The proportionalfair (PF) scheduling algorithm has been proposed as a technique to improve the throughput of multiple packet-data users sharing a wireless downlink channel while preserving fairness. It exploits the fact that the propagation channels between the base station (BS) and the mobile stations (MS) fade independently giving rise to "multi-user diversity". The gain offered is significant when the allowed packet delay exceeds the de-correlation time of the fading channels and the number of users is high. In this paper we investigate analytically the "time allocation" property and the multi-user diversity gain of the PF algorithm under certain conditions in the context of the HSDPA standard. Since in practical systems a certain latency is unavoidable, we have studied the use of a "backoff factor to reduce the probability that when the transmission takes place, the channel will not be able to support it.

We focus on maximum ratio combining at each base station and switching between base stations (BSs) as a simple macrodiversity technique. We obtain analytical results for pointwise outage probabilities for systems using one or a combination of both techniques to cover a desired area, assuming a certain correlation model for the set of path losses of the links connecting a terminal to the receiving BSs. Pointwise outage probabilities are averaged over the entire region of interest to get an estimate of the outage in the desired region. A comparison of micro-and macrodiversity schemes in terms of the outage gives insights as to the tradeoff between the two forms of diversity in the design of a cellular system.

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Coexistence of different access technologies, hierarchical cellular deployment, a wide variety of data services, requirements for transparent operation across different technologies, adaptivity to varying network conditions and mobility and quality of service (QoS) constraints introduce a number of challenges in the design of future generation systems and the specification of new air interfaces, such as efficiency and flexibility in the utilization of spectrum, dynamic resource allocation and exploitation of the multiuser diversity and reconfigurable interference management and inter-cell coordination. In this paper, three critical issues for the design of next generation systems are addressed: (i) duplexing, (ii) scheduling and resource allocation and (iii) interference and inter-cell coordination. A number of research directions are presented, which constitute promising potential candidates for next generation systems specification.