Zheng Du

Selective Time-Domain Filtering for Reduced-Complexity PAPR Reduction in OFDM

A new selective time-domain filtering scheme for reducing the peak-to-average power ratio (PAPR) in orthogonal frequency-division multiplexing (OFDM) systems is proposed. The proposed scheme not only relies on conventional pilot symbol channel estimation and demodulation techniques to recover the data in OFDM systems but also uses additional pilot symbols. The proposed scheme has greatly reduced complexity compared with the selective mapping (SLM) scheme and only slightly poorer performance than the SLM scheme. The new scheme achieves significant PAPR reduction at a cost of typically 0.1-0.75 dB in signal-to-noise ratio (SNR) relative to pilot symbol OFDM not having PAPR reduction.

Maximum Likelihood Based Channel Estimation for Macrocellular OFDM Uplinks in Dispersive Time-Varying Channels

Coherent modulation is more effective than differential modulation for orthogonal frequency division multiplexing (OFDM) systems requiring high data rate and spectral efficiency. Channel estimation is therefore an integral part of the receiver design. Two iterative maximum likelihood (ML) based channel estimation algorithms are proposed for OFDM uplinks in dispersive time-varying channels. The uplink multipath fading channel is modeled such that the channel state can be determined by estimating the unknown channel parameters. A second-order Taylor series expansion is adopted to simplify the channel estimation problem. Based on the system model, an iterative ML-based algorithm is first proposed to estimate the discrete-time channel parameters. The mean square error performance of the proposed algorithm is analyzed using a small perturbation technique. Based on a convergence rate analysis, an improved iterative ML channel estimation algorithm is presented using a successive overrelaxation method. Numerical experiments are performed to confirm the theoretical analyses and show the improvement in convergence rate of the improved algorithm.

BER Analysis of BPSK Signals in Ricean-Faded Cochannel Interference

The bit error rate of a binary phase shift keying (BPSK) signal in Ricean-faded cochannel interference is studied. A precise bit error rate expression is derived for a bandlimited BPSK signal corrupted by an arbitrary number of asynchronous Ricean-faded interfering signals. An error rate estimation based on a saddlepoint approximation is also provided. It is shown that this approximation is highly accurate. For the special case, when there is one synchronous Ricean-faded interfering signal in an interference-limited environment, an asymptotic error rate analysis based on the saddle point approximation reveals that fading in the cochannel interfering signal can have different impacts on the desired users error rate performance. Our study shows that when the signal-to-interference ratio (SIR) is sufficiently large, the error floor of the desired user signal decreases with an increase of the Rice factor in the interfering users fading channel. However, the opposite phenomenon can happen when the SIR is small.

Error rate of OFDM signals on frequency selective Nakagami-m fading channel

The error rates of orthogonal frequency division multiplexing (OFDM) signals on frequency selective Nakagami-m fading channels are considered. The exact probability density function of a sum of Nakagami-m random vectors is used to derive a closed-form expression for the error rate of OFDM signals. The exact error rate analysis is also extended to a system using multichannel reception with maximal ratio combining. Our analysis and numerical results show that the error rates obtained using a previous Nakagami-m approximation can be unreliable. It is observed that, depending on the number of channel taps, the error rate performance may degrade with increasing values of Nakagami fading parameters.

Asymptotic Error Rate Analysis of Dual-Branch Diversity over Correlated Rician Channels

Asymptotic error rate expressions are derived for equal gain combining and selection combining on dual-branch correlated Rician channels. The asymptotic error rate performances of three popular diversity combining techniques are compared and some insights are obtained. The impacts of various channel parameters on the asymptotic performances of these diversity combining techniques are studied comprehensively. Some previous observations on diversity performances on correlated Rician channels are unified and further extended.

Decision-feedback detection for block differential space-time modulation

Time variation on fading channels hinders accurate channel estimation in differential space-time modulation and deteriorates the performance. Decision-feedback differential detection is studied for block differential space-time modulation, and compared with conventional differential space-time modulation. It is observed that the proposed scheme does not suffer effective fading bandwidth expansion, as does the conventional scheme. An improved effective signal-to-noise ratio approach is proposed for analyzing the performance of the proposed scheme in time-varying flat Rayleigh fading. Theoretical analysis and simulations show the improved performance of the proposed scheme over the conventional scheme.

A Convergence Study of Iterative Channel Estimation Algorithms for OFDM Systems in Dispersive Time-Varying Channels

Maximum-likelihood based channel estimation is considered for orthogonal frequency division multiplexing systems in dispersive time-varying channels. A successive overrelaxation approach is adopted to analyze the convergence rate of an iterative channel estimation algorithm. Based on the analysis, an improved fast converging iterative channel estimation algorithm is proposed. Numerical tests are performed to show the improvement of the proposed algorithm over a recently proposed channel estimation algorithm.

A new two level differential unitary space-time modulation

Differential unitary space-time modulation is an attractive space-time technique because it does not require channel estimation. In this paper, we propose a new differential space-time code with rate 2.5 bit/s/Hz for two transmit antennas. In contrast to the original unitary space-time codes with one level of amplitude, the powers of the matrices in this new differential space-time code are different. We design a decoding algorithm based on the Viterbi algorithm to differentially detect the transmitted data. Simulation results show that the new two amplitude level differential space-time code outperforms the original unitary space-time code with rate 2.5 bit/s/Hz.

BER analysis of BPSK signaling in Ricean-faded cochannel interference

The bit error rate of a binary phase shift keying signal in Ricean-faded cochannel interference is studied. A precise bit error rate expression based on a characteristic function method is derived for a bandlimited binary phase shift keying signal corrupted by an arbitrary number of asynchronous Ricean-faded interfering signals. For the special case when there is one synchronous Ricean-faded interfering signal, a Chernoff bound analysis is performed and it predicts that the error floor of the desired user signal decreases with an increase of the Rice factor in the interfering users fading channel. However, our precise bit error rate analysis results reveal that the opposite phenomenon can also happen, in particular when the signal-to-interference power ratio is low. A saddle-point approximation based error rate analysis is also provided. It is shown that this approximation is highly accurate. An asymptotic analysis based on the saddle-point approximation further reveals that a minimum signal-to-interference power ratio is required to have the desired users error rate performance improved by a less-faded interfering signal.

Block differential space-time modulation with decision-feedback detection in Rayleigh fading

Time variation on fading channels hinders accurate channel estimation in differential space-time modulation and deteriorates the performance. Decision-feedback differential detection is considered for block differential space-time modulation and compared with conventional differential space-time modulation. It is observed that the proposed scheme does not suffer effective fading bandwidth expansion as does the conventional scheme. An improved effective signal-to-noise ratio approach is proposed for analyzing the performance of the system. Theoretical analysis and simulation show the improved performance of the proposed scheme over the conventional scheme