1 / 21

Wireless Access Systems: Exercise 2 – Coded OFDM Modem

Wireless Access Systems: Exercise 2 – Coded OFDM Modem. Schedule:. Coded OFDM Modem. We will simulate a complete coded OFDM (COFDM) modem . . Input bits. Estimated bits. Coded OFDM Modem. We will simulate a complete coded OFDM (COFDM) modem . . Task 2: OFDM. Task 1:

mora
Download Presentation

Wireless Access Systems: Exercise 2 – Coded OFDM Modem

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. Wireless Access Systems:Exercise 2 – Coded OFDM Modem

  2. Schedule: Communication Technology Laboratory Wireless Communication Group

  3. Coded OFDM Modem • We will simulate a completecoded OFDM (COFDM) modem. Input bits Estimated bits Communication Technology Laboratory Wireless Communication Group

  4. Coded OFDM Modem • We will simulate a completecoded OFDM (COFDM) modem. Task 2: OFDM Task 1: Channel Coding From Exercise 1 Input bits Estimated bits Communication Technology Laboratory Wireless Communication Group

  5. Coded OFDM Modem • Task 1– Channel Coding: Groups 1, 3 and 5 • Getfamiliarwithchannelcoding: convolutionalcodes, Viterbidecoder and bit-interleaver • Task 2 – OFDM: Groups 2, 4 and 6 • Getacquaintedwithconceptsof OFDM modulation/demodulation, cyclicprefix, frequencyselectivefadingchannel, channelestimation. • Combination Task – COFDM: Group {1,2}, {3,4} and {5,6} Communication Technology Laboratory Wireless Communication Group

  6. Task 1 – Convolutional Coding • Channel codingenablesthecorrectionand/ordetectionofbiterrorsintroducedbytransmissionof a modulatedsignalthrough a wirelesschannel. • Block codesandconvolutionalcodes. • A rate R = k/n convolutionalencoderwithmemorym isnothing but a k-input, n-output linear sequentialcircuitwithmemorym. Communication Technology Laboratory Wireless Communication Group

  7. Task 1 – Convolutional Coding k = 1, n = 2, m= 3 • Implementation: [0 0 1 0 1 1] Modulo 2 Conv g(0) Modulo 2 Conv g(1) [0 0 1 1 1 1] Communication Technology Laboratory Wireless Communication Group

  8. Task 1 – Interleaving • Channel codes are designed for good performance in AWGN channels. • In fading channels, when the channel is in deep fade, the errors usually occur in long bursts. • To improve coding performance in fading channels, interleaving has been introduced. • The basic idea: Spread error bursts due to deep fades over many codewords such that the received codeword exhibits at most a few simultaneous errors. Communication Technology Laboratory Wireless Communication Group

  9. Task 1 – Interleaving Without interleaver: Transmitted: 1 2 3 4 5 6 7 … Burst Error: 1 2 3 x x x 7 … Cannot be corrected Example: Hamming code (7,4) can correct only one error Communication Technology Laboratory Wireless Communication Group

  10. Task 1 – Interleaving read write With interleaver: Example: Hamming code (7,4) can correct only one error Communication Technology Laboratory Wireless Communication Group

  11. Task 1 – Interleaving With interleaver: Transmitted: 1 8 15 22 29 36 43 … Burst Error: 1 8 15 x x x 43 … Example: Hamming code (7,4) can correct only one error Communication Technology Laboratory Wireless Communication Group

  12. Task 1 – Interleaving With interleaver: Transmitted: 1 8 15 22 29 36 43 Burst Error: 1 8 15 x x x 43 Only one error  can be corrected Example: Hamming code (7,4) can correct only one error Communication Technology Laboratory Wireless Communication Group

  13. Task 1 – Viterbi Decoding • The convolutionalencodershave a naturaltrellisstructure. • Maximum likelihooddetectionof a convolutionalcodeentailsfindingthemostlikelysequenceofcodedsymbolsgiventhereceivedsequenceofcodedsymbols. • The Viterbialgorithmreducesthecomplexityofmaximumlikelihooddecodingbysystematicallyremovingpathsfromconsiderationthatcannotachievethehighestpathmetric. Communication Technology Laboratory Wireless Communication Group

  14. Task 2 - ODFM Modem Communication Technology Laboratory Wireless Communication Group

  15. Task 2 - ODFM Modem Circulant Channel Matrix Communication Technology Laboratory Wireless Communication Group

  16. Task 2 - ODFM Modem • Example: 2 taps channel • Remove Cyclic Prefix • Add Cyclic Prefix Circulant Channel Matrix Communication Technology Laboratory Wireless Communication Group

  17. Task 2 - ODFM Modem Diagonal Channel Matrix Communication Technology Laboratory Wireless Communication Group

  18. Task 2 – ODFM Modem • Let the vector of one OFDM symbol be: • The received vector (after CP removal): • Here, is circulant because of the cyclic prefix assumption. • Therefore, can be decomposed as: with a diagonal matrix • Hence, we perform an IDFT at the transmitter and a DFT at the receiver: with Communication Technology Laboratory Wireless Communication Group

  19. Task 2 – OFDM Detection and Estimation • After FFT, weshouldremovetheeffectofthechannelfromthedemodulatedsymbols. • Letbethedetectedk-thsymbolwhichisgivenby • The estimateofthechannelsisneededtobeperformedatthereceiver. • Howtoestimatethechannel? Communication Technology Laboratory Wireless Communication Group

  20. Task 2 – OFDM Detection and Estimation Communication Technology Laboratory Wireless Communication Group

  21. Combination Task – COFDM Same groups cooperate as last time. Combination Task Task 2: OFDM Task 1: Channel Coding Input bits Estimated bits Communication Technology Laboratory Wireless Communication Group

More Related