Page 276 - Satellite Communications, Fourth Edition

256 Chapter Nine

subcarrier is also used, and this practice will be followed here. Thus the
20-kHz carrier shown in Fig. 9.2 is a subcarrier.
Companded single sideband (CSSB) refers to a technique in which the
speech signal levels are compressed before transmission as a single side-
band, and at the receiver they are expanded again back to their original
levels. (The term compander is derived from compressor-expander). In one
companded system described by Campanella (1983), a 2:1 compression in
decibels is used, followed by a 1:2 expansion at the receiver. It is shown in
the reference that the expander decreases its attenuation when a speech
signal is present and increases its attenuation when it is absent. In this way
the “idle” noise on the channel is reduced, which allows the channel to
operate at a reduced carrier-to-noise ratio. This in turn permits more
channels to occupy a given satellite link, a topic which comes under the
heading of multiple access and which is described more fully in Chap. 14.

9.4 FDM Telephony
FDM provides a way of keeping a number of individual telephone sig-
nals separate while transmitting them simultaneously over a common
transmission link circuit. Each telephone baseband signal is modulated
onto a separate subcarrier, and all the upper or all the lower sidebands
are combined to form the frequency-multiplexed signal. Figure 9.3a
shows how three voice channels may be frequency-division multiplexed.
Each voice channel occupies the range 300 to 3400 Hz, and each is mod-
ulated onto its own subcarrier. The subcarrier frequency separation is
4 kHz, allowing for the basic voice bandwidth of 3.1 kHz plus an ade-
quate guardband for filtering. The upper sidebands are selected by
means of filters and then combined to form the final three-channel mul-
tiplexed signal. The three-channel FDM pregroup signal can be repre-
sented by a single triangle, as shown in Fig. 9.3b.
To facilitate interconnection among the different telecommunications
systems in use worldwide, the Comité Consultatif Internationale de
∗
Télégraphique et Téléphonique (CCITT) has recommended a standard
modulation plan for FDM (CCITT G322 and G423, 1976). The standard
group in the plan consists of 12 voice channels. One way to create such
groups is to use an arrangement similar to that shown in Fig. 9.3a,
except of course that 12 multipliers and 12 sideband filters are required.
In the standard plan, the lower sidebands are selected by the filters, and
the group bandwidth extends from 60 to 108 kHz.
As an alternative to forming a 12-channel group directly, the VF chan-
nels may be frequency-division multiplexed in threes by using the

∗
Since 1994, the CCITT has been reorganized by the International Telecommunications
Union (ITU) into a new sector ITU-T.