Employing multiple channels to improve the multi-hop wireless network performance has recently attracted a lot of attention. Given that hidden node collision is a key cause of the performance degradation in multi-hop wireless networks, we propose a solution to minimize the hidden node collision probability. It involves the use of a coordination mechanism, which tries to assign different frequency channels to two nodes that may be involved in hidden node collision. Also, a reserved bit in 802.11 MAC frame format is used to coordinate the channel assignments thereby minimizing the scheme's overhead. Simulation results show that an improved throughput can be achieved with our proposed scheme.

It is well known that dual-branch receivers suffer from in-phase (I) and quadrature (Q) imbalance problems resulting in distortion and poor signal quality. This paper presents a novel technique, incorporating adaptive estimation and compensation, to mitigate the combined effects of I/Q imbalance and multipath channel on the received signal. Estimation of the channel and I/Q imbalance parameters is performed in time domain using a training sequence. Based on the estimated I/Q imbalance and channel parameters, signal compensation are performed in the frequency domain. Computer simulations have shown that the proposed scheme is effective in mitigating the combined effects of I/Q imbalance and multipath fading for QPSK and 16-QAM modulation. The Symbol Error Rate (SER) achieved for the compensated 16-QAM signal is 10-4, at an SNR of 20dB, which is the same for ideal 16-QAM signal, at an SNR of 19dB.

This paper studies the influence of tolerances in inter-element spacing and element gain on the operation of the LLMS adaptive beamforming algorithm. Both random and worst case scenarios of inter-element spacing and element gain variations have been considered. Computer simulations show that these practical tolerances have greater influence on the beam pattern than the error vector magnitude (EVM). Simulated results also confirm the superior performance of the LLMS algorithm over the recursive least square (RLS) and the constrained stability LMS (CSLMS) algorithms when operating in the presence of multiple sources of co-channel interference.

A new adaptive algorithm, called LLMS, which employs an array image factor, AI, sandwiched in between two Least Mean Square (LMS) sections, is proposed for different applications of array beamforming. The convergence of LLMS algorithm is analyzed, in terms of mean square error, in the presence of Additive White Gaussian Noise (AWGN) for two different modes of operation; namely with either an external reference or self-referencing. Unlike earlier LMS based schemes, which make use of step size adaptation to enhance their performance, the proposed algorithm derives its overall error signal by feeding back the error signal from the second LMS stage to combine with that of the first LMS section. This results in LLMS being less sensitive to variations in input signal-to-noise ratio as well as the step sizes used. Computer simulation results show that the proposed LLMS algorithm is superior in convergence performance over the conventional LMS algorithm as well some of the more recent LMS based algorithms, such as constrained-stability LMS (CSLMS), and Modified Robust Variable Step Size LMS (MRVSS) algorithms. Also, the operation of LLMS remains stable even when its reference signal is corrupted by AWGN. It is also shown that LLMS performs well in the presence of Rayleigh fading.

A novel multiple access protocol, called DAMMA, is proposed for efficient handling of heterogenous traffic encountered in emerging wireless multimedia communications. To minimise contention collisions, DAMMA adopts random time slot selection in addition to the adaptive procedure for maintaining sufficient time slots available for contention. With DAMMA, time slots are selectively allocated to traffic of different service classes and priority levels to meet the diverse QoS requirements. Computer simulation results indicate that DAMMA can significantly enhance the throughput performance of data traffic. Under a given condition, the DAMMA protocol can achieve a maximum throughput of 91%, which compares well with 70% for the master-slave dynamic rate access (MSDRA) protocol adopted in the General Radio Packet Service (GPRS) system.

This paper examines the influence of fast Rayleigh fading and cochannel interference on GTFM with discriminator-MLSE detection using a hardware fading simulator. The degradation in BER performance, due to fading, can be largely recovered by using two-branch selection diversity. The resultant bit error patterns are also presented.

Continued investigation of tamed FM (TFM) has led to an extension called generalized tamed FM (GTFM). The very narrow spectral property of a GTFM signal meets the stringent requirements on bandwidth utilization in modern mobile radio applications. The principle of GTFM is first described. It is then shown that the generation of GTFM signals can be made very simple by the use of an ROM table-lookup technique. By an appropriate choice of parameters in GTFM signal generation at the transmitter, a joint optimization for coherent and noncoherent detection with regard to BER performance in the presence of additive white Gaussian noise can be achieved. For mobile radio applications, the robustness of the detection scheme to fast fading is important. A low-complexity noncoherent receiver is presented, Comprising a frequency discriminator and a simple maximum-likelihood sequence estimator. This receiver performs well in a fast fading environment. Computer simulated results of BER performance of this scheme are given. These results are confirmed by experimental measurements.

Continued investigation of Tamed FM (TFM) has led to an extension, called Generalized Tamed FM (GTFM). The features of GTFM are based on the possibility to select different combinations of parameters which characterize the process of waveform generation. This still allows for the generation of a constant envelope modulated signal with a spectrum similar to TFM. However, a joint optimization of coherent and noncoherent detection with regard to error performance can be achieved. Attention is given to a low-complexity, noncoherent receiver, comprising a frequency discriminator and a maximum likelihood estimator. The generation of GTFM signals can largely be based on a programmable LSI circuit.