31.6 MOBILE SATELLITE SYSTEMS

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To provide voice communication, the PSTN and the PLMN have been widely deployed all over the world. However, their reach is mainly limited to cities and towns in developed countries. In developing countries, particularly in Asia and Africa, remote and rural areas remain isolated with no communication facilities.

To lay cable to villages, where the subscriber density will be low and also the traffic is low will, not be a good business proposition because the return on investment will be very low. Satellite communications, its distance insensitivity, provide a great scope to provide communications anywhere on the earth. A number of projects have been initiated to provide voice communications using a satellite as the medium. Geostationary satellites-low Earth orbiting (LEO) satellites and medium Earth orbiting (MEO) satellites-can be used for providing satellite-based telephony services, also known as mobile satellite services (MSS). An overview of the various projects taken up for MSS is given in this section.

A number of mobile communication systems have been developed in which low Earth orbiting (LEO) and medium Earth orbiting (MEO) satellites are used. However, a few such projects did not become a commercial success.

31.6.1 Iridium

A very ambitious program launched by Motorola, the leading manufacturer of mobile equipment, is Iridium. Iridium's plan was to establish a global mobile personal communication system (GMPCS) through a constellation of 66 low Earth orbiting (LEO) satellites to provide telephony services anywhere on the Earth. The operations of Iridium began in January 1999, but the project ran into troubles-technical snags and lack of enough customers because of high prices. Initially the Iridium's phone was priced at about US$3000, with connection charges of US$7 per minute. Subsequently, prices were slashed, and the subscriber base increased, but Iridium could never get over the financial debts and by mid-2000, Iridium became bankrupt. "Geography is now History" was the slogan of Iridium, but Iridium became history. All financial institutions now look at MSS with skepticism. However, MSS still provides hope, but we need to learn many lessons from Iridium.

The Iridium system was conceived and developed by Motorola. It used 66 satellites to provide global mobile communication. Though it was a great technological marvel, it was a big commercial failure.

The salient features of Iridium are:

Altitude: 780 km
Visibility of the satellite: 11.1 minutes
Access method: FDMA/TDMA
Frequency of operation: 1.6138-1.6265 GHz
Intersatellite link frequency of operation: 23.18-23.38 GHz
Number of channels: 4,000
Services supported: voice, data, fax, and location services

31.6.2 Globalstar

Globalstar was officially launched in October 1999. Globalstar consists of a total of 48 satellites orbiting at an altitude of 1389 km. With an orbit inclination of 52^o and eight orbital planes, there are six satellites per orbital plane. The frequency bands of operation are 1610-1626.5 MHz and 2483.5-2500 MHz. The multiple access technique used is CDMA. The handsets available on the market are dual-band handsets that can be used with both the land mobile cellular system as well as the satellite-based system.

Globalstar is a mobile satellite system that uses CDMA as the access technique. It provides global connectivity. However, it is yet to become a commercial success.

Though the present subscriber base is very low and the first year operations resulted in losses for Globalstar, there is still hope for Globalstar to succeed. The salient features of Globalstar are:

Number of satellites: 48
Altitude: 1,389 km
Access method: CDMA
Number of channels: 2,700
Frequency of operation: 1.61-1.6265 GHz
2.4835-2.5 GHz

31.6.3 ICO/Teledesic

ICO Global Communications Holding Ltd. was established in 1995 to develop satellite-based mobile telephony services. ICO stands for intermediate circular orbit to reflect the location of the satellites-the orbit is also known as medium Earth orbit (MEO).

ICO/Teledesic is called "Internet in the sky" because it aims at providing Internet connectivity with speeds ranging from 19.2kbps to 2.048Mbps.

ICO went almost bankrupt in 1999 but reemerged in June 2000, with support from financial institutions and Craig McCaw, a pioneer in mobile communications. The name has been changed to New ICO to reflect a new business plan and new architecture so that not just voice but wireless Internet services can be provided through the system.

There is now a proposal to merge New ICO and Teledesic Corporation to form ICO-Teledesic Global. If the merger is approved, New ICO and Teledesic will be subsidiaries of ICO-Teledesic Global. New ICO plans to offer its services, a satellite equivalent of 3G services, worldwide in 2004.

The salient features of Teledesic are:

Number of satellites: 288
Altitude: 700 km
Access method: FDMA/TDMA
Frequency of operation: 28.6-29.1 GHz 18.8-19.3 GHz
Bit rates: 64Mbps downlink; 19.2kbps-2.048Mbps uplink

Note

The mobile satellite systems' objective is to provide telephone connectivity to remote and rural areas. Most of the projects did not become a commercial success because in many rural areas, two square meals are more important than communication.

Technically, mobile satellite services are the best bet for providing rural telecommunications. However, economics plays the most important role-in a village where the average daily income of a family is less than $2, if the communication cost is $2 per minute, it is not an attractive proposition. The need to fill the stomach takes priority, not communicating!

Summary

Cellular mobile communications systems provide voice communication services while on the move. In this chapter, the details of the digital cellular system based on GSM standards are presented. The analog mobile communication systems were replaced by the digital systems in the 1990s. Among the digital systems, the system based on GSM standards attained popularity not only in Europe, but in Asia and Africa as well. The various network elements of the GSM system are the mobile station carried by the subscribers, the base transceiver subsystem (BTS) that consists of the radio modules, the base station controller (BSC), which controls multiple base transceivers, and the mobile switching center (MSC), which carries out the switching operations. The MSC contains a number of databases. The HLR is a database that stores the subscriber information. VLR is a database that contains the current users in the service area. The EIR database contains the mobile equipment identity. AuC is the database that contains information to authenticate the user. The MSC is connected to the PSTN. In addition to voice communication, GSM supports short messaging service, with which short messages limited to 160 bytes can be sent using a store-and-forward mechanism. GSM systems are now being upgraded to provide high data rate services to access the Internet.

The salient features of mobile satellite communication systems are also presented in this chapter. Iridium, ICO Globalstar, and Teledesic are the major projects for providing global mobile communications. However, their success is yet to be ascertained.

References

Dreamtech Software Team. WAP, Bluetooth and 3G Programming. Wiley Dreamtech India Pvt. Ltd. This book contains an overview of the GSM system and also third generation wireless networks.
http://www.3gpp.org The official site of the 3G Partnership Programme. You can obtain the GSM standards documents from this site.
http://www.cdg.org Web site of CDMA development group. The competition to GSM is CDMA-based systems, which are extensively deployed in the U.S. You can get the details from this site.
http://www.qualcomm.com Qualcomm Corporation is a pioneer in CDMA technology. Qualcomm is the leading supplier of CDMA-based cellular mobile communication systems. You can get the product details from this site.

Questions

Explain the advantages of multicell systems for mobile communications as compared to single-cell systems.
Describe the architecture of a GSM-based mobile communication system. Explain the functionality of each subsystem.
What are the additional enhancements for the original GSM standards? What additional services can be provided through these enhancements?
Explain the operation of mobile satellite systems. List the salient features of Iridium, Globalstar, and Teledesic.

Exercises

1.	In the GSM system, speech is coded at 13Kbps data rate. Obtain freely available source code for coding the speech at this data rate and experiment with it.
2.	Study the details of the TDMA time slots of the GSM system.
3.	Write a technical paper on the mobile communication systems based on CDMA used in North America and China.
4.	Explain how a stolen mobile phone can be traced.

Answers

1.	You can obtain the GSM codec software from the Web site kbs.cs.tu-berlin.de/~jutta/toast.html.
2.	You can obtain the details of the GSM standard at http://www.3gpp.org.
3.	You can obtain the details form http://www.qualcomm.com.
4.	When a mobile phone is lost, the first thing to do is to call that number and check whether it is ringing. If the thief is using the mobile phone without removing the SIM card, the operator can trace approximately, but not precisely, where the thief is located. However, the operator can disable the phone so that both incoming and outgoing calls be barred. The GSM standard provides another feature using which lost mobile phones cannot be used. If the IMEI is registered with the operator, then the thief cannot use the mobile phone.

Projects

Develop a database of about 100 mobile subscribers using any RDBMS package (such as Oracle or MS SQL). The database should contain the name and address of the subscriber, the mobile number, and the details of the calls made by them. The details of the calls include the called number, time and date of initiating the call, and the duration. Using this database, develop a billing system. The billing system should have provision for normal rates, half the normal rates during night hours, and one-fourth the normal rates during Sundays and public holidays. Note that the database generated by you manually is automatically generated by the MSC. It is called the call details record (CDR). The CDR is processed offline to generate the bills. Your job is to develop the billing system. Make appropriate assumptions while developing the software.
Carry out the system design for providing a mobile service for your city. Assume that 10% of the population will use mobile phones. Based on the mobile subscribers density in different areas of the city, you can divide the city into different cells. Use the city map to decide the cells and location of BTS. If there are hills or lakes in the city, you need to consider the terrain-related issues while planning the number of cells.

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