1. ADSL Transceivers Dr. A. Falahati BSc, C.Eng, MSc, MIC, PhD, MIEE Dept. of Electrical & Electrical Eng. Iran University of Sceince & Technology afalahati@iust.ac.ir

2. 18 - 2 Outline Introduction Modulation Asymmetric DSL (ADSL) Transceivers –Multicarrier Modulation in ADSL –ADSL Transceiver Block Diagram Combating Inter-symbol Interference –Single-carrier communication systems –Multicarrier communication systems Equalization in ADSL Transceivers –Time-Domain Equalization –Frequency-Domain Equalization Conclusion

3. 18 - 3 Digital Subscriber Line (DSL) Broadband Access Customer Premises downstream upstream Voice Switch Central Office DSLAM DSL modem LPF Interne t DSLAM - Digital Subscriber Line Access Multiplexer LPF – Low Pass Filter (passes voiceband frequencies) Telephone Network Introduction

4. 18 - 4 Discrete Multitone (DMT) DSL Standards ADSL – Asymmetric DSL Maximum data rates supported in G.DMT standard (ideal case) Echo cancelled: 14.94 Mbps downstream, 1.56 Mbps upstream Frequency division multiplexing: 13.38 Mbps downstream, 1.56 Mbps up Widespread deployment in US, Canada, Western Europe, Hong Kong Central office providers only installing frequency-division multiplexed (FDM) ADSL:cable modem market 1:2 in US & 5:1 worldwide ADSL+ 8 Mbps downstream min. ADSL2 doubles analog bandwidth VDSL – Very High Rate DSL Asymmetric Faster G.DMT FDM ADSL 2 m subcarriers m  [8, 12] Symmetric: 13, 9, or 6 Mbps Optional 12-17 MHz band Introduction

5. 18 - 5 Spectral Compatibility of xDSL 10k100k1M10M100M Plain Old Telephone Service ISDN ADSL - USA ADSL - Europe HDSL/SHDSL HomePNA VDSL UpstreamDownstreamMixed Frequency (Hz) 1.1 MHz 12 MHz Any overlap with the AM radio band? Any overlap with the FM radio band? Introduction

6. 18 - 6 Message Source Modulator Encoder ChannelDemodulator Decoder Message Sink Noise Transmitter Receiver A Digital Communications System Encoder maps a group of message bits to data symbols Modulator maps these symbols to analog waveforms Demodulator maps received waveforms back to symbols Decoder maps the symbols back to binary message bits Modulation

7. 18 - 7 Amplitude Modulation by Cosine Function Example: y(t) = f(t) cos(  0 t) f(t) is an ideal lowpass signal Assume  1 <<  0 Y(  ) is real-valued if F(  ) is real-valued Demodulation is modulation then lowpass filtering Similar derivation for modulation with sin(  0 t) 0 1  --  F()F()  0 ½ -   -   -   +       -     +    ½F    ½F    Y()Y() Modulation

8. 18 - 8 Amplitude Modulation by Sine Function Example: y(t) = f(t) sin(  0 t) f(t) is an ideal lowpass signal Assume  1 <<  0 Y(  ) is imaginary-valued if F(  ) is real-valued Demodulation is modulation then lowpass filtering 0 1  --  F()F()  j ½ -   -   -   +       -     +    -j ½F    j ½F    -j ½ Y()Y() Modulation

9. 18 - 9 –Single carrier –Single signal, occupying entire available bandwidth –Symbol rate is bandwidth of signal being centered on carrier frequency Quadrature Amplitude Modulation (QAM) Bits Constellation encoder Bandpass Lowpass filter I Q Transmit cos(2  f c t) sin(2  f c t) - Modulator I Q 00110 frequency channel magnitude fcfc Modulation

10. 18 - 10 Multicarrier Modulation Divide broadband channel into narrowband subchannels –No ISI in subchannels if constant gain in every subchannel and if ideal sampling –Each subchannel has a different carrier Discrete multitone modulation –Based on fast Fourier transform –Standardized for ADSL and VDSL subchannel (QAM signal) frequency magnitude carrier DTFT -1 pulse sinc  n cc  c channel Subchannels are 4.3 kHz wide in ADSL and VDSL Modulation

11. 18 - 11 Multicarrier Modulation by Inverse FFT x x x + g(t)g(t) g(t)g(t) g(t)g(t) x x x + Discrete time g(t) : pulse shaping filter X i : i th symbol from encoder I Q Modulation

12. 18 - 12 Multicarrier Modulation in ADSL N-point Inverse Fast Fourier Transform (IFFT) X1X1 X2X2 X1*X1* x0x0 x1x1 x2x2 x N-1 X2*X2* X N/2 X N/2-1 * X0X0 N real- valued time samples forms ADSL symbol N/2 subchannels (carriers) QAM 00101 I Q Mirror complex data (in red) and take conjugates: ADSL Transceivers

13. 18 - 13 Multicarrier Modulation in ADSL CP: Cyclic Prefix N samplesv samples CP s y m b o l ( i ) s y m b o l ( i+1) copy D/A + transmit filter Inverse FFT ADSL frame is an ADSL symbol plus cyclic prefix ADSL Transceivers

14. 18 - 14 Multicarrier Demodulation in ADSL N-point Fast Fourier Transform (FFT) N time samples N/2 subchannels (carriers) S/P ADSL Transceivers

15. 18 - 15 P/S QAM decoder invert channel = frequency domain equalizer S/P quadrature amplitude modulation (QAM) encoder mirror data and N -IFFT add cyclic prefix P/S D/A + transmit filter N -FFT and remove mirrored data S/P remove cyclic prefix TRANSMITTER RECEIVER N/2 subchannelsN real samples N/2 subchannels time domain equalizer (FIR filter) receive filter + A/D channel Data Transmission in an ADSL Transceiver Bits 00110 P/S parallel-to-serial S/P serial-to-parallel FFT fast Fourier transform ADSL Transceivers

16. 18 - 16 Bit Manipulations Serial-to-parallel converter Example of one input bit stream and two output words Parallel-to-serial converter Example of two input words and one output bit stream S/P Bits 0 0 1 1 0 1 1 0 0 Words S/P Words 0 0 1 1 0 1 1 0 0 Bits ADSL Transceivers

17. 18 - 17 Inter-symbol Interference (ISI) Ideal channel –Impulse response is an impulse –Frequency response is flat Non-ideal channel causes ISI –Channel memory –Magnitude and phase variation Received symbol is weighted sum of neighboring symbols –Weights are determined by channel impulse response 111 1.7.4.1 1 1.7 2.1 1111 Channel impulse response Received signal Threshold at zero Detected signal * = 1.7 1 Combating ISI

18. 18 - 18 Single Carrier Modulation Ideal (non-distorting) channel over transmission band –Flat magnitude response –Linear phase response: delay is constant for all spectral components –No intersymbol interference Impulse response for ideal channel over all frequencies –Continuous time: –Discrete time: Equalizer –Shortens channel impulse response (time domain) –Compensates for frequency distortion (frequency domain) g  [k-  Discretized Baseband System g  (t-  z-z- h + w - xkxk ykyk ekek rkrk nknk + Equalizer Channel g Ideal Channel + Combating ISI

19. 18 - 19 Combat ISI with Equalization Problem: Channel frequency response is not flat Solution: Use equalizer to flatten channel frequency response Zero-forcing equalizer –Inverts channel (impulse response forced to impulse) –Flattens frequency response –Amplifies noise Minimum mean squared error (MMSE) equalizer –Optimizes trade-off between noise amplification and ISI Decision-feedback equalizer –Increases complexity –Propagates error Channel frequency response Zero-forcing Equalizer frequency response MMSE Equalizer frequency response Combating ISI

20. 18 - 20 Cyclic Prefix Helps in Fighting ISI subsymbols to be transmitted mirrored subsymbols cyclic prefix equal to be removed Combating ISI

21. 18 - 21 Cyclic Prefix Helps in Fighting ISI Provide guard time between successive symbols –No ISI if channel length is shorter than +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 N samplesv samples CP s y m b o l ( i ) s y m b o l ( i+1) copy Combating ISI

22. 18 - 22 Channel Impulse Response frequency (kHz) Combating ISI

23. 18 - 23 Channel Impulse Response frequency (kHz) Combating ISI

24. 18 - 24 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 channel impulse response shortened channel impulse response Combating ISI

25. 18 - 25 Eliminating ISI in Discrete Multitone Modulation Time domain equalizer (TEQ) –Finite impulse response (FIR) filter –Effective channel impulse response: convolution of TEQ impulse response with channel impulse response Frequency domain equalizer (FEQ) –Compensates magnitude/phase distortion of equalized channel by dividing each FFT coefficient by complex number –Generally updated during data transmission ADSL G.DMT equalizer training –Reverb: same symbol sent 1,024 to 1,536 times –Medley: aperiodic sequence of 16,384 symbols –At 0.25 s after medley, receiver returns number of bits on each subcarrier that can be supported channel impulse response effective channel impulse response   : transmission delay : cyclic prefix length ADSL Equalization

26. 18 - 26 Time-Domain Equalizer Design Minimizing mean squared error –Minimize mean squared error (MMSE) method [Chow & Cioffi, 1992] –Geometric SNR method [Al-Dhahir & Cioffi, 1996] Minimizing energy outside of shortened channel response –Maximum Shortening SNR method [Melsa, Younce & Rohrs, 1996] –Minimum ISI method [Arslan, Evans & Kiaei, 2000] Maximizing achievable bit rate –Maximum bit rate method [Arslan, Evans, Kiaei, 2000] –Maximum data rate method [Milosevic, Pessoa, Evans, Baldick, 2002] –Bit rate maximization [Vanblue, Ysebaert, Cuypers, Moonen & Van Acker, 2003] Other equalizer architectures –Dual-path (DP) design uses two TEQs [Ming, Redfern & Evans, 2002] –TEQ filter bank design [ Milosevic, Pessoa, Evans, Baldick, 2002] –Per tone equalization [Acker, Leus, Moonen, van der Wiel, Pollet, 2001] ADSL Equalization

27. 18 - 27 Minimize E{e k 2 } [Chow & Cioffi, 1992] –Chose length of b (e.g. in ADSL) to shorten length of h * w –b is eigenvector of minimum eigenvalue of channel-dependent matrix –Minimum MSE achieved when where Disadvantages –Does not consider bit rate –Deep notches in equalizer frequency response (zeros out low SNR bands) –Infinite length TEQ case: zeros of b on unit circle (kills subchannels) Minimum Mean Squared Error TEQ Design z-z- h + w b - xkxk ykyk ekek rkrk nknk + b k-  TEQ Channel Amenable to real-time fixed- point DSP implementation ADSL Equalization

28. 18 - 28 Maximum Shortening SNR Solution Minimize energy leakage outside shortened channel length For each possible position of a window of +1 samples, Disadvantages –Does not consider channel capacity –Requires Cholesky decomposition and eigenvector calculation –Does not consider channel noise Amenable to real-time fixed-point DSP realization h w ADSL Equalization

29. 18 - 29 Maximum Shortening SNR Solution h win, h wall : equalized channel within and outside the window Objective function is shortening SNR (SSNR) h + w xkxk ykyk rkrk nknk Choose w to minimize energy outside window of desired length –Locate window to capture maximum channel impulse response energy ADSL Equalization

30. 18 - 30 Simulation Results for 17-Tap TEQs Parameters Cyclic prefix length 32 FFT size (N) 512 Coding gain (dB) 4.2 Margin (dB) 6 Input power (dBm) 23 Noise power (dBm/Hz) -140 Crosstalk noise 24 ISDN disturbers Figure 1 in [Martin, Vanbleu, Ding, Ysebaert, Milosevic, Evans, Moonen & Johnson, submitted] Downstream transmission ADSL Equalization

31. 18 - 31 Simulation Results for 17-Tap TEQs (con’t) Parameters Cyclic prefix length 32 FFT size (N) 512 Coding gain (dB) 4.2 Margin (dB) 6 Input power (dBm) 23 Noise power (dBm/Hz) -140 Crosstalk noise 24 ISDN disturbers Figure 3 in [Martin, Vanbleu, Ding, Ysebaert, Milosevic, Evans, Moonen & Johnson, submitted] Downstream transmission ADSL Equalization

32. 18 - 32 Single-path, dual-path, per-tone & TEQ filter bank equalizers Available at http://www.ece.utexas.edu/~bevans/projects/adsl/dmtteq/ Matlab DMT TEQ Design Toolbox 3.1 various performance measures default parameters from G.DMT ADSL standard different graphical views -140 23 ADSL Equalization

33. 18 - 33 Multicarrier Modulation Advantages –Efficient use of bandwidth without full channel equalization –Robust against impulsive noise and narrowband interference –Dynamic rate adaptation Disadvantages –Transmitter: High signal peak-to-average power ratio –Receiver: Sensitive to frequency and phase offset in carriers Open issues for point-to-point connections –Pulse shapes of subchannels (orthogonal, efficient realization) –Channel equalizer design (increase bit rate, reduce complexity) –Synchronization (timing recovery, symbol synchronization) –Bit loading (allocation of bits in each subchannel) Open issues for coordinating multiple connections

34. 18 - 34 Residential Business Applications of Broadband Access Notes

35. 18 - 35 DSL Broadband Access Standards Courtesy of Mr. Shawn McCaslin Notes

36. 18 - 36 ADSL and Cable Modems Need for high-speed (broadband) data access –Voiceband data modems can yield 53 kbps (kilobits per second) –Telephone voice channel capacity ois 64 kbps (the Central Office samples voice signals at 8 kHz using 8 bits/sample) –Integrated Services Digital Network (ISDN) modems deliver 128 kbps –New modem standards are necessary to meet the demand for higher bandwidth access for telecommuting, videoconferencing, video-on- demand, Internet service providers, Internet access, etc. Two standards tested in 1998 and now widely available –Cable modems –Asymmetric Digital Subscriber Line (ADSL) modems Cable Modems –Always connected to the Internet –Your neighbors on the same local area network share the bit rate –Local area network provides either 27 or 36 Mbps downstream, and between 320 kbps and 10 Mbps upstream. Notes

37. 18 - 37 ADSL Modems ADSL modems –Always connected to the Internet –Call central office using a dedicated telephone line which also supports a conventional Plain Old Telephone Service (POTS) line for voice –Connection time is 5-10 seconds –ADSL modems are capable of delivering 1-10 Mbps from the central office to the customer (downstream) and 0.5-1 Mbps from the customer to the central office (upstream) –Although ADSL lines have been available from Southwestern Bell since the Fall of 1997, ADSL modems were not commercially available until Fall of 1999. Notes

38. 18 - 38 Discrete Multitone (DMT) Modulation DMT uses multiple harmonically related carriers –Implemented as inverse Fast Fourier Transform (FFT) in transmitter –Implemented using forward FFT in receiver Transmission bandwidth –1.1 MHz downstream and 256 kHz upstream –Limit of 1.1 MHz is due to power constraints imposed by the FCC –For 18 kft telephone lines, the attenuation at 1.1 MHz is -120 dBm. Frequency domain is divided into 256 4.3-kHz bins –Channel 0 is dedicated to voice –Channels 1-5 are not used due to compatibility with ISDN services. Notes

39. 18 - 39 Two Types of Transmission Two versions of ADSL 1.Frequency Division Multiplexing: the upstream and downstream channels do not overlap: the upstream uses channels 6-31 and the downstream uses channels 32-255. 2.Echo Cancelled: the upstream and downstream channels overlap: the upstream uses channels 6-31 and the downstream uses channels 6-255. According to available SNR in each bin, bin carries –QAM signal whose constellation varies from 2-15 bits or –no signal if SNR is less than 12 dB in that subchannel Constellations chosen so that overall bit error rate < 10 -7 Maximum transmission rate with symbol rate of 4 kHz –Downstream: 248 channels x 15 bits/channel x 4 kHz = 14.88 Mbps –Upstream: 24 channels x 15 bits/channel x 4 kHz = 1.440 Mbps Notes

40. 18 - 40 Channel Attenuation Reliable transmission of high-frequency information over a telephone line is wrought with several challenges. –Telephone lines are unshielded and bundled 50 wires to a trunk. The other lines in the bundle can cause severe crosstalk –Telephone lines attenuate signals. The attenuation increases with increasing frequency. At 1.1 MHz, which is the highest transmitted frequency, the attenuation of a 24 gauge wire is 10 kft -70 dBm/Hz 16 kft -110 dBm/Hz 12 kft -90 dBm/Hz 18 kft -120 dBm/Hz 14 kft -100 dBm/Hz Because of severe effects in the channel, the ADSL standard defines channel coding using cyclic prefixes and employs error correcting codes Notes

41. 18 - 41 Bridge Taps Bridge Taps are unterminated lines –During modem initialization, effect of bridge taps is included in channel estimate. Their effect would be to lower the possible channel capacity. –During data transmission, bridge taps may saturate the front-end and at a least will be unpleasant for the echo canceller. The echo canceller should have an estimate of the echo channel including the bridge taps. Given that the reflected echo is almost instantaneous than the echo canceller channel estimate should capture them too. In G.lite, echo cancellation is optional –Modems who use it can still use it –A bigger problem in G.lite is the phone due to the splitterless environment –Transmitters that do not have an echo canceller system can rely on their receive filters to reduce the echo. Notes

42. 18 - 42 ADSL Modems ADSL modem consists of a line driver plus 3 subsystems: 1.analog front end (15 V) 2.digital interface (3 V) 3.discrete multitone processor (3 V) Analog front end provides the analog-to-digital and digital- to-analog interfaces to the telephone line. Digital interace manages the input and output digital message streams. Discrete multitone processor implements the digital communications and signal processing to support the ADSL standard. An ADSL modem requires much greater than 200 Digital Signal Processor MIPS. Notes

43. 18 - 43 Motorola CopperGold ADSL Chip Announced March 1998 5 million transistors, 144 pins, clocked at 55 MHz 1.5 W power consumption DMT processor contains –Motorola MC56300 DSP core –Several application specific ICs 512-point FFT 17-tap FIR filter for time-domain channel equalization based on MMSE method (20 bits precision per tap) DSP core and memory occupies about 1/3 of chip area It gives up to 8 Mbps upstream and 1 Mbps downstream Notes

44. 18 - 44 Motorola Copper Gold ADSL Transceiver Contains all 3 ADSL modem subsystems on a single chip. –Has programmable bit to tell it whether it is at customer's or central office site –Analog front end operates at a sampling rate of 2.208 MHz and gives 16 bits/sample of resolution. It uses sigma-delta modulation with an oversampling factor of 55 / 2.208 = 25. Discrete multitone processor consists of a Motorola MC56300 DSP Onyx core and several application-specific digital VLSI circuits to implement –256-point FFT for downstream transmission or 512-point FFT for downstream reception if it is at the central office or customer's site, respectively –17-tap adaptive FIR filter for channel equalization (20 bits of precision per tap) running at 2.208 MHz –DSP core computes the 32-point FFT for the downstream transmission or the 64-point FFT for the downstream reception. Notes

45. 18 - 45 Minimum Mean Squared Error TEQ Matrix O selects the proper part out of R x|y corresponding to the delay  z-z- h + w b - xkxk ykyk ekek zkzk rkrk nknk + Notes

46. 18 - 46 Simulation Results for 17-Tap TEQ Cyclic prefix length 32 FFT size (N)512 Coding gain 4.2 dB Margin 6 dB Input power 23 dBm Noise power -140 dBm/Hz Crosstalk noise 8 ADSL disturbers POTS splitter5 th order Chebyshev Notes

47. 18 - 47 Simulation Results for Three-Tap TEQ Cyclic prefix length 32 FFT size (N)512 Coding gain 4.2 dB Margin 6 dB Input power 23 dBm Noise power -140 dBm/Hz Crosstalk noise 8 ADSL disturbers POTS splitter5 th order Chebyshev Notes