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Unit - 5 & 6 Earth Segment Introduction, receive only home TV system, out door unit, indoor unit, MATV, CATV, Tx – Rx earth station.
Interference and Satellite access Introduction, interference between satellite circuits, satellite access, single access, pre-assigned FDMA, SCPC (spade system), TDMA, pre-assigned TDMA, demand assigned TDMA, down link analysis, comparison of uplink power requirements for TDMA & FDMA, on board signal processing satellite switched TDMA. 9 Hours Text Book: 1. Satellite Communications, Dennis Roddy, 4th Edition, McGraw-Hill International edition, 2006.
References books: 1. Satellite Communications, Timothy Pratt, Charles Bostian and Jeremy Allnutt, 2nd Edition, John Wiley & Sons, 2003. 2. Satellite Communication Systems Engineering, W. L. Pitchand, H. L. Suyderhoud, R. A. Nelson, 2nd Ed., Pearson Education., 2007.
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5.1 Introduction The earth segment of a satellite communications system consists of the transmit and receive earth stations. The simplest of these are the home TV receive-only (TVRO) systems, and the most complex are the terminal stations used for international communications networks. Also included in the earth segment are those stations which are on ships at sea, and commercial and military land and aeronautical mobile stations.
5.2 Receive-Only Home TV Systems Planned broadcasting directly to home TV receivers takes place in the Ku (12-GHz) band. This service is known as direct broadcast satellite (DBS) service. There is some variation in the frequency bands assigned to different geographic regions. The comparatively large satellite receiving dishes (about 3-m diameter) which are a familiar sight around many homes are used to receive downlink TV signals at C band (4 GHz). Such downlink signals were never intended for home reception but for network relay to commercial TV outlets (VHF and UHF TV broadcast stations and cable TV “head end” studios). Although the practice of intercepting these signals seems to be well established at present, various technical and commercial and legal factors are combining to deter their direct reception. The major differences between the Ku-band and the Cband receive-only systems lies in the frequency of operation of the outdoor unit and the fact that satellites intended for DBS have much higher EIRP.
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5.3The outdoor unit This consists of a low noise amplifer/converter combination. A parabolic reflector is used along with horn mounted at focus. The downlink frequency band of 12.2 to 12.7 GHz spans a range of 500 MHz, which accommodates 32 TV/FM channels, each of which is 24 MHz wide. Obviously, some overlap occurs between channels, but these are alternately polarized left-hand circular (LHC) and righthand circular (RHC) or vertical/horizontal, to reduce interference to acceptable levels. This is referred to as polarization interleaving. A polarizer that may be switched to the desired polarization from the indoor control unit is required at the receiving horn. The receiving horn feeds into a low-noise converter (LNC) or possibly a combination unit consisting of a low-noise amplifier (LNA) followed by a converter. The combination is referred to as an LNB, for lownoise block. The LNB provides gain for the broadband 12-GHz signal and then converts the signal to a lower frequency range so that a low-cost coaxial cable can be used as feeder to the indoor unit. The standard frequency range of this downconverted signal is 950 to 1450 MHz. The coaxial cable, or an auxiliary wire pair, is used to carry dc power to the outdoor unit. Polarization-switching control wires are also required. The low-noise amplification must be provided at the cable input in order to maintain a satisfactory signal-to-noise ratio. A low-noise amplifier at the indoor end of the cable would be of little use, because it would also amplify the cable thermal noise. Of course, having to mount the LNB outside means that it must be able to operate over a wide range of climatic conditions, and homeowners may have to contend with the added problems of vandalism and theft.
5.4 The indoor unit for analog (FM) TV The signal fed to the indoor unit is normally a wideband signal covering the range 950 to 1450 MHz. This is amplified and passed to a tracking filter which selects the desired channel. As previously mentioned, polarization interleaving is used, and only half the 32 channels will be present at the input of the indoor unit for any one setting of the antenna polarizer. This eases the job of the tracking filter, since alternate channels are well separated in frequency. The selected channel is again downconverted, this time from the 950- to 1450-MHz range to a fixed intermediate frequency, usually 70 MHz although other values in the VHF range are also used. The 70-MHz amplifier amplifies the signal up to the levels required for demodulation. A major difference between DBS TV and conventional TV is that with DBS, frequency modulation is used, whereas with conventional TV, amplitude modulation in the form of vestigial single sideband (VSSB) is used. The 70-MHz, frequency-modulated IF carrier therefore must be demodulated, and the baseband information used to generate a VSSB signal which is fed into one of the VHF/UHF channels of a standard TV set.
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5.5 Master Antenna TV System A master antenna TV (MATV) system is used to provide reception of DBS TV/FM channels to a small group of users, for example, to the tenants in an apartment building. It consists of a single outdoor unit (antenna and LNA/C) feeding a number of indoor units. It is basically similar to the home system already described, but with each user having access to all the channels independently of the other users. The advantage is that only one outdoor unit is required, but as shown, separate LNA/Cs and feeder cables are required for each sense of polarization. Compared with the single-user system, a larger antenna is also required (2- to 3-m diameter) in order to maintain a good signal-to-noise ratio at all the indoor units. Where more than a few subscribers are involved, the distribution system used is similar to the CATV system described in the next section.
5.6 Community Antenna TV System The community antenna TV system employs a single outdoor unit, with separate feeds available for each sense of polarization, like the MATV system, so that all channels are made available simultaneously at the indoor receiver. Instead of having a separate receiver for each user, all the carriers are demodulated in a common receiver-filter system. The channels are then combined into a standard multiplexed signal for transmission over cable to the subscribers. In remote areas where a cable distribution system may not be installed, the signal can be rebroadcast from a low-power VHF TV transmitter. With the CATV system, local programming material also may be distributed to subscribers, an option which is not permitted in the MATV system.
5.7 Transmit-Receive Earth Stations
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In some situations, a transmit-only station is required,for example, in relaying TV signals to the remote TV receive-only stations already described. Transmit-receive stations provide both functions and are required for telecommunications traffic generally, including network TV.
It may be that groupings different from those used in the terrestrial network are required for satellite transmission, and the next block shows the multiplexing equipment in which the reformatting is carried out. Following along the transmit chain, the multiplexed signal is modulated onto a carrier wave at an intermediate frequency, usually 70 MHz. Parallel IF stages are required, one for each microwave carrier to be transmitted. After amplification at the 70MHz IF, the modulated signal is then upconverted to the required microwave carrier frequency. A number of carriers may be transmitted simultaneously, and although these are at different frequencies they are generally specified by their nominal frequency, for example, as 6-GHz or 14-GHz carriers.
It should be noted that the individual carriers may be multi destination carriers. This means that they carry traffic destined for different stations. For example, as part of its load, a microwave Page 36
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carrier may have telephone traffic for Boston and New York. The same carrier is received at both places, and the designated traffic sorted out by filters at the receiving earth station. The station’s antenna functions in both the transmit and receive modes, but at different frequencies. In the C band, the nominal uplink, or transmit, frequency is 6 GHz and the downlink, or receive, frequency is nominally 4 GHz. In the Ku band, the uplink frequency is nominally 14 GHz, and the downlink, 12 GHz. High-gain antennas are employed in both bands, which also means narrow antenna beams. A narrow beam is necessary to prevent interference between neighboring satellite links. In the case of C band, interference to and from terrestrial microwave links also must be avoided. Terrestrial microwave links do not operate at Ku-band frequencies. In the receive branch, the incoming wide-band signal is amplified in a low-noise amplifier and passed to a divider network, which separates out the individual microwave carriers. These are each down converted to an IF band and passed on to the multiplex block, where the multiplexed signals are reformatted as required by the terrestrial network. It should be noted that, in general, the signal traffic flow on the receive side will differ from that on the transmit side. The incoming microwave carriers will be different in number and in the amount of traffic carried, and the multiplexed output will carry telephone circuits not necessarily carried on the transmit side. 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 which is discussed further in Chap. 14. Antenna sizes range from 3.6 m (11.8 ft) for transportable stations up to 30 m (98.4 ft) for a main terminal. A medium-route circuit also provides multiple access, either on the basis of frequency-division multiple access (FDMA) or time-division multiple access (TDMA), multiplexed baseband signals being carried in either case. Antenna sizes range from 30 m (89.4 ft) for a main station to 10 m (32.8 ft) for a remote station.
Interferance and satellite acess Interference may be considered as a form of noise, and as with noise, system performance is determined by the ratio of wanted to interfering powers, in this case the wanted carrier to the interfering carrier power or C/I ratio. The single most important factor controlling interference is the radiation pattern of the earth station antenna. Comparatively large-diameter reflectors can be used with earth station antennas, and hence narrow beamwidths can be achieved. For example, a 10-m antenna at 14 GHz has a 3-dB beamwidth of about 0.15°. This is very much narrower than the 2° to 4° orbital spacing allocated to satellites. To relate the C/I ratio to the antenna radiation pattern, it is necessary first to define the geometry involved. The orbital separation is defined as the angle subtended at the center of the earth, known as the geocentric angle. However, from an earth station at point P the satellites would appear to subtend Page 37
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an angle ß. Angle ß is referred to as the topocentric angle. In all practical situations relating to satellite interference, the topocentric and geocentric angles may be assumed equal, and in fact, making this assumption leads to an overestimate of the interference (Sharp, 1983).
6.1 Single Access With single access, a single modulated carrier occupies the whole of the available bandwidth of a transponder. Single-access operation is used on heavy-traffic routes and requires large earth station antennas such as the class A antenna. As an example, Telesat Canada provides heavy route message facilities, with each transponder channel being capable of carrying 960 one-way voice circuits on an FDM/FM carrier. The earth station employs a 30-m-diameter antenna and a parametric amplifier, which together provide a minimum [G/T] of 37.5 dB/K.
6.2Preassigned FDMA Frequency slots may be preassigned to analog and digital signals, and to illustrate the method, analog signals in the FDM/FM/FDMA format ill be considered first. As the acronyms indicate, the signals are frequency-division multiplexed, frequency modulated (FM), with frequencydivision multiple access to the satellite. In Chap. 9, FDM/FM signals are discussed. It will be recalled that the voice-frequency (telephone) signals are first SSBSC amplitude modulated onto voice carriers in order to generate the single sidebands needed for the frequency-division multiplexing. For the purpose of illustration, each earth station will be assumed to transmit a 60channel supergroup. Each 60-channel supergroup is then frequency modulated onto a carrier which is then upconverted to a frequency in the satellite uplink band.
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6.3 Spade System The word Spade is a loose acronym for single-channel-per-carrier pulse-code-modulated multiple-access demand-assignment equipment. Spade was developed by Comsat for use on the INTELSAT satellites (see, e.g.,Martin, 1978). However, 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. Recalling from Fig. 14.5 that channel 400 also must be left vacant, 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.(An alternative arrangement is shown in Freeman, 1981). 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. To illustrate the procedure, suppose that a call to station F is initiated from station C in Fig. 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. Cities 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. 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 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. Time division multiple access, described in Sec. 14.7.8, is used for this purpose, allowing up to 49 earth stations to access the common signaling channel.
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6.4 TDMA With time-division multiple access, only one carrier uses the transponder at any one time, and therefore, inter modulation products, which result from the nonlinear amplification of multiple carriers, are absent. This leads to one of the most significant advantages of TDMA, which is that the transponder traveling-wave tube (TWT) can be operated at maximum power output or saturation level. Because the signal information is transmitted in bursts, TDMA is only suited to digital signals. Digital data can be assembled into burst format for transmission and reassembled from the received bursts through the use of digital buffer memories. Figure 1 illustrates the basic TDMA concept, in which the stations transmit bursts in sequence. Burst synchronization is required, and in the system illustrated in Fig. 1, one station is assigned solely for the purpose of transmitting reference bursts to which the others can be synchronized. The time interval from the start of one reference burst to the next is termed a frame. A frame contains the reference burst R and the bursts from the other earth stations, these being shown as A, B, and C in Fig. 1. Figure 2 illustrates the basic principles of burst transmission for a single channel. Overall, the transmission appears continuous because the input and output bit rates are continuous and equal. However, within the transmission channel, input bits are temporarily stored and transmitted in bursts.
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Fig 1
Fig2
Figure 3 shows some of the basic units in a TDMA ground station, which for discussion purposes is labeled earth station A. Terrestrial links coming into earth station A carry digital traffic addressed to destination stations, labeled B, C, X. It is assumed that the bit rate is the same for the digital traffic on each terrestrial link. In the units labeled terrestrial interface modules (TIMs), the incoming continuous-bit-rate signals are converted into the intermittent-burst-rate mode. These individual burst-mode signals are time-division multiplexed in the time- division Page 41
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multiplexer (MUX) so that the traffic for each destination station appears in its assigned time slot within a burst. Certain time slots at the beginning of each burst are used to carry timing and synchronizing information. These time slots collectively are referred to as the preamble. The complete burst containing the preamble and the traffic data is used to phase modulate the radiofrequency (rf) carrier. Thus the composite burst which is transmitted at rf consists of a number of time slots, as shown in Fig. 4. These will be described in more detail shortly. The received signal at an earth station consists of bursts from all transmitting stations arranged in the frame format shown in Fig. 4. The rf carrier is converted to intermediate frequency (IF), which is then demodulated. A separate preamble detector provides timing information for transmitter and receiver along with a carrier synchronizing signal for the phase demodulator, as described in the next section. In many systems, a station receives its own transmission along with the others in the frame, which can then be used for burst-timing purposes.
Fig 3
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Fig 4
Recommended Questions: 1. Explain a MATV system, with a neat diagram 2. With a neat block diagram explain the outdoor and indoor unit for a analog FM TV 3. A FM/TV carrier is specified as having a modulation index of 2.571 and top modulating frequency of 4.2MHz. Calculate theprotection ratio required to give a quality impairment factor of (a) 4.2 (b) 4.5 4. Explain possible interference nodes between satellite circuits and a terrestrial station. Explain spade system. 5. With a neat block diagram explain frame and burst formats for a TDMA system 6. Explain carrier recovery circuit with single tuned circuit having AFC.
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