User's Manual

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PRELIMINARY Elements of a Link Budget for CDMA Standards
Other CDMA Issues
• Never combine multiple sectors (more than one CDMA signal at the same fre-
quency) into a Unison system. The combined CDMA signals will interfere with
each other.
• Try to minimize overlap between in-building coverage areas that utilize different
sectors, as well as in-building coverage and outdoor coverage areas. This is impor-
tant because any area in which more than one dominant pilot signal (at the same
frequency) is measured by the mobile will result in soft-handoff. Soft-handoff
decreases the overall network capacity by allocating multiple channel resources to
a single mobile phone.
Eb/No This is the energy-per-bit divided by the received noise and interference. It’s the CDMA equivalent of sig-
nal-to-noise ratio (SNR). This figure depends on the mobile’s receiver and the multipath environment. For
example, the multipath delays inside a building are usually too small for a rake receiver in the mobile (or
base station) to resolve and coherently combine multipath components. However, if artificial delay can be
introduced by, for instance, using different lengths of cable, then the required E
b
/N
o
will be lower and the
multipath fade margin in the link budget can be reduced in some cases.
If the receiver noise figure is NF (dB), then the receive sensitivity (dBm) is given by:
P
sensitivity
= NF + E
b
/N
o
+ thermal noise in a 1.25 MHz band – P
G
= NF + E
b
/N
o
– 113 (dBm/1.25 MHz) – P
G
Noise Rise On the uplink, the noise floor is determined not only by the Unison system, but also by the number of
mobiles that are transmitting. This is because when the base station attempts to de-spread a particular
mobile’s signal, all other mobile signals appear to be noise. Because the noise floor rises as more mobiles
try to communicate with a base station, the more mobiles there are, the more power they have to transmit.
Hence, the noise floor rises rapidly:
noise rise = 10log
10
(1 / (1 – loading))
where loading is the number of users as a percentage of the theoretical maximum number of users.
Typically, a base station is set to limit the loading to 75%. This noise ratio must be included in the link
budget as a worst-case condition for uplink sensitivity. If there are less users than 75% of the maximum,
then the uplink coverage will be better than predicted.
Hand-off Gain CDMA supports soft hand-off, a process by which the mobile communicates simultaneously with more
than one base station or more than one sector of a base station. Soft hand-off provides improved receive
sensitivity because there are two or more receivers or transmitters involved. A line for hand-off gain is
included in the CDMA link budgets worksheet although the gain is set to 0 dB because the in-building
system will probably be designed to limit soft-handoff.
Table 8-30 Additional Link Budget Considerations for CDMA Systems
Consideration Description