1. EE 6331, Spring, 2009 Advanced Telecommunication Zhu Han Department of Electrical and Computer Engineering Class 18 Apr. 2 rd, 2009

2. ECE6331 Outline Review –Eye Diagram –Equalization Diversity Midterm 1 –Highest 98 –Lowest 47 –Mean 79 –Variance 6.3

3. ECE6331 Interpretation of Eye Diagram 10 points in the final

4. ECE6331 Linear Modulation with Nyquist Impulse Shaping QPSK diagram under limited bandwidth conditions  if system (tx and rx filter) meets 1st Nyquist : 4 sharp signal points (right diagram)

5. ECE6331 Equalization, Diversity, and Channel Coding Three techniques are used independently or in tandem to improve receiver signal quality Equalization compensates for ISI created by multipath with time dispersive channels (W>B C ) –Change the overall response to remove ISI Diversity also compensates for fading channel impairments, and is usually implemented by using two or more receiving antennas –Multiple received copies: Spatial diversity, antenna polarization diversity, frequency diversity, time diversity. –Reduces the depth and duration of the fades experienced by a receiver in a flat fading (narrowband) channel Channel Coding improves mobile communication link performance by adding redundant data bits in the transmitted message –Channel coding is used by the Rx to detect or correct some (or all) of the errors introduced by the channel (Post detection technique) –Block code and convolutional code

6. ECE6331 Equalization Techniques The term equalization can be used to describe any signal processing operation that minimizes ISI Two operation modes for an adaptive equalizer: training and tracking Three factors affect the time spanning over which an equalizer converges: equalizer algorithm, equalizer structure and time rate of change of the multipath radio channel TDMA wireless systems are particularly well suited for equalizers Symbol Mapper ISI Channel Equalizer Decision Device

7. ECE6331 Math Derivation Optimum weight vector Minimum mean square error (MMSE) Minimizing the MSE tends to reduce the bit error rate Example 7.1, 7.2 Training Sequence then Data transmission within each frame Training Sequence Data transmission Training Sequence Data transmission

8. ECE6331 Classification of Equalizer if d(t) is not the feedback path to adapt the equalizer, the equalization is linear if d(t) is fed back to change the subsequent outputs of the equalizer, the equalization is nonlinear

9. ECE6331 Overcoming Channel Impairments Deep FadingChannel Coding

10. ECE6331 Diversity Techniques Requires no training overhead Can provides significant link improvement with little added cost Diversity decisions are made by the Rx, and are unknown to the Tx Diversity concept –If one radio path undergoes a deep fade, another independent path may have a strong signal –By having more than one path to select from, both the instantaneous and average SNRs at the receiver may be improved, often by as much as 20 dB to 30 dB –Diversity order u How many independent copies u How many links to bring down the system

11. ECE6331 Diversity Motivation Aim: Reduce effects of fast fading –Concept: u Multiple branches, independent fading u Process branches to reduce fading probability –If probability of a deep fade on one channel is p, probability on N channel p N. –e.g. 10% chance of losing contact for one channel becomes 0.1 3 =0.001=0.1% with 3 channels Requirements for Diversity –Multiple branches –Low correlation between branches –Similar mean powers: –Efficient combiner

12. ECE6331 Diversity Example

13. ECE6331 Different Diversity Spatial Diversity –Multiple input multiple out system (MIMO) –Beamforming, smart antenna –Space time coding –Horizontal and Vertical Combining Frequency diversity –Frequency diversity transmits information on more than one carrier frequency –Frequencies separated by more than the coherence bandwidth of the channel will not experience the same fads Time diversity –Time diversity repeatedly transmits information at time spacings that exceed the coherence time of the channel Polarization diversity Multi-user diversity

14. ECE6331 Space Diversity Large antenna spacing or large scatterer spacing produce large path length differences Hence multipath will combine differently at each antenna

15. ECE6331 Analysis of Space Diversity Phase difference: Signals from one scatterer: Signals from n s scatterer: Correlation: Evaluate expectation Angle-of-arrival PDF

16. ECE6331 Horizontal Space Diversity

17. ECE6331 Vertical Space Diversity Restricted vertical angle spread, so greater separation needed in vertical direction

18. ECE6331 Polarisation Diversity Scattering shifts and decorrelates polarisation Advantage: Very compact Disadvantage: Unequal branch powers - less diversity gain

19. ECE6331 Polarization diversity Theoretical model for polarization diversity –the signal arrive at the base station –the correlation coefficient can be written as

20. ECE6331 Polarization diversity Theoretical Model for base station polarization diversity based on [Koz85]

21. ECE6331 Time Diversity Retransmit with Time Separation Advantage: Need only one receiver Disadvantage: Wastes bandwidth, adds delay

22. ECE6331 Frequency Diversity Wideband Channel Simultaneous Transmission Wastes power and bandwidth Equalizers Channel Spectrum Frequency

23. ECE6331 Combining Techniques How to combine the multiple received copies –Selection diversity –Feedback diversity –Maximal ration combining –Equal gain diversity

24. ECE6331 Selection diversity The receiver branch having the highest instantaneous SNR is connected to the demodulator The antenna signals themselves could be sampled and the best one sent to a single demodulation

25. ECE6331 Selection Combining

26. ECE6331 Derivation of Selection Diversity Microscopic diversity and Macroscopic diversity –The former is used for small-scale fading while the latter for large-scale fading –Antenna diversity (or space diversity) Performance for M branch selection diversity

27. ECE6331 Performance Example 7.4 Graph of probability distributions of SNR=  threshold for M branch selection diversity. The term  represents the mean SNR on each branch

28. ECE6331 Effect of Varying Branch Mean Powers

29. ECE6331 Maximal Ratio Combining Diversity The signals from all of the M branches are weighted according to their signal voltage to noise power ratios and then summed Like stock investigation

30. ECE6331 Varying Branch Correlations

31. ECE6331 Effect of Non-zero correlation on MRC

32. ECE6331 SNR for BPSK with MRC

33. ECE6331 Feedback diversity The signal, the best of M signals, is received until it falls below threshold and the scanning process is again initiated

34. ECE6331 Switched Combining Avoids multiple receivers Switch and stay strategy Must set appropriate threshold relative to mean level Performance always worse than selection combining

35. ECE6331 Equal Gain Combining The branch weights are all set to unity but the signals from each are co-phased to provide equal gain combining diversity Make use of energy in all branches

36. ECE6331 Equal Gain Combining Performance Received signals: Combiner output: SNR:

37. ECE6331 Comparison of Combining Techniques

38. ECE6331 RAKE Receiver 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.

39. ECE6331 RAKE Receiver 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.

40. ECE6331 RAKE Receiver 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.

41. ECE6331 RAKE Receiver Block Diagram

42. ECE6331 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.

43. ECE6331 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. An M-branch (M-finger) RAKE receiver implementation. Each correlator detects a time shifted version of the original CDMA transmission, and each finger of the RAKE correlates to a portion of the signal which is delayed by at least one chip in time from the other finger.

44. ECE6331 Interleaving Block interleaver where source bits are read into columns and out as n-bit rows