1. Spread spectrum systems II: WCDMA
WCDMA basic properties
Channel mapping
Chip sequence processing
Soft handover
Power control

2. 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

3. 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

4. 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.

5. Spectrum allocation for UTRA FDD
(Europe & part of Asia)
Uplink
Downlink
MHz
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.

6. Spreading in WCDMA Usage of code Uplink Downlink 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

7. Spreading in WCDMA SF = Spreading factor
Chip rate = SF x channel bit rate
Uplink: DPCCH SF = 256, DPDCH SF =
Downlink: DPCH SF = (512)
One bit consists
of 4 chips
One bit consists
of 256 chips

8. Channel bit rate (kb/s)
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

9. How many control channel bits does one time slot contain?
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:
Pilot
TFCI
FBI
TPC
3GPP TS Slot format 3

10. Channel bit rate (kb/s)
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 64
120
approx. 45 32
240
approx. 105 16
480
approx. 215 8
960
approx. 456 4
1920
approx. 936

11. 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)

12. 2. Channel mapping Issues / important concepts: Physical channels
Transport channels
Logical channels
DPDCH/DPCCH multiplexing in uplink
DPCH user/control data multiplexing in downlink

13. Logical / transport / physical channels: :
RLC
RLC
Logical channels
MAC
MAC
Transport channels
Phy
Phy
Lower layers
Lower layers
WCDMA
Physical channels UE
Base station
RNC

14. Logical / transport channel mapping
Uplink
Downlink
CCCH
DCCH
PCCH
BCCH
CCCH
CTCH
DCCH
DTCH
DTCH
Logical channels
Transport channels
RACH
CPCH
DCH
PCH
BCH
FACH
DSCH
DCH
Note the different possibilities for transmitting user data over transport channels

15. Transport / physical channel mapping
Uplink
Downlink
RACH
CPCH
DCH
PCH
FACH
BCH
DSCH
DCH
Transport channels
PRACH
PCPCH
DPDCH
SCCPCH
PCCPCH
DPCH
AICH
CSICH
DPCCH
Physical channels
CD/CA -ICH
PICH
CPICH
SCH
PDSCH
These channels are only for transport of information in the physical layer at the air (radio) interface

16. DPDCH / DPCCH structure in uplink
Dual-channel QPSK modulation:
Data
Pilot
TFCI
FBI
TPC
DPDCH (I-branch)
10 ms radio frame (38400 chips) 1 2 14
Time slot containing 2560 chips
DPCCH (Q-branch)

17. DPCH structure in downlink
QPSK modulation,
time multiplexed data and control information:
TFCI
Data
TPC 1 2 14
Pilot
Time slot containing 2560 chips
10 ms radio frame (38400 chips)

18. 3. Chip sequence processing
Issues / important concepts:
Spreading, scrambling, multiplexing and modulation
Uplink and downlink processing somewhat different
Channelization codes vs. spreading codes

19. 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.

20. Complex-valued signal I + jQ
Uplink multiplexing
Weight 1
DPDCH
I branch +
DPCCH
Q branch
Complex-valued signal I + jQ
Weight 2 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.

21. Complex-valued signal S
Uplink scrambling
Scrambling code
Re{S}
The complex signal I+jQ is multiplied by the complex-valued, UE specific scrambling code. +
Im{S}
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 (=> m-sequence or Gold codes).

22. To RF part and UE antenna
Uplink modulation
cos(t)
Re{S}
Pulse shaping
To RF part and UE antenna +
Pulse shaping
Im{S}
-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.

23. At the receiver side
At the transmitter side, signal formats and processing details are standardised (see 3GPP TS ).
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.

24. Complex-valued signal I + jQ
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.

25. 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.

26. Downlink multiplexing
Weight n
Re{S} 
Im{S}
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.

27. Downlink multiplexing
Weight n
Re{S} 
Im{S}
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 ).

28. To RF part and base station antenna
Downlink modulation
cos(t)
Re{S}
Pulse shaping 
To RF part and base station antenna +
Pulse shaping
Im{S}
-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.

29. 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

30. 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)

31. 4. Soft handover Issues / important concepts:
Serving RNC, Drift RNC, SRNS Relocation
Micro/macrodiversity combining
Soft handover in uplink
Soft handover in downlink

32. Serving RNC and Drift RNC in UTRAN
SRNC
Core network BS
Iub
RNC Iu UE
Iur BS
Iub
RNC
DRNC
Concept needed for:
Soft handover between base stations belonging to different RNCs

33. SRNS Relocation SRNC provides: 1) connection to core networkBS
Iub
RNC Iu UE
Iur BS
Iub
RNC Iu
DRNC
SRNC
SRNC provides: 1) connection to core network
2) macrodiversity combining point

34. Micro- / macrodiversity combining
(uplink)
SRNC BS
Iub
RNC Iu
Core network
Iur
Macrodiversity combining point in SRNC
Rake receiver
RNC UE
Iub
DRNC
Multipath propagation BS
Microdiversity combining point in base station

35. 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

36. 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
Different code sequences BS

37. 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

38. Why is power control needed?
Near-far problem arises in uplink when all UEs use the same transmit power: UE BS
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.

39. Uplink SIR expression SIR 
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
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.

40. 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).

41. 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 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.

42. 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)

43. 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
>
Target SIR
(N-1).Ps + Pn
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.

44. 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

45. 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)

46. 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 BS
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

47. 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