GPRS Radio Interface: GPRS Slot & Burst

The GPRS radio interface has to accommodate GSM voice as well as packet data and this requires updates including to the slot & burst.

GPRS General Packet Radio Service Tutorial Includes:
GPRS basics Network architecture Mobile / multislot classes Radio interface Coding Channels: physical, logical, control GPRS protocol stack Operational states

The GPRS radio interface has to accommodate GSM voice as well as packet data and this requires updates including to the slot & burst.

One requirement for GPRS was that it would be able to operate alongside the GSM system with mobiles for both types being able to access the radio access network.

GPRS modulation scheme

GPRS builds on the basic GSM structure. It uses the same signal format having 200 kHz channel bandwidths. It also has the same modulation scheme and using GMSK modulation. Retaining the same modulation scheme means that the level of upgrade required to be able to support GPRS in addition to GSM is minimised.

GMSK modulation was chosen for GSM originally because it offered a number of advantages including good spectral efficiency, resilience to interference, low levels of interference outside the wanted bandwidth, and the ability to use a non-linear RF power amplifier. This last point is of great importance because the use of a non-linear power amplifier brings greater levels of efficiency and this results in longer battery life - an important factor for mobile phones.

GPRS frame and slot structure

Again the GRPS air interface employs the same basic structure as that adopted for GSM. The overall slot structure for this channel is the same as that used within GSM, having the same power profile, and timing advance attributes to overcome the different signal travel times to the base station dependent upon the distance the mobile is from the base station. This enables the burst to fit in seamlessly with the existing GSM structure.

Diagram of the GSM slot arrangement showing the offset between transmit and receive — GSM slots
*Note: offset between transmit and receive*

GPRS burst structure

Each GPRS burst of information is 0.577 mS in length and is the same as that used in GSM. The GPRS burst carries two blocks of 57 bits of information in line with a GSM burst, giving a total of 114 bits per burst. It therefore requires four GPRS bursts to carry each 20 mS block of data, i.e. 456 bits of encoded data. Slots can be assigned dynamically by the BSC to GPRS dependent upon the demand, the remaining ones being used for GSM traffic.

GPRS Normal Burst showing the training, information and overhead bits — GPRS Normal Burst

Where:
T = tail bit
F = coding flag

The BSC assigns PDCHs to particular time slots, and there will be times when the PDCH is inactive, allowing the mobile to check for other base stations and monitor their signal strengths to enable the network to judge when handover is required. The GPRS slot may also be used by the base station to judge the time delay using a logical channel known as the Packet Timing Advance Control Channel (PTCCT).

The GPRS radio interface is very similar to that of GSM and this enables both GSM and GPRS to operate via the same radio access network. They can operate together on the same carrier, bursts of GSM and GPRS occupying the same frame.

This enabled GPRS to be an evolution of GSM and base stations to steadily have GPRS incorporated into them.