In this paper, we derive expressions for the bit error probability of QPSK/OFDM on frequency-selective Rayleigh fading channels. In the OFDM system, ICI (interchannel interference) caused by Doppler spread of the channel degrades the error performance of the system and introduces the error floor even for coherent detection. Analysis results show that the error performance of QPSK/OFDM can be degraded as the normalized maximum Doppler frequency fD /Bsub is increased where fD is the maximum Doppler frequency and Bsub is the subchannel bandwidth. Computer simulations confirm the theoretical analysis results for BPSK and QPSK signals.

We propose a new approach associated with the use of some selected sets of Walsh Hadamard codes for joint estimation of channels and the number of transmit antennas by employing only one OFDM pilot symbol. This allows transmit antenna diversity to be applied in systems which have a limited number of training symbols (preambles), e.g. HIPERLAN/2. The proposed approach does not require any a priori knowledge about the number of transmit antennas, providing flexibility in the number of antennas to be used. In addition, adaptive scheme associated with the proposed approach provides more accurate estimations of the channels. The effectiveness of the proposed approach is evaluated through simulation. Results show that the proposed scheme provides significant improvement over previous channel estimation schemes and has almost the same performance as the ideal system with the full knowledge of the channel state information.

In this letter, we present a normalized least-mean-square algorithm of blind estimator for carrier frequency offset estimation of orthogonal frequency division multiplexing systems. In conjunction with the closed-loop estimate structure, the proposed efficient algorithm eliminates the inter-carrier interference for time varying carrier frequency offset. The proposed algorithm offers faster convergence speed and more accuracy to the carrier frequency offset estimate. Several computer simulation examples are presented for illustrating and effectiveness of the proposed algorithm.

In this letter, we propose a robust IFDD scheme employing an interference canceller, which is used for mitigating interferences from the transmitting signal instead of complex filter bank to reduce the complexity, for the OFDM system using feedback information. According to simulation results, the proposed IFDD OFDM system does not show significant performance degradation but maintains the robustness against the fast time-varying multipath channel, while the TDD OFDM system estimating feedback information from receiving block makes serious performance degradation.

Carrier frequency and symbol timing errors may greatly degrade the performance of the orthogonal frequency division mulitplexing (OFDM) system, especially in multipath environment. In this paper, we explore the cyclostationarity of OFDM signals, which only relies on second order statistics, to estimate the synchronization offset. First, a coarse carrier frequency offset estimator for multipath environment is developed using the second order statistics of the received OFDM signal. It has a wide capture range though not accurate. Second, we introduce a new synchronization algorithm based on cyclostationarity and matched filter theories, which can get the maximal estimation SNR in multipath environment. Both estimators utilize channel state information to achieve better estimation performance and are non-pilot aided. They can be combined to form a whole OFDM synchronizer for multipath environment. Finally, simulations confirm the performance of the estimation algorithm.

In this letter, we study the performance of the iterative receiver as applied to the space division multiplexing/orthogonal frequency division multiplexing (SDM/OFDM) systems. The iterative receiver under consideration employs the soft in/soft out (SISO) decoding process, which operates iteratively in conjunction with channel estimation for performing data detection and channel estimation at the same time. As opposed to the previous studies in which the perfect channel state information is assumed, the effects of channel estimation are taken into account for evaluating the performance of the iterative receiver and it is shown that the channel estimation applied in every iteration step of the iterative receiver plays a crucial role to warrant the performance, especially at a low signal-to-noise power ratio (SNR).

In an orthogonal frequency division multiplexing (OFDM) system, the bit error performance is degraded in the presence of multiple propagation paths whose excess delays are longer than the Guard Interval (GI), because the orthogonality between subcarriers cannot be maintained. In this paper, we propose a new OFDM demodulation method with a variable-length effective symbol and a multi-stage inter-carrier interference (ICI) canceller, in order to improve the bit error performance in the presence of multipaths whose excess delays are longer than the GI. The influence of the inter-symbol interference (ISI) is eliminated by the variable-length effective symbol, and then the ICI component is reduced by the multi-stage ICI canceller. The principle of the proposed method is explained, and the performance of the proposed method is then evaluated by computer simulation. The results show that the proposed method improves the system availability under more various multipath fading environments without changing the system parameters.

In an orthogonal frequency division multiplexing (OFDM) system, the bit error performance is degraded in the presence of multiple propagation paths whose excess delays are longer than the Guard Interval (GI), because the orthogonality between subcarriers cannot be maintained. Therefore, the GI has to be long enough for an expected delay spread of the channel. On the other hand, a long GI causes a decrease in transmission efficiency. In this paper, we propose a new OFDM demodulation method with a variable effective symbol duration, in order to improve the bit error performance in the presence of multipaths whose excess delays are longer than the GI. The proposed method can realize more stable radio communication systems under a multipath propagation environment even if a propagation path whose excess delay is longer than the GI exists. In other words, the proposed method can improve transmission efficiency without performance degradation by a shortened GI under the same environment. The principle of the proposed method is explained, and the bit error probability of the proposed method is analyzed theoretically in an AWGN channel and a multipath fading channel. The performance of the proposed method is then evaluated by computer simulation. The results show that the proposed method improves the system availability under more various multipath fading environments without changing the system parameters.

We propose a subchannel power control scheme in the OFDM system, which transmits data with a variable power level for each subchannel based on the received SNR. The OFDM system, employing the D-QPSK modulation and the proposed subchannel power control with a grouping coefficient equal to 3, gives about 2.3 dB gain in Eb/N0 comparing with the conventional OFDM system, under the two-ray multipath channel with the mean value of the second-ray's attenuation coefficient equal to 0.25, for the required BER equal to 10-5.

This paper focuses on the comparison of OFDM system channel estimation using time domain techniques and using frequency domain techniques. The channel model is based on the Taiwan DTV field-testing results, with static and dynamic multipath distortion. The simulation results prove that the channel estimation performance of the OFDM system in the time domain is better than in the frequency domain.

We evaluate the BER performance of the OFDM system with the one-tap equalizer bank under the two-ray multipath channel with the frequency offset by the simple Gaussian analysis method and by a proposed modified Gaussian analysis method. The proposed analysis method considers two adjacent inter-channel interferences, separately, and models the other inter-channel interferences as a Gaussian noise. It is shown that the proposed analysis method affords much closer results to the simulations than those by the simple Gaussian analysis method, when the frequency offset exists.

In this paper the novel method of "weighted OFDM" is addressed. Different types of weighting factors (including Rectangular, Bartlett, Gaussian, Raised cosine, Half-sin and Shanon) are considered. The impact of weighting of OFDM on the peak-to-average power ratio (PAPR) is investigated by means of simulation and is compared for the above mentioned weighting factors. Results show that by weighting of the OFDM signal the PAPR reduces. Bit error performance of weighted multicarrier transmission over a multipath channel is also investigated. Results indicate that there is a trade off between PAPR reduction and bit error performance degradation by weighting.

Orthogonal frequency division multiplexing (OFDM) has been receiving a lot of attention in the field of broadcasting because of its ruggedness under multipath environments. One of important issues to realize high quality reception of OFDM signals is to correct frequency and timing offsets between the transmitter and receiver so that orthogonality of the carriers can be maintained. This paper discusses a frequency and timing period acquisition technique for OFDM systems. A new offset estimation technique is introduced that detects both the frequency and timing peirod offsets at the same time by using only one pilot symbol with its suitable frequency assignment. A pseudo noise (PN) sequence is also introduced to assign these frequencies of the pilot symbol so that the frequency acquisition range can be widened. Numerical examples are given to show the estimate variances of the proposed frequency and timing period estimator over both additive white Gaussian noise (AWGN) and multipath fading channels. Also the bit error rate (BER) performance for an open loop acquisition system is examined.