Transmit Diversity for Wireless Communications

1、a simple transmit diversity technique for wireless communications -m. alamouti 1998 chou-pin wu 09/06/2006 abstract nusing two transmit antennas and one receive antenna the scheme provides the same diversity order as maximal-ratio receiver combining (mrrc) with one transmit antenna, and two receive

2、antennas. nadvantage: 1. redundancy is applied in space across multiple antennas, not in time or frequency. =doesnt require any bandwidth expansion . 2. it doesnt need any feedback from the receiver to the tx. 3. its computation complexity is similar to mrrc. 4. two transmit antennas and m receive a

3、ntennas provides a diversity order of 2m. introduction nthe remote units are supposed to be small lightweight pocket communicators and provide high bit rate data services. nthe communication systems are supposed to have better quality and coverage, be more power and bandwidth efficient. nthe fundame

4、ntal phenomenon which makes reliable wireless transmission difficult is time-varying multipath fading . ex: ber= 10-2 = 10-3 environmentsnr increasing awgn1 or 2 db multipath10db introduction (cont) ntransmitter power control (the most effective ) : if channel conditions are known at the transmitter

5、 ,the transmitter can predistort the signal in order to overcome the effect of the channel at the receiver. ntwo problems: transmitter dynamic range: a. for the transmitter to overcome a certain level of fading, it must increase its power by that same level . b. it is not practical because of radiat

6、ion power limitations and the size and cost of the amplifiers. channel information: a. the channel information has to be fed back to the transmitter. b. throughput degradation and added complexity to both the transmitter and the receiver. introduction (cont) ntime and frequency diversity: time inter

7、leaving and error correction coding resulting in large delays when the channel is slowly varying. spread spectrum techniques are ineffective when the coherence bw of the channel is larger than the spreading bw. nantenna diversity : a. use multiple antennas at the rx (rx diversity) and perform combin

8、ing or selection and switching to improve the quality of the received signal. b. makes the remote units larger and more expensive. mrrc (maximum receive ratio combining) note1 mrrc (cont) nusing euclidean distance to detect: chose xi if d2(xi ,y) = d2(xk ,y) i != k ps. d2(x ,y) = (x - y) (x*-y*) nmr

9、rc: so= (02 + 12) so + h0* n0 + h1* n1 chose si if d2(s0 , (02 + 12) si ) = d2(s0 , (02 + 12) sk ) if signals are equal constellations: d2(s0 , si ) 0 1 q q q * akb * 00* * 1 1 qh qh q h 00 11 hn rh sns hn 2 2 q hs snr q n 22 2 a bab 22 2 a bab o sq rq hsq n the new transmit diversity scheme nencodi

10、ng and transmission sequence ncombining scheme nmaximum likelihook decision rule new scheme (cont) timeantenna0antenna1 t s0 s1 t+t -s1* s0* ncombining scheme:nmaximum likelihood decision rule: chose si if lit is equal to two-branch mrrc. nencoding and transmission sequence : assumption: d2(s0 , (02

11、 + 12) si ) using two transmit and m receive antennas ,we can use the combiner to obtain the same diversity order as 2m-branch mmrc. d2(s0 , (02 + 12+22 + 32) si ) = d2(s0 , (02 + 12+22 + 32) sk ) d2(s0 , si ) the performance of the new scheme with two transmitters and a single receiver is 3 db wors

12、e than two-branch mrrc. implement issues npower requirements: 1. the 3-db reduction of power in each transmit chain translates to cheaper, smaller, or less linear power amplifiers. 2. to employ two half-power amplifiers rather than a single full power amplifier. nsensitivity to channel estimation er

13、rors: 1. the rx extracts the samples and interpolates them to construct an estimate of the channel for every data symbol transmitted. 2. with 2(m) tx and 1 rx antennas, however, the estimates of the 2(m) channels must be derived from a single received signal. =2(m) times as many pilots are needed im

14、plement issues (cont) ndelay effects 1.with 2-branch transmit diversity, if the transformed copies of the signals are transmitted at distinct intervals from all the antennas, the decoding delay is symbol 2 periods. 2. if the copies are sent at the same time and on different carrier frequencies, then

15、 the decoding delay is only one symbol period. nantenna configurations 1. the propagation medium between the tx and tx in either direction are identical. 2. to provide sufficient decorrelation between the signals transmitted from the two transmit antennas at base station = 10 wavelengths. 3. the tra

16、nsmit antennas at the remote units must be separated by about 3 wavelengths to provide diversity at the base station. implement issues (cont) nsoft failure 1.one of the channel fail, and the other channel is operational, then the performance loss is on the order of the diversity gain. 2. adding reli

17、ability due to multiple tx chains. nimpact on interference 1. simultaneous transmission of signals from two antennas. =it appears that the number of potential interferers is doubled ,however, each with half the interference power. 2. if interference has properties where interference cancellation schemes may be effectively used. conclusion and discussions nusing 2 tx antennas and 1 rx antenna the scheme provides the same dive