Welcome to the World of CDMA
Good handoff performance in CDMA is not just a nicety; it
is absolutely essential. A mobile that is being served by
one base station when another is closer in terms of path loss
will be transmitting more power than would be necessary were
it using the "right" cell. The fact that that mobile,
and all others like it radiate excess power raises the overall
interference level. The higher overall interference level
increases the effective reuse factor, and thus reduces overall,
average reverse link capacity. Sloppy, late, or slow handoffs
thus should be kept to an absolute minimum.
What happens during soft handoff?
CDMA soft handoff is a call state in which two or more base stations support a mobile station. Those stations can be either separate sectors of separate cells, or they can be multiple sectors of the same cell, or any combination of these. The inter-sector handoff, in fact, is very common because of the broad antenna gain patterns and the vagaries of urban multipath propagation.
Forward CDMA Channel
Each participating cell in a soft handoff transmits the same traffic stream to the mobile, bit-for-bit. They do so on any available code channel. Each base station chooses a code channel simply on the basis of availability. The mobile station must implement, in its Rake receiver, multiple fingers that are capable of "tuning" to any of the 63 available code channels. The outputs of those Rake fingers must be combined for good Eb/N0 performance. The presence of a pilot in the Forward CDMA Channel allows optimum coherent combining of those Rake outputs.
Reverse Power Control
Embedded in the Forward CDMA Channel are the reverse power
control bits. These occur in pseudo-random positions in each
1.25 ms interval (power control group), or 16 times per frame.
Each power control bit is interpreted as a command to raise
or lower power by an increment of approximately 3/4 dB.
Reverse CDMA Channel
Spreading of the Reverse CDMA Traffic Channel is mobile-unique.
There is nothing about the coding and modulation that depends
in any way on the base stations that are serving the mobile.
The mobile thus needs do nothing special about handoff, aside
from proper interpretation of the power control bits.
What is "softer" handoff?
All of this is modified a bit if the participants in a soft
handoff are sectors of the same cell. This situation has come
to be known as "Softer" handoff. Collocated stations
permit combining to be done in a CDMA modem that has visibility
of multiple sectors. Such combining can be done on a symbol-by-symbol
basis, rather than by selection of entire frames.
What initiates soft handoff?
CDMA is said to use Mobile Assisted Handoff (MAHO). In practice
this means that the mobile station continuously searches for
the pilot code using a PN correlator specifically designated
for this purpose. Universality of the pilot code (or Short
Code) facilitates the search. All base stations use the same
code. The mobile station can search in timing hypothesis without
having to change the PN sequence.
The mobile reports pilots on the basis of their pilot-to-interference
ratio (PIR). The PIR (called, strangely, Ec/I0
in much of the literature and standards) is compared to an
absolute threshold to determine when it should be reported
as a handoff candidate. That threshold is a parameter that
the mobile obtains from the overhead messages broadcast by
the base stations. When a pilot crosses the first threshold,
T_ADD, then its presence is reported, via a message, to the
network. The network will normally add that base station to
the so-called Active Set, that is, the set of base stations
that are participants in the soft handoff to the mobile in
Does CDMA always use soft handoff?
No. It does so whenever possible because the performance is very much superior to other forms of handoff. However there are several forms of handoff that cannot be done "softly".
If there are multiple CDMA carrier frequencies active, then
handoffs between them must be hard. While inter-frequency
handoff is physically possible, the decision was made in the
standards committees to not require it. To implement it would
very much complicate the mobile station. Multiple (well at
least two) independent frequency synthesizers would be required.
Pilot searches would have to span all active frequencies.
All of this would very much increase the cost of the subscriber
sets, this in a very much cost-driven marketplace. While soft
inter-frequency handoff would certainly improve performance,
the marginal benefit of it was deemed to not justify the cost.
The CDMA air interfaces, to allow load balancing on network transmission facilities, include the ability to offset traffic frame timing from system time. Those timing offsets are accomplished by a hard handoff.
Quite obviously there is no practical way to do soft handoffs between a CDMA digital system and an analog FM system. These handoffs are thus inherently hard.
What about handoffs between FM and CDMA systems?
CDMA to analog handoffs are carried out by sending the same
information that is required in a normal analog to analog
handoff. Analog to digital handoffs are not allowed.
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