1 / 32

Introduction to ADSL Modems

Introduction to ADSL Modems. Prof. Brian L. Evans Dept. of Electrical and Comp. Eng. The University of Texas at Austin http://signal.ece.utexas.edu. Last modified August 27, 2005. UT graduate students: Mr. Zukang Shen, Mr. Daifeng Wang, Mr. Ian Wong

mliss
Download Presentation

Introduction to ADSL Modems

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. Introduction to ADSL Modems Prof. Brian L. EvansDept. of Electrical and Comp. Eng.The University of Texas at Austinhttp://signal.ece.utexas.edu Last modified August 27, 2005 UT graduate students: Mr. Zukang Shen, Mr. Daifeng Wang, Mr. Ian Wong UT Ph.D. graduates:Dr. Güner Arslan (Silicon Labs), Dr. Biao Lu (Schlumberger), Dr. Ming Ding (Bandspeed), Dr. Milos Milosevic (Schlumberger) UT senior design students:Wade Berglund, Jerel Canales, David J. Love,Ketan Mandke, Scott Margo, Esther Resendiz, Jeff Wu Other collaborators:Dr. Lloyd D. Clark (Schlumberger), Prof. C. Richard Johnson, Jr. (Cornell), Prof. Sayfe Kiaei (ASU), Prof. Rick Martin (AFIT),Prof. Marc Moonen (KU Leuven), Dr. Lucio F. C. Pessoa (Motorola),Dr. Arthur J. Redfern (Texas Instruments)

  2. Outline • Broadband Access • Applications • Digital Subscriber Line (DSL) Standards • ADSL Modulation Methods • ADSL Transceiver Block Diagram • Quadrature Amplitude Modulation • Multicarrier Modulation • ADSL Transceiver Design • Inter-symbol Interference • Time-Domain Equalization • Frequency-Domain Equalization • Conclusion

  3. Applications of Broadband Access

  4. downstream upstream DSL Broadband Access Internet DSLAM Central Office ADSL modem ADSL modem Voice Switch LPF LPF Customer Premises PSTN

  5. DSL Broadband Access Standards Courtesy of Mr. Shawn McCaslin (National Instruments, Austin, TX)

  6. 1.1 MHz POTS ISDN ADSL - USA ADSL - Europe HDSL/SHDSL HomePNA VDSL - FDD optional 10k 100k 1M 10M 100M Frequency (Hz) 12 MHz Upstream Downstream Mixed Spectral Compatibility of xDSL

  7. ADSL Modem N/2 subchannels N real samples S/P quadrature amplitude modulation (QAM) encoder mirror data and N-IFFT add cyclic prefix P/S D/A + transmit filter Bits 00110 TRANSMITTER channel RECEIVER N/2 subchannels N real samples P/S time domain equalizer (FIR filter) QAM demoddecoder N-FFT and remove mirrored data S/P remove cyclic prefix receive filter + A/D invert channel = frequency domain equalizer

  8. Serial-to-parallel converter Example of one input bit stream and two output words Parallel-to-serialconverter Example of two input words and one output bit stream Bit Manipulations S/P S/P 110 110 00110 00110 00 00 Words Bits Bits Words

  9. Amplitude Modulation by Cosine Function • Multiplication in time is convolution in Fourier domain • Sifting property of the Dirac delta functional • Fourier transform property for modulation by a cosine

  10. F(w) 1 w -w1 w1 0 ½F(w + w0) ½F(w - w0) ½ -w0 - w1 -w0 + w1 w0 - w1 w0 + w1 0 -w0 w0 Amplitude Modulation by Cosine Function • Example:y(t) = f(t) cos(w0 t) • f(t) is an ideal lowpass signal • Assume w1 << w0 • Y(w) is real-valued if F(w) is real-valued • Demodulation is modulation then lowpass filtering • Similar derivation for modulation with sin(w0t) Y(w) w

  11. Amplitude Modulation by Sine Function • Multiplication in time is convolution in Fourier domain • Sifting property of the Dirac delta functional • Fourier transform property for modulation by a sine

  12. j ½F(w + w0) -j ½F(w - w0) j ½ w0 w0 - w1 w0 + w1 w -w0 - w1 -w0 + w1 -w0 -j ½ Amplitude Modulation by Sine Function • Example: y(t) = f(t) sin(w0 t) • f(t) is an ideal lowpass signal • Assume w1 << w0 • Y(w) is imaginary-valued ifF(w) is real-valued • Demodulation is modulation then lowpass filtering F(w) 1 w -w1 w1 0 Y(w)

  13. Q Modulator cos(2pfct) Lowpassfilter I TX I Bits Constellation encoder Bandpass - 00110 Q Lowpassfilter sin(2pfct) channel magnitude fc frequency Quadrature Amplitude Modulation (QAM) • One carrier • Single signal, occupying the whole available bandwidth • The symbol rate is the bandwidth of the signal being centered on carrier frequency

  14. Multicarrier Modulation • Divide broadband channel into narrowband subchannels • Discrete Multitone (DMT) modulation • Based on fast Fourier transform (related to Fourier series) • Standardized for ADSL • Proposed for VDSL every subchannel behaves like QAM channel carrier magnitude subchannel frequency Subchannels are 4.3 kHz wide in ADSL

  15. Multicarrier Modulation by Inverse FFT Q g(t) x x I Discrete time g(t) x x + + g(t) x x g(t) : pulse shaping filterXi: ith symbol from encoder

  16. N-point Inverse FastFourierTransform(IFFT) Q X0 X1 x0 X2 x1 I x2 XN/2 XN/2-1* X2* xN-1 X1* Multicarrier Modulation in ADSL QAM 00101 N/2 subchannels (carriers) N time samples

  17. v samples N samples s y m b o l ( i+1) CP CP s y m b o l ( i ) copy copy Multicarrier Modulation in ADSL Inverse FFT D/A + transmit filter CP: Cyclic Prefix

  18. Multicarrier Demodulation in ADSL S/P N-point FastFourierTransform(FFT) N/2 subchannels (carriers) N time samples

  19. Ideal channel Impulse response is an impulse Frequency response is flat Non-ideal channelcauses ISI Channel memory Magnitude and phase variation Received symbol is weighted sum of neighboring symbols Weights are determined by channel impulse response 2.1 1.7 1 1 1 1 1 1 .7 .7 .4 .1 = * Received signal Channelimpulseresponse -1 Threshold at zero 1 1 1 1 1 Detected signal Inter-symbol Interference (ISI)

  20. Channel Impulse Response

  21. Channel Impulse Response

  22. v samples N samples s y m b o l ( i+1) CP CP s y m b o l ( i ) copy copy Cyclic Prefix Helps in Fighting ISI • Provide guard time between successive symbols • No ISI if channel length is shorter than n+1 samples • Choose guard time samples to be a copy of the beginning of the symbol – cyclic prefix • Cyclic prefix converts linear convolution into circular convolution • Need circular convolution so that symbol  channel  FFT(symbol) x FFT(channel) • Then division by the FFT(channel) can undo channel distortion

  23. Cyclic Prefix Helps in Fighting ISI Repeated symbol cyclic prefix * to be removed = equal

  24. channel Shortened channel Combat ISI with Time-Domain Equalizer • Channel length is usually longer than cyclic prefix • Use finite impulse response (FIR) filter called a time-domain equalizer to shorten channel impulse response to be no longer than cyclic prefix length

  25. Discrete-timeconvolution For every k, we compute a new summation Continuous-time convolution For every value of t, we compute a new integral x[k] x(t) h[k] h(t) y[k] y(t) Representedby its impulseresponse Representedby its impulseresponse Convolution Review

  26. Finite Impulse Response (FIR) Filter • Assuming that h[k] is causal and has finiteduration from k = 0, …, N-1 • Block diagram of an implementation (called a finite impulse response filter) x[k] z-1 z-1 … z-1 … h[0] h[1] h[2] h[N-1] S y[k]

  27. nk rk yk ek xk w h + + - b z- zk Example Time-Domain Equalizer • Minimize mean squared errorE{ek2} whereek=bk-- hk*wk • Chose length of bk to shorten length of hk*wk • Disadvantages • Does not consider channel capacity • Deep notches in equalizer frequency response

  28. Frequency Domain Equalizer in ADSL • Problem: FFT coefficients (constellation points) have been distorted by the channel. • Solution: Use Frequency-domain Equalizer (FEQ) to invert the channel. • Implementation: N/2 single-tap filters with complex coefficients.

  29. FEQ FEQ Q Yi I Frequency Domain Equalizer in ADSL Y0 Y1 N/2 subchannels (carriers) YN/2-1 Yi QAM decoder 0101

  30. ADSL Modem N/2 subchannels N real samples S/P quadrature amplitude modulation (QAM) encoder mirror data and N-IFFT add cyclic prefix P/S D/A + transmit filter Bits 00110 TRANSMITTER channel RECEIVER N/2 subchannels N real samples P/S time domain equalizer (FIR filter) QAM demoddecoder N-FFT and remove mirrored data S/P remove cyclic prefix receive filter + A/D invert channel = frequency domain equalizer

  31. Near-End echo Near-End echo f f FEXT f f NEXT f f Crosstalk and Near-End Echo TX TX H cable H RX RX TX TX H cable H RX RX

  32. FDM EC DS DS US US f f ADSL vs. FEXT, NEXT, Near-end Echo • ADSL with Freq. Division Multiplexing - FDM • Near-End Echo filtered out • Self-NEXT (NEXT from another ADSL) mostly filtered out • FEXT and NEXT (from another type of DSL) are problems • ADSL with overlapped spectrum (Echo Cancelled) • Near-End Echo Eliminated using an echo canceller • FEXT, NEXT and self-NEXT are a problem • Larger Spectrum available for downstream – higher data rate

More Related