1. WCDMA Air Interface Training Part 2 CDMA Power Control, RAKE Receiver, and Soft Handover

2. CDMA Reception Issues
Unequal received power levels degrade SSMA performance
Near-Far Ratio, terrain, RF obstacles, “Turn-the-Corner” effects, ...
Multipath fading cancellation
Time of Arrival delay spread

3. Effective Power Control Required
Near-far Problem
Path Loss
Fading

4. The Power Control Solution
Open loop
Fast closed loop

5. CDMA Power Control Open-Loop Power Control
Compute Initial Transmit Power
Measure received power from BS
Read BS transmit power from Broadcast Channel
Transmit Access Preamble
Access Acknowledged?
Increase Transmit Power by 1 dB No
Yes
MS Begins Uplink TCH Transmission
Outer-Loop (slow) Power Control
Inner-Loop (fast) Power Control
FER Acceptable?
Raise Rx Power Target
Lower Rx Power Target No
Yes
Received power > target?
Increase MS Transmit Power by 1 dB
Decrease MS Transmit Power by 1 dB

6. CDMA Power Control BS Receive Power time BS Receive Power Target
Inner-loop power Control (Initial Receive Power Target)
Inner-loop power Control (Updated Receive Power Target)
Open-loop Power Control Access Preambles
800 updates/sec (IS-95, cdma2000) 1500 updates/sec (WCDMA)
time
BS Receive Power Target

7. Multipath Fading Fast (Rayleigh) Fading
Time between fades is related to
RF frequency
Geometry of multipath vectors
Vehicle speed: Up to 2 fades/sec per kilometer/hour
Fast (Rayleigh) Fading
Composite Received Signal Strength
msec
Deep fade caused by destructive summation of two or more multipath reflections
time (mSec)

8. Why Spread Spectrum ? f Channel Quality f Channel Quality
Frequency selective fading - Frequency Diversity
Interference Averaging
Forward Error Correction f
Channel
Quality f
Channel
Quality
In FDMA/TDMA some users on a certain frequency suffer more from fading than other users on other frequencies.
Interference averaging is when you can filter out narrow band interference and other wideband interferers.
The coding gives us the possibility of Forward Error Correction (FEC). It could be described as adding extra information bits wchich immediately show what the correct bits should be.
In spread spectrum techniques al users will suffer the same from fading problems.
Frequency diversity in WCDMA is accomplished with spread spectrum technique.

9. t t Radio Environment h() Multipath Propagation Time Dispersion 1 2 32 3 t
h() 1 2 3
Multipath propagation is usually a problem.
Signals reach the receiver at different time, time dispersion.
In WCDMA the RAKE receiver benefits from the multipath propagation.

10. Delay Profiles a. Impulse Response b. Coherence Windows Narrow band
Time Delay
Power (dB)
b. Coherence Windows
Narrow band
Broad band

11. The RAKE Receiver CDMA Mobile Station RAKE Receiver Architecture
Each finger tracks a single multipath reflection
Also be used to track other base station’s signal during soft handover
One finger used as a “Searcher” to identify other base stations
Finger #1
Combiner
Sum of individual multipath components
Finger #2
Finger #N
Power measurement of Neighboring Base Stations
Searcher Finger

12. The RAKE Receiver The CDMA Pilot Channel
All CDMA standards include a full-time “Pilot” Channel
Broadcast by the Base Station
Unique to each cell (or sector)
Serves as precision coherent phase reference for Downlink channels
Equivalent to a continuous-loop transmission of the Cell’s PN code
‘I’ PN Code
I/Q
Modulator
FIR Filter
Data
All 0’s
Pilot Channel Output
FIR Filter
Orthogonal Code 0
‘Q’ PN Code

13. CDMA RAKE Receiver Architecture
Carrier Frequency Tracking Loop
Rake Receiver “Finger”
cos(2fIFt)
“I” PN Code (+1/-1)
bit rate = chip rate / SF
I/Q
Demod D
Integrate over ‘SF’ chips 
De- Interleave Data
Viterbi/ Turbo Decoder
Decoded Output Bits
BPF
LPF  D
Orthogonal Code (+1/-1)
cos(2fRFt)
“Q” PN Code (+1/-1)
CRC Verification
Timing Adj.
Correlator
Error Indication
Pilot Orthogonal Code (all zeros)
Other Rake Receiver Finger

14. The RAKE Receiver  Composite Received Signal time To Viterbi Decoder50
100
150
200
250
300
350
400 -2 2 4 6 8 10 12 14 16 18 2 3 1
time
To Viterbi Decoder 50
100
150
200
250
300
350
400 -2 2 4 6 8 10 12 14 16 18 2 3 3
+ Interference 1
1/2-chip delay
Correlator Ai 50
100
150
200
250
300
350
400 -2 2 4 6 8 10 12 14 16 18 2 3 2
+ Interference 1
1/2-chip delay 
Correlator Ai
The 1/2-chip delays shown in this diagram have "..." symbols between them, indicating that any arbitrary number of 1/2-chip delays may need to be used. In other words, the receiver selects the delayed multipath signal in any multiple of 1/2-chip which is needed to equalize the delays of each multipath ray which is to be tracked.
Note: The reason you need 1/2-chip steps is that if you had 1-chip steps, you may not be able to center the delay of the received signal to within +/- 1/2 chip of the receiver's internal PN generator. And if the received signal is not centered to within +/- 1/2 chip, it will be reduced to interference rather than correlated as signal.
Also, the range of the delay line is typically 50 to 100 usec in most RAKE receivers, in steps of 1/2 chip.
(ML=Maximum Likelyhood, which is the same as Maximum Ratio combining) 50
100
150
200
250
300
350
400 -2 2 4 6 8 10 12 14 16 18 2 3 1
+ Interference 1
1/2-chip delay
Correlator Ai
PN, Walsh Codes
Equal Combining, ML Combining, or Select Strongest

15. Handover Inter-System Handover Hard Handover Soft Handover
Handover from a CDMA system to an Analog or TDMA system
Traffic and Control Channels are Disconnected and must be Reconnected
Hard Handover
When the MS must change CDMA carrier frequency during the Handover
Soft Handover
Unique to CDMA
During Handover, the MS has concurrent traffic connections with two BS’s
Handover should be less noticeable
Softer Handover
Similar to Soft Handover, but between two sectors of the same cell
Handover is simplified since timing sectors have identical timing

16. Soft Handover

17. CDMA Without Soft Handover
MS responding to BS1 power control bits
MS responding to BS2 power control bits
BS1 Receive Power Target
time
BS2 Receive Power Target
time
Trouble zone: Prior to Hard Handover, the MS causes excessive interference to BS2

18. Monitor Neighbor BS Pilots
CDMA Soft Handover
CDMA Soft Handover
One finger of the RAKE receiver is constantly scanning neighboring Pilot Channels.
When a neighboring Pilot Channel reaches the t_add threshold, the new BS is added to the active set
When the original Base Station reaches the t_drop threshold, originating Base Station is dropped from the active set
Monitor Neighbor BS Pilots
Add Destination BS
Drop Originating BS

19. Soft Handover Add/Drop Thresholds
Soft Handover Measurement and Decision
Eb / N0 t t t
Cell 1
T_DROP
T_ADD
T_ADD
Cell 2
Cell 3
time
Cell 1 Connected
Add Cell 2
Drop Cell 1 Add Cell 3
Drop Cell 3

20. CDMA With Soft Handover
MS responds to power control commands from both BS1 and BS2
MS responding to BS1 power control commands
BS1 BS2 Action 0 0 Reduce power 0 1 Reduce power 1 0 Reduce power 1 1 Increase power
MS responding to BS2 power control commands
BS1 Receive Power Target 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
time
BS2 Receive Power Target 2 2 2 2 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
time

21. CDMA Soft Handover Key points to remember about Soft Handover
SSMA used to distinguish all transmitters in a Cellular CDMA system
Fast power control is required to sustain SSMA performance
When fast power control is used, soft handover is essential
Allows MS to operate in most conservative power control mode
Soft handover provides performance benefits
“Seamless” coverage at cell fringes
Handover may be less noticeable to the user
Increases apparent system capacity when system is lightly loaded
Soft handover also degrades system capacity
Uses redundant physical layer resources from adjacent or overlapping cells