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with a code of SF = 128. Please note that this is a simpli¬ed form of the process and is
indicative only.
If there is more than one transport channel, the rate matching process will consider all
the blocks to be concatenated into a CCTrCH, and rate match appropriately such that the
resulting CCTrCH is of an appropriate bit length.
It may seem a waste of valuable resources to perform such bit addition. However,
the repetition of bits does make the data more robust, thus requiring less transmission
power.



6.8.4 Spreading
The spreading process takes the CCTrCH in each radio frame, spreads it to the system
chip rate of 3.84 Mcps and transmits it across the air at the same frequency and time as all
the other common channels and currently active users. Each of these must be separated
from each other. To provide this signal separation, two types of code are used in the
WCDMA system. These are scrambling and channelization codes.
292 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


244 bits


TB


244 bits 8 bits


TB CRC


504 bits


1/2 Rate Convolution Coding


600 bits


Rate Matching


300 bits 300 bits


Radio Frame Radio Frame


10ms 10ms

Figure 6.20 Rate matching process (simpli¬ed)

Scrambling code
In the downlink the scrambling code (or long code) is used to identify a particular
cell. Each cell generally uses a single primary code. For greater capacity more than
one scrambling code may be used in a cell. This scrambling code is not used in any
neighbouring cells and identi¬es the cell within the vicinity. The codes can be reused
in another geographical area as long as it is not within neighbouring cells. In the uplink
the scrambling code is used to identify the particular user since each mobile device will
have its own allocated scrambling code. This code allocation for the mobile device is
performed by the RNC.
The use of a code to spread a signal over a wide bandwidth should result in a signal
which without knowing the code is only interpreted as noise. In fact, the scrambling codes
are Gold codes, as described in Chapter 2, where the codes are generated using two 25-
stage shift registers. Both the BTS and the UE are required to have this mechanism. Since
these codes are near orthogonal, if the receiver does not know the scrambling code used
for transmission then the data transmitted will simply be received as extra noise. In the
3G WCDMA context, many signals are sent out from different mobile devices at the same
time, i.e. a cell contains many mobile devices which will transmit simultaneously, over
the same wide frequency band, to the base station. The base station needs to know how
6.8 WCDMA PHYSICAL LAYER 293


to distinguish each of these transmissions from the next and therefore each of the mobile
devices needs to use a different scrambling code. Each of these coded signals introduces
additional noise into the system and this noise will reduce the capacity of the cell. The
higher the data rate, the more noise they will introduce.
In the downlink, the shift register can generate 218 ’ 1 codes, truncated to a length
of one 10 ms radio frame, so that each code is 38 400 chips long. However, only the
¬rst 8192 codes are used, and these 8192 codes are broken down into 512 groups each
consisting of a primary code and 15 secondary codes. Each of these will have a separate
channelization code tree available. The limiting of codes to 512 groups is to reduce the
number of codes that the mobile device needs to check through when it ¬rst enters a cell
and thus speed up the cell search procedure. Currently in most UMTS implementations,
only the primary code is used. In the uplink, there are 224 possible codes that may be
used. However, there is also an option to use simpler short codes which are only 256
chips long. These may be used to simplify the design of multi-user receivers at the BTS.


Channelization code
Since there is only one scrambling code in a cell in the downlink, user data and system
information still need to be separated. This is the job of the channelization code. Each
channel will represent a different mobile device or different control channel such as the
BCCH. In the uplink the different DPDCHs and the DPCCH for a single mobile device are
identi¬ed by their different channelization codes. These codes are also known as Walsh
codes or orthogonal codes, as explained in Chapter 2.
It can be seen from Figure 6.21 that a number of different data streams (these may
be user data or control information) each have their own channelization code. Channels
with different data rates will have different length channelization codes, ranging from the
lowest data rates at SF = 256 to the highest at SF = 4. These channels are combined
in the adder and mixed with a single scrambling code. In the downlink the scrambling


Channelisation
Code 1


Data A

Channelisation
Code 2 Scrambling Code
A
d
Data B Combined Scrambled A
d Transmitted
Data Data d
e Data
d
Channelisation r
e
Code 3 SCH
r

Data C


Figure 6.21 Downlink code usage in WCDMA
294 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


code does not include coverage of the synchronization channel (SCH), which has its own
system-wide code.
For high data rates, a single subscriber can use a maximum of three out of four of the
possible codes at SF = 4. In this case, the data is split and transferred over all three codes
in parallel. The fourth code may not be used in its entirety since some codes are reserved
for transmitting common channel information.
The usage of the different codes is summarized in Table 6.8 for both the uplink and
downlink.
Figure 6.22 illustrates how the different codes are used within the WCDMA system.
It is usual that cells 1 and 2 will be transmitting and receiving on the same frequency.
BS1 transmits a single signal which includes the channelization code for user 1 (Ccu1)
and the channelization code for user 2 (Ccu2), both of which are uni¬ed under a single
scrambling code (Csbs1). In the downlink for cell 2 the same situation occurs with the
exception that the scrambling code (Csbs2) must be different to that of cell 1. However,
the same channelization codes which are used in cell 1 can also be used in cell 2. In
the uplink each of the mobile devices is identi¬ed by its scrambling code (Csu1..4). The
mobile devices can all use the same channelization codes since these are concealed by
the unique scrambling code.

Table 6.8 Code usage table
Channelization code Scrambling code

Uplink Separation of physical data and control channels Separation of users
from one user
Downlink Separation of dedicated user channels Separation of cells
Code length Variable according to the required data rate Fixed at one radio frame (10 ms
or 38 400 chips)




Csbs1 Ccu1 +
Ccu1 + Csbs2
Cc + C
su1 su3
Cc + C
User 1 User 1
Ccu
sbs2
2+
2+C
C
Cc + sbs1 Ccu 4
Csu
Csu
2
c+
C



User 2 User 2
BS1 BS2

Cell 1 Cell 2

Figure 6.22 Example code usage
6.8 WCDMA PHYSICAL LAYER 295


6.8.5 Modulation and transmission

UMTS de¬nes the use of quadrature phase shift keying (QPSK) modulation for the air
interface. With a QPSK modulation scheme, the complex signal that results from the
spreading function is split by a serial to parallel converter into a real and an imaginary
branch, each of which is multiplied with an oscillator signal. However, the imaginary
branch is 90—¦ out of phase with the real branch. When summed, the resulting signal
can have four possible phase angles, each of which represents two data bits. Figure 6.23
illustrates the general principle.
QPSK modulation is speci¬ed for use in both the uplink and the downlink; however,
the use of the QPSK modulation scheme does present some dif¬culties in the uplink.
Consider that the ampli¬er is at a maximum output power and needs to change its signal
by 180—¦ . This consumes a considerable amount of power in the ampli¬er to retain the
linearity of the signal, particularly across such a wide frequency band, and most of this
power ends up wasted as heat. This is not so dif¬cult a problem at the BTS, but is quite
impractical at the UE, where cost, power consumption, battery life and heat dissipation are
all signi¬cant issues. A common solution to this problem is to use offset quadrative phase
shift keying (QPSK) in the uplink instead. With offset QPSK, there is a delay introduced
into the imaginary branch to offset the phase shifting of this branch relative to the real
branch. The result is that when a 180—¦ phase shift is required, the shift is performed in
two steps of 90—¦ .
QPSK modulation provides a one-to-one relationship between the bit rate of an unmodu-
lated signal and the symbol rate after modulation. In practice, this means that a 3.84 Mcps
spread signal entering the modulator will emerge as a 3.84 MHz signal.
In the course of the modulation process, pulse shaping is also performed. WCDMA
uses a root-raised cosine ¬lter with a roll-off of 0.22. A modulated signal with this roll-
off, plus the provision of a guard band between neighbouring frequencies, equates to the
5 MHz of spectrum allocated per WCDMA carrier (Figure 6.24). For frequencies licensed
to the same operator, there can be less than 5 MHz spacing between carriers. However,
the centre frequency must lie on a 200 kHz raster. This means that if, for example, the


cos (ωt )


Real
Serial to
Complex
Parallel
Spread RF out
Converter
Signal
Imaginary



-sin (ωt )

Figure 6.23 QPSK modulation
296 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM




3.84MHz


roll-off




centre
frequency
e.g. 2112.2MHz freq
5MHz

Figure 6.24 5 MHz frequency band

operator had been allocated 2010“2015 MHz downlink channel, the centre frequency
could be 2012.4 MHz or 2012.6 MHz.


6.8.6 Common channels
The primary common control physical channel (P-CCPCH) is the channel that carries the
system broadcast information. Since all users must be able to decode this channel, it is
given a ¬xed channelization code of SF = 256, which is a system constant. The broadcast
information is transmitted across two radio frames, and then repeats.

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