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Global System for Mobile Communication (GSM)
PDF of this tutorial5. GSM Specifications
Before looking at the GSM specifications, it is important to understand the following basic terms:
- bandwidth—the range of a channel's limits; the broader the bandwidth, the faster data can be sent
- bits per second (bps)—a single on-off pulse of data; eight bits are equivalent to one byte
- frequency—the number of cycles per unit of time; frequency is measured in hertz (Hz)
- kilo (k)—kilo is the designation for 1,000; the abbreviation kbps represents 1,000 bits per second
- megahertz (MHz)—1,000,000 hertz (cycles per second)
- milliseconds (ms)—one-thousandth of a second
- watt (W)—a measure of power of a transmitter
Specifications for different personal communication services (PCS) systems vary among the different PCS networks. Listed below is a description of the specifications and characteristics for GSM.
- frequency band—The frequency range specified for GSM is 1,850 to 1,990 MHz (mobile station to base station).
- duplex distance—The duplex distance is 80 MHz. Duplex distance is the distance between the uplink and downlink frequencies. A channel has two frequencies, 80 MHz apart.
- channel separation—The separation between adjacent carrier frequencies. In GSM, this is 200 kHz.
- modulation—Modulation is the process of sending a signal by changing the characteristics of a carrier frequency. This is done in GSM via Gaussian minimum shift keying (GMSK).
- transmission rate—GSM is a digital system with an over-the-air bit rate of 270 kbps.
- access method—GSM utilizes the time division multiple access (TDMA) concept. TDMA is a technique in which several different calls may share the same carrier. Each call is assigned a particular time slot.
- speech coder—GSM uses linear predictive coding (LPC). The purpose of LPC is to reduce the bit rate. The LPC provides parameters for a filter that mimics the vocal tract. The signal passes through this filter, leaving behind a residual signal. Speech is encoded at 13 kbps.
