A combined DMT/DWMT system for DSL application

doi:10.1016/S0165-1684(99)00120-6

Signal Processing

Volume 80, Issue 1, January 2000, Pages 185-195

https://doi.org/10.1016/S0165-1684(99)00120-6 Get rights and content

Abstract

A drawback of discrete multitone transmission (DMT) using discrete Fourier transform (DFT) bases as carriers is its sensitivity to narrow band interference. With overlapped orthogonal wavelet bases as carriers, discrete wavelet multitone transmission (DWMT) possesses better spectral containment than the DFT-based DMT, and can achieve a high level of immunity to narrow band interference at the cost of higher system delay and complexity. In this paper, we propose the use of overlapped pseudo-orthogonal bases for DWMT, which can be easily implemented on current DMT system. The resulting combined system can adapt itself to different channel environments by selecting the better transmission mode from DMT and DWMT, thereby combating narrow band interference effectively.

Zusammenfassung

Ein Manko der diskreten Mehrträger-Übertragung (Discrete Multitone Transmission, DMT), die die diskrete Fouriertransformation (DFT) als Basis für die Trägergenerierung benutzt, ist ihre Empfindlichkeit gegenüber schmalbandigen Störungen. Durch Verwendung von überlappenden, orthogonalen Wavelets zur Trägererzeugung besitzt das, nun als diskrete Wavelet-Mehrträger-Übertragung (DWMT) bezeichnete Verfahren, eine verbesserte spektrale Zurückweisung als die auf der DFT basierenden DMT und kann so einen höheren Grad an Sicherheit gegen Schmalbandinterferenz auf Kosten einer erhöhten Systemlaufzeit und -komplexität erzielen. In diesem Artikel schlagen wir die Verwendung von überlappenden pseudo-orthogonalen Basen für die DWMT vor, welche leicht auf einem bestehenden DMT System implementicrt werden können. Das resultierende zusammengesetzte System besitzt durch Auswahl des besseren der beiden möglichen Übertragungsverfahren (DMT oder DWMT) die Fähigkeit, sich an unterschiedliche Kanalsituationen anzupassen und auf diese Weise Schmalbandstörungen effektiv zu unterdrücken.

Résumé

Un inconvénient de la transmission multi-tons directe (TMD) utilisant des bases de la transformée de Fourier discrète (TFD) comme porteuses est sa sensitivité à des interférences à bande étroite. Avec des bases d'ondelettes orthogonales qui se recouvrent comme porteuses, la transmission multi-tons en ondelettes discrètes (TMOD) possède un contenu spectral meilleur que la TMD basée sur la TFD, et peut atteindre un haut niveau de résistance aux interférences à bande étroite au prix d'un plus haut délai et d'une plus grande complexité du système. Dans cet article, nous proposons l'utilisation de bases pseudo-orthogonales qui se recouvrent pour la TMOD, qui peuvent être aisément mises en œuvre sur des systémes actuels de TMD. Le système combiné résultant peut s'adapter tout seul à des environements de canaux différents en sélectionnant le meilleur mode de transmission pour la TMD et pour la TMOD, combattant dès lors efficacement les interférences à bande étroite.

Introduction

Discrete multitone transmission (DMT) and discrete wavelet multitone transmission (DWMT) [12] are two modulation schemes for digital subscriber line (DSL) application. In this paper, DMT refers to multitone modulation with DFT bases as the carriers. DMT has many advantages over the traditional single-carrier modulation [5] and has been adopted as an ANSI standard for asymmetric digital subscriber line (ADSL) [4]. However, the performance of DMT may degrade significantly in the presence of narrow band interference. Due to the high degree of spectral overlap among the DFT bases employed by DMT, the energy of a single-tone interference may spread into many adjacent sub-channels and this cannot be avoided by simply switching-off the sub-channel where the interference lies [3]. In addition, the performance of DMT is sensitive to the non-perfect settings of the pre-detection equalizer caused by channel variation and finite training time [12].

In recent years, there has been a lot of interest in the application of wavelets and filter bank to communication [1], [2], [3], [8], [9], [12], [14]. One such application is DWMT, which is proposed for very-high-rate digital subscriber line (VDSL) applications, where there may be some narrow band interferences such as AM radio transmissions and other single and multitone noise sources. DWMT uses overlapped orthogonal wavelet bases as carriers, which have high stop-band attenuation, and therefore possesses a high level of immunity to these narrow band interferences. In addition, it is more robust to non-perfect settings of the pre-detection equalizer compared to DMT [11]. These advantages are gained at the cost of an increase in the length of the bases and consequently, the adjacent modulated blocks overlap. The length of the bases is L=gM, where M denotes the sub-channel number and g is the overlap factor. Though there exist methods to design orthogonal bases with high-stop band attenuation [11], the value of g is usually large (>4) in order to get the desired stopband attenuation. But, for a given number of sub-channels, the larger the overlap factor, the higher the system delay and complexity of the modulator and the demodulator [14]. Therefore, the ideal bases for DSL applications should have both a small overlap factor and high stopband attenuation.

In this paper, we propose a combined DMT/DWMT system, which introduces immunity to narrow-band interference into current DMT system without much additional penalty on system complexity. The DWMT mode of the system employs complex pseudo-orthogonal bases as carriers, which can be designed with sufficiently good stopband attenuation with g=2. As a result, the fast implementation structure of DWMT is similar to that of DMT, and it can be easily implemented on current DMT system. In Section 2, we investigate the use of complex pseudo-orthogonal bases and in Section 3, we develop fast implementation of the proposed DWMT system with pseudo-orthogonal bases. In Section 4, we discuss the combined DMT/DWMT system. Section 5 gives some simulation results and Section 6 concludes the paper.

Section snippets

Pseudo-orthogonal bases

The basic multitone transceiver from the filter bank point of view is given in Fig. 1. In DMT, the synthesis filters {F_k(z)} in the modulator correspond to DFT bases, whose length L is equal to M, the number of sub-channels. In DWMT, {F_k(z)} form a set of perfect reconstruction paraunitary filter bank whose impulse responses constitute the wavelet bases. The impulse responses ${f_{k} (n)}$ of the synthesis filters {F_k(z)}, used in the modulator, satisfy $∑ n f_{k} (n) f_{l} (n+rM)=δ(k−l)δ(r), 0⩽k, l⩽M−1.$ The impulse

Efficient implementation

Cosine-modulated filter banks can be implemented efficiently using polyphase structure as shown in [13]. If we choose the length of the complex bases to be 2M and design them according to Section 2, the fast implementation of the modulator of Fig. 1 becomes very attractive, as illustrated in Fig. 3. A DWMT modulator with M sub-channels transmits M/2 QAM signals per symbol block. The modulator can be implemented by 2M-point FFT plus an overheard of 4M complex multiplications and 2M complex

A combined DMT/DWMT system

The M sub-channel DWMT modulator shown in Fig. 3 is very similar to a 2M sub-channel DMT modulator. If the DWMT modulator uses IFFT, sets the coefficients d_k and t_n to 1, and uses 2M sub-channels and 2M up-sampling, it will reduce to a DMT modulator. Similar mapping holds between the DWMT demodulator and the DMT demodulator. Therefore, a combined DMT/DWMT system can be designed as shown in Fig. 5. The system can work in either the DMT or DWMT mode, with a mode controller controlling the system.

Simulation results

In our simulations, we used M=128 in the DWMT mode, M=256 in the DMT mode, and chose the length of the DWMT bases to be L=256. Complex orthogonal bases and pseudo-orthogonal bases were designed according to Section 2. The channel was taken to be from a 500-tap FIR filter with frequency response as shown in Fig. 6. The impulse response of the channel was normalized to unity.

We used QAM-16 and chose the transmitted average signal energy in each sub-channel to be unity. The pre-detection equalizer

Conclusions

In this paper, we motivated a DWMT system with pseudo-orthogonal bases of a small overlap factor, discussed its performance, and developed a fast implementation structure. We then presented a combined DMT/DWMT system. The simulation results show that the DWMT system with pseudo-orthogonal bases gives better ISI suppression compared to that with the same length orthogonal bases. More importantly, because of good spectral containment of the bases, the DWMT system can combat narrow band

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