GSM radio frequency spectrum In the frequency range specified for GSM-900 System mobile radio networks, 124 frequency channels with a bandwidth of 200 KHz are available for both the uplink and downlink direction. The uplink (mobile station to BTS) uses the frequencies between 890 MHz and 915 MHz and the downlink (BTS to mobile station) uses the frequencies between 935 MHz and 960 MHz. The duplex spacing, the spacing between the uplink and downlink channel, is 45 MHz. The E-GSM band adds 50 frequency channels and the R-GSM another 20 frequency channels to the spectrum.
Multiple access techniques In cellular and cordless terminology the three main types of multiple access used to divide the radio frequency spectrum spectru m between the cell site radios and the mobile stations are: Frequency Division Multiple Access (FDMA): Frequency Division Multiple Access (FDMA): A transmission technique where the assigned frequency band for a network is divided into sub-bands which are all ocated to a subscriber for the duration of their calls. Each call is carried on a separate frequency channel.
Time Division Multiple Access (TDMA): Each frequency channel is further divided into a set of timeslots; each timeslot carries the data of a voice call. ca ll. Code Division Division Multiple Mu ltiple Access (CDMA): A spread-spectrum technology is used, in which t he radio signals associated with a call are spread across a single broad frequency spectrum (1.25 MHz). Each ca ll in the spectrum is differentiated from other calls in that spectrum by assigning a unique code to each call's signal. At the receiving rece iving end (mobile station or cell site), the specific call's signal is isolated by decoding the full received signal using the code cod e assigned to that call's signal.
Multiplexing techniques The two multiplexing techniques used in cellular and cordless terminology are: Frequency Division Duplexing (FDD) In FDD two symmetric frequency bands are used, o ne containing the uplink channels and the other the downlink channels. Time Division Duplexing (TDD) TDD means that the uplink of the voice call is time multiplexed on the same frequency channel as the downlink of the voice call.
Techniques used in GSM In the GSM system, TDMA in combination with FDMA is used; t he usage of each radio channel is partitioned into multiple (eight) timeslots, and each user is assigned a specific frequency/ timeslot combination. Thus, only a single mobile is using a given frequency/timeslot combination at any particular time. Also the FDD technique is in use, t hat is two symmetric frequency band, one ba nd containing the uplink channels and the other the downlink channels. With the FDMA (Frequency Division Multiple Access) multiple access method the total transmission bandwidth B available in the DCM is divided into NF contiguous subscriber frequency bands (also referred to as frequency channels) of width Bu, as shown in Figure 1 and Figure2. Width Bu is the subscriber bandwidth. All the subscribers are active at the same time and without interruption in time, and each of the subscribers has a single subscriber frequency band. Mobile radio systems which use FDMA exclusively cannot be u sed for the third generation of mobile radio. Combination with at least one of the multiple access methods TDMA and CDMA described below is essential.
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Figure 1: Multiple access method FDMA
Figure 2: Multiple access method FDMA
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Time division multiple access (TDMA) is a channel access method for shared medium (usually radio) networks. It allows several users to share the same frequency channel by dividing the signal into different timeslots. The users transmit in rapid succession, one after the other, each using his own t imeslot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only the part of its bandwidth they require.
