GSM PHYSICAL CHANNEL

GSM Bandwidth Allocation

GSM uses Frequency Division Duplexing(FDD) where the uplink and downlink of each channel operates on a different frequency . Therefore , two frequency band are allocated to GSM, separated by 20 MHz.

                 The following frequency band were initially allocated to GSM.

•    Uplink sub-band 890 MHz to 915 MHz
•    Downlink sub-band 935 MHz to 960 MHz

GSM FDMA Structure

Each band is divided into number of frequency channels, each channel carries having a 200 KHz bandwidth. Therefore 124 carriers have available within each of the up and down link bands, allowing for 2*100 khz guard bands. 

                                                    The up and downlink frequency channel pair allocation has been arranged such that the two frequencies comprising a channel pair are 45 MHz apart. Each of these frequency pairs are identified by an "Absolute Radio Frequency Carrier Number"(ARFCN) in the range 1-124 for GSM.   Up link and Downlink channel frequencies can be calculated as follows:

                   Up link frequencies : Fu(n)= 890+0.2n (1<=n<=124)

                   Downlink frequencies : Fd(n)=Fu(n)+45

GSM TDMA Structure

Each GSM carrier channel is subdivided by time into 8 timeslots. Timeslots are repeated in frames, each frame comprising 8 timeslots. The duration of single timeslot is 0.577ms. Therefore each TDMA frames repeat every 4.615ms. These timeslot are known as 'Physical channels'.     

Within each timeslot, a radio 'burst' is transmitted. This burst is comprises 8*156.25=1250 bit periods.If a frame is transmitted every 4.615ms , the row data rate over the carrier channel is (1\4.615ms)*1250=270kbps. The corresponding physical raw data rate is 270/8=33.75kbps.

Logical channels

The reason logical channels are used in because GSM has a number of control channels which do not require to operate at the full rate of a physical channel. Most of these control channels can therefore multiplexed onto single physical channel known as the 'control channel'. In a single carrier cell, this is always allocated to the physical channel 'timeslot 0'.

GSM Logical channel

• There  are two types of logical channel are defined: Traffic channel and Control channel.
• Each is further sub-divided as follows:

All the control and traffic channels are described as follow link GSM CHANNEL