Code Assignment for Downlink Transmission of Broadband OFCDM Systems

As a wireless access technique for future broadband mobile communications, orthogonal frequency and code division multiplexing (OFCDM) systems employ multi-code transmission to provide high speed mobile services. Since the multiple code channels are no longer orthogonal in the broadband wireless channel, multi-code interference (MCI) is caused. The variance of the MCI is dependent on which codes to be used. This paper investigates the code assignment in the multi-code OFCDM system, aiming to mitigate the MCI at the receiver. Since two-dimensional (2-D) spreading is employed in the OFCDM system, the code assignment is carried out in time and frequency domains, separately. In order to avoid large interference from the powerful code- multiplexed pilot channel, the time spreading code adjacent to the pilot code in the code tree is suggested not be used when the time domain spreading factor is large. The performance of the proposed code assignment scheme is analytically evaluated. It has been shown that the system performance can be improved significantly by the proposed scheme under various channel conditions, especially when a low to medium system load is assumed.