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.

1. AJAL.A.J ASST PROFESSOR METS SCHOOL OF ENGG MALA RAKE RECEIVER "To trip twice on the same rake", which means "to repeat the same silly mistake". Russian saying

2. Why the name rake receiver ? The rake receiver is so named because it reminds the function of a garden rake , each finger collecting symbol energy similarly to how a rake collect leaves.

3. Steel Rake Leaf Rake

4. SNOW RAKE

5. GARDEN RAKE

6. GARDEN RAKE

7. Multi-path Energy Capture In multi-path environments, the RMS delay spreads for a given channel can be large (14 ns for CM3, 25 ns ). Un captured multi-path energy results in loss in performance of the communication device. One method for energy collection is to use a RAKE receiver.

8. Propagation of Tx Signal

9. Multipath Multipath occurs when RF signals arrive at a location via different transmission paths due to the reflection of the transmitted signal from fixed and moving objects. The combination of the direct and reflected signals most often leads to significant signal loss due to mutual cancellation.

10. Rake Receiver – Multipath fading Rake receiver mitigates multipath fading effect Multipath fading is a major cause of unreliable wireless channel characteristic x(t) y(t) = a 0 x(t) y(t) = a 0 x(t)+a 1 x(t-d 1 ) y(t) = a 0 x(t)+a 1 x(t-d 1 )+a 2 x(t-d 2 )

11. RAKE Receiver: Basic Idea The RAKE receiver was designed to equalize the effects of multipath. It uses a combination of correlators, code generators, and delays, or “fingers”, to spread out the individual echo signals of the multipath. Each signal is then delayed according to peaks found in the received signal.

12. Overview of Rake Receiver A rake receiver is a radio receiver designed to counter the effects of multipath fading. It does this by using several "sub-receivers" each delayed slightly in order to tune in to the individual multipath components. Each component is decoded independently, but at a later stage combined in order to make the most use of the different transmission characteristics of each transmission path. This could very well result in higher SNR (or Eb/No) in a multipath environment than in a "clean" environmen

13. RAKE Receiver Continued The same symbols obtained via different paths are then combined together using the corresponding channel information using a combining scheme like maximum ratio combining (MRC). The combined outputs are then sent to a simple decision device to decide on the transmitted bits.

14. RAKE Receiver Block Diagram

15. Maximum Ratio Combining of Symbols MRC corrects channel phase rotation and weighs components with channel amplitude estimate. The correlator outputs are weighted so that the correlators responding to strong paths in the multipath environment have their contributions accented, while the correlators not synchronizing with any significant path are suppressed.

16. End Result of RAKE Receiver By simulating a multipath environment through a parallel combination of correlators and delays, the output behaves as if there existed a single propogation path between the transmitter and receiver.

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 !

25. Architecture(1) Conventional Rake Receiver W1 W2 W3

26. Architecture(2) Proposed Rake Receiver correlator move behind multipliers 3 fingers adopted W1 W2 W3

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)

38. Maximum Ratio Combining of Symbols MRC corrects channel phase rotation and weighs components with channel amplitude estimate. The correlator outputs are weighted so that the correlators responding to strong paths in the multipath environment have their contributions accented, while the correlators not synchronizing with any significant path are suppressed.

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

40. Maximum Ratio Combining Diversity Various techniques are known to combine the signals from multiple diversity branches. In Maximum Ratio combining each signal branch is multiplied by a weight factor that is proportional to the signal amplitude. That is, branches with strong signal are further amplified, while weak signals are attenuated.

42. Rake Receiver - Functions Ideally the function of rake receiver is to aggregate the signal terms with proper delay compensation y(t) = a 0 x(t)+a 1 x(t-d 1 )+a 2 x(t-d 2 ) r(t) = a 0 x(t-t dealy )+a 1 x(t-d 1 -d est1 )+a 2 x(t-d 2 -d est2 ) = (a 0 +a 1 +a 2 ) * x(t-t delay ) Rake receiver We need to know delay spread of received signal that randomly varies

43. Rake Receiver – Detect Delay Spread Scan the received signal in frame buffer while computing correlation with scrambling code sequence. Received signal Correlation window Correlation Result a 0 a 1 a 2 0 d 1 d 2

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

45. Multipath Diversity: Rake Receiver Instead of considering delay spread as an issue, use multipath signals to recover the original signal Used in IS-95 CDMA, 3G CDMA, and 802.11 Invented by Price and Green in 1958 R. Price and P. E. Green, &quot; A communication technique for multipath channels ,&quot; Proc. of the IRE, pp. 555--570, 1958

46. Multipath Diversity: Rake Receiver Use several &quot;sub-receivers&quot; each delayed slightly to tune in to the individual multipath components Each component is decoded independently, but at a later stage combined LOS pulse multipath pulses This could very well result in higher SNR in a multipath environment than in a &quot;clean&quot; environment

47. Rake Receiver: Matched Filter Impulse response measurement Tracks and monitors peaks with a measurement rate depending on speeds of mobile station and on propagation environment Allocate fingers: largest peaks to RAKE fingers

48. Rake Receiver: Combiner The weighting coefficients are based on the power or the SNR from each correlator output If the power or SNR is small out of a particular finger, it will be assigned a smaller weight:

49. RAKE DEMODULATOR

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

51. The demodulator structure shown in above Figure is called a RAKE demodulator . Because this demodulator has equally spaced taps with tap coefficients that essentially collect all the signal components in the received signal, its operation has been likened to that of an ordinary garden rake.

52. Let us use the correlator structure that is illustrated in Figure The received signal is passed through a tapped delay-line filter with tap spacing of 1/ W , as in the channel model The number of taps is selected to match the total number of resolvable signal components. At each tap, the signal is multiplied with each of the two possible transmitted signals s 1( t ) and s 2( t ), and, then, each multiplier output is phase corrected and weighted by multiplication with c ( t ), n = 1, 2, …, L .

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