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transmit window can advance or not. A 0 indicates that the window is not stalled.


Table 4.16 Coding scheme
Family Coding Payload Payload Data bits in
scheme size (bits) units each RLC block
A MCS-3 296 1 296
MCS-6 296 2 592
2 — 544
MCS-8 272 4
2 — 592
MCS-9 296 4
B MCS-2 224 1 224
MCS-5 224 2 448
2 — 448
MCS-7 224 4
C MCS-1 176 1 176
MCS-4 176 2 352
4.11 OTHER CELLULAR HIGH-SPEED DATA TECHNOLOGIES 157


• Retry (R): this single-bit ¬eld indicates whether the mobile station had sent the channel
packet request message more than once. A 0 indicates that it was sent once and a 1
indicates that it was sent multiple times.
• PFI indicator (PI): this single-bit ¬eld indicates the presence of the optional PFI ¬eld.
A 0 indicates that it is not present.
• Resent block bit (RSB): this ¬eld indicates whether there are any RLC data blocks
within the radio block that have already been transmitted before.

The EGPRS RLC/MAC data block is illustrated in Figure 4.57. This data block ¬ts inside
the RLC/MAC block and the size of the actual data block for each of the coding schemes
is highlighted in Table 4.16.
The following identify the ¬elds within the RLC data block illustrated in Figure 4.57:

• Final block indicator (FBI): this single-bit ¬eld indicates the last downlink RLC data
block. A 0 indicates that there are more blocks to come whereas a 1 indicates that this
is the last block in this TBF.
• Extension (E): this single-bit ¬eld indicates the presence of the optional byte in the
data block header. A 0 indicates that an extension byte follows.
• Length indicator (LI): this optional 7-bit ¬eld is used to delimit LLC PDUs within a
single RLC data block by identifying the last byte of the LLC PDU. If there are a
number of LLC PDUs within an RLC data block the ¬rst LI will indicate the length
of the ¬rst LLC PDU, the second LI will indicate the length of the second LLC PDU
etc. Only the last segment of a LLC PDU has an associated LI.
• TLLI Indicator (TI): this bit, which is only present in the uplink, indicates whether the
TLLI ¬eld is present or not. A 1 indicates that the TLLI is present.
• Temporary logical link identi¬er (TLLI): this 32-bit ¬eld is optional and is used while
contention resolution is required.

TI E

Length Indicator E



Length Indicator E

FBI E

TLLI Length Indicator E



PFI E Length Indicator E

RLC Data RLC Data
Uplink Downlink

Figure 4.57 RLC data block
158 GENERAL PACKET RADIO SERVICE


• Packet ¬‚ow identi¬er (PFI): this 7-bit ¬eld is assigned by the SGSN and used to
identify a particular ¬‚ow context and QoS value. Legitimate identi¬ers can be best
effort, signalling and SMS, which are prede¬ned, or it can be dynamically assigned
and offer a particular QoS for this speci¬c context.
• RLC data: this ¬eld contains the LLC PDU or part of it if it has been segmented. The
amount of data transferred depends on whether there are any optional RLC headers in
place and also on the coding scheme used. Figure 4.57 indicates the number of bits
each coding scheme can carry.



4.11.4 Channel coding for PDTCH
The nine different modulation and coding schemes (MCS) shown in Table 4.15 are divided
into different families as indicated below:

• family A consists of MCS-3, MCS-6, MCS-8 and MCS-9
• family B consists of MCS-2, MCS-5 and MCS-7
• family C consists of MCS-1 and MCS-4.

Each of these families has a ¬xed payload size: family A is 296 (and 2727 ) bits, B is
224 bits and C is 176 bits. This allows different code rates within a family to be achieved
through transmitting a number of blocks (known as payload units) within a radio block.
MCS-7, 8 and 9 each consist of four payload units, MCS-5 and 6 each consist of two
payload units and MCS-1, 2 and 3 each consist of one payload unit. This is highlighted
in Table 4.16. It can also be seen from this table that MCS-7, 8 and 9 actually transfer
two RLC blocks within a single radio block.
Figure 4.58, is a simpli¬ed view of how data is transferred in the downlink using MCS-
9. The stealing bit (SB) ¬eld is used to indicate the header format used. The numbers
in the diagram indicate the number of bits associated with each ¬eld. It can be seen that
there are actually two RLC blocks that are carried. Since MCS-9 uses 8PSK the four
radio bursts at the physical layer across the air transfer 1392 bits of data. MCS-6, which
is in the same family, will transfer one RLC block (612 bits) with more protection and
since it also uses 8PSK will transfer 1392 bits at the physical layer. MCS-3, which is also
in the same family, actually carries 316 bits of data within a block. It does not use 8PSK,
therefore at the physical layer 464 bits are transferred. The HCS is a header checksum
and the BCS is a checksum over the data.
Figure 4.59 shows another example; this time MCS-1 is being used. In both this and
the preceding diagram each of the payload data blocks (612 bits in Figure 4.58 and
372 bits in Figure 4.59) is punctured with an individual puncturing scheme. There are
up to three puncturing schemes available and in the case where two RLC blocks are
transmitted within a single radio block (MCS-7, 8 and 9) they may be subjected to

7
MCS-8 is only 272 bits, therefore when switching between MCS-3 or MCS-6 padding is required
to ensure that the length of 296 bits is consistent throughout the family.
4.11 OTHER CELLULAR HIGH-SPEED DATA TECHNOLOGIES 159


Header 48 bits RLC 1 Data = 612 bits RLC 2 Data = 612 bits

RLC/ Data 2 blocks Data 2 blocks
USF HCS E FBI BCS TB E FBI BCS TB
x 296 bits x 296 bits
MAC hdr



36 135 1836 bits after 1/3 convolution coding 1836 bits after 1/3 convolution coding

Puncturing

SB
36 124 612 612 612
612 612 612
=8

Data is not transferred directly into the 4 bursts but actually spread across them


Radio Burst Radio Burst Radio Burst Radio Burst

Since 8-PSK is used = 1392 bits


Figure 4.58 Coding and puncturing for MCS-9

39 196 bits

RLC/ Data =176
USF HCS E FBI BCS TB
MAC hdr bits


12 108 588 bits after 1/3 convolution coding

Puncturing

SB = 12 12 68 372 372

Data is not transferred directly into the 4 bursts but actually spread across them


Radio Burst Radio Burst Radio Burst Radio Burst

Since G-MSK is used = 464 bits

Figure 4.59 Coding and puncturing for MCS-1

different puncturing schemes. The coding and puncturing scheme (CPS) indicator ¬eld in
the RLC/MAC header indicates to the receiver which scheme has been implemented.


4.11.5 Link adaptation and incremental redundancy
As discussed, EDGE introduces nine modulation and coding schemes, each of which is
designed to deliver the optimal throughput under different radio conditions. As the radio
environment changes during a connection, then the coding scheme may change, enabling
more ef¬cient use of the air resources and improving performance and robustness. The
selection of this MCS is determined by the network and takes into consideration the link
160 GENERAL PACKET RADIO SERVICE



MCS-6 header

Padding: 6 bytes
Length: 40




RLC data=74
MCS-8 header
end of LLC1
Length: 40 byte 8-47
RLC data=68




(length 40 bytes)
end of LLC1
byte 2-41 start of LLC2
(length 40 bytes) byte 48-74
(length 27 bytes)
start of LLC2
byte 42-68
(length 27 bytes)
Length: 33 MCS-6 header
RLC data=68




end of LLC2
byte 2-34 Padding: 6 bytes
(length 33 bytes)
Length: 33
RLC data=74
start of LLC3
byte 33-68 end of LLC2
(length 34 bytes) byte 8-40
(length 33 bytes)
start of LLC3
byte 41-74
(length 34 bytes)

Figure 4.60 Example of IR retransmission

quality measurements (LQM) carried out by the mobile device and the BTS. This link
quality may be based on a number of parameters such as block error rate (BLER) or
channel to interference ratio (CIR).
When acknowledged mode is selected, EDGE may use the same ARQ mechanism as
used in GPRS but it can also use incremental redundancy (IR), which is also referred
to as hybrid ARQ type 2. In the traditional ARQ mechanism the data is segmented at
the transmitter, convolution encoded and a checksum added. At the receiver the block
is decoded and the CRC checked. If the computed checksum does not equal the CRC
transmitted then the data is discarded and a retransmission is requested.
When IR is selected, the block of data received in error is not discarded but retained.
There are a number of alternative puncturing schemes and each block of data is trans-
mitted with redundant information using a different puncturing scheme. In the case of an
erroneous block being received, additional coded bits are retransmitted using an alternative
puncturing scheme and together with the previously errored block are used to reconstruct
the data. If the block is received in error then it can be resent using the same MCS or
an alternative MCS within the same family. For example, if MCS-6 had been initially
selected and the block was received in error, then any MCS in family A including MCS-6
can be used for the retransmission.
Figure 4.60 shows an example of IR retransmission, the original transmission using
MCS-8 and the retransmission using MCS-6. In this example there are three LLC partial
or full frames to be transferred; LLC data 1 consists of the last 40 bytes of the LLC,
LLC data 2 consists of a complete LLC of 60 bytes and LLC data 3 consists of the ¬rst
34 bytes of the LLC.
4.11 OTHER CELLULAR HIGH-SPEED DATA TECHNOLOGIES 161


The MCS-8 block can carry two RLC data blocks each consisting of 68 bytes. This
value includes the length indicator and not just the actual data. The ¬rst length indicator
¬eld with the value 40 indicates where LLC1 data ends. The LLC2 data follows this
directly within the same RLC, i.e. the RLC block is not speci¬cally for one single LLC
frame and can span multiple frames. The LLC2 data does not ¬t completely into the ¬rst
RLC block and thus the second length indicator indicates the last byte of this LLC frame.
Again, LLC3 data follows on directly.
In the case of the retransmission, it can be seen that MCS-6 does not have such a large
payload capacity and as such two RLC/MAC blocks have to be sent. If MCS-3 had been
chosen for the retransmission then four RLC/MAC blocks would have been required,
introducing a large transfer overhead due to the four headers required. It can be seen
from Figure 4.60 that padding has been introduced for alignment purposes. This padding
takes up the initial part of the RLC data and consists of 6 bytes. Since the RLC data unit
can be 74 bytes this can now transport the ¬rst RLC data unit of the MCS-8 RLC/MAC

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