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Wire and Wireless Communication Applications Chap. 8
Satellite communication systems do differ from terrestrial microwave links in the
techniques used for multiple access of a single transponder by multiple uplink and multiple
downlink stations. Four main methods are used for multiple access:
1. Frequency-division multiple access (FDMA), which is similar to FDM.
2. Time-division multiple access (TDMA), which is similar to TDM. †
3. Code-division multiple access (CDMA) or spread-spectrum multiple access (SSMA).
4. Space-division multiple access (SDMA) where narrow-beam antenna patterns are
switched from one direction to another.
In addition, either of the following may be used:
1. A fixed-assigned multiple-access (FAMA) mode using either FDMA, TDMA, or
CDMA techniques.
2. A demand-assigned multiple-access (DAMA) mode using either FDMA, TDMA, or
CDMA.
In the FAMA mode, the FDMA, TDMA, or CDMA format does not change, even
though the traffic load of various Earth stations changes. For example, there is more telephone
traffic between Earth stations during daylight hours (local time) than between these stations in
the hours after midnight. In the FAMA mode, a large number of satellite channels would be
idle during the early morning hours, since they are fixed assigned. In the DAMA mode, the
FDMA and TDMA formats are changed as needed, depending on the traffic demand of the
Earth stations involved. Consequently, the DAMA mode uses the satellite capacity more efficiently,
but it usually costs more to implement and maintain.
In CDMA the different users share the same frequency band simultaneously in time, as
opposed to FDMA and TDMA, where users are assigned different frequency slots or time
slots. With CDMA, each user is assigned a particular digitally encoded waveform w j (t) that is
(nearly) orthogonal to the waveforms used by the others. (See Secs. 2–4 and 5–13.) Data may
be modulated onto this waveform, transmitted over the communication system, and recovered.
For example, if one bit of data, m j , is modulated onto the waveform, the transmitted
signal from the jth user might be m j w j (t), and the composite CDMA signal from all users
would be w(t) =© j m j w j (t). The data from the jth user could be recovered from the CDMA
t
waveform by evaluating 0 w(t)w j (t) dt = mj , where T is the length of encoded waveform
w j (t). Gold codes are often used to obtain the encoded waveforms.
Example 8–1 SPADE SYSTEM
The Intelsat series satellites may also be operated in a DAMA mode using an FDMA format
consisting of a single QPSK carrier for each telephone (VF) channel. This type of signaling is
called single channel per carrier (SCPC), in which 800 QPSK signals may be accommodated in
† Satellite-switched time-division multiple access (SS-TDMA) can also be used. With SS-TDMA satellites,
different narrow-beam antennas are switched in at the appropriate time in the TDMA frame period to direct the transmit
and receive beams to the desired direction. Thus, with multiple SS-TDMA beams, the same transmit and receive
frequencies may be reused simultaneously, so that the capacity of the SS-TDMA satellite is much larger than that of
a TDMA satellite. The Intelsat VIII is an SS-TDMA satellite.