Page 91 - From GMS to LTE

General Packet Radio Service (GPRS) and EDGE 77

TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7
Signaling TCH PDTCH PDTCH PDTCH PDTCH TCH

GSM/GPRS GSM GPRS GSM
signaling
Figure 2.5 Shared use of the timeslots of a cell for GSM and GPRS.

2.3.2 Mixed GSM/GPRS Timeslot Usage in a Base Station
As GPRS is an addition to the GSM network, the eight timeslots available per carrier
frequency on the air interface can be shared between GSM and GPRS. Therefore, the
maximum GPRS datarate decreases as more GSM voice/data connections are needed.
The network operator can choose how to use the timeslots, as shown in Figure 2.5.
Timeslots can be assigned statically, which means that some timeslots are reserved for
GSM and some for GPRS. The operator also has the option of dynamically assigning
timeslots to GSM or GPRS. If there is a high amount of GSM voice traffic, more times-
lots can be used for GSM. If voice traffic decreases, more timeslots can be given to
GPRS. It is also possible to assign a minimum number of timeslots for GPRS and
dynamically add and remove timeslots depending on voice traffic.

2.3.3 Coding Schemes
Another way to increase the data transfer speed besides timeslot aggregation is to use
different coding schemes. If the user is at close range to a base station, the data trans-
mitted over the air is less likely to be corrupted during transmission than if the user is
farther away and the reception is weak. As has been shown in Chapter 1, the base sta-
tion adds error detection and correction to the data before it is sent over the air. This is
called coding and the method used to code the user data is called the coding scheme. In
GPRS, four different coding schemes (CS‐1 to 4) can be used to add redundancy to the
user data depending on the quality of the channel [2]. Table 2.2 shows the properties of
the different coding schemes.
Figure 2.6 shows how CS‐2 and CS‐3 encode the data before it is transmitted over the
air interface. CS‐4 does not add any redundancy to the data. Therefore, CS‐4 can only
be used when the signal quality between the network and the mobile device is very good.

Table 2.2 GPRS coding schemes.

Number of user data bits per block Transmission speed per
Coding scheme (four bursts with 114 bits each) timeslot (kbit/s)

CS‐1 160 8
CS‐2 240 12
CS‐3 288 14.4
CS‐4 400 20