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...
First TB of DCH n First TB of DCH n
...




...
First TB of DCH n Pad First TB of DCH n Pad
...




...
Last TB of DCH n Last TB of DCH n
...




...
CRCI
Last TB of DCH n Pad Last TB of DCH n Pad
First
TB QE Payload Checksum
DCH 1 CRCI
... Payload Checksum
Last TB
...




DCH n
... Pad
Payload Checksum
Payload Checksum
(a) (b)

Figure 6.73 (a) Uplink and (b) downlink FP data frame structure


The purpose of the uplink frame protocol is to transport additional information that the
SRNC needs for such tasks as handover evaluation, macrodiversity and channel quality
estimates for outer loop power control. Figure 6.74, summarizes the role the BTS plays
in forming the frame protocol. Across the air, the BTS receives a physical dedicated data
channel and a control channel, DPDCH and DPCCH. From the control channel, it must
pass the TFCI for each physical data channel. The BTS will extract the TFCI for the
CCTrCH and decode it to the TFI for each transport block. From the data channel, it
extracts each of the transport blocks to the frame protocol, checks the physical layer CRC
(if present) and makes an evaluation of the transport/physical channel BER, if requested
to do so. This information is also included in the frame protocol.
For uplink data transmission, there are two modes of operation de¬ned: normal mode
and silent mode. Which is used is decided by the SRNC at the time the transport channel
is established. For silent mode, the BTS will only transmit data frames to the RNC if it
has received transport blocks over the air, and if it receives an indication that there are no
transport blocks, it will not send a transmission. For normal mode, the BTS will always
6.18 FRAME PROTOCOLS 351




Frame
protocol
TFI (TB1)
UE BTS
...
DPCCH
TFI (TBn)
pilot TFCI FBI TPC
Transport
Blocks
DPDCH (TB1..TBn)
data
QE
BER count CRCI

CRC Check

Figure 6.74 Role of uplink frame protocol


send an uplink data frame, even if it has not received a transport block over the air. In this
case, it will send an empty data frame. This is to preserve a timing relationship between
the BTS and RNC.


6.18.1.2 Macrodiversity

This mechanism enables a reduction in the required Eb /No when soft handover is used as
compared to having a single radio link. For soft handover, macrodiversity is performed
at the SRNC, which increases traf¬c over the Iub and possibly Iur (if a radio link is via
a D-RNC). This therefore introduces a tradeoff between decreased interference over the
air and increased traf¬c volume on the ¬xed network. This tradeoff has to be carefully
managed and planned for, but with the air interface generally being a limiting factor in
CDMA capacity, any reduction in interference over the air is seen as advantageous.
In a simple implementation a method of selection combining can be used at the SRNC to
ensure that a signal with a successful CRC check is passed onto the core network whereas
any frames with a CRC error are discarded. A more advanced system of recombining will
make use of the QE that has been passed to the SRNC in the frame protocol. Using this
in addition to the CRC check will enable the SRNC to pass data with errors to the core
network from the radio path with the best quality, with reference to SIR, and hence BER.
Consider that the UE is in soft handover, and data from two active connections is being
received by the SRNC. For each received TB, the SRNC can compare the CRC checks
using the CRCI bits in the frame protocol. If both pass, then either TB can be passed to
the CN. If one fails, then the TB which is correct can be passed. However, if both fail,
then the SRNC can resort to the QE value, if available, and select the best quality TB
to send.
352 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


Although this may not be useful when dealing with TCP/IP data packets, packets with
errors can still be useful for voice traf¬c over both the CS-CN and the PS-CN. Simulations
have shown that for soft handover to be effective there should be limited difference in the
signal power from all links. It is suggested that this limit be around 3 dB but of course
this depends on the actual implementation.


6.18.1.3 Downlink data frame
The downlink data frame, shown in Figure 6.73(b), is almost identical to the uplink
frame aside from the absence of the QE and CRCI, since these are only relevant to uplink
information extracted from the Uu interface performance.


6.18.1.4 Control frames
The control frame procedures transfer control information regarding the DCH from
the SRNC to the base station. The general format of a control frame is illustrated in
Figure 6.75.
The control frame type ¬eld can have the values shown, to indicate the nature of
the control information contained within. Only a subset of the control procedures are
discussed here. For further information, the reader is referred to TS25.427.


6.18.1.5 Synchronization
To ensure that the network is stable and that information is distributed with correct
timing, synchronization is an important feature of the UMTS network. It is particularly
important for the downlink delivery of information to the mobile device, to minimize the
transmission delay and buffering time. The procedures for synchronization can be broken
down into a number of important areas, as discussed in the following subsection.

Value Control Frame Type
1 Outer loop power control
2 Timing adjustment
3 DL TrCH synchronisation
4 UL TrCH synchronisation
Frame CRC FT 5 DSCH TFCI
header
6 DL node synchronisation
Control Frame Type
7 UL node synchronisation
Control Information 8 Received timing deviation
9 Radio interface parameter update
...




10 Timing advance
Control Information 11-255 Not currently used

Figure 6.75 Control frame general format
6.18 FRAME PROTOCOLS 353


BTS timing derived from Reference Clock
E1 link e.g. Atomic Clock,
GPS Signal
BTS

RNC

BTS

MSC

BTS

RNC
RNC timing derived
from STM-1 link
BTS


Figure 6.76 UMTS network synchronization


Network synchronization
This involves the distribution of a common reference clock to all the nodes throughout the
system. This is performed in a hierarchical manner. An example is shown in Figure 6.76.

Node synchronization
This provides a measure of the timing differences between two nodes. For example,
BTS-RNC node synchronization enables the RNC to know what are the time differences
between itself and the BTSs connected to it. This is essential for correct delivery of
information, especially in soft handover situations. Node synchronization is achieved by
the RNC sending an FP control frame to the BTS(s) containing a sent time reference, T1.
Upon receipt, the BTS will respond, echoing T1, and supplying T2, the time the control
frame from the RNC was received, and T3, the time the BTS responded. When the RNC
receives this reply it notes the time, T4. Now the RNC can simply calculate the round
trip time (RTT) according to

RTT = (T2 ’ T1) + (T4 ’ T3)

This now allows the RNC to factor in the link delay, particularly in soft handover when
communicating simultaneously with two or three BTSs with different RTTs.

Transport channel synchronization
The role of the transport channel synchronization procedure is twofold. First, it achieves
or restores synchronization between nodes by establishing a common frame numbering
between the two. Second, it acts as a keep-alive procedure across the Iub and Iur interfaces.
Figure 6.77 shows a synchronization exchange between an SRNC and a base station.
First, the RNC sends a downlink synchronization message to the BTS. This message
contains a connection frame number (CFN) to be used for the data transfer, the format is
shown in Figure 6.78(a).
354 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM



BTS SRNC



downlink sync


uplink sync




Figure 6.77 Iub synchronization procedure


Frame CRC FT
header
UL Synchronisation

Frame CRC FT CFN
header
DL Synchronisation ToA

CFN ToA

(a) (b)

Figure 6.78 Downlink synchronization

Once the BTS has received this message, it immediately responds with an uplink syn-
chronization message, which echoes the CFN received, and also contains a time of arrival
(ToA) value (Figure 6.78(b)). This is an indication of the time difference between the
arrival time of the downlink frame and the de¬ned time of arrival window end point
(ToAWE). During the NBAP establishment phase of the radio bearer, a timing window
is de¬ned during which a frame may arrive, so that a correct timing for downlink data
transfer is established between the RNC and the BTS. This timing window is de¬ned in
terms of a time of arrival window startpoint (ToAWS) and a ToAWE.
Should a data frame arrive outside this window, either before or after, a timing adjust-
ment procedure is invoked and a message sent to indicate the ToA of the frame. This
ToA can be either a negative value, indicating a frame received after the ToAWE, or a
positive value, indicating a frame received before the ToAWE. This allows the RNC to
adjust its transfer time of data frames to keep it within the window. The ToA ¬eld is a
16-bit ¬eld that covers the range from ’1280 ms to +1279.875 ms in steps of 0.125 ms.
Figure 6.79 shows a simpli¬ed form of this in practice. The receiving window is de¬ned
between ToAWS and ToAWE. The RNC sends a frame with CFN number 30 to arrive at
the BTS within this window to provide the BTS enough time to process it and transmit
it across the air interface to reach the UE at the expected time. Beyond the ToAWE is
another value, the latest time of arrival (LToA). This is the last possible time that the
BTS can receive the frame and still have enough time to handle it. Any frames received
after LToA are discarded. The LToA is set at time Tproc before the transmit point, where
Tproc is the BTS processing time. This ¬gure will be naturally vendor dependent.

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