29 Mobile communications Satellite systems (Part-2)

Suchit Purohit

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Learning Objectives

  • Essential subsystems communication satellite
  • Basics Communications Satellites Basics
  • Satellite Transponders
  • Communication Payloads
  • Fixed Satellite Services
  • Broadcast Satellite Services
  • Mobile Satellite Services
  • Routing
  • Localization
  • Handover in satellite systems
  • MEO and LEO Systems examples
  • Regenerative Satellite Payload
  • NextGen High Capacity Service Paylaods
  • Advance Communication Satellite
  • Advanced Multi-beam MSS System
  • Free Space INTER SATELLITE LINKS ISL/IOL
  • Summary

Introduction

 

Satellite communication supports the mobile Communications. Satellites offer global coverage without wiring costs for base stations and are almost independent of varying population densities.

 

Essential subsystems of any communication satellite

  • Spacecraft Structure
  • Propulsion
  • Communication Payload
  • Attitude & Orbit Control System (AOCS)
  • Deployment Mechanism
  • Thermal Control System
  • Power System
  • Telemetry, Tracking & Command System (TTCS)

The satellites and Communication involves multidisciplinary engineering Knowledge e.g. Spacecraft Structure requires civil engg. Propulsion – Chemical, Mechanism, Antenna structure, Momentum wheels – mechanical engg. : Thermal Engg: Power systems and Solar Array, Battery, -Electricalengg.; Telemetry, Tracking & Command System (TTCS) requires advance software and servo systems, transponder Communications – Electronics, Computer and Information Technology.

 

Figure 1 : INSAT/GSAT  Spacecraft

 

Communications Satellites Basics

  • Elliptical or Circular Orbits
  • Complete rotation time depends on distance satellite-earth
  • LOS (Line of Sight) to the satellite necessary for connection
  • High elevation is needed for less absorption due to buildings etc.
  • Uplink: Connection base station – satellite
  • Cn 3.7-4.2 GHz , Cext 4.4GHz-4.8GHz , Ku 10.7-11.7GHz
  • Downlink: Connection satellite and base station
  • Cn. 5.9 -6.4 GHz, Ku 13.75-14.5Ghz, ka 29.6-30.2Ghz
  • Typically separate frequencies for uplink and downlink are used.
  • The  operating  frequency  band  of  a  satellite  is  divided  into  several  channels,  each offering specific bandwidth.
  • The bandwidth of each channel is typically 40 MHz in C-Band, 40 MHz & 80 MHz in Ku- Band, 10 MHz & 20 MHz in S-Band. Some channels also have bandwidths smaller than 40 MHz
  • Each channel (also known as Transponder) in satellite is dedicated to a specific service or a bundle of services.
  • Number of Transponders or Channels available in a satellite determines the service capacity of a satellite.
  • The  information  travels  through  varieties  of  electronic  equipment  on  the  satellite platform prior to their transmission towards the receiving station on the earth.
  • A chain of electronics hardware used for collecting and redistributing the    signal is known as payload or transponder.
  • Each Transponder is designed to provide services with predetermined flux-density of RF power energy over a specific area or geographical coverage.
  • The total amount  of radiated power is decided by the  power amplifiers & area  of coverage and, available power-flux density in it is governed by size, shape and design of Antenna.
  • EIRP – Equivalent Isotropic Radiated Power and antenna radiation pattern depicting the coverage zone are two critical parameter in a satellite.

Satellite Transponders Type

 

Repeater subsystem receives the uplink RF signals, and convert these signals to the appropriate downlink frequency and power for transmission towards service area. ‘Transponder’ is a part (unit block) of the Repeater.

 

Transparent Transponder: called bent-pipe Only translates the uplink frequency to suitable downlink frequency & power without any baseband processing

Regenerative Transponder: Along with frequency translation and amplification provides capabilities of demodulation, baseband processing and re-modulation

 

Figure 2 : Typical Communication Payload

 

Figure 3 : Antenna Elements output side

 

Figure 4 : Typical C-Band and Ku –Band Payload Coverage for Indian Region.

 

Communication Payloads

 

Table 1 :

 

The key specifications for payloads are EIRP, G/T, Frequency bands, Transponders quantity, Saturation Flux density, Coverage Regions (as tabulated above) depends on hardware design. The Essential payload subsystems (shown in following Fig. 19. ) are responsible for performances e.g. Receiver’s Low noise Amplifiers and Receive (Rx) Antenna together contributes for G/T, Power Amplifiers-TWTAs/SSPAs and Transmit Antenna (Tx) contributes for EIRP, ALC Channel amplifiers control the Transponder gain variation during rain, Input Multiplexers divides the Receive band in 40/80 MHz (Typical) frequency band before amplification by Power Amplifiers and again combined by Output Multiplexers before feeding to Transmit Antenna.

 

Figure 5 : Payload hard ware Subsystems

 

Fixed Satellite Services

 

Definition:  Pair  of  Transmitting  &  Receiving  stations  are  at  the  fixed  location  on  earth  when communication link is established. One to One communication, Stationary user terminals..  Applications:

  • Long distance telephony trunk routs.
  • Business networking for data communication (VSAT based)
  • Broad-band internet access to remote areas.
  • Satellite News Gathering, Search & Rescue.
  • Tele-medicine, Tele-education, Disaster Management

 

Figure 6 : FSS

 

Broadcast Satellite Services

 

Definition: A radio communication service in which signals transmitted or re-transmitted by space stations are intended for indirect or direct reception by the general public.One to One many, stationary user terminals.

Applications: Feed for terrestrial TV transmitters, Direct To Home TV, Digital Audio Broadcast

 

Figure 7 : BSS

Mobile Satellite Services

 

Definition: Pair of Transmitting & Receiving stations are on move when communication link is established.One to One many, stationary user terminals.

Applications: Voice & Data communication for: Land Mobile, Aero-mobile, Maritime Mobile

 

Figure 8 : MSS

 

Advanced Multi-beam MSS System

 

Mobile Satellite Number of spot beams and size: 500 #, 0.4°, Typ. Antenna System:~ 22m

 

Figure 9 : Mobile Satellite Venture (MSV)

 

Figure 10 : Hybrid System – Cellular & Satellite with Ancillary Terrestrial Components.

 

Routing

  • One solution: inter satellite links (ISL)
  • Reduced number of gateways needed
  • Forward connections or data packets within the satellite network as long as possible
  • Only one uplink and one downlink per direction needed for the connection of two mobile phones Problems
  • More complex focusing of antennas between satellites
  • High system complexity due to moving routers
  • Higher fuel consumption, thus shorter lifetime

 

A satellite system together with gateways and fixed terrestrial networks as shown in Fig. 3. has to route data transmissions from one user to another as any other network does. Routing in the fixed segment (on earth) is achieved as usual, while two different solutions exist for the satellite network in space. If satellites offer ISLs, traffic can be routed between the satellites. If not, all traffic is relayed to earth, routed there, and relayed back to a satellite.

 

Assume two users of a satellite network exchange data. If the satellite system supports ISLs, one user sends data up to a satellite and the satellite forwards it to the one responsible for the receiver via other satellites. This last satellite now sends the data down to the earth. This means that only one up-link and one down link per direction is needed. The ability of routing within the satellite network reduces the number of gateways needed on earth.

 

If a satellite system does not offer ISLs, the user also sends data up to a satellite, but now this satellite forwards the data to a gateway on earth. Routing takes place in fixed networks as usual until another gateway is reached which is responsible for the satellite above the receiver. Again data is sent up to the satellite which forwards it down to the receiver. This solution requires two uplinks and two down links. Depending on the orbit and the speed of routing in the satellite network compared to the terrestrial network, the solution with ISLs might offer lower latency. The drawbacks of ISLs are higher system complexity due to additional antennas and routing hard- and software for the satellites.

Localization

 

Localization of users in satellite networks is similar to that of terrestrial cellular networks. One additional problem arises from the fact that now the ‘base stations’, i.e., the satellites, move as well. The gateways of a satellite network maintain several registers. A home location register (HLR) stores all static information about a user as well as his or her current location. The last known location of a mobile user is stored in the visitor location register (VLR). Functions of the VLR and HLR are similar to those of the registers in. A particularly important register in satellite networks is the satellite user mapping register (SUMR). This stores the current position of satellites and a mapping of each user to the current satellite through which communication with a user is possible.

 

Registration of a mobile station is achieved. The mobile station initially sends a signal which one or several satellites can receive. Satellites receiving such a signal report this event to a gateway. The gateway can now determine the location of the user via the location of the satellites. User data is requested from the user’s HLR, VLR and SUMR are updated.

 

Calling a mobile station is again similar to GSM. The call is forwarded to a gateway which localizes the mobile station using HLR and VLR. With the help of the SUMR, the appropriate satellite for communication can be found and the connection can be set up.

Handover in satellite systems

 

Several additional situations for handover in satellite systems compared to cellular terrestrial mobile phone networks caused by the movement of the satellites Intra satellite handover

  • Handover from one spot beam to another
  • Mobile station still in the footprint of the satellite, but in another cell Inter satellite handover
  • Handover from one satellite to another satellite
  • Mobile station leaves the footprint of one satellite Gateway handover
  • Handover from one gateway to another
  • Mobile station still in the footprint of a satellite, but gateway leaves the footprint Inter system handover
  • Handover from the satellite network to a terrestrial cellular network
  • Mobile station can reach a terrestrial network again which might be cheaper, has a lower latency etc.

 

An important topic in satellite systems using MEOs and in particular LEOs is handover. Imagine a cellular mobile phone network with fast moving base stations. This is exactly what such satellite systems are – each satellite represents a base station for a mobile phone. Compared to terrestrial mobile phone networks, additional instances of handover can be necessary due to the movement of the satellites.

  • Intra-satellite handover: A user might move from one spot beam of a satellite to another spot beam of the same satellite. Using special antennas, a satellite can create several spot beams within its footprint. The same effect might be caused by the movement of the satellite.
  • Inter-satellite handover: If a user leaves the footprint of a satellite or if the satellite moves away, a handover to the next satellite takes place. This might be a hard handover switching at one moment or a soft handover using both satellites(or even more) at the same time (as this is possible with CDMA systems). Inter-satellite handover can also take place between satellites.If they support ISLs. The satellite system can trade high transmission quality for handover frequency. The higher the transmission quality should be, the higher the elevation angles that are needed. High elevation angles imply frequent handovers which in turn, make the system more complex.
  • Gateway handover: While the mobile user and satellite might still have good contact, the satellite might move away from the current gateway. The satellite has to connect to another gateway.
  • Inter-system handover: While the three types of handover mentioned above take place within the satellite-based communication system, this type of handover concerns different systems. Typically, satellite systems are used in remote areas if no other network is available. As soon as traditional cellular networks are available, users might switch to this type usually because it is cheaper and offers lower latency. Current systems allow for the use of dual-mode (or even more) mobile phones but unfortunately, seamless handover between satellite systems and terrestrial systems or vice versa has not been possible up to now.

MEO and LEO Systems examples

 

Table.3 shows four examples (two in operations, two planned) of MEO/LEO satellite networks. One system, which is in operation, is the Iridium system. This was originally targeted for 77 satellites (hence the name Iridium with its 77 electrons) and now runs with 66 satellites plus seven spare satellites (was six, Iridium, 2002). It is the first commercial LEO system to cover the whole world. Satellites orbit at an altitude of 780 km, the weight of a single satellite is about 700 kg. The fact that the satellites are heavier than, e.g., the competitor Globalstar results from their capability to route data between Iridium satellites by using ISLs, so a satellite needs more memory, processing power etc. Mobile stations (MS in Table 3.0) operate at 1.6138–1.6265 GHz according to an FDMA/TDMA scheme with TDD, feeder links to the satellites at 29.1–29.3 GHz for the uplink and 19.4–19.6 GHz for the downlink. ISLs use 23.18–23.38 GHz. The infrastructure of Iridium is GSM-based.

 

Table 2 : Example of MEO and LEO Systems

 

A direct competitor of Iridium is Globalstar (Globalstar, 2002). This system, which is also operational, uses a lower number of satellites with fewer capabilities per satellite. This makes the satellites lighter (about 450 kg weight) and the overall system cheaper. Globalstar does not provide ISLs and global coverage, but higher bandwidth is granted to the customers. Using CDMA and utilizing path diversity, Globalstar can provide soft handovers between different satellites by receiving signals from several satellites simultaneously. Globalstar uses 1.61– 1.6265 GHz  for uplinks from mobile stations to the  satellites and 2.4835–2.5 GHz for the downlink. Feeder links for the satellites are at 5.091–5.250 GHz gateway to satellite and 6.875–7.55 GHz satellite to gateway.

 

While the other three systems presented in Table 3.0 are LEOs, Intermediate Circular Orbit, (ICO) (ICO, 2002) represents a MEO system as the name indicates. ICO needs less satellites, 10 plus two spare are planned, to reach global coverage. Each satellite covers about 30 per cent of earth’s surface, but the system works with an average elevation of 40°. Due to the higher complexity within the satellites (i.e., larger antennas and larger solar paddles to generate enough power for transmission), these satellites weigh about 2,600 kg. While launching ICO satellites is more expensive due to weight and higher orbit, their expected lifetime is higher with 12 years compared to Globalstar and Iridium with eight years and less. ICO satellites need fewer replacements making the whole system cheaper in return. The start of ICO has been delayed several times. The ICO consortium went through bankruptcy and several joint ventures with other satellite organizations, but still plans to start operation of the system within the next few years. The exact number of satellites is currently unclear, however, the system is shifted towards IP traffic with up to 144 kbit/s.

 

A very ambitious LEO project was Teledesic which plans to provide high bandwidth satellite connections worldwide with high quality of service (Teledesic, 2002). In contrast to the other systems, this satellite network is not primarily planned for access using mobile phones, but to enable worldwide access to the internet via satellite. Primary customers are businesses, schools etc. in remote places. Teledesic wants to offer 64 Mbit/s downlinks and 2 Mbit/s uplinks. With special  terminals  even  64  Mbit/s  uplinks  should  be  possible.  Receivers  will  be,  e.g.,  roof-mounted laptop-sized terminals that connect to local networks in the building. Service start was targeted for 2003, however, currently only the web pages remained from the system and the start was shifted to 2005. The initial plans of 840 satellites plus 84 spares were dropped, then 288 plus spares were planned, divided into 12 planes with 24 satellites each. Considering an expected lifetime of ten years per satellite, this still means a new satellite will have to be launched at least every other week. Due to the high bandwidth, higher frequencies are needed, so Teledesic operates in the Ka-band with 28.6–29.1 GHz for the uplink and 18.8–19.3 GHz for the downlink. At these high frequencies, communication links can easily be blocked by rain or other obstacles. A high elevation of at least 40° is needed. Teledesic uses ISL for routing between the satellites and implements fast packet switching on the satellites.

 

Only Globalstar uses CDMA as access method, while the other systems rely on different TDMA/FDMA schemes. ICO satellites for example are more complicated compared to Iridium, so the ICO system has similar initial costs. Smaller and simpler Globalstar satellites make the system cheaper than Iridium.

 

Regenerative Satellite Payload

 

Figure 11 : Regenrative Payload Systems

 

Onboard processing (intelligence) satellite using multiple beam antennas and onboard switching

 

NextGen High Capacity Service Paylaods

 

Figure 12 : High Throughput Satellite

 

Advance Communication Satellite

 

Figure 13 : ACS

 

Advanced Multi-beam MSS System

 

Figure 14 : INMARSAT -4

 

Free Space INTER SATELLITE LINKS – ISL and IOL

 

Figure 15 : FSO and ISL

  • Potential for high data rates.
  • Small size, low mass, low prime power.
  • Negligible interlink interference.
  • Spectral separation  from  existing  ground-based  and  space-based  communication system.
  • Optical Spectrum is currently un regulated whereas RF spectrum is regulated by national and international agencies

Summary

 

The trend for communication satellites is moving away from big GEOs, toward the smaller MEOs and LEOs mainly for the reason of lower delay which is essential for  voice communication. Different systems will offer global coverage with services ranging from simple voice and low bit rate data up to high bandwidth communications with quality of service. However, satellite systems are not aimed at replacing terrestrial mobile communication systems but at complementing them. Special problems for LEOs in this context are the high system complexity and the relatively short lifetime of the satellites. Before it is possible to offer any service to customers the whole satellite system has to be set up. Operators install new terrestrial networks in densely populated areas first to get a quick return on investment. Most LEO satellites fly over non- or sparsely populated areas (sea, deserts, polar regions etc.) and can not generate any revenue. A new application for satellite systems is their use as an addition to terrestrial and can not generate any revenue.

 

A new application for satellite systems is their use as an addition to terrestrial mobile communication systems in the following way. Point-to-point communication services are handled by, e.g., UMTS, additional multicast or broadcast delivery of multimedia content is performed by a satellite system. In this scenario the role of a satellite is similar to terrestrial broadcasters.

 

Yet another market for new systems might appear between the low orbiting LEOs and terrestrial antennas. Several companies are planning to use high altitude aircraft or Zeppelins for carrying base stations, so-called high-altitude platforms these base stations could be placed high above large cities offering high-quality transmission at lower costs compared to satellite systems.

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Suggested Reading:

  1. Mobile Communication 2nd edition by Jochen Schiller, Pearson education
  2. Mobile Computing by Asoke Talukder, Roopa Yavagal (Tata McGraw Hill)
  3. “Wireless communication and networking” by William Stallings
  4. Mobile Cellular Telecommunications — W.C.Y. Lee, Mc Graw Hill
  5. Wireless Communications – Theodore. S. Rapport, Pearson Education
  6. Reza B’Far (Ed), “Mobile Computing Principles”, Cambridge University Press.