A space-time block coded orthogonal frequency division multiple access (STBC-OFDMA) scheme based on combination of user grouping (UG) and frequency reuse (FR) is proposed for WiMAX uplink transmission where each user employs an STBC scheme. First, a direction-of-arrival (DOA)-based intra-cell UG model is combined with the idea of inter-cell FR so as to produce a UGFR-STBC-OFDMA uplink access scheme which satisfies with both UG constraints and the inter-cell FR planning. Furthermore, a Capons beamforming-based receiver with a smart antenna array is proposed for directional reception of signals coming from different user groups as well as multiple access interference (MAI) suppression at the base station. Both theoretical analysis and simulation results show that the proposed uplink access scheme not only achieves a well-behaved tradeoff in ideal average spectral efficiency, bit error rate (BER) and computational complexity, but also suppresses inter-cell interferences without power control, compared with some conventional schemes for uplink transmission.

The authors propose a pseudo-noise (PN) sequence-based carrier frequency offset (CFO) estimation algorithmfor multiuser OFDMuplink. In the proposed algorithm, a PN sequence and its circular-shifting versions are considered as training sequences for different users, and then channel parameters with CFO factors from each user to a central base station can be estimatedby virtue of autocorrelation characteristic of the PN sequence. Finally all users’CFO estimates can be acquired simultaneously by calculating channel phase-angle difference between two training periods. The simulation result shows that the proposed algorithm achieves better estimation performance.