andoff is an essential compo- nent of mobile cellular communication systems. Mobility caus- es dynamic variations in link quality and interference levels in cellular systems, sometimes requiring that a particular user change its serving base station. This change is known as a handoff. In first-generation cellular systems like the Advanced Mobile Phone System (AMPS) [1, 2], handoffs were relatively simple. Second-generation cellular systems like the Global System for Mobile Communications (GSM) and the Personal Access Communication System (PACS) [2, 3] are superior to first-generation ones in many ways, including the handoff algorithms used. More sophisticated signal processing and handoff decision procedures have been incorporated in these systems. The control/decision structures have been improved so that in progressing from network-controlled toward mobile- assisted handoffs (MAHO) or mobile-controlled handoffs (MCHO), the handoff decision delay has been substantially reduced. Another idea that has been proposed for improving the handoff process is soft handoff, the subject of this article. Our purpose is to provide an overview of soft handoff, from the point of view of the performance benefits available and the trade-offs involved in selecting system parameters. Recent research on various aspects of soft handoff are reviewed and discussed. It is not the objective of this article to provide comprehensive coverage of every paper referenced, and the interested reader is encouraged to refer to the tables themselves for details. We do present a broad survey of the technical issues involved, with some details of the highlighted issues to assist the reader in developing a good understanding of those issues, and understanding the place of soft handoff in modern cellular systems. This article is organized as follows. In the rest of this intro- ductory section, we will discuss what soft handoffs are and why one might want to implement them in a cellular system. In the second section, we will look at benefits of soft handoff. However, in order to reap the benefits of soft handoff, it is necessary that the handoff parameters be well set. So in the third section we will examine the tradeoffs involved and dis- cuss the setting of soft handoff parameters based on an under- standing of the tradeoffs. A general discussion and conclu- sions will be given in the fourth section. What Is Soft Handoff? Soft handoff is so called to distinguish it from the more tradi- tional hard handoff process. With hard handoff, a definite decision is made on whether to handoff or not. On a positive decision, the handoff is initiated and executed without the user attempting to have simultaneous traffic 1 channel commu- nication with the two base stations. 2 With soft handoff, a con- ditional decision is made on whether to hand off. Depending on the changes in pilot signal strength from the two or more base stations involved, a hard decision will eventually be made to communicate with only one. This normally happens after it is clear that the signal from one base station is considerably stronger than those from the others. In the interim period, the user has simultaneous traffic channel communication with all candidate base stations. The difference between hard and soft handoffs is like the dif- ference between swimming relay events and track-and-field relay events. In swimming relays, the next swimmer starts just as the preceding one touches the wall, analogous to the switch from one base station to another in a hard handoff. In track-and- field relays, the baton is passed from one runner to the next after the second runner starts running, and so for a short time they are both running together, analogous to a soft handoff. IEEE Personal Communications • December 1997 6 1070-9916/97/$10.00 © 1997 IEEE H Soft Handoffs in CDMA Mobile Systems Daniel Wong, Stanford University Teng Joon Lim, National University of Singapore Abstract This article presents an overview of soft handoff, an idea which is becoming quite important because of its use in the IS-95 code-division multiple access (CDMA) cellular phone standard. The benefits and disadvantages of using soft handoff over hard handoff are discussed, with most results drawn from the available literature. The two most well-known benefits are fade margin improvement and higher uplink capacity, while disadvan- tages include increased downlink interference and more complex implementation. Handoff parameter optimization is extremely important, so various studies on the trade-offs to be considered when selecting these parameters are surveyed, from both the link quality and resource allocation perspectives. Finally, research directions and future trends are discussed. 1 In both soft and hard handoffs, there will normally be some simultane- ous control channel communication between the two base stations and the user according to the signaling protocol in use, so we must look at traf- fic channels to distinguish between hard and soft handoffs. 2 Handoff can also be between two channels at one base station. If the two channels are in two sectors of a sectorized cell, and the kind of handoff used is soft, this is sometimes known as “softer handoff’’ [4]. The IS-95 cellular standard does not discuss intersector handoff in much detail, and mostly assumes that all handoffs are intercell. In this article, we take hand- off to involve two or more base stations unless otherwise stated.