Manora Caldera

On the development of a reduced-reference perceptual image quality metric

User-oriented image quality assessment has become a key factor in wireless multimedia communications. In particular, perceptual quality assessment methods are required to measure the overall perceived service quality based on the grading given by human subjects. This paper focuses on the development of a reduced-reference perceptual image quality metric, which can be applied for in-service quality monitoring and link adaptation purposes. In contrast to the conventional image fidelity metrics such as the peak signal-to-noise ratio (PSNR), the proposed hybrid image quality metric (HIQM) takes the human perception into account. In addition, HIQM does not rely on the availability of the full reference of the original image at the receiver.

Unequal error protection for ROI coded images over fading channels

Region of interest (ROI) coding is a feature supported by the Joint Photographic Experts Group 2000 (JPEG 2000) image compression standard and allows particular regions of interest within an image to be compressed at a higher quality than the rest of the image. In this paper, unequal error protection (UEP) is proposed for ROI coded JPEG 2000 images as a technique for providing increased resilience against the effects of transmission errors over a wireless communications channel. The hierarchical nature of an ROI coded JPEG 2000 code-stream lends itself to the use of UEP whereby the important bits of the code-stream are protected with a strong code while the less important bits are protected with a weaker code. Simulation results obtained using symbol-by-symbol maximum a posteriori probability (MAP) decoding demonstrate that the use of UEP offers significant gains in terms of the peak signal to noise ratio (PSNR) and the percentage of readable files. Moreover the use of ROI-based UEP leads to reduced computational complexity at the receiver.

Utilising objective perceptual image quality metrics for implicit link adaptation

An implicit link adaptation technique based on hybrid automatic repeat request (H-ARQ) using a soft-combining algorithm is considered for image transmission over wireless channels. Instead of the conventional link-level metrics such as bit error rate and frame error rate that do not correlate well with the quality as perceived by the end user, adaptation is carried out using metrics obtained from models that take the human perception into account. In this paper, a novel reduced-reference hybrid image quality metric (RR-HIQM) is proposed and used as a criterion to terminate the retransmission request in link adaptation. Numerical results show that the use of RR-HIQM in link adaptation provides robust link performance while meeting satisfactory quality constraints.

Performance of an adaptive QAM scheme over correlated Rayleigh fading with non-zero delay

Link adaptation techniques have recently been proposed as a spectrally efficient method of obtaining high quality service for mobile communication systems. These schemes aim to utilise channel capacity better than fixed transmission schemes, by adapting signal transmission parameters, such as modulation constellation and transmit power. Adaptive modulation schemes, which adapt the modulation constellation, have gained considerable favour for exploiting time-varying channel conditions without increasing the level of cochannel interference. However, the traditional adaptive modulation schemes are designed assuming zero delay between the channel estimation and the modulation mode adaptation. In the case of non-zero delay, the transmitter would update the modulation mode based on outdated channel state information from the receiver. We investigate an adaptive quadrature amplitude modulation (AQAM) scheme, which, by way of retransmissions, allows a targeted reliability level to be met, even in the presence of non-zero delay. The effect of non-zero delay on performance is then investigated. Closed form expressions for the average number of bits per symbol (BPS) throughput and the average bit error rate (BER) of the proposed scheme are derived.

APP decoding of non-binary block codes on Gilbert-Elliott channels using generalized weight polynomials

In this paper, we present an a posteriori probability (APP) decoding algorithm for non-binary block codes on non-binary Gilbert-Elliott channels (GECs) using generalized weight polynomials. The proposed approach is based on a single-sweep APP decoding technique that utilizes matrix multiplications. By fixing the crossover probability in the dasiabadpsila state of the non-binary GEC such that for a given transmitted symbol, all symbols are equally likely to be received, an APP decoding decision can efficiently be reached by evaluating trivariate polynomials. In this case, the non-binary GEC is described by three variables that are referred to as the average fade to connection time ratio, the burst factor, and the channel reliability factor. The application of the generalized weight polynomial approach is demonstrated with respect to numerical performance results obtained for simple non-binary block codes from computer simulations.

Prefiltered Turbo Equalization With SINR Mismatch

In this paper, we consider the use of a channel shortening prefilter in conjunction with a turbo equalizer, in order to allow its use with arbitrarily long channel impulse responses. We show that the residual intersymbol interference (ISI), caused by imperfect channel shortening, results in considerable performance loss. However, by intentionally introducing a particular signal-to-interference plus noise ratio (SINR) mismatch, some of the penalty incurred can be overcome. We also show that the coloring of the noise through the prefllter results in a significant performance loss, which is insensitive to SINR mismatch and cannot be improved by choosing an appropriate SINR mismatch.

Analysis of an adaptive QAM scheme with non-zero delay for Rayleigh fading channels

Link adaptation techniques have recently been proposed as a spectrally efficient method of obtaining high quality service for mobile communication systems. These techniques adapt signal transmission parameters, e.g. modulation constellation and transmit power, according to time-varying channel conditions, in order to better utilise channel capacity compared to fixed transmission schemes. Adaptive modulation schemes, which adapt modulation constellation, exploit time-varying channel conditions without increasing the level of co-channel interference, and have gained considerable favour for this reason. However, one drawback is the effect of estimation delay, whereby the transmitter updates modulation mode based on outdated channel state information from the receiver. In this paper, we propose an adaptive quadrature amplitude modulation (AQAM) scheme, which by way of retransmissions, allows a targeted reliability level to be met even in the presence of non-zero delay. The effect of non-zero delay on performance is then investigated, resulting in a closed form expression for the average number of bits per symbol (BPS) throughput.

Call quality monitoring for VoIP

A method of monitoring the speech quality of a Voice-over-IP call is described in this paper. This method is attractive because it employs the ITU-T standard P.862 for Perceptual Evaluation of Speech Quality which is well-known for its accuracy. In addition, with the proposed method, call quality is monitored without interfering with the call while it is in progress. The results corresponding to an implementation of this method shows call quality can be measured with excellent accuracy under typical network delays.

On in-service perceptual speech quality monitoring in cellular radio systems

A method for in-service monitoring of the end-user perceptual speech quality in cellular radio systems is proposed. This method incorporates the perceptual evaluation of speech quality (PESQ) algorithm to monitor the quality experienced by the end-user. Here, the monitoring is carried out at the transmitting side. In this case, the speech signal received by the end-user is estimated at the transmitter in accordance with a feedback signal. The performance of the proposed scheme has been investigated through extensive simulations for the Universal Mobile Telecommunication System (UMTS) using different speech coding rates and channel conditions. The results indicate that the proposed scheme can predict end-user quality with a root-mean-squared error (RMSE) of at most 0.15 using the mean opinion score (MOS) rating scheme. Such accuracy can be beneficial in applications such as radio resource management for satisfying the desired level of quality of service.

Enhanced channel shortened turbo equalization

In this paper, the use of a channel shortening prefilter in conjunction with a maximum a-posteriori probability (MAP) based turbo equalizer is considered. The prefilter shortens the effective channel, thereby reducing the number of equalizer states. As a result of channel shortening, residual intersymbol interference (ISI) appears at the input to the turbo equalizer and the noise becomes colored. To account for the ensuing performance loss, two enhancements to the scheme are proposed. Firstly, a feedback path is used to cancel residual ISI. Secondly, a carefully selected value for the variance of the noise assumed by the MAP-based turbo equalizer is used. Simulations are performed over the highly dispersive Proakis C channel. It is shown that the proposed enhancements give an improvement of approximately 0.65 dB with respect to the unmodified channel shortened turbo equalizer at a bit error rate (BER) of 10-5.