Slawomir Stanczak

Are LAS-codes a miracle ?

Large area synchronized (LAS)-CDMA has been proposed to enhance third generation and fourth generation wireless systems. LAS-CDMA is based on multiple access codes that result from a combination of LA codes and pulse compressing LS codes. To reduce multiple access interference and intersymbol interference in time dispersive channels, LS codes have perfect auto-correlation and cross-correlation functions in a certain vicinity of the zero shift. In this paper, we provide systematic methods and the underlying theory for the construction of such codes that go far beyond the examples revealed by LinkAir (2000).

Convexity of some feasible QoS regions and asymptotic behavior of the minimum total power in CDMA systems

Link scheduling and power control are efficient mechanisms to provide quality-of-service (QoS) to individual users in wireless networks. When developing optimal access-control strategies, a good understanding of the geometry of the feasible QoS region is essential. In particular, if the feasible QoS region is a convex set, the effect on scheduling is to prefer simultaneous transmission of users. Moreover, the convexity property plays a key role in the development of optimal power-control strategies. This paper provides sufficient conditions for the convexity of the feasible QoS region in systems with and without power constraints. Furthermore, we prove necessary conditions for the feasibility of QoS requirements to better understand the optimal QoS tradeoff. Finally, the paper provides insight into the interrelationship between QoS requirements and the minimum transmission power necessary to meet them. Although the results are obtained in the context of a power-controlled code-division multiple-access system, they also apply to some other communications systems. A key assumption is that there is a one-to-one relationship between a QoS parameter of interest (data rate, service delay, and etc.) and the signal-to-interference ratio at the output of a linear receiver.

Achievable Rates for the Restricted Half-Duplex Two-Way Relay Channel

The main result of the paper is a new achievable rate region for the two-phase two-way relay channel with half duplex nodes. The new region is obtained using a compress- and-forward like protocol. Even though all information passes the relay, there are channels where decoding at the relay is suboptimal. The proposed protocol establishes a virtual two-way channel by forwarding a good enough version of the MAC output at the relay to both the receivers. In effect, the side information at the receivers can be used for the decoding of the transmission in the first phase.

Toward Energy-Efficient 5G Wireless Communications Technologies: Tools for decoupling the scaling of networks from the growth of operating power

The densification and expansion of wireless networks pose new challenges on energy efficiency. With a drastic increase of infrastructure nodes (e.g. ultradense deployment of small cells), the total energy consumption may easily exceed an acceptable level. While most studies focus on the energy radiated by the antennas, the bigger part of the total energy budget is actually consumed by the hardware (e.g., coolers and circuit energy consumption). The ability to shut down infrastructure nodes (or parts of it) or to adapt the transmission strategy according to the traffic will therefore become an important design aspect of energy-efficient wireless ?architectures. Network infrastructure should be ?regarded as a resource that can be occupied or released on demand, and the modeling and optimization of such systems are highly nontrivial problems. In particular, elements of the network infrastructure should be released by taking into account traffic forecasts to avoid losing the required coverage and capacity. However, even if traffic profiles were perfectly known, the determination of the elements to be released is complicated by the potential interference coupling between active elements and the sheer size of the optimization problems in dense networks.

Distributed Utility-Based Power Control: Objectives and Algorithms

This paper deals with the problem of medium access control (MAC)-layer fair power control in a wireless mesh network with an established network topology. The notion of MAC-layer fairness is defined along similar lines as end-to-end fairness for elastic traffic, except that instead of end-to-end flows, MAC-layer flows are considered, being that one hop flows between neighboring nodes. In this paper, we identify a class of utility functions of link rates that allows for a convex problem formulation. The convexity property is a key prerequisite for implementing power control algorithms in practice. We present a novel distributed algorithmic solution to the power control problem based on gradient-projection methods, prove its global convergence, and provide sufficient conditions for a geometric convergence rate. The main novelty of our scheme lies in the use of the so-called adjoint network in such a way that each transmitter can estimate its current update direction from the received signal power. This mitigates the problem of global coordination of the transmitters when carrying out gradient-projection algorithms in distributed wireless networks. The price for this are possible estimation errors so that the proposed scheme is analyzed within the framework of stochastic approximation.

Log-convexity of the minimum total power in CDMA systems with certain quality-of-service guaranteed

In this correspondence, we consider a code-division multiple-access (CDMA) channel with a linear receiver structure whose inputs are subject to a total power constraint. Each user is required to satisfy a certain quality-of-service (QoS) requirement expressed, for instance, in terms of data rate or delay. The set of all feasible QoS requirements is called the feasibility region. It is shown that if the signal-to-interference ratio (SIR) at the output of each linear receiver is a bijective and log-convex function of the QoS parameter of interest, the minimum total power needed to satisfy the QoS requirements is a jointly log-convex function of the QoS parameters. Furthermore, in two special cases of practical interest, we show that the minimum total power is strictly log-convex. These results imply that the corresponding feasibility regions are convex sets. The convexity property is a key ingredient in the development of access control strategies for wireless communications systems.

Fundamentals of Resource Allocation in Wireless Networks

The wireless industry is in the midst of a fundamental shift from providing voice-only services to offering customers an array of multimedia services, including a wide variety of audio, video and data communications capabilities. Future wireless networks will be integrated into every aspect of daily life, and therefore could affect our life in a magnitude similar to that of the Internet and cellular phones. This monograph demonstrates that these emerging applications and directions require fundamental understanding on how to design and control wireless networks that lies far beyond what the currently existing theory can provide. It is shown that mathematics is the key technology to cope with central technical problems in the design of wireless networks since the complexity of the problem simply precludes the use of engineering common sense alone to identify good solutions. The main objective of this book is to provide tools for better understanding the fundamental tradeoffs and interdependencies in wireless networks, with the goal of designing resource allocation strategies that exploit these interdependencies to achieve significant performance gains. The book consists of three largely independent parts: theory, applications and appendices. The latter contain foundational aspects to make the book more understandable to readers who are not familiar with some basic concepts and results from linear algebra and convex analysis.

Robust Analog Function Computation via Wireless Multiple-Access Channels

Wireless sensor network applications often involve the computation of pre-defined functions of the measurements such as for example the arithmetic mean or maximum value. Standard approaches to this problem separate communication from computation: digitized sensor readings are transmitted interference-free to a fusion center that reconstructs each sensor reading and subsequently computes the sought function value. Such separation-based computation schemes are generally highly inefficient as a complete reconstruction of individual sensor readings at the fusion center is not necessary to compute a function of them. In particular, if the mathematical structure of the channel is suitably matched (in some sense) to the function of interest, then channel collisions induced by concurrent transmissions of different nodes can be beneficially exploited for computation purposes. This paper proposes an analog computation scheme that allows for an efficient estimate of linear and nonlinear functions over the wireless multiple-access channel. A match between the channel and the function being evaluated is thereby achieved via some pre-processing on the sensor readings and post-processing on the superimposed signals observed by the fusion center. After analyzing the estimation error for two function examples, simulations are presented to show the potential for huge performance gains over time- and code-division multiple-access based computation schemes.

Base station selection for energy efficient network operation with the majorization-minimization algorithm

In this paper, we study the problem of reducing the energy consumption in a mobile communication network; we select the smallest set of active base stations that can preserve the quality of service (the minimum data rate) required by the users. In more detail, we start by posing this problem as an integer programming problem, the solution of which shows the optimal assignment (in the sense of minimizing the total energy consumption) between base stations and users. In particular, this solution shows which base stations can then be switched off or put in idle mode to save energy. However, solving this problem optimally is intractable in general, so in this study we develop a suboptimal approach that builds upon recent techniques that have been successfully applied to, among other problems, sparse signal reconstruction, portfolio optimization, statistical estimation, and error correction. More precisely, we relax the original integer programming problem as a minimization problem where the objective function is concave and the constraint set is convex. The resulting relaxed problem is still intractable in general, but we can apply the majorization-minimization algorithm to find good solutions (i.e., solutions attaining low objective value) with a low-complexity algorithm. In contrast to state-of-the-art approaches, the proposed algorithm can take into account inter-cell interference, is suitable for large-scale problems, and can be applied to heterogeneous networks (networks where base station consume different amounts of energy).

On Function Computation via Wireless Sensor Multiple-Access Channels

In wireless sensor networks, the identity of a particular sensor node and a complete reconstruction of sensed data at a designated sink node may be not needed. Indeed, the objective is often to compute a certain function of the sensed data. Such desired functions can be for example the arithmetic mean, the geometric mean, polynomials and other functions that adequately match the mathematical characteristic of the underlying multiple-access channel. In this paper, we propose a simple practical approach to compute desired functions of sensor network data, exploiting explicitly the mathematical characteristic of the wireless sensor multiple-access channel (WSMAC). In contrast to traditional schemes that are designed to combat interference caused by other connections, we exploit this interference with the goal of computing the desired functions, which is in a sense a paradigm shift. This leads directly to a higher data rate in terms of function computation or a higher SNR in comparison to other schemes like time division multiple access (TDMA). Our approach needs no extensive symbol or phase synchronization, since the measured values are converted into the tranmit power of a specific random transmit sequence with unit norm. Only a coarse block synchronization is necessary so that the proposed scheme is easy to implement.