WO2008032973A1

WO2008032973A1 - Method for performing cooperative transmission in multi-carrier system

Info

Publication number: WO2008032973A1
Application number: PCT/KR2007/004382
Authority: WO
Inventors: Seung Woo Ko; Min Seok Oh; Seung Hee Han; Seong Lyun Kim; Yeong Hyeon Kwon
Original assignee: Lg Electronics Inc.; Industry-Academic Cooperation Foundation, Yonsei University
Priority date: 2006-09-11
Filing date: 2007-09-11
Publication date: 2008-03-20
Also published as: KR20080023505A; KR101293360B1

Abstract

There is provided a method for enabling a user equipment to perform cooperative transmission in multi-carrier system. The method includes obtaining a listened signal by listening to a signal transmitted from another user equipment to a base station and transmitting the listened signal to the base station. The signal and the listened signal is allocated to the same subcarriers. By co-operative transmission in multi-carrier system additional time diversity gain and ser diversity gain can be obtained.

Description

METHOD FOR PERFORMING COOPERATIVE TRANSMISSION IN MULTI-CARRIER SYSTEM

Technical Field

[1] The present invention relates to wireless communication, and more particularly, to a method for performing cooperative transmission in multi-carrier system. Background Art

[2] With the advent of various multimedia services and high-quality services, a next- generation wireless communication system requires high-speed data transmission and guarantee of discriminative quality of service (QoS).

[3] As the post-third-generation system requires a higher data transmission rate, multipath delay becomes a serious problem because it causes inter-symbol interference (hereinafter, referred to as 'ISI'). To reduce the ISI effect with low complexity, attention has been focused on orthogonal frequency division multiplexing (OFDM).

[4] The OFDM converts serially inputted data symbols into N parallel data symbols, respectively loads the N parallel data symbols on N subcarriers, and transmits the subcarriers having the parallel data symbols loaded thereon. The subcarriers maintain orthogonality in the frequency domain. Each orthogonal channel undergoes independent frequency selective fading and an interval of the transmitted data symbols is increased. Thus the ISI can be minimized. Orthogonal frequency division multiple access (hereinafter, referred to as 'OFDMA') is a multiple access scheme which provides a part of available subcarriers to each of users to achieve multiple accesses in a OFDM-based system.

[5] One of techniques for mitigating multipath delay is a diversity technique which repeatedly transmits the same data. If a plurality of same data are independently transmitted through multipath, even though some path has a very lowe level, other paths can have a high level. Accordingly, the diversity technique combines a plurality of same data to achieve stable transmission and reception. An example of the diversity technique includes frequency diversity for transmitting data at different frequencies, time diversity for transmitting data at different points of time, and spatial diversity for transmitting data using multiple antennas.

[6] Cooperative transmission using a relay station is known as one of diversity techniques. In the cooperative transmission, the relay station is arranged between a base station and a user equipment so that the relay station relay and transmit data from the base station or the user equipment. The relay station first receives information for cooperative transmission, and then sequentially transmits the information. [7] However, the cooperative transmission is based on a single carrier and a cooperative transmission technique suitable for a multi-carrier system is not known.

[8] In a single-carrier system, data transmission delay caused by the relay station deteriorates the system performance. In the multi-carrier system, data rates may be remarkably decreased when the channel gain between the base station and the user equipment is low.

[9] There is a need for a cooperative transmission technique suitable for the multi-carrier system.

Disclosure of Invention Technical Problem

[10] The present invention is to provide a cooperative transmission method suitable for a multi-carrier system. Technical Solution

[11] In one aspect, there is provided a method for enabling a user equipment to perform cooperative transmission in multi-carrier system. The method includes obtaining a listened signal by listening to a signal transmitted from another user equipment to a base station and transmitting the listened signal to the base station. The signal and the listened signal are allocated to the same subcarriers.

[12] In another aspect, there is provided a method for enabling a user equipment to perform cooperative transmission in multi-carrier system. The method includes generating a listened signal by listening to a signal transmitted from another user equipment to a base station and transmitting the listened signal to the base station. The signal and the listened signal are allocated to the different subcarriers.

[13] In still another aspect, there is provided a method for relaying a signal transmitted from a source station to a target station in wireless communication system. The method includes obtaining a listened signal by listening the signal transmitted from the source station to the target station and transmitting the listened signal to the target station using the same subcarriers as the subcarriers of the the signal.

[14]

Advantageous Effects

[15] Cooperative transmission using a plurality of user equipments is achieved in multi- carrier system to obtain additional time diversity gain and ser diversity gain. The system capacity can be improved. Brief Description of the Drawings

[16] FIG. 1 illustrates a wireless communication system.

[17] FIG. 2 is a block diagram of a base station according to an embodiment of the present invention. [18] FIG. 3 is a block diagram of a user equipment according to an embodiment of the present invention. [19] FIGS. 4 and 5 illustrate a cooperative transmission method according to an embodiment of the present invention. [20] FIGS. 6 and 7 illustrate a cooperative transmission method according to another embodiment of the present invention. [21] FIGS. 8 and 9 illustrate a cooperative transmission method according to another embodiment of the present invention. [22] FIGS. 10 and 11 illustrate a cooperative transmission method according to another embodiment of the present invention. [23] FIGS. 12 and 13 illustrate a cooperative transmission method according to another embodiment of the present invention. [24] FIGS. 14 and 15 illustrate a cooperative transmission method according to another embodiment of the present invention. [25] FIGS. 16 and 17 illustrate a cooperative transmission method according to another embodiment of the present invention. [26] FIG. 18 is a graph illustrating the relationship between user density and system capacity. [27] FIG. 19 is a graph illustrating the relationship between user density and system capacity per user.

Mode for the Invention [28] A technique which will be described below can be used in various multiple access communication systems using multi-carrier such as OFDM systems or multi-carrier code division multiple access (MC-CDMA) systems. OFDM partitions a system bandwidth into a plurality of subcarriers having orthogonality. A subcarrier can be called a subband or a tone. [29] FIG. 1 illustrates a wireless communication system. Referring to FIG. 1, the wireless communication system includes a base station (BS) 100 and a user equipment (UE)

200. The wireless communication system is widely deployed to provide various communication services including audio data and packet data. [30] The BS 100 corresponds to a fixed station which communicates with the UE 200.

The BS 100 can be called other terminologies such as a node-B, a base transceiver system and an access point. [31] The UE 200 can be fixed or moved and it can be called a mobile station, a user terminal, a subscriber station or a wireless device. [32] Downlink means communication from the BS 100 to the UE 200 and uplink means communication from the UE 200 to the BS 100. [33] FIG. 2 is a block diagram of a BS according to an embodiment of the present invention.

[34] Referring to FIG. 2, the BS 100 includes a transmitter 120, a receiver 140 and a controller 160. The transmitter 120 includes a channel encoder 121, a symbol mapper 122, an inverse fast Fourier transform (IFFT) unit 123, and a cyclic prefix (CP) insertion unit 124. The transmitter 120 transmits data received from a data source 110.

[35] The channel encoder 121 encodes a stream of information bits provided by the data source 110 according to a predetermined coding scheme to generate coded data. The information bits can include text, audio, video or other data. The channel encoder 121 may add error detection bits such as a cyclic redundancy check (CRC) code to the information bits. The channel encoder 121 can process using an error correction code. The error correction code can be a turbo code, a low density parity check code (LDPC) or a convolution code.

[36] The symbol mapper 122 maps the coded data to a modulated symbol on constellation points. The modulation scheme used by the symbol mapper 122 is not restricted and m-PSK (m-quadrature phase shift keying) or m-QAM (M- Quadrature amplitude modulation) can be used. The m-PSK can include BPSK or 8-PSK as well as QPSK. The m-QAM can include 256-QAM as well as 16-QAM and 64-QAM.

[37] The IFFT unit 123 performs IFFT on the modulated symbol to transform the modulated symbol into a time domain sample. The time domain sample may be called as a OFDM symbol. The symbols inputted to the IFFT unit 123 can include not only the modulated symbol but also a pilot symbol. The pilot symbol is a symbol a priori known to both the BS 100 and the UE 200.

[38] The CP insertion unit 124 adds a CP to the time domain sample. The CP may be referred to as a guard interval. A sample signal output from the CP insertion unit 124 is converted into an analog signal and transmitted through the antenna 130.

[39] The receiver 140 includes a CP removal unit 141, a FFT unit 142, a symbol demapper 143 and a channel decoder 144. The receiver 140 decodes the signal received from the antenna 130 and provides the decoded signal to a data sink 150 or the controller 160.

[40] The signal received from the antenna 130 is digitalized and the CP removal unit 141 removes a CP from the digital signal. The FFT unit 142 performs FFT on the CP- removed signal to transform the signal into a frequency domain symbol. A demodulation scheme corresponds to the modulation scheme of the symbol mapper 122. The channel decoder 144 decodes the frequency domain symbol and provides decoded data to the data sink 150.

[41] The controller 160 controls the operation of the BS 100. A memory 170 can store a program for operating the BS 100. Furthermore, the controller 160 receives a channel quality indicator from the UE 200 to adaptively adjust modulation and coding scheme.

[42] FIG. 3 is a block diagram of a UE according to an embodiment of the present invention.

[43] Referring to FIG. 3, the UE 200 includes a transmitter 220, a receiver 240, and a controller 260. The configurations and operations of the transmitter 220 and the receiver 240 of the UE 200 are identical to those of the transmitter 120 and the receiver 140 of the BS 100 illustrated in FIG. 2. The transmitter 220 encodes and modulates data provided by a data source 210 and transmits the data through an antenna 230. The receiver 240 demodulates and decodes the signal received through the antenna 230 and provides decoded data to a data sink 250.

[44] The controller 260 controls the operation of the UE 200. A memory 270 can store a program for operating the UE 200. Furthermore, the controller 260 can estimate a channel quality indicator and feed back the channel quality indicator to the BS 100.

[45] A signal received by the UE 200 can include a signal transmitted from other UE to the BS or a signal transmitted from the BS to the other UE. The controller 260 listens to the signal and cooperatively transmits the signal to the other UE or the BS using a subcarrier allocated to the UE 200.

[46] FIGS. 4 and 5 illustrate a cooperative transmission method according to an embodiment of the present invention. FIGS. 4 and 5 show uplink transmission. For clarity of description, it is assumed that there are three UEs UEl, UE2 and UE3 and a BS. The number of BSs and the number of UEs are not limited and can be greater than 1.

[47] Referring to FIG. 4, a first UE UEl transmits a signal to the BS using subcarriers

(radio resources) allocated thereto. Here, the first UE UEl becomes a source station and the S becomes a target station. The source station corresponds to a point from which a signal is transmitted and the target station corresponds to a point to which the signal is transmitted.

[48] While the first UE UEl transmits the signal to the BS, neighboring second and third

UEs UE2 and UE3 listen to the signal of the first UE UEl. A dotted line represents that a UE listens to the signal transmitted from another UE.

[49] Referring to FIG. 5, the second and third UEs UE2 and UE3 cooperatively transmit the signal of the first UE UEl to the BS using the same subcarriers as the subcarriers of the first UE UEl. A time diversity gain according to a transmission interval and a user diversity gain according to a plurality of users can be additionally obtained through the signals cooperatively transmitted from the second and third UEs UE2 and UE3. The BS can combine the cooperatively transmitted signals to reproduce the original signal. The BS can use a maximum ratio combining (MRC) technique to combine the cooperatively transmitted signals. [50] Each of the second UEs UE2 and UE3 can be called as a relay station which relay the signal between the source station and the target station. A plurality of UEs can cooperatively transmit a signal using the same subcarriers to obtain the time diversity gain and the user diversity gain.

[51] Cooperatively transmitted signals can be processed by the relay station in various manners. In an embodiment, the second and third UEs UE2 and UE3 can simply amplify received signals and cooperatively transmit the amplified signals. The relay station generates the listened signal by amplifying the received signal and cooperatively transmit the listened signal. In another embodiment, the second and third UEs UE2 and UE3 can reprocess the received signals and cooperatively transmit the reprocessed signals. That is, the relay station decodes the signal of the source station, reprocess the decoded signal, and cooperatively transmit the reprocessed signal. Reprocessing can be performed in various ways. For example, the second and third UEs UE2 and UE3 can demodulate and decode the received signals, encode the decoded data and modulate the encoded data. Otherwise, the second and third UEs UE2 and UE3 can demodulate and decode the received signals, and then transmit only additional information such as incremental redundancy (IR) in hybrid automatic repeat request (HARQ) scheme. In still another embodiment, the second and third UEs UE2 and UE3 can select whether they simply amplify and transmit the received signals or whether they reprocess and transmit the received signals. In still another embodiment, the second and third UEs UE2 and UE3 can use different processing methods. For example, the second UE UE2 can simply amply and cooperatively transmit a received signal and the third UE UE3 can reprocess and cooperatively transmit a received signal.

[52] In general, the BS can recognize a channel gain when a UE correctly estimates the channel gain and transmits feedback information on the channel gain to the BS. Data rates can be stabilized without any feedback information through cooperative transmission.

[53] The relay station can dynamically determine whether the relay station cooperatively transmits signals according to a command signal of the target station. For example, the target station broadcasts success when a signal received from the source station is higher than a predetermined signal-to-noise ratio (SNR), and cooperative transmission is not performed. If the SNR is lower than the predetermined value, the target station broadcasts failure and the relay station can cooperatively transmit listened signals.

[54] FIGs. 6 and 7 illustrate a cooperative transmission method according to another embodiment of the present invention.

[55] Referring to FIG. 6, a first UE UEl transmits a signal to a BS using subcarriers allocated thereto. The first UE UEl corresponds to a source station and the BS corresponds to a target station. While the first UE UEl transmits the signal to the BS, neighboring second and third UEs UE2 and UE3 listen to the signal of the first UE UEl.

[56] Referring to FIG. 7, the second and third UEs UE2 and UE3 cooperatively transmit the listened signal to the BS using different subcarriers. Each of the first and second UEs UE2 and UE3 corresponds to a relay station which relays the listened signal. The BS combines cooperatively transmitted signals for the respective subcarriers to reproduce the original signal.

[57] When the first and second UEs UE2 and UE3 performs cooperative transmission using different subcarriers, frequency diversity gain in addition to time diversity gain can be obtained.

[58] FIGs. 8 and 9 illustrate a cooperative transmission method according to another embodiment of the present invention. FIGs. 8 and 9 show that a plurality of UEs perform uplink transmission. A plurality of source stations is arranged.

[59] Referring to FIG. 8, a BS respectively allocates subcarriers to first, second and third

UEs UEl, UE2 and UE3. The first, second and third UEs UEl, UE2 and UE3 transmit signals to the BS using the subcarriers allocated thereto. The first, second and third UEs UEl, UE2 and UE3 correspond to source stations and the BS corresponds to a target station. While the first UE UEl transmits its own signal to the BS, the neighboring second and third UEs UE2 and UE3 listen to the signal of the first UE UEl. Furthermore, the first and third UEs UEl and UE3 listen to the signal of the second UE UE2 while the second UE UE2 transmits its own signal to the BS. The first and second UEs UEl and UE2 listen to the signal of the third UE UE3 while the third UE UE3 transmits its own signal to the BS.

[60] Referring to FIG. 9, the first UE UEl cooperatively transmits the listened signals of the second and third UEs UE2 and UE3 to the BS. The first UE UEl can transmit the listened signals through the the subcarriers allocated to the listened signals. For example, the first UE UEl can transmit the listend signal of the second UE UE2 on the same subcarriers as the subcarriers of the second UE UE2 and transmit the listened signal of the third UE UE3 on the same subcarriers as the subcarriers of the third UE UE3. The second UE UE2 cooperatively transmits the listened signals of the first and third UEs UEl and UE3 to the BS respectively using the same subcarriers as the subcarriers of the first and third UEs UEl and UE3. The third UE UE3 cooperatively transmits the listened signals of the first and second UEs UEl and UE2 to the BS respectively using the same subcarriers as the subcarriers of the first and the second UEs UEl and UE2.

[61] When there are multiple source stations, listened signals can be cooperatively transmitted using subcarriers respectively corresponding to the multiple source stations to obtain additional diversity gain.

[62] FIGs. 10 and 11 illustrate a cooperative transmission method according to another embodiment of the present invention.

[63] Referring to FIG. 10, a BS respectively allocates subcarriers to first, second and third

[64] Referring to FIG. 11, the first, second and third UEs UEl, UE2 and UE3 cooperatively transmit the listened signals using the subcarriers respectively allocated thereto. The first UE UEl transmits the listened signals of the second and third UEs UE2 and UE3 to the BS. The first UE UEl can transmit an identifier for discriminating the signal of the second UE UE2 from the signal of the third UE UE3. The second UE UE2 transmits the listened signals of the first and third UEs UEl and UE3 to the BS. The third UE UE3 transmits the listened signals of the first and second UEs UEl and UE2 to the BS.

[65] FIGs. 12 and 13 illustrate a cooperative transmission method according to another embodiment of the present invention.

[66] Referring to FIG. 12, a BS transmits a signal to a first UE UEl using allocated subcarriers. The BS corresponds to a source station and the first UE UEl corresponds to a target station. While the BS transmits the signal to the first UE UEl, neighboring second and third UEs UE2 and UE3 listen to the signal of the BS.

[67] Referring to FIG. 13, the second and third UEs UE2 and UE3 cooperatively transmit the listened signal to the first UE UEl using the same subcarriers as the subcarriers of the listened signal. The second and third UEs UE2 and UE3 become relay stations for relaying the signal between the source station and the target station. The first UE UEl combines the signals cooperatively transmitted from the second and third UEs UE2 and UE3 to reproduce the original signal. An MRC technique can be used to combine the cooperatively transmitted signals.

[68] Signals can be cooperatively transmitted using the same subcarriers to obtain time diversity gain.

[69] FIGs. 14 and 15 illustrate a cooperative transmission method according to another embodiment of the present invention. [70] Referring to FIG. 14, a BS transmits a signal to a first UE UEl using allocated subcarriers. The BS corresponds to a source station and the first UE UEl corresponds to a target station. While the BS transmits the signal to the first UE UEl, neighboring second and third UEs UE2 and UE3 listen to the signal of the BS.

[71] Referring to FIG. 15, the second and third UEs UE2 and UE3 cooperatively transmit the listened signal to the first UE UEl using different subcarriers. The first UE UEl combines the signals cooperatively transmitted from the second and third UEs UE2 and UE3 for respective subcarriers to reproduce the original signal. An MRC technique can be used to combine the cooperatively transmitted signals.

[72] The second and third UEs UE2 and UE3 can cooperatively transmit a signal using different subcarriers to obtain frequency diversity gain in addition to time diversity gain.

[73] FIGs. 16 and 17 illustrate a cooperative transmission method according to another embodiment of the present invention. FIGs. 16 and 17 show that a BS performs downlink transmission to a plurality of UEs and a plurality of target stations are arranged.

[74] Referring to FIG. 16, first, second and third UEs UEl, UE2 and UE3 respectively receive signals from a BS. The BS can transmit the same signal or different signals to the first, second and third UEs UEl, UE2 and UE3. The first, second and third UEs UEl, UE2 and UE3 receive the signals from the BS using subcarriers respectively allocated thereto. The BS corresponds to a source station and the first, second and third UEs UEl, UE2 and UE3 correspond to target stations. While the BS transmits a signal to the first UE UEl, the neighboring second and third UEs UE2 and UE3 listen to the signal transmitted from the BS to the first UE UEl. While the BS transmits a signal to the second UE UE2, the neighboring first and third UEs UEl and UE3 listen to the signal transmitted from the BS to the second UE UE2. While the BS transmits a signal to the third UE UE3, the first and second UEs UEl and UE2 listen to the signal transmitted from the BS to the third UE UE3.

[75] Referring to FIG. 17, the first UE UEl transmits the listened signal of the second UE

UE2 to the second UE UE2 and cooperatively transmits the signal of the third UE UE3 to the third UE UE3. The first UE UEl can transmit the listened signal on the subcarriers of the listened signal. For example, the first UE UEl can transmit the listened signal of the second UE UE2 on the same subcarriers as the subcarriers of the second UE UE2 and transmit the listened signal of the third UE UE3 on the same subcarriers as the subcarriers of the third UE UE3.

[76] The second UE UE2 cooperatively transmits the listened signal of the first UE UEl to the first UE UEl using the same subcarriers as the subcarriers of the first UE UEl and cooperatively transmits the listened signal of the third UE UE3 to the third UE UE3 using the same subcarriers as the subcarriers of the third UE UE3.

[77] The third UE UE3 cooperatively transmits the listened signal of the first UE UEl to the first UE UEl using the same subcarriers as the subcarriers of the first UE UEl and cooperatively transmits the listened signal of the second UE UE2 to the second UE UE2 using the same subcarriers as the subcarriers of the second UE UE3.

[78] When there are multiple target stations, relay stations can cooperatively transmit signals using subcarriers respectively corresponding to the target stations to obtain diversity gain.

[79] Simulation results with respect to cooperative transmission according to the present invention and a conventional art are described below. A system environment includes a cell diameter of IKm, and user density is controlled while the number of users in a cell is increased. The user density corresponds to the number of users/cell width.

[80] FIG. 18 is a graph showing the relationship between user density and system capacity, and FIG. 19 is a graph showing the relationship between user density and system capacity per user. The system capacity per user is obtained by dividing system performance by the number of users.

[81] Referring to FIGs. 18 and 19, in the proposed technique the system capacity abruptly increases as the user density increases, compared to the conventional technique. In the conventional technique which does not perform cooperative transmission, a data rate per user is remarkably reduced when the number of users becomes greater than the number of subcarriers. However, when cooperative transmission is carried out, the system capacity increases as the number of users increases because of a user diversity gain.

[82] While a single-input single-output (SISO) system having a single transmission antenna and a single receiving antenna has been described above, the technical spirit of the present invention can be applied to a multiple-input multiple-output (MIMO) system.

[83] The steps of a method described in connection with the embodiments disclosed herein may be implemented by hardware, software or a combination thereof. The hardware may be implemented by an application specific integrated circuit (ASIC) that is designed to perform the above function, a digital signal processing (DSP), a programmable logic device (PLD), a field programmable gate array (FPGA), a processor, a controller, a microprocessor, the other electronic unit, or a combination thereof. A module for performing the above function may implement the software. The software may be stored in a memory unit and executed by a processor. The memory unit or the processor may employ a variety of means that is well known to those skilled in the art.

[84] As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims. Therefore, all changes and modifications that fall within the metes and bounds of the claims, or equivalence of such metes and bounds are intended to be embraced by the appended claims.

[85]

[86]

[87]

[88]

Claims

[1] A method for enabling a user equipment to perform cooperative transmission in multi-carrier system, the method comprising: obtaining a listened signal by listening to a signal transmitted from another user equipment to a base station; and transmitting the listened signal to the base station, wherein the signal and the listened signal are allocated to the same subcarriers. [2] The method of claim 1, wherein the listened signal is generated by amplifying the signal transmitted from another user equipment to the base station. [3] The method of claim 1, wherein the listened signal is generated by reprocessing the signal transmitted from another user equipment to the base station. [4] A method for enabling a user equipment to perform cooperative transmission in multi-carrier system, the method comprising: generating a listened signal by listening to a signal transmitted from another user equipment to a base station; and transmitting the listened signal to the base station, wherein the signal and the listened signal are allocated to the different subcarriers. [5] A method for relaying a signal transmitted from a source station to a target station in wireless communication system, the method comprising: obtaining a listened signal by listening the signal transmitted from the source station to the target station; and transmitting the listened signal to the target station using the same subcarriers as the subcarriers of the the signal. [6] The method of claim 5, the source station is a user equipment and the target station is a base station. [7] The method of claim 5, the source station is a base station and the target station is a user equipment. [8] The method of claim 5, the source station is a first user equipment and the target station is a second user equipment.