SATELLITE COMMUNICATIONS

Transcription

1 SATELLITE COMMUNICATIONS Masters of Science Course ELE616 For Electrical Engineering Department UNIVERSITY OF DURBAN-WESTVILLE, SA Prepared and Presented By Dr. MUCEMI GAKURU Phd. (Cantab.) Senior Lecturer Electrical & Electronic Engineering Department University of Nairobi, KENYA October 1999

2 1.0 Introduction 1.1 Topic coverage - Introduction - Synchronous satellites, orbital relations, stabilisation and station keeping. - Communication payload, system link model and parameters - Link budget computation - Access techniques: FDMA, TDMA, CDMA, packet switched - Direct Broadcast by satellite - VSAT s 1.2 References - KAMILO FEHER, Digital communications, Pentice-Hall, ROBERT L. GOODMAN, Digital satellite service, McGraw-Hill, DANIEL MINOLI, Telecommunications Technology handbook, Artech house, SAMUEL Y. LIAO, Microwave devices and circuits, Prentice-Hall, Definition The term satellite refers to an un-manned spacecraft in orbit or an electronic system, e.g. transmitter or power station, which is subordinate to or dependent on a larger or more complete system. 1.4 History of satellite systems The first satellite to be launched was the Soviet Sputnik that orbited the earth in Subsequently, a communication satellite, echo, was launched in 1962, followed by weather satellites; Nimbus and Tiros in Throughout the 1960 s, satellites were largely experimental, and were launched on rockets that would be destroyed once the launching had been accomplished. The first communication satellites were passive reflectors, more like large metal balloons, that would be put in low orbits to reflect sufficient power. The 1970 s saw active, analogue, commercial systems in place, operating at 3.7 GHz, thus requiring large antenna dishes. Many digital systems have since the 1980 s been put in place, with a wide range of applications. Launching is nowadays carried out using space shuttles, which come back to earth and can be re-used. 2

3 1.5 Uses of satellite systems - Communications: Radio, telephone, mobile communications, News gathering (replacing outside broadcasting) and broadcast - Navigation: Reference for aircraft, ships, moving vehicles through Radio Determination Satellite Service (RDMSS which uses an earth based computer to receive information via satellite), Global Positioning System (Uses 21 Navstar Navigation Satellites) - Weather: Monitoring of atmospheric and surface conditions for weather forecasting. - Surveillance: These are mainly Military applications where objects on or near the earth s surface are monitored. Low orbiting satellites that repeatedly circle the area of interest usually monitor the reflected or emitted radio signals. - Remote sensing: This is recording and relaying of emission and energy reflections from the earths surface by satellites fitted with infrared sensors and photographic cameras. It can be used for determining weather conditions, movement of objects and military reconnaissance. - Research: The satellites are fitted with high-resolution cameras, or telescopes for space observations. These photographs were previously restricted to military operations but are now available commercially for surveying etc. 3

4 1.6 Altitude bands The satellites are to be found in certain distances from the earth s surface, depending on their use. These are as follows: Distance from the earth s surface in miles Use of satellite Classification Imaging and reconnaissance LEO Weather sensor LEO Surveillance monitoring and communications LEO Scientific Research MEO Exclusively Navigation MEO Broadcast, communication and surveillance Geosynchronous 4

5 1.7 Communications frequency bands Band Frequency 1 P MHz 2 L MHz 3 S MHz MHz 4 C MHz MHz 5 X MHz ` MHz 6 Ku GHz 7 Kc GHz 8 K GHz Higher frequencies imply higher gain and smaller antenna dishes. However they are more prone to rain, snow and water vapour with Ku being 5 times more susceptible than C band. 5

6 1.8 Components of a satellite system Space Segment Solar panel Solar panel Antenna Uplink C Band Ku Band Downlink C Band Ku Band Earth station Earth station (Tracking system) An active satellite receives a signal and re-transmits it at a different frequency. The combination of the receiver-transmitter system is called the transponder. The separation allows for maximum frequency and prevents un-desirable feedbacks. The signal is usually very weak as a result of propagation through the atmosphere. 6

7 1.9 Recent advances in satellite communications - Most satellites use 14/12 GHz band hence greater amplifier power - Solid state amplifiers have improved hence replacing TWT s, more reliable, cheaper and compact - On board facilities for switching and re-using frequencies are now included - Mobile satellites are in place - VSAT s are also now in use for handling point to point data. 7

8 2.0 GEOSYNCHRONOUS SATELLITES A geosynchronous satellite rotates in such a way that it appears stationary with respect to a given position on the earth s surface. It therefore takes 24 hours to go round its orbit. The motion is governed by Keplerian laws, which govern the planetary motion. The orbits is circular hence a special case. Recalling that the centrifugal force has to balance the gravitational attraction, then Since the period, t, is 24 hrs, then mv 2 /r = GmM/r 2, Hence v 2 =GM/r, where GM=3.99x10 14 t= 2πr/v Therefore r=42,188,000 metres and v = 10,800 km/h. Given that the radius of the earth is 6,378, 000 metres, then the distance of geosynchronous orbit above the earths surface is 35,810 Km or 22,380 miles. In reality, the earth is not a perfect sphere and gravitational effects of the moon and the sun have to be taken into consideration. These are taken care of by signals sent from the surface of the earth and use of on board jets to make orbital corrections. The position of the satellite is measured by the longitude (degrees west). The angle with which the orbit is tilted with respect to the equator is known as the inclination, which can be greater than 90 degrees. The zero inclination geosynchronous satellites are referred to as geostationary. The distance from the earth station to the satellite is referred to as slant range and can be computed using the cosine rule. Elevation is the angle made by the tangent on the earth s surface and the slant range. Satellites are powered by solar cells that are mounted in two ways: One is to mount the cells on the outside of a cylindrical satellite body, which is continuously rotated so that the cells always face the sun, while the Antenna is kept pointed at the earth s surface. The spinning body provides the gyroscope effect that stabilizes the satellite in space. 8

9 The other way is to use build in gyroscopes in both the body and the large solar panel arrays mounted on the satellite. This way, more power is achieved because large solar panels, with large areas, can be positioned so that the sun continuously illuminates them. Such satellites are known as three-axis stabilized, and have more complex control than the spin stabilized. There are a limited number of satellites that can be packed into the geostationary orbited because they have to have sufficient spatial separation to avoid interference. The desired separation is 3-6 degrees, and the actual separation will depend on the earth station beamwidth, carrier frequency, modulation techniques and permissible level of signal degradation due to interference. Currently, there are about 120 geostationary satellites in orbit. In summary the following are the characteristics of geostationary satellites: - Broad earth coverage, degrees, hence cover more than 30% of the earth s surface - No need for expensive tracking equipment as they are stationary with respect to the earth s surface. - The round trip full duplex delay is 600 ms. The adverse effects of this delay can be eliminated by echo cancellation circuits. 9

10 3.0 System link models and parameters These models and parameters are defined for computation of system performance. The model include the uplink, transponder and the down link model discussed as follows: 3.1 Uplink Model This comprises of the stages the signal has to pass through before reaching the transponder. The signal is first passed through an IF modulator, then a bandpass filter. It is then taken through an upconveter and a High Power Amplifier before launching it to the antenna. BPF HPA PtGt input IF modulator Upconveter Pt (dbw) The signal travels through the atmosphere and experiences loss due to water vapour, oxygen, fog, rain, snow hail and discharges in space. These can be summed up as Lu. It is also subject to the spread factor Lsf. The power density or flux at the receiving end is denoted as Ωu, while flat noise characteristics Nu are assumed Transponder model Pu BPF LNA (Tunnel diode) BPA Limiter TWT PsGd f 10

11 The Transponder receives the power Pu and noise, which go through the BPF and a low noise Tunnel diode Amplifier. Frequency translation is then effected, which prevents interference otherwise an isolation of db would be required. The signal is then passed through the High power TWT amplifier and fed to the downlink. 3.2 Downlink model On the downward propagation, the signal experiences loss due to spreading factor and the atmospheric effects. It is received through a low noise amplifier, then a down converter to provide the IF stage that is fed to the demodulator. LNA Demod PsGs Gd Downconverter 11

12 3.4 System parameters Antenna gain. For an isotropic antenna, the gain is given by G=η(ϖd/λ) 2, =η(4ϖf 2 Ae/c) where η is the efficiency. EIRP. Effective Isotropic Radiated Power is the product of the power at the antenna input and the Gain of the antenna, i.e. EIRP = PG or in db, P+G db. Space path loss. This is defined as the ratio of power transmitted by an isotropic antenna to the power received by an isotropic antenna. Pr=Ae x Pdensity =(λ 2 /4ϖ) x (P/4ϖr 2 ) Therefore Lfs= P/Pr = (4ϖr/λ) 2 Loses due to water and oxygen are predominant at 22 GHz and 60 GHz. Bit Energy. This is the product of saturated output power Posat and the bit duration Tb. Eb = Posat x Tb = Carrier Power x Tb = C x Tb Since the HPA s are non-linear devices, they are operated at near saturation to avoid distortion. Noise temperature. Noise performance of terrestrial devices is adequately measured by noise figure, NF, the ratio of noise power from a practical amplifier to noise power from an ideal amplifier. However, for satellite systems, noise budget has to be verified within fractions of decibels as it can be very expensive. A suitable parameter used for the noise performance is the noise temperature. NF = Npractical/Nideal=(Npractical)/A ktow The Npractical can be expressed as, Nideal + Receiver Noise. The receiver noise can be expressed in terms of equivalent noise temperature, thus: 12

13 Receiver Noise = A ktew, where Te is the equivalent noise temperature. The noise figure becomes: NF = 1 + Te /TO Typical noise temperatures for satellite systems are 1000 K and K for receiver stations. Noise density. This is the noise power normalized at 1 Hz. N0 = Npower/W = kte Carrier to noise-density ratio. This is the average wideband carrier power to noise power ration, normalized to 1 Hz Bandwidth i.e. C/N0=C/kTe Figure of merit. This is used to express the efficiency of a satellite and that of the earth station. It is the ration of the gain to equivalent noise temperature. Figure of merit = G/Te Typical figures of merit for earth station are 37.7 dbk -1 and 5.3 db dbk -1 for satellite transponders. Bit energy to noise density ratio. This figure is enables comparisons of systems having variable transmission rates and different modulation and coding schemes. It is given by: Eb/N0, where Eb = CTb 13

14 3.5 Link equations The power density or flux in an uplink model is given by: Ωu= PtGtLu/4ϖRu 2 The power received is thus by the satellite is Pu = Ωu x Asu = Ωu x Gsuλu 2 /4ϖ The noise Nou at the satellite receiver has flat spectral characteristics and thus the carrier power to noise density, C/ Nou, can be expressed as Pu/kTe. This gives the basic uplink equation: (C/Nou)dB= 10log PtGt 20log(4ϖRu/λu) + 10log(Gsu/Ts) + 10log Lu - 10log k EIRP Free space loss Satellite figure other losses merit Problem: Derive the basic downlink equation. The overall carrier power to noise density ratio is given by the addition of downlink and uplink noise thus: C/N0= 1/(Nou/Cu + Nod/Cd) This gives the overall Bit energy to Noise density ratio as: Eb/N0= 1/(Nou/Ebu + Nod/Ebd) 14

15 Link Budget Calculations This can be illustrated by calculating the link budget for a satellite system at 14/12 GHz with 2 KW transmitter power for earth station and 12 KW at the transponder output, using a 15m parabolic antenna dishes. The efficiency of the earth s transmitter has back-off and combining loses of 7dB and the transponder s loses are 0.6 db. The desired bit rate is 120 Mb/s. Clear weather losses of 0.6 db on the uplink and 0.4 on the downlink are assumed, as well as typical values of satellite and earth station figure of merit. UPLINK 1 Transmitter power at saturation 2 Back-off and combining Losses 3 Transmitter Antenna Gain 4 EIRP 5 Free space loss 6 Atmospheric loss 7 Satellite figure of merit 8 Bit duration 9 Bit energy to noise power density 15

16 DOWNLINK 1 Transmitter power at saturation 2 Modulation and bandlimiting Losses 3 Free space loss 4 Atmospheric loss 5 Power received at antenna 6 Receive Antenna Gain 7 Earth station figure of merit 8 Bit energy to noise power density Problem: How would these figures change if the elevation of the satellite was 60 0? Problem: In the INTELSAT transmission, the power output from the earth station is 3 KW, the uplink frequency 6GHz and the antenna diameter 30m. Using the typical satellite figure of merit, calculate the bit energy to noise density ratio for a transmission rate of 60 Mb/s. 16

17 4.0 ACCESS TECHNIQUES The limiting factor in satellite communication is the number of transponders and frequencies. A satellite may carry both C and Ku band transponder, thus giving it a high channel capacity. It would however require multiple antennas, hence expensive. Frequency reuse is therefore implemented to obtain higher bandwidth. This implemented through two methods: spatial and polarization reuse. A single antenna can be made to cover a large area, without compromising the gain by using multiple feeds. Spatial reuse takes advantage of the fact that the antenna is directional and thus multiple antennas with non-overlapping coverage areas can use the same frequencies, hence more effective bandwidth. Polarization of the field can further be used to enhance the spatial frequency reuse. Antenna will receive fields polarized in a give direction and reject those orthogonal to it. This way adjacent areas of spatial coverage could be made to have orthogonal polarization, thus reducing possible interference. To maximum on the use of the transponders, sharing by different subscribers has therefore to effected. To avoid mutual interference, a set of rules are established which are referred to as the access techniques FMDA (SCPC) In the 1970 s and 80 s, FDM/FM dedicated systems were used with single or multiple destinations. This involved pre-assigning a given set of circuits some a specific uplink and down link carrier frequency. The frequency remains un-used, whenever there is signal originating from the circuit even though other users would need to transmit. This resulted in poor utilization of the bandwidth. Further, at full power, the TWT amplifiers are non-linear and therefore would cause inter-modulation of signals. They were therefore operated at back-off power of 3dB, thereby reducing the EIRP from the satellite, with possible signal degradation. Demand assignment in FDMA is effected through allocation of a higher frequency, and the satellite channels would remain unused when change over is being effected. In demand assigned systems, the channels are assigned on temporary basis. Multiple access schemes are therefore optimized for % traffic so that few channel assignments are required. 17

18 4.2 DEMAND ASSIGNMENT MULTIPLE ACCESS Dynamic frequency assignment is carried out within each station from a pool of available frequencies. First a common channel signaling (CSC) system exists through which each station continuously updates its channel assignment data. When two stations wish to communicate, a given station, say X, examiners its pool for available frequencies and requests station Y for possible communication through the CSC, which operates in a TDMA mode. If Y responds positively, communication resumes otherwise it examines other frequencies. Sometime delays of 280 ms may mean the frequency in A has already been taken and a new search begins. In this method neither of the circuits is fully occupied. When the circuit is not in use, its power is switched off, only put on by voice activation. 4.3 TDMA In TDMA each earth station is assigned a time slot, through which it sends its data. The data comprises a header that has timing and supervisory information. The frames do not have to have the same length but could depend on the bandwidth requirement of the system. This scheme provides each station with full use of the transponder for a time interval and no inter-modulation distortion or back off is required. The actual traffic frame comprises of a reference burst and traffic bursts from the various earth stations. The reference burst is issued from the MES and is used for synchronizing other stations. The traffic burst from the other station are referenced to this reference burst, which contains in addition, the duration of each traffic burst in case of demand assignment. It also carries the carrier and bit timing recovery bits, used to carry the receiver carrier frequency and its bit timing clocks. The number of bit rates assigned to the reference burst can be varied to compensate for low bit energy to noise density ratio. In addition each traffic-burst has its own preamble, which contains the network control and signaling information. An earth station can also send or receive traffic bursts to more that one transponder in what is know as transponder hoping. Different uplink and downlink frequencies and polarization are required, and the process is only practical when the reference bursts in all the transponders are synchronized to a common time reference. Digital systems are more desirable as they could be used to offer solutions to the problems of satellite delays. In full duplex transmission, the delay is 600 ms, which comprises the send time and time taken to acknowledge receipt of error 18

19 free data. Forward-error-correction technique (FEC), where sufficient information is built into the frame to enable error correction is commonly used in satellite communications. This is followed by satellite delay compensator, which operates at each end of the satellite path and provides local acknowledgement of information sent on behalf of each earth station. The available bit rate in a TDMA system is obviously determined by the available bandwidth, power and the transponder s figure of merit. Rates achieved so far are up to 120 MB/s. All the transmission is carried out using the common TDMA terminal equipment (CTTE), which does the following: Transmit time start, preamble generation, FEC coding, data scrambling and terminal frequency hoping. It detects UW s, receives bursts and process CSC for network control. The TDMA is currently being used by most satellite systems. However, the analogue systems implemented over the s are cheaper and are used by many countries for their local communications DEMAND ASSIGNMENT IN TDMA SYSTEMS This requires only a method of assigning time-slots to active channels and is carried out in one of the four methods: Polled allocation systems. Here a reference master earth station (MES) invites the other stations to transmit. Those with data will seize the channel, while those without respond negatively. The method is not suitable for high capacity service with a large number of systems. Central allocation on demand. The MES is contacted for time slot allocation, using terrestrial link or supervisory channels. These two methods rely on the MES and communication is completely lost when the MES goes down. In this case, the traffic burst from one of the stations is used as the reference burst. Distributed allocation. A station seizes the channel and results to arbitration in the case of collision. It is less prone to loss of the MES and its actual location. Contention. The satellite space is treated similar to a CSMA/CD local area network, with each station transmitting when the channel is free. The satellite however listens only to the downlink path as the uplink traffic from other satellites is weak. It sends its own traffic, listens for it and if it does not appear after a certain delay, it assumes a collision has occurred. 19

20 4.5. ADVANTAGES OF TDMA OVER FDMA - It has one carrier at a time in the transponder hence minimizing intermodulation. - The signals are time separated as opposed to frequency separated, hence no need for very many filters and down converters. Therefore it is simpler to implement. - TDMA is suitable a digital data transmission technique, hence suitable for on-board processing and switching, and also for demand assignment where duration of traffic bursts can be easily changed to accommodate more demand. - Satellite switches and frequency reuse systems can be used to selectively connect down beams and up beams in the case of multiple beam antennas, thus reducing the effect of adding up all the noise as in FDMA SPREAD SPECTRUM SYSTEMS In spread spectrum systems, a far higher channel capacity than signals bandwidth is used to transmit the signal. Each bit is coded with a pseudorandom noise sequence (microbits) thus spreading the information into a bandwidth many times wider than that of the data alone. Receivers using the same sequence can decode the encoded data bits. This makes the signal less vulnerable to noise and interference, whose power is limited to a fraction of the signals bandwidth. It was first adopted by the military to avoid deliberate jamming. Because of the limitations of noise effects and interference, low powers can be transmitted using small antennas and inexpensive earth equipment. It is costeffective for low duty cycles, low data rate earth stations. The signals are in the same frequency channel, are time independent and suffer much less from collision-caused transmission delays. No network-wide synchronization is needed. 20

21 5.0 DIRECT BROADCAST SATELLITE In standard satellite applications, the user is provided with a permanent leased communication or a switched path, so that power earth stations support high capacity transponders. Satellite service is usually sold through tail end carriers and time slots. These systems are known as point-to-point systems. The capacity of the earth station becomes un-economical, if it is very low as indeed the arbitration costs are high. Systems below 56KBPs are considered unreasonable. In other systems, the MES also behaves as a host of the information desired by a series of earth stations, referred to as slave stations. All the stations receive the same information, with each rejecting the data that it does not need. The forward channel traffic to the slave stations is high, while the back channel traffic from the stations is low. The back channel traffic mainly comprises of requests for transmission from the MES, which allows contention-controlled access. However, when there is a lot of traffic, other methods such as terrestrial methods are used. This technology is known as multipoint access. In some cases the back channel traffic in a multipoint access system has zero volume, resulting in what is known as direct broadcasting system (DBS). The earth stations are therefore small and high EIRP is provided in the satellite to support them. Typically this is in the range of 52dBW, the diameter of earth station ~1.0 M and transmission frequencies / GHz. The receiver are therefore inexpensive and comprise of the antenna, a down converter and an LNA at the antenna site, which is fed through a large coaxial cable known as heliax. An ordinary cable carries an RF signal at 70MHz to the processing unit. Un-authorized reception is prevented through encryption methods. Specific high power zones have been reserved at W, W & W, with satellite spacing of 2 0. When installing these receivers, the pointing requires that both the azimuth and elevation are set. The azimuth angle is the angle of rotation in the horizontal direction, through that a ground based parabolic antenna must be rotated to point to a specific satellite. It is usually worked out from the latitude and longitude and is measured due north. 21

22 6.0 Very Small Aperture Terminals (VSAT s) These refer to earth stations with antenna diameters of m and have therefore low power output. They have gained prominence due to improvement on transponder technology and the use of spot beams. The first VSAT systems were introduced in the C band around 1984, and were used for receiving only. In 1984 systems using the Ku band frequency band were introduced. With the development of random access techniques, two-way systems became possible for low data rate transmission. Currently, VSAT s incorporate switching and management systems and are in the Ku band ( / GHz) using contention access techniques. The VSAT s are essentially multipoint satellite systems, where the back channel traffic is very low compared to forward channel traffic. A complete VSAT system thus comprises of a master earth station that is connected to a customer host computer. This computer may contain the data desired at several VSAT sites. The MES has a high gain antenna (5-9 m diameter) and powerful amplifier as it carries a huge volume of traffic. This enables good reception by the earth stations. The transponders also produce a lot of power so that, coupled with the spot beams with high gains, a large EIRP is achieved for the downlink. Some VSAT s are used for receive only, more like a DBS. Data communication can also be carried out between two specific VSAT s. Here, the respective VSAT s send data through the transponder then to the MES, where switching is done, then the traffic routed to the designated VSAT. Only very low data rates are possible say 64KBPS. The whole interconnection is a start network topology, with the MES being the hub, and the customer data bank becomes like the server. The MES performs a variety of functions such as transponder monitoring and host interfacing. It also has an RF-IF stage, network switching system and network management. The earth stations comprise of outdoor and indoor units, with former being the antenna dish, mounting frame and the RF stage, while the latter has a controller; a microprocessor based data unit for data communication and protocol handling for the terminal equipment. Installation of these units is often hampered by obstruction from buildings, lack of space for running cables etc. More than 30,000 VSAT s have been installed so far, and are mainly used for data communication, with only 25% being used for video and 5 % for voice. They find their applications in interconnection of widely dispersed operations and often act as data tributaries to terrestrial backbone systems. They are subject to adverse weather effects, especially at Ku band, while C-band systems are subject to interference from terrestrial microwave systems. 22