User's Manual

Designing a Fusion Wideband Solution
Page 94 InterReach Fusion Wideband Installation, Operation, and Reference Manual
© 2015 TE Connectivity D-620616-0-20 Rev K • TECP-77-044 Issue 9 • March 2015
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 reception, the CDMA signal despread. In the
despreading 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 despread
signal is higher than that of the CDMA signal before despreading. 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, of which Table 84 shows an example.
Table 84 assumes that there are 15 active traffic channels operating with 50% v
oice activity (so
that the total power adds up to 100%). Notice that the pilot and syn
c channels 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, for
example, that you should not exceed 10 dBm for a CDMA signal, 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 s
ystems is that the uplink and downlink paths should be
gain and noise balanced. This is required for proper operation of soft-handoff 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 equations are as follows:
where P
TX
is the mobile’s transmitted power and P
RX
is the power received by the mobile.
Table 84.
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
• for Cellular, IS-95: P
TX
+ P
RX
= –73 dBm
• for PCS, J-STD-008: P
TX
+ P
RX
= –76 dBm