Variable Spreading Factor Orthogonal Frequency and Code Division Multiplexing (VSF-OFCDM)

Abstract

This paper proposes Variable Spreading Factor — Orthogonal Frequency and Code Division Multiplexing (VSF-OFCDM) as the most promising forward link wireless access method in broadband packet wireless transmission using an approximate 50 – 100 MHz bandwidth. OFCDM is originally based on multicarrier CDMA where the spreading sequence is multiplied in the frequency domain, and OFCDM employing VSF can flexibly realize near optimum wireless access with higher link capacity by adaptively changing the appropriate spreading factor, SF, in the frequency domain based on the cell structure and radio link conditions such as the delay spread. Furthermore, by establishing the spreading factor of SF = 1, i.e., no spreading mode, VSF-OFCDM can be used as orthogonal frequency division multiplexing (OFDM). Simulation results demonstrate that, while SF = 1 (OFDM) achieves higher link capacity than SF > 1 in an isolated-cell environment, OFCDM with the optimized SF value over 1 achieves approximately 1.4 times higher capacity compared with OFDM in a multi-cell environment associated with the advantageous one-cell frequency reuse. Consequently, VSF-OFCDM can provide seamless deployment of broadband packet wireless access with high link capacity, that is, OFDM in an isolated-cell environment such as hot spot areas or indoor offices, and OFCDM with the adaptively optimized SF value over 1 in a multi-cell environment such as cellular systems according to the radio link conditions such as measured delay spread, by only changing the spreading factor.