the estimated channel information.

7. The communication method according to claim 6, wherein in the reception signal processing step, as the maximum likelihood decision, the remote unit apparatus detects a code word that corresponds to each transmission signal using a hard decision output, and decodes each code word into a bit sequence.

8. The communication method according to claim 6, wherein in the reception signal processing step, as the maximum likelihood decision, the remote unit apparatus detects log likelihood ratios of bits corresponding to each transmission signal using a soft decision output, and decodes each log likelihood ratio into a bit sequence.

9. The base station apparatus according to claim 2, wherein the signal selection unit obtains each propagation loss between each antenna of the wireless terminal and each antenna of the base station apparatus based on the acquired channel information, and selects a transmission signal with the minimum propagation loss from among common transmission signals as a reception signal that corresponds to the common transmission signal.

10. The base station apparatus according to claim 3, wherein the signal selection unit obtains each propagation loss between each antenna of the wireless terminal and each antenna of the base station apparatus based on the acquired channel information, and selects a transmission signal with the minimum propagation loss from among common transmission signals as a reception signal that corresponds to the common transmission signal.

11. A wireless communication system comprising: the base station apparatus according to claim 2; and the wireless terminal.

12. A wireless communication system comprising: the base station apparatus according to claim 3; and the wireless terminal.

13. A wireless communication system comprising: the base station apparatus according to claim 4; and the wireless terminal.

14. A wireless communication system comprising: the base station apparatus according to claim 9; and the wireless terminal.

15. A wireless communication system comprising: the base station apparatus according to claim 10; and the wireless terminal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] FIG. 1 shows a first example of a system configuration for performing communication between a central unit and remote units related to the present invention.

[0051] FIG. 2 shows a second example of a system configuration for performing communication between a central unit and remote units related to the present invention.

[0052] FIG. 3 shows a third example of a system configuration for performing communication between a central unit and remote units related to the present invention.

[0053] FIG. 4 shows an example of a technique of implementing CoMP when base stations operate in cooperation with each other.

[0054] FIG. 5 shows an example of a technique of implementing CoMP when a central unit and remote units operate in cooperation with each other.

[0055] FIG. 6 shows an example of a system configuration for performing CoMP using JR in a full centralization system configuration.

[0056] FIG. 7 shows an example of a system configuration for performing CoMP using JR in a partial centralization system configuration.

[0057] FIG. 8 shows an example of a wireless communication system in accordance with Embodiment 1.

[0058] FIG. 9 shows an example of a wireless communication system in accordance with Embodiment 2.

[0059] FIG. 10 shows an example of a wireless communication system in accordance with Embodiment 3.

[0060] FIG. 11 shows an example of a wireless communication system in accordance with Embodiment 4.

MODES FOR CARRYING OUT THE INVENTION

[0061] Hereunder, embodiments of the present invention will be described with reference to the drawings. It is to be noted that the present invention is not limited to the embodiments shown below. These embodiments are illustrated merely as examples, and various modifications and/or improvements may be made to the present invention based on the knowledge of one skilled in the art. It is to be noted that in the present specification and in the drawings, constituents having the same reference symbols mutually denote the same constituents.

Embodiment 1

[0062] FIG. 8 shows an example of a wireless communication system in accordance with Embodiment 1. The wireless communication system in accordance with the present embodiment is provided with a central unit 10 that functions as a central unit apparatus, remote units 20 that function as remote unit apparatuses, and a terminal 30 that functions as a wireless terminal. The terminal 30 performs CoMP signal transmission to the plurality of remote units 20.

[0063] The wireless communication system in accordance with the present embodiment has the terminal 30 (wireless terminal) provided with one or more antennas. As an example, FIG. 8 shows a case in which the number of wireless terminals is one and the number of antennas is two. The wireless communication system in accordance with the present embodiment is provided with one or more remote units 20 each provided with one or more antennas. As an example, FIG. 8 shows a case in which the number of antennas is one and the number of apparatuses is two. It is to be noted that in FIG. 8, the number of the terminals 30 is one, the number of the antennas provided in each terminal 30 is two, the number of cooperating remote units 20 is two, and the number of antennas provided in each remote unit 20 is one. However, these numbers are not limited.

[0064] In the present embodiment, the remote unit 20 performs a receiving process using hard decision. The central unit 10 is provided with signal conversion units 11 that function as inter-unit receiving units, a signal selection unit 16, and a MAC function unit 15. Each remote unit 20 is provided with an antenna 21, an RF receiving unit 22, a channel estimation unit 24, a distributed MLD unit 27 that functions as a reception signal processing unit, a decoding unit 26, and a signal conversion unit 23 that functions as an inter-unit transmission unit. The terminal 30 is provided with a transmission function unit (not shown in figure) and antennas 31.

[0065] The communication method in accordance with the present embodiment performs a channel estimation step, a reception signal processing step, and a signal selection step, sequentially.

[0066] In the channel estimation step, the remote units 20 receive transmission signals s.sub.1 and s.sub.2, and use reception signals to estimate channel information between the antennas 31 and the antennas 21. In the present embodiment, the RF receiving unit 22 of the remote unit #1 receives the transmission signals s.sub.1 and s.sub.2 and outputs a reception signal r.sub.1. The channel estimation unit 24 of the remote unit #1 uses the reception signal r.sub.1 to estimate channel information h.sub.e11 and h.sub.e12. The RF receiving unit 22 of the remote unit #2 receives the transmission signals s.sub.1 and s.sub.2 and outputs a reception signal r.sub.2. The channel estimation unit 24 of the remote unit #2 uses the reception signal r.sub.2 to estimate channel information h.sub.e21 and h.sub.e22.

[0067] In the reception signal processing step, the remote units 20 detect the reception signals corresponding to the transmission signals by performing maximum likelihood decision on the reception signals using the estimated channel information, and decodes the detected signals. In the present embodiment, the distributed MLD unit 27 of the remote unit #1 detects a code word c.sub.11 that corresponds to the transmission signal s.sub.1 and a code word c.sub.12 that corresponds to the transmission signal s.sub.2, using a hard decision output as maximum likelihood decision. Then the decoding unit 26 of the remote unit #1 decodes the code words c.sub.11 and c.sub.12 into bit sequences b.sub.11 and b.sub.12. The distributed MLD unit 27 of the remote unit #2 detects a code word c.sub.21 that corresponds to the transmission signal s.sub.1 and a code word c.sub.22 that corresponds to the transmission signal s.sub.2, using a hard decision output as maximum likelihood decision. Then the decoding unit 26 of the remote unit #2 decodes the code words c.sub.21 and c.sub.22 into bit sequences b.sub.21 and b.sub.22.

[0068] In the signal selection step, the central unit 10 selects signals corresponding to the transmission signals s.sub.1 and s.sub.2 from the detected signals, using the channel information estimated by the remote units 20. In the present embodiment, the bit sequence b.sub.11 that corresponds to the transmission signal s.sub.1 and the bit sequence b.sub.22 that corresponds to the transmission signal s.sub.2 are selected from the bit sequences b.sub.11, b.sub.12, b.sub.21, and b.sub.22, using the channel information h.sub.e11, h.sub.e12, h.sub.e21, and h.sub.22.

[0069] The transmission signals s.sub.1 and s.sub.2 transmitted from the terminal 30 undergo MIMO transmission expressed as Equation (1), the RF receiving unit 22 of the remote unit #1 receives the reception signal r.sub.1 expressed as Equation (14) below, and the RF receiving unit 22 of the remote unit #2 receives r.sub.2 expressed as Equation (15) below.

[Equation 14]

r.sub.1=h.sub.11s.sub.1+h.sub.12s.sub.2+n.sub.1(14)

[Equation 15]

r.sub.2=h.sub.21s.sub.1+h.sub.22S.sub.2+n.sub.2(15)

[0070] The channel estimation unit 24 of the remote unit #1 can estimate the channel information h.sub.e11 and h.sub.e12 using the reception signal r.sub.1 based on Equation (14). As with the remote unit #1, the channel estimation unit 24 of the remote unit #2 can estimate the channel information h.sub.e11 and h.sub.e22 using the reception signal r.sub.2 based on Equation (15). Specifically, a known signal sequence is inserted into the transmission signals s.sub.1 and s.sub.2, and differences from received signal sequences that have undergone channel variation due to wireless transmission are calculated, to thereby estimate the channel information.

[0071] Focusing on the remote unit #1, the distributed MLD unit 27 performs a distributed MLD process on the reception signal r.sub.1. In the distributed MLD, if the transmission signals s.sub.1 and s.sub.2 are both modulated by means of BPSK, four transmission signal vector candidates s.sub.c1, s.sub.c2, s.sub.c3, and s.sub.c4 expressed as Equation (2) to Equation (5) are multiplied by a channel vector h.sub.e1 which is estimated from the reception signal r.sub.1 and is expressed as Equation (16) below, and reception replicas r.sub.1c1, r.sub.1c2, r.sub.1c3, and r.sub.1c4 expressed as Equation (17) to Equation (20) below are generated.

[Equation 16]

h.sub.e1=[h.sub.e11h.sub.e12](16)

[Equation 17]

r.sub.1c1=h.sub.e1s.sub.c1=h.sub.e11e.sup.j0+h.sub.e12e.sup.j0(17)

[Equation 18]

r.sub.1c2=h.sub.e1s.sub.c2=h.sub.e11e.sup.j0+h.sub.e12e.sup.j(18)

[Equation 19]

r.sub.1c3=h.sub.e1s.sub.c3=h.sub.e11e.sup.j+h.sub.e12e.sup.j0(19)

[Equation 20]

r.sub.1c4=h.sub.e1s.sub.c4=h.sub.e11e.sup.j+h.sub.e12e.sup.j(20)

[0072] As with conventional MLD, squared Euclidean distances E.sub.1, E.sub.2, E.sub.3, and E.sub.4 between the reception replicas and the reception signal r.sub.1 expressed as Equation (14) are calculated as Equation (21) to Equation (24) below, and the transmission signal vector candidate that corresponds to the reception replica with the minimum squared Euclidean distance is determined as an estimated transmission signal.

[Equation 21]

E.sub.1=r.sub.1r.sub.1c1.sup.2(21)

[Equation 22]

E.sub.2=r.sub.1r.sub.1c2.sup.2(22)

[Equation 23]

E.sub.3=r.sub.1r.sub.1c3.sup.2(23)

[Equation 24]

E.sub.4=r.sub.1r.sub.1c4.sup.2(24)

[0073] Through the above processing, the distributed MLD unit 27 of the remote unit #1 performs a receiving process using hard decision, and acquires code words c.sub.11 and c.sub.12 that correspond to the transmission signals s.sub.1 and s.sub.2. Specifically, a bit or bit sequence indicating a modulation symbol of the estimated transmission signal s.sub.1 is obtained as c.sub.11, and a bit or bit sequence indicating a modulation symbol of the estimated transmission signal s.sub.2 is obtained as c.sub.12. Then after having performed a decoding process of the code words c.sub.11 and c.sub.12 into bit sequences b.sub.11 and b.sub.12 in the decoding unit 26, they undergo signal conversion in the signal conversion unit 23, and are transmitted as the bit sequences b.sub.11 and b.sub.12 to the central unit 10. Examples of the decoding process include a technique in which transition of bits in the received bit sequences is observed, and the bit sequence with the highest probability of having been transmitted, is determined. Moreover, a similar process is performed in the remote unit #2. Here code words c.sub.21 and c.sub.22 are acquired through the distributed MLD process in the distributed MLD unit 27, a process in which the code words c.sub.21 and c.sub.22 are decoded into bit sequences b.sub.21 and b.sub.22 is performed in the decoding unit 26, and then they undergo signal conversion in the signal conversion unit 23 to be transmitted as the bit sequences b.sub.21 and b.sub.22 to the central unit 10.

[0074] In the central unit 10, two types of bit sequences that correspond to the transmission signals s.sub.1 and s.sub.2 are collected. Then, the signal selection unit 16 selects, from the two types of bit sequences corresponding to the transmission signals s.sub.1 and s.sub.2, signals that correspond to the transmission signals s.sub.1 and s.sub.2, and outputs, for example, the bit sequences b.sub.11 and b.sub.22 and discards the other signals. As a result, even if the transmission signals s.sub.1 and s.sub.2 are MIMO transmission signals as expressed as Equation (1), the reception signal can be detected for each transmission signal. The MAC function unit 15 performs a MAC process of the data link layer, using the bit sequences b.sub.11 and b.sub.22. The MAC process, for example, includes a process of requesting re-transmission of a signal that includes an error.

[0075] Here, the signal selection process selects the bit sequences with small propagation loss between the antennas based on propagation loss between the antennas obtained from the channel information, which is fedback from the channel estimation units 24 of the remote units 20. For example, b.sub.11 and b.sub.22 are selected. In this case, it corresponds to the process when: propagation loss between the antenna 31 which transmitted transmission signal s.sub.1 and the antenna 21 which received the reception signal r.sub.1 obtained from the channel information h.sub.e11 is smaller than propagation loss between the antenna 31 which transmitted the transmission signal s.sub.1 and the antenna 21 which received the reception signal r.sub.2 obtained from the channel information h.sub.e21; and propagation loss between the antenna 31 which transmitted the transmission signal s.sub.2 and the antenna 21 which received the reception signal r.sub.2 obtained from the channel information h.sub.e22 is smaller than propagation loss between the antenna 31 which transmitted the transmission signal s.sub.2 and the antenna 21 which received the reception signal r.sub.1 obtained from the channel information h.sub.e12. Depending on propagation loss, the bit sequences b.sub.11 and b.sub.12 received from the remote unit #1 may be both selected, and the bit sequences b.sub.21 and b.sub.22 received from the remote unit #2 may be both discarded.

[0076] It is to be noted that for the signal conversion in the signal conversion units 11 and the signal conversion units 23 used for signal transmission between the central unit 10 and the remote units 20, an existing interface may be used, or a unique interface may be used. Furthermore, after RF reception has been performed at each remote unit 20, a process for performing reception of multi-carrier signals such as those in OFDM on reception signals may be performed. Moreover, in the process of distributed MLD, the number of transmission signal vector candidates increases exponentially in accordance with the number of modulation levels and the number of transmission antennas, and thus a technique of reducing the amount of computation may be used in the process of the distributed MLD unit 27 at each remote unit 20.

Embodiment 2

[0077] In the case of using the technique of Embodiment 1, the number of terminals, the number of antennas of each terminal, the number of remote units, and the number of antennas of each remote unit may be set to arbitrary numbers, as will be described in Embodiment 2. FIG. 9 shows CoMP signal transmission in Embodiment 2. Here, the number of terminals is K, the number of antennas of terminals k (k is an integer from 1 to K) is L.sub.k, the number of remote units is M, and the number of antennas of remote units m (m is an integer from 1 to M) is N.sub.m. It is to be noted that the addition of noise in each RF receiving unit is omitted in FIG. 9.

[0078] The distributed MLD unit 27 of each remote unit 20 acquires code words that correspond to the transmission signals transmitted from all antennas 31 of all terminals 30, by means of a process similar to that of Embodiment 1. For example, c.sub.1ij (i is an integer from 1 to K, j is an integer from 1 to L.sub.i) code words are acquired in the remote unit #1, and c.sub.Mij code words are acquired in the remote unit #M. These code words undergo a decoding process in the decoding units 26 where the code words c.sub.1ij and c.sub.Mij are decoded into bit sequences b.sub.1ij and b.sub.Mij, subjected to signal conversion, and are transmitted to the central unit 10 as bit sequences b.sub.1ij and b.sub.Mij.

[0079] Then, the signal selection unit 16 of the central unit 10 selects bit sequences b.sub.sij that correspond to the transmission signals based on the propagation loss between the antennas obtained from the channel information fedback from the channel estimation units 24, and outputs the selected bit sequences b.sub.sij. When the number of terminals, the number of antennas of each terminal, the number of remote units, and the number of the antennas of each remote unit are arbitrary, values each obtained by adding propagation loss between a given antenna 31 of a given terminal 30 and all of the antennas 21 of each remote unit 20 are compared with one another, and a bit sequence delivered from the remote unit 20 with the minimum total propagation loss is selected, while all other bit sequences are discarded. A similar process is performed also for the other antennas.

[0080] It is to be noted that for the signal conversion in the signal conversion units 11 and the signal conversion units 23 used for signal transmission between the central unit 10 and the remote units 20, an existing interface may be used, or a unique interface may be used. Furthermore, after RF reception has been performed at each remote unit 20, a process for performing reception of multi-carrier signals such as those in OFDM on reception signals may be performed. Moreover, in the process of distributed MLD, the number of transmission signal vector candidates increases exponentially in accordance with the number of modulation levels and the number of transmission antennas, and thus a technique of reducing the amount of computation may be used in the process of the distributed MLD unit 27 at each remote unit 20. Furthermore, instead of distributed MLD, code words may be obtained using a process such as MMSE or SIC and hard decision demodulation.

Embodiment 3

[0081] In contrast with Embodiment 1, as will be described in Embodiment 3, likelihood for each bit of a transmission signal may be estimated by performing a receiving process using soft decision at each remote unit 20, and a bit sequence may be detected from the output likelihood information by means of a decoding process using a soft input decoder. FIG. 10 shows CoMP signal transmission in Embodiment 3. Each remote unit 20 is provided with an antenna 21, an RF receiving unit 22, a channel estimation unit 24, a soft output distributed MLD unit 28 that functions as a reception signal processing unit, a decoding unit 26, and a signal conversion unit 23 that functions as an inter-unit transmission unit.

[0082] In the reception signal processing step of a communication method in accordance with the present embodiment, the soft output distributed MLD unit 28 of the remote unit #1 detects log likelihood ratios R.sub.11 of the bits corresponding to the transmission signal s.sub.1 and log likelihood ratios R.sub.12 of the bit corresponding to the transmission signal s.sub.2, using a soft decision output as maximum likelihood decision. Then, the decoding unit 26 of the remote unit #1 decodes the log likelihood ratios R.sub.11 and R.sub.12 into bit sequences b.sub.11 and b.sub.12. The soft output distributed MLD unit 28 of the remote unit #2 detects log likelihood ratios R.sub.21 of the bits corresponding to the transmission signal s.sub.1 and log likelihood ratios R.sub.22 of the bits corresponding to the transmission signal s.sub.2, using a soft decision output as maximum likelihood decision. Then the decoding unit 26 of the remote unit #2 decodes the log likelihood ratios R.sub.21 and R.sub.22 into bit sequences b.sub.21 and b.sub.22.

[0083] The soft output distributed MLD unit 28 of the remote unit #1 outputs, as likelihood information, the log likelihood ratios R.sub.11 and R.sub.12 of the bits corresponding to the transmission signals s.sub.1 and s.sub.2, by means of a soft output distributed MLD process. Here, one example of a specific processing method of soft output MLD is such that the nearest reception replica having the minimum squared Euclidean distance from the reception signal is identified, and then a reception replica having the minimum squared Euclidean distance from the reception signal, among reception replicas corresponding to bit sequences obtained by inverting one of bits of a bit sequence that corresponds to the nearest reception replica, is identified. The squared Euclidean distances between the two identified reception replicas and the reception signal are calculated, and the difference therebetween is obtained as a log likelihood ratio.

[0084] The decoding unit 26 decodes the log likelihood ratios R.sub.11 and R.sub.12 into the bit sequences b.sub.11 and b.sub.12, using a soft decision input decoder such as a turbo decoder.

[0085] The signal conversion unit 23 transmits the bit sequences b.sub.11 and b.sub.12 to the central unit 10.

[0086] It is to be noted that when the number of modulation levels of a transmission signal increases, the number of bits that express the transmission signal also increases, and thus the number of log likelihood ratios also increases. For example, when the transmission signals s.sub.1 and s.sub.2 are both modulated by means of BPSK, the number of pieces of log likelihood ratio information becomes two as described above, whereas when they are modulated by means of QPSK, the total number of bits becomes 4 bits, and the number of pieces of log likelihood ratio information also becomes four. The decoding unit 26 of the remote unit #1 performs a soft input decoding process into the bit sequences b.sub.11 and b.sub.12, based on the acquired log likelihood ratios R.sub.11 and R.sub.12, and the output bit sequences b.sub.11 and b.sub.12 undergo signal conversion in the signal conversion unit 23 to be transmitted to the central unit 10. Moreover, a similar process is performed in the remote unit #2, and the log likelihood ratios R.sub.21 and R.sub.22 are acquired by means of a soft output distributed MLD process, a decoding process is performed based thereon, and the output bit sequences b.sub.21 and b.sub.22 undergo signal conversion to be transmitted to the central unit 10.

[0087] In the central unit 10, two types of bit sequences that correspond to the transmission signals s.sub.1 and s.sub.2 are collected, signals that correspond to the transmission signals s.sub.1 and s.sub.2 are selected from the two types of bit sequences based on the channel information fedback from the remote units 20, and, for example, b.sub.11 and b.sub.22 are output while all other signals are discarded. In this case, it corresponds to the process when: propagation loss between the antenna 31 which transmitted transmission signal s.sub.1 and the antenna 21 which received the reception signal r.sub.1 obtained from the channel information h.sub.e11 is smaller than propagation loss between the antenna 31 which transmitted the transmission signal s.sub.1 and the antenna 21 which received the reception signal r.sub.2 obtained from the channel information h.sub.e21; and propagation loss between the antenna 31 which transmitted the transmission signal s.sub.2 and the antenna 21 which received the reception signal r.sub.2 obtained from the channel information h.sub.e22 is smaller than propagation loss between the antenna 31 which transmitted the transmission signal s.sub.2 and the antenna 21 which received the reception signal r.sub.1 obtained from the channel information h.sub.e12. Depending on propagation loss, the bit sequences b.sub.11 and b.sub.12 received from the remote unit #1 may be both selected, and the bit sequences b.sub.21 and b.sub.22 received from the remote unit #2 may be both discarded.

[0088] It is to be noted that for the signal conversion used for signal transmission between the central unit 10 and the remote units 20, an existing interface may be used, or a unique interface may be used. Furthermore, after RF reception has been performed at each remote unit, a process for performing reception of multi-carrier signals such as those in OFDM on reception signals may be performed. Moreover, in the process of the soft output distributed MLD, the number of transmission signal vector candidates increases exponentially in accordance with the number of modulation levels and the number of transmission antennas, and thus a technique of reducing the amount of computation may be used in the process of the soft output distributed MLD at each remote unit.

Embodiment 4

[0089] In the case of using the technique of Embodiment 3, as will be described in Embodiment 4, the number of terminals, the number of antennas of each terminal, the number of remote units, and the number of antennas of each remote unit may be set to arbitrary numbers. FIG. 11 shows CoMP signal transmission in Embodiment 4. Here, the number of terminals is K, the number of antennas of terminals k (k is an integer from 1 to K) is L.sub.k, the number of remote units is M, and the number of antennas of remote units m (m is an integer from 1 to M) is N.sub.m. It is to be noted that the addition of noise in each RF receiving unit is omitted in FIG. 11.

[0090] In each remote unit 20, log likelihood ratios that correspond to the bits of the transmission signals transmitted from all of the antennas 31 of all of the terminals 30 are acquired by a soft output distributed MLD process of the soft output distributed MLD unit 28. For example, R.sub.1ij (i is an integer from 1 to K, j is an integer from 1 to L.sub.i) log likelihood ratios are acquired in the remote unit #1, and R.sub.Mij pieces of information are acquired in the remote unit #M. The decoding units 26 perform a process of decoding into bit sequences b.sub.1ij to b.sub.Mij, by means soft input decoders based on these log likelihood ratios. The bit sequences b.sub.1ij to b.sub.Mij output from the decoding units 26 undergo signal conversion in the signal conversion units 23, and are then transmitted to the central unit 10.

[0091] Then, the signal selection unit 16 of the central unit 10 selects a bit sequence b.sub.sij that corresponds to each transmission signal based on the channel information fedback from the channel estimation units 24, and outputs the selected bit sequence b.sub.sij. When the number of terminals, the number of antennas of each terminal, the number of remote units, and the number of the antennas of each remote unit are arbitrary, values each obtained by adding propagation losses between a given antenna 31 of a given terminal 30 and all of the antennas 21 of each remote units 20 are compared with one another, and a bit sequence delivered from the remote unit 20 with the minimum total propagation loss is selected, while all other bit sequences are discarded. A similar process is performed also for the other antennas 31.

[0092] It is to be noted that for the signal conversion in the signal conversion units 11 and the signal conversion units 23 used for signal transmission between the central unit 10 and the remote units 20, an existing interface may be used, or a unique interface may be used. Furthermore, after RF reception has been performed at each remote unit 20, a process for performing reception of multi-carrier signals such as those in OFDM on reception signals may be performed. Moreover, in the process of the soft output distributed MLD, the number of transmission signal vector candidates increases exponentially in accordance with the number of modulation levels and the number of transmission antennas, and thus a technique of reducing the amount of computation may be used in the process of the soft output distributed MLD at each remote unit. Furthermore, instead of distributed MLD, log likelihood ratios may be obtained using a process such as MMSE or SIC and soft decision demodulation.

INDUSTRIAL APPLICABILITY

[0093] The present invention can be applied to the information communication industry.

DESCRIPTION OF REFERENCE SYMBOLS

[0094] 10 Central unit [0095] 11 Signal conversion unit [0096] 12 Channel estimation unit [0097] 13 MIMO signal detection unit [0098] 14 Decoding unit [0099] 15 MAC function unit [0100] 16 Signal selection unit [0101] 20 Remote unit [0102] 21 Antenna [0103] 22 RF receiving unit [0104] 23 Signal conversion unit [0105] 24 Channel estimation unit [0106] 25 MIMO signal detection unit [0107] 26 Decoding unit [0108] 27 Distributed MLD unit [0109] 28 Soft output distributed MLD unit [0110] 30 Terminal [0111] 31 Antenna [0112] 91 Core network [0113] 92 Cell