Link Budget

The link budget process, as defined in previous sections of this book, is
essential for the establishment of a valid RF design to take place. The link
budget helps define the cell-size spacing. The cell-site spacing is determined
by the link budget using a signal level that is exceeded by 50 percent of the
time.
There are two links that need to be determined in the establishment of
a link budget: forward and reverse. The forward and reverse links utilize
different coding and modulation formats. The first step in the link budget
process is to determine the forward before the reverse links maximum path
losses. The link budget is defined previously in an earlier chapter.
CDMA2000-1X has a better link budget than IS-95A/B at the same traffic
loading therefore offering a high overall capacity at the same traffic
load due to vocoder improvements as well as utilizing a coherent demodulation
for the reverse link. However, for the link budget that will be used
for the design, the link budget parameters primarily associated with IS-95
are utilized due to the prevalence of the RC1 and RC2 subscriber units in
the market.
Regarding packet-data services, due to the improved modulation and
coding scheme (resulting in a lower target Eb/No), the 38.4-Kbps packet data
rate for CDMA2000-1x has approximately the same link budget as IS-95
13K voice vocoder, but at higher data rates the service coverage will shrink
due to a variety of factors that include process gain as well as power allocation.
With 1xRTT, voice is given a priority and therefore data petitions for
all available remaining power. Therefore for the design effort put forth a
lower data rate of 38.4 Kbps was used per packet data subscriber in the link
budget calculations, but 70 Kbps was used for subscriber packet throughput.
The disparity was done for ease of discussion.
As stated previously, the link budget calculations utilized directly influence
the performance of the CDMA system because it is used to determine
power setting and capacity limits for the network. Proper selection of the
variables that comprise the link budget is a very obvious issue due to its
impact on a successful design.

The following Tables (13-24 and 13-25) represent the link budgets for a
CDMA2000 system. Obviously the issue of differing data rates, and subscriber
and base radio configurations makes the possible combinations
daunting. However, the basic principals that comprise the link budget
tables presented in Tables 13-24 and 13-25 can be modified with different
process gains as well as a different spreading rate for the uplink path when,
and, if a 3X system is deployed. 538