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previously (see Figure 6.30). This is expected to be transmitted in all 15 slots within a


normal mode compressed mode



transmission
data data
gap
data

10ms radio frame 10ms radio frame 10ms radio frame

Figure 6.37 Compressed mode
6.11 DOWNLINK TRANSMIT DIVERSITY TECHNIQUES 307


12 bits 4 4 52 bits 8




TFCI
TPC
Data 1 Data 2 Pilot

Slot Format 9B

Figure 6.38 Downlink compressed mode slot format 9B

radio frame. If compressed mode using SF reduction was indicated, slot format 9B would
then be used (Figure 6.38), but only sent in 8“14 slots of the frame.
Slot format 9 uses SF = 128, channel bit rate = 60 kbps, whereas 9B uses SF = 64,
channel bit rate = 120 kbps, and the respective ¬elds within the slots are exactly doubled.



6.11 DOWNLINK TRANSMIT DIVERSITY TECHNIQUES
Both open loop and closed loop transmitter diversity techniques can be used at the BTS to
improve the quality of the radio interface. The open loop modes are space time transmit
diversity (STTD), time switched transmit diversity (TSTD) and site selection diversity
transmit (SSDT). The simultaneous use of these techniques on the same physical channels
is not allowed. There are also two modes of operation in the closed loop system. All of
these are described below.


6.11.1 Space time transmit diversity (STTD)
Figure 6.39(a) illustrates how STTD functions. The symbols to be transmitted are grouped
into blocks of four, b0“b3. Antenna 1 transmits these symbols in the same order and in
the same polarity as they arrive. Antenna 2 modi¬es the order prior to transmission and
also modi¬es the polarity of symbols 1 and 2. This method of open loop transmit diversity
can be used on all channels except for SCH. The UE will then regenerate the original
signal, now having two sources to use. Support for this open loop method of transmit
diversity is mandatory in the mobile device and optional within UTRAN.


6.11.2 Time switched transmit diversity (TSTD)
TSTD is only used on the synchronization channel (SCH), and functions by alternating
which of the two antennas will transmit the SCH. If TSTD is utilized, one antenna
(antenna 1) will transmit both the primary and secondary SCH in even slots and the
other antenna (antenna 2) will transmit both the primary and secondary SCH in odd slots.
When an antenna is not transmitting the SCH channels, its transmitter will be switched
off, thus reducing interference. In a situation where TSTD is not used, a single antenna
will transmit the primary and secondary SCH on both the even and odd slots. Although
TSTD support is mandatory in the mobile device, it is again optional to use this method
within UTRAN.
308 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM




DPCCH




2
Tx




DP
b3




b1
na
DC
An




ten
H
b2




b0
ten




BTS1 &
An UE DP
na




CC Primary
b1




b3
Tx

H
1




Cell
H
b0




b2
C
PC
D
BTS3
b0 b1 b2 b3
symbols BS BTS2

(a) (b)

Figure 6.39 (a) STTD and (b) SSDT


6.11.3 Site selection diversity transmit (SSDT)
SSDT is a form of power control that can be used on the downlink while a mobile device
is in the soft handover mode of operation. The UE will select the best cell in its active set
on the basis of the quality of the CPICH channel. This is then deemed to be the primary
cell, and is used to transmit the DPDCH. All of the other cells in the active set will now
only transmit the DPCCH, so as to enable the mobile device to perform measurements
and maintain synchronization. Each of the cells is given a temporary identi¬er with the
primary cell referred to as the primary id. This identi¬er is transmitted on the uplink
DPCCH to each BTS in the active set. The cell that has been selected as the primary cell
will then begin transmitting at a suf¬cient power to maintain the desired SIR target. The
primary cell can be periodically changed by the UE. This period is set by the UTRAN
and can be 5, 10 or 20 ms. To enable SSDT requires each BTS involved to support
this mode of operation, and its use is speci¬ed during the channel establishment phase.
Simulations have demonstrated that SSDT is particularly suited to mobile subscribers who
are moving at slow speed, where capacity gains of up to 50% can be achieved (TS25.922
Appendix D). As the speed of the user increases, the ef¬ciencies diminish due to the
limited frequency of update of the primary cell identi¬er.
Figure 6.39(b) illustrates the concept of SSDT. In the diagram, BTS3 is the primary cell
and as such transmits both the data channel (DPDCH) and the control channel (DPCCH).
The other two base stations simply transmit the control channel. On the uplink, the mobile
device will transmit both the data and control channels, which will be received by all of
the base stations and fed to the SRNC.


6.11.4 Closed loop mode transmit diversity
Figure 6.40 shows a general architecture for the closed loop mode. The DPCH channel
is fed to both antennas for transmission on the downlink. There are actually two modes
6.12 RADIO INTERFACE PROTOCOL ARCHITECTURE 309


W1 Antenna-1
CPICH-1


DPDCH
DPCH
Antenna-2
DPCCH

UE
CPICH-1/2
Spreading and W2
scrambling
Weight
CPCCH: FBI information
Generator
W1 & W2

Figure 6.40 Closed loop transmit diversity

of closed loop, mode 1 where different CPICH channels are used and mode 2 where the
same CPICH is fed to both of the summation units. The mobile device uses the CPICH
it receives to estimate the adjustments that should be made to the transmitted signal and
this information is fed back to the BTS. The value of the weight factors (w1 and w2 ) are
determined through use of this feedback via the FBI bits from the mobile device on the
DPCCH. Antenna-1 is used as a reference and in mode 1, the signal transmitted from
antenna-2 is phase shifted from that of antenna-1. In mode 2 the signal transmitted from
antenna-2 is phase shifted and also modi¬ed in amplitude.



6.12 RADIO INTERFACE PROTOCOL ARCHITECTURE
The radio interface is a layered structure, which forms the access stratum connection
between the mobile devices and the RNC. It has three layers and roughly follows the OSI
model, consisting of physical, datalink and network layers. The physical layer, shaded in
Figure 6.41, consists of WCDMA and ATM. The datalink layer is comprised of a number
of sublayers: the media access control (MAC), radio link control (RLC), packet data con-
vergence protocol (PDCP) and the broadcast/multicast control (BMC) (see Figure 6.41).
The PDCP layer is shown only half-way across the protocol stack. This is because it is
only used for packet data such as IP, between the UE and the SRNC. The PDCP provides
mechanisms for upper-layer header compression.
The network layer, layer three, is not shown but consists of the NAS signalling2 between
the UE and the core network.
The user plane is complemented by a control plane, shown in Figure 6.42. As can be
seen, the control plane also uses the MAC and RLC layers. However, it has an additional
protocol, considered to reside at the lower part of layer three, known as the radio resource

2
NAS signalling consists of mobility management (MM), connection management (CM) and session
management (SM).
310 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


IP voice IP voice
packets CODEC packets CODEC

PDCP PDCP

RLC RLC

MAC MAC

FP FP FP FP
AAL2 AAL2 AAL2 AAL2
WCDMA WCDMA
ATM ATM ATM ATM
L1 L1 L1 L1
UE SRNC
DRNC
Node B

Figure 6.41 Radio network user plane protocol stack


signalling connection
RRC RRC


RLC RLC


MAC MAC

FP FP FP FP
AAL2 AAL2 AAL2 AAL2
WCDMA WCDMA
ATM ATM ATM ATM
L1 L1 L1 L1

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