Spread spectrum
systems II: WCDMA
• WCDMA basic properties
• Channel mapping
• Chip sequence processing
• Soft handover
• Power control
1. WCDMA basic properties
Issues / important concepts:
• Two duplex alternatives: UTRA FDD vs. UTRA TDD
• Spectrum allocation (UTRA FDD)
• Spreading in WCDMA
• DPDCH/DPCCH/DPCH channel bit rates
Two duplex alternatives: FDD vs. TDD
In UTRA FDD (Frequency Division Duplex), uplink
and downlink are separated in frequency domain:
UL
DL
frequency
In UTRA TDD (Time Division Duplex), uplink and
downlink are separated in time domain:
...
UL
DL
UL
DL
...
time
Two duplex alternatives: FDD vs. TDD
UTRA FDD will be more widely used in the near future,
since UTRA TDD technology is more complex.
However, UTRA TDD offers some benefits:
1. More flexible UL/DL capacity allocation
(in non-voice applications, DL usually demands
more capacity than UL)
2. Channel reciprocity (channel estimation in one
direction could be used in the other direction)
3. No need for duplex filter.
Spectrum allocation for UTRA FDD
(Europe & part of Asia)
Uplink
Downlink
1920 - 1980 MHz
2110 - 2170 MHz
60 MHz
5 MHz
Spectrum is
allocated to
operators at
this level
Chip sequnces are multiplexed in code domain
and transmitted within a 5 MHz frequency slot.
The chip rate is always 3.84 Mchips/s.
Spreading in WCDMA
Channelization
code
Channel
data
Scrambling
code
QPSK
Channel
bit rate
Usage of code
Chip rate
Chip rate
(always 3.84 Mchips/s)
Uplink
Channelization code
Scrambling code
Downlink
User separation
User separation
Cell separation
Spreading in WCDMA
SF = Spreading factor
Chip rate = SF x channel bit rate
Uplink: DPCCH SF = 256, DPDCH SF = 4 - 256
Downlink: DPCH SF = 4 - 256 (512)
One bit consists
of 4 chips
One bit consists
of 256 chips
Uplink DPDCH bit rates
SF
Channel bit rate
(kb/s)
User data rate
(kb/s)
256
15
approx. 7.5
128
30
approx. 15
64
60
approx. 30
32
120
approx. 60
16
240
approx. 120
8
480
approx. 240
4
960
approx. 480
Uplink DPCCH bit rate
SF
Channel bit rate
256
15 kb/s
How many control
channel bits does one
time slot contain?
Each 10 ms radio frame (38400 chips long) is divided
into 15 time slots (2560 chips long).
Since SF = 256, each time slot contains 10 control
channel bits that can be used, for example, like this:
Pilot
TFCI
3GPP TS 25.211 Slot format 3
FBI
TPC
Downlink DPCH bit rate
SF
Channel bit rate
(kb/s)
User data rate
(kb/s)
512
15
approx. 1-3
256
30
approx. 6-12
128
60
approx. 20-24
64
120
approx. 45
32
240
approx. 105
16
480
approx. 215
8
960
approx. 456
4
1920
approx. 936
User data rate vs. channel bit rate
User data rate (kb/s)
Interesting
for user
Channel coding
Interleaving
Bit rate matching
Channel bit rate (kb/s)
Important
for system
2. Channel mapping
Issues / important concepts:
• Physical channels
• Transport channels
• Logical channels
• DPDCH/DPCCH multiplexing in uplink
• DPCH user/control data multiplexing in downlink
Logical / transport / physical channels
:
:
RLC
RLC
Logical channels
MAC
MAC
Transport channels
Phy
WCDMA
Phy
Lower
layers
Lower
layers
Physical channels
UE
Base station
RNC
Logical / transport channel mapping
Uplink
CCCH
Downlink
DCCH
PCCH
BCCH
CCCH
CTCH
DTCH
DCCH
DTCH
Logical channels
Transport channels
RACH
CPCH
DCH
PCH
BCH
FACH
DSCH
DCH
Note the different possibilities for transmitting
user data over transport channels
Transport / physical channel mapping
Uplink
RACH
CPCH
Downlink
DCH
PCH
FACH
BCH
DSCH
DCH
Transport channels
PRACH
PCPCH
DPDCH
AICH
CSICH
DPCCH
PICH
CD/CA
-ICH
SCCPCH
PCCPCH
Physical channels
CPICH
SCH
PDSCH
These channels are only for transport of information in
the physical layer at the air (radio) interface
DPCH
DPDCH / DPCCH structure in uplink
Dual-channel QPSK modulation:
Time slot containing 2560 chips
DPDCH (I-branch)
Data
Pilot
TFCI
FBI
TPC
DPCCH (Q-branch)
0
1
2
14
10 ms radio frame (38400 chips)
DPCH structure in downlink
QPSK modulation,
time multiplexed data and control information:
Time slot containing 2560 chips
TFCI
0
1
Data
TPC
Data
2
Pilot
14
10 ms radio frame (38400 chips)
3. Chip sequence processing
Issues / important concepts:
• Spreading, scrambling, multiplexing and modulation
• Uplink and downlink processing somewhat different
• Channelization codes vs. spreading codes
Uplink spreading
OVSF Code 1
DPDCH
I branch
DPCCH
Q branch
OVSF Code 2
In the UE, the user data
(DPDCH) and control data
(DPCCH) signals are spread
to the chip frequency of
3.84 Mchips/s using
different channelisation
codes, also called OVSF
(Orthogonal Variable
Spreading Factor) codes.
The DPCCH is spread on the Q-branch using SF = 256.
In case of very high user bit rates, up to six DPDCH
channels can be used in parallel by distributing the signals
to the I and Q branches using additional OVSF codes.
Uplink multiplexing
Weight 1
DPDCH
I branch
+
DPCCH
Q branch
Weight 2
Complex-valued
signal I + jQ
j
The DPCCH signal and DPDCH signal (or up to 6 DPDCH
signals) are synchronously combined, i.e. ”multiplexed in
code domain”, to form the complex signal I+jQ.
Uplink scrambling
Scrambling code
Re{S}
+
I + jQ
Im{S}
Complex-valued
signal S
The complex signal
I+jQ is multiplied
by the complexvalued, UE specific
scrambling code.
After scrambling, signals from different UEs can be separated
at the base station, since each UE uses a different scrambling
code.
Scrambling codes must have good correlation properties even
when not synchronized (=> m-sequence or Gold codes).
Uplink modulation
cos(t)
Re{S}
Pulse
shaping
+
Im{S}
Pulse
shaping
To RF part
and
UE antenna
-sin(t)
The real and imaginary parts of the scrambled signal S are
fed to the I and Q branches of the modulator and are
modulated by sinusoids with a 90-degree phase shift to
achieve the desired QPSK modulation.
The QPSK signal is transmitted from the UE antenna.
At the receiver side
At the transmitter side, signal formats and processing
details are standardised (see 3GPP TS 25.213).
At the receiver side, base station manufacturers are free to
implement any receiver structure they wish.
In general terms, the code processing is in the reverse
order (demodulation, despreading, demultiplexing ...) and
makes use of a Rake receiver able to resolve and despread
separate multipath replicas of the transmitted signal.
Channel estimation and phase synchronisation is based on
pilot bits transmitted in the DPCCH signal.
Downlink spreading
OVSF Code n
Any downlink
physical
channel
except SCH
S/P
+
Complex-valued
signal I + jQ
OVSF Code n
j
Serial/parallel conversion is applied to two consecutive
channel bits. The bits in the I and Q branches are then
spread using the same OVSF (channelisation) code.
Downlink scrambling
Scrambling code
+
The spreaded, complex-valued signal is chipwise multiplied
with a complex-valued scrambling code.
Scrambling codes are selected from a base station specific
code set. A scrambling code can be shared among several
physical channels.
Adjacent base stations use different (sets of) scrambling
codes.
Downlink multiplexing
Weight n
Re{S}

Other downlink
physical channels
SCH signal(s)
Im{S}
Complex-valued
signal S
Before the spreaded and scrambled physical channels are
”multiplexed in code domain”, signal powers are adjusted to
the appropriate levels determined by the downlink closed
loop power control (on a channel-by-channel basis).
Note that all channels are multiplexed synchronously.
Downlink multiplexing
Weight n
Re{S}

Other downlink
physical channels
Im{S}
SCH signal(s)
Synchronisation channels (SCH) are spread using special
code sequences (i.e. no OVSF codes are involved).
SCH is first multiplexed with CCPCH in time domain. The
composite signal is then added to the other channels in code
domain (see 3GPP TS 25.211).
Downlink modulation
cos(t)
Re{S}

Pulse
shaping
+
Im{S}
Pulse
shaping
To RF part
and
base station
antenna
-sin(t)
The real and imaginary parts of the multiplex signal S are
fed to the QPSK modulator like in uplink.
Note that this signal contains information for many UEs.
Synchronous / non-s. chip sequences
Chip Sequence = encoded bit/symbol
Two synchronous
chip sequences
Two non-synchronous
chip sequences
Chips
Sequences start here
Sequences end here
One sequence starts here
Another sequence starts here
Synchronous / non-s. chip sequences
Different effect
on different
types of codes:
Synchronous
chip
sequences
Non-synchronous
chip
sequences
Channelization
(Hadamard-Walsh)
codes
No interference
(sequences are all
orthogonal)
Large interference
Scrambling
(m-sequence, Gold)
codes
Little interference
(sequences are
near orthogonal)
Little interference
(sequences are
near orthogonal)
4. Soft handover
Issues / important concepts:
• Serving RNC, Drift RNC, SRNS Relocation
• Micro/macrodiversity combining
• Soft handover in uplink
• Soft handover in downlink
Serving RNC and Drift RNC in UTRAN
SRNC
BS
Iub
UE
RNC
Iur
BS
Iub
Iu
Core
network
RNC
DRNC
Concept needed for:
Soft handover between base stations belonging to different RNCs
SRNS Relocation
SRNC
BS
Iub
UE
RNC
Iu
Core
network
Iur
BS
Iub
RNC
Iu
DRNC
SRNC
SRNC provides: 1) connection to core network
2) macrodiversity combining point
Micro- / macrodiversity combining
(uplink)
SRNC
BS
Iub
RNC
Iu
Core
network
Iur
Rake
receiver
UE
Multipath
propagation
Iub
RNC
DRNC
Macrodiversity
combining point
in SRNC
BS
Microdiversity combining point in base station
Diversity combining, soft handover
(uplink)
Microdiversity combining: multipath signal
components are processed in Rake branches and
combined (MRC = Maximum Ratio Combining)
Macrodiversity combining: bit sequences carrying
the same signal (but with different bit error
positions) are either combined at SRNC (bit-by-bit
majority voting), or best quality signal is selected.
Hard handover: slow, complex signalling
Soft handover: fast selection in SRNC is possible
due to macrodiversity combining
Microdiversity combining, soft handover
(downlink)
Rake
receiver
BS
Advantage: number of
multipath components is
increased
UE
Different
code
sequences
Soft handover: same
signal is transmitted via
several base stations
BS
Draw-back: in downlink,
soft handover decreases
capacity
5. Power control
Issues / important concepts:
• Near-far problem
• Uplink SIR expression; what means Target SIR?
• Open loop power control
• Inner loop (closed loop) power control
• Outer loop (closed loop) power control
Why is power control needed?
Near-far problem arises in uplink when all UEs use the
same transmit power:
UE
Weak signal will be
drowned
BS
UE
Strong signal
dominates
Rather, UEs should adjust their transmit power levels
so that the received power levels are approximately the
same at the base station.
Uplink SIR expression
In uplink, in case of same received power levels (and
ignoring interference from UEs located in other cells)
the signal-to-interference ratio for the k:th user is
SIR 
Ps . SF
(N-1).Ps + Pn
This simple rule-of-thumb expression
• is useful for estimating capacity in uplink
• is the basis of admission control in uplink
• explains “Target SIR” used in power control.
Analysis of uplink SIR expression
SIR 
Ps . SF
(N-1).Ps + Pn
Signal-to-interference ratio (SIR) is a very important
parameter in a DS-CDMA system.
SIR describes the situation after despreading in the
CDMA receiver.
The corresponding ratio before despreading is called
CIR (carrier-to-interference ratio).
Analysis of uplink SIR expression
SIR 
Ps . SF
(N-1).Ps + Pn
SF = Spreading Factor.
We assume here that the power of the desired signal
(of k:th user) after despreading is SF times the power
of interfering signal (of another user) after despreading
if the powers before despreading are the same.
In other words, this is a crude model for estimating the
level of cross-correlation in the CDMA receiver.
Analysis of uplink SIR expression
SIR 
Ps . SF
(N-1).Ps + Pn
It is assumed that the received signal power (before
despreading) of all N active users in the cell is Ps.
Pn is the thermal noise power in the receiver.
(In case there are users with different bit rates - and
thus different spreading factors - this expression must
obviously be modified)
Analysis of uplink SIR expression
The SIR for the k:th user must be larger than a certain
value, Target SIR. In other words, the total interference
in the system must remain below a certain target level.
Ps . SF
(N-1).Ps + Pn
> Target SIR
First, we see that the best case is when Ps >> Pn . In
other words, CDMA systems are interference limited,
not noise limited.
Second, the inequality above is valid for values of N up
to a maximum value Nmax , the capacity of the cell.
Analysis of uplink SIR expression
The target SIR value depends on various issues, such
as required Bit Error Ratio (BER) or Frame Error Ratio
(FER), user/channel bit rate of k:th user, etc.
BER
High BER means
low target SIR
Low BER means
high target SIR
SIR
Open loop power control
This simple and inaccurate power control scheme must be
used during the random access process at the beginning of
a connection until more accurate control information is
available.
UE
UE estimates the average
path loss in downlink ...
BS
... and adjusts the uplink
transmission power accordingly
(Note: uplink / downlink fading in UTRA FDD is not the same)
Inner loop power control
Inner loop power control (also called fast power control) is
used both in uplink (shown in this figure) and downlink.
UE
UE transmits
initial signal
BS
Is measured SIR
larger (smaller)
than Target SIR?
If answer is yes: decrease (increase) power
UE decreases (increases) transmit power
This loop is performed 1500 times per second
Outer loop power control
Outer loop power control is used both in uplink (shown in
this figure) and downlink.
UE
BS
RNC
If not, increase
or decrease
Target SIR
Is signal
quality
(BER) ok?
Inner loop power control
uses new Target SIR value