EE 6331, Spring, 2009 Advanced Telecommunication

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

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

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

4. 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) ECE6331

5. 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>BC) 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 ECE6331

6. 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 ECE6331

7. 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 ECE6331

8. 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 ECE6331

9. Overcoming Channel Impairments Deep Fading Channel Coding ECE6331

10. 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 How many independent copies How many links to bring down the system ECE6331

11. Diversity Motivation Aim: Reduce effects of fast fading Concept: Multiple branches, independent fading Process branches to reduce fading probability If probability of a deep fade on one channel is p, probability on N channel pN . e.g. 10% chance of losing contact for one channel becomes 0.13=0.001=0.1% with 3 channels Requirements for Diversity Multiple branches Low correlation between branches Similar mean powers: Efficient combiner ECE6331

12. Diversity Example ECE6331

13. 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 ECE6331

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

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

16. Horizontal Space Diversity ECE6331

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

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

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

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

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

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

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

24. 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 ECE6331

25. Selection Combining ECE6331

26. 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 ECE6331

27. 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 ECE6331

28. Effect of Varying Branch Mean Powers ECE6331

29. 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 ECE6331

30. Varying Branch Correlations ECE6331

31. Effect of Non-zero correlation on MRC ECE6331

32. SNR for BPSK with MRC ECE6331

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

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

35. 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 ECE6331

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

37. Comparison of Combining Techniques ECE6331

38. 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. ECE6331

39. 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. ECE6331

40. 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. ECE6331

41. RAKE Receiver Block Diagram ECE6331

42. 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. ECE6331

43. 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. ECE6331

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