Modeling and Simulation of an ADSLTransmitterbyKripa Venkatachalam and Qiu WuAbstractThis paper gives a short introduction of Asymmetric Digital Subscriber Line (ADSL)technology and briefly discusses its importance. ADSL uses Discrete MultitoneModulation technique and enables very high rate data throughput over normal telephonelines. This paper discusses at length the modulation scheme and also describes in detailthe functioning of an ADSL transmitterAsymmetric Digital Subscribe Line (ADSL) is used to deliver high-rate digital dataover existing ordinary telephone lines. ADSL facilitates use of normal telephone services,ISDN and high-speed data transmission simultaneously. ETSI and ANSI have defined thecurrent ADSL standard. OFDM or Discrete Multi-tone technology is the modulationtechnology that allows the transmission of high-speed data. The high bandwidthprovided by ADSL makes technologies like Video-on-demand and video-conferencingpossible. ADSL is seen as a smooth transition between present day telephone lines to thefiber optic links of the future. It is assuming lot of importance because of its economicviability.DSL ArchitectureADSL allows data rates of 6Mbps downstream and about 640Kbps upstream. Thetransmission bandwidth is 1.1MHz downstream and 256KHz upstream. The limit of1.1MHz is due to power constraints imposed by FCC. The frequency domain is dividedinto 256 sub-channels of 4kHz. The 0-4kHz channel is dedicated for voice, 2-6 channelsare not used. There are two versions of ADSL:• Frequency Division Multiplexed: Upstream and downstream channels usedifferent frequency range. Upstream uses 7-32 and downstream uses 32-256• Echo cancelled: Upstream and downstream channels overlap. Upstream useschannels 7-32 and downstream uses channels 7-256. So echo-cancellationtechniques are used to make the transmitted and received signals independent ofeach otherADSL System Engineering RequirementsThe twisted-pair telephone loop between a central office and a subscriber premises canhave a length of a few kilofeet to a few tens of kilofeet. A twisted telephone loop consistsof many cable sections of sizes from 19 gauge to 26 gauge. Due to installation practice,there are bridged taps associated with telephone subscriber loops. This causes magnitudeand phase distortion for the ADSL bandwidth. Reflections due to the bridged taps andimpedance mismatch are also potential cause of intersymbol interference. Countermeasures such as adaptive channel equalization and multi-carrier modulation becomenecessary.Many twisted pairs share the same electrical sheath and plastic covering within a singletwisted pair cable. Crosstalk exists between adjacent cables. There is Near End crosstalk(NEXT) and Far End crosstalk (FEXT).Apart from these, telephone lines attenuate signals and attenuation increases withincreasing frequencies. At 1.1MHz, the attenuation of a 24 gauge wire isTable 1: Attenuation of a 24 gauge wire signal at 1.1MHzLength of line (in kft) Attenuation (in dBm/Hz)10 -7012 -9014 -10016 -11018 -120Thus, very efficient schemes for modulation and equalization are a must to counter severechannel attenuation (due to loading coils), inter-symbol interference, cross-talk noise,additive white Gaussian noise and impulse noise. Extensive Digital processingtechniques, such as adaptive channel equalizers and adaptive echo cancellers have beenadapted for ADSL systems. The advancements in VLSI and the presence of high-speedDSPs have made realization of the above mentioned techniques. Implementation of verygood error correction techniques is also made possible.Discrete Multi-tone Modulation: Discrete Multi-tone Modulation (DMT) technique ischosen as an ANSI standard line code for ADSL system. Multi-carrier modulationtechniques are primarily used to partition data transmission channels with inter-symbolinterference, into a set of parallel sub-channels, each with its own carrier, while thecarriers form an orthogonal set [1]. Data transmission in one sub-channel is (ideally)independent of the data transmitted in other sub-channels and channel is partitioned suchthat each sub-channel has a near flat response. DMT modulation [4] is the most preferredbecause of its computational ease. For DMT, the modulation and demodulation(transmitter) schemes can be carried out with Inverse FFT (transmitter), and FFT(receiver). To implement an N sub-channel DMT system, a 2N size FFT is required. Theforced conjugate symmetry in the frequency domain will result in real-valued timedomain samples.The general structure of a DMT system is illustrated in Figure 1, where {X0, X1,…, Xn-1}Are the original complex, input data symbols, {xk} is the modulated data sequence, {yk}is the received sequence (after the removal of cyclic prefix, which will be discussed verysoon), and {Y0, Y1,…., Yn-1} are the decoded complex data symbols.{pi} and {pi*} arethe modulating and demodulating orthonormal vectors. p0 p0* X0 Y0 X1 p1 p1* Y1 pn-1 pn-1*Xn-1 Yn-1Figure 1: Basic Discrete Multitone Modulation Scheme DMT also allows for a variable data rate based on signal to noise ratio of individual sub-channel. This allows for optimal usage of bandwidth and is suitable for applications thatrequire bandwidth on demand.Architecture of an ADSL transmitter:The block diagram of a basic ADSL DMT transmitter [3] is illustrated in Figure 2. N/2 sub-channels N/2 QAM signalsInputFigure 2: Block Diagram of a DMT TransmitterEach sub-channel has a width of ∆f=4kHz and T=250µsecond is the block symbol periodof each multi-carrier channel. To maximize the transport capacity of any given loop, bitBit parsingand InputbufferingCoset -Encoder(plus Toneshuffle)InverseLength-NFFTV-tap Cyclicprefix(parallel/serial)DAC andTransmitfilterChannelloading algorithm [5] is used to allocate “number of bits” to the channels according to thechannel noise conditions. The gap approximation is given by: b(i) = log2(1+ ( SNR(i) / Γ ))Where b(i) is the number of bits that can be supported by the ith subchannel, SNR(i) isthe signal to noise ratio of the ith subchannel, and Γ is the SNR gap, which is a function oftarget bit error rate (10-7), the chosen coding scheme and the desired system performancemargin. The bit
View Full Document
Unlocking...