Page 136 - Intro to Space Sciences Spacecraft Applications

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Communications
where bps stands for bits per second. The transmission rate is an impor-
tant parameter of a digital communication system, as it is a measure of the
capabilities of the system to transmit information. Most modern space-
craft use digital communication methods, and in many cases the trans-
mission rate is a major factor limiting the system capability. Examples are
telecommunications satellites which can only handle so many telephone
calls at a time or remote sensing systems that can only observe so much
information due to data storage and transmitting limitations. The major
limiting factor to transmission rate (and subsequently, the limit to the sam-
ple rate and number of Q-levels) is the carrier wave’s ability to transport
digital information, as described in the next few sections.

Example Problem:

Satellite transmission of telephone calls allots a bandwidth (range
of frequencies) of 4 kHz (associated with the minimum bandwidth
required to reproduce the human voice) for each call. If eight Q-lev-
els are used to digitize each call, determine the minimum sampling
rate (frequency), sampling period, and transmission rate associated
with one conversation.

Solution:

fsmin = 2 fmax = 8,000 samples/sec
t, = l/f, = 0.125 x lod3 sec
n = 1.4427 In(k) = 3 bits/sample
TR = n x f, = n/t, = 24,000 bits/sec (bps)
= 24 Kbps

Digital Baseband Signals

The output of an AD converter is a string of “one” and “zero” values
representing the bits assigned to each successive sample of the original
signal. These values are usually output as voltages, where a positive volt-
age may correspond to a “one” and either a negative or zero voltage would
represent a “zero” bit. Figure 5-10 shows these two common methods of
representing a digital string of values.