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block. The second MCS-6 RLC/MAC block can also transport the second RLC data unit
of the MCS-8 RLC/MAC block completely. At the receiver, both the MCS-8 block and
the MCS-6 blocks can be used to reassemble the LLC data frames.


4.11.6 Compact EDGE
UWC-136 EDGE classic was speci¬ed by ETSI and uses the standard GSM carriers and
control channels. A BCCH is located in each cell; it transmits continuously and unlike data
channels does not hop between frequencies. This system typically requires 2.4 MHz of
spectrum in both directions. Currently it is typical for a GSM system to adopt a frequency
reuse pattern of 3/9 (three BTS and three sectors each) or 4/12 (four BTS and three sectors
each). It is expected that EDGE systems will be co-located and thus also use the same
reuse pattern.
In the USA, where there is limited spectrum available, a deployment that uses 1 MHz
in both directions is being considered which puts considerable constraints on the system
implementation. This will require a frequency reuse pattern that is optimal and spacing of
carriers will be much closer than with the classic model. In fact, it requires a frequency
reuse of 1/3 whereby a BTS that has three cells using F1, F2 and F3 will be adjacent
to other BTSs using the same three frequencies. This reuse pattern is seen as possible
with the dedicated channels which can withstand lower signal to noise ratios. However,
to ensure that the system works effectively, control signalling such as system broadcast,
paging and packet access will require to be extremely robust. The robustness cannot come
from frequency separation due to the limited spectrum but rather this is achieved through
the time domain. This system is known as compact EDGE. To achieve the reliability and
robustness of the common signalling channels through the time domain requires that all
the BTSs are frame synchronized. This is not a requirement for classic EDGE. This can
be achieved through the use of GPS.
In compact EDGE, for control signalling purposes, each cell is not only identi¬ed by its
frequency but also by the time group to which it belongs and there are four time groups
speci¬ed (although not all of these need to be implemented). Essentially this means that
although cells using the exact same frequency will be in close proximity geographically
162 GENERAL PACKET RADIO SERVICE


and will cause a certain amount of interference, it is a design consideration to ensure
that they belong to different time groups. When it is one time group™s turn to use the
common channel signalling channel other cells that are not in the same group but are at
the same frequency will not transmit at all, thus reducing the interference. This mechanism
is illustrated in Figure 4.61. It can be seen that there are three frequencies and that there
are four time groups. Also highlighted is a group of twelve cells which includes only one
cell using frequency 1 and timing group 4.
The actual time groups are identi¬ed by the time slot number in the GSM/EDGE frame.
There are eight time slots in a GSM frame and time slots 1, 3, 5 and 7 are used for these
four time groups. The time group is not ¬xed to a particular time slot and is rotated over
the time slots, so at any one time, for example, time group 2 may use slot 1, 3, 5 or 7.
The time groups are synchronized so that the other time groups will use the other slots
that are available. The time groups are also only transmitted at certain times within the
hyperframe (52-frame); these are frames 0“3, 21“24, 34“37 and 47“50. When they are
not being used for common channel signalling these slots (1, 3, 5 and 7) and the other
even time slots can be used for data transfer (DTCH) or dedicated signalling (PACCH).
The only exception as mentioned is when another time group is using the common control
channel and to reduce interference all other slots at that time will be idle and will not
transmit. This is highlighted in Figure 4.62, where the synchronized downlink time slots
of four time groups all using the same frequency are illustrated.


4.11.7 GSM/EDGE radio access network (GERAN)
The next phase of EDGE for R4/5 is the GSM EDGE radio access network (GERAN),
which is closely aligned with UTRAN. It is designed to give better QoS than the existing


F2 F2 F2
TG 1 TG 2 TG 1
F1
F1 F1
BTS BTS BTS
TG 4
TG 4 TG 3
F3
F3 F3
TG 1
TG 2 TG 2
F2 F2
F2 F2
TG 4 TG 3
TG 3 TG 3
F1
F1 F1 F1
BTS BTS BTS BTS
TG 1
TG 2 TG 2 TG 2
F3 F3 F3 F3
TG 4 TG 3 TG 4 TG 4
F2 F2
F2 F2
TG 1
TG 2 TG 2
TG 1
F1 F1 F1 F1
BTS
BTS
BTS BTS
TG 3 TG 4 TG 3 TG 4
F3 F3 F3 F3
TG 1 TG 2 TG 1 TG 2



Figure 4.61 Compact EDGE frequency and time reuse
4.11 OTHER CELLULAR HIGH-SPEED DATA TECHNOLOGIES 163


0 1 2 3 4 5 6 7
Time
CPBCCH
Data Data Idle Data Idle Data Idle
Group
0
1
0 1 2 3 4 5 6 7
Time CPBCCH
Data Idle Data Data Idle Data Idle
Group 1
2
0 1 2 3 4 5 6 7
Time CPBCCH
Data Idle Data Idle Data Data Idle
Group 2
3
0 1 2 3 4 5 6 7
Time CPBCCH
Data Idle Data Idle Data Idle Data
Group 3
4

Figure 4.62 Compact EDGE downlink time slots

EDGE systems and will enable such services as VoIP. Since in this case the voice data will
be transported over the Iu-PS, it will be possible to multiplex other data services within
the same time slot as long as they do not interfere with the real-time QoS requirements of
the voice. Figure 4.63 illustrates a generalized GERAN architecture. It can be seen that
there are options for using both the A/Gb interface and/or the Iu interface for circuit- and
packet switched connections to the core network. Which of these interfaces is supported
is indicated in the broadcast system information messages. The actual user and control
plane protocol stacks depend on which interfaces (A/Gb or Iu) are implemented within
the GERAN. For example, on the user plane for the packet core, if the Gb interface is
implemented then SNDCP/LLC will be used and if Iu-PS is implemented then PDCP
will be used. The protocol stacks for the control plane and user plane for the Gb mode
of operation are illustrated in Figures 4.7 and 4.8, respectively. For the Iu-PS mode of
operation please refer to Chapter 6.
It is anticipated that additional transport options rather than just ATM will be utilized,
such as IP.


4.11.7.1 Iur-g interface8
The Iur-g interface is an open standard allowing for multi-vendor interoperability. Imple-
menting this optional Iur-g interface to connect different RANs together, or even to other
BSSs, enables the GERAN routing area (GRA) to be introduced. This has a similar func-
tion to the URA within UTRAN and enables the RAN to assist the core network in the
mobility management related to each of the mobile devices. It is used if the Iu mode is
implemented but is not used for the A/Gb mode of operation. The GRA consists of one or
more cells which may overlap BSS/RNS or LA/RAs. To enable this overlapping feature,
a single cell may broadcast a number of GRA identi¬ers.
The RNSAP protocol will be used across the interface but will support only a subset of
those procedures that are incorporated in the Iur in UTRAN. For example, there is no soft

8
The Iur-g is similar to the Iur interface, which is described in detail in Section 6.19.3.
164 GENERAL PACKET RADIO SERVICE


GERAN
BSC Circuit Switched Core Network
BSS




ce
BSS




rfa
Iur-g
MSC/VLR GMSC




e
int
CS
-
Iu
A/
Abis Gb
/I u-P
S
Mobile int
erfa Packet Switched Core Network
Station BSC
ce
BTS
Iur-g




SGSN GGSN



UTRAN

RNC

Figure 4.63 Generalized GERAN architecture

handover in GERAN, therefore this does not need to be supported. The interface allows
for paging and SRNS relocation, as well as uplink and downlink signalling transfers. The
protocol stack for RNSAP is the same as used in UMTS later releases. It has the option
of being carried over MTP3-B or M3UA/SCTP.
A mobile device which has an RRC connection will be issued a G-RNTI, which consists
of the serving BSC identi¬er (SBSC-id) as well as a unique identi¬er, within the BSC
serving area, for the mobile device (S-RNTI).


4.11.7.2 Radio resource connection modes
This is when the Iu interface is being used and the mobile device can be in either of two
modes: idle or connected.
In idle mode the CN knows the location of the mobile device to the current LA or RA.
There is no knowledge of the mobile device within the BSS, and for downlink transfers
the mobile device needs to be paged. The mobile device will return to this state from
connected mode when the RRC connection is released.
The RRC connected mode is entered when the mobile device is assigned a G-RNTI.
There are three states de¬ned for this mode:

• RRC-cell shared: no dedicated basic physical subchannel is allocated and the mobile
device is known to the cell by GERAN.
FURTHER READING 165


• RRC-cell dedicated: one or more dedicated basic physical subchannels has been allo-
cated in the uplink and downlink, which it can use at any time. The mobile device may
also be assigned resources on one or more shared channels. Again, the mobile device
is known to the cell by GERAN.
• RRC-GRA PCH: no channels are allocated and no uplink activity is possible. The
location of the mobile device is known to the GRA level by GERAN.

In both RRC-cell shared and RRC-GRA PCH mode, the core network will know the
location of the mobile to the serving BSC. GERAN will know the location to the cell and
GRA, respectively.




4.12 SUMMARY

This chapter explains the evolution of the basic GSM network to effectively handle
non-real-time data traf¬c with the introduction of the GPRS infrastructure. The required
changes and additions throughout the network are described, as well as the new protocols
introduced. The dynamic allocation of resources on the air interface and the new coding
schemes introduced are also explained. Like GSM, the GPRS system forms a central part
of the UMTS core network. The key procedures are explained with use of examples of
network traces to illustrate their operation in a real environment. The evolution of GPRS
may be to an EDGE network, so, in this context, the operation of EDGE is described
in some detail, along with its use in the evolved GSM network, GERAN, which will
eventually be integrated into a ubiquitous 3G cellular environment.




FURTHER READING

T. Halonen, J. Romero, J. Melero (2002) GSM, GPRS and EDGE Performance. John
Wiley & Sons, Chichester.
3GPP TS03.64: General Packet Radio Service (GPRS); Overall description of the GPRS
radio interface; Stage 2.

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