11/5/00 p. 1 Postacademic Course on Telecommunications Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven/ESAT-SISTA Module-3 : Transmission Lecture-8 (11/5/00) Marc Moonen Dept. E.E./ESAT, K.U.Leuven

Postacademic Course on Telecommunications 11/5/00 p. 2 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Lecture 8 : Overview ADSL/VDSL revisited Specs, spectra, FDM vs. echo, bit loading, 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’

Postacademic Course on Telecommunications 11/5/00 p. 3 Module-3 Transmission Marc Moonen Lecture-8 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 Telecommunications Special thanks to G. Van der Plas (Alcatel) -Lecture partly co-authored by Thierry Pollet, Alcatel Telecom, CRC, Antwerp (B)

Postacademic Course on Telecommunications 11/5/00 p. 4 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA ADSL/VDSL Revisited ADSL-DMT Transmission block scheme (see Lecture-7)

Postacademic Course on Telecommunications 11/5/00 p. 5 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA ADSL/VDSL Revisited : Specs ADSL specs (=ANSI standard) 256 carriers 512-point (I)FFT’s (or `similar’) prefix length = 32 carrier spacing fo= kHz basic sampling rate is 2.21 MHz (=512*4.3215k) VDSL specs (=proposal) up to 4096 carriers same carrier spacing etc.

Postacademic Course on Telecommunications 11/5/00 p. 6 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA ADSL/VDSL Revisited : Spectrum ADSL spectrum :

Postacademic Course on Telecommunications 11/5/00 p. 7 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA ADSL/VDSL Revisited : Spectrum ADSL spectrum : FDD-ADSL (=frequency division duplex) EC-ADSL (=overlapping up- and downstream band)

Postacademic Course on Telecommunications 11/5/00 p. 8 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA ADSL/VDSL Revisited : Spectrum VDSL spectrum : Power spectrum unused tones due to line conditions tone spacing (4.3 kHz) POTS/ ISDN 17.6 MHz 1.1 MHz Frequency Up Down

Postacademic Course on Telecommunications 11/5/00 p. 9 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA ADSL/VDSL Revisited : Spectrum VDSL spectrum : flexible setting up- & downstream rates Upstream Downstream Frequency Medium line Long line Short line

Postacademic Course on Telecommunications 11/5/00 p. 10 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA ADSL/VDSL Revisited : Channel ADSL channel attenuation, crosstalk, noise

Postacademic Course on Telecommunications 11/5/00 p. 11 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA ADSL/VDSL Revisited : Channel VDSL channel attenuation, crosstalk, noise, RFI: Next dominated environment + several RF interferers

Postacademic Course on Telecommunications 11/5/00 p. 12 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA ADSL/VDSL Revisited: Bit Loading Bit loading and gain settings : QAM constellation size for each tone depends on SNR measured at the receiver Guarantees optimal capacity in all operation conditions (loops, noise).

Postacademic Course on Telecommunications 11/5/00 p. 13 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA ADSL/VDSL Revisited: Bit Loading Bit loading and gain settings Transmit power optimization to obtain BER=10^-7 on each tone

Postacademic Course on Telecommunications 11/5/00 p. 14 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA ADSL/VDSL Revisited: Bit Loading Bit loading and gain settings Transmit power optimization to obtain BER=10^-7 on each tone (continued) SNR gain 5 bits 4 bits 3 bits 2 bits 1 bits

Postacademic Course on Telecommunications 11/5/00 p. 15 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA ADSL/VDSL Revisited: P-to-A Peak-to-average problem DMT signals have large dynamic range :

Postacademic Course on Telecommunications 11/5/00 p. 16 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA ADSL/VDSL Revisited: P-to-A Peak-to-average problem (continued) large dynamic range implies large quantization steps in A/D & D/A, etc. Remedies tone structure : redundant tones, modulated with linear combination of other tones, such that time- domain signals have lower dynamic range receiver re-design : receiver compensates for clipping non-linearity (part of `channel model’)...

Postacademic Course on Telecommunications 11/5/00 p. 17 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Equalization Problem Statement : ISI/ICI-free transmission if L is channel impulse response length is prefix length In ADSL : prefix-length is 32 samples channel length is….500 samples (approx.) !!! Solution : Insert time-domain equalizer for channel shortening

Postacademic Course on Telecommunications 11/5/00 p. 18 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Equalization Time-domain equalization : ps: in Lecture 5, (zero-forcing) equalization means `shortening to 1 non-zero tap’ (cfr. H(z).C(z)=1) ps: complexity ! Example: MHZ -> 70 Mflops...

Postacademic Course on Telecommunications 11/5/00 p. 19 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Equalization Time-domain equalization :

Postacademic Course on Telecommunications 11/5/00 p. 20 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Equalization Time-domain equalization additional design parameter : synchronization delay (=`positioning’ of shortened channel impulse response)

Postacademic Course on Telecommunications 11/5/00 p. 21 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Time-Domain Equalization (TEQ) Equalizer design : - Possible optimization criterion is overall capacity, i.e. sum of transmitted bits (with BER 10^-7) on all carriers together. - Number of bits loaded onto 1 carrier is a function of the received SNR for that carrier. - SNR for 1 carrier is function of SNR at TEQ-output. - Leads to highly non-linear optimization criterion. Iterative non-linear optimization procedures usually lead to (sub-optimal) local minima. - Need simpler optimization criterion.

Postacademic Course on Telecommunications 11/5/00 p. 22 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Time-Domain Equalization (TEQ) Equalizer design (continued): - Alternative optimization criterion is time-domain minimum mean squared error criterion based on…. channel impulse response target impulse response time-domain equalizer synchronization delay (p20)

Postacademic Course on Telecommunications 11/5/00 p. 23 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Time-Domain Equalization (TEQ) Equalizer design (continued): - minimum mean squared error criterion is... i.e., for a given TEQ-length (e.g. 16 taps), find TEQ-coefficients such that cascade (`convolution’) of CIR and TEQ is as close as possible to a (delayed) TIR with non-zero coefficients ( = prefix length)

Postacademic Course on Telecommunications 11/5/00 p. 24 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Time-Domain Equalization (TEQ) Equalizer design (continued): -This leads to a least squares problem a la the least squares problems for equalizer design in Lecture-6. - Have to include a constraint to avoid trivial (TEQ=TIR=0) solution, e.g. impose `normalized’ TIR or `normalized’ TEQ - Constrained least squares problem equivalent to eigenvalue problem (solved iteratively)

Postacademic Course on Telecommunications 11/5/00 p. 25 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Time-Domain Equalization (TEQ) Resulting structure (TEQ + 1-taps FEQ) example: ADSL-TEQs with 16…64 taps

Postacademic Course on Telecommunications 11/5/00 p. 26 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Time-Domain Equalization (TEQ) Problem with TEQ-receiver : - MMSE criterion does correspond to optimizing overall bit-rate. Leads to unpredictable behavior (sometimes lower bit-rate with longer TEQ…) - Obtained bit-rate heavily (and unpredictably) depends on chosen synchronization delay - In practice : modem start-up procedure does not allow for searching for optimal delay within a range. Hence can pick only one delay, and then have to be lucky..

Postacademic Course on Telecommunications 11/5/00 p. 27 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Frequency-Domain Equalization (FEQ) Alternative structure based on `per-tone equalization’ : - each tone had its own TEQ, but implemented after the FFT (and combined with 1-taps FEQ) - overall `FEQ’ (=TEQ + 1-taps FEQ) can be designed to optimize SNR for each tone separately. Hence true overall bit-rate optimization. - Smooth behavior (as a function of synchronization delay) - Computational complexity = comparable - Implementation based on sliding FFT properties

Postacademic Course on Telecommunications 11/5/00 p. 28 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Frequency-Domain Equalization (FEQ) `Per-tone’ equalization structure : `difference terms’ cfr. sliding FFT properties

Postacademic Course on Telecommunications 11/5/00 p. 29 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Windowing (VDSL) Problem Statement : Transmit spectrum for 1 tone has high side-lobes, that generate interference (e.g.NEXT) in other systems. Receiver DFT may be viewed as filter bank, each filter has high side-lobes, hence picks up interference in a broad frequency range Solution : Reduce transmit/receive side-lobes by (time-domain) windowing

Postacademic Course on Telecommunications 11/5/00 p. 30 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Windowing (VDSL) Windowing basic principle = extend symbol length + apply `symmetric’ weighting around original symbol edges

Postacademic Course on Telecommunications 11/5/00 p. 31 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Windowing (VDSL) Windowing at the transmitter: Reduces frequency domain side-lobes Reduces the out of band power of the NEXT signal Allows for digital duplexing Guarantees spectral compatibility with ADSL, HPNA,… Symbol body CPCS DMT symbol CP: cyclic prefix CS: cyclic suffix

Postacademic Course on Telecommunications 11/5/00 p. 32 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA DMT symbol Symbol body Windowing (VDSL) Windowing at the receiver : Reduces Inter Symbol Interference (ISI) Reduces susceptibility to RFI ps: This is equivalent to per-tone equalization (=per-tone optimized window).

Postacademic Course on Telecommunications 11/5/00 p. 33 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Windowing (VDSL) DMT-based VDSL-transceiver : additional features : analog & digital RFI-cancellation, egress spectrum control,...

Postacademic Course on Telecommunications 11/5/00 p. 34 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Echo Cancellation Problem Statement : ADSL with overlapping up- and downstream bands requires echo cancellation (adaptive filter) Main problem = complexity example : echo path with MHz ->...Gflops

Postacademic Course on Telecommunications 11/5/00 p. 35 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Echo Cancellation Time-domain echo cancellation : Adaptive filter (see Lecture 6) has complexity > 2M (M is number of filter taps). Leads to too high computational complexity Time + Frequency domain echo cancellation : Part of the cancellation is brought to the frequency- domain (after the FFT). Leads to complexity reduction with a factor..4…8. Conclusion : Overlapping up- & downstream bands difficult to implement cost) PS : EC in FDD-system with reduced front-end filter specs

Postacademic Course on Telecommunications 11/5/00 p. 36 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Echo Cancellation PS: alternative solutions - `Joint Shortening’ : Design TEQ such that echo channel impulse response length is shortened, together with transmission channel impulse response length. Allows for shorter (cheaper) echo cancellation filter. - `Per-tone’ echo cancellation : optimize TEQ + echo filter for each tone separately (=true SNR optimization!). Allows for shorter (cheaper) echo cancellation filter

Postacademic Course on Telecommunications 11/5/00 p. 37 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Conclusion Investigated additional features in ADSL/VDSL systems - Equalization - Windowing - Echo Cancellation - … Confirms importance of digital signal processing techniques in present-day high- performance communication systems (`box full of maths/DSP’,cfr. Lecture-1)

Postacademic Course on Telecommunications 11/5/00 p. 38 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Assignment 4.1 Recommended (optional) reading: J.A.C. Bingham, `Multicarrier modulation for data transmission, an idea whose time has come’, IEEE Communications Magazine, May 1990, pp.5-14

Postacademic Course on Telecommunications 11/5/00 p. 39 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Assignment 4.2 Echo Cancellation in an FDD/ADSL-system ? Assume an FDD/ADSL transceiver sending an upstream signal (lower tones) and receiving a downstream signal (higher tones) Assume the downstream path as well as the echo path is shorter than the prefix length, and the up- & downstream signals being perfectly (symbol-)synchronized. Investigate the effect of the echo... Assume the echo path is longer than the prefix. Investigate the effect of the echo... Assume the echo path is shorter than the prefix, but that the two signals are not (symbol-)synchronized. Investigate the effect of the echo…

Postacademic Course on Telecommunications 11/5/00 p. 40 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA Assignment 4.2 Echo Cancellation in an FDD/ADSL-system ? (continued) Is it possible to have (symbol-)synchronization at the two ends of the line (i.e. in the remote terminal as well as in central office) ? Conclusion: which extra features to be introduced in FDD modems. Echo cancellation or… ? To be solved by using your brains and/or pen&paper (i.e. write down a few formulas) and/or your favorite simulation tool….