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6.18 FRAME PROTOCOLS 355


time
RNC
CFN: 30
early on time late too late


TOAWS TOA TOAWE LTOA




+ve TOA -ve TOA
BTS Receiving window Tproc

27 28 29 30 CFN



CFN: 30
27 28 29 30
UE


Figure 6.79 UTRAN transport channel synchronization

It should be noted that on the air interface, frames are numbered according to the cell
SFN which is sent on the BCH. The CFN is not transmitted over the air but rather is
mapped by layer 1 to the SFN of the ¬rst radio frame being transmitted. The SFN cycles
every 4096 frames.
Figure 6.80 shows an example exchange of synchronization information between a
SRNC and a BTS.

Radio interface synchronization
Radio interface synchronization is used to ensure that when the UE is receiving frames
from several cells, it gets the correct frames synchronously so as to minimize the buffering
requirements in the UE. During the establishment of a radio link, RRC signalling will
inform the UE of when the frames it will receive in the downlink can be expected. At
handover, the UE will measure the time difference between its existing dedicated physical
channel and the SFN in the target cell. It will then report the timing deviation to the SRNC,
which will in turn use this information to calculate appropriate timings for the new BTS,
and deliver this information to the BTS.


6.18.1.6 Outer loop power control
As previously discussed, the outer loop power control is used by the SRNC to modify
the SIR target used at the BTS for received user data. The SIR is modi¬ed to adjust the
quality of the user signal (either reduce or increase) depending on the BER of the signal
received. The uplink QE and CRCI can be used in the RNC power control algorithm to
356 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


Frame CRC FT Frame CRC FT
010 0010 1 011 1000 1
SRNC
DL Synchronisation UL Synchronisation
Conn:4 VPI:1 VCI:120 CID:9
0000 0011 0000 0100
AAL2 CFN CFN
UUI: 0 (00h) 0000 1000 0000 1000
SSSAR-PDU: 45 03 08
ToA
DOWNLINK SYNC
00 66h = 102
Control Frame CRC : 34 (22h)
102 * 0.125ms = 12.75ms
Control Frame Type : 3 (3h)
BTS
Conn. Frame number : 8 (8h)
Conn:4 VPI:1 VCI:120 CID:9
AAL2
UUI: 26 (1Ah)
SSSAR-PDU: 71 04 08 00 66
UPLINK SYNC
Control Frame CRC : 56 (38h)
Control Frame Type : 4 (4h)
Conn. Frame number : 8 (8h)
Time of Arrival : 12.750 ms


Figure 6.80 Trace example of synchronization procedure. Reproduced by permission of
NetHawk Oyj



Frame CRC FT
header
Outer Loop PC


UL SIR Target


Figure 6.81 Outer loop power control frame

determine the required adjustment necessary to the SIR target. Once an outer loop power
control message is received, the BTS will then update its SIR target for the user, and
perform the necessary inner loop power control adjustments. The frame format is shown
in Figure 6.81.
The UL SIR target value is an 8-bit ¬eld, encoding a value in dB in the range ’8.2 dB
to +17.3 dB in 0.1 dB steps. For example the value 98 implies a SIR target of +1.6 dB.
An example trace of outer loop power control is shown in Figure 6.82.


AAL2 2Dh = 45 = -8.2 - 45*0.1 = -3.7dB
UUI: 0 (00h)
SSSAR-PDU: D3 01 2D
OUTER LOOP PWR CTRL: D3 01 2D
Control Frame CRC : 105 (69)
Control Frame Type : 1 (1)
UL SIR TARGET : -3.7 (X) dB


Figure 6.82 Sample trace of outer loop power control
6.18 FRAME PROTOCOLS 357


6.18.2 User data on Iub common channels
The FP format for data transport on the common channels is slightly different to that of the
dedicated channels. This covers transport on both the FACH/RACH and the shared data
channels DSCH/CPCH. The differences are since the termination point of these channels
is the controlling RNC (CRNC) and not the serving RNC (SRNC), although they may
often be the same entity. Recall that these channels are handled by the MAC-c/sh layer
at the CRNC, which separates the control and data, with the data (DTCH/DCCH) being
passed to the MAC-d layer of the SRNC. If the MAC-c/sh and MAC-d are in separate
network elements then data frames will be transferred over the Iur using the Iur FP,
which is described later in this section. For this reason, extra information is required.
There is no need for a quality estimate on uplink common channels since there will be
no macrodiversity performed because there is no soft handover on common channels.


6.18.2.1 RACH/FACH transport
For the RACH, which is an uplink channel, the data frame is almost the same as shown
in Figure 6.73, without the QE and CRCI ¬elds but with the addition of a propagation
delay ¬eld at the end of the header. This is an 8-bit ¬eld which indicates, in numbers of
chips, the radio interface delay during the RACH access. Another difference is that the
TFCI ¬eld in the channel will only be a transport format indicator (TFI) here since only
one transport channel is permitted per frame. For the FACH, the additional header ¬eld is
a transmit power level, which encodes a negative offset to the maximum power for this
channel, indicating to the BTS the power level recommended for use for transmission
of this transport block across the Uu interface. This is an 8-bit ¬eld, with a range of
0“25.5 dB in 0.1 dB steps. Figures 6.83 and 6.84 show examples of a RACH and a
FACH exchange.


6.18.2.2 CPCH/DSCH transport
On the common packet channel, which is an uplink channel, the format is the same as
the RACH frame. For the downlink shared channel, the header format is as shown in
Figure 6.85.
The additional ¬elds are de¬ned as follows:

• Code number (8 bits): indicates the code number of the physical downlink shared chan-
nel, since there may be more than one.
• Spreading factor (SF, 3 bits): indicates which SF should be used on the physical DSCH.
The mapping is as shown in Table 6.29.
• Multicode information (MC info, 4 bits): indicates the number of parallel physical chan-
nel codes on which the data will be transported. For data rates higher than 384 kbps, a
user must use more than one CDMA code in parallel, with the data distributed across
these codes on the Uu interface. If more than one channel code is used, the SF of all
codes is the same.
358 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


Header CRC FT
UL: UE/BTS
010 0111 0
Conn:12 VPI:3 VCI:120 CID:8
CFN
AAL2 0100 1100 (4C)
UUI: 26 (1Ah)
TFI
spare
SSSAR-PDU 0 0000
4E 4C 00 0B 0C 25 48 Propagation Delay
06 04 08 54 44 00 58 0Bh = 11 = 33 chips
05 00 00 00 80 02 00
Transport Block
00 01 41 4C 00 96 4F
RACH/CPCH DATA FRAME
0C 25 48 06 04 08 54
Header CRC : 39 (27)
Conn. Frame number : 76 (4Ch) 44 00 58 05 00 00 00
Transport Format Ind : 0 (0)
80 02 00 00 01 41 4C
dynamic part = {1 block, 168 bits/block}
FDD - Propagation Delay : 33 chips
Payload Checksum
Transport Blocks
96 4F
1. Transport Block
0C 25 48 06 04 08 54
44 00 58 05 00 00 00
80 02 00 00 01 41 4C
CRC Indicators : 00
Payload CRC : 38479 (96 4Fh)

Figure 6.83 RACH data frame trace


6.18.2.3 Control frames

As with the dedicated channels, there are similar control procedures for the common
channels, however not all procedures are applicable to each channel. Table 6.30 details
the procedures, and the channels to which they apply.
The DSCH TFCI signalling procedure is sent every 10 ms on the TFCI2 transport
bearer when there is DSCH data to be sent. It provides the BTS with information to
allow it to generate the TFCIs to be transmitted on the DPCCH.


Table 6.29 SF mapping
SF value SF on PDSCH
0 4
1 8
2 16
3 32
4 64
5 128
6 256
6.18 FRAME PROTOCOLS 359


6.18.3 User data on Iur common channels
The FP across the Iur interface is a little more complex. It is required to provide the
following services:

• transfer of MAC-c/sh SDUs from DRNC to SRNC for RACH and CPCH channels;
• transfer of MAC-c/sh SDUs from SRNC to DRNC for FACH and DSCH channels;
• ¬‚ow control between MAC-c/sh and MAC-d.

The reason for the signi¬cant difference here is that the MAC-c/sh for common/shared
channels is terminated at the CRNC whereas the MAC-d terminates at the SRNC. Recall
that the CRNC controls the BTS, and thus the common channels, whereas the SRNC
manages the UE. Most of the time, calls on common channels will have coincident
CRNC and SRNC, as shown in Figure 6.86 for the RACH channel. The diagram for the
FACH is identical except for the directions of the arrows.

Header CRC FT
DL: CRNC
001 1000 0
Conn:13 VPI:3 VCI:120 CID:9
CFN
AAL2 0111 0010 (72h)
UUI: 0 (00h) TFI
spare
SSSAR-PDU 0 0010
30 72 02 CD 40 51 F8 Tx Power Level
CDh = 205 = 20.5dB
30 E7 12 A4 03 02 04
2A 22 00 20 37 00 20 First Transport Block
40 96 06 91 40 52 A5
40 51 F8 30 E7 12 A4
C1 09 70 85 98 D4 28
80 30 B8 95 0D 2E 28
03 02 04 2A 22 00 20
4B 8C 4C C6 63 28
FACH DATA FRAME 37 00 20 40 96 06 91
Header CRC : 24 (18h) Second Transport Block
Conn. Frame number : 114 (72h)
40 52 A5 C1 09 70 85
Transport Format Ind : 2 (2h)
dynamic part = {2 blocks, 168 bits/block}
98 D4 28 80 30 B8 95
Transmit Power Level : 20.5 dB
Transport Blocks 0D 2E 28 4B 8C 4C C6
1. Transport Block
Payload Checksum
40 51 F8 30 E7 12 A4
63 28h

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