On Linear Transmission Systems

This thesis is divided into two parts. Part I analyzes the information rate of single antenna, single carrier linear modulation systems. The information rate of a system is the maximum number of bits that can be transmitted during a channel usage, and is achieved by Gaussian symbols. It depends on the underlying pulse shape in a linear modulated signal and also the signaling rate, the rate at which the Gaussian symbols are transmitted. The object in Part I is to study the impact of both the signaling rate and the pulse shape on the information rate. Part II of the thesis is devoted to multiple antenna systems (MIMO), and more specifically to linear precoders for MIMO channels. Linear precoding is a practical scheme for improving the performance of a MIMO system, and has been studied intensively during the last four decades. In practical applications, the symbols to be transmitted are taken from a discrete alphabet, such as quadrature amplitude modulation (QAM), and it is of interest to find the optimal linear precoder for a certain performance measure of the MIMO channel. The design problem depends on the particular performance measure and the receiver structure. The main difficulty in finding the optimal precoders is the discrete nature of the problem, and mostly suboptimal solutions are proposed. The problem has been well investigated when linear receivers are employed, for which optimal precoders were found for many different performance measures. However, in the case of the optimal maximum likelihood (ML) receiver, only suboptimal constructions have been possible so far. Part II starts by proposing new novel, low complexity, suboptimal precoders, which provide a low bit error rate (BER) at the receiver. Later, an iterative optimization method is developed, which produces precoders improving upon the best known ones in the literature. The resulting precoders turn out to exhibit a certain structure, which is then analyzed and proved to be optimal for large alphabets

Optimal time-frequency occupancy of finite packet OFDM

In this paper we consider the least time--frequency product necessary to transmit a small finite symbol packet such that the symbols can be independently detected. The system model assumed is offset QAM-OFDM, based on a finite duration pulse shape. The outcome is that the optimal pulse shape is of very short duration and that the optimal symbol allocation strategy is often to use as many subcarriers as there are symbols to transmit. Symbol packets up to 150 symbols are considered

Physical layer security for massive MIMO: An overview on passive eavesdropping and active attacks

This article discusses opportunities and challenges of physical layer security integration in MaMIMO systems. Specifically, we first show that MaMIMO itself is robust against passive eavesdropping attacks. We then review a pilot contamination scheme that actively attacks the channel estimation process. This pilot contamination attack not only dramatically reduces the achievable secrecy capacity but is also difficult to detect. We proceed by reviewing some methods from literature that detect active attacks on MaMIMO. The last part of the article surveys the open research problems that we believe are the most important to address in the future and give a few promising directions of research to solve them

Faster-than-Nyquist modulation based on short finite pulses

We investigate faster-than-Nyquist modulation based on short finite pulses over the AWGN channel. We consider several pulse shapes and compare their information rates for several system setups. We compare the effect of increasing the alphabet size versus of increasing the signaling rate. The outcome is that for these pulses the FTN symbol rate is of greater importance than the alphabet size. Finally we test some concatenated coding schemes where faster-than-Nyquist modulation constitutes the innermost encoder; the outcome is very good

Cooperative Power and DoT Estimation for a Directive Source

Reliable estimation of the source power as well as the direction of transmission (DoT) is required in a large number of applications, e.g. radio environment mapping for cognitive radios, security, system performance and interference monitoring. In this paper, we develop a multi-sensor cooperative estimation algorithm for joint power and DoT estimation of a source with a known location and equipped with a directive antenna pattern. The source signal is assumed to be known, e.g. a training sequence, and the channel is modeled by the free-space path loss. Simulation results show that the developed algorithm can deliver a reliable estimation accuracy

On precoder design under maximum-likelihood detection for quasi-stationary MIMO channels

We consider the problem of constructing linear precoders for quasi-stationary multiple-input multiple-output channels. Maximum-likelihood detection is assumed and the objective of the precoding is to maximize the minimum Euclidean distance of the signaling. Since the channel remains constant for some time, the precoding is performed spatially as well as across time. As will be shown, the precoder design is tightly connected to the theory of partial response signaling and precoders can be designed by usage of existing methods. The decoding complexity will be controlled and can be maintained small

The Effect of Signaling Rate on Information Rate for Single Carrier Linear Transmission Systems

We consider the effect of signaling rate (baud rate) on the information rate of single carrier linear transmission systems with Gaussian inputs. Several different communication scenarios are investigated: correlated or uncorrelated symbols, a fixed modulation pulse or a modulation pulse varying with the signaling rate and frequency selective or flat channels. For uncorrelated symbols, we show that the information rate grows monotonically with signaling rate for some cases while it can in fact decrease in other cases. Sufficient conditions on the modulation pulse and the channel impulse response are derived so that the information rate is increasing with increased signaling rate. Especially, these conditions give criterias for when non-orthogonal signaling is beneficial compared to orthogonal signaling in the case of flat fading. For modulation pulses varying with the signaling rate, it is shown that there are pulses for which the information rate is non-decreasing with increasing signaling rate. When correlation between symbols is allowed, we show that one can guarantee increasing information rate with increased signaling rate, no matter the pulse-channel shape (except for some hypothetical special cases), by signaling with an SNR above a certain finite threshold

Design of close to optimal Euclidean distance MIMO-precoders

In this work we study the problem of constructing precoders for spatially multiplexed multiple-input multiple output (MIMO) channels with close to optimal minimum Euclidean distance. In order to exploit the full potential of such designs, an ML detector must be used. Our design takes the decoding complexity into account and constrain it to a reasonable level. For our simplest case, the ML detector can be implemented by a Viterbi algorithm operating on a state space of size equal to the size of the modulation alphabet. The design problem will be relaxed by using precoders $F$ such that $F^{ast}H^{ast}HF$ is a cyclic Toeplitz matrix. Within this class of precoders, the optimal precoder can be found via linear programming. Of uttermost practical importance is the discovery that there only exist very few different effective channels $HF$ even for large MIMO setups; thus, the optimization at the transmitter side reduces into choosing the best precoder from a small list. Receiver tests will verify that our method improves upon the currently best precoder designs