A Novel Finger Assignment Algorithm for RAKE Receivers in CDMA Systems

1. 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

2. Outline • Scope • Introduction • Proposed FAA • Simulation Results • Conclusions

3. 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

4. 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)

5. 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

6. Received Signal RAKE Receiver Combiner Output Searcher PN code Finger delays Channel estimates Per path SIR Estimator FAA Search results SIR estimates 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

7. 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

8. 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

9. 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)

10. Bit Error Rate: Pfa=1%

11. Probability of Detection First Path

12. Probability of Detection All Four Paths

13. Receiver Operational Characteristics (ROC)

14. Sensitivity to Threshold Value SIR = 0 dB, SNR = 10 dB

15. 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.

16. Thank You!