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EE 615 Lecture 6

October 5, 2006. HW . Due today, Channel estimation. October 5, 2006. Diversity (Background) . Potential for capacity through change of the channel (either different channels or variation)Increase link performance or throughput Limits of Capacity?. October 5, 2006. Capacity. [Shannon] Maximum a

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EE 615 Lecture 6

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    1. October 5, 2006 EE 615 Lecture 6 Diversity Space (Antenna) Time (Coding)

    2. October 5, 2006 HW Due today, Channel estimation

    3. October 5, 2006 Diversity (Background) Potential for capacity through change of the channel (either different channels or variation) Increase link performance or throughput Limits of Capacity?

    4. October 5, 2006 Capacity [Shannon] Maximum achievable throughput without distortion (asymptotically error free data) Multiple parallel channel capacity. If Hn=I

    5. October 5, 2006 Additive White Gaussian Noise Channel Capacity scales linearly with number of channels rather than logarithmically Benefit of parallel transmission! The assumption: independent channels, in reality interference / correlation between channels What to do if channel is not AWGN?

    6. October 5, 2006 Modulation mapping the digital information to analog form demodulation, done by the receiver to recover the transmitted digital information constellation is the set of points that can be transmitted on a single symbol

    7. October 5, 2006 Modulation Bit error rate (BER) of PB of a constellation given by calculating Q-function Different constellations have different dmin values for the same Constellation with the largest dmin for a given has the best performance. To make comparisons between constellations fair, averaging the power of all the M points Ck of the constellation

    8. October 5, 2006 Constellation

    9. October 5, 2006 Distance properties of PSK

    10. October 5, 2006 BER performance PSK BPSK Higher order PSK modulation Ps appoximated

    11. October 5, 2006 Quadrature Amplitude Mod.

    12. October 5, 2006 Distance properties QAM

    13. October 5, 2006 Detection

    14. October 5, 2006 Channel coding Capacity is a rate limit for error free communications. The channel codes that can achieve this bound is not given! GOAL: Enable signal to counter channel impairments, noise, fading, jamming Block coding vs Channel coding

    15. October 5, 2006 Channel Coding (ctnued) Decrease BER of power/bandwidth limited channel Add structured redundancy into signal Examples: Linear Block Codes

    16. October 5, 2006 Interleaving Most codes are designed for AWGN channel. Performance rapidly degrades in frequency selective channels with correlated channels.

    17. October 5, 2006 Convolutional Interleaver

    18. October 5, 2006 Interleaving in IEEE 802.11a Block interleaver, limited to one OFDM symbol (read the book for details) Performance effect of interleaving in IEEE 802.11a is due to frequency diversity. Channel assumed to be quasi static and assumed to stay the same for duration of a transmitted packet.

    19. October 5, 2006 Interleaving in 802.11a

    20. October 5, 2006 IEEE 802.11a 12 Mbits mode with and without interleaving in 75ns rms delay spread Rayleigh fading channel.

    21. October 5, 2006 Interleaving in OFDM % Interleaver from OFDM Wireless LANS % A Theoretical and Practical Guide function interleaved_bits = tx_interleaver(in_bits, sim_options) global sim_consts; interleaver_depth = sim_consts.NumDataSubc * get_bits_per_symbol(sim_options.Modulation); num_symbols = length(in_bits)/interleaver_depth; % Get interleaver pattern for symbols single_intlvr_patt = tx_gen_intlvr_patt(interleaver_depth, sim_options); % Generate intereleaver pattern for the whole packet intlvr_patt = interleaver_depth*ones(interleaver_depth, num_symbols); intlvr_patt = intlvr_patt*diag(0:num_symbols-1); intlvr_patt = intlvr_patt+repmat(single_intlvr_patt', 1, num_symbols); intlvr_patt = intlvr_patt(:); % Perform interleaving interleaved_bits(intlvr_patt) = in_bits;

    22. October 5, 2006 OFDM Standard 802.11a Generating 12 Mbit/s involves: a. Encoding the 24 bit signal field with a rate convolutional encoder (yields 48 bits) b. Interleaving the encoded bits with a 2 step interleaver c. QPSK modulating of the interleaved bits d. Padding the modulated bits with zeros, and skipping 4 locations (yields a total of 64 bits) e. Inserting pilots in the 4 skipped locations f. Applying the IFFT to the final bits.

    23. October 5, 2006 Systemview OFDM w/ interlea.

    24. October 5, 2006 Deinterleaving (MATLAB) Deinterleaving is exactly same as interleaving reversed

    25. October 5, 2006 Convolutional Coding Most widely used channel codes (GSM, IS-95), IEEE 802.11a has eight different data rates (conv. Coding rate adjustment)

    26. October 5, 2006 Punctured Codes

    27. October 5, 2006 Ddistances of the 64 State Convolutional Codes Used in IEEE 802.11a

    28. October 5, 2006 Decoding Convolutional Code Viterbi algorithm, ML sequential detector (hard or soft decision decode) In coherent OFDM system channel estimate is necessary. This can be incorporated ito Viterbi algorithm to improve channel by weighting the squared Euclidean distance

    29. October 5, 2006 Decoding Conv. Code Hard or Soft decision demodulation Coherent OFDM system needs to estimate frequency response of system. Performance effect of weighting matters because of frequency diversity

    30. October 5, 2006 Viterbi Decoding Trellis, using state transition. For K memory element conv. Encoder, there are 2^K states.

    31. October 5, 2006 Effect of Soft vs Hard decode

    32. October 5, 2006 Effect of Interleaving Interleaving improves BER for convolutionaly coded (r=1/2)

    33. October 5, 2006 Coding Gain BER and PER w/ and w/out coding

    34. October 5, 2006 Decoding BER and PER improvement due to soft decoding

    35. October 5, 2006 Effect of metric weighting The performance of coding changes in fading channel.

    36. October 5, 2006 Next time Space Diversity (receiver, transmit diversity) HW due next week: Read Chapter 3, (see below) Implement OFDM with 9Mbits/sec (1/3 code), and check performance with different rms delay spread values. (Use the code simulator.gz of OFDM Wireless Lans: A Theoretical and Practical Guide (Sams Publishing)

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