Air Interface Coding Schemes and Channel Types

 With the
advent of EDGE, we find a number of new channel coding schemes in addition
to the coding schemes that exist for GSM voice and GPRS. For packet
data services in an EDGE network, we refer to Enhanced GPRS (EGPRS)
and the new coding schemes for EGPRS are termed Modulation and Coding
Scheme-1 to Modulation and Coding Scheme-9 (MCS-1 to MCS-9). The
reason why they are not just called coding schemes is the fact that for
MCS-1 to MCS-4, GMSK modulation is used, whereas 8-PSK modulation
is used for MCS-5 to MCS-9.
Table 5-4 shows the modulation scheme and data rate applicable to each
MCS.

It should be noted that MCS-4 offers no error protection for the user
data, nor does MCS-9. Given that MCS-4 offers no error protection and uses
GMSK, one would expect that it would provide the same data rate as CS-4, as used in standard GPRS. The difference is due to the fact that in EGPRS,
the RLC/MAC header is coded differently from the rest of the PDU and contains
additional bits for error correction. The objective is to ensure that at
least the header can be decoded. The same does not apply for CS-4.
The channel types applicable to EGPRS are the same as those applicable
to GPRS�"we have a number of PDCHs that carry PCCCH, PBCCH,
PDTCHs, and so on. In fact, these channels are shared among GPRS and
EGPRS users. Thus, both GPRS users and EGPRS users can be multiplexed
on a given PDTCH. Of course, during those radio blocks when the
PDTCH is used by an EGPRS user, the modulation may be either GMSK or
8-PSK, whereas it must be GMSK when used by a GPRS user.
Similar to the manner in which the network controls the coding scheme
to be used by a GPRS user, the network also controls the MCS to be used by
an EGPRS user in both the uplink and downlink. This is done through the
addition of new information elements in the Packet Uplink Assignment and
Packet Downlink Assignment messages.

One important aspect of GPRS and EGPRS users sharing the same
PDTCH on the uplink is the use of the USF. Recall that the USF is used
with dynamic allocation, is sent on the downlink, and is used to indicate
which MS has access to the next RLC/MAC block on the uplink. If a given
PDTCH is being used for both GPRS and EGPRS MSs, then it is important that both types of MS be able to decode the USF, so that they may appropriately
schedule uplink transmissions. Consequently, when a PDTCH is
used for both GPRS and EGPRS, GMSK modulation must be used for any
radio block that assigns uplink resources to a GPRS MS. All other radio
blocks may use 8-PSK modulation. Note that this forced use of GMSK for
radio blocks destined for an 8-PSK MS only applies with dynamic allocation.