GSM is the digital cellular telephone system which is widely used throughout Europe and other parts of the world. Its deployment gathered pace around the world, and this growth continues. It has been far successful than ever expected. More than 80% of all mobile users in the world are served by it, and every month approximately 40 million new users join this community and it was the mandated standard. As a mobile technology, it has unmatched coverage, economies of scale, simplicity and maturity. Steeply falling tariffs like ‘pay per sec’ made a revolutionary change in mobile community.
The evolution of mobile communication networks has started from Advanced Mobile Phone Service (AMPS)-1G to Global System for Mobile Communications (GSM)-2G to General Packet Radio Service (GPRS)-2.5G and to Third Generation(3G) and finally to Fourth Generation(4G). Some network operators may also choose to implement High Speed Circuit Switched Data (HSCSD) or Enhanced Data Rate (EDGE) for GSM in this process.
In the beginning, the analog systems named High Capacity Mobile Telephone System (HCMTS) were developed at Bell Labs around 1960’s which used FM modulation with a band width of 30 kHz. The signalling rate was 10kbps. Since there was no standard organisation for mobile communication system, the standard EIA named it as IS-3, and then it merged with TIA and named the system as AMPS was used since 1976.
In 1983, Europe started developing GSM (the original name was Group of Special Mobile was then changed to Global System for Mobile) and was first deployed in Germany in 1991.It was the first digital system with better bandwidth and data rate than that of the analog. It uses the 890-915 MHz band for uplink signals (signals from cell user to cell site) and 935-960 MHz band for downlink signals (from cell site to user). In newer GSM systems, referred to as DCS-1800, range is 1.71-1.785GHz and 1.805-1.88GHz for uplink and downlink respectively, spacing between them is 45MHz on the lower and 95 MHz on the upper frequencies. Each channel has a band-width of 25KHz. In order to achieve full duplex operation, these signals occur simultaneously in their own 25 KHz channel. The carrier spacing of the channel is 200 KHz. There are 124 channels in Lower Frequency Band and 374 channels in Upper Frequency Band. It uses Time Division Multiplexing to allow 8 simultaneous telephone calls to use each channel. Voice signals are digitized by A/D convertor and converted to serial format (basic bit rate: 13kbps). The serial voice data is time multiplexed into 8 channels. Each voice channel contains one digital sample of the audio signal where unused channels are blank. The type of modulation used is Gaussian Minimum Shift Keying (GMSK), a form of FSK in which two frequencies selected to represent binary 0 and 1 are related in such a way that their zero crossing times are same. Switching between frequencies takes place at the zero crossing points, this reduces the number of harmonics and hence the bandwidth is narrower than a standard FSK signal.
It had a data transmission enhancement called GPRS which is capable of transmitting the digital signals and the data. The data rate was 14-64kbps. There is another high-speed data enhancement called EDGE with a transmission data rate up to 500 kbps.
The next step towards 3G for GSM networks increased data rate up to 348kbps by bundling up to 8 channels of 48 kbps/channel. It is capable of transmitting multimedia signals, which enables streaming of a video or a movie.
Despite of this development BSNL has started working on 4G which provides wireless broadband. In 2003, ITU declared the requirements for a 4G network as, at a standstill condition, the transmission rate should be 1 Gbps and at a moving condition, the rate should be 100Mbps.
While the 3G is being introduced in India, it could take some more years for us to welcome another generation of mobile evolution. Although there is still room for some growth in most areas, the systems are reaching their limits mainly due to the lack of available spectrum space for expansion.
GSM has been the prime growth driver of Telecom services and would continue to be the same for new evolving technologies and will contribute to the growth and prosperity of the industry.
Thank You,
P. Siva Prasad
III B. Tech, ECE
sivaprasad_pusarla@ece.sastra.edu
Ph.: 9262401505
