FMAC: a highly flexible multiple-access protocol for wirelesscommunications systems

Due to the limited bandwidth of wireless networks, an efficient medium-access control protocol is essential to meet the growing demand of wireless access. Most multiple-access protocols require contentions (collisions) in the process of acquiring the transmission medium. While collisions cannot be avoided, successive collisions that consist of the same group of active stations are totally unnecessary. Successive collisions not only waste bandwidth, but also raise the concern of saturation in the channel. In this paper, we solve the problem of repetitive contentions involving the same set of stations by using the theory of finite projective planes. Due to the property of single-point intersection for an arbitrary pair of sets in the finite projective plane, we can minimize the number of unnecessary collisions. Protocol finite projective plane-based medium access control (FMAC) is highly flexible and has many features including adaptation for a mobile environment, support for priority assignment and handoffs in cellular networks, and extension of asynchronous transfer mode (ATM) services to mobile users. A performance evaluation shows that the throughput of the system is higher than that of slotted ALOHA. By dynamically adjusting the retransmission probability and the order of the finite projective plane, protocol FMAC can be stabilized     

A Novel Convergence Accelerator for the LMS Adaptive Filter

Due to its ease of implementation, the least mean square (LMS) algorithm is one of the most well-known algorithms for mobile communication systems. However, the main limitation of this approach is its relatively slow convergence rate. This paper proposes a booster using the Markov chain concept to speed up the convergence rate of LMS algorithms. The nature of Markov chains makes it possible to exploit past information in the updating process. According to the central limit theorem, the transition matrix has a smaller variance than that of the weight itself. As a result, the weight transition matrix converges faster than the weight itself. Therefore, the proposed Markov-chain based booster is able to track variations in signal characteristics and simultaneously accelerate the rate of convergence for LMS algorithms. Simulation results show that the Markov-chain based booster allows an LMS algorithm to effectively increase the convergence rate and further approach the Wiener solution. This approach also markedly reduces the mean square error while improving the convergence rate.     

Bandwidth allocation for TDMA-based wireless local loops

Wireless local loops are of interest due to their many advantages over the wireline systems. However, the bandwidth allocation schemes in wireless local loops are different from that of the cellular networks in some aspects since the terminals are fixed. In this paper, we propose a strategy of bandwidth allocation for time-division multiple-access based wireless local loops. We align the sectors assigned the same channels such that the cochannel interference is reduced. In the performance evaluation, we demonstrate that the proposed scheme improves the bandwidth efficiency and increases system capacity with minimal loss of signal-to-interference ratio     

Bit error rate reduction scheme for direct sequence CDMA in aRayleigh fading environment

A finite projective plane based time hopping scheme is presented for direct sequence CDMA with non-ideal power control in a Rayleigh fading environment. The advantages are two fold. First, the number of error bits is reduced significantly; secondly, the throughput is increased     

Distributed Power Balancing with Linear Prediction and Adaptive On-Off Strategies for CDMA Cellular Mobile Systems

Several distributed balance (DB) power control algorithms that can achieve SIRbalance have already been proposed for cellular mobile systems [1–3].In the present paper, two simple strategies, namely linear prediction andadaptive on-off strategies, are further applied to forward link distributedbalance (DB) power control in DS/CDMA cellular mobile systems. The linearprediction is used to track the variance of the short-term fading andcompensate it in advance, and the adaptive on-off strategy is applied to solvethe problem that the received SIR of all the communication links less than theminimum required SIR. Simulation results indicate that DB power control withthese two strategies can achieve much better performance than the original DBpower control.     

Design and Performance Evaluation of Space–time Receivers with Dynamically Adjustable Tap Delays

The delay elements of a conventional two-dimensional space–time receiver are fixed. The rather inflexible design is inappropriate since the received signals among the antenna branches are correlated due to limited spacing, and the fading environment is time-varying. In this paper we propose a dynamic finger assignment strategy for a space–time receiver, where the delay taps are adjusted dynamically, based on the theory of finite projective planes, to improve the signal-to-noise ratio in correlated antenna branches. The proposed finite projective plane (FPP) based finger assignment scheme maximizes the collected energy by dynamically setting the tap delays depending on the correlation among antenna branches and the delay spread of the channel. In the performance evaluation, we demonstrate that the FPP based finger assignment scheme can improve the signal-to-noise ratio significantly in comparison to the conventional mesh-grid configuration when the correlation among antenna branches is high and the channel dispersion is relatively long in comparison to the overall span of the fingers. However, the gain is moderate if the channel dispersion ratio is short, where the gain of reduced correlation among antenna branches must compensate for the reduced signal-to-noise ratio due to the missed signals.

Space Partitioning Hybrid Channel Assignment Scheme for Cellular Networks

The conventional approach of hybrid channel assignment strategy in cellular networks is rather inefficient due to the fact that it does not take advantage of the FCA scheme to the extreme. In this paper, we divide a cell into two parts: inner cell region and outer cell region, and apply the dynamic channel assignment and the fixed channel assignment schemes to the inner region and out region, respectively, in an attempt to fully utilize the strengths of the channel assignment schemes. In the performance evaluation, we demonstrate that the channel reuse efficiency has been improved compared to the FCA and DCA schemes. We also calculate the probability of an intracell handoff due to the use of the space partitioning. The proposed scheme can be adapted to a multi-tier structure with high/low speed mobile users, and hot spots.     

Orthogonal variable spreading codes for wideband CDMA

Orthogonal codes of variable lengths are essential for multirate services in the Third-Generation Partnership Project (3GPP). We propose an alternative to Walsh functions for variable spreading codes. There are three important points. First, we design an orthogonal code of constant length by mapping the points of a set of finite projective planes to chips of high/low amplitudes. Second, we develop variable spreading codes using multiple layers of recursion. Last, we compare and contrast the proposed variable spreading technique with Walsh functions in terms of ease of synchronization, blind-rate detection and wideband characteristics     

HRLS: a more efficient RLS algorithm for adaptive FIR filtering

The fast convergence rate and its immunity to the eigenvalue spread of the input correlation matrix make the RLS algorithm particularly attractive. However, the computational complexity is high. We propose using a hierarchical approach to reduce the computational complexity and further increase the convergence rate. The results of simulation runs and theoretical justifications confirm our claims     

A novel switching architecture for ATM networks

It is known that the flexibility and capacity of asynchronous transfer mode (ATM) networks can meet the bandwidth requirements of multimedia applications. In ATM networks, switching is one of the major bottlenecks of end-to-end communication. We propose using a multiple partitionable circular bus network (MPCBN) as an ATM switch. Connection requests are first transformed into a graph where vertices and edges represent connection requests and conflicts among connection requests, respectively. We then use a graph traversal algorithm to select a maximal set of requests for execution in physically partitioned buses. An approach of using finite projective planes is then used to reduce the number of switch points from O(N²) to O(N √N), where N is the number of ports of a switch. A performance evaluation for both uniform and bursty data sources shows that the approach of using finite projective planes to reduce the number of switch points results in a small increase of cell loss probability