Abstract

Along with the fast growing demand of information exchange, telecommunication systems are required to provide fast and reliable service to high-data-rate applications such as video conference, real-time broadcasting, and on-line gaming. In downlink transmission, orthogonal frequency and code division multiplexing (OFCDM) has been an attractive technique for high-data-rate applications. With two-dimensional spreading, in both time domain and frequency domain, OFCDM achieves diversity gains in multiuser scenarios. Moreover, the adjustable spreading factors (SF) give OFCDM systems the flexibility in transmission rate and diversity gain. In this thesis, we focus on the downlink of OFCDM communication systems. The performance of OFCDM systems is investigated over Ricean fading channels with Rayleigh fading as special case. Code division multiple access (CDMA) technique is used to support multiuser communications, where users can transmit at the same time using the same frequency with the help of code sequences. We compare different combining methods that are employed to achieve diversity gain. Moreover, channel correlation is examined to see its effect on the system performance. We also propose to combine multiple-input and multiple-output (MIMO) techniques, specifically space-time block coding (STBC), with OFCDM systems. By adding spatial diversity, a MIMO system can provide more reliable transmission compared to a single-input and single-output (SISO) system. The space-time scheme used in our study is Alamouti scheme [1], which employs N = 2 and M antennas at the transmitter side and receiver side respectively. In the thesis, we explain the system structure, transmission and detection methods, and system performance of such MIMO-OFCDM systems. In our study, the expressions of system bit error rate (BER) are considered under the condition that no multi-code interference (MCI) is present. The accuracy of the BER expressions is verified when compared with the simulated ones for both SISO and MIMO-OFCDM systems with different combining methods. These comparisons are carried over different channels and with different system parameters to explore the benefits of OFCDM based systems. Both analytical and simulation results show the large diversity gains achieved when incorporating STBC with OFCDM.