Superimposed Frequency Symbol Based Adaptive Downlink OFDM with Frequency Spreading and Equalization

Chang-Jun AHN
Hiroshi HARADA
Yukiyoshi KAMIO

Publication
IEICE TRANSACTIONS on Communications   Vol.E89-B    No.2    pp.500-508
Publication Date: 2006/02/01
Online ISSN: 1745-1345
DOI: 10.1093/ietcom/e89-b.2.500
Print ISSN: 0916-8516
Type of Manuscript: PAPER
Category: Wireless Communication Technologies
Keyword: 
AMS/OFDM,  frequency symbol spreading,  frequency equalization,  MMSEC,  ORC,  

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Summary: 
In AMS/OFDM systems, a base station controls the modulation level of each subcarrier with feedback information (FBI), and then, adaptive modulated packets are transmitted from the base station to the mobile station. In this case, the mobile station requires modulation level information (MLI) to demodulate the received packet. The MLI is generally transmitted as a data symbol, so the throughput is degraded. To overcome this problem and increase the total throughput, in this paper, we propose superimposed frequency symbol based adaptive OFDM with frequency spreading and equalization. In the proposed system, each S/P transformed signal is spread by orthogonal spreading codes and combined. This means that each subcarrier holds several superimposed S/P transformed signals with the same power rate. In this case, the frequency-selective faded subcarriers obtain the same power rate for each S/P transformed signal. Therefore, the detected signals also obtain the same SINR, and as a result, we can assign the same modulation level for each frequency symbol spreading block. Hence, the proposed system requires only one piece of FBI and MLI for each frequency symbol spreading block, as compared with conventional adaptive OFDM.