OFDM-CDMA with Uniform Time-Frequency Spreading

• Main Authors: Hsin-Yao Wang, 汪新堯
• Other Authors: Ming-Xian Chang
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/52234148221171778910

碩士 === 國立成功大學 === 電腦與通信工程研究所 === 94 === Orthogonal multi-carrier modulation combined with CDMA is well known as Orthogonal Frequency Division Multiplexing – Code Division Multiple Access (OFDM-CDMA) and has drawn a lot of interest in modern wireless communications. On top of that, the permutation of spreading codes has impact on the system performance and is studied in this thesis. In early OFDM-CDMA systems, the spreading code is put along time slots on a fixed sub-channel, or along sub-channels at a fixed time slot. Such arrangement can be regarded as one-dimensional (1-D) spreading, i.e., spreading in either time domain or frequency domain. Recently, there are schemes proposed that allocate the spreading code in both time and frequency domains, which can be considered as two-dimensional (2-D) spreading. The 2-D spreading schemes exploit both time and frequency diversity. Examples of 2-D spreading schemes are Joint Frequency Time Spreading (JFTS) and Random Frequency Time Spreading. In this thesis, we propose a Uniform 2-D Spreading Scheme and study the effects of channel time and frequency correlations on the 2-D spreading schemes. Hence, the channel correlation function [10] is cited in this thesis to see the correlation among the position of spreading code in different permutation method. Besides, we use the independent complex Gaussian process to obtain a channel with fully uncorrelated time and frequency fading, and the corresponding performance is served as a lower bound of 2-D spreading schemes. It is observed that the performance of the Uniform 2-D Spreading Scheme approaches the lower bound when the channel has higher Doppler shift and larger delay spread.