User's Manual

Link Budget Analysis
InterReach Fusion Wideband Installation, Operation, and Reference Manual Page 95
D-620616-0-20 Rev K • TECP-77-044 Issue 9 • March 2015 © 2015 TE Connectivity
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 85). The difference between these power levels, Δ
P
, can
be estimated by comparing the power radiated from the RAU, P
downink
, to the minimum received
signal, P
uplink
, at the RAU:
It’s a good idea to keep –12 dB <
Δ
P
< 12 dB.
Table 85 provides link budget consider
ations for CDMA systems.
for Cellular:
Δ
P
= P
downink
+ P
uplink
+ 73 dBm
for PCS:
Δ
P
= P
downink
+ P
uplink
+ 76 dBm
Table 85.
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
Carrier, downlink
This depends o
n 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 (representing how
much signal a mobile will receive on the Walsh code that it is despreading) and the total power are used.
The channel power is needed to determine the maximum
path loss, and the
total power is needed to
determine how hard the Fusion Wideband system is being driven.
The total power for a fully-loaded CDMA signal is given by (approximately):
total power =
voice channel power + 13 dB + 10log
10
(50%)
= voice channel power + 10 dB
Information Rate This is simply
10log
10
(9.6 Kbps) = 40 dB for rate set 1
10log
10
(14.4 Kbps) = 42 dB for rate set 2
Process Gain The process of despreading the desired signal boosts th
at 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
= 10log
10
(1.25 MHz / 9.6 Kbps) = 21 dB rate set 1
P
G
= 10log
10
(1.25 MHz / 14.4 Kbps) = 19 dB rate set 2
Note that the process gain can also be expressed as 10log
10
(CDMA bandwidth) minus the information rate.
Eb/No This is the energy-per-bit divided by the received noise
and interference. It’s the CDMA equivalent of
Signal-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
is 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