Adaptive channel equalization in WCDMA downlink with turbo coding (original) (raw)

Decision Feedback Equalization Receiver for DS-CDMA with Turbo Coded Systems

2005

 In this paper, adaptive equalizer receiver for a turbo code direct sequence code division multiple access (DS-CDMA) by using least mean square (LMS) adaptive algorithm is presented. The proposed adaptive equalizer is using soft output of decision feedback adaptive equalizer (DFE) to examines the output of the equalizer and the Log-maximum a posteriori (Log-MAP) algorithm for the turbo decoding process of the system. The objective of the proposed equalizer is to minimize the bit error rate (BER) of the data due to the disturbances of noise and intersymbol interference (ISI) phenomenon on the channel of the DS-CDMA digital communication system. The computer program simulation results shown that the proposed soft output decision feedback adaptive equalizer provides a good BER than the others one such as conventional adaptive equalizer, infinite impulse response adaptive equalizer.

Design rules for adaptive turbo equalization in fast-fading channels

2002

This paper uses the mutual information (MI) transfer metric to characterize the behavior of an adaptive turbo equalization scheme. We consider recursive systematic convolutional coding at the input to a multipath fastfading channel. The equalizer is a recent adaptive, nearoptimal APP algorithm that incorporates MMSE channel estimation. We develop design rules for this turbo equalizer using EXIT charts. We optimize the use of pilot bits and determine the best ratio of code bits to pilot bits, for fixed data and symbol rates.

Adaptive Complexity Equalization for the Downlink in WCDMA Systems

IEEE Vehicular Technology Conference, 2006

We consider the issue of terminal reconfigurability in the downlink of WCDMA. For the purpose of optimizing power consumption in mobile terminals, we propose an adaptivecomplexity equalization algorithm, which adapts the equalization length to the environment. A simple approach in WCDMA systems consists of computing the equalizer coefficients in frequency domain, and carry out channel equalization in time domain. The equalizer can be easily computed in frequency domain from channel estimates, which are generally obtained through pilot symbols. In our work, we decouple the task of adaptive complexity equalization in two parallel operations: variable length equalization and equalization length control. We propose a practical scheme to reduce equalization complexity by prewindowing in frequency domain and performing IFFT of variable length. An element of length control monitors the equalizer coefficients, updating the equalizer length at each stage. Both simulation and experimental results in outdoor-toindoor scenarios show good performance and significant power savings with respect to full length equalization.

Low-complexity turbo equalization and multiuser decoding for TD-CDMA

IEEE Transactions on Wireless Communications, 2004

The authors propose a low complexity multiuser joint parallel interference cancellation (PIC) decoder and turbo decision feedback equalizer for code division multiple access (CDMA). In their scheme, an estimate of the interference signal (both multiple-access interference and intersymbol interference) is formed by weighting the hard decisions produced by conventional (i.e., hard-output) Viterbi decoders. The estimated interference is subtracted from the received signal in order to improve decoding in the next iteration. By using asymptotic performance analysis of random-spreading CDMA, they optimize the feedback weights at each iteration. Then, they consider two (mutually related) performance limitation factors: the bias of residual interference and the ping-pong effect. The authors show that the performance of the proposed algorithm can be improved by compensating for the bias in the weight calculation, and they propose a modification of the basic PIC algorithm, which prevents the ping-pong effect and allows higher channel load and/or faster convergence to the single-user performance. The proposed algorithm is validated through computer simulation in an environment fully compliant with the specifications of the time-division duplex mode of third-generation systems, contemplating a combination of time-division multiple access and CDMA and including frequency-selective fading channels, user asynchronism, and power control. The main conclusion of this work is that, for such application, soft-input soft-output decoders (e.g., implemented by the forward-backward BCJR algorithm) are not needed to attain very high spectral efficiency, and simple conventional Viterbi decoding suffices for most practical settings.

Performance Analysis of MIMO Equalization Techniques with Highly Efficient Channel Coding Schemes

To combat the wireless fading impairment in the high network demand environment, various coding schemes have been implemented. MIMO techniques are still the powerful techniques along with source coding. This paper focuses on coherent implementation of high performance turbo codes with MIMO equalization techniques. It is proposed to achieve optimum BER value at very low values of SNR in a noisy environment.

Frequency-Domain Turbo Equalisation in Coded SC-FDMA Systems: EXIT Chart Analysis and Performance

2012 IEEE Vehicular Technology Conference (VTC Fall), 2012

In this paper, we investigate the achievable performance of channel coded single-carrier frequency-division multiple-access (SC-FDMA) systems employing various detection schemes, when communicating over frequency-selective fading channels. Specifically, three types of minimum mean-square error (MMSE) based frequency-domain (FD) turbo equalisers are considered. The first one is the turbo FD linear equaliser (LE). The second one is a parallel interference cancellation (PIC)-assisted turbo FD decision-feedback equaliser (DFE). The final one is the proposed hybrid interference cancellation (HIC)-aided turbo FD-DFE, which combines successive interference cancellation (SIC) with iterative PIC and decoding. The benefit of interference cancellation (IC) is analysed with the EXtrinsic Information Transfer (EXIT) charts. The performance of the coded SC-FDMA systems employing the abovementioned detection schemes is investigated with the aid of simulations. Our studies show that the IC techniques achieve an attractive performance at a moderate complexity.

Chip-Level Channel Equalization in WCDMA Downlink

EURASIP Journal on Advances in Signal Processing, 2002

The most important third generation (3G) cellular communications standard is based on wideband CDMA (WCDMA). Receivers based on TDMA style channel equalization at the chip level have been proposed for a WCDMA downlink employing long spreading sequences to ensure adequate performance even with a high number of active users. These receivers equalize the channel prior to despreading, thus restoring the orthogonality of users and resulting in multiple-access interference (MAI) suppression. In this paper, an overview of chip-level channel equalizers is delivered with special attention to adaptation methods suitable for the WCDMA downlink. Numerical examples on the equalizers' performance are given in Rayleigh fading frequency-selective channels.

Combined Channel Estimation and Turbo Equalization on Wireless Channels

2008 IEEE 68th Vehicular Technology Conference, 2008

To date most frequency-domain (FD) turbo equalization schemes assume ideal channel state information (CSI) is available. In this paper, a system combining FD turbo linear equalization with time-domain channel estimation is developed and evaluated for single-carrier modulation formats. The effect of estimated CSI on the equalizer form is shown. Performance results employing convolutionally encoded QPSK and 16-QAM transmissions show the efficacy of the proposed system and its capability to operate in different wireless scenarios.