Page 92 - From GMS to LTE

78 From GSM to LTE-Advanced Pro and 5G

RLC/MAC LLC Data Spare
24 240/288 Bits 7/3

271/315 Bits

USF precoding
6 268/312 Data bits 16 Bits parity 0000
294 / 338 Bits

½ Rate convolutional coder

588 / 676 Coded bits

Punctured to 456 bits

Figure 2.6 CS‐2 and CS‐3 channel coder.

GPRS uses the same 1/2‐rate convolutional coder as already used for GSM voice traffic.
The use of the convolutional coding in CS‐2 and CS‐3 results in more coded bits than
can be transmitted over a radio block. To compensate for this, some of the bits are
simply not transmitted. This is called ‘puncturing’. As the receiver knows which bits are
punctured, it can insert 0 bits at the correct positions and then use the convolutional
decoder to recreate the original data stream. This, of course, reduces the effectiveness
of the channel coder as not all the bits that are punctured are 0 bits at the sender side.

2.3.4 Enhanced Datarates for GSM Evolution (EDGE)
To further increase data transmission speeds, an additional modulation and coding
scheme, which uses 8 Phase Shift Keying (8‐PSK), has been introduced into the stand-
ards. The new coding scheme is the basis of the ‘enhanced datarates for GSM evolution’
package, which is also called EDGE. The packet‐switched part of EDGE is also referred
to in the standard as Enhanced‐GPRS or EGPRS. In the GPRS context, EGPRS and
EDGE are often used interchangeably. By using 8‐PSK modulation, EDGE transmits
three bits in a single transmission step. This way, data transmission can be up to three
times faster compared to GSM and GPRS, which both use Gaussian minimum shift
keying (GMSK) modulation, which transmits only a single bit per transmission step.
Figure 2.7 shows the differences between GMSK and 8‐PSK modulation. While with
GMSK the two possibilities 0 and 1 are coded as two positions in the I/Q space, 8‐PSK
codes the three bits in eight different positions in the I/Q space.
Together with the highest of the nine new coding schemes introduced with EDGE, it
is possible to transfer up to 60 kbit/s per timeslot. Similar to CS‐3 and CS‐4, the use of
these coding schemes requires an update of the backhaul connection, and in addition,
new transceiver elements in the base stations that are 8‐PSK capable. From the network
side, the mobile device is informed of the EDGE capability of a cell by the EDGE