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11/5/00 p. 1 Postacademic Course on Telecommunications Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven/ESAT-SISTA Module-3.

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Presentation on theme: "11/5/00 p. 1 Postacademic Course on Telecommunications Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven/ESAT-SISTA Module-3."— Presentation transcript:

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2 11/5/00 p. 1 Postacademic Course on Telecommunications Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven/ESAT-SISTA Module-3 : Transmission Lecture-7 (11/5/00) Marc Moonen Dept. E.E./ESAT, K.U.Leuven

3 Postacademic Course on Telecommunications 11/5/00 p. 2 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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

4 Postacademic Course on Telecommunications 11/5/00 p. 3 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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’

5 Postacademic Course on Telecommunications 11/5/00 p. 4 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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)

6 Postacademic Course on Telecommunications 11/5/00 p. 5 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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,

7 Postacademic Course on Telecommunications 11/5/00 p. 6 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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) : Semiconductor technology (size & power reduction) : Introduction of coding techniques (-> … 19.2 kbits/sec) : 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),...

8 Postacademic Course on Telecommunications 11/5/00 p. 7 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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

9 Postacademic Course on Telecommunications 11/5/00 p. 8 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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) updown frequency updown time up& down time & frequency

10 Postacademic Course on Telecommunications 11/5/00 p. 9 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Driver Application: ADSL (VDSL) Modems PS: echo cancellation for full-duplex (see Lecture-6/8)

11 Postacademic Course on Telecommunications 11/5/00 p. 10 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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 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 100Hz3600Hz

12 Postacademic Course on Telecommunications 11/5/00 p. 11 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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

13 Postacademic Course on Telecommunications 11/5/00 p. 12 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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 : ADSL deployment, reoriented to data applications, as telco’s reaction to cable operators offering high- speed internet access with cable modems.

14 Postacademic Course on Telecommunications 11/5/00 p. 13 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Driver Application: ADSL (VDSL) Modems ADSL : `Asymmetric Digital Subscriber Line’ VDSL : `Very High Speed Digital Subscriber Line’ Backbone Network Access Node max 3.5…5 km Backbone Network Access Node max 0.3…1.5 km cabinet (ONU) optical fiber copper wire

15 Postacademic Course on Telecommunications 11/5/00 p. 14 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Driver Application: ADSL (VDSL) Modems Distance/bitrate trade-off (upstream) distance (km) bitrate (Mbits/sec) ADSL VDSL

16 Postacademic Course on Telecommunications 11/5/00 p. 15 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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) useful signal FEXTNEXT

17 Postacademic Course on Telecommunications 11/5/00 p. 16 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Driver Application: ADSL (VDSL) Modems Communication Impairments :

18 Postacademic Course on Telecommunications 11/5/00 p. 17 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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. !

19 Postacademic Course on Telecommunications 11/5/00 p. 18 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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..

20 Postacademic Course on Telecommunications 11/5/00 p. 19 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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)

21 Postacademic Course on Telecommunications 11/5/00 p. 20 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Driver Application: ADSL (VDSL) Modems ADSL (Preview) DAC S/PS/P FFT FEQ IFFT P/SP/S Tx clock Discrete equivalent channel Rx clock p(t) Tx filterChannelRx filter ch(t) r(t)ADC

22 Postacademic Course on Telecommunications 11/5/00 p. 21 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA DMT Principles: Preliminaries DMT `inspiration’ : see Lecture 2

23 Postacademic Course on Telecommunications 11/5/00 p. 22 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA DMT Principles : Preliminaries DMT `inspiration’ : see Lecture 2

24 Postacademic Course on Telecommunications 11/5/00 p. 23 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA DMT Principles : Preliminaries DMT `inspiration’ : see Lecture 3

25 Postacademic Course on Telecommunications 11/5/00 p. 24 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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’)

26 Postacademic Course on Telecommunications 11/5/00 p. 25 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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

27 Postacademic Course on Telecommunications 11/5/00 p. 26 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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-domain symbols are transmitter over channel Receiver : demodulation with inverse operation, i.e. FFT (i.o. I-FFT)

28 Postacademic Course on Telecommunications 11/5/00 p. 27 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA DMT Principles: IFFT/FFT-based modulation DMT-transmission block scheme (revisited): DAC Tx clock S/PS/P FFT FEQ IFFT P/SP/S 0 Discrete equivalent channel Rx clock p(t) Tx filterChannelRx filter ch(t) r(t)ADC

29 Postacademic Course on Telecommunications 11/5/00 p. 28 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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 :

30 Postacademic Course on Telecommunications 11/5/00 p. 29 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA DMT Principles: The Magic Prefix Trick Prefix insertion :

31 Postacademic Course on Telecommunications 11/5/00 p. 30 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA DMT Principles: The Magic Prefix Trick Prefix insertion : in the receiver, the samples corresponding to the prefix are removed (=unused) : S/PS/P FFT FEQ IFFT P/SP/S 0 Discrete equivalent channel

32 Postacademic Course on Telecommunications 11/5/00 p. 31 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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-sideRx-side Tone 3 Tone 2 Tone 1 Tone 0 Tone 3 Tone 2 Tone 1 Tone 0 PrefixFrom IFFTGuardbandTo FFT * ch(t) r(t)s(t)

33 Postacademic Course on Telecommunications 11/5/00 p. 32 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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)

34 Postacademic Course on Telecommunications 11/5/00 p. 33 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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)

35 Postacademic Course on Telecommunications 11/5/00 p. 34 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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

36 Postacademic Course on Telecommunications 11/5/00 p. 35 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA DMT Principles: The Magic Prefix Trick Received samples for symbol m, after removing prefix :

37 Postacademic Course on Telecommunications 11/5/00 p. 36 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA DMT Principles: The Magic Prefix Trick...This is equivalent with :

38 Postacademic Course on Telecommunications 11/5/00 p. 37 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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) !

39 Postacademic Course on Telecommunications 11/5/00 p. 38 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA DMT Principles: The Magic Prefix Trick …By substituting this :

40 Postacademic Course on Telecommunications 11/5/00 p. 39 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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.

41 Postacademic Course on Telecommunications 11/5/00 p. 40 Module-3 Transmission Marc Moonen Lecture-7 Multi-tone Modulation K.U.Leuven-ESAT/SISTA 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) S/PS/P FFT FEQ IFFT P/SP/S 0 Discrete equivalent channel


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