Performance Analysis of Transmit Diversity STBC-OFDM and Differential STBC-OFDM Over Fading Channels

ABSTRACT: Alamouti space-time block coding (A-STBC) is a relatively well known coding technique that is employed to enhance the capacity of wireless communication systems without affecting the bandwidth efficiency. Furthermore, A-STBC is known to have a linear decoding complexity which has made it one of the most popular among space time codes (STCs). The decoding of space-time block codes, however, requires knowledge of channel state information (CSI) at the receiver and in general, channel parameters are assumed to be known (assumes that channel estimation is possible). However, when there is high mobility and the channel conditions are fluctuating rapidly it may be difficult to obtain perfect or close to perfect estimates for the channel. To alleviate this problem another space-time block coding technique known as Differential Space-Time Block Coding (DSTBC) has been proposed. Differential phase shift keying (DPSK) which is employed by DSTBCs is a common form of phase modulation that conveys data by changing the phase of the carrier wave. The decoder in DPSK modulation does not require CSI since the transmitted symbols depend on the previous symbol and the decoder would sense the data from symbols that come one after another. When channel fluctuations are very high DSTBCs need to be used at the expense of lower bit error rate. The work presented herein, provides a link level bit error rate analysis of none line- of-sight (NLOS) Alamouti space-time block coded with Orthogonal Frequency Division Multiplexing (OFDM) using either BPSK or QPSK modulation. OFDM is a multicarrier modulation technique where a high rate data stream is sub-divided into a number of lower rate data streams and transmitted over a number of subcarriers. The performance increment due to OFDM comes from the fact that the usage of a guard interval that help reduce or completely eliminate the interference between symbols due to multipath effect. The simulation results presented in this thesis have all been obtained using the readily available MATLAB platform and writing dedicated functions for different tasks. The results have been presented in four parts. The first part provides the bit error rate performance for BPSK modulated data transmitted over a Rayleigh fading channel. This is then followed by a performance analysis of OFDM over the AWGN channel using either BPSK or QPSK modulation. Third part demonstrates the BER vs. SNR for Alamouti STBC and Alamouti DSTBC coded data transmitted over a Rayleigh fading channel without using OFDM. Finally, part four will provide STBC and DSTBC coded OFDM performance when BPSK and QPSK are the preferred modulation and the channel is again the Rayleigh fading channel. The work presented clearly demonstrates that even though OFDM by itself is giving good performance by mitigating inter-symbol interference, combining OFDM with a transmit diversity technique like Alamouti STBC helps further improve the link level BER performance. The simulation results indicate that when no CSI is available and DSTBC needs to be employed, the system performance will degrade approximately by 2.2dB at a BER of 10-4 (in comparison to STBC-OFDM using BPSK). Keywords: STBC, DSTBC, OFDM, Rayleigh Fading, BER. …………………………………………………………………………………………………………………………