Page 94 - From GMS to LTE

80 From GSM to LTE-Advanced Pro and 5G

Table 2.3 EDGE modulation and coding schemes (MCS).

Speed per Coding rate (user bits to Coding rate with
Modulation timeslot (kbit/s) error correction bits) one retransmission

MCS‐1 GMSK 8.8 0.53 0.26
MCS‐2 GMSK 11.2 0.66 0.33
MCS‐3 GMSK 14.8 0.85 0.42
MCS‐4 GMSK 17.6 1.00 0.50
MCS‐5 8‐PSK 22.4 0.37 0.19
MCS‐6 8‐PSK 29.6 0.49 0.24
MCS‐7 8‐PSK 44.8 0.76 0.38
MCS‐8 8‐PSK 54.4 0.92 0.46
MCS‐9 8‐PSK 59.2 1.00 0.50

with the GPRS coding schemes, some error detection and correction bits produced by
the convolutional decoder are punctured and therefore not put into the final block
that is sent over the air interface. With the incremental redundancy scheme it is
possible to send the previously punctured bits in a second or even a third attempt.
On the receiver side, the original block is stored and the additional redundancy
information received in the first and second retry is added to the information. Usually
only a single retry is necessary to allow reconstruction of the original data based on
the additional information received. Figure 2.8 shows how MCS‐9 uses a 1/3 convo-
lutional decoder to generate three output bits for a single input bit. For the final
transmission, however, only one of those three bits is sent. In case the block was not
received correctly, the sender will use the second bits that were generated by the
convolutional decoder for each input bit to form the retry block. In the unlikely
event that it is still not possible for the receiver to correctly decode the data, the
sender will send another block containing the third bit. This further increases the
probability that the receiver can decode the data correctly by combining the infor-
mation that is contained in the original block with the redundancy information in
the two additional retransmissions.
Another way of retransmitting faulty blocks is to split them up into two blocks for a
retransmission that uses a different MCS. This method is called re‐segmentation. As
can be seen in Table 2.4, the standard defines three code families. If, for example, a
block coded with MCS‐9 has to be retransmitted, the system can decide to send the
content of this block embedded in two blocks, which are then coded using MCS‐6. As
MCS‐6 is more robust than MCS‐9, it is much more likely that the content can be
decoded correctly. In practice, it can be observed that the incremental redundancy
scheme is preferred over re‐segmentation.
The interleaving algorithm, which reorders the bits before they are sent over the air
interface to disperse consecutive bit errors, has been changed for EDGE as well. GSM
voice packets and GPRS data blocks are always interleaved over four bursts as described
in Section 1.7.3. As EDGE notably increases the number of bits that can be sent in a