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In this contribution we consider a communication system consisting of a cascade of a passive optical network (PON) link with on/off keying, and a short twisted pair (TP) link with discrete multitone modulation (DMT). Three coding configurations are investigated. In the first configuration (C1), the PON link and the TP link are protected by a Reed-Solomon (RS) code and trellis-coded modulation (TCM), respectively. In the second configuration (C2), the PON link is the same as with C1, but the TP link is now protected by the concatenation of an outer RS code and inner TCM. We compare these conventional coding configurations C1 and C2 with an alternative configuration (C3), in which an outer RS code protects the cascade of the uncoded PON and the TCM-encoded TP. Whereas the configuration C3 has the same complexity as the configuration C1, we show that the former is able to achieve approximately the same information bitrate as the (more complex) configuration C2, at the expense of a slightly (about 1 dB) higher optical power. Our numerical results for a typical setting indicate that the configurations C2 and C3 can provide information bitrates beyond 10 Gbps, which are about 20–25% larger than for configuration C1.

In this paper, we first review the baseband modulation techniques intended for use in short-reach, high-speed electrical interconnects. Then we briefly introduce the high-level design concepts of the investigated electrical interconnect, indicating the main limitations and outlining the transceiver complexity related to the advanced modulation designs. We further investigate finite-complexity linear pre-equalization under an average transmit power constraint of both full-response and precoded partial response signaling with pulse amplitude modulation (L-PAM) mapping. For a representative electrical interconnect, we argue that the constellation size, the type of modulation, the detection method as well as the number of pre-equalizer taps should be carefully selected in order to achieve target error performance at data rates between 100Gbit/s and 200Gbit/s. We show that for many combinations of above mentioned parameters, precoded duobinary 4-PAM yields the best error performance for a fixed average transmit power.

The Monte Carlo (MC) simulation of the error performance of a concatenated coding system with finite interleaving depth between the outer and inner codes is time-consuming, especially when targeting low error rates and examining several interleaver settings. In this letter, we present a semi-analytical evaluation of the word error rate (WER) performance of a system with Reed–Solomon (RS) outer coding, finite block interleaving and systematic low-density parity check (LDPC) inner coding. The proposed evaluation method relies on a simple semi-analytical statistical model for the number of bit errors in a segment of the information word after LDPC decoding on the AWGN channel. Only the WER and bit error rate of the inner subsystem (LDPC code and the considered constellation) are required to compute the WER of the concatenated code, corresponding to different parameters of the RS code and the interleaver. We show that the semi-analytical WER of the concatenated system closely matches the WER resulting from MC simulations for both the AWGN and Rayleigh block-fading channels.

While the requirements for delivering high throughputs increase exponentially with every generation of access node hardware, the device cost is of primary concern. As a result, multiple- input multiple-output (MIMO) equalization, which has been shown to facilitate multi-Gbit/s communication over low-cost parallel electrical interconnects, is emerging as an attractive high- speed interconnect solution for next-generation access nodes. Because of the high operating frequencies, however, the transfer functions of the on- and off-chip interconnects become highly susceptible to manufacturing tolerances (MTs); hence, the equalization filters must be adjusted to the specific channel realization to achieve optimal performance, which involves a high implementation and computational complexity. Considering that the MTs are usually limited, we propose a robust low-complexity transceiver consisting of a fixed MIMO linear pre-equalizer (which avoids the need for feeding back the channel state information to the transmitter), with either a fixed or adjustable MIMO decision- feedback equalizer (DFE). For a specific chip-to- chip interconnect operating at 75 Gbit/s per line and a 26 dB signal-to-noise ratio, we show that the resulting bit error rate does not exceed 10^(-12) for MTs up to 10.5% (fixed DFE) and 17.7% (adjustable DFE) of the nominal line width.

There are two types of crosstalk in digital subscriber line (DSL) systems, namely near-end crosstalk (NEXT) and far-end crosstalk (FEXT). NEXT is usually much stronger than FEXT. Therefore, high-speed DSL systems transmit in preplanned disjoint downstream (DS) and upstream (US) frequency bands to avoid NEXT. Although easy for implementation, such a fixed bandplan can lead to inefficient bandwidth usage, depending on the DS and US bit rate requirements and the loop topology, particularly in so-called vectored DSL systems, which include signal coordination for FEXT cancellation. In dynamic bandplanning (DBP), each frequency band is allocated to either DS, US, or to both directions depending on the aforementioned parameters. In this paper, we consider optimal DBP for vectored DSL with linear as well as with nonlinear transmitter/receiver structures. We propose an optimal DBP algorithm for systems with disjoint DS and US bands. For systems with overlapping bands, the problem of finding the optimal transmitter/receiver filters is nonconvex and we propose two iterative algorithms based on recent optimum spectrum balancing schemes from the literature. We also study the effect of echo and US NEXT cancellation on the system performance. Finally, simulation results are provided to demonstrate that our algorithms can indeed significantly increase the achievable bit rates.

Continuous improvements in communication technologies over copper pair access networks have enabled hybrid fiber/copper access networks that provide ever increasing broadband data rates. Most recently, field trials have demonstrated that crosstalk cancellation, also referred to as vectoring, and bonding two pairs to push data rates of very high speed DSL above 100 Mb/s on a single copper pair and above 200 Mb/s on two copper pairs. Looking ahead, the G.fast project group of the ITU-T is currently defining a new copper access technology to provide aggregate (upstream and downstream) data rates of up to 1 Gb/s over distances up to 250 m. This article provides an overview of some key challenges that need to be addressed for G.fast.

The continued growth of broadband traffic pushes wireline access network operators to continuously offer better service. Due to the large investments needed to upgrade to an all-fiber network, operators seek to boost performance over their existing copper-based infrastructure, while gradually evolving to a fiber network. As a result, Digital Subscriber Line (DSL) technology, which offers broadband over copper, has been drastically improving. The next generation of DSL will enable the continued cost-effective deployment of fiber in the access network by providing aggregate data rates up to 1 Gb/s. To keep operational costs down, this new DSL generation should enable self-installation. Therefore, it should be robust and adaptive to a wide range of time-varying channels. In this paper, we propose a novel technique based on hierarchical modulation to achieve this robustness. It adapts quickly to noise and interference dynamics in frequency and in time and allows the transmitter to select its preferred configuration without the need for an explicit communication of this configuration to the receiver. We present an overview of hierarchical modulation and how we aim to use these concepts in DSL. We further show with simulations that the receiver is able to blindly track the changes introduced by the transmitter and discuss how this protocol can be easily introduced in state-of-the-art systems.

This contribution considers the word error rate (WER) performance of a concatenated coding scheme in the presence of impulsive noise (IN), which is modeled as gated white Gaussian noise, with on- and off-times governed by a 2-state Markov model. The scheme consists of a Reed-Solomon (RS) outer code and a low-density parity-check (LDPC) inner code, which are separated by a block interleaver with finite depth. The Monte Carlo (MC) simulation of the WER of such communication systems is time-consuming, especially when targeting low error rates and examining several interleaver settings. We present a semi-analytical evaluation of the WER, which relies on a simple semi-analytical statistical model for the number of byte errors in a segment of the information word after LDPC decoding. To compute the error performance of the concatenated code corresponding to different parameters of the RS code and the interleaver, we require only the WER and byte error rate (ByteER) of the inner subsystem, determined by the LDPC code and the considered constellation, in the presence of stationary white noise. We show that the semi-analytical WER of the concatenated system closely matches the WER resulting from MC simulations and use the proposed model to investigate the effect of the interleaver depth on the WER performance.

In this paper, we investigate joint level 2 and 3 dynamic spectrum management (joint DSM 2/3) for downstream (DS) DSL. We consider a DS scenario in which users are divided into a few separate groups, where vector encoding based signal coordination can be applied in each group and spectrum coordination is possible for all users. This can be seen as a mixed interference/broadcast channel (IF/BC) scenario. In order to obtain the optimal transmitter structure, we develop a generalized duality between the vector broadcast and multiple-access channel (MAC) for scenarios in which partial signal coordination is available among users. This theory together with optimal spectrum balancing (OSB) is exploited to calculate the jointly optimal filters and transmit powers for non-linear vector dirty paper coding structures (in the form of Tomlinson-Harashima precoder (THP)) in the groups. The proposed scheme is compared to several other joint DSM 2/3 algorithms for DS DSL. Simulation results show that the proposed scheme (referred to as the IF/BC-OSB algorithm) achieves considerably higher bit rates than the other schemes. IF/BC-OSB encompasses the earlier developed BC-OSB algorithm as a special case. A simplified version of IF/BC-OSB avoiding exhaustive search with near-optimal performance is also proposed. © 2012 IEEE. ispartof: IEEE Transactions on Communications vol:60 issue:10 pages:3111-3122 status: published

Downstream (DS) crosstalk channels of digital subscriber line systems can be estimated based on the reported signal-to-noise ratio (SNR). In the case of an already active victim line (VL) impacted by the joining of a new disturber line (DL), sending perturbing signals on the VL and reporting the SNRs allow the estimation of the crosstalk channel from the DL to the VL. This paper generalizes that concept to include full startup, tracking and joining scenarios as well as the impact of different perturbation signal choices. Additionally, the sensitivity of the algorithm to different sources of imperfection is analyzed. Simulation results reveal that starting without precoding, and updating the precoder matrix based on the DS crosstalk channel estimates, the far-end crosstalk-free SNR can be reached in a limited number of iterations (e.g., 36 SNR measurements are needed for the four lines case). The SNR-assisted method is documented in an informative appendix of the draft G.vector recommendation as a backward compatible method for very high speed digital subscriber lines (VDSL2) and, hence, can be used for channel acquisition today.

To gradually expand their networks, operators deploy new xDSL technologies from remote terminals (RT's) closer to the customer. However, since xDSL lines deployed from an RT can share a binder with lines deployed from the central office (CO), a near-far problem is introduced and crosstalk may cause a severe performance degradation for the CO-deployed lines. RT lines have to be careful about how to allocate transmit power to the transmission frequencies, such that the impact on the CO lines is acceptable. Noise impact (NI) methods have been proposed based on a target noise shape generated by the disturbers (RT lines). In this paper new methods based on the actual impact on the bit rate of the victims (CO lines) are proposed. These bit impact (BI) methods closely approximate the optimal power allocation.

In xDSL systems, crosstalk can be separated into two categories, namely in-domain crosstalk and out-of-domain crosstalk. In-domain crosstalk is also refered to as self crosstalk. Out-of-domain crosstalk is crosstalk originating from outside the multi-pair system and is also denoted as external noise (alien crosstalk, radio frequency interference,...). While self crosstalk in itself can easily be canceled by a linear detector like the ZF detector, the presence of external noise requires a more advanced processing. Coordination between transmitters and receivers enables the self crosstalk and the external noise to be mitigated using MIMO signal processing, usually by means of a whitening filter and SVD. In this paper, we investigate the problem of finding the optimal power allocation in MIMO xDSL systems in the presence of self crosstalk and external noise. Optimal Tx/Rx structures and power allocation algorithms will be devised under practical limitations from xDSL systems, namely per-modem total power constraints and/or spectral mask constraints, leading to a generalized SVD-based transmission. Simulation results are given for bonded VDSL2 systems with external noise coming from ADSL2+ or VDSL2 disturbing lines, along with a comparison between algorithms with one-sided signal coordination either only at the transmit side or the receive side., Comment: 8 pages, 6 figures

In this contribution, we propose a novel code-aided channel tracking and carrier frequency offset (CFO) and/or phase noise (PN) estimation algorithm for orthogonal frequency-division multiplexing (OFDM) systems. Starting from the maximum-likelihood (ML) principle, we derive a low complexity estimator based on the expectation-maximization (EM) algorithm. It turns out that the estimator receives soft information from the channel decoder to refine the CFO, PN, and channel estimates. Simulation results revealed that the proposed algorithm outperforms the conventional techniques in terms of bit-error rate (BER).

The performance of multicarrier systems depends on the propagation channel behavior. The latter is subject to time and/or frequency selectivity. The designer has to select properly the guard interval and the number of carriers for a given system bandwidth to combat the channel dispersiveness. In this paper, we investigate the sensitivity of downlink multicarrier code-division multiple-access (MC-CDMA) performance to these parameters in different environments. We derive closed-form expressions of the useful and the different interference powers after maximum ratio combining and despreading. The optimum parameters correspond to the minimum of the signal-to-noise-ratio degradation. It turns out that the derivations in the paper of Steendam and Moeneclaey ( IEEE Trans. Commun., 1999) that are restricted to the orthogonal frequency-division multiplexing (OFDM) system are a particular case of our results. Numerical evaluation of the analytical expressions reveals that the optimum parameters of the MC-CDMA and its corresponding OFDM system are similar and depend in the same way on the channel characteristics. We show in this paper that the load of the MC-CDMA system only has small influence on the optimization of the parameters. Therefore, from the point of view of parameter optimization, single-user transmission is sufficient to find the optimum parameters. Furthermore, as the optimum parameters mainly depend on the channel characteristics, similar conclusions can be drawn for the uplink transmission

In order to achieve high speed on electrical interconnects, channel attenuation at high frequencies must be dealt with by proper transceiver design. In this paper we investigate finite-complexity MMSE pre-equalization under an average transmit power constraint, to compensate for channel distortion in the case of both full-response and precoded partial response signaling with L-PAM mapping, and consider the resulting error performance for symbol-by-symbol detection and sequence detection. For a representative electrical interconnect, we point out that the constellation size (2-PAM or 4-PAM), the type of signaling (full response or partial response), the detection method (symbol-by-symbol detection or sequence detection) and the number of pre-equalizer taps should be carefully selected in order to achieve satisfactory error performance at high data rates. For several scenarios, precoded duobinary 4-PAM is found to yield the best error performance for given average transmit power.

This paper deals with joint data detection, synchronization and channel parameter estimation for direct-sequence code-division multiple-access systems over frequency-selective channels. In low-signal-to-noise ratio environments, conventional data-aided (DA) estimation algorithms may require an unacceptably large number of pilot symbols in order to obtain sufficiently accurate estimates of the channel and synchronization parameters. Especially, frequency offset estimation results in a significant loss in spectral efficiency. In this contribution, we consider several code-aided estimation schemes which can be incorporated in an iterative turbo detection scheme. We consider the expectation maximization algorithm, as well as the space alternating generalized expectation maximization algorithm as tools to develop code-aided estimation algorithms for a variety of scenarios. We pay special attention to the issue of computational complexity, and propose some complexity-reducing approximations. We show through computer simulations that the proposed code-aided estimation techniques considerably outperform their conventional DA counterparts.

This paper addresses the issue of channel parameter estimation for uplink direct-sequence code-division multiple-access (DS-CDMA) system. A frequency-selective multipath channel is considered, and a feedback (FB) structure to jointly estimate path delays and complex gains is developed. The proposed algorithms are extensions of those from , where only path-delay estimation is considered. In the current paper, a detailed mathematical analysis of the coupling between the different parallel feedback loops is carried out. From this analysis, results are derived pertaining to system dynamics and jitter variance. Furthermore, a novel operating mode, called improved decision-directed (IDD), is proposed as an improvement of the standard decision-directed (DD) mode. The resulting performance will be measured in terms of mean-square estimation errors (MSEEs) and benchmarked against the corresponding modified Crame/spl acute/r-Rao bounds (MCRBs). Finally, the impact of the accuracy of the estimated complex gains on the jitter of the path delay trackers (and vice versa) is investigated.

In this paper, we consider iterative space-time multiuser detection and channel parameter estimation in a bit-interleaved coded modulation scheme for asynchronous direct-sequence code division multiple access (DS-CDMA) transmission over frequency selective, slowly fading channels. Accurate estimation of the channel parameters is critical as it has great impact on the overall BER performance. We present an iterative space-time multiuser (STMU) turbo detection and estimation scheme, based on space alternating generalized expectation maximization (SAGE) algorithm. This algorithm operates on the coded symbols by exchanging soft information between the detector and the estimator. We show through computer simulations that the proposed low complexity STMU receiver considerably outperforms conventional estimation schemes and achieves excellent performance, both in terms of BER and estimation error variance. Finally, we will consider different mapping strategies and investigate their impact on the performance and complexity of the estimator.

Traditionally, copper network operators complement a Fiber-to-the-Home (FTTH) strategy with a hybrid fiber-copper deployment in which fiber is gradually brought closer to the consumer, and Digital Subscriber Line (DSL) technology is used for the remaining copper network. In this paper, we propose the system concepts of XG-FAST, a technology capable of delivering 10 Gb/s connection speeds over short copper pairs. With a hardware proof-of-concept platform, it is shown that multi-gigabit speeds are achievable over typical drop lengths from front yard to customer premises equipment of up to 70 m. When an additional pair is available, 10 Gb/s service is shown to be possible up to 30 m by exploiting bonding and phantom mode. The XG-FAST technology will make Fiber-to-the-Door deployments feasible, which avoids many of the hurdles accompanying a traditional FTTH roll-out. Single subscriber XG-FAST devices would be an integral component of FTTH deployments, and as such help accelerate a worldwide roll-out of FTTH services. Standardization is required to specify the physical layer and the interface with the fiber network.

We extend studies on power optimization in VDSL2 to G.fast, the technology under definition. In vectored G.fast the power of the signals before precoding needs to be optimized to meet power spectral density and maximum aggregate tarnsmit power constraints per line. We formulate this interplay between vectoring and power optimization, with non-vectored transmission as a particular case. Three power allocation algorithms are devised that allow a tradeoff between optimality and computational complexity. Measured channel data from a European operator up to 106 MHz is considered. Numerical evaluation reveals that low complex heuristics for power optimization yield near optimal solutions both with perfect and imperfect channel state information. Similar conclusions hold for the 212 MHz profile.1

The objective of this paper is to propose a reduceddimension resource allocation scheme in the context of cognitive radio system in presence of co-channel interference between users. We assume a multicarrier transmission for both the primary and secondary systems. Instead of optimizing the powers over all sub-carriers, the sub-carriers are grouped into clusters of sub-carriers, where the power of each sub-carrier is directly related to the power of the correspondent cluster. The power optimization is done only over the set of clusters instead of all sub-carriers which can significantly reduce the complexity of the resource allocation problem. The performance loss of the reduced dimension solution with respect to the optimal solution, where the optimization is carried over all active sub-carriers, allows trading-off complexity versus performance. Numerical evaluation indeed revealed that a limited performance loss occurs by optimizing over a reduced set of clusters instead of the full optimization in the context of cognitive radio systems.

We present a measurement study of the energy consumption of an operator-side digital subscriber line (DSL) board under various conditions of data rate and power spectral density, with and without vectoring. The results highlight practical opportunities and challenges for optimizing rate-power-stability tradeoffs in DSL access systems, complementing simulation-based studies focused on energy reduction through spectral optimization. We validate models for line board consumption that can be tied with line driver consumption based on the aggregate transmit power of each line, and demonstrate that near-optimal rate-power-stability tradeoffs can be obtained through external line management of data rate, Signal-to-Noise-Ratio margin and power spectral density parameters.

Single-carrier transmission is considered in the general finite impulse response inter-symbol interference (ISI) channel. In an ISI channel with a matched filter, the folded spectrum of the received pulse can be factored into a minimum phase causal part and a maximum phase anticausal part corresponding to the postcursor and precursor ISI, respectively. In this paper, zero-forcing ISI cancellation is considered. In a direct implementation, the precursor equalization is carried out based on truncating and delaying the ideal anticausal precursor equalizer impulse response. In the proposed scheme, a block transmission is adopted, and the precursor equalization is carried out by a time reversal within each block and using a practical minimum phase filter. We show that the ISI can be removed perfectly using the proposed scheme. By means of a numerical example, it is shown that the proposed scheme achieves improved performance compared to the truncate- and delay-based equalizer in terms of transmission rate, delay, and implementation complexity.

We consider Discrete Multitone Transmission over short Digital Subscriber Lines using bandwidths applicable to G.fast, in the presence of impulsive noise. Various combinations of coding and retransmission schemes are compared in terms of bit error ratio performance and error-free information bitrate (goodput). Our study shows that, under an error performance constraint, retransmission schemes allow to achieve larger good-put with smaller latency as compared to the traditional combination of Reed-Solomon coding and Trellis Coded Modulation with a large interleaver in between.

The second part of this chapter focuses on deployment practices and describes how different access network architectures can improve the energy consumption, when considering both the telecom equipment and its supporting functions. The authors show that introducing an access network architecture that distributes more functions in the outside plant does not negatively impact energy consumption of the access network. A use case for the Benelux is worked out and a related innovation in the Swisscom access network shows that also in the more centralized architectures further optimizations are possible.1

The ever growing demand for higher data rate is pushing existing electrical broadband communication systems, using coax or twisted pair cabling, to the limit. While optical access systems inherently have a much higher transmission capacity, upgrading the access network entirely to fiber remains a costly and time-consuming effort. Therefore, many operators deploy fiber gradually closer to the end user, while bridging the remaining distance through copper technologies such as VDSL2. In the next step of this evolutionary path, fiber is brought to distribution points that are in the range of no more than 200 m from the end user, with a copper access technology called G. fast delivering data rates of 500 Mb/s to 1 Gb/s over the remaining copper stretch. In this paper, we give an overview of the G. fast technology that is currently being standardized in ITU. We show the feasibility of its main technical requirements, namely i) powering of the access equipment through the customer premises modem, ii) high transmission efficiency for orthogonal frequency division multiplexing over short copper loops and iii) offering high net data rates to end users.

In this paper, we consider optimal dynamic spectrum management (DSM) for a downstream (DS) DSL scenario in which users are divided into a few separate groups, where vector encoding based signal coordination can be applied in each group and spectrum coordination is possible for all users. This can be seen as a mixed interference/broadcast channel (IF/BC) scenario. We investigate several candidates for vector encoding the signals inside the groups, including the linear zero-forcing (ZF) pre-compensator, ZF Tomlinson-Harashima pre-coder (THP), optimal linear pre-compensator (OLP), and THP with optimal transmit filters. The calculation of the optimal transmit filters (for both the linear pre-compensator and THP) is a non-convex problem. To resolve this problem, we develop a generalized duality between the IF/BC and IF/multiple-access channel (MAC). In order to achieve the highest data rates, optimal spectrum balancing (OSB) is applied to all users on top of the vector encoding inside the groups. Simulation results show that the grouped THP with optimal transmit filters achieves considerably higher bit rates than the other schemes. The resulting algorithm is referred to as the IF/BC-OSB algorithm, and encompasses the OSB (i.e., IF-OSB) and the earlier developed BC-OSB algorithm as special cases.

The continued growth of multimedia traffic pushes access network operators to further boost performance over their copper infrastructure network. As a result, digital subscriber line technology has evolved substantially. Field and lab trials have proven that recent innovations, such as crosstalk cancellation and phantom mode transmission, can push data rates of VDSL2 higher than 100 Mb/s on a single twisted pair and 300 Mb/s on two pairs. In ITU-T Q4, a new access technology to provide aggregate data rates in the range of 500 Mb/s to 1 Gb/s per copper loop of up to a few hundred meters is being defined under the G.fast project. G.fast enables the continued cost-effective deployment of fiber in the access network. In this paper, we give an overview of some key challenges this new access technology needs to overcome. By taking into account the expected access network topology, we give guidelines for the node architecture and deployment of G.fast.

While in-home and access technologies are located at different places in the end-to-end communication chain, they have been shown to mutually interfere. In this contribution, we quantify the mutual impact of an in-home network and a Digital Subscriber Line access link. Both regular and vectored VDSL2 links are considered. For newer in-home technologies, which are able to use a large bandwidth, the access network becomes the bottleneck in the end-to-end communication. In this case, we recommend removing the interference from the in-home distribution network to the access network by shaping the transmit spectrum of the in-home distribution network. We show that this does not impact the performance of the end-to-end communication link. For older in-home technologies, which use a lower bandwidth, the attainable data rate of the in-home and access networks should be balanced by adapting the transmit power spectral density on the overlapping frequency band.

Recently, it has been shown that spectral optimization in copper access networks can significantly reduce the transmit power required, resulting in significant reductions in the line driver energy consumption. Building on these results, we develop an improved model for line driver power usage which focuses on the aggregate transmit power, the supply voltage, and on constraints that connect these two. We show that an ideally tuned supply voltage can save significant power relative to a fixed supply voltage. Much of these gains may be achievable with a small set of discrete supply voltage options. The line driver model is simple enough to incorporate into a full spectral optimization framework, in which line driver power usage and rate are traded-off in a utility-based optimization framework. Benefits of power supply tuning in combination with spectral optimization are illustrated in numerical examples.

The design of crosstalk mitigation techniques for DSL broadband access systems has mainly focused on physical layer transmit rate maximization. However, for certain applications, upper-layer performance metrics like network throughput, stability and delay performance may be more relevant. In this paper, we present a number of dynamic resource allocation based algorithms for partial crosstalk cancellation (PCC) that focus on these upper-layer metrics. A first algorithm focuses on preserving transmission queueing stability while maximizing the transmit rate. This is then extended towards budget adaptive algorithms, which dynamically adapt the PCC so as to improve the resource efficiency and to obtain a desirable trade-off between delay performance and resource consumption. Simulation results demonstrate the improved stability of the proposed algorithms and the obtained trade-off between delay performance and resource consumption.

It has been shown recently that power-efficient digital subscriber line (DSL) access networks can be designed via an optimization problem with an objective that assigns a value to user's data rate while assessing a penalty proportional to the line driver energy consumption. An optimization algorithm was proposed that combines elements of linear programming and trust region optimization, having a complexity that scales linearly with the number of users and the number of used tones. Motivated by the expansion of DSL technology from hundreds to thousands of tones, and by operator's preference to implement power spectral profiles that can be specified with a small number of parameters, we propose in this paper a reduced dimensional formulation of the optimization problem. The new formulation reduces complexity with little loss in performance, by exploiting the smoothness of the problem data as a function of frequency. Using the new formulation, we compare the efficiency achievable using two classes of line drivers: class AB and class G.

The minimum mean squared error (MMSE)-generalized decision feedback equalizer (GDFE) provides a high multi-user crosstalk cancellation performance in upstream digital subscriber lines (DSL) scenarios. The weighted sum rate maximization approach can be used to find the points located on the boundary of the corresponding rate region (RR). The performance of the MMSE-GDFE depends on the decoding order of the users. Unfortunately, unlike in ideal multiple access channels (MAC), the optimal decoding order is not determined by the order of weight factors in practical DSL MACs. To reach the boundary of the RR, a tonal exhaustive search (TES) with computational complexity O(N2N!) is required for each tone, where N is the number of users. In this paper, we investigate the main deviations of the DSL receiver from the ideal MAC receiver and model their effects as a set of update factors on the weight factors. The order of the updated weight factors is then used to determine the decoding order of the users in an iterative fashion. The computational complexity of our updated weight factor ordering (UWFO) algorithm is O(N4) which is significantly smaller than that of TES. Moreover, our simulation results show that the algorithm achieves bit rates very close to the boundary of the RR leading to approximately the same performance as TES. UWFO can be applied to wireless MACs and zero-forcing GDFEs as well.

Although a strong growth in fiber based access technology is projected, still over half of of the fixed broadband users will use a form of copper based access technology such as digital subscriber line (DSL) in the coming years. This growth is further asserted by the latest breakthrough boosting the capacity of copper access such as Vectoring and Phantom mode [2]. While most of the state of the art power efficiency improvement strategies for communication systems focuses on the optimization of one specific building block at a time, we take another approach based on the deployment practices and how different access network architectures can improve the energy consumption. To achieve substantial energy savings, we look at the complete broadband access network and its supporting functions. Not only the access network equipment such as the DSL Access Multiplexer (DSLAM) is considered but also the supporting functions which are required to make sure that the equipment can operate. It will be shown in this paper that up to 50 % of the energy consumption in the distributed deployment can be saved compared to the centralized architecture.

Recently, the duality between Multiple Input Multiple Output (MIMO) Multiple Access Channels (MAC) and MIMO Broadcast Channels (BC) has been established under a total power constraint. The same set of rates for MAC can be achieved in BC exploiting the MAC-BC duality formulas while preserving the total power constraint. In this paper, we describe the BC optimal power allo- cation applying this duality in a downstream x-Digital Subscriber Lines (xDSL) context under a total power constraint for all modems over all tones. Then, a new algorithm called BC-Optimal Spectrum Balancing (BC-OSB) is devised for a more realistic power allocation under per-modem total power constraints. The capacity region of the primal BC problem under per-modem total power constraints is found by the dual optimization problem for the BC under per-modem total power constraints which can be rewritten as a dual optimization problem in the MAC by means of a precoder matrix based on the Lagrange multipliers. We show that the duality gap between the two problems is zero. The multi-user power allocation problem has been solved for interference channels and MAC using the OSB algorithm. In this paper we solve the problem of multi-user power allocation for the BC case using the OSB algorithm as well and we derive a computational efficient algorithm that will be referred to as BC-OSB. Simulation results are provided for two VDSL2 scenarios: the first one with Differential-Mode (DM) transmission only and the second one with both DM and Phantom- Mode (PM) transmissions., Comment: 8 pages, 4 figures

Traditionally, dynamic spectrum management (DSM) deals with the rate maximization of digital subscriber lines subject to a per modem total transmit power constraint. Motivated by the desirability of reducing total power consumption, we reformulate the problem as one of minimizing the transmit power subject to minimum target rates. Both versions of the problem are special cases of a unified framework in which both rate maximization and power minimization are objectives. We also consider less complex formulation where a flat power backoff is applied relative to a fixed power profile.

In this paper, we present a new method to estimate the crosstalk channels in VDSL systems, allowing to implement well-known pre-compensation schemes. As opposed to previously presented methods, this estimation only requires minimal changes to the current standard and equipments. It is based on the concept of adding small, controlled, perturbations to the transmitted signal and observing the related changes to the SNR at the receiver. It is shown in this paper that, based on a limited amount of SNR measurements from the receivers, it is possible to accurately estimate the crosstalk channels, both in amplitude and phase.

In this paper we investigate the tracking performance of the downstream (DS) crosstalk (XT) channel estimation based on the reported signal to noise ratio (SNR), in particular for digital subscriber line (DSL) systems. Aiming to its simplicity, the SNR-assisted XT estimation, has been recognized in ITU as a backward compatible method that does not require any change in the very high speed digital subscriber line 2 (VDSL2) standard. This low complex algorithm can be used for XT channel estimation in dynamic spectrum management (DSM) techniques today. The algorithm as proposed recently, relies on sending perturbing signals on the victim lines (VLs) and reporting the SNRs by those lines to acquire the crosstalk channel from some disturber line (DL) to the VLs. We generalize this concept to include full startup, tracking and joining scenarios as well as the impact of different perturbation signal choices. Simulation results reveal that starting from no crosstalk precompensation, and updating a precoder matrix based on the DS crosstalk channel estimates, the far-end crosstalk (FEXT) free SNR can be reached in few iterations (36 SNR measurements for the four lines case).

Spectrum-overlay scenarios for wide-band multi-carrier (MC) systems bring new technical challenges along that must be considered during the system design. In such scenarios, the receiver has to perform actions, such as channel estimation and synchronization, in the presence of possibly strong in- band interference. In this paper, we investigate the impact of digitally modulated narrowband interference (NBI) on the iterative algorithm for joint channel estimation and frame synchronization as proposed in (Guenach et al., 2008) for uplink MC-CDMA. Moreover, we modify the algorithm to have better performance in the presence of NBI. The performance of the modified algorithm taking into account the statistics of NBI is verified through computer simulations and compared with the results from (Guenach et al., 2008).

Rapid acquisition of accurate crosstalk estimates is a core requirement for effective preceding in digital subscriber line (DSL) systems. It is shown that signal-to-noise ratio (SNR) reports provided by customer premises equipment (CPE) can be used to perform this task by "tuning" the precoder, i.e., iterating alternate steps of estimation and precoder adaptation. Such an approach has the advantage that it can be applied to legacy CPEs. Estimation algorithms are designed using techniques from stochastic control.

This paper deals with space-time detection and channel parameter estimation in a bit-interleaved coded modulation (BICM) scheme for asynchronous direct-sequence code division multiple access (DS-CDMA) transmission over frequency selective channels. Using a pilot sequence to obtain sufficiently accurate channel estimates for this system requires an unacceptably large number of pilot symbols. In this contribution, we consider several code-aided estimation schemes which can be incorporated in an iterative space-time (ST) turbo detection scheme. We point out that the EM algorithm, which has recently been addressed as a convenient tool for code-aided estimation, has some serious drawbacks because of the large number of parameters to be estimated. These can be resolved using the space alternating generalized expectation maximization (SAGE) algorithm. We show through computer simulations that the proposed low complexity ST receiver with SAGE estimation considerably outperforms conventional estimation schemes.

B-VHF is a proposal for a future aeronautical communication system in the VHF band based on an overlay concept, i.e. during the transition phase the B-VHF system shares the same frequency band with legacy VHF systems without interfering with them. In this paper, the overlay concept is evaluated by simulations of the physical and higher layers. Simulation results show that the B-VHF overlay system works in presence of interference from legacy VHF systems. The protocol is designed to allow using the available resources very efficiently and to provide voice and data services with the required quality of service.

In this paper, we consider synchronization and detection in a space-time bit-interleaved coded modulation (BICM) scheme for an asynchronous direct-sequence code division multiple access (DS-CDMA) uplink transmission over static frequency selective channels for fixed wireless transmission. We propose a strategy to synchronize and detect a new user accessing the system by exploiting the knowledge of the synchronization parameters of previously synchronized users. In this way, we can synchronize one user at a time and hereby avoid complex multi-dimensional optimization problems. To limit the number of pilot symbols required for this problem, we exploit all available information, i.e., pilot symbols and the unknown coded symbols of all users. We resort to the iterative space alternating generalized expectation maximization (SAGE) algorithm to tackle this problem in a systematic manner.

The performance of multi-user direct-sequence code division multiple access (DS-CDMA) transmission over frequency selective, slowly fading channels strongly depends on the reliability of the channel parameter estimates: state-of-the-art detection algorithms that exploit multiple-access-interference and inter-symbol-interference require very powerful estimation algorithms. In this contribution we consider a system with bit-interleaved coded modulation (BICM) with an iterative detector consisting of minimum mean squared error (MMSE) interference cancellation, followed by a soft demapper and a MAP decoder. We derive an estimator that makes use of the a posteriori probabilities computed by the detector. This multi-user estimator is based on the iterative space alternating generalized expectation maximization (SAGE) algorithm. We show through computer simulation that the proposed receiver considerably outperforms conventional channel estimation schemes. Finally, the impact of different mappings in the BICM scheme is evaluated.

We describe a class of low-complexity channel parameter and frequency offset estimation algorithms, applied to a space-time bit-interleaved coded modulation (ST-BICM) scheme for burst-mode asynchronous DS-CDMA uplink transmission over frequency selective channels. Conventional estimation algorithms rely mainly on the presence of training symbols within the data stream, reducing both the energy- and bandwidth efficiency. Our main goal is to remove the need for long training sequences by exploiting the presence of an underlying error-correcting code. We derive an iterative estimation technique, based on the space alternating generalized expectation maximization (SAGE) algorithm, which iterates between detection and estimation stages. Performance results are illustrated through computer simulations

This contribution deals with the performance of joint space-time multi-user turbo carrier phase and propagation delay estimation in a bit interleaved coded modulation scheme for asynchronous direct-sequence code division multiple access (DS-CDMA) transmission over frequency selective, slowly fading channels. As it has great impact on the overall BER performance, accurate estimation of the channel parameters is critical. We present an iterative space-time multi-user (STMU) turbo detection and estimation scheme. We show through computer simulation that the proposed low complexity STMU synchronizer considerably outperforms conventional synchronization schemes and achieves excellent performance, both in terms of BER and estimation error variance.

In this paper, we investigate the performance of a turbo estimation scheme based on the expectation maximization (EM) algorithm for M-PSK modulation and a convolutionally coded system. Estimates of the path delay and the complex gain are updated jointly by combining the training sequence and the soft information provided by the MAP channel decoder. Initial estimates may be obtained through either a data aided or a non data aided algorithm. We show through computer simulation that the iterative process between turbo detection and estimation leads to performances close to those obtained for perfect channel state information in terms of bit error rate (BER). On the other hand, the mean square estimation errors (MSEE) of path delay and the complex gain reach the classical modified Cramer-Rao bound. Comparisons with conventional estimation schemes are carried out.

In this paper, we present a theoretical framework for code-aided synchronisation and channel estimation for multi-carrier (MC) systems. Our endeavor is to exploit all available information from the received signal for the estimation of the unknown parameters. By means of the expectation--maximisation (EM) formalism, we will show how soft information of the coded symbols computed by the detector can be combined with pilot symbols in a systematic and efficient manner in order to improve the overall system performance. The resulting code-aided algorithm comes with a negligible increase in processing demands compared to its data-aided counterpart, since the synchroniser and the estimator can be embedded in the iterative ‘turbo’ detector. For sake of simplicity, we develop our estimation framework for a simple orthogonal frequency division multiplexing (OFDM) transceiver. As far as the estimation is concerned, extension to other MC set-ups using more advanced detectors is conceptually very straightforward. Computer simulations show that the proposed algorithm outperforms the conventional data-aided estimation algorithms. Copyright © 2006 AEIT.

Earlier work on crosstalk mitigation techniques for DSL access networks has focused on physical layer transmit rate maximization or transmit power minimization through static resource allocation, i.e., static allocation of transmit power and/or computational complexity to the individual user in the network. However, such static resource allocation does not allow to consider upper-layer metrics such as network throughput, stability and delay performance. In this article, we develop a set of dynamic resource allocation algorithms for linear zero-forcing partial crosstalk cancellation that allow to include these upper-layer metrics, so as to dynamically allocate computational complexity to the individual users. A first algorithm focuses on preserving dynamic transmission queueing stability while maximizing the transmit rates. This algorithm is then extended towards budget adaptive algorithms, which improve resource efficiency, so as to obtain a trade-off between delay performance and resource consumption. The improved stability and resource efficiency of the proposed algorithms are verified by simulations.