The uplink and downlink carriers of the satellite are required to be modulated by the signal to be transmitted. The modulation can be analog or digital. In the earlier satellites analog modulations were dominant and therefore they were used in communication satellites.
The analog voice and video transmission systems use frequency. modulation and frequency division multiplexing techniques. Since the frequency modulation techniques has no amplitude fluctuation, the transmitter could be operated efficiently at full power capacity. As we have seen the bandwidths of the satellite systems are from 500MHz to upto 5GHz, this complete band width is not needed for the single channel.
Thus the complete satellite bandwidth can be used by several channels. Each channel is given a frequency slot in the satellite bandwidth. Thus it is possible to transmit and receive several channels simultaneously through these frequency slots. The channels are transmitted continuously. This is called Frequency Division Multiplexing (FDM).
The modern communication satellites use digital modulation methods. Since the signals to be transmitted using digital modulation are sampled in time, multiplexing in time can be used. Digital modulation has been very attractive since the availability of fast speed and advanced computers.
Frequency Division Multiple Access (FDMA)
For the analog communication, frequency modulation is used in the satellites. Several users share satellite link so that many signals from the earth station are transmitted. These FM signals are in FDM. For digital modulation, the satellite link uses time division multiplexing (TDM). Fig shows the FDM scheme for C-Band of the satellite. The receiver in the satellite transponder covers the complete uplink range of that band.
Fig (a) shows that the uplink frequency range of 5.925 MHz to 6.425 MHz of C-Band has 12 channels. The receiver receives all these 12 channels. Each channel is allotted the bandwidth of 36 MHz with 4 MHz spacing between the channels. Thus 12 channels take (36+4 = 40; 2×40 = 480 MHz) 480 MHz bandwidth of the satellite.
Remaining 20 MHz is used for transmission of command and control of telemetry channel.
The transponder converts the uplink frequencies to downlink frequencies. The channels has same bandwidth of 36 MHz and spacing of 4 MHz, as shown in Fig (b).
The advantage of frequency division multiplexing is that the power required for the transmission at satellite as well as earth station is dependent on the number of channels being transmitted.
The signal to noise ratio of frequency modulation is improved, but it needs wider bandwidth. Therefore the number of channels to be accommodated in the band of satellite are reduced.
Time Division Multiple Access (TDMA)
In this method, the complete channel is allocated to particular earth station for some time slot. The earth stations access the transponder as shown in Fig.
At any time, the power amplifier in the transponder amplifies only one carrier signal. Hence intermodulation is absent. TDMA is mainly suitable for Transmission of Digitally Modulated Signals.
TDMA needs accurate synchronization of the earth stations, whereas in FDMA it is not required. Therefore TDMA is very complex compared to FDMA.
FDMA Frame Structure
In TDMA, a preamble or reference burst is transmitted at the beginning of every TDMA frame. The reference burst is normally transmitted by one of the earth stations to designate it as master station. Fig shows the typical TDMA reference burst.
The segments CTBR and UW contains –
- Carrier recovery waveform.
- Bit clock synchronization.
- Unique word.
- Station identifier.
CTBR (carrier and bit timing recovery) : It enables the receiving earth station to recover remainder of the burst. CTBR contains the sequence of predetermined signals. These signals are used for synchronization of the bit clock.
Unique word (UW) : Length of the unique word is 20 to 48 bits. The unique word serves following purposes :
- Unique word acts as station identifier.
- It provides Start Of Frame (SOF).
- It acts as burst marker.
- It acts as a carrier phase ambiguity detector.
For each earth station there is separate unique word. The unique word can be detected from TDMA frame with the help of unique word correlator. If there are large number of stations, then single unique word is used. The stations are then identified by separate station identifier word.
Teletype Link (TTY) : These are 16 bits in each burst. They are used to identify the teletype link between the earth stations.
Service Channel (SC) : These are 16 bits. They are used for management of the TDMA system alongwith TTY and VOW.
Voice Order Wire (VOW) : This field is 64 bits. These are two fields of VOW. They are used in digital voice links between the earth stations.
The remaining part of the TDMA frame consists of data from various channels.
Demand Access Systems – SPADE
Here we will discuss mainly SPADE, which is one of the demand access systems. The satellite capacity of all the stations in FDMA network is pooled and it is used on SCPC basis. When the call is to be made, a pair of frequencies is assigned to the duplex circuit (i.e. two channels). This is called SCPC demand access method. The SCPC demand access method is used in the networks where there are many small stations and the total traffic on each station is very light.
The Single channel per carrier Pulse code modulation multiple Access (MA) Demand assignment Equipment (SPADE) system is basically SCPC demand access network that uses distributed control. This system allows circuits to be selected by an terminal on demand. The circuits are not permanently associated with the terminals. They are assigned from the satellite channel pool as required. Each channel of the voice circuit occupies 45 kHz bandwidth in SPADE. There are two channels per circuit. Hence there are paired RF carriers for these channels corresponding to one circuit. These RF carriers are separated by 18.045 MHz. There is a common signaling channel (CSC). It occupies 16kHz of RF bandwidth.
Fig shows the SPADE frequency plan. As shown in this figure, the center of CSC is at 18.045 MHz from pilot frequency. The pilot frequency is 70 MHz IF. There are total 800 RF channels of 45 kHz each. SPADE has all the advantages of SCPC over FDM/FM system. That is reduced transponder power consumption, reduced cost of earth stations etc.