Spread%20spectrum%20systems%20II:%20WCDMA

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Spread spectrum systems II WCDMA

• WCDMA basic properties
• Channel mapping
• Chip sequence processing
• Soft handover
• Power control

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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

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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
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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 More flexible UL/DL capacity
allocation (in non-voice applications, DL
usually demands more capacity than UL) Channel
reciprocity (channel estimation in one direction
could be used in the other direction) No need
for duplex filter.
1. 2. 3.
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Spectrum allocation for UTRA FDD
(Europe part of Asia)
Uplink
Downlink
1920 - 1980 MHz
2110 - 2170 MHz
60 MHz
Spectrum is allocated to operators at this level
5 MHz
Chip sequnces are multiplexed in code domain and
transmitted within a 5 MHz frequency slot. The
chip rate is always 3.84 Mchips/s.
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Spreading in WCDMA
Channelization code
Scrambling code
Channel data
QPSK
Channel bit rate
Chip rate
Chip rate
(always 3.84 Mchips/s)
Usage of code
Uplink
Downlink
Channelization code
User separation
Scrambling code
User separation
Cell separation
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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 256 chips
One bit consists of 4 chips
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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
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Uplink DPCCH bit rate
SF
Channel bit rate
How many control channel bits does one time slot
contain?
256
15 kb/s
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
FBI
TPC
TFCI
Pilot
3GPP TS 25.211 Slot format 3
10
Downlink DPCH bit rate
SF
Channel bit rate (kb/s)
User data rate (kb/s)
15
approx. 1-3
512
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
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User data rate vs. channel bit rate
User data rate (kb/s)
Interesting for user
Channel coding
Interleaving
Bit rate matching
Important for system
Channel bit rate (kb/s)
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2. Channel mapping

• Issues / important concepts
• Physical channels
• Transport channels
• Logical channels
• DPDCH/DPCCH multiplexing in uplink
• DPCH user/control data multiplexing in downlink

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Logical / transport / physical channels


RLC
RLC
Logical channels
MAC
MAC
Transport channels
Phy
Lower layers
Phy
Lower layers
WCDMA
Physical channels
UE
Base station
RNC
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Logical / transport channel mapping
Uplink
Downlink
CCCH
DCCH
CTCH
BCCH
PCCH
DCCH
CCCH
DTCH
DTCH
Logical channels
Transport channels
PCH
DCH
DSCH
FACH
BCH
DCH
CPCH
RACH
Note the different possibilities for transmitting
user data over transport channels
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Transport / physical channel mapping
Uplink
Downlink
RACH
PCH
DCH
DSCH
FACH
BCH
CPCH
DCH
Transport channels
PRACH
PCPCH
SCCPCH
PCCPCH
DPDCH
DPCH
DPCCH
AICH
CSICH
Physical channels
CD/CA-ICH
SCH
CPICH
PICH
PDSCH
These channels are only for transport of
information in the physical layer at the air
(radio) interface
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DPDCH / DPCCH structure in uplink
Dual-channel QPSK modulation
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DPCH structure in downlink
QPSK modulation, time multiplexed data and
control information
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3. Chip sequence processing

• Issues / important concepts
• Spreading, scrambling, multiplexing and
  modulation
• Uplink and downlink processing somewhat
  different
• Channelization codes vs. spreading codes

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Uplink spreading
OVSF Code 1
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.
DPDCH
I branch
DPCCH
Q branch
OVSF Code 2
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.
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Uplink multiplexing
Weight 1
DPDCH
I branch

DPCCH
Q branch
Complex-valued signal I jQ
j
Weight 2
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 IjQ.
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Uplink scrambling
Scrambling code
ReS
The complex signal IjQ is multiplied by the
complex-valued, UE specific scrambling code.

ImS
I jQ
Complex-valued signal S
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 (gt m-sequence or Gold
codes).
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Uplink modulation
cos(?t)
ReS
Pulse shaping
To RF part and UE antenna

Pulse shaping
ImS
-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.
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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.
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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.
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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.
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Downlink multiplexing
Weight n
ReS
?
ImS
Other downlink physical channels
Complex-valued signal S
SCH 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.
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Downlink multiplexing
Weight n
ReS
?
ImS
Other downlink physical channels
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).
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Downlink modulation
cos(?t)
ReS
Pulse shaping
?
To RF part and base station antenna

Pulse shaping
ImS
-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.
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Synchronous / non-s. chip sequences
Chip Sequence encoded bit/symbol
Two synchronous chip sequences
Two non-synchronous chip sequences
Chips
Sequences start here
One sequence starts here
Sequences end here
Another sequence starts here
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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)
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4. Soft handover

• Issues / important concepts
• Serving RNC, Drift RNC, SRNS Relocation
• Micro/macrodiversity combining
• Soft handover in uplink
• Soft handover in downlink

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Serving RNC and Drift RNC in UTRAN
SRNC
Core network
Iu
Iub
BS
RNC
UE
Iur
Iub
BS
RNC
DRNC

Concept needed for Soft handover between base
stations belonging to different RNCs
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SRNS Relocation
SRNC
Core network
Iu
Iub
BS
RNC
UE
Iur
Iub
BS
RNC
Iu
DRNC
SRNC
SRNC provides 1) connection to core network
2) macrodiversity combining point
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Micro- / macrodiversity combining
(uplink)
SRNC
Iu
Iub
BS
RNC
Core network
Iur
Macrodiversity combining point in SRNC
RNC
Rake receiver
UE
Iub
DRNC
Multipath propagation
BS
Microdiversity combining point in base station
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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
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Microdiversity combining, soft handover
(downlink)
Soft handover same signal is transmitted via
several base stations Advantage number of
multipath components is increased Draw-back in
downlink, soft handover decreases capacity
Rake receiver
BS
UE
BS
Different code sequences
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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

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Why is power control needed?
Near-far problem arises in uplink when all UEs
use the same transmit power
BS
UE
Weak signal will be drowned
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.
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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 kth user is
Ps . SF
?
SIR
(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.

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Analysis of uplink SIR expression
Ps . SF
?
SIR
(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).
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Analysis of uplink SIR expression
Ps . SF
?
SIR
(N-1).Ps Pn
SF Spreading Factor. We assume here that the
power of the desired signal (of kth 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.
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Analysis of uplink SIR expression
Ps . SF
?
SIR
(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)
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Analysis of uplink SIR expression
The SIR for the kth 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
Target SIR
gt
(N-1).Ps Pn
First, we see that the best case is when Ps gtgt 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.
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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 kth
user, etc.
BER
High BER means low target SIR
Low BER means high target SIR
SIR
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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 estimates the average path loss in downlink ...
BS
UE
... and adjusts the uplink transmission power
accordingly
(Note uplink / downlink fading in UTRA FDD is
not the same)
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Inner loop power control
Inner loop power control (also called fast power
control) is used both in uplink (shown in this
figure) and downlink.
BS
UE
UE transmits initial signal
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
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Outer loop power control
Outer loop power control is used both in uplink
(shown in this figure) and downlink.
BS
UE
RNC
Is signal quality (BER) ok?
If not, increase or decrease Target SIR
Inner loop power control uses new Target SIR value