Beatriz Soret

Analysis of QoS parameters in fading channels based on the effective bandwidth theory

Providing Quality of Service (QoS) guarantees is an important challenge in the design of next generations of wireless networks. In particular, real-time services involving stringent delay constraints are expected to be increasingly popular among users of mobile equipments. In Rayleigh channels, the delay requirement is usually expressed in terms of a probabilistic delay constraint composed by two terms: the target delay and the probability of exceeding the target delay.

In this thesis, a variable-rate multiuser and multichannel system using adaptive modulation is addressed. Specifically, the tradeoff among information source, fading channel and delay is analyzed, based on the effective bandwidth theory. Within the effective bandwidth framework, expressions of the channel effective bandwidth function (also known as effective capacity) are obtained on the channel side. Several scenarios are addressed: uncorrelated and time-correlated flat Rayleigh channels and an OFDM system under a frequency-selective Rayleigh channels. The procedure to obtain these functions is generic and could be applied to other channel models and scenarios.

The effective bandwidth theory makes feasible the analysis of the distribution tail of the delay. The percentile of the delay and the maximum information rate that can be transmitted over the channel under a target BER and a probabilistic delay constraint are evaluated. The delay suffered by certain information flow depends not only on the transmission rate but also on the distribution and self-correlation of the information process. Even in wired systems (constant rate channels) different distributions of the information process having the same average rate will cause different delays. Indeed, the better conditions for the delay are obtained when the incoming user traffic is constant. For any other source process, the delay performance degrades. Besides, the correlation of the channel process, in time or in frequency, has also a negative impact on the delay.

In the last part of the thesis, multiplexing of users over multiple shared fading channel is addressed. A new element comes up in this case: the scheduling algorithm. We calculate the maximum rate that each user can transmit by fulfilling a target BER and its own delay constraint, and under a given scheduling discipline. The analysis is done first in a single channel link and later on generalized to multiple shared channels employing OFDMA as multiplexing mechanism. Now it is not only the delay constraint and the channel and source process that influence the source rate, but also the discipline that rules the system. Three representative multiplexing algorithms are analyzed: Round Robin, Best Channel and Proportional Fair. The results make possible the comparison of the algorithms in terms of throughput, delay and fairness.

In summary, this thesis shows the high sensitivity of the delay to the burstiness of the traffic, to the time or frequency correlation of the channel and to the scheduling discipline. The proposed procedure is generic and can be extended to other disciplines and traffic and channel models. Nevertheless, the effective bandwidth function of the source and the channel process cannot always be explicitly evaluated. For such cases, a semi-analytical strategy is also proposed.