Gsm tdm-frame structure
GSM implements SDMA using cells with BTS and assigns an MS to a BTS. Furthermore, FDD is used to separate downlink and uplink. Media access combines TDMA and FDMA. In GSM 900, 124 channels, each 200 kHz wide, are used for FDMA, whereas GSM 1800 uses, 374 channels. Typically, 32 channels are reserved for organizational data; the remaining 90 are used for customers. Channels 1 and 124 are not used for transmission in GSM 900.
GSM TDMA frame, slots, and bursts:
Figure 8.4 GSM TDM Frame structure
Figure8.4 shows Each of the 248 channels is separated in time via a GSM TDMA frame, i.e., each
200 kHz carrier is subdivided into frames that are repeated continuously.
- The duration of a frame is 4.615 ms.
- A Frame is again subdivided into 8 GSM time slots, where each slot represents a physical TDM channel and lasts for 577 μs.
- Each TDM channel occupies the 200 kHz carrier for 577 μs every 4.615 ms.
- Data is transmitted in small portions, called bursts.
- The burst is only 546.5 μs long and contains 148 bits. The remaining 30.5 μs are used as guard space to avoid overlapping
Each physical TDM channel has a raw data rate of about 33.8 Kbit/s, each radio carrier transmits approximately 270 Kbit/s over the Um interface.
- The first and last three bits of a normal burst (tail) are all set to 0.
- The training sequence in the middle of a slot is used to adapt the parameters of the receiver to the current path propagation characteristics and to select the strongest signal in case of multi-path propagation.
- S indicates whether the data field contains user or network control data.
Types of burst:
- Frequency correction: Allows the MS to correct the local oscillator to avoid interference with neighboring channels.
- Synchronization burst: has an extended training sequence synchronizes the MS with the BTS in time.
- Access burst: Used for the initial connection setup between MS and BTS.
- Dummy burst: used if no data is available for a slot