Both adaptive modulation and diversity combining are attractive techniques to combat fading and these two can be applicable to each digital-modulated symbol in OFDM transmission. In this letter, aiming to combat severe fading more effectively than the adaptive modulation, we theoretically analyze the benefit of a frequency diversity scheme within one OFDM symbol, which is a simple kind of coded OFDM (COFDM) based on IEEE 802.16 protocols. A simple closed form equation of bit error rate (BER) is derived, and then the advantages of correlated diversity gain and interference suppression by the diversity scheme are verified by both theoretical analysis and Monte Carlo simulation.

This paper presents a performance comparison between carrier interferometry coded OFDM (CI/COFDM) and typical coded OFDM (COFDM) using multiple receive antennas. The feature of CI/COFDM with diversity reception is to combine the time diversity benefit introduced by channel coding with the frequency and space diversity benefits created by frequency-domain equalization (FDE). Simulation results for QPSK showed that, at the perfect channel estimation, CI/COFDM outperforms COFDM under frequency and time selective fading channels because of the powerful time diversity benefit introduced by channel coding with time interleaving. However, at the imperfect channel estimation, this advantage of CI/COFDM over COFDM becomes very limited.

The Multiple Input Multiple Output (MIMO) technique is motivating the world-wide researchers to realize the next generation wireless LANs with the higher channel capacity and higher signal quality. This paper proposes and analyzes the Space Division Multiplexing Single Carrier OFDM (SDM-SCOFDM) system with adaptive modulation (AM) method over MIMO channels. The salient features of the proposed method are to enable the significant reduction of the number of required feedback adaptive modulation information (AMI) bits and the improvement of the PAPR (peak to average power ratio) performance at a cost of little degradation of channel capacity as comparing with the AM aided SDM-OFDM system. We also propose the Carrier-to-Noise Power Ratio (CNR) estimation method for SDM-SCOFDM signals over multi-path fading channels, which can be used for the assignment of the optimal modulation scheme in each transmit antenna. This paper presents the various computer simulation results to verify the proposed method under a typical wireless LAN environment.

Decision-directed, pilot-symbol-aided channel estimation (PSACE) for coded orthogonal frequency division multiplexing (COFDM) systems has structurally unavoidable processing delay owing to the generation of new reference data. In a fast fading environment, the channel condition which varies during the delay induces channel estimation error. This paper proposes a method of reducing this estimation error. In this method, channel equalization is performed for the received signal twice. One is done as pre-equalization with the delayed estimates of channel frequency response in order to update them periodically. At the same moment, the other is done as post-equalization for the received signal that is delayed by the processing delay time, with the same estimates as the pre-equalization. By the proposed method, more accurate channel estimation can be realized without significant output delay. Computer simulations are performed by utilizing the IEEE 802.11a packet structure of 24 Mbit/s. The result shows that the proposed OFDM transmission scheme having the delay time of 20 µs offers 2.5 dB improvement in the required Eb/N0 at PER = 10-2 in the ESTI-BRAN model C Rayleigh fading channel with fd = 500 Hz.

This paper proposes a space division multiplexed - coded orthogonal frequency division multiplexing (SDM-COFDM) scheme for multi-input multi-output (MIMO) based broadband wireless LANs. The proposed scheme reduces inter-channel interference in SDM transmission with a simple feed-forward canceller which multiplies the received symbols by the estimated propagation inverse matrix for each OFDM subcarrier. This paper proposes a new preamble pattern in order to improve power efficiency in the estimation of the propagation matrix. Moreover, the proposed likelihood-weighting scheme, which is based on signal-to-noise power ratio (SNR) of each OFDM subcarrier, improves the error correction performance of soft decision Viterbi decoding. Computer simulation shows that the proposed SDM-COFDM scheme with two transmitting/receiving antennas doubles the transmission rate without increasing the channel bandwidth and achieves almost the same PER performance as the conventional single-channel transmission in frequency selective fading environments. In particular, it achieves more than 100 Mbit/s per 20 MHz by using 64QAM with the coding rate of 3/4.

In this paper, we propose a new coded orthogonal frequency division multiplex (COFDM)-based ubiquitous antenna system, which is composed of multiple radio base stations (RBSs) deployed over the service area and Radio-on-Fiber (RoF) link that connects RBSs to the central control station (CCS). The proposed system is capable of receiving multiple mobile terminals simultaneously operating at the same frequency by making effective use of joint detection. However, the propagation delay due to the RoF link could be a major problem for realizing the ubiquitous antenna system. In order to overcome this delay problem, we assume that the guard interval of COFDM is longer than the delay difference. Furthermore, in order to improve BER performance in a multipath Rayleigh fading channel, we also propose the MSE normalization scheme followed by the use of an MMSE-based joint detector. Computer simulation results show that the proposed system can improve the frequency utilization efficiency of the broadband wireless access system.