Mumbai University > EXTC > Sem 8 > Satellite Communication and Networks

Marks: 10M

The word SPADE is a loose acronym for Single-channel-per-carrier Pulse- code-modulated multiple Access Demand assignment Equipment.

SPADE carries traffic using Demand assignment SCPC FDMA with QPSK modulation of the individual carriers. In fact SPADE is a SCPC DAMA satellite using distributed control. There is no central controller or master control station. It uses a computer controlled Demand Assignment and Switching System (DASS) at each earth station for self assignment of circuits (each circuit consists of two channels) via a Common Signalling Channel (CSC). DASS is a kind of data base that keeps a record of status of all the channels.

The distributed-demand assignment facility requires a common signaling channel (CSC). The CSC bandwidth is 160 kHz, and its center frequency is 18.045 MHz below the pilot frequency. 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. Channel 400 also must be left vacant to prevent interference with the pilot carrier. This requires that channel 800 be left vacant for duplex 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.

All the earth stations are permanently connected through the common signaling channel (CSC). This is shown diagrammatically in Fig. for six earth stations A, B, 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.

Illustration

Suppose that a call to station F is initiated from station C. 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 back 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.

In addition to 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 signalling information in the Spade system is routed through the CSC rather than being sent over a voice channel. Each earth station has equipment called the demand assignment signalling and switching (DASS) unit which performs the functions required by the CSC.

Some type of multiple accesses to the CSC must be provided for all the earth stations using the Spade system. A number of different classes of earth stations are available, depending on the service requirements. Traffic can be broadly classified as heavy route, medium route, and thin route.

In a thin-route circuit, a transponder channel (36 MHz) may be occupied by a number of single carriers, each associated with its own voice circuit. This mode of operation is known as single carrier per channel (SCPC), a multiple-access mode. SCPC systems are widely used on lightly loaded routes, this type of service being referred to as a thin route service. It enables remote earth stations in sparsely populated areas to connect into the national telephone network in a reasonably economical way.

SCPC/FDMA along with Demand Assignment is most suitable for thin route service as the traffic density is low.