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General Packet Radio Services GPRS and EDGE
General Packet Radio Services GPRS and EDGE. GPRS is a packet switched service in GSM. The smallest entity is called a radio block and it consists of four normal bursts. A radio block can be transmitted over the radio interface using any of the four coding schemes that are available. The coding schemes are CS-1 to CS-4. The lower coding schemes have a high amount of channel coding and give a low data rate. The higher coding schemes have less channel coding and give a higher data rate.
The most robust coding scheme (CS-1) is used for transmission of all radio blocks that carry RLC/MAC control messages. Radio blocks can also carry RLC data blocks, and in such case any of the four coding schemes can be used. The coding
schemes are summarized in table below:
The coding scheme that offers the best performance varies depending on the radio link quality. CS-1 offers the highest throughput when the radio link quality is poor while CS-4 offers the best performance in good radio conditions. The BLER reflects the performance of the coding scheme in relation to the radio link quality.
Enhanced Data Rates for GSM Evolution (EDGE)
Edge supports the GMSK and 8-PSK modulation methods on the radio interface and defines nine Modulation and Coding Schemes (MCSs). MCS-1 to MCS-4 are modulated with GMSK and MCS-5 to MCS-9 are modulated with 8-PSK.
The maximum data rates for the GMSK based MCSs are in general reached at a lower radio link quality than for the MCSs that are based on 8-PSK. 8-PSK is less robust than GMSK but gains from an improved radio link quality where the gain for GMSK is negligible.
For each modulation method the low MCSs have high amounts of channel coding, giving low data rates. The high MCSs have less channels coding which gives high data rates. In the end, it is the combination of modulation method and amount of channel coding that determines the characteristics of an MCS. The MCSs are summarized in table:
Coding and modulation scheme (MCS) Speed (kbit/s/slot) Modulation
MCS-1 8.80 GMSK
MCS-2 11.2 GMSK
MCS-3 14.8 GMSK
MCS-4 17.6 GMSK
MCS-5 22.4 8- PSK
MCS-6 29.6 8 PSK
MCS-7 44.8 8- PSK
MCS-8 54.4 8- PSK
MCS-9 59.2 8- PSK
EDGE uses nine modulation and Coding Schemes (MCS-1 to 9).
In EDGE the RLC protocol is enhanced with the possibility to resegment data within the same coding family. Hence, it is possible to retransmit a radio block with a different MCS. The enhanced RLC protocol also makes it possible for the receiver to store and use information (soft values) from previous transmissions of the same RLC data block in order to increase the probability of successful decoding. This is called Incremental Redundancy (IR). The old soft values can be combined with new soft values from the same RLC data block if the RLC data block has not been resegmented. The receiver will store the soft values until the RLC data block has been successfully decoded.
EDGE also supports the Bit Error Probability (BEP) measurement, which is an improved radio link quality measurement. The mean value and the coefficient of variation of BEP reflect not only the C/I, but also factors like time dispersion and interleaving gain caused by velocity and frequency hopping.