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14.4 Demand-Assigned FDMA In the demand-assigned mode of operation, the transponder frequency bandwidth is subdivided into a number of channels. A channel is assigned to each carrier in use, giving rise to the single-channel-per-carrier mode of operation discussed in the preceding section. As in the preassigned access mode, carriers may be frequency modulated with analog information signals, these being designated FM/SCPC, or they may be phase modulated with digital information signals, these being designated as PSK/SCPC. Demand assignment may be carried out in a number of ways. In the polling method, a master earth station continuously polls all the earth stations in sequence, and if a call request is encountered, frequency slots are assigned from the pool of available frequencies. The polling delay with such a system tends to become excessive as the number of participating earth stations increases. Instead of using a polling sequence, earth stations may request calls through the master earth station as the need arises. This is referred to as centrally controlled random access. The requests go over a digital orderwire, which is a narrowband digital radio link or a circuit through a satellite transponder reserved for this purpose. Frequencies are assigned, if available, by the master station, and when the call is completed, the frequencies are returned to the pool. If no frequencies are available, the blocked call requests may be placed in a queue, or a second call attempt may be initiated by the requesting station. As an alternative to centrally controlled random access, control may be exercised at each earth station, this being known as distributed control random access. A good illustration of such a system is provided by the Spade system operated by INTELSAT on some of its satellites. This is described in the following section. 14.5 Spade System The word Spade is a loose acronym for SCPC pulse-code-modulated multiple-access demand-assignment equipment. Spade was developed by Comsat for use on the INTELSAT satellites (see, e.g., Martin, 1978) and is compatible with the INTELSAT SCPC preassigned system described in Sec. 14.3. However, the distributed-demand assignment facility requires a common signaling channel (CSC). This is shown in Fig. 14.8. The CSC bandwidth is 160 kHz, and its center frequency is 18.045 MHz below the pilot frequency, as shown in Fig. 14.8. To avoid interference with the CSC, voice channels 1 and 2 are left vacant, and to maintain duplex matching, the corresponding channels 1 and 2 are also left vacant. Recalling from Fig. 14.5 that channel 400 also must be left vacant, this requires that channel 800 be left vacant for duplex
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matching. Thus six channels are removed from the total of 800, leaving a total of 794 one-way or 397 full-duplex voice circuits, the frequencies in any pair being separated by 18.045 MHz, as shown in Fig. 14.8. (An alternative arrangement is shown in Freeman, 1981.) All the earth stations are permanently connected through the CSC. This is shown diagrammatically in Fig. 14.9 for six earth stations A, B,
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C, D, E, and F. Each earth station has the facility for generating any one of the 794 carrier frequencies using frequency synthesizers. Furthermore, each earth station has a memory containing a list of the frequencies currently available, and this list is continuously updated through the CSC. To illustrate the procedure, suppose that a call to station F is initiated from station C in Fig. 14.9. Station C will first select a frequency pair at random from those currently available on the list and signal this information to station F through the CSC. Station F must acknowledge, through the CSC, that it can complete the circuit. Once the circuit is established, the other earth stations are instructed, through the CSC, to remove this frequency pair from the list. The round-trip time between station C initiating the call and station F acknowledging it is about 600 ms. During this time, the two frequencies chosen at station C may be assigned to another circuit. In this event, station C will receive the information on the CSC update and will immediately choose another pair at random, even before hearing from station F. Once a call has been completed and the circuit disconnected, the two frequencies are returned to the pool, the information again being transmitted through the CSC to all the earth stations. As well as establishing the connection through the satellite, the CSC passes signaling information from the calling station to the destination station, in the example above from station C to station F. Signaling information in the Spade system is routed through the CSC rather than being sent over a voice channel. Each earth station has an equipment called the demand assignment signaling and switching (DASS) unit which performs the functions required by the CSC. Some type of multiple access to the CSC must be provided for all the earth stations using the Spade system. This is quite separate from the SCPC multiple access of the network s voice circuits. TDMA, described in Sec. 14.7.8, is used for this purpose, allowing up to 49 earth stations to access the common signaling channel. 14.6 Bandwidth-Limited and Power-Limited TWT Ampli er Operation A transponder will have a total bandwidth BTR, and it is apparent that this can impose a limitation on the number of carriers that can access the transponder in an FDMA mode. For example, if there are K carriers each of bandwidth B, then the best that can be achieved is K BTR/B. Any increase in the transponder EIRP will not improve on this, and the system is said to be bandwidth-limited. Likewise, for digital systems, the bit rate is determined by the bandwidth, which again will be limited to some maximum value by BTR.
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