Performance of a Novel Fuzzy-Based Code Allocator for Two-Dimensional Spreading OFCDM Communication Systems

Moreover, the two-dimensional (2-D) spreading codes, spreading on both time and frequency domain, has been applied to OFDM system [5-6]. Using the orthogonality property between the 2-D codes, the orthogonal frequency coded division multiplexing (OFCDM) is performed in the downlink for mobile communication systems [7-8]. The diversity gain can be obtained from frequency domain or time domain, respectively referring frequency diversity and time diversity. The frequency diversity depends on the correlation of the combining elements in frequency bands while the time diversity depends on the correlation of the combining elements in time slices. From the 2-D codes, the diversity gain can be obtained from frequency domain which the spreading codes are all spread in the subcarriers for the multi-carrier CDMA systems [8]. However, the 2-D codes could also be spread in the time-domain to obtain the time diversity [9]. As we know that the both frequency and time diversity gain depend on the 2-D spreading codes and the response of the frequency and time selective fading channels [10]. Moreover, the time diversity not only depends on the time spreading factor in the 2-D spreading codes but also related with the Doppler shift incurred by the speed of mobiles. Therefore, with the flexibility to the diversity effects, the 2-D spreading codes are proposed to perform the future 4G wireless mobile communication systems. In this paper, we investigate the multi-user performance in a two-dimensional (2-D) spreading orthogonal frequency coded division multiplexing (OFCDM) communication system over frequency and time selective fading channels. To compromise between diversity gain and multiple access interference (MAI), we propose an adaptive fuzzy inference system to infer adequate 2-D spreading factors and to assign the orthogonal variable spreading factor (OVSF) codes to active users for OFCDM mobile communication systems. Simulation results show that the proposed fuzzy-based code allocation (FBCA) scheme not only suppresses the MAIs induced by non-orthogonality but also exploit the diversity gain over time and frequency selective fading channels and then outperforms the orthogonal code allocation (OCA) scheme at all system loads.