Abstract-We present a narrowband interference (NBI) canceller that suppresses spectral leakage in an orthogonal frequency-division multiplexing (OFDM)-based system caused by a narrowband (NB) signal. In our scenario, we assume that the spectrum of the NB signal is within the spectrum of the OFDM signal. This can be the case, e.g., on digital subscriber lines (DSL) and in new unlicensed frequency bands for radio transmission.The canceller makes linear minimum mean-square error estimates of the spectral leakage by measuring the NBI on a few modulated or unmodulated OFDM subcarriers. It uses a model of the NB signal's power spectral density as a priori information. Using frequency invariant design it is possible to cancel NBI from signals that are changing their frequency location with significantly reduced complexity overhead. The operational complexity of the canceller can be lowered by using the theory of optimal rank reduction and using the time-bandwidth product of the NB signal.Analytical performance evaluations, as well as Monte Carlo simulations, also show that without perfect a priori information this canceller can suppress the spectral leakage from a strong NB signal (e.g., with equal power as the OFDM signal) to well below the background noise floor for typical applications where it causes negligible signal to noise ratio and symbol error rate degradation.Index Terms-Digital subscriber lines (DSL), discrete multitone (DMT), industrial-scientific-medical (ISM) band, orthogonal frequency-division multiplexing (OFDM), radio frequency interference (RFI), wireless local area network (WLAN).

In this paper, we present a new duplex scheme, called Zipper, for discrete multitone (DMT)-based very high bit-rate digital subscriber line (VDSL) systems on copper wires. This scheme divides the available bandwidth by assigning different subcarriers for the upstream and downstream directions. It has high flexibility to divide the capacity between the up and downstream, as well as good coexistence possibilities with other systems such as ADSL. Simulation results show high bit-rate performance in different environments such as mixed ADSL and VDSL traffic under radio frequency interference and with different background noise sources.

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We present an algorithm that autonomously synchronizes all DMT-Zipper based VDSL modems in an unbundled access network, solving the problem with nonorthogonal NEXT that appears in systems with unsynchronized modems. The algorithm we present runs autonomously in each VDSL-modem in the central office or in the street cabinet. We determine the other modems' relative frame offsets by exploiting their NEXT signal using the inherent cyclic redundancy found in DMT signals. By estimating the relative frame offsets of the other users, we can adjust a given user's own frame-timing relative to the mean of the others. With our method all modems in the network will be synchronized to within a small fraction of the total DMT frame-length, suppressing the nonorthogonal NEXT to a level far below the background noise-floor. This means that we can achieve the same performance in an unbundled access network without any master clock reference as in a system where all modems are perfectly synchronized using a master clock.