The document discusses the rake receiver, which is used to mitigate multipath fading effects in wireless communications. It operates by using multiple "fingers" to capture signal energy from different propagation paths. Each finger correlates the received signal with a reference signal and applies appropriate delays. The outputs of the fingers are then combined, such as by maximum ratio combining, to improve the overall signal quality by constructively adding the energy from different paths. The rake receiver allows energy from all propagation paths to be captured and combined, increasing the signal-to-noise ratio compared to a single propagation path.

3. Steel Rake Leaf Rake

4. SNOW RAKE

5. GARDEN RAKE

6. GARDEN RAKE

8. Propagation of Tx Signal

14. RAKE Receiver Block Diagram

17. Fading in CDMA System ... Because CDMA has high time-resolution, different path delay of CDMA signals can be discriminated. Therefore, energy from all paths can be summed by adjusting their phases and path delays. This is a principle of RAKE receiver. CDMA Receiver CDMA Receiver ••• Synchronization Adder interference from path-2 and path-3 ••• Path Delay Power path-1 path-2 path-3 Path Delay Power CODE A with timing of path-1 path-1 Power path-1 path-2 path-3 Path Delay Power CODE A with timing of path-2 path-2

18. Fading in CDMA System (continued) In CDMA system, multi-path propagation improves the signal quality by use of RAKE receiver. Time Power Detected Power RAKE receiver Less fluctuation of detected power, because of adding all energy . Power path-1 path-2 path-3

19. Rake finger selection Delay (  ) Channel estimation circuit of Rake receiver selects strongest samples (paths) to be processed in the Rake fingers: In the Rake receiver example to follow, we assume L = 3. Only one path chosen, since adjacent paths may be correlated L = 3 Only these paths are constructively utilized in Rake fingers

20. Received multipath signal Received signal consists of a sum of delayed (and weighted) replicas of transmitted signal. All replicas are contained in received signal and cause interference : Signal replicas: same signal at different delays, with different amplitudes and phases Summation in channel <=> “smeared” end result Blue samples (paths) indicate signal replicas detected in Rake fingers Green samples (paths) only cause interference

21. Rake receiver Finger 1 Finger 2 Channel estimation Received baseband multipath signal (in ELP signal domain) Finger 3  Output signal (to decision circuit) Rake receiver Combining (MRC) (Generic structure, assuming 3 fingers) Weighting

22. Rake Receiver Blocks Correlator Finger 1 Finger 2 Finger 3 Combiner

23. Delay Rake finger processing  Received signal To MRC Stored code sequence (Case 1: same code in I and Q branches) I branch Q branch I/Q Output of finger: a complex signal value for each detected bit

24. Correlation vs. matched filtering Received code sequence Stored code sequence Basic idea of correlation: Same result through matched filtering and sampling: Received code sequence Matched filter Sampling at t = T Same end result !

27. Rake finger processing Correlation with stored code sequence has different impact on different parts of the received signal = desired signal component detected in i :th Rake finger = other signal components causing interference = other codes causing interference (+ noise ... )

28. Rake finger processing Illustration of correlation (in one quadrature branch) with desired signal component (i.e. correctly aligned code sequence) Desired component Stored sequence After multiplication Strong positive / negative “correlation result” after integration “ 1” bit “ 0” bit “ 0” bit

29. Rake finger processing Illustration of correlation (in one quadrature branch) with some other signal component (i.e. non-aligned code sequence) Other component Stored sequence After multiplication Weak “correlation result” after integration “ 1” bit “ 0” bit

30. Rake finger processing Mathematically: Correlation result for bit between Interference from same signal Interference from other signals Desired signal

31. Rake finger processing Set of codes must have both: - good autocorrelation properties (same code sequence) - good cross-correlation properties (different sequences) Large Small Small

32. Delay Rake finger processing Received signal Stored I code sequence (Case 2: different codes in I and Q branches) I branch Q branch I/Q Stored Q code sequence To MRC for I signal To MRC for Q signal Required: phase synchronization

33. Rake finger processing Case 1: same code in I and Q branches Case 2: different codes in I and Q branches - for purpose of easy demonstration only - the real case in IS-95 and WCDMA - no phase synchronization in Rake fingers - phase synchronization in Rake fingers

34. Phase synchronization I/Q When different codes are used in the quadrature branches (as in practical systems such as IS-95 or WCDMA), phase synchronization is necessary. Phase synchronization is based on information within received signal (pilot signal or pilot channel). Signal in I-branch Pilot signal Signal in Q-branch I Q Note: phase synchronization must be done for each finger separately!

35. Weighting Maximum Ratio Combining (MRC) means weighting each Rake finger output with a complex number after which the weighted components are summed “on the real axis”: Component is weighted Phase is aligned Rake finger output is complex-valued real-valued (Case 1: same code in I and Q branches) Instead of phase alignment: take absolute value of finger outputs ...

36. Phase alignment The complex-valued Rake finger outputs are phase-aligned using the following simple operation: Before phase alignment: After phase alignment: Phasors representing complex-valued Rake finger outputs

37. Maximum Ratio Combining The idea of MRC: strong signal components are given more weight than weak signal components. The signal value after Maximum Ratio Combining is: (Case 1: same code in I and Q branches)

39. Maximum Ratio Combining Output signals from the Rake fingers are already phase aligned (this is a benefit of finger-wise phase synchronization). Consequently, I and Q outputs are fed via separate MRC circuits to the quaternary decision circuit (e.g. QPSK demodulator). (Case 2: different codes in I and Q branches) Quaternary decision circuit Finger 1 Finger 2 MRC  MRC  : I Q I Q I Q

44. Rake Receiver – Overall Architecture Detects delay spread Compensates propagation delay recombine signal terms without delay

49. RAKE DEMODULATOR

50. RAKE DEMODULATOR RAKE demodulator for signal transmitted through a frequency selective channel.