Page 138 - Intro to Space Sciences Spacecraft Applications

Communications

CI-) f ‘‘cant" = 1/2t b 125
fb “cont”

The maximum transmission rate a digital system could have, based on
a given carrier wave frequency (fc), can be found from the following rela-
tionships:

(5 - 18)

where fb“c,,nt” = % tb, representing the equivalent “continuous’’ frequency
of the digital waveform. Once a maximum transmission rate is deter-
mined, the bit period, sampling frequency, and sample period can be
established.

Example Problem:

A communications satellite uses a 6 GHz down-link frequency to
transmit digitized telephone calls. Using the results of the last exam-
ple problem, determine the maximum transmission rate and the
number of calls that may be transmitted simultaneously.

Solution:

From the last example problem we found that for each call:

fSmin = 8,000 samples/sec
t, = 0.125 x lop3 sec
n = 3 bitdsample
TR = 24 Kbps