The act of transferring support of a mobile from one base station
to another is termed handoff (handover, in some parts of the
world). The implementation of handoff is radically different
between the narrowband standards and the CDMA standards.
AMPS handoffs fail frequently, causing dropped calls. This
contributes to perceptions of poor service quality. Moreover
each handoff is preceded and followed by long intervals of
poor link quality, resulting in annoying noise and distortion.
By comparison, CDMA is specifically designed not only to reduce
handoff failures but also to provide (pardon the cliché)
seamless service. CDMA maintain good voice quality
at all times: before, during, and after handoff. The handoffs
are undetectable, even by skilled listeners.
How much of a problem is this? After all, if handoffs are rare,
doing them poorly is a less important consideration.
The number of cells transited during a call can be crudely estimated
with the following assumptions:
- Cells are circular with a uniform radius R.
- Mobile station trajectory is a straight line.
- Mobile enters each cell at a random location on the periphery,
with a random direction within ±90 degrees from radial.
With these assumptions the average path within a cell is easily
shown to be about pi*R/2. The number of cells transited during
a call of duration T, by a vehicle with speed v is thus crudely
estimated at about 0.65*v*T/R. Some representative cases are
shown in the table for three minute calls.
||Cell Radius (km)
|Surface streets, urban
The number of handoff is seen to be anywhere from perhaps
one every 8-10 calls to perhaps 3-4 per call. Although this
is only a crude estimate, it does indicate that handoff is
frequent enough that good performance is important.
It is perhaps noteworthy that CDMA cell radii can be considerably
larger than AMPS cell radii for any particular load distribution.
This reduces handoff rates simply on the basis of geometry,
independent of the many other performance advantages that
acrue to CDMA.
Steps in a Handoff
Regardless of technology, the following steps are part of handoff
of any call.
- Starting in a state where only one cell is supporting
the call in question.
- Determining that over-the-air link conditions between
the mobile and the old serving cell are deteriorating, and
that there is a potentially better link to a new, candidate
- Informing the candidate cell of the imminent handoff,
including parameters needed to identify the mobile and execute
- Signaling the mobile to begin executing the handoff.
- New cell beginning to service the mobile
- Mobile beginning to use the new cell
- Entering the mid-handoff state (prolonged only in CDMA)
- Mobile discontinuing use of the old cell
- Old cell stopping service to the mobile
- Ending in a state where one cell, the new one, is supporting
the call in question
Real network exchanges are much more complex, but these are
the major events. Some steps entail different activities in
AMPS versus CDMA. For example, CDMA handoffs do not normally
require frequency tuning; instead they require change of the
code channel in the Forward CDMA Channel. No tuning, either
frequency or code channel is required in the Reverse CDMA
Channel at any time.
Handoff - The Old Way
Handoff in AMPS has several significant aspects that create
- It is "hard", meaning that communication is
interrupted briefly while control is transferred. Mobiles
can tune to only one channel pair at a time, and a switch
of pairs is required because adjacent cells never use the
same frequency sets.
- There is no diversity during handoff in the sense that
all communication is with one base station or the other.
At no time is there simultaneous communication with more
than one station.
- The base stations, not the mobiles do the signal quality
measurements that lead to the decision to do a handoff.
The handoff trigger in an AMPS system can be several things:
- Absolute received signal level as measured by the current
serving cell receiver. This can be misleading because the
signal strength measurement does not distinguish between
the desired signal and interference; the latter can sometimes
be large. If it is, the measurement can be misleading and
will fail to initiate the handoff.
- Signal power difference between the current serving cell
and a candidate cell. This tends to work better than absolute
threshold because it tends to remove the effects of variations
in mobile antenna gain, placement, and power output.
- Receiver quieting as measured, for example, by the post-detection
signal-to-noise ratio of the SAT. This does not seem to
be used much in practice.
The trigger mechanism must anticipate the need for handoff with
enough margin for execution. It is not possible to do this with
perfect reliability based only on signal strength measurements.
If the trigger is too sensitive, then handoffs will be initiated
when they are not needed. Mobiles transiting areas of low signal
strength well within a cell may trigger a handoff, followed
immediately by a handoff back. Spurious handoffs will reduce
system capacity somewhat, and will increase the likelihood of
a failure for other reasons, such as lack of resources in the
The concept of an abrupt handoff is fundamentally flawed in
the sense that static signal strength measurements cannot foretell
the future. Even if one could make assumptions about the dynamics
of the mobile station, extracting information about that motion
from the communication signal is all but impossible. Moreover,
calculating propagation loss from the station's location is
an extremely difficult problem, even in principle. Doing such
a calculation "on the fly" in a fraction of a second
is not remotely practical.
The solution that has generally been adopted by the infrastructure
manufacturers is to monitor signal strength (RSSI) on the current
serving cell. When the RSSI drops below a threshold, then measurements
are requested from predetermined handoff candidate cells. After
the measurement reports are collected the messaging to carry
it out is initiated. Even when this works, it is unsatisfactory
because the initial RSSI threshold has to be set so low that
the voice quality has generally deteriorated badly before the
process is even triggered.
The ensuing handoffs often fail, even if they were warranted.
Some of the most frequent causes are:
- There was no channel available in the target cell due
to traffic loading.
- The RSSI measurement in a candidate was erroneous due
to interference, leading to the incorrect target being selected.
- The link quality deteriorated so badly prior to the need
for handoff that the signaling needed to carry it out failed.
- The 5 second fade timer expired due to low signal before
the handoff could be executed.
- The mobile was not really at a handoff boundary but rather
was in a coverage hole within a cell so no handoff target
could be found.
And the voice quality just before and just after the handoff,
even when it is successful, is often poor because the handoff
boundary is the point where there is large loss to both cells.
Nothing in the system design can make up for large loss at
the handoff boundaries.
Handoff - The CDMA Way
CDMA handoff differs from AMPS in several key aspects:
- It is "soft", meaning that communication is
not interrupted by the handoff. This is sometimes called
"make before break." But it is more than that.
- The handoff is not abrupt, but rather it is a prolonged
call state during which there is communication via two or
more base stations. The multi-way communication diversity
improves the link performance during the handoff. The diversity
gain partially compensates for the large path loss at the
- The signal measurement that triggers the handoff is performed
by the mobile stations, not the base stations.
There is no handoff boundary in CDMA but rather
a handoff region. The distributed handoff alleviates
most of the shortcomings of the AMPS-style hard handoffs.
For example, a decision that handoff should be initiated means
only that another base station is added to the active
set of base stations for this mobile. The handoff can
be completed either by the mobile moving completely into the
new cell, or by the mobile returning to the original serving
cell. In either case the call is never in jeopardy due to
The diversity during handoff improves link performance to
the point where not only are the handoffs not disruptive,
they are not even detectable in the voice quality,
even by skilled observers. This also very much reduces the
likelihood of dropped calls due to signaling failures that
disrupt handoff coordination.
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Copyright © 1996-1999 Arthur H. M. Ross, Ph.D., Limited