Requirements and Limitations

Comparing market estimates for wireless personal communication and considering recent proposals for wideband multimedia services with the existing spectrum allocations for these types of systems show that spectrum resource management remains an important topic in the near and distant future. In this article the authors star t out by presenting a quite general formulation of the radio resource management problem where the three key allocation decisions are concerned with waveforms ("channels"), access ports (or base stations), and, finally, with the transmitter powers. Some current approaches to these problems found in the literature are reviewed. In particular, the principles of random channel allocation schemes, as found in frequency-hopping or direct-sequence CDMA systems, are compared with deterministic dynamic channel allocation schemes. The article closes by giving an outlook of some of the key problems in resource management in future wireless multimedia systems. Radio Resource Management in Future Wireless Net works : Requirements and Limitations Jens Zander, Royal Institute of Technology he rapid increase in the size of the wireless mobile community and its demands for high-speed multimedia communications stands in clear contrast to the rather limited spectrum resources that have been allocated in international agreements. Efficient spectrum or radio resource management (RRM) is of paramount importance due to these increasing demands. Figure 1 illustrates the principles of wireless network design. The network consists of a fixed network part and a wireless access system. The fixed network provides connections between bast. stations or radio access ports (RAPS), which in turn provide the wireless "connections" t o the mobiles. The RAPs are distributed over the geographical area where we wish to provide the mobile users with communication services. We will refer to this area simply as the service area. The area around an RAP where transmission conditions are favorable enough to maintain a connection ol the required quality between a mobile and the RAP is denoted the coverage area of the RAP. The transmission quality and thus the shape of these regions will, as may be expected, depend heavily on propagation conditions and current interference from otlier users in the system. Coverage areah are therelore usually highly irregular in shape. The fraction of the service area where communication with some required quality of service (QoS) is possible is called the coverage or area availability of the system. In two-way communication systems (such as mobile telephone systems), links have t o be established both from the RAP to the mobile (downor forward link) and from the mobile terminal to the RAP (upor reverse link). A t first glance these two links seem to have very similar properties, but there are some definite differences from a radio communication perspective. The propagation situation is quite different, particularly in widearea cellular phone systems, where the RAP (base station) usually has its antennas at some elevated location, free of obstacles. The terminals, on the other hand, are usually located amid buildings and other obstacles, creating shadowing and multipath reflections. Also, the interference situation in the upand downlinks will be different since there are many terminals and varying locations, and quite a few RAPs at fixed locations. For obvious economic reasons, we would like our wireless network to provide ample coverage with as few RAPs as possible. Clearly, this would not only minimize the cost of the RAP hardware and installation, but also limit the extent of the fixed wired part of the infrastructure. Coverage problems due to various propagation effects put a lower limit on the number of RAPS required. Although not quite correct, one could say that the range of RAPs is too small for the interRAP distance. The type of system in which this kind of problem is dominant is called range-limited. As the number of transmitters in the system becomes large within some fixed chunk of available radio frequency (RF) spectrum, the number of simultaneous connections (links) will become larger than the number of orthogonal signals the available bandwidth may provide. In order to provide service for such a large population of users, it is obvious that the bandwidth used by the RAPS and terminals has to be reused in some clever way at the cost of mutual interference. The system is said to be bandwidthor inle$erence-limited. Absolutely vital to the study of any resource management problem is a thorough understanding of the user requirements (i.e,. the required QoS and the traffic characteristics). All resource management schemes are designed (01optimized) using some model for the traffic. The resulting performance will clearly be a function of not only how well our design has been adapted to the traffic model, but also how accurate the traffic model is. Most wireless systems today use circuit-switched speech as the main design model (e.g., Global System for Mobile Communications, or GSM). This does not prevent such systems from carrying other types of traffic, but they always do this at a performance penalty. Future wireless access systems 30 0163-6804/97/$10.00