27-08-2013, 03:19 PM
Soft Handoffs in CDMA Mobile Systems
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.
INTRODUCTION
Handoff 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.
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 traffic1 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.
Fade Margin Improvement
The outage probability in a system is defined as the probabili-
ty of dropping a call before it is terminated by either party,
and is denoted by Pout. This quantity is a function of distance
from the base station r, the minimum acceptable received sig-
nal power Sr,min, the path loss exponent μ (the power of the
signal decays as r –μ ), the shadow fading component ζ, the
availability of macroscopic diversity, and the transmitted sig-
nal power St. A system’s outage probability must satisfy Pout
(r0) ≤ Pout,max, where r0 is the radius of the service area within
which a minimum quality of service (QoS) — of which Pout is
a parameter — is guaranteed. The smallest S t required to
meet the Pout requirement is the parameter of interest in anal-
yses of soft handoff algorithms focusing on fade margins, a
term to be defined shortly.
Discussion and Conclusion
Soft handoff is an intriguing technology. It promises better
performance than hard handoff, through the exploitation
of macroscopic diversity and not having to use hysteresis
margins. Attempts have been made to substantiate these
claims by augmenting the qualitative arguments with quantita-
tive results. Several papers have shown that soft handoff has
some fade margin gain over hard handoff, and that there is a
possible uplink capacity increase in power-controlled systems
with soft handoff over power-controlled systems with hard
handoff. However, these are limited comparisons which do
not adequately balance the main advantages and disadvan-
tages of soft handoff.
Because of the complexity of soft handoff, another main
area of research on soft handoff is on the different trade-offs
involved in the handoff parameter settings. Most of the
research in this area, with the notable exception of [24], is in
the form of simulations. There are many variables and perfor-
mance indicators involved, and it is often unclear what is opti-
mal for a particular system. However, studies have indicated
that system performance may be very sensitive to the settings
of some parameters, so a deeper understanding of the trade-
offs and optimal parameter settings is essential to the success-
ful implementation of soft handoff.