Frequency Selective Fast Fading Channels Research Articles

Tutorial for Space-Time ICI Parallel Cancellation Techniques for OFDM Systems

Orthogonal frequency division multiplexing (OFDM) expects the subcarriers to be orthogonal. However, the factors, such as residual carrier frequency, offset time variations due to Doppler shift or phase noise leads to a loss in the orthogonality between subcarriers and results in inter-carrier interference (ICI). Further developing the parallel cancellation (PC) scheme to mitigate the ICI of OFDM systems, the authors expand this OFDM symbol-based PC scheme into a space-time (ST) coded system. This simple space-time parallel cancellation (STPC) scheme is a technique that combines the useful properties of ST and PC schemes together. Computer simulations indicate that OFDM systems using the STPC scheme outperform the regular PC and ST systems in slow and fast frequency selective fading channels, specifically at a high signal-noise ratio (SNR). Furthermore, the error floor of the STPC-OFDM system is significantly lower than that of the regular ST systems without increasing computational load.

Estimation of Rice Factor Ratio for Doubly Selective Fading Channels

In wireless communication systems, Rice factor ratio (RFR) defined as K/(1 + K) is a key parameter not only to evaluate the quality of communication channel since it can reveal the severity of the small-scale fading, but also to be employed as a priori information for estimation of other parameters such as frequency. Consequently, its estimation is important for a variety of wireless application scenarios. In this paper, we propose an estimation algorithm on the RFR for the received signals that are disturbed by the Rician doubly selective fading channels and additive noise. During the estimation periods, we initially utilize the known signals to multiply the received signals. Second-order and fourth-order statistics are then employed to further deal with the processed signals mentioned above, which disposes of influence of some unnecessary parameters, e.g., indistinguishable multipaths, maximum Doppler shift, Doppler shift, and noise variance. Finally, a useful expression on the RFR estimation is derived for the Rician frequency selective fast fading channels by flexibly mathematical calculation. Furthermore, the presented method only uses the maximum estimation values of the second-order and fourth-order statistics defined in this paper, which can reduce the computational complexity. Importantly, the investigated scheme is robust to the signal-to-noise ratio over 0 dB and frequency offset (maximum Doppler shift and Doppler shift), and shows a slight improvement on the estimation performance with an increase of the aided data length. The performance and benefits of the proposed approach are verified and evaluated through computer simulations.

Turbo-Estimation for CPM Over Frequency-Selective Fast Fading Channels

The benefits of continuous phase modulation (CPM), namely high power and spectral efficiencies, are often overshadowed by the high complexity that comes along with the nonlinearity of the modulation format. We tackle the problem of detection of a quaternary CPM signal in a frequency-selective fast fading channel. In particular, we derive a joint detector-equalizer using the Laurent principal pulse approximation. To handle time-selectivity, we adopt the well-known turbo-estimation approach. However, it turns out that classical turbo-estimation cannot be directly applied, but has to be modified beforehand. With the help of Monte Carlo bit error rate (BER) simulations, we show that using the modified turbo-estimation approach, the receiver is able to track channel variations within a transmit burst under fast fading conditions. This allows for a larger payload-to-overhead ratio. Since joint detection-equalization is practically feasible only for channels with short memory, we also consider a suboptimal approach which decouples detection from equalization.

An Improved Channel Estimation Algorithm Based on Cubic Convolution Interpolation for LOWPM System

L ift ing scheme based orthogonal wavelet packet multiplexing (LOWPM) system can increase the computation efficiency and save storage space. B y using the orthogonality of the translational wavelet function s , LOWPM system efficiently avoids inter-symbol interference (ISI) and inter-channel interference (ICI) . Compared with the orthogonal frequency division multiplexing (OFDM) system, LOWPM system has higher spectrum efficiency and anti-jamming performance. However, in frequency-selective fast-fading channel , the performance of system is still not ideal. In order to implement high bit rate transmission, channel estimation technique is needed to combat noise of the channel. This paper presents an improved channel estimation algorithm in which the cubic convolution interpolation is applied to the pilot based linear minimum mean square error ( L MMSE) channel estimation algorithm so as to get higher estimation accuracy . Simulation results show that, the algorithm can effectively improve the performance of the LOWPM system.

Research on Complex Channel Characteristic for Hypersonic Vehicles

Hypersonic vehicles experience complex and bad communication environment including high dynamic Doppler and unique plasma sheath. Characteristics of the complex channel environment, which is divided into two distinguished channel part, will be discussed. The atmospheric propagation part of the complex channel is modeled as frequency-selective fast fading channel based on analysis of its coherence time and coherence bandwidth. The plasma sheath channel transmission (attenuation) and phase shift characteristics when radio wave pass through it is obtained by the scatter matrix method and FDTD algorithm. Finally a typical blunt conical hypersonic vehicle model is simulated to evaluate plasma sheath channel transmission and phase shift features, results show that communications might completely interrupted under high speed above 16 Mach because of small transmission coefficient and large phase shift.

Precise BER analysis of π/4-DQPSK OFDM with carrier frequency offset over frequency selective fast fading channels

An exact closed-form bit error rate (BER) expression is derived for an orthogonal frequency-division multiplexing system with π/4-shifted differentially encoded quadrature phase shift keying (π/4-DQPSK) in the presence of carrier frequency offset over frequency-selective fast Rayleigh fading channels. Different system configurations, including time domain differential modulation, frequency domain differential modulation, single channel reception, and multi-channel reception with maximal ratio combining diversity, are considered in the exact BER analysis. For a small number of subcarriers, the BER expression can be calculated directly. A Monte Carlo method is designed to evaluate the BER for a large number of subcarriers. The analytical expression can be used to investigate the effect of several channel parameters, including mean delay spread and maximum Doppler spread, on the system BER performance. Particularly, the effect of carrier frequency offset on the system performance, and the differences and opportunities to use frequency domain differential modulation or time domain differential modulation, can be studied in a quantitative way for a more realistic wireless channel environment model. By using the exact closed-form BER expression, an optimum number of subcarriers can be found, and the allowable carrier frequency offset, Doppler spread, and mean delay spread for given operating conditions can be determined.

Pilot Symbol Assisted Channel Estimation Techniques for OFDM Systems: A Review

Orthogonal frequency division multiplexing (OFDM) Is a potential technique for transmitting high rate data over wireless channels. Satisfactory performance can be achieved by using coherent modulation techniques together with the channel estimation. For frequency selective fast fading channels, the task of channel estimation becomes quite challenging for an OFDM system because channel response appears unequal in both frequency and time domain. Therefore, an independent estimation of the channel is necessary at each of the subcarriers in each individual OFDM symbol. This article describes various pilot symbol assisted (PSA) channel estimation techniques. The Minimum mean square error (MMSE) channel estimation is superior in performance as it exploits prior information about the channel. Different models of MMSE estimator, which are based on different approximations of channel correlation matrix, are also discussed. Finally, a brief description of Kalman filter based estimation is presented for the situations where inter carrier interference (ICI) can not be neglected.

Iterative Channel Estimation, Equalization, and Decoding for Pilot-Symbol Assisted Modulation Over Frequency Selective Fast Fading Channels

A pilot-based channel estimation scheme is proposed for frequency selective Rayleigh fading channels which work in conjunction with the existing paradigm of turbo equalization. The iterative nature of the channel estimation technique provides substantial gain over noniterative methods and makes it a suitable choice for iterative equalization and decoding. The channel estimator has low complexity, is decoupled from the equalizer soft-input soft-output module, and is capable of tracking significant channel variations within a codeword. The scheme is compatible with quadratic-amplitude modulation and with both parallel concatenated convolutional and low-density parity checks coding. The proposed scheme provides an attractive low-complexity alternative to iterative receivers based on state-space models for channel parameter evolution. The use of pilot symbols is demonstrated to aid the equalizer both directly through channel estimation and indirectly through pilot message insertion.

Adaptive Signal Detection Over Fast Frequency-Selective Fading Channels Under Class-A Impulsive Noise

The problem of adaptive detection of signals contaminated with Middleton's class-A impulsive noise and transmitted over a fast time-varying frequency-selective fading channel is addressed. Adaptive algorithms are derived to update the estimate of the channel parameters to the detector. A theoretical performance evaluation of the detector is provided. Computer simulations are performed to validate the theoretical developments.

Algoritmo LMS com Passo Adaptativo para Utilização em Equalizadores Cegos Bayesianos

A blind equalizer that performs joint estimation of channel and symbols is proposed. Its origin comes from two blind equalization schemes proposed in [1] which are based on the Kalman filter and the LMS algorithm. The performance of the most complex equalizer under multipath channel models is much higher than the least complex one. A better trade-off between performance and computational complexity was searched through an investigation of strategies to improve the LMS performance. An original result obtained from this investigation is the new blind equalization algorithm here proposed which uses LMS filters with adaptive step-size parameter based on a metric used in another work for blind detection of equalization errors. An extensive performance evaluation of these equalizers under fast frequency-selective fading channels is presented. The results reached by the proposed algorithm show a remarkable improvement in the performance compared with the scheme based on standard LMS, and a reduction of the complexity compared with the one based on Kalman filters.

DPSK versus pilot-aided PSK MAP equalization for fast-fading channels

This letter compares pilot-aided phase-shift keyed (PSK) and differential PSK (DPSK) modulation when using new maximum a posteriori joint channel estimation and equalization receivers with frequency-selective fast-fading channels. We conclude that pilot-aided PSK has superior bit-error rate performance in this case. However, at low signal-to-noise ratio, performance is similar, and DPSK is competitive due to reduced receiver complexity.

Simplified per-survivor Kalman processing in fast frequency-selective fading channels

Per-survivor processing (PSP) is now seen as an attractive approach to performing maximum-likelihood sequence estimation (MLSE) over mobile radio channels that are rapidly time varying. An optimal PSP strategy incorporates statistical channel modeling and Kalman filtering. For severely time-dispersive channels, this approach becomes prohibitively complex. A novel filtering algorithm is presented to approximate Kalman PSP. MLSE with the new scheme offers a large reduction in computational complexity, and achieves performance close to the optimal Kalman approach and superior to existing PSP schemes in rapidly fading channels. The exact expressions presented for the pairwise error probability of MLSE with Kalman PSP may be used to predict the detector performance without resorting to lengthly simulations.