Understanding EDGE (EGPRS): Enhanced Data Rates for GSM Evolution

This page explains what EDGE is, and outlines the features of EDGE (Enhanced Data rates for GSM Evolution) or EGPRS (Enhanced GPRS). This technology was introduced to extend the capabilities of existing GSM/GPRS technologies.

EDGE stands for Enhanced Data rates for GSM Evolution, or Enhanced GPRS.

EDGE technology significantly increases the data rate supported by the GPRS system. EDGE introduced modulation schemes that carry more bits per symbol to increase the data rate, such as 8-PSK and 16QAM. With 8-PSK, for example, three bits are carried over one symbol. While GPRS allows data rates up to 115kbps, EDGE or EGPRS is capable of providing data rates of approximately 384 kbps.

EDGE supports various modulation-coding schemes (MCS) from MC1 to MC9. MCS1 to MCS4 use GMSK modulation, and MCS5 to MCS9 use 8PSK modulation. These schemes are divided among four families:

• Family A: MCS 3, 6, 9
• Family A’: MCS 3, 6, 8
• Family B: MCS 2, 5, 7
• Family C: MCS 1, 4

The following figure describes various MCS families with their respective payload units. From this, it’s clear that falling back from one MCS to another within the same family is easier based on channel conditions, due to the same size of payloads.

EDGE can retransmit a packet with a more robust coding scheme, meaning re-segmentation is possible. This improves throughput, which isn’t possible in GPRS. In EDGE, packets are addressed up to 2048 and the window size is 1024. In GPRS, packets were numbered from 1 to 128 and the addressing window size was 64.

Interleaving is changed in EDGE to increase performance. Higher coding schemes, such as MCS7 to MCS9, actually transmit 2 radio blocks over 4 bursts. Interleaving occurs over 2 bursts instead of 4. This reduces the number of bursts that need to be re-transmitted when errors occur.

To achieve the maximum possible throughput over the wireless link, EDGE/EGPRS uses a combination of link adaptation and incremental redundancy functionalities.

Evolved EDGE

In EDGE, it takes about 20 ms to transmit one radio block in 4 consecutive TDMA frames. This Transmission Time Interval (TTI) is reduced to half, i.e., 10 ms, in Evolved EDGE, also called Reduced Transmission Time Interval (RTTI).

Fast Ack/Nack reporting is done by inserting a fixed-size piggy-backed Ack/Nack field in the RLC radio block. Higher orders of modulation, such as 16-QAM and 32-QAM, are introduced to achieve even higher data rates.

Downlink Dual Carrier (DLDC), meaning different frequencies at the same time, is introduced in Evolved EDGE. DLDC means that 2 different carrier frequencies are transmitted/received by the same mobile terminal.

References

• 3GPP TS 44.060