Practical ADSL Kit

AbstractIn an ongoing effort to study and improve the

performance of ADSL modems, the effect of NEXT and FEXT interference is examined. The ultimate purpose is to introduce filters for NEXT and FEXT interference mitigation that would improve the signal to noise ratio at the input of the ADC in the receiver. Towards this end the entire ADSL environment is modeled ie. transmitter, channel, noise and receiver. The Matlab simulation model was implemented in a TI TMS320VC5416 DSP platform, which was subsequently used to investigate the effects of NEXT and FEXT interference at the receiver. An analysis of SNR profile with interference is presented for various lengths for the three popular twisted pair copper wires 22AWG, 24AWG and 26AWG. Also parameter of user interest is Bit Error Rate (BER) for the ADSL link, have been obtained from the implementation and presented. The transmitter and receiver blocks include all the standard blocks of an ADSL modem. Channel is modeled using transfer function of twisted pair copper wire. FEXT and NEXT interference are modeled using their power spectral densities (PSD) and added to the PSD of AWGN to obtain total Noise.

KeywordsSNR Profile , DMT, ADSL, FEXT, NEXT, BER

I. INTRODUCTION SYMMETRICAL Digital Subscriber Line (ADSL) is a high speed data transmission technology that allows

reuse of existing copper phone line [1,11]. It allows up to 9 Mbps data transmission on downstream and 640 Kbps on upstream. The high throughputs require implementation of modems based on Discrete Multitone modulation (DMT) [9]. ADSL uses two separate frequency bands as shown in figure1. With standard ADSL, the band from 25.875 kHz - 138 kHz is used for upstream communication, while 138 kHz 1104 kHz is used for downstream communication.

The ADSL uses multicarrier modulation technique known as DMT which transforms the wideband channel into 256 sub-channels in parallel, with downstream having 224 tones,

K.V. Padmaja is working as Assistant Professor in the Department of

Electronics & communication in R.V. College of Engg., Bangalore, India and pursuing PhD. (phone: 080 28601701; fax: 080 28600337; e-mail: [email protected]).

B.V. Uma is working as Assistant Professor in the Department of Electronics & communication in R.V. College of Engg., Bangalore, India and pursuing PhD (phone: 080 28601701; fax: 080 28600337; e-mail: [email protected]).

S. Ravishankar is working as Professor in the Department of Electronics & communication in R.V. College of Engg., Bangalore, India and has 4 patents to his credit. Published significant number of IEEE papers (phone:080 28601701; fax: 080 28600337; e-mail: [email protected]).

upstream having 26 tones and the other 6 tones used for POTS[8]. Each tone has a frequency range of 4.3125 KHz.

Fig. 1 Frequency plan for ADSL

The SNR depends on both distance and frequency. The

signal gets attenuated due to noise in the channel, which can be classified as AWGN and disturbances that include crosstalk i.e NEXT (Near End Crosstalk) and FEXT (Far End Crosstalk). NEXT is defined as the cross talk between a receiving path and a transmitting path of DSL transreceivers at the same end of two different subscriber loops within the same twisted pair cable. FEXT is the noise detected by the receiver located at the far end of the cable from the transmitter that is the noise source [1,6]. NEXT is a major impairment for systems that share the same frequency band for upstream and downstream transmission. .

II. MODELING OF ADSL SYSTEM

Transmitter: The transmitter consists of the following functional blocks[3,8].

(a) Pseudo Random Sequence(PN) Generator This block generates the input bit stream(PN sequence) for

the transmitter. A PN sequence of maximal length 15 is so chosen such that the output sequence repeats after every 225 bits. The algorithm to generate the PN sequence is given below. 1.Generate a number between 0 and 14 using rand() function. 2.Add 1 to exclude 0 (To never enter lock-out state i.e. 0th state). 3.Convert to binary and apply to the 4 flip-flops. 4.Generate the 15-length PN sequence. 5.If the number of bits for the input bit stream generated is not equal to target go to step 1 else go to step 6. 6.Exit

Simulation and Implementation of a Practical ADSL Environment on DSP Kit

K. V. Padmaja, B. V. Uma, and S. Ravishankar

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World Academy of Science, Engineering and Technology 24 2006

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Fig. 2 Block Diagram of ADSL environment

(b) Reed Solomon Encoder Reed-Solomon (RS) codes are used in DSL because they

provide coding gain against random errors (of about 3 db). We have used GF256 RS code with each symbol having 8 bits. In our block we have used RS (255,249) code with error correcting capability of 3 bytes/symbol.

(c) Constellation Encoder This block maps the parallel data into 256 multiple bit sub

channels with each sub channel being represented by a QAM signal constellation. We have uniformly assigned 8 bits per sub channel, i.e. we have considered only 256 QAM constellations with 8 bits per constellation point/symbol. Algorithm: 1.Make QAM constellation by assigning the constellation points using gray code. 2.Store in a lookup table. 3.Take the incoming parallel bit stream and compare with the look up table. 4.Assign the equivalent point to that byte of data. 5.If all input bytes are not assigned go to step 1 else exit.

(d) Inverse Discrete Fourier Transformer The set of complex numbers are duplicated in conjugate

symmetry and Inverse Discrete Fourier Transform is performed on them. This results in a sequence of real numbers in time domain xn.

1-N 1,....., 0,n 2 j

e 1

0===

NknN

k KXnx

(1)

In our implementation we have forced the first element to zero. Since in an ADSL system the first frequency band is used for POTS (plain old telephone service), this 0th frequency is of no consideration to us as it carries only the voice data and not the data actually transmitted. Hence the value of the data carried in it is of no consequence to the ADSL system. Thus the forcing of the first element to 0 is justified. The output of this block is 512 real values which correspond to the 256 complex values at the input. Algorithm 1. Find the complex conjugate of the input sequence 2. Flip the conjugate sequence and append to the original sequence such that for the kth element the (N-k)th is its complex conjugate. 3. Force the 0th and (N/2)th element to zero to ensure that the output consists of only real values. 4. Find the IDFT of the sequence using the FFT algorithm.

(e) Channel The channel is Copper twisted pair. The transmitter signal

undergo convolution with the channel transfer function [3,5]. This results in change of original signal by addition of noise and attenuation of signals. At high frequency, f > 250 kHz, the twisted pair transfer function can be simplified as follows

H (d, f ) = e d ( k1f + k2 f ) e j d k3 f (2)

where f is the frequency in Hz, d is the length of the cable in miles while k1, k2 and k3 are proportional constants determined empirically[1,2]. The values of these constants depend upon the gauge of twisted pair cable.

In our implementation we have modeled three types of noise namely NEXT, FEXT and Additive White Gaussian Noise (AWGN).


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(f) Noise Model Disturber Noise (Downstream) The PSD of the ADSL transceiver disturbers for downstream is given by equation3 [4,7]:

PSDADSL.ds_Disturber = KADSL.ds )0(

1

1

1

1sin

3

12

3

2

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III. HARDWARE IMPLEMENTATION All transmitter and receiver blocks modeled in MATLAB

were implemented in hardware using the Texas Instruments TMS320VC5416 DSP.

Code Composer Studio was used as a cross compiler between C and assembly language. The transmitter blocks were implemented in C and downloaded onto the DSP Kit using CCStudio.

The output of the transmitter was written into a file TXOUT.txt. The output of the pseudo random generator was also written into a file TX.txt to enable a comparison between TX and RX data.

The Channel coefficients were simulated in MATLAB and the resulting table was stored as a file NOISE.txt used for channel simulation on the kit.

The Receiver blocks were implemented in C Code and Downloaded onto the DSP kit. The RX output is written onto a text file RX.txt to enable comparison.

IV. SIMULATION RESULTS Matlab simulation results: The following waveforms are

for 24 gauge, 18Kft. Equation 2 is used to simulate Channel characteristic. RS coding has no influence on the channel characteristics.. The channel is attenuating for higher frequencies thereby reducing received signal power.

Fig. 3 Total noise. The two graphs on observation show that noise is identical for both cases ie. with and without RS Coding

Fig. 4 SNR profile. We observe from figure that SNR decreases with increases frequency


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Fig. 5 The transmitter output Vs receiver input. We can see that receiver input signal power improves with RS coding. This is because RS

Code increases coding gain thereby increasing the receiver input power

V. HARDWARE RESULTS

TABLE I BER WITH AND WITHOUT RS CODING

Gauge Length of Cable BER with RS coding BER without RS encoding

22 AWG 12 Kft 15 Kft 18 Kft

0.00133 0.00260 0.00066

0.004 0.002 0.002


0.0020 0.0006 0.0026

0.0033 0.0053 0.0730


0.00066 0.00660 0.09200

0.0053 0.0640 0.2230

The Transmitter and Receiver blocks were implemented in

C Code and Downloaded onto the DSP kit. The RX output is written onto a text file RX.txt to enable comparison. Bit Error Rate is tabulated (Table I).

VI. CONCLUSION The ADSL environment ie; Transmitter, receiver, channel

and noise were modeled using Matlab as well as implemented in a DSP platform. This was used to investigate effects of the NEXT and FEXT interference at the receiver in a real practical environment. The metric at the receiver chosen to show an ADSL performance improvement was SNR profile and also a user related metric BER.

From BER Table I for a particular length, the BER decreases with RS coding.

In most cases of cables two or three types of interferences

need to be considered and hence a systematic analysis like the above can be used to provide an adaptive filter which mitigates multiple cross talk.

REFERENCES [1] Thomas Starr, John M. Cioffi, Peter J. Silverman, Understanding

Digital Subscriber Line Technology . 1999 prentice Hall PTR. [2] Walter Chen, DSL Simulation of the Channel, 2nd Edition. [3] Simon Haykin, Communication Systems, 4th Edition. [4] ITU - Telecommunication Standardization Sector, G.test.bis: G.test.bis

draft Recommendation, Draft new recommendation g.992.1: asymmetrical.

[5] Raju Hormis, Dong Guo and Xiaodong Wang, Monte carlo FEXT cancellers for DSL channels IEEE transactions on circuits and systems, Vol. 52, Sept. 2005.

[6] Bingham, John A.C., "Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come", IEEE Communications Magazine, May 1990.


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[7] J. Cioffi, P. Silvermann, T.Starr, Digital Subscriber Lines, Article Computer Networks, Vol. 31 25th Feb 1999.

[8] J. Karpez, Kamran Sistanizadeh, High bit rate ADSL Communication over ADSL Loops, IEEE Transactions on Communications Vol. 43, Issue 6, June 1995.

[9] Jean-Jacques Werner, The HDSL Environment, IEEE Transactions on selected areas in Communications, August 1991.

[10] Peter s. Chow, John M Cioffi, John A.C. Bingham, DMT based ADSL: Concept, Architecture and Performance, IEEE Transactions, 1994.

[11] Walter Y. Chen , The development and standardization of ADSL, IEEE Communications Magazine, May 1999.


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Practical ADSL Kit

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