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The result of this multiplication process is referred to as double-sideband suppressed-carrier (DSBSC) modulation, since only the sidebands, and not the carrier, appear in the spectrum. Now, all the information in the original telephone signal is contained in either of the two sidebands, and therefore, it is necessary to transmit only one of these. A filter may be used to select either one and reject the other. The resulting output is termed a single-sideband (SSB) signal. The SSB process utilizing the lower sideband is illustrated in Fig. 9.2, where a 20-kHz carrier is used as an example. It will be seen that for the lower sideband, the frequencies have been inverted, the highest baseband frequency being translated to the lowest transmission frequency at 16.6 kHz and the lowest baseband frequency to the highest transmission frequency at 19.7 kHz. This inversion does not affect the final baseband output, since the demodulation process reinverts the spectrum. At the receiver, the SSB signal is demodulated (i.e., the baseband signal is recovered) by being passed through an identical multiplying process. In this case the multiplying sinusoid, termed the local oscillator (LO) signal, must have the same frequency as the original carrier. A low-pass filter is required at the output to select the baseband signal and reject other, unwanted frequency components generated in the demodulation process. This single-sideband modulation/ demodulation process is illustrated in Fig. 9.2. The way in which SSB signals are used for the simultaneous transmission of a number of telephone signals is described in the following section. It should be noted at this point that a number of different carriers are likely to be used in a satellite link. The radiofrequency (rf) carrier used in transmission to and from the satellite will be much higher in frequency than those used for the generation of the set of SSB signals. These latter carriers are sometimes referred to as VF carriers. The term
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A basic SSB transmission scheme.
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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 sideband, 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 signals 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 modulated onto its own subcarrier. The subcarrier frequency separation is 4 kHz, allowing for the basic voice bandwidth of 3.1 kHz plus an adequate guardband for filtering. The upper sidebands are selected by means of filters and then combined to form the final three-channel multiplexed signal. The three-channel FDM pregroup signal can be represented by a single triangle, as shown in Fig. 9.3b. To facilitate interconnection among the different telecommunications systems in use worldwide, the Comit Consultatif Internationale de T l graphique et T l phonique (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 channels may be frequency-division multiplexed in threes by using the
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Since 1994, the CCITT has been reorganized by the International Telecommunications Union (ITU) into a new sector ITU-T.
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