Wai Man Tam

Exact analytical bit error rates for multiple access chaos-based communication systems

In this paper, exact analytical expressions for the bit error rates (BERs) in a multiple-access chaos-based digital communication system are derived. Comparisons are made with those obtained using traditional approximation methods which assume a Gaussian distribution for the conditional decision parameter. The obtained results are compared to the results of brute-force (BF) numerical simulations. It is found that the exact analytical BERs are in perfect agreement with BF simulations and hence provide better prediction of the BER performance than those given by traditional Gaussian-approximation-based methods.

Analysis of power control and its imperfections in CDMA cellular systems

We develop a general model to study the forward-link capacity of a code-division multiple access (CDMA) cellular system with power control. Using this model, two forward-link power control schemes (nth-power-of-distance power control scheme and optimum power control scheme) are examined. The increase in capacity by using power control has been studied. The capacities of the forward link are also compared with those of the reverse link with perfect power control. In the case of imperfect power control, an analytical model is presented to study the effect of power control error in a CDMA cellular system. The effect of the dynamic range of transmission, moreover, has also been analyzed.

A multiple access scheme for chaos-based digital communication systems utilizing transmitted reference

In this paper, a multiple access (MA) technique is proposed for chaos-based communication systems, in which chaotic reference signals are transmitted followed by the information-bearing signals. Chaotic reference signals modulated by a binary training sequence are sent periodically. The same chaotic signals are then modulated by the binary data and transmitted. To achieve MA, different chaotic signals and training sequences are assigned to different users. Two types of receivers are proposed. For the first one, an adaptive filter is employed which aims to reduce the inter-user interference. For the second receiver type, a simple inverse-and-average method is used in an attempt to recover the chaotic reference signal, which is then used to correlate with the information-bearing signals for determining the received symbols. The performance bounds of these two schemes are also derived. Finally, the bit error rates of the proposed system are simulated and compared.

A class of QC-LDPC codes with low encoding complexity and good error performance

We propose a novel approximate lower triangular structure for the parity part of the parity-check matrix of QC-LDPC codes. About half of the non-zero elements in the parity part are set to locate on the upper diagonal while the remaining non-zero elements can be located almost anywhere within the lower triangular area, provided certain rules are observed. Compared with the typical dual-diagonal structure, the proposed structure requires very similar encoding complexity and produces lower error rates over an AWGN channel.

Complex-Network Modeling of a Call Network

Recently, real-life data have revealed that the number of calls originating from or received by a telephone number in a network follows a power-law distribution. They show that a few telephone numbers make or receive a very large number of calls, whereas a large number of telephone numbers make or receive very few calls. The data have overthrown the general assumption that all telephone numbers are similar in generating telephone traffic. The first objective of this paper is to therefore construct a telephone call network (TCN) with connection properties following power-law distributions. With a more realistic TCN, researchers and engineers will be able to evaluate the telephone traffic behavior more accurately. Having constructed the aforementioned TCNs, we then consider the scenario when there is a sudden surge in the number of telephone calls, for example, during natural or man-made disasters. Under such a condition, the telephone network is usually overloaded and cannot operate properly. To mitigate the problem, we propose a preferential call blocking (PCB) scheme, aiming at blocking calls to target telephone numbers which have large numbers of incoming calls (in-strengths). We will investigate the effect on the carried traffic intensity when the PCB scheme is applied. We will compare the results with a benchmark, which corresponds to the case when all calls are blocked with equal probability. For the sake of completeness, we will also study the effectiveness of the blocking schemes when applied to a traditional TCN, in which all telephone numbers can call one another with equal probability.

Novel SIR-estimation-based power control in a CDMA mobile radio system under multipath environment

A novel decisive algorithm based on signal-to-interference power ratio estimation has been proposed to control the power transmission of the mobiles in a code-division multiple-access mobile radio system under multipath environment. A simple second-order equation, together with three different sets of measurement and power-control periods, is first used to predict the power gains of the channels. Based on the predicted power gains, five decision rules have been proposed to determine the power-control commands for the mobiles. The novel algorithm with the best prediction method and decision rule has been observed to accomplish a lower outage probability when compared with the conventional power-control algorithm with optimum power-control threshold.

A 2.0 Gb/s Throughput Decoder for QC-LDPC Convolutional Codes

This paper proposes a decoder architecture for low-density parity-check convolutional code (LDPCCC). Specifically, the LDPCCC is derived from a quasi-cyclic (QC) LDPC block code. By making use of the quasi-cyclic structure, the proposed LDPCCC decoder adopts a dynamic message storage in the memory and uses a simple address controller. The decoder efficiently combines the memories in the pipelining processors into a large memory block so as to take advantage of the data-width of the embedded memory in a modern field-programmable gate array (FPGA). A rate-5/6 QC-LDPCCC has been implemented on an Altera Stratix FPGA. It achieves up to 2.0 Gb/s throughput with a clock frequency of 100 MHz. Moreover, the decoder displays an excellent error performance of lower than 10-13 at a bit-energy-to-noise power-spectral-density ratio (Eb/N0) of 3.55 dB.

Analysis of telephone network traffic based on a complex user network

The traffic in telephone networks is analyzed in this paper. Unlike the classical traffic analysis where call blockings are due to the limited channel capacity, we consider here a more realistic cause for call blockings which is due to the way in which users are networked in a real-life human society. Furthermore, two kinds of user network, namely, the fully connected user network and the scale-free network, are employed to model the way in which telephone users are connected. We show that the blocking probability is generally higher in the case of the scale-free user network, and that the carried traffic intensity is practically limited not only by the network capacity but also by the property of the user network.

Novel predictive power control in a CDMA mobile radio system

A novel prediction-based decisive power control algorithm has been proposed in the reverse link of a CDMA mobile radio system. An adaptive transversal filter, with the tap weights updated using the recursive least-squares (RLS) algorithm is constructed to predict the power gain in the next power measurement period. With two different measurement periods defined, the power gain in the next power control period is then predicted. Based on the predicted power gains, we signal-to-interference power ratio (SIR) estimation-based decision rules have been proposed to determine the power control commands for the mobiles. The novel algorithm has been observed to accomplish a lower outage probability when compared with the conventional power control algorithm with optimum power control threshold. Moreover, the effects of the number of measurement periods used for prediction, the mobile speeds, the prediction methods and the decision algorithms have been examined.

Traffic congestion analysis in complex networks

The problem of traffic congestion in complex networks is studied. Two kinds of complex network structures, namely random graphs and scale-free networks, are considered. In terms of the structure of connection, random graphs are homogeneous networks whereas the scale-free networks are heterogeneous networks. For both types of networks, we introduce an additional scale-free feature in the load generation process such that a small number of nodes are more heavily loaded than others. A traffic model similar to the routing algorithm in computer networks is used in our simulation study. We show how the network structures and parameters influence the traffic congestion status