Abstract

Future wireless communications systems need a high quality of service coupled with high data rate transmission for multimedia services. Achieving this goal in the hostile wireless environment with its limited spectrum has several challenges and implies the necessity of a communication system that is able to increase the channel capacity and overcome the difficulties of the wireless transmission environment with reasonable system complexity. Two of the most enabling technologies for the next generation of wireless systems are orthogonal frequency division multiplexing (OFDM) and multiple-input multiple output (MIMO) systems. MIMO systems have been originally designed for known flat fading channels. In this research, some novel MIMO-OFDM schemes for broadband wireless applications are developed and presented. The objective of the proposed schemes is to enhance the performance of OFDM systems over multipath fading channels by using antenna diversity techniques, and also to make MIMO systems applicable to frequency selective multipath fading channels. For the performance evaluation, both bit error rate (BER) and channel capacity analysis are considered. The channel capacity of MIMO-OFDM systems is analytically evaluated and it is shown that the channel capacity of the these systems can be dramatically increased as a function of the number of antennas. The BER performance of the MIMO-OFDM systems is analytically evaluated. New closed-form expressions for the BER performance of the MIMO-OFDM systems over frequency selective fading channels are derived. On the other hand, the growing popularity of both MIMO and OFDM systems creates the need for adaptive modulation to integrate temporal, spatial and spectral components together. The performance improvement offered by adaptive modulation over non-adaptive systems is remarkable. Furthermore, other dimensions such as frequency and space may yield further gains by providing additional degrees of freedom that can be exploited by adaptive modulation. In this research, a new adaptive modulation scheme for our MIMO-OFDM system (SFBC-OFDM) is presented. The proposed scheme exploits the benefits of space-frequency block codes (SFBC), OFDM and adaptive modulation to provide a high quality of transmission for wireless communications over frequency selective fading channels. It is shown that adaptive modulation can greatly improve the performance of the conventional SFBC-OFDM systems. Finally, a novel antenna selection algorithm is proposed for our MIMO-OFDM system. Three different forms of antenna selection are considered: transmit antenna selection, receive antenna selection, and joint transmit/receive antenna selection. The coding and diversity advantages of the MIMO-OFDM system with antenna selection are examined using average SNR gain, outage probability and BER analysis. The system performance of different forms of the proposed scheme is evaluated and compared. It is shown that the proposed scheme can greatly improve the performance of the conventional SFBC-OFDM systems.