On the performance of interference subspace rejection for next generation multicarrier CDMA
Besma Smida, Sofiène Affes · 2006
A multicarrier-CDMA receiver with full interfer- ence suppression capabilities, named multi-carrier interference subspace rejection (MC-ISR), has recently been proposed and assessed by simulations for high-rate transmissions over next- generation CDMA systems. In this paper, we derive a link/system- level performance analysis of MC-ISR based on the Gaussian assumption (GA) and validate it by simulations. In addition, we provide a comparative study of the two potential next- generation multicarrier CDMA air-interface configurations: MT- CDMA and MC-DS-CDMA. Simulations show that for both DBPSK and DQPSK modulations, MT-CDMA has the best link- level performance and the highest throughput. With two receiving antennas and nine MT-CDMA subcarriers in 5 MHz bandwidth, MC-ISR provides about 1.4 bps/Hz at low mobility for DBPSK, i.e., an increase of 170% in spectrum efficiency over a DS-CDMA system with MRC. I. INTRODUCTION Although multi-carrier (MC)-CDMA systems are promis- ing, challenges remain before they can achieve their full potential. One of the major obstacles in detecting MC-CDMA signals is interference. The multiple access interference (MAI) and the inter-symbol interference (ISI), which are inherited from conventional DS-CDMA, affect likewise the performance of MC-CDMA systems. In addition, MC-CDMA capacity is limited by the inter-carrier interference (ICI) due to the use of multicarrier modulation. Indeed, the imperfect frequency down-conversion due to the instability of local oscillators combined with the multipath effect disturbs the subcarriers orthogonality thereby causing ICI. Since MC-CDMA systems also contain a DS-CDMA com- ponent, traditional multiuser detection techniques can be per- formed on each carrier with some form of adaptation. An efficient multiuser detection technique, denoted interference subspace rejection (ISR), first proposed for DS-CDMA (1), has been recently developed for multicarrier systems (2). The performance of multi-carrier (MC)-ISR was evaluated there through simulations using very realistic link-level simulation setups that take into account time and frequency mismatch, imperfect power control, channel identification errors etc. Simulation results confirm the net advantage of the full in- terference suppression capabilities of MC-ISR. In this paper, we develop a theoretical link/system-level performance analysis of MC-ISR based on the Gaussian assumption (GA), under the condition of perfect channel identification. In addition, we provide a comparative study of the two potential next-generation multicarrier CDMA air- interface configurations: MT-CDMA and MC-DS-CDMA.