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Broadband internet access via satellite: How it works
The large central teleport hub station
The hub station is a large satellite dish, typically 6m to 32m diameter. The large size means high receive gain and sensitivity and this minimises the transmit power and dish size required by the remote customer terminals. The large size also means high transmit gain and this reduces the transmitter power at the hub, where a high speed outlink carrier, similar to a satellite digital TV type carrier, needs to be transmitted. High speed means anything from 2 to 60 Mbits/s
The transmit Hub Common Equipment (HCE) consists of a router to interface to the external ISP network, a DVB-IP encapsulator to embed the IP data into an MPEG-2 format, a DVB multiplexer, a continuous modulator, a timing clock, an up-converter and high power amplifier.
The receive HCE consists of a low noise amplifier, down-converter, timing and distribution MF-TDMA demodulator unit and multiple MF-TDMA demodulator units for each inbound carrier to be received simultaneously. Each demodulator receives an inbound link from a large number of remote RCSTs. The number of optional demodulators depends on the number of inbound links.
The satellite
To work well for small dish transmit services the satellite uplink needs to have high sensitivity (gain to noise temperature ratio, G/T). This is most readily achieved if the uplink beam coverage area is small. Also the satellite transponder has to have a sufficiently high gain setting. This is not technically difficult but does need to be specified before satellite construction starts. The gain of a satellite may be adjusted in orbit by remote control of a gain step attenuator. For large dish services a lower gain setting is attractive.
Frequency bands
The frequency bands can be any of C band 4/6 GHz, Ku band 10-12/14 GHz or higher (Ka band). Ku band is most popular. C band has merit in tropical areas with heavy rainfall. The higher Ka bands, 18-30 GHz have come into use in recent years and offer lower costs as more bandwidth is available and this bandwidth may be re-used many times using small spot beams. Rain margin is a problem but can be compensated for using adaptive modulation and coding and uplink power control.
Transmission technique for the outlink (Hub to Remote terminals)
A large outlink carrier from the hub is shared amongst all the customer terminals. The bit rate is up to 60 Mbits/s rate and is ETSI-compliant (EN-301210) for modulation and FEC. The format of this carrier is Quadrature or 8 phase Phase Shift Keying (QPSK or 8-PSK). These are ways of transmitting digital data. For a particular bit rate both need the approx the same power from the satellite but 8-PSK concentrates the power into half the bandwidth. If you have a really powerful satellite and perhaps slightly larger remote terminals, then 8-PSK becomes feasible with a doubling of the satellite transponder bit rate capacity. The carrier is formed by a series of symbols where each symbol has 4 or 8 possible states, thus conveying 2 or 3 binary bits per symbol. The symbol rate is approximately the same as the carrier -3 dB bandwidth. To achieve a negligible bit error rate for the information, forward error correction systems are used which add extra bits to create a higher transmission bit rate. With QPSK a convolutional inner code is concatenated with an outer Reed Soloman block code. With 8-PSK the inner code is a pragmatic Trellis type. The data stream format is MPEG-2 with DVB Multi-Protocol Encapsulation (DVB-MPE) format for IP data.
At the customer receiver, the extra forward error correction bits are used to detect errors and normally to correct all of them.
Return Channel Satellite Terminals (RCST) outdoor unit (ODU)
The outdoor ODU equipment comprises a parabolic
antenna reflector, feed, ortho-mode transducer, filters and transmit / receive
radio frequency modules.
The reflector collects the received downlink signal from the
satellite. The larger the size the better. Most dishes used are of offset front
fed parabolic shape with the feed at the bottom on an arm. The beam comes off
the dish at an upwards angle. Reflectors vary:
| Dish dia | C band Gain | C band antenna Noise Temp | Ku band Gain | Ku band antenna Noise Temp |
| m | rx / tx dBi | (el=20deg) K | rx / tx dBi | (el=20deg) K |
| 0.96 | 39.7 / 41.2 | 39 | ||
| 1.2 | 42 / 43.5 | 36 | ||
| 1.8 | 35.4 / 39.3 | 36 | 45.3 / 46.8 | 36 |
| 2.4 | 38.0 / 42.0 | 35 | 47.6 / 49.3 | 34 |
The dish size must be large enough to receive the signal, according to site location in the satellite coverage beam. Further, in some cases, the next larger size may be selected if a high transmit bit rate is requested as there is a trade off between say doubling the dish area and doubling the transmit power amplifier size. A larger dish rather than a more powerful transmitter is my technical preference as this gives an improvement in the receive margin as a bonus.
The reflector surface must be accurate and not suffer from surface irregularities or from overall bending distortion. At 14 GHz the tolerance is +/- 1.5 mm.
The feed is where the radio signal emerges from the radio equipment and into the air. Its function is to distribute the power across the dish area. The feed must be at the focus of the parabolic dish shape. Behind the feed there is an ortho-mode transducer (OMT) which separates the two polarisations which are linear and at right angles. (Left hand and right hand polarisations are used in some applications). One polarisation is used for transmit and the other for receive. The feed and OMT assembly and feed must be rotated to line up the polarisation accurately with the satellite. Accuracy of +/- 1 degree is required to avoid interference with services on the other polarisation. Filters are inserted to avoid the transmitter interfering with the receiver and finally the receiver and transmitter are attached. The receiver module is called a low noise block down converter (LNB), similar to that used for digital satellite television. Typical LNB noise temperatures and gain are 59K and 54 dB. The transmit module (Block Up Converter BUC) up-converts the signals to the transmit frequency and amplifies them before transmission. Lower or higher powers are associated with lesser or greater transmit bit rates. Ku band BUCs are available with 0.5, 1, 2 or 4 watts. C band BUCs are available with powers of 2 or 5 watts.
Two coaxial cables with F type connectors join from the outdoor to indoor units. These cables, one for transmit and one for receive, each carry DC power and radio signals. It is vital that the F type terminations are made up correctly, the centre pin should stick out 2mm and, once assembled, waterproof tape must be applied.
Dish pointing
Find your site latitude and longitude by looking at a map or using a GPS receiver. Note the satellite orbit location (longitude above the equator) that you will be using. Go to determining your lat/long and dish pointing angles and polarisation. Set the feed polarisation angle. Set the elevation angle really accurately using an inclinometer (allowing for the dish offset) and then, using a compass, swing the antenna boldly around the required azimuth direction. You should find the satellite in a few seconds. Refine the pointing by making small offsets in all directions, which should all cause reductions. Peak up the polarisation with the assistance of the hub who will monitor your transmit carrier for cross polarisation quality.
Indoor equipment
The satellite modem, satellite router or Return Channel Satellite Terminal (RCST), is an integrated unit with connections for the one or two coax cables to the antenna and a CAT5 10/100 Base-T connection for an ethernet cable direct to the customers computing equipment. There is also the mains power input. Put the indoor box in a cool, clean, dry place. Apart from the initial set up, the box is entirely controlled from the hub. Popular equipment comes from Viasat (Surfbeam, Linkstar etc), HughesNet (HN9000 etc) and iDirect (Evolution etc). Some equipment is intended for home users, other for professional applications.
The received MPEG-2 or MPEG-S2 stream is recovered from the outbound signal by an integrated circuit consisting of a DVB-S demodulator and de-multiplexer. This logic demodulates the outbound signal and the demux recovers the IP packets intended for the specific customer terminal are then delivered to the external network via the ethernet interface
Note that if you fail to receive the outlink carrier from the satellite (e.g. due to dish damage in a gale or obstruction of the beam) the possibility of transmitting a data burst is inhibited. Also the hub can inhibit transmission in the event that off-axis or cross polarisation interference to other satellite services is detected.
When it is required to send data the indoor unit prepares the data into short packets or bursts which are transmitted in accordance with a time division multiple access (TDMA) system. Bursts are sent typically each time the mouse is clicked on a web page hyperlink, with relatively long non-transmitting gaps in between. Due to the long gaps, average individual terminal bit rates are typically less than 3 kbit/s. If a larger file is to be uploaded to the hub then a rapid sequence of longer bursts is typical. Modulation method and burst timing techniques are designed for maximum efficiency. Typically QPSK modulation is used with rate 2/3 Turbo Coding The transmission rates are shown below. You will see several throughputs, that vary with the nature of traffic. Doubling the return link carrier rates requires twice the transmit power or a dish with twice the area.
|
Return link Carrier Rate |
Min Thruput |
Nominal Thruput |
Max thruput |
Transmission rate |
|
k symbols / sec |
kbit/s |
kbit/s |
kbit/s |
kbit/s |
|
156.25 |
95 |
128 |
146 |
208 |
|
312.5 |
215 |
256 |
310 |
416 |
|
625 |
439 |
512 |
689 |
832 |
|
1250 |
870 |
1024 |
1404 |
1664 |
Latency
Satellites are a long way up and the speed of the radio waves (same as speed of light) means that there is some significant delay or latency, approx quarter of a second, associated with an end to end satellite transmission. This latency delay make satellite connection unsuitable for competitive computer games that require rapid click actions.
Safety
The dish assembly should be mounted securely with care so that it does not fall down on anyone. Take great care if working on a ladder or on a roof. The view upwards from the dish to the satellite should be clear of obstruction and the possibility of people, vehicles, animals, trees etc getting in the way of the beam. The radio power transmitted out of the feed is comparable to that from several cell phones and the head, eyes etc should not be put at the feed opening or between the feed and the dish. The beam up to the sky, which is like a torch beam approximately the same diameter as the dish, is also to be avoided, although the power density here is very much less than immediately at the feed opening.
Connecting your PC or local area network
The least complex installation will be a single PC connected with an ethernet cable. You just need to set the IP address and subnet mask. More commonly, a local router will be used to enable many computers on a local area network, cafe or wireless LAN to be connected.
Network Management
Management and control functions are performed by the NMS (Network Management System). It is a web-based design, user-friendly and flexible and allows Configuration, Control, Performance, and Alarm Management, as well as Accounting, and User Administration. It provides for the download of software to the remote terminals.
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Page updated 3 Nov 2014, updated 4 April 2021 |