Link Quality Measurement and Handoff Initiation

Cellular systems must have the ability to maintain a call while a mobile station (MS) moves throughout a cellular service area. This is accomplished by performing a handoff to an alternate base station (BS) whenever the link quality with the serving BS becomes unacceptable. A variety of parameters such as bit error rate (BER) [54], carrier-to-interference ratio (C/I) [103], distance [209], [85], traffic load, signal strength [209], [130], [132], [225], [41], [319], and various combinations of these fundamental schemes have been suggested for evaluating the link quality and deciding when a handoff should be performed. Of these, temporal averaging signal strength based handoff algorithms that measure the received carrier plus interference power (C+I) have received the most attention due to their simplicity and good performance in macrocellular systems. As a reflection of this trend, much of this chapter will be devoted to these types of handoff algorithms. However, spectrally efficient cellular systems are interference limited and a large C+I does not necessary imply a large C/I. Since radio link quality is more closely related to C/I than to C+I, it is apparent that C/I based handoff algorithms are highly desirable for microcellular systems with their characteristically erratic propagation environments. Unfortunately, C/I measurements can be quite difficult to obtain in practice [180], [348] and, hence, practical resource allocation algorithms often resort to received (C+I) measurements as a quality measure. Nevertheless, some discussion of C/I measurement techniques will be presented in this chapter.