International Conference on Information, Communications and Signal Processing ICICS '97 Singapore, 9-12 September 1997 Estimation of the Hysteresis Va 3D3.2 ue for Handover :Decision Algorithms using Bayes Criterion B.Senadji and B.Boashash Signal Processing Research Centre Queensland University of Technology 2, George Street Brisbane QLD 4001, Australia Abstract In mobile radio communications, inter-cell handover is the process whereby a call in progress is maintained while the mobile unit passes through different cells. Current handover decision algorithms compare the dif- ference between the received signal strengths from dif- ferent base-stations to a hysteresis value. The proposed paper is a contribution towards an optimal estimation of the hysteresis value using Bayes criterion [l]. An ex- pression of the threshold value h is derived in terms of the standard deviation IS of the log-normal shadowing affecting the received signals and the ratio 77 between the a priori probability of handover and probability of no handover. The performance of the handover deci- sion algorithm is evaluated in terms of the probability of false alarm, or probability of unnecessary handover, and probability of detection, or probability of successful handover. 1 Introduction In mobile radio communications, inter-cell handover for cellular, microcellular or picocellular is the process whereby a call in progress is maintained while the mo- bile unit passes through different cells. Handovers in cellular systems are less involved than in microcellu- lar and picocellular systems where the number of han- dovers per call is increased while the time available to process them is decreased [2]. Current handover deci- sion algorithms for microcellular and picocellular sys- tems are based on comparing the signal strengths re- ceived from the current and target base stations [3]. A handover is processed if the signal strength from one of the target base stations proves to be higher than the signal strength from the current base station. However, the fluctuations of the received signal strengths caused by Rayleigh fading arid Log-normal shadowing are re- sponsible for a high number of unnecessary handovers around the cell boundary (Ping-Pong effect). These al- gorithms have been improved by: 1. The use of window averaging for smoothing the received signals 2. The use of a hysteresis value for reducing the num- ber of unnecessary handovers. This technique is il- lustrated in Figure l for a simple situation where a mobile station is travelling from the current base- station BS1 towards a target base-station BS2. 'i lBS2,toBSl -h BSI to BS2 BSI to BS2 Figure 1: Handover Dlecision Algorithm: Use of a Hys- teresis Value The difference zk between the received signal strengths from the two base-stations is compared to a hysteresis value h. A handover is processed from BS1 to BS2 when the current base station is BS1 and zk < -h, and from BS2 to BS1 when the current base-sation is BS2 and Zk > h. The main drawback of these algorithms consist in the choice of the hysteresis value. When h is two small, a large number of unnecessary handovers may occur. When h is two large, the process of handover is delayed which may result in a drop of the signal quality, therefore in a lost call [4]. The proposed paper is a contribution towards an opti- mal estimation of the hysteresis value using Bayes cri- terion [l]. Each time measurements of signal strengths from the current and target base-stations are received, a 0-7803-3676-3/97/$10.00 0 1997 IEEE 1771