Patent Application
20060203711
This application claims priority under 35 U.S.C. §119 to an application filed in the Korean Intellectual Property Office on Mar. 9, 2005 and assigned Serial No. 2005-19849, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an apparatus and method for mapping data in a broadband wireless communication system, and in particular, to an apparatus and method for allocating space-time coded data to subcarriers in an Orthogonal Frequency Division Multiple Access (OFDMA) communication system.
2. Description of the Related Art
The fundamental issue in communications is how efficiently and reliably data is transmitted on channels. Today, mobile communication systems are being developed toward high-speed, high-quality wireless data packet communication systems to additionally provide data service and multimedia service beyond voice-oriented service.
In the wireless channel environment of a mobile communication system, unlike that of a wired channel environment, a transmission signal inevitably experiences loss due to several factors such as multipath interference, shadowing, wave attenuation, time-variant noise, and fading.
The information loss causes a severe distortion to the transmission signal, degrading the whole system performance. In order to reduce the information loss, many error control techniques may be adopted to thereby increase system reliability. The basic error control technique is to use an error correction code.
Multipath fading is relieved by diversity techniques in the wireless communication system. The diversity techniques are broken up into time diversity, frequency diversity, and antenna diversity.
The antenna diversity uses multiple antennas. This diversity scheme is further branched into receive (Rx) antenna diversity using a plurality of Rx antennas, transmit (Tx) antenna diversity using a plurality of Tx antennas, and Multiple-Input Multiple-Output (MIMO) using a plurality of Tx antennas and a plurality of Rx antennas.
The MIMO is a special case of Space-Time Coding (STC) that extends coding of the time domain to the space domain by transmission of a signal encoded in a predetermined coding method through a plurality of Tx antennas, with the aim to achieve a lower error rate.
V. Tarokh, et al. proposed Space-Time Block Coding (STBC) as one of methods of efficiently applying antenna diversity (see “Space-Time Block Coding from Orthogonal Designs”, IEEE Trans. On Info., Theory, Vol. 45, pp. 1456-1467, July 1999). The Tarokh STBC scheme is an extension of the transmit antenna diversity scheme of S. M. Alamouti (see, “A Simple Transmit Diversity Technique for Wireless Communications”, IEEE Journal on Selected Area in Communications, Vol. 16, pp. 1451-1458, October 1988), for two or more Tx antennas. The Alamouti STBC scheme advantageously offers a maximum diversity order, that is a diversity order equal to the number of transmit antennas without data rate loss, even though complex symbols are transmitted through two Tx antennas.
Referring to
The S/P converter 102 parallelizes the serial modulation symbols received from the modulator 100, s1, s2 , s3, s4. The STBC coder 104 creates eight symbol combinations by STBC-encoding the four modulation symbols, s1, s2 , s3, s4 and sequentially transmits them through the four Tx antennas 106 to 112. A coding matrix used to generate the eight symbol combinations is expressed as Equation (1):
where G4 denotes the coding matrix for symbols transmitted through the four Tx antennas 106 to 112 and s1, s2 , s3, s4 denote the input four symbols to be transmitted. The number of columns in the coding matrix is equal to that of the Tx antennas and the number of rows corresponds to the time required to transmit the four symbols. Thus, the four symbols are transmitted through the four Tx antennas for eight time intervals.
Specifically, for a first time interval, s, is transmitted through the first Tx antenna 106, s2 through the second Tx antenna 108, s3 through the third Tx antenna 110, and s4 through the fourth Tx antenna 112. In this manner, −s4*, −s3*, s2*, s1* are transmitted through the first to fourth Tx antennas 106 to 112, respectively for an eighth time interval. That is, the STBC encoder 104 sequentially provides the symbols of an ith column in the coding matrix to an ith Tx antenna.
As described above, the STBC encoder 104 generates the eight symbol sequences using the input four symbols and their conjugates and negatives, then transmits them through the four Tx antennas 106 to 112 for eight time intervals. Since the symbol sequences for the respective Tx antennas, that is the columns of the coding matrix are mutually orthogonal, as high a diversity gain as a diversity order is achieved. However, since four complex symbols are transmitted for eight time intervals, the data rate is ½. This implies that 2N time intervals are taken to transmit N symbols, thereby increasing latency and decreasing the data rate.
To solve the above problem, a Full Diversity Full Rate (FDFR) STBC scheme for four Tx antennas was proposed.
The modulator 200 generates complex symbols s1, s2 , s3, s4 by modulating input information data (or coded data) in a predetermined modulation scheme. The S/P converter 202 groups the four complex symbols by two and outputs two vectors each including two elements, [S1, s2]T and [s3, s4]T to the Alamouti encoder 206 and the delay 204, respectively.
The delay 204 delays the second vector [s3, s4]T for one time interval. Thus, the first vector [s1, s2]T is provided to the Alamouti encoder 206 in a first time interval and the second vector [s3, s4]T is provided to the Alamouti coder 208 in a second time interval. The Alamouti encoder refers to an encoder that operates in the Alamouti STBC scheme.
The Alamouti encoder 206 encodes [s1, s2]T so that it is transmitted through the first and second Tx antennas 210 and 212 for first and second time intervals. The Alamouti coder 208 encodes [s3, s4]T so that it is transmitted through the third and fourth Tx antennas 214 and 216 for third and fourth time intervals. A coding matrix used to transmit the outputs of the Alamouti encoders 206 and 208 through the multiple antennas is shown in Equation (2):
where an ith column represents an ith time interval and a jth row represents a jth transmit antenna.
To be more specific, s1 and s2 are transmitted through the first and second Tx antennas 210 and 212, respectively for a first time interval. Symbols −s2*, and s1* are transmitted through the first and second Tx antennas 210 and 212, respectively for a second time interval. Symbols s3 and s4 are transmitted through the third and fourth Tx antennas 214 and 216, respectively for a third interval. Symbols −s4* and s3* are transmitted through the third and fourth Tx antennas 214 and 216, respectively for a fourth time interval.
As described above, Spatial Diversity (SD) achieves transmit diversity by transmitting the same data through multiple antennas. However, the diversity order increases with the number of transmit antennas but decreases a gain increase rate. In other words, as the number of antennas increases, the diversity order is saturated rather than linearly increasing.
In contrast, Spatial Multiplexing (SM) offers the benefit of high-speed data transmission without increasing the bandwidth of the system by transmitting different data at the same time through multiple antennas in the transmitter and the receiver.
The modulator 300 modulates input information data (or coded data) in a predetermined modulation scheme. The S/P converter 302 spatially multiplexes four modulation symbols s1, s2 , s3, s4 as Equation (3):
where the number of columns is equal to that of Tx antennas, and the number of rows is equal to the time required for transmitting the four transmission symbols. Since four symbols are transmitted for one time interval, the data rate is 4.
Meanwhile, the reception part 312 of the receiver estimates the transmitted symbols s1, s2 , s3, s4 from signals received through the four Rx antennas 314, 316, 318 and 320.
Orthogonal Frequency Division Multiplexing (OFDM) is promising for reducing channel fading in 4th generation mobile communication systems. Particularly, multi-user OFDM systems are under consideration in which a plurality of users are simultaneously supported and identified in the frequency domain. Since channel variation in the frequency domain must be considered in the OFDM system, spatial antenna diversity is inevitable. Accordingly, there exists a need for a technique of effectively mapping STC encoded data symbols to subcarriers in the case where the above-described antenna transmission methods (MIMO or STC) are applied to OFDM systems.
An object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an object of the present invention is to provide an apparatus and method for mapping STC encoded data symbols to subcarriers in an OFDMA wireless communication system. The above object is achieved by providing a method of mapping STC encoded data symbols to subcarriers in a broadband wireless communication system using multiple antennas.
According to one aspect of the present invention, in a transmission method in a broadband wireless communication system using multiple antennas, transmission symbols are encoded according to a predetermined space-time coding matrix and the coded data symbols are mapped to time-space-subcarriers according to a mapping rule determined by the space-time coding matrix.
According to another aspect of the present invention, in a transmitter in a broadband wireless communication system using multiple antennas, a space-time encoder encodes transmission symbols according to a predetermined space-time coding matrix, and a mapper maps the coded data symbols received from the space-time encoder to time-space-subcarriers according to a mapping rule determined by the space-time coding matrix.
The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:
Embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.
The present invention provides a method of mapping STC (Space Time Coding) encoded data symbols to subcarriers in a MIMO-OFDMA wireless communication system. For example, the following description is made in the context of STC schemes compliant with IEEE 802.16e.
The STC encoded data symbols mapping will be described first in relation to Adaptive Modulation and Coding (AMC) permutation in STC zone and then in relation to diversity permutation (or Optional Full Usage of Subcarriers (O-FUSCs)). In the former case, a minimum unit (i.e. slot) that supports MIMO is a 2×6 (2 bins×6 OFDM symbols) structure. In the latter case, a 1×2 (1 subchannel×2 OFDM symbols) structure is used.
How pilots are mapped to subcarriers in a MIMO-OFDMA wireless communication system will be described below.
In MIMO for two Tx antennas, antenna #1 transmits pilots in odd-numbered OFDM symbols and antenna #2 transmits pilots in even-numbered OFDM symbols. As illustrated in
Hereinbelow, basic pilot positions are defined as the positions of subcarriers for antenna #1 and antenna #2, and additional pilot positions are defined as the positions of subcarriers for antenna #3 and antenna #4.
When pilots are mapped in the manners illustrated in
At first, a data mapping rule in the case of an AMC permutation will be described. First for two Tx antennas, a coding matrix A for a rate-1 STBC is given as Equation (4):
where the rows represent antenna indexes and the columns represent OFDM symbol indexes.
The STC encoded data symbols expressed as the above matrix is mapped to subcarriers in the following manner.
For Ant #2, encoded data expressed in the second row of the coding matrix of Equation (4) is mapped to subcarriers in the above manner.
For four Tx antennas, a coding matrix A for an STBC is given as Equation (5):
where the rows represent antenna indexes and the columns represent time and subcarriers. In a slot, the first two columns are mapped to odd-numbered subcarriers and the second two columns are mapped to even-numbered subcarriers. The first and third columns are mapped in odd-numbered OFDM symbols and the second and fourth columns are mapped in even-numbered OFDM symbols.
Subcarrier indexes are decided for data mapping by counting the remaining subcarriers except pilot subcarriers. If a CC (Convolutional code) is used, the remaining subcarriers except the basic pilot positions are counted. If a CTC (Convolutional Turbo Code) is used, the remaining subcarriers except the main and additional pilot positions are counted. Therefore, the odd-numbered subcarriers or the even-numbered subcarriers may change physically in position depending on the channel code used.
The STC encoded data symbols expressed as the above matrix is mapped to subcarriers in the following manner.
For Ant #2, Ant #3, and Ant #4, coded data expressed in the second, third and fourth rows of the coding matrix of Equation (5) are mapped to subcarriers, respectively in the above manner.
In
For an SM code, coding matrices C are given as Equation (6):
where the rows represent antenna indexes.
STC encoded data symbols expressed as the matrices C are mapped to subcarriers as follows.
In
In
As described above, the data mapping is performed in the same manner as illustrated in
Now a data mapping rule in the case of an O-FUSC permutation will be described. STC encoded data symbols expressed as Equation (4) is mapped to subcarriers as follows. As described above, the minimum MIMO-supporting unit (slot) is of a 1×2 (1 subchannel×2 OFDM symbols) structure on the O-FUSC permutation.
STC encoded data symbols are sequentially allocated on a slot basis, starting from a slot with the lowest index. Since two OFDM symbols are considered one group for an STBC, one time interval is defined in one slot. For SM, one OFDM symbol forms one group and thus two time intervals are defined in one slot.
Data is mapped in one slot in the following steps:
(1) A slot (1 subchannel×two symbols) is allocated according to the number of allocated subchannels.
(2) Data mapping starts from a slot with a lower index.
(3) Data mapping starts from the first time interval (the first symbol in SM).
(4) Data mapping is performed from a lower-index subcarrier to a higher-index subcarrier (48 subcarriers).
(5) Data mapping is performed according to a coding matrix based on MIMO. When the CC is used, no data is allocated to additional pilot positions (i.e. pilot positions for Ant #3 and Ant #4). Instead, data is allocated to the following data subcarrier positions. Consequently, the last part of the whole data is not transmitted.
(6) Data mapping of steps (4) and (5) is repeated for the second time interval (i.e. the second symbol), which is confined to SM.
(7) The above steps are sequentially performed for all other slots.
(8) When the CC is used, data at the additional pilot positions are punctured after the steps of (1) to (7). For Ant #3 and Ant #4, pilot symbols are allocated at the punctured positions. When the CTC is used, pilot symbols are allocated at the additional pilot positions for Ant #3 and Ant #4, after all steps.
Data mapping is carried for the O-FUSC permutation in the same manner as for the band AMC channel.
An example of the steps (5) to (8) for a system with four Tx antennas will be taken.
Parameters for the O-FUSC permutation are defined as
CELL_ID: 1
pilot subcarrier positions=9k+1(k=0, 1, 2, . . . )
subchannel index=0
the number of subchannels=1.
According to the above parameters, the following subcarrier position table is presented for 48 data subcarriers in subchannel #0:
{3 23 46 58 79 104 121 132 156 170 196 215 231 245 254 273 293 316 328 349 374 391 402 426 440 466 485 501 515 524 543 563 586 598 619 644 661 672 696 710 736 755 771 785 794 813 833 856}.
Among the subcarrier indexes, additional pilot subcarrier positions are {245 254 515 524 785 794}.
When the CC is used, after data is mapped to the above 48 subcarriers, data are punctured at the additional pilot subcarrier positions. For Ant #3 and Ant #4, additional pilot symbols are allocated at the punctured subcarrier positions.
When the CTC is used, data truncation is required. Thus, the above subcarrier positions are changed, thereby creating a new subcarrier position table. The additional pilot subcarrier positions {245 254 515 524 785 794} are skipped and as many Null data as the number of the additional pilot subcarrier positions are added at the last of the table.
For the CTC, the data subcarrier position table except the additional pilot subcarrier positions is: {3 23 46 58 79 104 121 132 156 170 196 215 231 273 293 316 328 349 374 391 402 426 440 466 485 501 543 563 586 598 619 644 661 672 696 710 736 755 771 813 833 856 Null Null Null Null Null Null}.
Null means shifting to the next subchannel without data allocation. That is, only 42 data among 48 data are allocated and the remaining data is allocated to the second subchannel. The number of Nulls may vary with parameters and thus it is preferable to define such tables as illustrated above for all parameters.
If the tables are prepared for all parameters (number of subchannels×pilot positions (3)), when the CTC is used and additional pilots are allocated, data mapping becomes easy.
A description will now be made of the configuration of a MIMO-OFDMA wireless communication system.
In operation, the encoder 1001 encodes an input information bit stream at a predetermined coding rate. Given k information bits and a coding rate R, the number of code symbols is k/R. The encoder 1001 can be a convolutional encoder, a turbo encoder, or a Low Density Parity Check (LDPC) encoder.
The modulator 1002 generates complex symbols by mapping the code symbols received from the encoder 1001 to signal points in a predetermined modulation scheme. The modulation scheme can be BPSK, QPSK, 8QAM, 16QAM or 64QAM. One bit (s=1) is mapped to one signal point (complex signal) in BPSK, two bits (s=2) to one complex signal in QPSK, three bits (s=3) to one complex signal in 8QAM, four bits (s=4) to one complex signal in 16QAM, and six bits (s=6) to one complex signal in 64QAM.
The STC encoder 1003 encodes the complex symbols according to a predetermined STC coding matrix, thereby mapping them in the time-space-frequency domain. Symbols mapped to the first antenna are provided to the first subcarrier mapper 1004 and symbols mapped to the Nth antenna are provided to the Nth subcarrier mapper 1005.
The subcarrier mappers 1004 to 1005 map the received symbols to subcarriers and output the mapped symbols to corresponding IFFT processors. More specifically, the subcarrier mappers 1004 to 1005 output the received symbols to corresponding subcarrier positions of the IFFT processors in corresponding time intervals according to a mapping rule based on the coding matrix. They use lookup tables (e.g. FIGS. 6 to 9) or indexes generated in a predetermined rule in symbol mapping in the time-frequency domain.
The IFFT processors 1006 to 1007 generate time-domain samples data (i.e. an OFDM symbol) by IFFT-processing the symbols received from the corresponding subcarrier mappers 1004 to 1005.
The filters 1008 to 1009 band-pass-filter the sample data. The DACs 1010 to 1011 convert the filtered sample data to analog signals. The RF processors 1012 to 1013 process the analog signals to RF signals and transmit the RF signals through the corresponding Tx antennas 1014 to 1015.
As described above, the present invention allows for MIMO in an OFDMA wireless communication by proposing the method of mapping STC encoded data symbols to subcarriers. Particularly, STC encoded data symbols can be efficiently transmitted through a plurality of antennas on a band AMC subchannel compliant with IEEE 802.16e for multiple antennas.
While the invention has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-0019849 | Mar 2005 | KR | national |
