Link Budget Analysis
6-30
InterReach Unison Installation, Operation, and Reference Manual
CONFIDENTIAL
D-620003-0-20 Rev J
P
TX
+ P
RX
= –76 dBm (for PCS, J-STD-008)
where P
TX
is the mobile’s transmitted power and P
RX
is the power received by the
mobile.
The power level transmitted under closed-loop power control is adjusted by the base
station to achieve a certain E
b
/N
0
(explained in Table 6-28 on page 6-30). The differ-
ence between these power levels,
Δ
P
, can be estimated by comparing the power radi-
ated from the RAU,
P
downink
, to the minimum received signal,
P
uplink
, at the RAU:
Δ
P
=
P
downink
+
P
uplink
+
73 dBm (for Cellular)
Δ
P
=
P
downink
+
P
uplink
+
76 dBm (for PCS)
It’s a good idea to keep –12 dB <
Δ
P
< 12 dB.
Table 6-28 provides link budget considerations for CDMA systems.
Table 6-28
Additional Link Budget Considerations for CDMA
Consideration
Description
Multipath Fade
Margin
The multipath fade margin can be reduced (by at least 3 dB) by using different lengths of optical fiber (this
is called “delay diversity”). The delay over fiber is approximately 5µS/km. If the difference in fiber
lengths to Expansion Hubs with overlapping coverage areas produces at least 1 chip (0.8µS) delay of one
path relative to the other, then the multipaths’ signals can be resolved and processed independently by the
base station’s rake receiver. A CDMA signal traveling through 163 meters of MMF cable is delayed by
approximately one chip.
Power per car-
rier, downlink
This depends on how many channels are active. For example, the signal is about 7 dB lower if only the
pilot, sync, and paging channels are active compared to a fully-loaded CDMA signal. Furthermore, in the
CDMA forward link, voice channels are turned off when the user is not speaking. On average this is
assumed to be about 50% of the time. So, in the spreadsheet, both the power per Walsh code channel (rep-
resenting how much signal a mobile will receive on the Walsh code that it is de-spreading) and the total
power are used.
The channel power is needed to determine the maximum path loss, and the total power is needed to deter-
mine how hard the Unison system is being driven.
The total power for a fully-loaded CDMA signal is given by (approximately):
total power =
voice channel power
+ 13 dB + 10
log
10
(
50%
)
= voice channel power + 10 dB
Information Rate
This is simply
10
log
10
(9.6 Kbps) = 40 dB for rate set 1
10
log
10
(14.4 Kbps) = 42 dB for rate set 2
Process Gain
The process of de-spreading the desired signal boosts that signal relative to the noise and interference.
This gain needs to be included in the link budget. In the following formulas, P
G
= process gain:
P
G
= 10
log
1
0
(
1.25 MHz / 9.6 Kbps
) = 21 dB rate set 1
P
G
= 10
log
1
0
(
1.25 MHz / 14.4 Kbps
) = 19 dB rate set 2
Note that the process gain can also be expressed as 10
log
1
0
(CDMA bandwidth) minus the information
rate.
