System analysis and performance comparison of different multi-carrier schemes

Multicarrier modulation techniques are attracting great interest in emerging wireless communication systems such as fourth generation cellular systems and evolutions of IEEE802.11 (WiFi) and IEEE802.16 (WiMax) air interfaces. The analysis is made complex as a result of many combinations of modulation scheme parameters such as number of subcarriers, degree of spreading, method of spreading in time and frequency, interplay between modulation and coding parameters, and synchronicity between the signals of different users. The goal of this thesis is to illustrate various multicarrier system structures and analyze parameter trade-offs with emphasis on systems with non-synchronous users in order to optimize system performance. There are three main results in this thesis. First we study a multicarrier CDMA system with a repetition code structure. In such a system, decreasing the repetition code rate can lead to more frequency diversity. However, to keep the same transmit bit rate, the spreading factor of each subcarrier needs to be reduced. We investigate the tradeoff between the spreading factor and the rate of repetition code in a system with fixed transmit bit rate, bit energy and allocated bandwidth. The system performance is analyzed and the outage probability is calculated. It is shown that when the number of users in the system becomes large, the code rate needs to be increased in order to maintain a low outage probability. Meanwhile, in a system with a given number of users, if a threshold SINR with large value is required, we should increase the spreading factor in order to achieve a low outage probability. Secondly, unlike the conventional multicarrier CDMA schemes, where spreading codes are utilized to realize multiple access (i.e., combining bit streams from different users), and frequency division is utilized for multiplexing (i.e., combining different bit sub-streams from the same user), we reverse the roles and utilize spreading codes for multiplexing and frequency division for multiple access. That is, bits from the same user are spread with different codes and transmitted on the same subcarrier. We propose a new scheme called CDM-OFDMA and make a comparison with other well known schemes. We place the four multicarrier schemes (MC-DS-CDMA, MC-CDMA, CDM-OFDMA, OFCDM) into a unified analytic framework. With a single-path channel model, the comparative performances of these four schemes in terms of bit error rate (BER) are as follows: OFCDM (better than) CDM-OFDMA (better than) MC-CDMA (almost equal to) MCDS-CDMA. Thirdly we study the peak power problem in the multicarrier systems. Different methods are listed and compared. We propose an algorithm which is used to find a generator matrix for a linear block code, while its codewords will not produce a large peak to average power ratio (PAPR) after the IFFT operation. The performances of the proposed algorithm and the random phase shift algorithm are compared numerically. It is shown that although the proposed algorithm does not control the peak power as good as the random phase shift algorithm, the systems with these two algorithms have close BER performances. Beside this, our algorithm comes with an advantage that it can be designed off-line.