Forcing Mobile Satellites to Provide Data Service

Over the past decade the digital revolution has transformed the way we think about communications services. Telecommunications companies have converted from analog to digital transmission. Data services are supplanting voice services. Always connected, on-demand services are replacing dial-up, dedicated circuits. Users are substituting e-mail messages for telephone calls and fax transmissions.

The current generation of mobile communications satellites was designed with an emphasis on circuit switched voice services. Data communications were not a high priority on Iridium and Globalstar. Everything was optimized for hand-held mobile voice. Designers concluded that voice capacity should be expanded by a factor of 100. Elaborate techniques were developed to squeeze more voice circuits through a satellite. Sophisticated speech compression processors were developed to reduce the bandwidth required for voice communications. These clever devices have reduced the data rate required for voice from 64 kbps to 2.4 kbps or less. These vocoders are also tolerant of bit errors. The voice quality is not significantly degraded if one bit in 100 is in error, a bit error rate (BER) of 10-2.

The advent of high-speed data communications is a challenge for mobile satellites. Data services require more bits per customer but bandwidth is a precious commodity. The throughput of a typical mobile communications satellite is a small fraction of the throughput of fixed and broadband satellites:

Data services not only require several times more bandwidth for speed, they require very low bit error rates. If the BER were 10-2 nearly every word would contain an incorrect letter. For data communications the BER must be at least 10-6 or better. This further increases the need for system resources by a factor of two or three. Globalstar expects to offer data services at only 9.6 kbps by the end of 2000. Nearly everyone expects much higher transmission rates today. However, a 9.6 kbps data rate connection requires the equivalent capacity of about 10 voice signals and it is unrealistic to expect customers to pay $15 per minute for data services.

Planning for the next generation of mobile communications (3G, UMTS, IMT2000, etc.) requires transmission from 384 kpbs up to 2 mbps. Few high data rate users could be supported on the current generation of mobile satellites. Inmarsat already offers 64 kbps service by using M-4 terminals that cost about $9000 and airtime rates of more than $6 per minute. In the past few months, ICO Global has redesigned its satellites and ground segment to carry much higher data rate services starting in 2002. Inmarsat has announced that its next generation Inmarsat 4 satellites will transmit at relatively high data rates by 2004.

In 1990, many thought that mobile services were a natural wireless application for satellites. Over time it has become obvious that mobile satellite services must struggle to carve out a small niche in the booming wireless industry. But it is also clear that there is a need for ubiquitous communications services that can only be met by satellites. With persistence and determination we can expect mobile satellites to expand their role by delivering universal communications service.