GPRS Radio Air Interface, GPRS Slot and Burst

- a summary or tutorial describing the basics of the GPRS radio or air interface describing the modulation, GPRS slot and burst structures.

In order to be able to carry the packet data and to increase the data rates that can be carried by the GPRS technology, it has been necessary to make some upgrades to the GPRS radio interface. In particular the GPRS slot or GPRS burst have been upgraded to accommodate the new scheme.

Despite the upgrades it was necessary to ensure that GPRS would operate alongside the existing GSM system, and this would require modifications to the air interface to accommodate both schemes.

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.

Note on GMSK:

GMSK, Gaussian Minimum Shift Keying is a form of phase modulation that is used in a number of portable radio and wireless applications. It has advantages in terms of spectral efficiency as well as having an almost constant amplitude which allows for the use of more efficient transmitter power amplifiers, thereby saving on current consumption, a critical issue for battery power equipment.

Click on the link for a GMSK tutorial

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.

GSM slots showing 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 burst stucture showing the various blocks of data transmitted including tail bits, data, training sequence, data, tail bits and guard period.
GPRS burst structure

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 used by GSM, enabling the two systems to operate together on the same carrier, bursts of GSM and GPRS occupying the same frame. In this way it is possible for a single base station to support both systems. As a result of the GPRS burst and GPRS slot fitting being compatible with operation on a GSM signal does place restrictions regarding what can be done, but it enables the systems to operate together which is one of the essential requirements.

By Ian Poole

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