For advanced mobile communication systems that adopt orthogonal frequency-division multiple access (OFDMA) technologies, intercarrier interference (ICI) significantly degrades performance when mobility is high. Standard specifications and concerns about complexity demand low-cost methods with deployment readiness and decent performance. In this paper, novel zero forcing (ZF) and minimum mean-square error (MMSE) equalizers based on per-subcarrier adaptive (PSA) processing and perturbation-based (PB) approximation are introduced. The proposed equalizers strike a good balance between implementation cost and performance; therefore they are especially suitable for OFDMA downlink receivers. Theoretical analysis and simulations are provided to verify our claims.

This letter presents a novel opportunistic cooperative positioning approach for orthogonal frequency-division multiple access (OFDMA) systems. The basic idea is to allow idle mobile terminals (MTs) opportunistically estimating the arrival timing of the training sequences for uplink synchronization from active MTs. The major advantage of the proposed approach over state-of-the-arts is that the positioning-related measurements among MTs are performed without the paid of training overhead. Moreover, Cramer-Rao lower bound (CRLB) is utilized to derive the positioning accuracy limit of the proposed approach, and the numerical results show that the proposed approach can improve the accuracy of non-cooperative approaches with the a-priori stochastic knowledge of clock bias among idle MTs.

Similar to orthogonal frequency-division multiplexing (OFDM) systems, orthogonal frequency-division multiple access (OFDMA) is vulnerable to carrier frequency offset (CFO). Since the CFO of each user is different, CFO compensation in OFDMA uplink is much more involved than that in OFDM systems. It has been shown that the zero-forcing (ZF) compensation method is a simple yet effective remedy; however, it requires the inversion of a large matrix and the computational complexity can be very high. Recently, we have developed a low-complexity iterative method to alleviate this problem. In this paper, we consider the theoretical aspect of the algorithm. We specifically analyze the output signal-to-interference-plus-noise-ratio (SINR) of the algorithm. Two approaches are used for the analysis; one is simple but approximated, and the other is complicated but exact. The convergence problem is also discussed. In addition to the analysis, we propose a pre-compensation (PC) method enhancing the performance of the algorithm. Simulations show that our analysis is accurate and the PC method is effective.

Our investigation is presented into analysis of the co-channel interference (CCI) statistic in orthogonal frequency-division multiple access (OFDMA) uplink systems. The derived statistic is then used to analyze the performance of reuse partitioning (RP)-based dynamic channel allocation (DCA). Analysis and simulation results show that the performance of DCA in multi-cell environments is noticeably dependent on the CCI. Finally, the results of the analysis yield the optimum RP area for achieving the maximum spectral efficiency.

This letter deals with our investigations into improving the performance of a wireless uplink system when an orthogonal frequency-division multiple access (OFDMA) is used as an access scheme. To do this, the OFDMA-based uplink system adopts a frequency diversity coupled with a cyclic time shift (CTS) at the transmitter, which is named as the FD-OFDMA system with CTS. It is found that the multi-user FD-OFDMA system equipping with CTS can decrease the probability of destroying the orthogonality among the users and provide the MAI-robust reception without decreasing the bandwidth efficiency of the system.