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 descr iption 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 carr ier 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 carr ier. 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.
