Radio Interface

 Radio Interface:

The GSM networks use two 25 MHz bandwidths globally in bands.

The 890-915 MHz band is set for the subscriber to base transmissions (reverse link).

The 935-960 MHz band is set for a base to subscriber transmissions (forward link).

In order to provide multiple access to the mobile users, the GSM uses FDD and a combination of TDMA and FHMA methods.

The 25 MHz bandwidth forward and reverse frequency bands are further divided into 125 carrier frequency channels (assuming no guard band) each 200 KHz called ARFCNs (Absolute Radio Frequency Channel Numbers).

The ARFCN indicates a pair of channels (forward and reverse) that are separated by 45 MHz. Each channel is time shared between 8 subscribers in the TDMA scheme with a frame duration of 4.615 ms.

The radio transmissions on the forward and reverse link are done at a rate of 270.833 Kbps using binary BT=0.3 GMSK modulation. Hence, the signaling bit period is 3.692 µs and the effective channel transmission rate per user is 33.854 kbps. The user data is sent at a maximum rate of 24.7 kbps because of the overload on the GSM system.

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Each Time slot has a time allocation of 156.25 channel bits of which 8.25 are guard bits, six starts, and stop bits to prevent overlap with adjacent time slots.

The below image shows a Time slot. Each Time Slot (TS) has a duration of 576.92 µs.

As each radio channel comprises 8-time slots, there are 1000 traffic channels within GSM. Practically a guard band of 100 KHz is provided at the upper and lower ends of the spectrum. Only 124 channels are implemented.

Radio Interface

The combination of the TS number and ARFCN constitutes a physical channel for the forward and reverse link. Each of the physical channels can be mapped to different logical channels at different instants of time.

The GSM specifications define various logical channels that can be used to link the physical layer with the data link layer of the GSM network. The logical channels transmit data and control the network on ARFCN. The GSM air interface specifications are given in Table.

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 GSM Air Interface / Radio Interface Specifications: