1 / 13

Time Diversity

Time Diversity. Interleaving Another form of time diversity that does not require transmission of extra data bits Used in tandem with digital speech coders that transform analog voice signals to digital data 2 nd generation wireless systems (TDMA, CDMA)

nile
Download Presentation

Time Diversity

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Time Diversity • Interleaving • Another form of time diversity that does not require transmission of extra data bits • Used in tandem with digital speech coders that transform analog voice signals to digital data • 2nd generation wireless systems (TDMA, CDMA) • Efficient digital speech encoding produces data stream where several “important” bits are in succession • Important bits susceptible to rapid small-scale fading  short duration deep fade ECE 4730: Lecture #19

  2. Time Diversity • Interleaver spreads the bits out in time so that a sequence of important bits are not all corrupted at the same time • Interleaving often combined with channel coding (error coding) • Two interleaver forms: 1) block 2) convolutional • Block  bits are grouped in distinct blocks (arrays) • Convolutional  bits are sequentially processed • Block & convolutional interleavers are best used with same form (block & conv.) of channel code ECE 4730: Lecture #19

  3. Time Diversity • Block Interleaver • m rows & n columns ; degree = m • mnbits interleaved at a time • Sequentially read in to columns • 1 row = 1 word • Sequentially read out of rows • Original source bits separated bym bits ECE 4730: Lecture #19

  4. Time Diversity Block Interleaver ECE 4730: Lecture #19

  5. Time Diversity • Interleaver delay • nm bits must arrive @ Rx before process is inverted  de-interleaved • Delay = nmTb • Perceived quality of real-time data affected by delay  voice, video, etc. • For voice  delay < 40 msec for acceptable quality ECE 4730: Lecture #19

  6. Channel Coding • Channel Coding • Sole purpose  protect digital data from MRC effects (multipath, fading, etc.) that corrupt information • Introduce redundant data bits (time diversity!) in selective manner  error correction after demodulation • Shannon’s Channel Capacity Bound ECE 4730: Lecture #19

  7. Channel Coding • Shannon’s Channel Capacity Bound • Fundamental tradeoff : 1) We can increase power to save BW • Tx power Eb then B to maintain same capacity (C) • m-ary modulation, e.g. more states/symbol 2) We can increase BW to save power • B then Eb to maintain same C • Channel Coding !! ECE 4730: Lecture #19

  8. Channel Coding • Redundant coding bits produce a) Lower data rate for same signal BW OR b) Higher signal BW for same data rate • Either way B = R/ B ** For low S / N the improvement in BER from coding is often worth the reduced B ** Non-linear improvement in BER vs. S / N ECE 4730: Lecture #19

  9. BER = 10-5 Good P Occupied BW Shannon’s Bound Good B dB 0 5 10 15 Eb / No x x BPSK   BFSK Channel Coding Graphical Representation of Shannon’s Bound “X” = after channel coding a lowerEb / Nois required for the same BER Difference in Eb / Nois called “coding gain” ECE 4730: Lecture #19

  10. Code Word k n - k Source Parity Code Block Channel Coding • Block Codes • Forward Error Correction (FEC) Codes • Detect & correct limited # errors • (n-k) parity bits added to k source bits to produce code word of length n n ECE 4730: Lecture #19

  11. Channel Coding • Block code = (n, k) code • Code rate = Rc = k / n • (n, k) code can correct burst (sequential) errors of length = b (n-k) / 2 • Interleaver/coder combination  length = mb(m = int. degree) • Hamming codes (n, k) = (2m – 1, 2m– 1 – m) with m = n–k • Golay codes (23, 12)  special block code that corrects all patterns of 1, 2, & 3 bit errors in 12-bit source data length • Very useful for random error correction like AGWN channels • Must use with interleaver for MRC b/c of “bursty” nature of errors ECE 4730: Lecture #19

  12. Channel Coding • BCH codes • Wide range of rates (Rc) • Large coding gains • High-speed implementation • Reed – Solomon codes • Correct for error bursts  sequential due to MRC fading • CD player (cut with scissors!) ECE 4730: Lecture #19

  13. Channel Coding • Convolutional Codes • Continuous sequence of input data mapped into output code bits using shift registers & algebraic functions • Much larger coding gains compared to block codes of similar complexity • Widely used in wireless systems • Viterbi decoding algorithm  CEO Qualcomm • Most widely used convolutional decoder ECE 4730: Lecture #19

More Related