M. P. Fitton

Evaluation of metrics for characterising the dispersion of the mobile channel

This paper analyses the suitability of the RMS delay spread and coherence bandwidth when characterising system quality-of-service of QPSK operating in a variety of wideband mobile channels. It is demonstrated that the RMS delay spread is unduly affected by the presence of relatively weak signals with large excess delay, limiting its effectiveness when characterising the time dispersion. The coherence bandwidth is shown to offer a more general dispersion metric. Furthermore it is indicated that the instantaneous system performance is poorly characterised by the dispersion parameters calculated from the discrete power delay profile (such as the RMS delay spread). The instantaneous delay spread incorporates the effects of pulse shaping and the vector summation which occurs in the instantaneous channel impulse response. Consequently, the instantaneous delay spread is demonstrated to provide a better indication of the instantaneous performance than the RMS delay spread.

Frequency hopping CDMA for flexible third generation wireless networks

Frequency hopping spread spectrum (FH-SS) has found a number of applications in cellular systems, wireless local loop, and wireless local area networks. The suitability of slow frequency hopping code division multiple access (SFH-CDMA) is characterised, for application in third generation wireless communications. An FH architecture displays inherent frequency diversity, and consequently is resilient to the effects of intersymbol interference arising from significant time dispersion in the channel. Furthermore, interference diversity of FH-CDMA results in a robust air interface technique. It is demonstrated that FH-CDMA can support the medium rate service bearers required for UMTS and PCS, whilst providing high cellular capacity in the urban environment.

Implementation issues of a frequency hopped modem

Investigations into code division multiple access (CDMA) have shown that frequency hopping (FH) possesses many of the advantages claimed by direct sequence (DS) spread spectrum techniques. This paper investigates the theoretical and practical implementation issues associated with designing a FH modem. The criteria which are explored include areas which are specifically associated with the frequency hopping nature of the modem, such as the the hopping synthesizer, hop duration, and hop bin separation. Furthermore, consideration is made of the impact of frequency hopping on more general design issues, such as coding, modulation, and multiple access. Work at the Centre for Communications Research (CCR) in Bristol has been carried out to develop a practical implementation of a prototype frequency hopped modem. The paper includes the modem design, and preliminary performance in an additive white noise channel.

The impact of spreading bandwidth on DS-CDMA power control requirements

Direct sequence code division multiple access (DS-CDMA) techniques have received a great deal of attention for current and future communication systems. One of the major considerations in implementing a DS-CDMA system is the necessity for accurate power control to ensure adequate quality-of-service and capacity. This paper investigates the power control errors that occur in a real microcellular environment for a variety of architectures. Furthermore, the mitigation of the effect of power control errors with varying spreading bandwidth is quantified.

Propagation aspects of mobile spread spectrum networks

Publisher Summary In this chapter, the propagation aspects of frequency hopping and direct-sequence spread spectrum are characterized in terms of providing a high-quality, high-capacity wireless communication network. In particular, the impact of the mobile channel on service provision is examined. It is indicated that both DS and FH-SS can be employed to mitigate the deleterious effects of the mobile environment, resulting in improved quality of service for a wireless multimedia network. Comparison of direct sequence and frequency hopping CDMA indicates similar voice-channel-capacity performance. In addition, FH-CDMA displays a greater soft capacity facility than DS-CDMA, which is important in the provision of a flexible cellular system. Furthermore, the nature of FH-CDMA eases the frequency planning requirements in a cellular network, as the interference pattern is randomized at each hop and consequently, no one interferer can dominate. In contrast, DS-CDMA requires complex closed-loop power control to mitigate the effects of the near-far effect. However, it is easier to exploit soft handover in a DS-CDMA network to provide an enhancement in the quality of service. The application of FH-CDMA is limited to medium data-rate services in high-time dispersion, such as urban macrocellular environments.

Frequency Hopping Code Division Multiple Access for Wireless Local Access

In this paper, the suitability of Slow FrequencyHopping Code Division Multiple Access (SFHCDMA) isevaluated for wireless local access applications.Investigations of the wireless channel indicate that frequency hopping mitigates the poorpropagation characteristics associated with low mobilitycommon to fixed wireless applications. Employingstatistical analysis and simulation models, it is shownthat the frequency hopped channel displays inherentfrequency diversity. Consequently, an FH architecture isresilient to the effects of intersymbol interferencearising from significant time dispersion frequently experienced in the wireless channel.Furthermore, interference diversity of SFH-CDMA resultsin a robust air interface technique. With simulationtechniques it is demonstrated that SFH-CDMA can support the medium rate service bearers (approximately1 Mbps) required for wireless local access, whilstproviding high capacity.

Propagation aspects of frequency hopping spread spectrum

Frequency hopping spread spectrum (FH-SS) has found a number of applications in both CDMA and TDMA cellular systems, wireless local loop, and wireless local area networks. The effect of FH-SS on mobile channel characteristics is evaluated. Employing propagation studies, statistical analysis and simulation models, it is shown that the frequency-hopped channel displays improved characteristics when compared to the non-hopped case. The short term fading statistics are improved, which can be exploited to provide an overall increase in quality-of-service. The short term statistics of the frequency-hopped channel are derived, enabling prediction of the performance of an FH system.