1 / 40

Module-3 : Transmission Lecture-7 (11/5/00)

Module-3 : Transmission Lecture-7 (11/5/00). Marc Moonen Dept. E.E./ESAT, K.U.Leuven marc.moonen@esat.kuleuven.ac.be www.esat.kuleuven.ac.be/sista/~moonen/. Lecture 7-8 : Multi-tone Modulation. Lecture 7 : Overview Driver application : ADSL (VDSL) modems

gabrielg
Download Presentation

Module-3 : Transmission Lecture-7 (11/5/00)

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. Module-3 : TransmissionLecture-7 (11/5/00) Marc Moonen Dept. E.E./ESAT, K.U.Leuven marc.moonen@esat.kuleuven.ac.be www.esat.kuleuven.ac.be/sista/~moonen/ Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven/ESAT-SISTA

  2. Lecture 7-8 : Multi-tone Modulation Lecture 7 : Overview • Driver application : ADSL (VDSL) modems Modem technology : voice-band vs. xDSL Communication impairments • DMT principles Preliminaries IFFT/FFT-based modulation/demodulation The magic `prefix’ trick Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  3. Lecture 7-8 : Multi-tone Modulation Lecture 8 : Overview • ADSL/VDSL revisited Specs, spectrum, FDM vs. EC, P-to-A,… • Equalization Time-domain equalization Frequency-domain equalization • Windowing Receiver windowing, Transmitter windowing • Echo Cancellation Time-domain echo cancellation Time+frequency-domain echo cancellation `Joint shortening’ Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  4. Acknowledgement Lecture partly adopted from Module T2 `Digital Communication Principles’ M.Engels, M. Moeneclaey, G. Van Der Plas 1998 Postgraduate Course on Telecommunication Special thanks to G. Van der Plas (Alcatel) Lecture partly co-authored by Thierry Pollet, Alcatel Telecom, CRC, Antwerp (B) Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  5. Driver Application: ADSL (VDSL) Modems References: • J.A.C. Bingham, `The theory and practice of modem design’, Wiley, 1980 • J.A.C. Bingham, `Multicarrier modulation for data transmission, an idea whose time has come’, IEEE Communications Magazine, May 1990, pp.5-14 • W.Y. Chen, `DSL simulation techniques and standards development for digital subscriber line systems’, Macmillan, 1998 • ... Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  6. Driver Application: ADSL (VDSL) Modems Modem Technology : • …1970 : development of efficient modulation techniques for transmission over band limited channels, spurred by military applications (-> … 9.6 kbits/sec) • 1970-1980 : Semiconductor technology (size & power reduction) • 1980-1990 : Introduction of coding techniques (-> … 19.2 kbits/sec) • 1990-2000 : reaching Shannon limit (-> 33kbits/sec, asymmetric 56kbits/sec modems) • 1995-… : ADSL (8Mbits/sec downstream, 60 kbits/sec upstream) • 2000-… : VDSL (52Mbits/sec downstream, 6Mbits/sec upstream), ... Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  7. Driver Application: ADSL (VDSL) Modems Performance improvement results from: • improved telephone channel quality (digital switches, etc.) • improved modulation/coding techniques (QAM, trellis coded modulation (TCM), echo cancellation (EC), adaptive equalization,…) • semiconductor technology Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  8. Driver Application: ADSL (VDSL) Modems PS: duplexing techniques • Frequency division duplexing (FDD) `downstream’ = central office -> remote user `upstream’ = remote user -> central office • Time division duplexing (TDD) : `ping-pong’ • Full duplex with echo-cancellation (EC) (see also Lecture-6/8) up down frequency up down time up & down time & frequency Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  9. Driver Application: ADSL (VDSL) Modems PS: echo cancellation for full-duplex (see Lecture-6/8) Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  10. Driver Application: ADSL (VDSL) Modems Voice-band modems : Shannon capacity limit (Lecture-2) • In analog public switched telephone network SNR > 28dB (signal-to-noise ratio) BW > 2400 Hz (bandwidth) ...Hence capacity approx. 22.4 kbits/sec • In digital public switched telephone network SNR > 34 dB (signal-to-noise ratio) BW > 3200 Hz (bandwidth) ...Hence capacity approx. 35 kbits/sec 100Hz 3600Hz Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  11. Driver Application: ADSL (VDSL) Modems Digital Subscriber Line (DSL) Technologies: • HDSL : `High Speed Digital Subscriber Line’ …2Mbits/sec on 2 or 3 pairs • ADSL : `Asymmetric Digital Subscriber Line’ …8Mbits/sec down, 60kbits/sec up on 1 pair uses frequency band up to approx. 1 MHz • SDSL : `High Speed Digital Subscriber Line’ …2Mbits/sec symmetrical on 1 pair • VDSL : `Very High Speed Digital Subscriber Line’ …52Mbits/sec down, 6Mbits/sec up on 1 pair uses frequency band up to approx. 10 MHz Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  12. Driver Application: ADSL (VDSL) Modems • ADSL : `Asymmetric Digital Subscriber Line’ …-1993: ADSL spurred by interest in video-on-demand (VOD) 1995 : ADSL/VOD interest decline 1996 : ADSL technology trials prove viability. 1997-... : ADSL deployment, reoriented to data applications, as telco’s reaction to cable operators offering high- speed internet access with cable modems. Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  13. Access Node Backbone Network max 3.5…5 km Driver Application: ADSL (VDSL) Modems • ADSL : `Asymmetric Digital Subscriber Line’ • VDSL : `Very High Speed Digital Subscriber Line’ copper wire cabinet (ONU) Access Node Backbone Network optical fiber copper wire max 0.3…1.5 km Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  14. Driver Application: ADSL (VDSL) Modems • Distance/bitrate trade-off (upstream) bitrate (Mbits/sec) 40 VDSL 30 20 10 ADSL 1 2 3 4 distance (km) Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  15. useful signal NEXT FEXT Driver Application: ADSL (VDSL) Modems Communication Impairments : • Frequency-dependent cannel attenuation introduces inter-symbol interference (ISI) • Coupling between wires in same or adjacent binders introduces crosstalk Near-end Xtalk (NEXT) Far-end Xtalk (FEXT) Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  16. Driver Application: ADSL (VDSL) Modems Communication Impairments : Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  17. Driver Application: ADSL (VDSL) Modems Communication Impairments (continued) : • Xtalk from other systems e.g. HPNA (Home Phone Network Alliance) vs. VDSL • Radio Frequency Interference (RFI) e.g. AM broadcast, amateur radio • Noise e.g. impulsive noise (=high bursts of short duration) • Echo due to hybrid impedance mismatch Conclusion: Need advanced modulation, DSP,etc. ! Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  18. Driver Application: ADSL (VDSL) Modems Line Codes (modulation technique) : • ADSL (ANSI standard) DMT (`Discrete Multi-tone Modulation’, a.k.a. multi-carrier modulation, orthogonal frequency division multiplexing (OFDM)) 256 different `carriers’, each of them QAM-modulated, see below. • VDSL DMT or single carrier (CAP/QAM) systems ? =undecided.. Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  19. Driver Application: ADSL (VDSL) Modems Line Codes (modulation technique) : • DMT (Preview) : Multiple carriers are QAM-modulated with a low-rate bit stream. The high-rate bit stream is thus carried by dividing it into hundreds of low-rate streams. Modulation/demodulation by FFT/IFFT (see below) Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  20. Driver Application: ADSL (VDSL) Modems • ADSL (Preview) IFFT P/S S/P Discrete equivalent channel FFT FEQ r(t) ADC DAC p(t) ch(t) Tx filter Channel Rx filter Tx clock Rx clock Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  21. DMT Principles: Preliminaries • DMT `inspiration’ : see Lecture 2 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  22. DMT Principles : Preliminaries • DMT `inspiration’ : see Lecture 2 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  23. DMT Principles : Preliminaries • DMT `inspiration’ : see Lecture 3 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  24. DMT Principles: IFFT/FFT-based modulation `Multi-tone’/`multi-carrier’ = modulate different carriers at the same time: • (=complex envelope) modulates fo - subscript `m’ refers to m-th symbol (frame) - this is one `tone’ • similarly, modulate 2.fo, 3.fo, …, N.fo - N `tones’ - highest frequency N.fo = Fs/2, with Fs sampling frequency (for baseband DMT) • each tone carries a number of bits (0,1,2,…) depending upon its SNR (`bit loading’) Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  25. DMT Principles: IFFT/FFT-based modulation Modulation is realized by means of 2N-point Inverse Discrete Fourier Transform (I-DFT) :example N=4 `*’ = complex conjugate real real Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  26. DMT Principles: IFFT/FFT-based modulation • I-DFT is implemented by means of 2N-point (inverse) Fast Fourier Transform (I-FFT) - complexity is O(N.logN) instead of O(N^2) - fast hardware available • Time-domainsymbols are transmitter over channel • Receiver : demodulation with inverse operation, i.e. FFT (i.o. I-FFT) Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  27. 0 IFFT P/S S/P Discrete equivalent channel FFT FEQ DMT Principles: IFFT/FFT-based modulation • DMT-transmission block scheme (revisited): r(t) ADC DAC p(t) ch(t) Tx filter Channel Rx filter Tx clock Rx clock Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  28. DMT Principles: The Magic Prefix Trick Additional feature : before transmission, a `prefix’ is added to each time-domain symbol, i.e. the last samples are copied and put up front : Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  29. DMT Principles: The Magic Prefix Trick Prefix insertion : Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  30. 0 IFFT P/S S/P Discrete equivalent channel FFT FEQ DMT Principles: The Magic Prefix Trick Prefix insertion : • in the receiver, the samples corresponding to the prefix are removed (=unused) : Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  31. Tone 3 Tone 2 Tone 1 Tone 0 Tone 3 Tone 2 Tone 1 Tone 0 DMT Principles: The Magic Prefix Trick • if channel impulse response has length L (= L non-zero taps) and ( is prefix length), then all `transient effects’ between symbols are confined to the prefix period : Tx-side Rx-side ch(t) * s(t) r(t) Prefix From IFFT Guardband To FFT Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  32. DMT Principles: The Magic Prefix Trick • Magic trick fails if -> transient effects within sample sequence that is fed into FFT -> this results in inter-symbol-interference (ISI) = interference from previous symbol(s) (same carrier) inter-carrier interference (ICI) = interference from other carriers (`carrier orthogonality destroyed’) (see also Lecture 8) Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  33. DMT Principles: The Magic Prefix Trick • In the receiver, after removing the samples corresponding to the prefix, the i-th tone is observed, multiplied by a factor H(i.fo), i.e. the channel response for frequency f=i.fo (=`simplified statement’) • `Prefix trick’ based on a linear convolution (filtering by channel impulse response) being turned into a circular convolution, which corresponds to component-wise multiplication in frequency domain (=statement to be ignored) Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  34. DMT Principles: The Magic Prefix Trick The mathematics behind this are as follows : • assume N=4, prefix-length=2 (cfr. supra) assume channel impulse response is • received samples are… transmitted signal at time k received signal at time k Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  35. DMT Principles: The Magic Prefix Trick Received samples for symbol m, after removing prefix : Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  36. DMT Principles: The Magic Prefix Trick ...This is equivalent with : Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  37. DMT Principles: The Magic Prefix Trick • The modified channel matrix is a so-called `circulant’ matrix (constant along the diagonals & `wrapped around’) • Theorem : every circulant matrix C is diagonalized by a DFT & I-DFT matrix : PS: Proof by MatLab (try it, it always works!) PPS: This is an eigenvalue (singular value) decomposition (remember from high school maths?) • Diagonal matrix has DFT of first column of C on its main diagonal, i.e. Hi=H(i.fo) ! Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  38. DMT Principles: The Magic Prefix Trick …By substituting this : Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  39. DMT Principles: The Magic Prefix Trick • …which means that after removing the prefix-samples and performing a DFT in the receiver, the obtained samples are equal to the transmitted (`frequency-domain’) symbols, up to a channel attenuation Hi (for tone-i). • Hence `channel equalization’ may be performed `in the frequency domain’, by component-wise divisions (divide by Hi for tone-i). =`1-taps FEQ’ (frequency-domain equalization) • Conclusion: if , DMT-modulation leads to a simple (trivial) channel equalization problem. …otherwise, see Lecture-8. Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  40. DMT Principles: The Magic Prefix Trick • PS : compare with transmitter & receiver transformation for MIMO-transmission (singular value decomposition of channel response matrix, etc., see Lecture-2) 0 IFFT P/S S/P Discrete equivalent channel FFT FEQ Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

More Related