Patent Application
20060093066
This application claims priority under 35 U.S.C. § 119 to an application entitled “Apparatus And Method For Space-Time-Frequency Block Coding” filed in the Korean Intellectual Property Office on Nov. 4, 2004 and assigned Serial No. 2004-89507, the contents of which are herein incorporated by reference.
1. Field of the Invention
The present invention relates generally to a transmitter in a communication system supporting a data rate of 2 for three transmit antennas (a 3 Tx-rate 2 communication system), and in particular, to a transmitter for maximizing a coding gain by transmitting an input symbol sequence through a plurality of transmit antennas according to feedback Channel Quality Indicators (CQIs) from a receiver in a 3 Tx-rate 2 communication system.
Specifically, the present invention relates to a transmitter for space-time-frequency block coding (space-time block coding or space-frequency block coding) a transmission signal based on feedback channel information from a receiver and transmitting the space-time-frequency block coded signal (the space-time block coded signal or space-frequency block coded signal) through three antennas in order to realize a high-quality, highly reliable communication system.
A data rate is defined as the number of transmitted data units per unit of time in space-time block coding (STBC), as the number of transmitted data units per unit of resource (frequency and time) in space-time-frequency block coding (STFBC), and as the number of transmitted data units per frequency and time in space-frequency block coding (SFBC). In the case where eight data units are transmitted for four unit time intervals in STBC, eight data units are transmitted at two unit frequencies for two unit time intervals in STFBC, or eight data units are transmitted at four unit frequencies for one unit time interval in SFBC, the common data rate is 2.
2. Description of the Related Art
The fundamental issue in communications is how efficiently and reliably data is transmitted on channels. As high-speed communication systems capable of transmitting a variety of information including video and wireless data beyond the early-stage voice service is required for future-generation multimedia mobile communications, it is very significant to increase system efficiency by use of a channel coding method suitable for a system.
Generally, 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 can cause a severe distortion to the transmission signal, degrading the entire system performance. In order to reduce the information loss, many error control techniques are usually adopted to increase system reliability. One of the basic techniques is to use an error correction code.
Multipath fading is corrected for by diversity techniques in the wireless communication system. The diversity techniques are classified into time diversity, frequency diversity, and antenna diversity.
The antenna diversity uses multiple antennas. This diversity scheme is further classified into receive (Rx) antenna diversity using a plurality of Rx antennas, 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 transmitting 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 STBC as one of methods of efficiently applying antenna diversity, as disclosed in 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 for two or more Tx antennas as disclosed in A Simple Transmit Diversity Technique for Wireless Communications, IEEE Journal on Selected Area in Communications, Vol. 16, pp. 1451-1458, October 1988.
Another approach to the efficient application of the antenna diversity is to use STFBC.
Referring to
The S/P converter 104 converts the serial modulation symbols received from the modulator 102 into parallel modulation symbols, s1, s2, s3, s4, s5, s6, s7, s8. The STFBC coder 106 STFBC-encodes the eight modulation symbols and transmits them through the three Tx antennas 108, 110 and 112. A coding matrix used to generate a combination of the eight symbols at a data rate of 2 is expressed as
where A denotes the coding matrix for symbols to be transmitted through the three Tx antennas and s1, s2, s3, s4, s5, s6, s7, s8 denote the input eight symbols.
The rows of the coding matrix represent the Tx antennas and the columns represent time and frequencies at which the eight symbols are to be transmitted. The first two columns are transmitted at frequency f1, while the last two columns are transmitted at frequency f2. The first column of each of the first and last two columns is transmitted at time t1 and the second column is transmitted at time t2. Thus, the eight symbols are transmitted through the three Tx antennas at the two frequencies at the two time intervals.
Transmission of the columns is not limited to the time-frequency plane. The elements of the columns can be transmitted for different time intervals. That is, as the eight data units are transmitted for four time intervals through the three Tx antennas by mapping the four columns to time t1, t2, t3 and t4, respectively, the data rate of 2 is also achieved. Instead of the time plane, the columns can be transmitted in the frequency plane. Specifically, the four columns are transmitted at four frequencies f1, f2, f3 and f4, respectively, thus at the data rate of 2.
As described above, the STFBC coder 104 transmits the input eight symbols at two frequencies at two time intervals through the three Tx antennas 108, 110 and 112, or for four time intervals through the three Tx antennas 108, 110 and 112, or at four frequencies through the three Tx antennas 108, 110 and 112.
The Alamouti STFBC scheme offers the benefit of a maximum diversity order equal to the number of Tx antennas without a loss in the data rate even though complex symbols are transmitted through two Tx antennas. This implies that the symbols s1, s2, s3, s4 transmitted by the Alamouti scheme have the diversity effect and thus they can be effectively recovered. However, since the non-Alamouti symbols s5, s6, s7, s8 are excluded from the diversity effect, they are not effectively recovered and have a higher error probability than s1, s2, s3, s4.
The receiver is comprised of a plurality of Rx antennas 200, 202 and 204, an STFBC decoder 206, a channel estimator 208, a detector 210, and a decoder 212.
Referring to
The channel estimator 208 estimates channel coefficients representing channel gains from the Tx antennas 108 to 112 to the Rx antennas 200 to 204 using the signals received from the first to Pth Rx antennas 200 to 204.
The STFBC decoder 206 estimates the input data based on the signals received from the Tx antennas 108, 110 and 112 by STFBC decoding corresponding to the STFBC coding that maps four of eight symbols to two Tx antennas by the Alamouti scheme and maps the remaining four symbols to the other Tx antenna.
The detector 210 generates hypothesis symbols of the transmitted symbols based on the STFBC-decoded symbols and the channel coefficients. The hypothesis symbols are obtained by calculating a decision statistic over all possible symbols transmittable from the transmitter.
The decoder 212 decodes the hypothesis symbols in a decoding method corresponding to the coding scheme used in the encoder 100 of the transmitter, thereby recovering the original information data bits.
The distinctive drawback of the above-described conventional coding scheme is that diversity is achieved for the Alamouti-coded symbols s1, s2, s3, s4 but not for the non-Alamouti-coded symbols s5, s6, s7, s8 and thus s5, s6, s7, s8 are hard to recover and have a higher error probability, as compared to s1, s2, s3, s4.
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 a transmission apparatus and method with improved performance in a 3 Tx-rate 2 mobile communication system.
The above object is achieved by providing a transmitter for maximizing coding gain by transmitting an input symbol sequence through a plurality of transmit antennas according to feedback CQIs from a receiver in a 3 Tx-rate 2 communication system.
According to one aspect of the present invention, in a transmitter in a communication system supporting a data rate of 2 for three transmit antennas, an S/P converter converts serial modulated data to parallel modulated data. An STBC coder mode decider (an SFBC coder mode decider or an STFBC coder mode decider) determines a transmission mode based on the channel quality indicator of each transmit antenna fed back from a receiver. An STBC coder (an SFBC coder or an STFBC coder) transmits the parallel modulated data through the three transmit antennas according to the determined transmission mode.
According to another aspect of the present invention, in a transmission method in a communication system supporting a data rate of 2 for three transmit antennas, serial modulated data is converted to parallel modulated data. A transmission mode index indicating a transmission mode determined by the receiver is received from the receiver. The parallel modulated data is space-time block coded (space-frequency block coded or space-time-frequency block coded) according to the determined transmission mode, and transmitted through the three transmit antennas.
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:
A preferred embodiment 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 is intended to provide an STFBC coding apparatus in a transmitter of a Tx-rate 2 communication system, more specifically a method and apparatus for generating an STFBC in the manner that maximizes the coding gain of the STFBC in a transmission scheme where an input symbol sequence is transmitted through a plurality of Tx antennas.
While the present invention is described in the context of an STFBC coding apparatus, it is also applicable to an STBC coding apparatus or an SFBC coding apparatus.
Referring to
The components except the STFBC coder mode decider 314 operate in the same manner as their counterparts illustrated in
A coding matrix for transmission mode 3 in the STFBC coder 306 is given as
where the rows of the coding matrix represent the Tx antennas and the columns represents time and frequencies at which the eight symbols are transmitted. The data of the first two columns are transmitted at frequency f1, while that of the last two columns at frequency f2. The data of the first column in each pair is transmitted at time t1 and that of the second column is transmitted at time t2. Specifically, the data of the first column is carried at frequency f1 and time t1, the data of the second column at frequency f1 and time t2, the data of the third column at frequency f2 and time t1, and the data of the fourth column at frequency f2 and time t2. Thus, the eight symbols are transmitted through the three Tx antennas at two frequencies for two time intervals.
Alternatively, the data of the first two columns can be transmitted at time t1, while that of the last two columns at time t2. The data of the first column in each pair is transmitted at frequency f1 and that of the second column is transmitted at frequency f2. Therefore, the data of the first column is carried at frequency f1 and time t1, the data of the second column at frequency f2 and time t1, the data of the third column at frequency f1 and time t2, and the data of the fourth column at frequency f2 and time t2.
As stated earlier, data transmission according to the coding matrix is not limited to the above time-frequency plane.
Thus transmission of the four columns at different time can be contemplated. That is, the data of the first to fourth columns are carried at time t1 to t4, respectively. In this case, the STFBC coder 306 is replaced by an STBC coder.
Also the columns can be transmitted at different frequencies. That is, the data of the first to fourth columns are carried at frequencies f1 to f4; respectively. In this case, the STFBC coder 306 is replaced by an SFBC coder.
Since the rows of the coding matrix represent the Tx antennas, the data of the first row is transmitted through the first Tx antenna 308, the data of the second row through the second Tx antenna 310, and the data of the third row through the third Tx antenna 312.
Consequently, the symbols s5, s6, s7, s8 are transmitted through the third Tx antenna 312. In the case where the channel gain of the third Tx antenna 312 is low, the symbols s5, s6, s7, s8 are hard to recover at the receiver. The transmission performance can be improved by transmitting the symbols s5, s6, s7, s8 through a Tx antenna having the highest channel gain.
Assuming that the first Tx antenna 308 has the highest channel gain, the data is preferably transmitted by the following coding matrix so that data recovery is facilitated at the receiver and the gain is improved. This transmission mode is set as transmission mode 1.
where the rows of the coding matrix represent the Tx antennas and the columns represents time and frequencies used to transmit the eight symbols.
The data of the first two columns are transmitted at frequency f1, while that of the last two columns at frequency f2. The data of the first column in each pair is transmitted at time t1 and that of the second column is transmitted at time t2. Specifically, the data of the first column is carried at frequency f1 and time t1, the data of the second column at frequency f1 and time t2, the data of the third column at frequency f2 and time t1, and the data of the fourth column at frequency f2 and time t2. Thus, the eight symbols are transmitted through the three Tx antennas at two frequencies for two time intervals.
Alternatively, the data of the first two columns can be transmitted at time t1, while that of the last two columns at time t2. The data of the first column in each pair is transmitted at frequency f1 and that of the second column is transmitted at frequency f2. Therefore, the data of the first column is carried at frequency f1 and time t1, the data of the second column at frequency f2 and time t1, the data of the third column at frequency f1 and time t2, and the data of the fourth column at frequency f2 and time t2.
As stated earlier, data transmission according to the coding matrix is not limited to the above time-frequency plane.
Thus transmission of the four columns at different time can be contemplated. That is, the data of the first to fourth columns are carried at time t1 to t4, respectively. In this case, the STFBC coder 306 is replaced by an STBC coder.
Also the columns can be transmitted at different frequencies. That is, the data of the first to fourth columns are carried at frequencies f1 to f4, respectively. In this case, the STFBC coder 306 is replaced by an SFBC coder.
Since the rows of the coding matrix represent the Tx antennas, the data of the first row is transmitted through the first Tx antenna 308, the data of the second row through the second Tx antenna 310, and the data of the third row through the third Tx antenna 312.
Assuming that the second Tx antenna 310 has the highest channel gain, the data is preferably transmitted by the following coding matrix. This transmission mode is set as transmission mode 2.
where the rows of the coding matrix represent the Tx antennas and the columns represents time and frequencies taken to transmit the eight symbols.
The data of the first two columns are transmitted at frequency f1, while that of the last two columns at frequency f2. The data of the first column in each pair is transmitted at time t1 and that of the second column is transmitted at time t2. Specifically, the data of the first column is carried at frequency f1 and time t1, the data of the second column at frequency f1 and time t2, the data of the third column at frequency f2 and time t1, and the data of the fourth column at frequency f2 and time t2. Thus, the eight symbols are transmitted through the three Tx antennas at two frequencies for two time intervals.
Alternatively, the data of the first two columns can be transmitted at time t1, while that of the last two columns at time t2. The data of the first column in each pair is transmitted at frequency f1 and that of the second column is transmitted at frequency f2. Therefore, the data of the first column is carried at frequency f1 and time t1, the data of the second column at frequency f2 and time t1, the data of the third column at frequency f1 and time t2, and the data of the fourth column at frequency f2 and time t2.
Similarly to the transmission mode 1 and the transmission mode 3, data transmission according to the coding matrix is not limited to the above time-frequency plane in the transmission mode 2. Therefore, transmission of the four columns at different time or at different frequencies can be contemplated.
Since the rows of the coding matrix represent the Tx antennas, the data of the first row is transmitted through the first Tx antenna 308, the data of the second row through the second Tx antenna 310, and the data of the third row through the third Tx antenna 312.
The STFBC coder mode decider 314 determines which Tx antenna is in the best channel condition based on the CQI of each Tx antenna fed back from the receiver, and notifies the STFBC coder 306 of the determined Tx antenna. The STFBC coder 306 selects one of the coding matrixes described in Equation (1), Equation (2) and Equation (3) in correspondence with the Tx antenna.
Alternatively, the receiver can be so configured as to determine which Tx antenna is in the best channel condition based on the CQI of each Tx antenna and transmits to the STFBC coder 306 a Tx mode index indicating a transmission mode for use in STFBC coding. That is, the receiver decides on a Tx antenna to carry non-Alamouti data based on the CQIs of the Tx antennas and tell the STFBC coder 306 an index indicating the Tx antenna.
Referring to
The STFBC coder STFBC-encodes the eight symbols according to the determined transmission mode in step 410 and transmits the STFBC-coded signals through the three Tx antennas in step 412.
In the case where the receiver is configured to determine which Tx antenna has the best channel condition based on the CQI of each Tx antenna and to notify the STFBC coder of a Tx mode index, the STFBC coder receives the Tx mode index and, rather than the STFBC code mode decider, determines the transmission mode, in step 408.
Meanwhile, many other coding matrices are available for the STFBC coder (STBC or SFBC coder) that transmits data at a data rate of 2 through three Tx antennas based on feedback CQIS.
An exemplary set of coding matrices are
The above coding matrices represent data re-grouped on a complex plane from constellation-rotated data in the STFBC coder.
In the case where the receiver decides a transmission mode and transmits a Tx mode index indicating the determined transmission mode to the transmitter, a Tx mode index 0b000, 0b1010 or 0b110001 indicates the coding matrix of Equation (5), a Tx mode index 0b001, 0b1011 or 0b110010 indicates the coding matrix of Equation (6), and a Tx mode index 0b010, 0b1100 or 0b110011 indicates the coding matrix of Equation (7).
As described above, the STFBC coding apparatus and method for a 3 Tx-rate 2 transmitter of the present invention enables realization of a more reliable communication system using a transmission scheme where an input symbol sequence is transmitted through a plurality of Tx antennas based on feedback CQIs from a receiver.
While the invention has been shown and described with reference to a certain preferred embodiment 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 |
|---|---|---|---|
| 2004-0089507 | Nov 2004 | KR | national |
