EDGE

EDGE is an improvement to the GPRS air interface that enables higher user bit-rates and greater system capacity by enhancing the physical layer. The higher bit-rates place extra demands on parts of the GPRS network.

The core GPRS nodes, SGSN and GGSN, are more or less independent of user bit-rates and no new hardware is required. In the radio network, base station transceivers need to be EDGE-capable, and base station and BSC/PCU software needs to be updated.

One fundamental characteristic of cellular systems is that different users experience different channel quality, as a result of differences in the distance to the serving base station, fading and interference. Radio network planning ensures that only a fraction of users ever experience low channel quality, but also means that many users have ‘excessive’ channel quality.
EDGE improves the situation by employing a new modulation method and link quality control. 8-PSK (as illustrated in Figure 9) is a high level linear modulation method that carries three times more information through an extended signal constellation. GMSK modulation, as defined in GSM/GPRS, is also part of EDGE.
Nine modulation and coding schemes are defined in EDGE. Link quality control dynamically selects the modulation and coding scheme for transmission of data over the air interface. The protection of the data is adapted to the channel quality to obtain optimal bit-rate. Standard GPRS bit-rate saturates at relatively low channel quality, whereas EDGE user bit-rates increase with better channel quality. Link quality control in EDGE uses both link adaptation and incremental redundancy where the initial coding is selected based on measurement of radio quality and additional redundanc y is sent if decoding fails.
EDGE Evolution makes substantial improvements in latency and perceived delay through reduced Transmission Time Interval (TTI) and additional protocol
enhancements. Radio blocks are currently transmitted over four consecutive bursts on one timeslot using a TTI of 20 ms. Reducing the TTI to 10 ms improves latency substantiall y, to below 100 ms. The four bursts are then transmitted on more than one timeslot (parallel timeslots on two carriers or dual timeslots on one carrier).

The most obvious improvement to peak bit-rates is through the introduction of dual carriers in the downlink, increasing the carrier bandwidth available above 200 kHz. EDGE terminals already use multiple timeslots for transmission and reception. Today’ s terminals receive on up to five timeslots. The introduction of dual carriers doubles the available bandwidth (to 400 kHz) as well as the practical peak bit-rate. Using dual carriers and five timeslots on each carrier provides bit-rates of almost 600 kbit/s, with no other changes to EDGE.

Higher average and peak bit-rates and improved spectrum efficiency are achieved through more advanced modulation, more efficient channel coding and an increased symbol rate (in practice, increasing the carrier bandwidth). Using 16QAM instead of 8-PSK modulation for some of the current Modulation and

Coding Schemes (MCS) improves robustness against interference and, as a result, increases the average bit-rates. In this case, the higher number of bits per symbol (see figure 9), is used to increase the channel coding. Using so called ‘turbo codes’, which handle error correction more efficiently than current convolutional codes, improves average bit rates even further. With twelve MCSs – three more than with regular EDGE – enabled by higher-order

modulations (16QAM and 32QAM in addition to GMSK and 8-PSK), the peak bit rate is boosted to 100 kbit/s per timeslot, equating to user bit-rates of 1 Mbit/s if dual carriers are used. The higher symbol rate enables higher bit-rates in the uplink, since dual carriers are only standardized for the downlink.

Dual-antenna terminals enable efficient interference rejection techniques, similar to those used in base station receivers. By combining signals from the two antennas, a large proportion of the interference can be cancelled out, significantly improving average bit-rates and spectrum efficiency. Dual antenna terminals can also improve service coverage. With two antennas and efficient combination methods, weaker signal transmissions can be captured. Around 3 dB less (roughly 50 per cent) signal power is needed to provide service, enabling larger cells or lower output power.

EDGE is delivering high bit-rates and spectrum efficiency for GSM operators around the world, and is becoming a standard capability of GSM and WCDMA phones. In effect, EDGE enables 3G applications over current GSM networks, and provides seamless services with 3G. Today, EDGE enables user bit-rates of 250 kbit/s and a latency of 150 ms. This means it can handle four times as much traffic as standard GPRS, increasing the usability of mobile data services, enhancing customer satisfaction and boosting data revenues.

EDGE Evolution, currently being standardized in 3GPP, will improve performance and coverage even further, with bit-rates of up to 1 Mbit/s and latency below 100 ms. For GSM operators around the world, this makes the business case for EDGE even stronger than it is today – whether or not they have access to 3G spectrum.