Ian Oppermann

Ping-pong effects in linear parallel interference cancellation for CDMA

The convergence behavior of linear parallel interference cancellation is investigated. Especially the so-called ping-pong effect, where the bit-error rate performance is found to oscillate between two different convergence patterns is studied in detail. This effect is shown to be a direct consequence of the extreme eigenvalues of the correlation matrix, allowing for an analytical approach. Intervals for the dominating eigenvalues within which ping-pong effects can occur are specified and illustrated by examples. It is shown that the decision statistic for traditional parallel cancellation will always exhibit oscillating behavior with either short or long codes. Relaxation factors, leading to weighted cancellation, are shown to be effective for alleviating oscillations and ping-pong effects at the expense of convergence rate. Asymptotic analysis for large systems is applied to uncover the convergence behavior for long code systems.

Convergence behaviour of linear parallel cancellation in CDMA

In this paper, the convergence behaviour of linear parallel interference cancellation is investigated. Especially the so-called ping-pong effect where the bit error rate performance is found to oscillate between two different convergence patterns is studied in detail. This effect is shown to be a direct consequence of the extreme eigenvalues of the correlation matrix, allowing for an analytical approach. Intervals for the dominating eigenvalues within which ping-pong effects can occur are specified and illustrated by examples. Relaxation factors, leading to weighted cancellation, are shown to be effective for alleviating oscillations and ping-pong effects at the expense of convergence rate.

Complex pseudo random spreading sequences with a wide range of correlation properties

This paper introduces a new family of complex valued pseudo random sequences, for use in coded multiple access communication systems. The sequences offer a very wide range of correlation properties. Based on a measure described, the correlation properties of sets of these sequences are compared to well known sequence sets and it is shown that the new family of sequences has superior qualities. Lower bounds on the aperiodic autocorrelation (AC) and aperiodic cross-correlation (CC) values for sequences of this family are introduced and it is shown that sequence subsets exist in which the average correlation values approach these bounds.

DS-CDMA performance with maximum ratio combining and antenna arrays

The mobile communication channel is very hostile to a DS-CDMA signal and therefore effective techniques are needed to enhance system performance and capacity. Further, since DS-CDMA capacity and performance is limited by the uplink, ways to improve the uplink performance is needed. By implementing antenna arrays, diversity schemes or a combination of antenna arrays and diversity techniques, the uplink performance can be improved substantially. In this study we consider a single cell with a base station at the center with mobiles uniformly distributed around it. As channel model a Nakagami distributed path gain is assumed. This model was chosen for flexibility (e.g., Rayleigh and Rice channel models can be approximated) and also since empirical data suggests that path fading statistics are adequately described by this distribution. At the receiver an array of M antennas is used to discriminate between the users based on their spatial diversity. The fading process at each of the antenna elements is statistically dependent and further improvements can be realized by making use of the independent fading characteristics of the received signal. To make use of this statistical independent information, the performance of a P branch Maximum Ratio Combining (MRC) receiver is also considered. We further investigate the performance of a combination of P clusters of M antennas separated by the coherence bandwidth of the channel, thereby making use of both forms of spatial diversity. A comparison of the three schemes (antenna arrays, MRC diversity and a combination of antenna arrays and MRC diversity) under equal complexity conditions are made under multipath fading conditions. It is shown that the performance and capacity of a MRC diversity receiver outperforms the other two methods when perfect power control is assumed.

Performance of decorrelating receivers in multipath Rician fading channels

This letter focuses on the performance analysis of the decorrelating receiver in multipath Rician faded CDMA channels. M-ary QAM scheme is employed to improve the spectral efficiency. Approximate expressions are first derived for the two performance indexes: the average symbol error rate (SER) and the average bit error rate (BER) when the decorrelating-first receiver perfectly knows the channel information of the user of interest. To achieve desirable closed-form expressions of the SER and the BER, we exploit results in large system analysis and make assumptions of a high signal-to-interference ratio (SIR) and/or a small Rician K-factor. To measure the receiver performance in the practical scenario, we further derive expressions to approximate the average SER and BER of the decorrelating-first scheme with channel uncertainty. Simulation results demonstrate that the analytical results can also be employed to evaluate the performance of the combining-first receiver

Modelling and simulation of an indoor radio channel at 20 GHz

This paper reports the results of measurements and analysis of the indoor radio propagation channel at 20 GHz. The experiment consisted of taking impulse response measurements in several locations corresponding to typical office environments while varying numbers of users carried out normal daily activities. This paper presents some of the parameters extracted from the measured impulse responses which characterise the channel and details the parameter extraction process used. These parameters are used in a channel simulation model and the output of this simulation is compared to the original data on the basis of direct and multipath amplitudes and probability density functions.

Wide-band fading channel model for micro-cellular systems

This paper presents a model for a wide-band fading channel for terrestrial mobile applications. The model is based on the results of measurements made in a heavily built-up urban environment using a 25 MHz (bandwidth) signal centred at approximately 2.6 GHz. This paper presents measured impulse responses and details the parameter extraction process used to determine the characteristics of the channel. These parameters are used in the channel simulation package and the output of these simulations are compared to the original data.

Capacity of digital cellular CDMA system with adaptive receiver

This paper presents the results of a capacity evaluation for a cellular code-division multiple access (CDMA) system over a wideband fading channel. The study compares the performance of a system based on a conventional matched filter with that based on an adaptive receiver. Trellis codes and various rate convolutional codes are investigated in an attempt to improve the system performance. The performance is measured in terms of the maximum number of simultaneous users per cell for a given bit error rate (BER). The channel model is developed from measured propagation data at 2.6 GHz in heavily built-up urban areas and includes the effect of both intra-cell and inter-cell interferers.

Performance of a CDMA System with an Adaptive Receiver and a New Familyof Complex Valued Spreading Sequences

This paper presents the results of a capacity evaluation for a direct-sequence (DS), code-division, multiple access (CDMA) system over both an additive white Gaussian noise (AWGN) channel and a Rayleigh fading channel. The system considered uses an adaptive receiver. The performance of sets of sequences from various binary and complex valued families is examined. The performance of the system is measured in terms of the total efficiency, and it is shown that the new sequence sets offer higher system capacity and lower signal-to-noise power spectral density ratios.

Coded direct sequence spread spectrum systems over satellite channels using an adaptive receiver

This paper examines the performance of a direct sequence spread spectrum multiple access (DS-SSMA) system used over two typical, frequency-selective, fading channels for intermediate circular orbit (ICO) satellites. In an attempt to increase the system efficiency, a timing and phase synchronisation insensitive, adaptive receiver described by Rapajic and Vucetic (1994) has been implemented. The receiver is a least-mean-squares (LMS) implementation of a minimum mean square error (MMSE) receiver. This system has been combined with soft-decision convolutional coding in order to improve the system performance under the fading conditions relative to the uncoded system and to allow as many simultaneous users as possible. Various code rates have been examined and the results are given. This paper specifically focuses on DS-SSMA systems with low spreading ratios. The satellite channels used in this paper were produced by models developed as a result of experimental measurements.