Telecom ABC - E
Enhanced Data rates for Global Evolution (EDGE) is a technology that can be used in existing GSM and D-AMPS (IS-136) networks to increase the data capacity. EDGE is an official member of the IMT-2000 family. however, in Europe, EDGE is seen as an intermediate technology in the evolution from GSM to UMTS and systems beyond.
EDGE can be used as an enhancement of the General Packet Radio Service (GPRS). This enhancement is known as Enhanced GPRS (EGPRS). EDGE can also be used as an enhancement of High Speed Circuit Switched Data (HSCSD). This enhancement is known as Enhanced Circuit-Switched Data (ECSD).
EDGE increases the capacity of the radio link within the excisting 200 kHz GSM radio channels. EDGE employs three techniques to increase the capacity on the radio link. The first technique is the introduction of a new modulation scheme, Octogonal Phase Shift Keying (8PSK). An 8PSK signal is able to carry three bits per symbol while the standard GSM Gaussian Minimum Shift Keying (GMSK) carries only 1 bit per symbol. This means that the data rate per timeslot will triple.
A comparison of GSM GMSK and 8-PSK is given in the table below.
The bit rate in the table is the gross bit rate. Part of the bits are used for error correction. In a standard GSM channel only 9.6 kbps (circuit switched) or 9.05 kbps (packet switched) is available to the user for data.
The second technique is the employment of different error correction coding schemes. The amount of error correction can be varied based on the quality of the radio link. In EGPRS there are nine different coding schemes defined; five coding schemes for 8-PSK and four coding schemes for GMSK. These coding schemes are given in the table below.
If all 8 time slots are used, the maximum data rate will reach 473.6 kbps.
In ECSD there are only five different data rates defined. On top of the excisting data rates of 9.6 and 14.4 kbps, there are 3 new data rates introduced. These are given in the table below.
The third technique is called link adaptation. Link adaptation means that the radio link quality, is measured either by the mobile station in a downlink transfer or by the base station in an uplink transfer, to select the most appropriate modulation coding scheme for transmission of the next sequence of packets. This increases the likelihood of a succesful transmission.
To introduce EDGE in an excisting GSM network, the base stations (and the mobile stations) have to be upgraded. The increase in data throughput does not come for free, the price being paid is the increased vulnerability of the 8-PSK signal. This affects, e.g. the radio network planning, and the highest data rates can only be provided with limited coverage.
EDGE is also designed as an enhancement in D-AMPS networks. However, D-AMPS uses 30 kHz radio channels comapred to 200 kHZ in GSM and EDGE. To ease the introduction of EDGE in an D-AMPS network a special version of EDGE is made: EDGE Compact. EDGE Compact is a packet-switched data-only overlay using EDGE that can be introduced in a D-AMPS network. To do so a small portion of the original D-AMPS frequency spectrum has to be released for the EDGE data network.
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