17
UMTS standard: WCDMA/FDD
Layer 1
17.1 TRANSPORT CHANNELS AND PHYSICAL
CHANNELS (FDD)
17.1.1 Transport channels
In the terminology used in wireless communications, ‘transport channels’ are the services
offered by Layer 1 to the higher layers. The purpose of this section is to introduce basic
terminology and abbreviations used in practice. For more details, see www.3gpp.org.We
start with the definition of the channels.
• Dedicated transport channel
DCH – Dedicated Channel DCH is a downlink or uplink transport channel that is used
to carry user or control information between the network and mobile station.
DTCH – Dedicated Traffic Channel.
SDCCH – Stand-Alone Dedicated Control Channel.
ACCH – Associated Control Channel defined within ITU-R M.1035.
The DCH is transmitted over the entire cell or over only a part of the cell using
lobe-forming antennas.
• Common transport channels
BCCH – Broadcast Control Channel. BCCH is a downlink transport channel that is used
to broadcast system and cell-specific information. The BCCH is always transmitted over
the entire cell.
FACH – Forward Access Channel. FACH is a downlink transport channel that is used
to carry control information to a mobile station when the system knows the location cell
of the mobile station. The FACH may also carry short user packets. The FACH is trans-
mitted over the entire cell or over only a part of the cell using lobe-forming antennas.
PCH – Paging Channel. PCH is a downlink transport channel that is used to carry
control information to a mobile station when the system does not know the location
cell of the mobile station. The PCH is always transmitted over the entire cell.
Adaptive WCDMA: Theory And Practice.
Savo G. Glisic
Copyright
¶ 2003 John Wiley & Sons, Ltd.
ISBN: 0-470-84825-1
592 UMTS STANDARD: WCDMA/FDD LAYER 1
RACH – Random-Access Channel. RACH is an uplink transport channel that is used
to carry control information from a mobile station.
The RACH may also carry short user packets. The RACH is always received from the
entire cell.
17.1.2 Physical channels
• The physical resources
The basic physical resource is the code/frequency plane.
On the uplink, different information streams may be transmitted on the I and Q branches.
Consequently, a physical channel corresponds to a specific carrier frequency, code, and,
on the uplink, relative phase (0 or 90
◦
).
• Uplink physical channels
Dedicated uplink physical channels.
Uplink DPDCH – The uplink Dedicated Physical Data Channel.
Uplink DPCCH – The uplink Dedicated Physical Control Channel.
There may be zero, one, or several uplink DPDCHs on each Layer 1 connection. The
Layer 1 control information consists of known pilot bits to support channel estimation
for coherent detection, transmit power-control (TPC) commands, an optional transport-
format indicator (TFI) and feedback information (FBI) (interference level). The TFI
informs the receiver about the instantaneous parameters of the different transport chan-
nels multiplexed on the uplink DPDCH. There is one and only one uplink DPCCH on
each Layer 1 connection.
Frame structure
Figure 17.1 shows the frame structure of the uplink DPCHs. Each frame of length
10 ms is split into 15 slots, each of length T
slot
= 0.666 ms, corresponding to one power-
control period.
A super frame corresponds to 72 consecutive frames, that is, the super frame length
is 720 ms.
The Spreading Factor (SF) may range from 256 down to 4. An uplink DPDCH and
uplink DPCCH on the same Layer 1 connection are generally of different rates, that is,
they have different SFs. Multicode operation is possible for the uplink DPCHs. When mul-
ticode transmission is used, several parallel (6) DPDCHs are transmitted using different
channelization codes. There is only one DPCCH per connection.
Common uplink physical channels
PRACH – Physical Random-Access Channel. PRACH is used to carry RACH. It is based
on a Slotted ALOHA. The different time offsets called access slots are used for trans-
mission of access bursts as shown in Figure 17.2.
Information on the access slots available in the current cell is broadcast on the BCCH.
The structure of the Random-Access burst is shown in Figure 17.3.
TRANSPORT CHANNELS AND PHYSICAL CHANNELS (FDD) 593
T
f
= 10 ms
Data,
N
data
bits
Pilot,
N
pilot
bits TPC
0.666 ms,
T
super
= 720 ms
FBITFCI
Frame #1 Frame #2 Frame #i Frame #72
DPDCH
DPCCH
Slot #1 Slot #2 Slot #i Slot #15
Figure 17.1 Frame structure for uplink DPDCH/DPCCH.
Random-access burst
Random-access burst
Random-access burst
Random-access burst
Offset of access slot #
i
Frame boundary
2 slots
Access slot #1
Access slot #2
Access slot #
i
Access slot #8
Figure 17.2 Access slots.
594 UMTS STANDARD: WCDMA/FDD LAYER 1
Random access burst
Preamble part
Message part
10 ms2 slots
Figure 17.3 Structure of the random-access burst.
• Preamble part
The preamble part of the random-access burst consists of a signature of length 16 com-
plex symbols (±1 ±j). Each preamble symbol is spread with a 256-chip real Orthogonal
Gold code. There are a total of 16 different signatures, based on the Orthogonal Gold
code set of length 16.
• Message part
The message part of the random-access burst has the same structure as the uplink
Dedicated physical channel (DPCH). It consists of a data part, corresponding to the
uplink DPDCH, and a Layer 1 control part, corresponding to the uplink DPCCH (see
Figure 17.4). The data and control parts are transmitted in parallel. The data part carries
the random access request or small user packets. The SF of the data part is limited to
SF ∈{256, 128, 64, 32} corresponding to channel bit rates. The control part carries
pilot bits and rate information, using a SF of 256. The rate information indicates
which channelization code (or rather the SF of the channelization code) is used on the
data part.
Random-access burst
It consists of the following fields (see Figure 17.5):
• Mobile station identification (MS ID) [16 bits]. The MS ID is chosen at random by the
mobile station at the time of each Random-Access attempt.
Data part
Pilot symbols
Rate information
10 ms
I
Q
Figure 17.4 The message part of the random-access burst.
MS ID Req. Ser. Optional user packet CRC
Figure 17.5 Structure of random-access burst data part.
TRANSPORT CHANNELS AND PHYSICAL CHANNELS (FDD) 595
• Required Service [3 bits]. This field informs the Base Station (BS) what type of service
is required (short packet transmission, DCH set-up and so on).
• An optional user packet.
• A cyclic redundancy check (CRC) to detect errors in the data part of the Random-Access
burst [8 bits].
Downlink physical channels
DPCH – Dedicated physical channels. Figure 17.6 shows the frame structure of the down-
link DPCH.
In the case of multicode transmission, the slot format is shown in Figure 17.7.
Frame #1 Frame #2 Frame #
i
Frame #72
Data Data
TPC
0.666 ms (2560 chips)
Slot #1 Slot #2 Slot #
i
Slot #15
T
f
= 10 ms
T
super
= 720 ms
Pilot
DPCCH DPCCHDPDCH DPCCH DPDCH
TFCI
Figure 17.6 Frame structure for downlink DPCH.
DPCCH DPDCH
Transmission
power
Transmission
power
Transmission
power
Physical channel 1
Physical channel 2
Physical channel
L
One slot (0.666 ms)
Figure 17.7 Downlink slot format in the case of multicode transmission.
596 UMTS STANDARD: WCDMA/FDD LAYER 1
Common physical channels
CCPCH – Primary Common Control Physical Channel. The Primary CCPCH is a fixed
rate (30 kbps, SF = 256) downlink physical channel used to carry the BCCH.
Figures 17.8 and 17.9 show the frame structure of the primary and the secondary com-
mon control physical channels, respectively. The frame structure differs from the downlink
DPCH in that no transmit power control (TPC) command or TFI is transmitted. The only
Layer 1 control information is the common pilot bits needed for coherent detection.
The secondary CCPCH is used to carry the FACH and PCH. It is of constant rate. In
contrast to the primary CCPCH, the rate may be different for different secondary CCPCH
within one cell and between cells, in order to be able to allocate different amounts of
FACH and PCH capacity to a cell.
The rate and SF of each secondary CCPCH is broadcast on the BCCH. The set of
possible rates is the same as for the downlink DPCH. The FACH and PCH are mapped
256 chips
pilot
2304 chips data
Frame #1 Frame #2 Frame #
i
Frame #72
0.666 ms
Slot #1 Slot #2 Slot #
i
Slot #15
T
f
= 10 ms
T
super
= 720 ms
Figure 17.8 Frame structure for primary common control physical channel.
Frame #1 Frame #2 Frame #
i
Frame #72
Data
TPC RI
0.666 ms
Slot #1 Slot #2 Slot #
i
Slot #15
T
f
= 10
T
super
= 720 ms
Pilot
N
pilot
bits
N
TPC
bits
N
RI
bits
N
data
DPCCH DPDCH
Figure 17.9 Frame structure for secondary common control physical channel.
TRANSPORT CHANNELS AND PHYSICAL CHANNELS (FDD) 597
to separate secondary CCPCHs. The main difference between a CCPCH and a downlink
DPCH is that a CCPCH is not power controlled. The main difference between the primary
and secondary CCPCH is that the primary CCPCH has a fixed predefined rate while the
secondary CCPCH has a constant rate that may be different for different cells, depending
on the capacity needed for FACH and PCH. A Primary CCPCH is continuously transmit-
ted over the entire cell while a secondary CCPCH is only transmitted when there is data
available and may be transmitted in a narrow lobe in the same way as a DPCH (only
valid for a secondary CCPCH carrying the FACH).
17.1.3 Synchronization channel (SCH)
SCH is a downlink signal used for cell search. The SCH consists of two subchannels, the
primary and secondary SCH. Details are given in Chapter 3.
17.1.4 Mapping of transport channels to physical channels
Figure 17.10 summarizes the mapping of a subset of transport channels to physical channels.
For additional details like high-speed downlink shared channel (HSDSCH) and related
control channels see www.3qpp.org.
17.1.5 Mapping of PCH to secondary common control physical channel
The mapping method is shown in Figure 17.11. The PCH is divided into several groups
in one super frame, and Layer 3 information is transmitted in each group. Each group
of PCH shall have information amount worth four slots and consists of a total of six
information parts:
– Two Paging Indication (PI) parts – for indicating whether there are MS-terminated
calls,
– Four Mobile User Identifier (MUI) parts – for indicating the identity of the paged
mobile user.
Transport
channels
Physical channels
BCCH
FACH
PCH
RACH
DCH
Primary Common Control Physical
Channel (Primary CCPCH)
Secondary Common Control Physical
Channel (Secondary CCPCH)
Physical Random Access Channel
(PRACH)
Dedicated Physical Data Channel
(DPDCH)
Synchronisation Channel (SCH)
Figure 17.10 Transport channel to physical channel mapping.
598 UMTS STANDARD: WCDMA/FDD LAYER 1
Frame (10 ms)
MUI
2
MUI
3
MUI
4
Superframe (720 ms)
PCH PCH PCH PCH PCH
Slot (0.265 ms)
PI1 PI2 MUI1 PI1 PI2 MUI1 PI1 PI2 MUI1
PCH information
for Group #1
PCH information
for Group #2
PCH information
for Group #3
MUI
2
MUI
3
MUI
4
MUI
2
MUI
3
MUI
4
Figure 17.11 PCH mapping method.
In each group, PI parts are transmitted ahead of MUI parts. In all groups, 6 information
parts are allocated with a certain pattern in the range of 24 slots. By shifting each pattern
by 4 slots, multiple 288 groups of PCH can be allocated on one Secondary Common
Physical Channel.
17.2 MULTIPLEXING, CHANNEL CODING
AND INTERLEAVING
Multiplexing of transport channels, coding and interleaving for a frequency division
duplexing (FDD) system is illustrated in Figure 17.12.
17.2.1 Channel coding
Channel coding is done on a per-transport-channel basis, that is, before transport channel
multiplexing. The following options are available (see Figure 17.13):
• Convolutional coding
• Outer Reed–Solomon coding + Outer interleaving + Convolutional coding
• Turbo coding
• Service-specific coding, for example, unequal error protection (UEP) for some types of
speech codecs
17.2.2 Convolutional coding
Generator polynomials for the convolutional codes are given below.
MULTIPLEXING, CHANNEL CODING AND INTERLEAVING 599
Rate Constraint Generator Generator Generator
length Polynomial 1 Polynomial 2 Polynomial 3
1/3 9 557 663 711
1/2 9 561 753 NA
Rate 1/3 convolutional coding is applied to dedicated transport channels (DCHs) in normal
(nonslotted) mode.
Rate 1/2 convolutional coding is applied to DCHs in slotted mode.
TC TC TC TC
Coding +
interleaving
Rate matching
Interleaving
Rate
matching
Interleaving
(optional)
Coding +
interleaving
Rate matching
Interleaving
(optional)
Multiplex
Channel coding +
optional TC multiplex
Static rate matching
Inner interleaving
(interframe)
Transport channel
multiplexing
Dynamic rate matching
(uplink only)
Inner interleaving
(intraframe)
Figure 17.12 Coding and multiplexing of transport channels.
Reed–Solomon
coding
10
−3
10
−6
High data rates
Convolutional
coding
Convolutional
coding
Outer
interleaving
Turbo
coding
Service-specific
coding
Figure 17.13 Channel coding in UTRA/FDD.
600 UMTS STANDARD: WCDMA/FDD LAYER 1
Info
bits
Interleaver
Constituent
encoder #
Constituent
encoder #2
Parity
bits
Parity
bits
Puncture
Figure 17.14 Block diagram of a turbo code encoder.
17.2.3 Other types of coding
• Outer Reed–Solomon coding and outer interleaving
The RS-coding is of approximate rate 4/5 using the 256-ary alphabet. The outer inter-
leaving is a symbol-based block interleaver with width equal to the block length of the
RS code. The interleaver span is variable and can be 10, 20, 40 or 80 ms.
• Turbo coding
The turbo coding is used for high-data rate (above 32 kbps), high-quality services.
Turbo code of rate 1/3 and 1/2 (for the highest data rates) have replaced the concate-
nation of convolutional and RS codes. The block diagram for the basic turbo encoder
is shown in Figure 17.14.
• Service specific coding
Additional coding schemes, in addition to the standard coding schemes listed above,
can be used. One example is the use of unequal error-protection coding schemes for
certain speech codecs.
17.3 SPREADING AND MODULATION
17.3.1 Uplink spreading and modulation
The block diagram of uplink spreading and modulation is shown in Figure 17.15.
For multicode transmission, each additional uplink DPDCH may be transmitted on
either the I or the Q branch. For each branch, each additional uplink DPDCH should be
assigned its own channelization code. Uplink DPDCHs on different branches may share
a common channelization code. The spreading and modulation of the message part of
the random-access burst is basically the same as for the uplink DPCHs, in Figure 17.15
in which the uplink DPDCH and uplink DPCCH are replaced by the data part and the
control part, respectively. The scrambling code for the message part is chosen on the
basis of the base-station-specific preamble code.
17.3.2 Channelization codes
Orthogonal Variable Spreading Factor (OVSF) codes, described in Chapter 2, are used
for channelization. All codes within the code tree cannot be used simultaneously by one
mobile station.
SPREADING AND MODULATION 601
C
D
,
C
C
: channelization codes
I
Q
I +
j
Q
DPDCH
DPCCH
C
D
C
C
c′
scramb
c′
scramb
: primary scrambling code
c′′
scramb
: secondary scrambling code (optional)
c′
scramb
(optional)
cos(w
t
)
sin(w
t
)
Imag
Channelization
codes (OVSF)
Real
p
(
t
) : pulse-shaping filter (root raised cosine, roll off 0.22)
p
(
t
)
p
(
t
)
∗
j
Figure 17.15 Spreading/modulation for uplink DPDCH/DPCCH.
A mobile station can use a code if and only if the same mobile station uses no other
code on the path from the specific code to the root of the tree or in the subtree below the
specific code. This means that the number of available channelization codes is not fixed
but depends on the rate and SF of each physical channel. Each connection is allocated at
least one uplink channelization code to be used for the uplink DPCCH. In most cases, at
least one additional uplink channelization code is allocated for an uplink DPDCH.
Further uplink channelization codes may be allocated if more than one uplink DPDCH
is required. All channelization codes used for the DPDCHs must be orthogonal to the
code used for the DPCCH.
As different mobile stations use different uplink scrambling codes, the uplink chan-
nelization codes may be allocated with no coordination between different connections.
The uplink channelization codes are therefore always allocated in a predefined order. The
mobile station and network only need to agree on the number and length (SF) of the
uplink channelization codes. The exact codes to be used are then implicitly given.
17.3.3 Scrambling codes
Either short or long scrambling codes should be used on the uplink. The short scrambling
code is typically used in cells where the BS is equipped with an advanced receiver, such
as a multiuser detector or an interference canceler.
With the short scrambling code, the cross-correlation properties between different phys-
ical channels and users does not vary in time in the same way as when a long code is
used. This means that the cross-correlation matrices used in the advanced receiver do not
have to be updated as often as for the long scrambling code case, thereby reducing the
complexity of the receiver implementation.
In cells where there is no gain in implementation complexity using the short scrambling
code, the long code is used instead because of its better interference averaging properties.
For the details of scrambling code construction, see www.3gpp.org.
602 UMTS STANDARD: WCDMA/FDD LAYER 1
These scrambling codes are designed such that at N − 1 out of N consecutive chip
times, they produce +/−90
◦
rotations of the In phase + Quadrature (IQ) multiplexed data
and control channels. At the remaining 1 out of N chip times, they produce 0, +/−90
◦
or 180
◦
rotations.
This limits the transitions of the complex baseband signal that is inputted to the root-
raised cosine pulse-shaping filter. This in turn reduces the peak to average ratio of the
signal at the filter output, allowing a more efficient power-amplifier implementation.
To guarantee these desirable properties, restrictions on the choice of uplink OVSF
codes are also required.
Short scrambling code
For code construction details, see www.3qpp.org. The network decides the uplink short
scrambling code. The mobile station is informed about which short scrambling code to
use in the downlink Access Grant message, which is the base station response to an uplink
Random-Access Request. The short scrambling code may, in rare cases, be changed during
the duration of a connection.
Long scrambling code
The long uplink scrambling code is typically used in cells without multiuser detection
(MUD) in the BS. The mobile station is informed if a long scrambling code should
be used in the Access Grant Message following a Random-Access Request and in the
handover message.
Which long scrambling code to use is directly given by the short scrambling code. No
explicit allocation of the long scrambling code is thus needed.
Modulation
• Modulating chip rate
The modulating chip rate is 3.84 Mcps. This basic chip rate can be extended to 2 ×
3.84 Mcps or 4 × 3.84 Mcps
• Pulse shaping
The pulse-shaping filters are root-raised cosine (RRC) with roll-off α = 0.22 in the
frequency domain
• Data modulation
For data, quadrature phase shift keying (QPSK) modulation is used. Phase transition
restrictions are introduced by the scrambling code design.
17.3.4 Downlink spreading and m odulation
Data modulation is QPSK, where each pair of two bits are serial-to-parallel converted
and mapped to the I and the Q branches, respectively.
The I and Q branch are then spread to the chip rate with the same channelization code
c
ch
(real spreading) and then scrambled by the same cell-specific scrambling code c
scramb
SPREADING AND MODULATION 603
DPDCH/
DPCCH
S P
I
c
ch (real)
p
(
t
)
cos(w
t
)
sin(w
t
)
c
ch
: channelization code,
p
(
t
)
c
scramb (complex)
c
scramb
: scrambling code
p
(
t
) : pulse-shaping filter (root raised cosine, roll-off 0.22)
Q
Figure 17.16 Spreading/modulation for downlink DPCH.
(complex scrambling). The different physical channels use different channelization codes,
while the scrambling code is the same for all physical channels in one cell. The system
block diagram is shown in Figure 17.16.
The multiplexing of the synchro channel (SCH)
The SCH is only transmitted intermittently (one code word per slot). The SCH is
multiplexed after the long code scrambling of the DPCH and CCPCH, as shown in
Figure 17.17. Consequently, the SCH is nonorthogonal to the other downlink physical
channels.
For code construction, based on Golay codes, see www.3gpp.org.
Modulation
• Modulating chip rate
The modulating chip rate is 3.84 Mcps. This basic chip rate can be extended to 2 ×
3.84 Mcps or 4 × 3.84 Mcps
0
c
p
SCH
0
1
d
j
To I Q
modulator
c
scramb
c
s
DPDCH/DPCCH
& CCPCH
Lower position during
256 chips per slot
c
ch, N
c
ch, 1
Σ
Σ
Σ
Figure 17.17 Multiplexing of SCH.
604 UMTS STANDARD: WCDMA/FDD LAYER 1
0
−2−4−6−8−10
246810
Frequency (MHz)
−10
−20
−30
−40
−50
−60
−70
0
Power density (dB/Hz) relative to the carrier
Transmitter masks
MS mask
BS mask
Figure 17.18 Assumed spectrum masks.
• Pulse shaping
The pulse-shaping filters are root-raised cosine (RRC) with roll-off α = 0.22 in the
frequency domain
• Modulation
For data, QPSK modulation is used
17.3.5 Output RF spectrum emissions
Out-of-band emissions are specified in Figure 17.18.
• Spurious emissions
The limits for spurious emissions at frequencies greater than ±250% of the necessary
bandwidth would be based on the applicable tables from the ITU-R recommendation
SM.329.
17.4 PHYSICAL LAYER PROCEDURES (FDD)
17.4.1 Uplink power control
• Closed-loop power control
The BS should estimate the received uplink DPCCH power after the RAKE combining
of the connection to be power controlled. Simultaneously, the BS should estimate the
total uplink received interference in the current frequency band and generate an SIR
estimate – SIR
est
. The BS then generates TPC commands. The algorithms are described
in Chapter 6. Upon reception of a TPC command, the mobile station should adjust the
transmit power of both the uplink DPCCH and the uplink DPDCH in the given direction
with a step of
TPC
dB. The step size
TPC
is a parameter that may differ between
different cells, in the region of 0.25 to 1.5 dB.
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