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WCDMA Air Interface Training
Part 2CDMA Power Control,
RAKE Receiver,and Soft Handover
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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
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Effective Power Control Required
• Near-far ProblemNear-far Problem• Path LossPath Loss• FadingFading
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The Power Control Solution
• Power ControlPower Control• Open loopOpen loop• Fast closed loopFast closed loop
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CDMA Power ControlOpen-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 BeginsUplink TCH
Transmission
Outer-Loop (slow) Power Control Inner-Loop (fast) Power Control
FER Acceptable
?
Raise RxPower Target
Lower RxPower Target
No
Yes
Received power
> target?
Increase MS Transmit Power
by 1 dB
Decrease MS Transmit Power
by 1 dB
No
Yes
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CDMA Power Control
Inner-loop power Control(Initial Receive Power Target)
BS Receive Power Target
Open-loop Power ControlAccess Preambles
Inner-loop power Control(Updated Receive Power Target)
BS Receive Power
time800 updates/sec (IS-95, cdma2000)1500 updates/sec (WCDMA)
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Multipath Fading
• Fast (Rayleigh) Fading
time (mSec)
CompositeReceived
Signal Strength
Time between fades is related to
• RF frequency
• Geometry of multipath vectors
• Vehicle speed: Up to 2 fades/sec per kilometer/hour
Deep fade caused by destructive summation of two or more multipath reflections
msec
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Frequency selective fading - Frequency Diversity
Interference Averaging
Forward Error Correction
f
ChannelQuality
f
ChannelQuality
Why Spread Spectrum ?
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Time DispersionMultipath Propagation
10
2
3
h()
10 2 3
Radio Environment
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Narrow band
Broad band
Delay Profiles
b. Coherence Windowsa. Impulse Response
Time Delay
Pow
er
(dB
)
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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
Finger #2
Finger #N
Searcher Finger
Combiner
Sum ofindividual multipath components
Power measurement of Neighboring Base Stations
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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
Pilot Channel Output
FIR Filter
FIR Filter
I/Q
Modulator
‘ I’ PN Code
‘ Q’ PN
Code
Orthogonal Code 0
Data
All 0’s
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BPF LPF
“ I” PNCode
(+1/-1)
“ Q” PNCode
(+1/-1)
Orthogonal
Code(+1/-1)
Integrateover
‘SF’ chips
De-Interleave
Data
Viterbi/Turbo
Decoder
CRCVerification
DecodedOutputBits
ErrorIndication
cos(2fRFt)
PilotOrthogonal
Code(all zeros)
TimingAdj.
bit rate = chip rate / SF
cos(2fIFt)
CarrierFrequencyTrackingLoop
Other Rake Receiver Finger
Rake Receiver “Finger”
D
D
I/Q
Demod
Correlator
CDMA RAKE Receiver Architecture
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0 50 100 150 200 250 300 350 400-2
0
2
4
6
8
10
12
14
16
18
0 50 100 150 200 250 300 350 400-2
0
2
4
6
8
10
12
14
16
18
0 50 100 150 200 250 300 350 400-2
0
2
4
6
8
10
12
14
16
18
The RAKE Receiver
1/2-chip delay
To Viterbi Decoder
Composite Received Signal
PN, Walsh Codes
1/2-chip delay
1/2-chip delay
Ai
Ai
Ai
Correlator
Correlator
Correlator
Equal Combining, ML Combining,
or Select Strongest
time
0 50 100 150 200 250 300 350 400-2
0
2
4
6
8
10
12
14
16
18
1
23
1
23
1
23
1
23 1
2
3 + Interference
+ Interference
+ Interference
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Handover
• Inter-System 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
Traffic and Control Channels are Disconnected and must be Reconnected
• 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
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Soft Handover
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CDMA Without Soft Handover
time
Trouble zone: Prior to Hard Handover, the MS causes excessive interference to BS2
BS2 Receive Power Target
MS responding to BS1power control bits
MS responding to BS2power control bits
time
BS1 Receive Power Target
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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
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Soft Handover Add/Drop Thresholds
• Soft Handover Measurement and Decision
Cell 1 Connected
Add Cell 2 Drop Cell 1Add Cell 3
time
Drop Cell 3
Eb / N0
Cell 1
T_DROP
Cell 2
Cell 3
T_ADD T_ADD
t t t
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CDMA With Soft Handover
time
BS2 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 2 2 2 2 2 2 1 1 1 12 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
MS responding to BS1power control commands
MS responding to BS2power control commands
time
BS1 Receive Power Target
1 1 11 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 12 2 2 2 2 2
1 1 12 2 2 2 2 2 2 2 2 2 2 2 2
2 2 2 2
BS1 BS2 Action0 0 Reduce power0 1 Reduce power1 0 Reduce power1 1 Increase power
MS responds to power control commandsfrom both BS1 and BS2
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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
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