05-09-2012, 03:38 PM
Handoff in Wireless Mobile Networks
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INTRODUCTION
Mobility is the most important feature of a wireless cellular communication system. Usually,
continuous service is achieved by supporting handoff (or handover) from one cell to
another. Handoff is the process of changing the channel (frequency, time slot, spreading
code, or combination of them) associated with the current connection while a call is in
progress. It is often initiated either by crossing a cell boundary or by a deterioration in
quality of the signal in the current channel. Handoff is divided into two broad categories—
hard and soft handoffs. They are also characterized by “break before make” and “make before
break.” In hard handoffs, current resources are released before new resources are
used; in soft handoffs, both existing and new resources are used during the handoff
process. Poorly designed handoff schemes tend to generate very heavy signaling traffic
and, thereby, a dramatic decrease in quality of service (QoS). (In this chapter, a handoff is
assumed to occur only at the cell boundary.) The reason why handoffs are critical in cellular
communication systems is that neighboring cells are always using a disjoint subset of
frequency bands, so negotiations must take place between the mobile station (MS), the
current serving base station (BS), and the next potential BS. Other related issues, such as
decision making and priority strategies during overloading, might influence the overall
performance.
HANDOFF INITIATION
A hard handoff occurs when the old connection is broken before a new connection is activated.
The performance evaluation of a hard handoff is based on various initiation criteria
[1, 3, 13]. It is assumed that the signal is averaged over time, so that rapid fluctuations due
to the multipath nature of the radio environment can be eliminated. Numerous studies
have been done to determine the shape as well as the length of the averaging window and
the older measurements may be unreliable. Figure 1.2 shows a MS moving from one BS
(BS1) to another (BS2). The mean signal strength of BS1 decreases as the MS moves away
from it. Similarly, the mean signal strength of BS2 increases as the MS approaches it. This
figure is used to explain various approaches described in the following subsection.
Relative Signal Strength
This method selects the strongest received BS at all times. The decision is based on a
mean measurement of the received signal. In Figure 1.2, the handoff would occur at position
A. This method is observed to provoke too many unnecessary handoffs, even when
the signal of the current BS is still at an acceptable level.
Relative Signal Strength with Hysteresis
This scheme allows a user to hand off only if the new BS is sufficiently stronger (by a hysteresis
margin, h in Figure 1.2) than the current one. In this case, the handoff would occur
at point C. This technique prevents the so-called ping-pong effect, the repeated handoff
between two BSs caused by rapid fluctuations in the received signal strengths from both
BSs. The first handoff, however, may be unnecessary if the serving BS is sufficiently
strong.
HANDOFF DECISION
There are numerous methods for performing handoff, at least as many as the kinds of state
information that have been defined for MSs, as well as the kinds of network entities that
maintain the state information [4]. The decision-making process of handoff may be centralized
or decentralized (i.e., the handoff decision may be made at the MS or network).
From the decision process point of view, one can find at least three different kinds of
handoff decisions.
Network-Controlled Handoff
In a network-controlled handoff protocol, the network makes a handoff decision based on
the measurements of the MSs at a number of BSs. In general, the handoff process (including
data transmission, channel switching, and network switching) takes 100–200 ms. Information
about the signal quality for all users is available at a single point in the network
that facilitates appropriate resource allocation. Network-controlled handoff is used in
first-generation analog systems such as AMPS (advanced mobile phone system), TACS
(total access communication system), and NMT (advanced mobile phone system).
Mobile-Assisted Handoff
In a mobile-assisted handoff process, the MS makes measurements and the network makes
the decision. In the circuit-switched GSM (global system mobile), the BS controller (BSC)
is in charge of the radio interface management. This mainly means allocation and release of
radio channels and handoff management. The handoff time between handoff decision and
execution in such a circuit-switched GSM is approximately 1 second.
Mobile-Controlled Handoff
In mobile-controlled handoff, each MS is completely in control of the handoff process.
This type of handoff has a short reaction time (on the order of 0.1 second). MS measures
the signal strengths from surrounding BSs and interference levels on all channels. A handoff
can be initiated if the signal strength of the serving BS is lower than that of another BS
by a certain threshold
Handoff Schemes in Single Traffic Systems
In this section, we introduce nonpriority, priority, and queuing handoff schemes for a single
traffic system such as either a voice or a data system [6–14]. Before introducing these
schemes, we assume that a system has many cells, with each having S channels. The channel
holding time has an exponential distribution with mean rate . Both originating and
handoff calls are generated in a cell according to Poisson processes, with mean rates O
and H, respectively. We assume the system with a homogeneous cell. We focus our attention
on a single cell (called the marked cell). Newly generated calls in the marked cell are
labeled originating calls (or new calls). A handoff request is generated in the marked cell
when a channel holding MS approaches the marked cell from a neighboring cell with a
signal strength below the handoff threshold.