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TABLE 1 MODULATION AND CODING SCHEMES FOR EGPRSSchemeMCS-9MCS-8MCS-7MCS-6MCS-5MCS-4MCS-3MCS-2MCS-1Modulation8PSK8PSK8PSK8PSK8PSKGMSKGMSKGMSKGMSKMaximumrate [kbit/s]59.254.444.829.622.417.614.811.28.8Code rate1.00.920.760.490.371.00.800.660.53Headercode rate0.360.360.361/31/30.530.530.530.53FamilyAABABCABCwhich means that it can request the retransmissionof erroneously received blocks.This mechanism is called automatic repeatrequest (ARQ). EGPRS uses an enhancedvariant of ARQ called incremental redundancy(IR). With IR, all information iscoded with a convolution code at a rate of1/3. The code is punctured to a certain rateand transmitted over the air. If the decodingfails, a retransmission is formed using adifferent puncturing scheme. Because theretransmission is combined with the previouslytransmitted block, the process yieldsa lower bit rate, which facilitates decoding.At present, nine coding schemes have beendefined for EGPRS, see Table 1.The "Family" column in Table 1 indicatesthe coding schemes that can be used for retransmission.For example, if the initialtransmission with MCS-9 fails and the qualityof the radio channel diminishes, the retransmissioncan use more robust codingschemes from the same family.Had the retransmission used a differentcoding scheme than the original transmission,then it would have to be resegmentedinto new radio link control (RLC) blocks.This is what limits the selection of codingschemes. Blocks that are initially transmittedwith MCS-8 can be retransmitted usingMCS-6 or MCS-3, by adding padding bitsto the data field.It should also be noted that even whenthere is nothing with which to combine aretransmission—that is, if the initial transmissionis lost altogether—it is nonethelesspossible to decode a retransmission. Thereis flexibility in the implementation of LQCfor EGPRS in the system: LA will be mandatory,while IR is optional.GPRS in TDMA systems—system architectureThe packet core network for COMPACTand Classic, which is based on the core networkarchitecture for GPRS networks, is integratedwith TIA/EIA-136 circuitswitchednetworks via a serving GPRS supportnode (SGSN). Figure 1 shows the referencemodel for the resulting network.Figure 1Reference model for an EGPRS-136 networkintegrated into a TIA/EIA-136 circuitswitchednetwork through an SGSN gatewayMSC/VLR.70 Ericsson Review No. 2, 2000