The Zipper duplex method in very high-speed digital subscriber lines

Abstract: This thesis deals with different aspects of digital subscriber lines (DSL). Over the years several DSL techniques for high-speed communication on the telephone access network have evolved that gradually have allowed higher data rates. In this thesis we focus on the latest technique, called Very high-speed Digital Subscriber Line (VDSL), which is intended to offer bit-rates up to 50-60 Mbits/s on ordinary twisted-pair wires. The major part of this thesis concerns a novel scheme called the Zipper duplex method, which is introduced herein. Zipper has been proposed as duplex method for VDSL, and represents one of the two leading proposals in the North American (ANSI T1E1.4), the European (ETSI TM6), and in the International (ITU SG15/Q4) standardization groups. The key advantages with the Zipper duplex method are superior flexibility, high duplex efficiency and compatibility with existing services. It is based on discrete multitone (DMT) modulation and uses different DMT-tones in the up- and the downstream directions. The method relies on an additional cyclic extension, a cyclic suffix, to ensure orthogonality between signals in opposite directions. Best performance is achieved when all transmitters in the access network are synchronized, but Zipper can also operate in an asynchronous mode. However the asynchronous mode comes with some bit-rate performance loss and this performance loss tends to increase with wire-length. So we have developed a method for self-synchronizing all Zipper-modems that guarantees best possible performance for long wires. This method uses the correlation present in DMT-signals, due to the cyclic extension, to estimate the other users' frame-offset relative to their own. It then adjusts the frame-timing accordingly. Problems with near-far far-end crosstalk (FEXT) in the upstream can severely limit the performance of modems on long wires. In this thesis we propose a bandwidth efficient power allocation method called the multi-rate power back-off. This method creates a spectrally shaped transmit power spectral density (PSD) that results in higher bit-rates for all users compared to using a flat transmit PSD. By incorporating target bit-rates, a system operator can better control the bit rate distribution among the users, and the FEXT is kept at a minimum since no user is allowed to use more transmit power than necessary to achieve the desired target rate. The unshielded wires in the access network are quite susceptible to radio frequency interference (RFI). We introduce an efficient digital RFI-cancellation method that operates in the frequency domain for modems using DMT-modulation. By deriving a parameterized model of the narrowband RFI-signal and measuring the RFI-signal on a few unmodulated DMT-tones, the RFI can be extrapolated to the other tones and cancelled. Simulations show that it is sufficient with two parameters and two measurement tones to almost completely eliminate the RFI. Impulse noise that originates from telephone activity can be very harmful to VDSL. The use of a system margin together with Reed-Solomon coding and interleaving have been proposed to handle the impulse noise. In this thesis we study the effect of impulse noise on DMT-based VDSL systems, and present a robust detection and erasure scheme that can reduce the interleaving depth required to cope with the strongest impulses.