Beamspace-Code Transmit Diversity for Two-Dimensional Spreading OFDM-CDM Systems

Masaaki FUJII  

IEICE TRANSACTIONS on Communications   Vol.E87-B   No.2   pp.257-265
Publication Date: 2004/02/01
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: PAPER
Category: Wireless Communication Technology
OFDM-CDM,  space-time block code,  transmit array,  inter-code interference,  Doppler spread,  

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This paper presents a transmit diversity scheme that allocates space-time block codes (STBC) to beamspace and spreading codes for two-dimensional spreading orthogonal frequency-division multiplexing code-division multiplexing (OFDM-CDM) downlink transmission. In this scheme, the STBC output symbols are beam-steered using a pair of neighboring beams selected via closed-loop beam selection. The beam-steered symbols in two adjacent time slots are spread by two distinct spreading codes and multiplexed in the same spreading segment. User signals transmitted from different pairs of beams, but that share the same beam, interfere with each other when decoding STBC. Spreading codes are thus allocated to users according to beam pairs used. This is to suppress the interference in time-direction despreading that precedes decoding of STBC. Simulation results demonstrated that the proposed scheme provides beam gains or beam diversity gains or both and that it alleviates inter-code interference by spatially separating user signals by using transmit beam. The proposed scheme also provides high tolerance to large Doppler spread.