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6-29
D-620003-0-20 Rev J
CONFIDENTIAL
Link Budget Analysis
6.4.3
Elements of a Link Budget for CDMA Standards
A CDMA link budget is slightly more complicated because you must consider the
spread spectrum nature of CDMA. Unlike narrowband standards such as TDMA and
GSM, CDMA signals are spread over a relatively wide frequency band. Upon recep-
tion, the CDMA signal is de-spread. In the de-spreading process the power in the
received signal becomes concentrated into a narrow band, whereas the noise level
remains unchanged. Hence, the signal-to-noise ratio of the de-spread signal is higher
than that of the CDMA signal before de-spreading. This increase is called
processing
gain
. For IS-95 and J-STD-008, the processing gain is 21 dB or 19 dB depending on
the user data rate (9.6 Kbps for rate set 1 and 14.4 Kbps for rate set 2, respectively).
Because of the processing gain, a CDMA signal (comprising one Walsh code channel
within the composite CDMA signal) can be received at a lower level than that
required for narrowband signals. A reasonable level is –95 dBm, which results in
about –85 dBm composite as shown below.
An important issue to keep in mind is that the downlink CDMA signal is composed of
many orthogonal channels: pilot, paging, sync, and traffic. The composite power
level is the sum of the powers from the individual channels. Table 6-27 shows an
example.
This table assumes that there are 15 active traffic channels operating with 50% voice
activity (so that the total power adds up to 100%). Notice that the pilot and sync chan-
nels together contribute about 25% of the power. When measuring the power in a
CDMA signal you must be aware that if only the pilot and sync channels are active,
the power level will be about 6 to 7 dB lower than the maximum power level you can
expect when all voice channels are active. The implication is that if only the pilot and
sync channels are active, and the maximum power per carrier table says that you
should not exceed 10 dBm for a CDMA signal, for example, then you should set the
attenuation between the base station and the Main Hub so that the Main Hub receives
3 dBm (assuming 0 dB system gain).
An additional consideration for CDMA systems is that the uplink and downlink paths
should be gain and noise balanced. This is required for proper operation of soft-hand-
off to the outdoor network as well as preventing excess interference that is caused by
mobiles on the indoor system transmitting at power levels that are not coordinated
with the outdoor mobiles. This balance is achieved if the power level transmitted by
the mobiles under close-loop power control is similar to the power level transmitted
under open-loop power control. The open-loop power control equation is
P
TX
+ P
RX
= –73 dBm (for Cellular, IS-95)
Table 6-27
Distribution of Power within a CDMA Signal
Channel
Walsh Code Number
Relative Power Level
Pilot
0
20%
–7.0 dB
Sync
32
5%
–13.3 dB
Primary Paging
1
19%
–7.3 dB
Traffic
8–31, 33–63
9% (per traffic channel)
–10.3 dB
