Sergey Zhidkov

Analysis and comparison of several simple impulsive noise mitigation schemes for OFDM receivers

In this paper, we analyze and compare the performance of OFDM receivers with blanking, clipping and combined blanking-clipping nonlinear preprocessors in the presence of impulsive noise. Closed-form analytical expressions for the signal-to-noise ratio at the output of three types of nonlinearity are derived. Simulation results are provided that show good agreement with theory.

Impulsive noise suppression in OFDM-based communication systems

Orthogonal frequency division multiplexing (OFDM) is a technique used for terrestrial digital video broadcasting (DVB-T) and many other modern applications. The longer OFDM symbol duration provides an advantage in a presence of weak impulsive noise, because impulsive noise energy is spread among simultaneously transmitted OFDM sub-carriers. However, it has been recently recognized that this advantage turns into a disadvantage if the impulsive noise energy exceeds certain threshold. In this paper the algorithm for impulsive noise suppression in OFDM receivers is proposed and investigated. Whereas traditional methods for impulsive noise suppression are implemented in a time domain before OFDM demodulation, proposed algorithm compensates impulsive noise in a frequency domain after OFDM demodulation and channel equalization. The method is applied to DVB-T and its performance is studied by means of simulation.

Performance analysis and optimization of OFDM receiver with blanking nonlinearity in impulsive noise environment

A simple method of improving orthogonal frequency division multiplexing (OFDM) receiver performance in an impulsive noise environment is to precede a conventional OFDM demodulator with blanking nonlinearity. This method is widely used in practice since it is efficient and very simple to implement. However, performance analysis of this scheme has not yet appeared. In this paper, we study performance of the OFDM receiver with blanking nonlinearity in the presence of impulsive noise. Closed form analytical expressions for the signal-to-noise ratio (SNR) at the output of blanking nonlinearity and the optimal blanking threshold that maximizes SNR are derived. Simulation results are provided that show good agreement with theory if the number of OFDM subcarriers is sufficiently large.

Low-Complexity Iterative Equalization for Symbol-Reconstruction-Based OFDM Receivers Over Doubly Selective Channels

Time selectivity of multipath channels introduces significant inter-carrier interference (ICI) in OFDM systems demanding high levels of mobility and capacity. In this paper, a piecewise channel approximation is presented for improving the structure of the frequency-domain channel gain matrix, leading to a low-complexity iterative equalization scheme for OFDM receivers. The equalization is a sequential data detection based on a parallel-filtering architecture, avoiding matrix inversion adopted by classical equalizers with ICI cancellation. Meanwhile, information redundancy in the traditional cyclically prefixed OFDM is investigated for time-invariant channels, then a method of symbol reconstruction (SR) is proposed to enhance OFDM system performance, including compensation for SNR loss and improvements on the accuracy of parameter estimation. On the basis of the appropriate channel modeling in this work, the SR processing is proved to be effective for OFDM receivers over doubly selective channels. The iterative equalization, combined with the SR processing, is applied to the DVB-H receiver design. Numerical simulation indicates that the proposed equalizer significantly improves the immunity of OFDM systems to time selectivity with very little complexity increased, in contrast to the conventional one-tap equalizer without ICI cancellation.

A Composite PN-Correlation Based Synchronizer for TDS-OFDM Receiver

In this paper, we presents a novel synchronizer dealing with carrier frequency offset (CFO), sampling frequency offset (SFO), as well as frame timing offset (FTO) in TDS-OFDM receiver. The proposed schemes are based on tracking the output waveform of a composite PN-correlator (CPC), which provides sufficient correlative gains to detect its peaks even in the presence of large CFOs. From the correlation peaks, we can extract useful information for estimating the synchronization offsets. CFO is recovered by a multi-stage CPC scheme, of which the parameters are adjustable for meeting the systems demands on the tracking range and accuracy. According to the inter-frame variations of correlation waveform, we estimate SFO for a large scale and correct SFO through an interpolator. Meanwhile, frame timing is investigated in this paper, and the analysis indicates a very fast and robust timing scheme is possible for TDS-OFDM receiver. The developed synchronizer is quite robust against a large CFO even in very adverse fading channels, and it is shown by computer simulation that the residual synchronization error has little effect on the performance of TDS-OFDM receiver.

Performance analysis of multicarrier systems in the presence of smooth nonlinearity

Most of the performance studies devoted to the analysis of multicarrier systems in the presence of nonlinear distortion assume that the multicarrier signal can be modeled as a random Gaussian process regardless of modulation format, guard interval duration, and number of subcarriers. On the contrary, we present an analysis based on the discrete model of the multicarrier signal. It is shown that the discrete model provides more accurate signal-to-distortion ratio and out-of-band spectrum prediction if the number of subcarriers is small. Only the effect of third-order nonlinearity is studied; however results can also be extended to the fifth-order polynomial model. Simulation results are provided which confirm analytical derivations.

An adaptive detection and suppression of co-channel interference in DVB-T/H system

Channel state information (CSI) is defined as the signal-to-noise ratio (SNR) for a sub-carrier position. In white noise channels it is proportional to magnitude of the channel frequency response (CFR). However, if the channel introduces frequency selective interference such as interference from existing analogue TV systems (PAL/SECAM/ NTSC) and narrowband interference due to inter-modulation distortions in receiver, magnitude of CFR does not represent SNR values and system performance is significantly degraded. On the other hand, conventional methods for SNR estimation (using received data) do not give good results in white noise channels. To improve the reliability of the channel state measurements, we propose an adaptive technique for coded OFDM receiver, particularly for Digital Video Broadcasting-terrestrial and handheld ((DVB-T/H) receiver. In this paper, two methods are employed for CSI generation. The first method is based on channel frequency response provided by channel equalizer, whereas in the second method, channel state is obtained by measuring distance between received and de-mapped values for each sub-carrier of the OFDM signal. In most situations the first method provides better results, except in situations when frequency selective co-channel inference (CCI) is present in the received signal. In order to improve channel state measurements, CCI detector is used, and based on its decisions appropriate method for channel state measurements is selected. Extensive simulation results verify that the proposed technique has good performance in channels with CCI and no performance loss in white-noise channels.

A practical noise mitigation scheme for DVB-T/H system using delay spread estimation

In order to improve receiver performance in multipath channels, signal-to-noise ratio (SNR) should be maximized. In this paper, we propose and analyze several noise mitigation techniques for orthogonal frequency division multiplexing (OFDM) receivers, namely, the adaptive guard interval recycling algorithm and the adaptive interpolationbased channel estimation scheme. In realistic multipath environment, these techniques require accurate estimation of channel delay profile. Therefore, we also propose a robust and low-complexity channel impulse response analyzer for OFDM systems with lattice-type pilot arrangement. Extensive simulation results verify that the proposed techniques can improve bit error rate (BER) performance by maximizing SNR for a typical OFDM system such as digital video broadcasting -terrestrial and handheld (DVB-T/H) system1.

Receiver synthesis for nonlinearly amplified OFDM signal

Receiver synthesis for a nonlinearly amplified orthogonal frequency division multiplexing (OFDM) signal is presented. An optimal maximum-likelihood (ML) receiver is proposed and its computational complexity is discussed. Further, a sub-optimal receiver suitable for OFDM signals with a large number of sub-carriers and high-order constellation is presented. The performance of optimal and sub-optimal receivers for a nonlinearly amplified m-QAM-OFDM signal is studied by means of simulation.

‘CIR Analyzer’-based OFDM receiver for improving noise performance in delay-spread channel

In order to improve receiver performance in multi-path channels, signal-to-noise ratio (SNR) should be maximized. Therefore, in this paper, we propose a channel impulse response analyzer (CIRA)-based adaptive methods1 which can effectively reduce the relative received noise for demodulation and channel estimation. Corresponding adaptive guard interval (GI) reutilization method increases the SNR for demodulation while frequency interpolation method with filter coefficient bank reduces channel estimation error. By using computer simulation, we show that the proposed method can improve bit error rate (BER) performance by maximizing the SNR for typical Orthogonal Frequency Division multiplexing (OFDM) systems.