Block-coded OFDM Research Articles

Adaptive Subcarrier and Power Allocation with Fairness for Multi-user Space-Time Block-Coded OFDM System

This paper investigates the subcarrier and power allocation problems of multi-user space-time block coded OFDM based cellular systems. Based on the tradeoff between the number of assigned subcarriers and the amount of allocated power for users, a less complexity algorithm that separates subcarrier allocation and power allocation is proposed. Simulation results show that the proposed resource allocation algorithm can improve the capacity significantly compared with static FDMA fixed allocation algorithm and the MIMO-OFDMA scheme, and the more important thing is that it can make the capacity be distributed more fairly, very close to the ideal rate constraints, among users than the scheme which maximizes the system capacity only.

Interference Mitigation in Cooperative SFBC-OFDM

We consider cooperative space-frequency block-coded OFDM (SFBC-OFDM) networks with amplify-and-forward (AF) and decode-and-forward (DF) protocols at the relays. In cooperative SFBC-OFDM networks that employ DF protocol, (i), intersymbol interference (ISI) occurs at the destination due to violation of the "quasistatic" assumption because of the frequency selectivity of the relay-to-destination channels, and (ii) intercarrier interference (ICI) occurs due to imperfect carrier synchronization between the relay nodes and the destination, both of which result in error-floors in the bit-error performance at the destination. We propose an interference cancellation algorithm for this system at the destination node, and show that the proposed algorithm effectively mitigates the ISI and ICI effects. In the case of AF protocol in cooperative networks (without SFBC-OFDM), in an earlier work, we have shown that full diversity can be achieved at the destination if phase compensation is carried out at the relays. In cooperative networks using SFBC-OFDM, however, this full-diversity attribute of the phase-compensated AF protocol is lost due to frequency selectivity and imperfect carrier synchronization on the relay-to-destination channels. We propose an interference cancellation algorithm at the destination which alleviates this loss in performance.

Performance Analysis of Two-Branch Transmit Diversity Block-Coded OFDM Systems in Time-Varying Multipath Rayleigh-Fading Channels

Based on Alamouti code, Lee and Williams proposed two-branch transmit diversity block-coded orthogonal frequency-division multiplexing (TDBC-OFDM) systems, namely, space-time block-coded OFDM (STBC-OFDM) and space-frequency block-coded OFDM (SFBC-OFDM). However, they employed the simple maximum-likelihood (SML) detector, which was designed under the assumption that the channel is static over the duration of a space-time/frequency codeword. Therefore, STBC-OFDM/SFBC-OFDM suffers from the high time/frequency selectivity of the wireless mobile fading channel. In this paper, besides the original SML detector, three detectors proposed by Vielmon et al. are applied to improve the two-branch TDBC-OFDM systems. Additionally, assuming sufficient cyclic prefix, the performances of all systems in spatially uncorrelated time-varying multipath Rayleigh-fading channels are evaluated by theoretical derivation and computer simulation, as well. According to the derived bit-error rate (BER), we further derive the bit-error outage (BEO) to provide a more object judgment on the transmission quality within a fading environment. Numerical results have revealed that significant performance improvement can be achieved even when the systems are operated in highly selective channels.

Space-time block-coded OFDM systems with RF beamformers for high-speed indoor wireless communications

A beamforming network is considered for a space-time block-coded orthogonal frequency-division multiplexing system in a high-speed indoor wireless network. We find that choosing the most powerful beams for transmission provide the best performance in the absence of interference. In the presence of interference, an iterative two-metric beam-selection method is proposed.

Iterative receivers for space-time block-coded OFDM systems in dispersive fading channels

We consider the design of iterative receivers for space-time block-coded orthogonal frequency-division multiplexing (STBC-OFDM) systems in unknown wireless dispersive fading channels, with or without outer channel coding. First, we propose a maximum-likelihood (ML) receiver for STBC-OFDM systems based on the expectation-maximization (EM) algorithm. By assuming that the fading processes remain constant over the duration of one STBC code word and by exploiting the orthogonality property of the STBC as well as the OFDM modulation, we show that the EM-based receiver has a very low computational complexity and that the initialization of the EM receiver is based on the linear minimum mean square error (MMSE) channel estimate for both the pilot and the data transmission. Since the actual fading processes may vary within one STBC code word, we also analyze the effect of a modeling mismatch on the receiver performance and show both analytically and through simulations that the performance degradation due to such a mismatch is negligible for practical Doppler frequencies. We further propose a turbo receiver based on the maximum a posteriori-EM algorithm for STBC-OFDM systems with outer channel coding. Compared with the previous noniterative receiver employing a decision-directed linear channel estimator, the iterative receivers proposed here significantly improve the receiver performance and can approach the ML performance in typical wireless channels with very fast fading, at a reasonable computational complexity well suited for real-time implementations.

A space-time block-coded OFDM scheme for unknown frequency-selective fading channels

We introduce a space-time block-coded orthogonal frequency-division multiplexing (STBC-OFDM) selective fading channels which does not require channel knowledge either at the transmitter or at the receiver. The decoding algorithm is based on generalized maximum-likelihood sequence estimation. We investigate the performance of the proposed scheme over two-tap Rayleigh fading channels. Simulation results show the performance to be near optimum.