Mikael Isaksson

Low-complex frame synchronization in OFDM systems

Orthogonal frequency-division multiplexing (OFDM) systems have gained an increased interest due to their use in wireless applications such as mobile communication systems. A novel data-based frame synchronization method for OFDM-systems is presented. OFDM frames are shown to contain sufficient information to synchronize a system without the use of pilots. The cyclic extension, preceding OFDM frames, is of decisive importance for this method. Based on only the sign bits of the in-phase and the quadrature components of the received OFDM signal, the maximum likelihood solution is derived. This solution basically consists of a correlator, a moving sum and a peak detector. The stability of the generated frame-clock is improved significantly by averaging over a few number of frames. Simulations show that this low-complex, averaging method can be used to synchronize an OFDM system on twisted pair copper wires and in slowly fading radio channels.

Zipper: a duplex method for VDSL based on DMT

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 the high bit-rate performance in different environments such as mixed ADSL and VDSL traffic under radio frequency interference and with different background noise sources.

Asynchronous Zipper [subscriber line duplex method]

Previously the authors presented a novel duplex method for very high bit-rate digital subscriber lines (VDSL) called Zipper. With this method all VDSL-modems on different wires in the same bindergroup have to be time-synchronized to avoid near-end cross-talk (NEXT). We describe a method which enables Zipper to run in a time-asynchronous mode. By introducing pulse-shaping in the transmitter and windowing in the receiver the NEXT is almost completely suppressed even though the synchronization between modems on neighboring lines is skipped. The remaining NEXT and efficiency loss due to pulse-shaping and windowing results in only a small bit-rate performance loss, typically less than 10% compared to the time-synchronized Zipper. However, with the new freedom of optimizing the lengths of the cyclic suffices with asynchronous Zipper, there may even be a small improvement in bit-rate performance for short wires.

An adaptive mixed-signal narrowband interference canceller for wireline transmission systems

Narrowband radio transmitters like radio amateurs and broadcast radio stations are considered to be a serious problem for high-bitrate data transmission over twisted pairs. Due to its high power level, radio frequency interference (RFI) has the potential of overloading the receivers analog-to-digital converter (ADC). Once the ADC is in saturation, any countermeasure taken in the digital domain will fail, so the problem has to be faced at least partly in the analog domain. This paper proposes an adaptive, mixed-signal, narrowband interference canceller employing a modified recursive least-squares (RLS) algorithm which is split into an analog and a digital part. The mixed-signal approach enables the circuit to generate an interference-cancelling signal of several MHz while operating the adaptive algorithm at some kHz. The structure is fast enough to prevent the ADC from overloading due to radio amateur interference, thus protecting the data transmission from interruption. Simulation results as well as measurements indicate a practical disturbance rejection potential of about 40-50 dB.

Performance evaluation of the Zipper duplex method

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

Self-synchronizing a DMT-based VDSL system

We present an algorithm for self-synchronizing all modems in a discrete multitone (DMT)-based very high-speed digital subscriber line (VDSL) system using the Zipper duplex method. This solves the problem with non-orthogonal near-end crosstalk (NEXT) that appears in systems with unsynchronized modems. The algorithm we present runs autonomously in each VDSL-modem. It uses the auto-correlation of the DMT-signal to determine the frame-offset of other users, and adjusts the own frame-timing to be better aligned with the other users. With our method all modems will be self-synchronized to within a small fraction of the total DMT frame-length. This self-synchronization suppresses the NEXT to a level far below the background noise-floor. This means that our self-synchronized system has the same performance as a system where all modems are perfectly synchronized to a master clock.