Crosstalk cancellation is a crucial issue for traditional digital subscriber line systems. For LTE-over-copper systems, however, the need for crosstalk cancellation varies depending on the analog fronthauling architecture and its parameters. A crosstalk handler that is decoupled as much as possible from the rest of the system architecture is thus preferred. Therefore, we propose a time-domain precoding scheme specifically in downstream direction to separate the precoding unit from the LTE signal flow, and perform crosstalk cancellation in an on-demand manner. Estimation of all direct and crosstalk paths is assisted by LTE cell-specific reference signals. The time-domain precoder transforms the interference channel into a crosstalk-free channel with identical direct paths, which allows for low-complexity inter-symbol interference mitigation. We evaluate the concept in terms of signal-to-noise ratio provided for LTE signals using measured wireline channel data.

Vectoring enables cooperative crosstalk mitigation in wideband wireline systems. A sudden termination change within a vectored group alters the entire perceived channel matrix and thus disturbs the vectoring operation, since the precoder, which is tuned to the channel before the change, is no longer up to date. We analyze the resulting residual crosstalk and its impact on performance degradation during showtime for both linear and non-linear precoding systems. To cover, a new channel matrix must be estimated to update the precoder accordingly. We use a model-based approach to track a vector instead of estimating the full matrix. Based on this vector, simplified precoder updating methods for both the precoding systems are presented accounting for a sudden line deactivation or reactivation in a vectored group. As an alternative to the state-of-the-art methods, the proposed procedures significantly reduce the complexity of both channel estimation and precoder updating and minimize the disturbed period for active users.

Next-generation wireline systems may exploit fre- quencies up to several hundred MHz on short lines. Strong crosstalk coupling, comparable to the direct paths, is one of the main channel characteristics at high frequencies. Instead of fully cancelling all crosstalk, we utilize strong crosstalk paths to boost data-rate for active users. Two linear precoding schemes, based on maximum ratio-combining (MRC) and convex optimization, respectively, are proposed and applied to a common network topology. The precoding schemes exploit constructive crosstalk signals on unused lines or in unused parts of the spectrum on neighboring lines to boost data-rate while still complying with the regulated spectral power limits per line. More than 500 Mbps throughput gain per active user can be achieved compared to state-of-the-art linear precoding.

To increase indoor coverage for mobile services, we propose a residential small cell infrastructure making use of the existing copper plant. The system is cabinet-based, collocated with VDSL2 and uses small pieces of spectrum next to VDSL2. Inspired by the Ericsson Radio Dot System, it challenges the femtocell paradigm offering full macro functionality in the small cells. An interesting service potential is offered albeit the added mobile traffic capacity is moderate as it is limited by the copper fronthaul.

Vectoring is a vital component of wideband wireline communication systems. A disorderly leaving event (DLE) disturbs the vectoring operation since the precoder, which was designed for the channel before the change, is no longer up to date. Measurements indicate that the impact of a DLE can be serious for frequencies beyond 30 MHz, which corresponds to the band used by emerging wideband communication systems over short multi-pair copper cables such as G.fast. As an alternative to the state-of-the-art update procedure, this paper presents an approach to mitigating the DLE problem. By interpreting DLE with the FEXT-reflected-NEXT (FRN) model, we propose a scheme that enables the showtime lines to return to disturbance-free transmission once the loss of signal on a certain line is detected while updating the precoder as a background process. Furthermore, the estimation complexity for a K-user vectoring group is reduced from O(K2) to O(K).

The densification of mobile networks in order to meet increased capacity demands is ongoing, needed and costly. A few papers have been published based on the insight that the fixed broadband networks offer a multitude of sites, for instance our homes, for potential small cell deployment providing backhaul capacity and power without site costs. However, in order to reach economical large-scale benefits, we explore the case when radio systems are deployed in coexistence with DSL. In this paper, we establish the feasibility of such a concept under constraints invoked by state-of-the-art and emerging systems (3GPP, VDSL2 and G.fast) and make statements about the required architecture. We also point out that the enthusiasm of previously published results should be lowered a notch.

Emerging wireline transmission systems such as G.fast use bands up to around 200 MHz on short cables. A key enabler for achieving the aspired throughput of several hundred Mbit/s is joint processing of transmit signals in downstream direction as well as joint processing of receive signals in upstream direction through techniques referred to as vectoring. A new challenge in such systems are sudden and severe changes in the channel matrix caused by changing terminations on lines outside the vectoring group. Such events can be caused by users disconnecting their modems, turning them on or off, or on-/off-hook events on lines that still support the plain old telephony service. This work presents channel measurements capturing the impact of termination changes caused by modems or handsets. An analysis of the impact of these sudden changes on the signal-to-noise-power-ratio in vectoring systems reveals that throughput and stability can be seriously degraded. The potential of decision-directed channel tracking based on least squares estimation is investigated.

Cooperation of transceivers on signal level at the remote terminal-often referred to as vectoring-is an effective approach to mitigate crosstalk in wireline transmission systems. While the gain in bitrate compared to crosstalk-limited transmission is substantial, the initialization of a vectoring system requires time and care to ensure system stability. In this contribution, a fast initialization scheme for vectored wireline systems based on spectral protection is analyzed. Two scenarios are considered, where in the first one no prior channel information is available, and in the second one erroneous prior knowledge of channel coefficients is utilized. Measurements on a commercial VDSL2 vectoring platform and throughput simulations demonstrate the potential of the fast initialization scheme.

Future wireline communication systems aspire to boost the throughput in two ways: First, they exploit higher frequencies to gain more bandwidth on shorter lines in combination with vectoring. Second, they use non-differential transmission modes (such as phantom modes, common modes, split-pair modes) to exploit more dimensions. Performance predictions for systems exploiting these techniques are of great importance for upgrading copper networks to provide Internet access or deploying copper-based backhaul systems to connect mobile base-stations. Good predictions require accurate channel models. However, channel modeling for higher frequencies and nondifferential modes is still in its infancy. A mixed deterministic/stochastic channel model is proposed to remedy this problem. The outage rate is derived based on an asymptotic (in the number of participating transceivers) analysis. As application examples, performance predictions in access networks using phantom modes and frequencies up to 200 MHz are presented.

This paper investigates common-mode propagation in shielded twisted pair cables. The common mode exhibits great potential for improving the throughput in emerging wireline systems. The design of corresponding transmission schemes over multipair copper cables requires accurate knowledge of the channel properties. We present measurement and modeling results and investigate the feasibility of using standard differential- mode models for data fitting in multiconductor transmission-line modelling of common-mode paths.

This paper derives a class of equalizer-based symbol-rate timing recovery algorithms, which are to be employed in digital subscriber line (DSL) systems. These algorithms are intended to break the coupling of the adaptive decision-directed timing recovery and the baud-spaced equalizer in tracking mode, once acquisition has been reached. As the derivation is based on a generalized complex-valued system model, the results can be applied for all common DSL systems employing baseband and passband modulation schemes, like PAM, QAM and CAP. Exemplarily, an algorithm based on the ratio of the largest precursor tap weight and the cursor tap weight is derived for very high data-rate DSL (VDSL) based on QAM transmission. It is shown by means of simulations, that destructive coupling is prevented. A simple modification of the complex-valued equalizer structure with cursor-normalized tap weights allows an easy implementation of the derived timing recovery algorithm.

The common-mode (CM) signal in wireline transmission systems has proven to provide valuable information exploited for mitigating narrowband noise at the receive side. We focus on the case of broadband noise. Treating the CM signal as an additional receive signal, we investigate the capacity of the copper cable channel for different levels of coordination among the users. We introduce a channel model which includes the common-mode paths and derive a suitable form of the channel capacity formula. CM crosstalk measurement results, essential for evaluation of the channel capacity, are presented. Using the measurement data, exemplary results of capacity gain achievable by CM-aided data transmission over the copper cable are shown.

This paper introduces an algorithm for blind adaptation of decision feedback equalizers (DFE), which is applied for equalization in the passband domain. The transmission scheme, which is assumed to be unknown at the receiver side, can either be carrierless amplitude/phase (CAP) modulation or quadrature amplitude modulation (QAM). As the input signal of the passband equalizer is real-valued, the tap weights of the complex-valued feedback filter cannot be identified by a standard whitening approach. We propose therefore an algorithm, which determines a starting coefficient set by means of an infinite impulse response (IIR) equalizer updated by constant modulus algorithm (CMA). For stability reasons, the equalizer's feedback filter is implemented in lattice structure during acquisition. In case of QAM transmission, its tap weights have to be rotated before switching to the DFE. The feedback filter is then fed by the soft decision in order to minimize the probability of wrong solutions.

This paper presents techniques for digital interpolation in the passband domain, exemplarily for a carrierless amplitude modulation/phase modulation (CAP) transmission system. Well-known techniques for designing baseband interpolation filters are extended to signal processing at nonzero center frequencies. The resulting filters are either real- or complex-valued. Real-valued interpolation filters are shown to suffer from increasing center frequencies, while their complex-valued counterparts keep their characteristics. For implementations based on the Farrow structure, however, the required polynomial degree grows with increasing center frequency.

Among all noise sources present in wireline transmission systems we focus on one special type: narrowband radio frequency interference generated by radio amateurs (RAM) and broadcast radio stations. This disturbance, characterized by high power and narrow bandwidth, has the potential of overloading the receiver's analog-to-digital converter (ADC). Once the ADC is in saturation, any countermeasure taken in digital domain will fail. A viable way to face this problem is cancellation using the common-mode signal as a reference. This paper describes in detail an adaptive, mixed-signal, narrowband interference canceller employing a modified recursive least-squares 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 kilohertz. Simulation as well as measurement results show a steady-state disturbance suppression of about 35 dB. The convergence speed is high enough to protect the ADC from overloading due to time-variant HAM interference.