User's Manual Part 1

Designing a Fusion Wideband Solution
Page 94 InterReach Fusion Wideband Installation, Operation, and Reference Manual
© 2013 TE Connectivity Ltd D-620616-0-20 Rev H • TECP-77-044 Issue 7 • May 2013
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6‐34onpage6‐32).Thedifferencebetweenthese
powerlevels,

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<

P
<12dB.
Table80provideslinkbudgetconsiderationsforCDMAsystems.
forCellular:

P
=P
downink
+P
uplink
+73dBm
forPCS:

P
=P
downink
+P
uplink
+76dBm
Table80.
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,
down
link
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 (representing 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
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 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
= 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), the
n 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