transmit diversity techniques for wireless communication

Transmit Diversity Technique for Wireless Communications : 

By, Undety Srinu, EC094220(ACS) Transmit Diversity Technique for Wireless Communications

Abstract : 

Abstract Using 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 antennas. Advantage: 1. Redundancy is applied in space across multiple antennas, not in time or frequency. =>Doesn’t require any bandwidth expansion . 2. It doesn’t need any feedback from the receiver to the TX. 3. Its computation complexity is similar to MRRC. 4. Two transmit antennas and M receive antennas provides a diversity order of 2M.

Introduction : 

Introduction The remote units are supposed to be small lightweight pocket communicators and provide high bit rate data services. The communication systems are supposed to have better quality and coverage, be more power and bandwidth efficient. The fundamental phenomenon which makes reliable wireless transmission difficult is time-varying multipath fading . BER= 10-2 => 10-3

Introduction (cont) : 

Introduction (cont) Transmitter power control (the most effective ) : If channel conditions are known at the transmitter ,the transmitter can predistort the signal in order to overcome the effect of the channel at the receiver. Two 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 radiation power limitations, 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 transmitter and receivers.

Introduction (cont) : 

Introduction (cont) Time and frequency diversity: time interleaving and error correction coding resulting in large delays when the channel is slowly varying. Antenna diversity : a. use multiple antennas at the RX (RX diversity) and perform combining 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) : 

MRRC (Maximum Receive Ratio Combining) note1

MRRC (cont) : 

MRRC (cont) Using Euclidean distance to detect: chose xi if d2(xi ,y) <= d2(xk ,y) i != k ps. d2(x ,y) = (x - y) (x*-y*) MRRC: SO’= (α02 + α12) sO + h0* n0 + h1* n1 chose si if d2(s0’ , (α02 + α12) si ) <= d2(s0’ , (α02 + α12) sk ) if signals are equal constellations: d2(s0’ , si ) <= d2(s0’ , sk )

The new transmit diversity scheme : 

The new transmit diversity scheme encoding and transmission sequence combining scheme maximum likelihood decision rule

New scheme (cont) : 

New scheme (cont) Encoding and transmission sequence :

: 

Combining scheme: The combiner builds the following two combined signals that are sent to the maximum likelihood detector: New scheme (cont)

New scheme (cont) : 

d2(s0’ , (α02 + α12) si ) <= d2(s0’ , (α02 + α12) sk ) d2(s0’ , si ) <= d2(s0’ , sk ) The resulting diversity order from the new branch transmit diversity scheme with one receiver is equal to two branch MMRC New scheme (cont) Maximum likelihood decision rule: Choose si if

New scheme (cont)- two-branch transmit diversity with M antennas : 

New scheme (cont)- two-branch transmit diversity with M antennas

New scheme (cont)- two-branch transmit diversity with M antennas : 

New scheme (cont)- two-branch transmit diversity with M antennas Assumption: Encoding and transmission sequence: like 2-to-1 transmission diversity. Combining scheme:

New scheme (cont)- two-branch transmit diversity with M antennas : 

New scheme (cont)- two-branch transmit diversity with M antennas Maximum likelihood decision rule: chose si if if signals are equal constellations: The combined signals from the two RX antennas are the addition of the combined signals from each antenna ,i.e., the combining scheme is identical to a single RX antenna . =>using two transmit and M receive antennas ,we can use the combiner to obtain the same diversity order as 2M-branch MMRC. d2(s0’ , (α02 + α12+α22 + α32) si ) <= d2(s0’ , (α02 + α12+α22 + α32) sk ) d2(s0’ , si ) <= d2(s0’ , sk )

Error performance simulation : 

Error performance simulation The total radiated power at Tx are equal. => the performance of the new scheme with two transmitters and a single receiver is 3 dB worse than two-branch MRRC.

Implement issues : 

Implement issues Power 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. Sensitivity to Channel Estimation Errors: 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

Implement issues (cont) : 

Implement issues (cont) Delay 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 the decoding delay is only one symbol period. Antenna configurations 1. the propagation medium between the TX and RX 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 transmit antennas at the remote units must be separated by about 3 wavelengths to provide diversity at the base station.

Implement issues (cont) : 

Implement issues (cont) Soft failure 1.one of the receive chain fail, and the other receive chain is operational, then the performance loss is on the order of the diversity gain. 2. adding reliability due to multiple RX chains. Impact 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 : 

Conclusion and discussions Using 2 TX antennas and 1 RX antenna the scheme provides the same diversity order as maximal-ratio receiver combining (MRRC) with 1 TX antenna, and 2 RX antennas. 2 TX antennas and M RX antennas provides a diversity order of 2M. To employ two half-power amplifiers rather than a single full power amplifier. The scheme does not require any feedback from the receiver to the transmitter and its computation complexity is similar to MRRC.

References : 

References S. M. Alamouti, “A simple transmitter diversity scheme for wireless communications,” EEE J. Select. Areas Commun., vol. 16, pp. 1451–1458, Oct. 1998. Wittneben, “A new bandwidth efficient transmit antenna modulation diversity scheme for linear digital modulation,” in Proc. 1993 IEEE International Conf. Communications (ICC’93), May 1993, pp.1630–1634.   V. Tarokh, A. Naguib, N. Seshadri, and A. R. Calderbank, “Spacetime codes for wireless communication: Combined array processing and space time coding,” IEEE Trans. Inform. Theory, Mar. 1998.   V. Tarokh, N. Seshadri, and A. R. Calderbank, “Space-time codes for high data rate wireless communication: Performance criteria and code construction,” IEEE Trans. Inform. Theory,

Slide 21: 

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