DSP 2 Project ADSL Equalization Students Dung Nguyen

DSP 2 Project ADSL Equalization Students: Dung Nguyen Quoc- Master Student Tudor Tesu- Erasmus Student Supervisor Jan Vangorp

ADSL Equalization- General Topics n Asymmetric Digital Subscriber Line (ADSL) is a form of DSL (Digital Subscriber Line), a data communication technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem. n Broadband communications over telephone lines is summarized in the next table: n A gateway is commonly used to make an ADSL connection; the application of ADSL is on GSTN (General Switched Telephone Network), FDM (Frequency Division Multiplexed) group-band cellular radio. n ADSL has several similarities to ISDN (Integrated Services Digital Network or Isolated Subscriber Digital Network). 12 November 2008 DSP 2 Project 2

ADSL Equalization- General Topics n ADSL provides access to digital services at several Mb/s over the ordinary wire-pair connection between customers and the local telephone company central office. n Available transmission band is divided in a certain number of subbands (sending data in interleaved bit streams), each one transmitting different substreams over different channels. n DMT voiceband modems have been adopted for the Asymmetric Digital Subscriber Line (ADSL), which provides digital communication at several Mb/s from a telephone company central office to a subscriber, and a lower rate in the reverse direction, over a normal twisted pair of wires in the loop plant. 12 November 2008 DSP 2 Project 3

ADSL Equalization- General Topics General Trasmission Channel for ADSL 12 November 2008 DSP 2 Project 4

ADSL Equalization- General Topics Power Spectrum for ADSL Transmission Channel Including examples for 3 schemes of QAM modulation 12 November 2008 DSP 2 Project 5

MCM Technique n MCM (Multicarrier modulation) is the general generic name for QAM and Orthogonal FDM Modulation technique). n Parameters that have to be taken into consideration when talking about MCM: mc=bits per symbol, the proportion, the total transmitted power—all these refer to multicarrier signal. n MCM is good because in modern receivers the MCM signal has to be processed (in digital domain) without enhancement of noise and interference caused by linear equalization of a single carrier signal. n MCM in good because the symbols generated are immune to noise and fast fades. n Multiplexing: needs the separation of carriers at the receiver (for future demodulation). n The performance of sending/receiving data is evaluated using SNR (Signal To Noise Ratio) 12 November 2008 DSP 2 Project 6

MCM Technique n Problems that can appear with the MCM: impulse noise, phase jitter, nonlinear distortion, single frequency interference fades. n Reduction in bit rate caused by the use of general period can be done by linearly equalizing the received signal. n TDL (Tapped Delay Line is the most suitable for Equalization) n FFT/IFFT MCM based methods: OFDM (Orthogonal Division Frequency Multiplexing) and DMT( Discrete Multitone). n The key components of an OFDM system are the Inverse DFT(IFFT Block) in the transmitter and the DFT(FFT Block) at the receiver. 12 November 2008 DSP 2 Project 7

ADSL Equalization - Why equalizing filters? n n Compensate for the unequal frequency response of some other signal processing circuit or system (e. g. improve the fidelity of sound, to remove undesired noises) When the channel consists of telephone lines, the channel transfer function changes from call to call and the equalizing filter needs to be an adaptive filter 12 November 2008 DSP 2 Project 8

ADSL Equalization- Types of equalizations in time domain n Time domain equalization is used to restrict Inter-symbol Interference to a tolerable extent - CF: Impulse shortening filter - CB: Reference filter with reference tap values - Estimator: Adaptive filter to find the optimum equalizer coefficients) Equalizer configuration in training mode 12 November 2008 DSP 2 Project 9

ADSL Equalization- Types of equalizations in frequency domain n Frequency domain equalization is used to compensate for channel complex gain (correct for signal phase rotation and signal amplitude attenuation) at each sub-carrier frequency. - Y: Tap Input - D: Desired response - Decision: Making desired response - X: Adaptive weight-control filter - A: Transversal filter with optimum multiplier A (in case of 1 tap filter) LMS adaptation of a frequency domain equalizer multiplier. 12 November 2008 DSP 2 Project 10

ADSL Equalization- Conclusions: n General transmission channel for ADSL lines/power spectrum for upstream/downstream data n MCM Technique used in ADSL modems n Equalization methods in time/frequency domain Further Studies (Second presentation- 4 th December 2008) n Full block simulation in MATLAB/SIMULINK for the communication channel (both the transmitter and the receiver). n Adaptive frequency-domain Equalization Filters (N 1 -tap adaptive filters). n Equalizer in time domain (The channel impulse response is modeled as ARMA system - autoregressive moving average model). Bibiliography n John A. C Bingham- Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come, IEEE Communications Magazine, May 1990 n Koen Vanbleu, Geert Ysebaert, Gert Cuypers and Marc Moonen- On the relationship between time domain equalizers and PER Tone Equalizers for DMT Based Systems IEEE Benelux Signal Processing Symposium , April 2004 12 November 2008 DSP 2 Project 11