A Novel Finger Assignment Algorithm for RAKE Receivers in CDMA Systems
Mohamed Abou-Khousa
Department of Electrical and Computer Engineering, Concordia University, Montreal, QC, Canada
Mohamed El-Tarhuni
Department of Electrical and Engineering, American University of Sharjah, Sharjah, UAE
Ali Ghrayeb
Department of Electrical and Computer Engineering, Concordia University, Montreal, QC, Canada
Outline
Scope
Introduction
Proposed FAA
Simulation Results
Conclusions
Scope
Finger assignment is very crucial for CDMA RAKE receiver performance
Conventional schemes use average signal strength criterion
We propose a new finger assignment
strategy based on signal-to-interference
ratio
Introduction
Most third generation (3G) wireless networks use CDMA
Improving the performance of CDMA systems is very crucial for successful deployment
Wideband transmission using PN spreading codes
Channel appears frequency selective with many paths to detect
A RAKE receiver is essential to exploit multipath diversity
The RAKE consists of several correlators (fingers)
Fingers are time aligned with “Potential” paths
After despreading, outputs of fingers are properly combined (MRC)
Conventional FAA
Finding “potential” paths to be combined impacts the overall performance of the receiver
Finger Assignment Algorithm (FAA)
Conventionally, paths with maximum signal strength are assigned to fingers
Works well if interference seen at all paths is the same
This condition does not hold always, hence the conventional solution is suboptimal
Proposed FAA
A better FAA should consider SIR rather than signal strength alone
Practically, paths with maximum SIR should be combined
Problem: we need to estimate the SIR per path
Channel estimates
PN code Finger
delays
Combiner Output
SIR estimates Search
results Received
Signal RAKE
Receiver
Searcher
Per path SIR
Estimator
FAA
System Model and Per Path SIR Estimation
Received base-band signal:
The searcher: an acquisition circuit that provides correlation results over a window of delays. Searcher output:
For a given delay, many search results are used
The signal-to-interference per path is estimated as
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k n E n I n n N
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(n E 1 1 n I n
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(
k b
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SIR
NA
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FAA Steps
• In order for a path to be detected, it should have an SIR value that exceeds the threshold
• In order for a path to be assigned, its SIR value should be one of the L maximum values among all the detected paths
The FAA steps are:
Perform a search over the window of possible delays
Accumulate search results over a period of time
Estimate the SIR per path for every delay
Assign fingers to the paths with maximum SIR that exceed a certain threshold for a given probability of False Alarm
If none of the delays in the current search window has an SIR value that passes the threshold, assign a detected path from the previous frame
Otherwise, assign the delay with maximum SIR from the current search window
The above steps are repeated for every frame of received data
Simulation Results
Two-user CDMA system
Random spreading codes with spreading factor of 128
frequency-selective Rayleigh fading channel
4 paths of relative delays 0, 2, 4, and 6 chips
Uniform PDP
Normalized Doppler frequency: 0.01
Frame duration: 20 ms
Search window: 32 offsets
Search step: ½ chip
Number of Accumulations: 13
False Alarm Probability: 1%
Pilot channel gain compared to data channel: 0 dB
The same channel estimation algorithm is implemented for both
schemes (Moving average filter over the despread values for the whole frame period)
Bit Error Rate: P
fa=1%
Probability of Detection First Path
0 2 4 6 8 10 12 14 16
10-2 10-1
100 Probability of False Alarm = 0.01
Eb/No dB Probability of Detection (P D)
Conventional SIR=-10 dB Conventional SIR=0 dB Conventional SIR=10 dB Proposed SIR=-10 dB Proposed SIR=0 dB Proposed SIR=10 dB
Probability of Detection All Four Paths
0 2 4 6 8 10 12 14 16
10-4 10-3 10-2 10-1 100
E /N dB Probability of Detection (P D)
Probability of False Alarm = 0.01
Conventional SIR=-10 dB Conventional SIR=0 dB Conventional SIR=10 dB Proposed SIR=-10 dB Proposed SIR=0 dB Proposed SIR=10 dB
Receiver Operational Characteristics (ROC)
10-4 10-3 10-2 10-1
10-1 100
SNR = 12 dB
False Alarm Probability
Probability of Detection
Conventional SIR=-10 dB Conventional SIR=0 dB Conventional SIR=10 dB Proposed SIR=-10 dB Proposed SIR=0 dB Proposed SIR=10 dB
Sensitivity to Threshold Value
Scheme Conventional Proposed No Threshold 91.05% 99.30%
PFA= 0.01 74.85% 91.85%
SIR = 0 dB, SNR = 10 dB
Conclusions
An improved FAA algorithm is proposed for RAKE receivers
Per-path signal-to-interference ratio criterion
A simple per-path SIR estimator is developed
Significant performance improvement over the conventional signal-strength based FAA
Future work should consider the potential of the proposed scheme in the forward link.
