Medium Access Control Algorithms for Wireless Personal Area Networks Based on Channel State Estimation


Rico Schwendener


PhD thesis, Technische Wissenschaften ETH Z├╝rich, Nr. 15063, 2003, Series in Broadband Communications, Vol. 14, Hartung-Gorre Verlag Konstanz, ISBN 3-89649-850-9, pp. 119, 2003.

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The subjectof this thesis is the investigation of novel medium access control (MAC) algorithms which use channel state information (CSI) to improve the efficiency of wireless transmissions over time variant channels. The evaluation of the proposed algorithms is performed assuming high rate wireless personal area networks (WPANs). These short range radio systems are intended to provide high data rates for portable and wearable devices on an ad hoc network basis. Due to the mobility of WPAN nodes, fast changes of the transmission quality may occur. In order to reduce the probability of errors in a network with time variant links, the allocation of resources is based on the estimated instantaneous bit error ratio (BER) or packet error ratio (PER) of each contending link. If several concurrent requests for transmission are pending, one can exploit the multiuser diversity in the indoor environment. The throughput of error free packets in the network is considerably higher if CSI is available , even for small numbers of concurrent scheduled links. As an instrument for hte evaluation of MAC algorithms, we propose a novel finite state channel model. It is based on a semi-Markov process used to describe the quantized channel impulse response energy, and it includes the bias of the fading process. Compared to a finite state model which does not include this bias, our channel model leads to a good approximation of the time variant BER. Furthermore, the influence of impairments, like the error of a low complexity signal-to-noise ratio estmator, and of outadated CSI estimates is evaluated for a centralized MAC protocol. According to the presented results, system performance does not suffer much from the aforementioned impairments as long as the scheduling frame length is much smaller than the coherence time of the channel. Important issues related to channel state dependent MAC protocols are fairness and the provision of quality of service (QoS). Several of the presented scheduling rules, achieve fair scheduling with limited delay. Moreover, they do not cause a significant decrease in the throughput of error free packets compared to a scheduler without QoS support. The exploitation of multiuser diversity is not restricted to centralized scheduling. Thus, a novel distributed access scheme is introduced. Channel state dependent access times combined with a random backoff time allow to use CSI in a distributed manner. The throughput of error free packets is considerably increased when compared to that achieved by classic distributed access schemes based on a random backoff time.

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