Module3 Transmission Lecture8 11500 Marc Moonen Dept E

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

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 Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 2

Acknowledgement -Lecture partly adopted from Module T 2 `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 Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 3

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

ADSL/VDSL Revisited : Specs ADSL specs (=ANSI standard) • • • 256 carriers 512 -point (I)FFT’s (or `similar’) prefix length = 32 carrier spacing fo= 4. 3215 k. Hz basic sampling rate is 2. 21 MHz (=512*4. 3215 k) VDSL specs (=proposal) • up to 4096 carriers • same carrier spacing • etc. Postacademic Course on Telecommunications Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 5

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

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

ADSL/VDSL Revisited : Spectrum • VDSL spectrum : Power spectrum unused tones due to line conditions Up Down POTS/ ISDN Frequency 17. 6 MHz 1. 1 MHz tone spacing (4. 3 k. Hz) Postacademic Course on Telecommunications Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 8

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

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

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

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 Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 12

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 Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 13

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

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

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 timedomain signals have lower dynamic range • receiver re-design : receiver compensates for clipping non-linearity (part of `channel model’) • . . . Postacademic Course on Telecommunications Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 16

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 Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 17

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: 32 taps @ 2. 2 MHZ -> 70 Mflops. . . Postacademic Course on Telecommunications Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 18

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

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

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 Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 21

Time-Domain Equalization (TEQ) • Equalizer design (continued): - Alternative optimization criterion is time-domain minimum mean squared error criterion based on…. channel impulse response time-domain equalizer synchronization delay (p 20) target impulse response Postacademic Course on Telecommunications Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 22

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 Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 23

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 Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 24

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

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 Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 26

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 Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 27

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

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 Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 29

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

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 CP CS DMT symbol Postacademic Course on Telecommunications Module-3 Transmission Lecture-8 Multi-tone Modulation CP: cyclic prefix CS: cyclic suffix Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 31

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

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

Echo Cancellation • Problem Statement : ADSL with overlapping up- and downstream bands requires echo cancellation (adaptive filter) Main problem = complexity example : echo path with 500 taps @ 2. 2 MHz ->. . . Gflops Postacademic Course on Telecommunications Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 34

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

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 Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 36

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

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 Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 38

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 Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 39

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…. Postacademic Course on Telecommunications Module-3 Transmission Lecture-8 Multi-tone Modulation Marc Moonen K. U. Leuven-ESAT/SISTA 11/5/00 p. 40