Bringing physical layer cooperation closer to practical wireless systems

Nowadays the demand for communication over the wireless medium is significantly increasing, while the available wireless spectrum is already limited. In this thesis, we introduce new practical physical layer designs that employ node cooperation for more efficient wireless spectrum utilization. First, a cooperative scheme is introduced that takes advantage of the existence of simultaneous multiple frequency band operation in modern wireless devices. The main idea of this scheme is to use one frequency band for sharing data among collaborating nodes, and another one for cooperatively transmitting to a common destination. The benefits of such a cooperative design across frequency bands are evaluated on a testbed of software-defined radios, where we investigate and identify the possible network conditions under which our scheme enables better throughput compared to the traditional operation. Second, we present a practical design that facilitates physical layer cooperation in Long Term Evolution (LTE) networks. The main idea here is to adapt the inherent Hybrid Automatic Repeat-reQuest (H-ARQ) in standard LTE operation, so that multiple relays can cooperate over the transmission of one encoded block. Performance evaluation of the design reveals significant throughout gains under asymmetric channel conditions for parallel relay networks in LTE framework. Finally, we look at the node cooperation from a privacy perspective in wireless sensor networks. To this end, we design and evaluate a low-overhead message encryption protocol based on network coding. The main idea of the protocol is to employ node cooperation to increase the inherent weak security that network coding offers. Results show up to 60% increase in terms of extra security added on top of the inherent one.

Low Complexity Scheduling and Coding for Wireless Networks

Siddhartha Brahma

The advent of wireless communication technologies has created a paradigm shift in the accessibility of communication. With it has come an increased demand for throughput, a trend that is likely to increase further in the future. A key aspect of these challenges is to develop low complexity algorithms and architectures that can take advantage of the nature of the wireless medium like broadcasting and physical layer cooperation. In this thesis, we consider several problems in the domain of low complexity coding, relaying and scheduling for wireless networks. We formulate the Pliable Index Coding problem that models a server trying to send one or more new messages over a noiseless broadcast channel to a set of clients that already have a subset of messages as side information. We show through theoretical bounds and algorithms, that it is possible to design short length codes, poly-logarithmic in the number of clients, to solve this problem. The length of the codes are exponentially better than those possible in a traditional index coding setup. Next, we consider several aspects of low complexity relaying in half-duplex diamond networks. In such networks, the source transmits information to the destination through

n

half-duplex intermediate relays arranged in a single layer. The half-duplex nature of the relays implies that they can either be in a listening or transmitting state at any point of time. To achieve high rates, there is an additional complexity of optimizing the schedule (i.e. the relative time fractions) of the relaying states, which can be

2^n

in number. Using approximate capacity expressions derived from the quantize-map-forward scheme for physical layer cooperation, we show that for networks with

n\leq 6

relays, the optimal schedule has atmost

n+1

active states. This is an exponential improvement over the possible

2^n

active states in a schedule. We also show that it is possible to achieve at least half the capacity of such networks (approximately) by employing simple routing strategies that use only two relays and two scheduling states. These results imply that the complexity of relaying in half-duplex diamond networks can be significantly reduced by using fewer scheduling states or fewer relays without adversely affecting throughput. Both these results assume centralized processing of the channel state information of all the relays. We take the first steps in analyzing the performance of relaying schemes where each relay switches between listening and transmitting states randomly and optimizes their relative fractions using only local channel state information. We show that even with such simple scheduling, we can achieve a significant fraction of the capacity of the network. Next, we look at the dual problem of selecting the subset of relays of a given size that has the highest capacity for a general layered full-duplex relay network. We formulate this as an optimization problem and derive efficient approximation algorithms to solve them. We end the thesis with the design and implementation of a practical relaying scheme called QUILT. In it the relay opportunistically decodes or quantizes its received signal and transmits the resulting sequence in cooperation with the source. To keep the complexity of the system low, we use LDPC codes at the source, interleaving at the relays and belief propagation decoding at the destination. We evaluate our system through testbed experiments over WiFi.

EPFL2015

On uncoordinated wireless ad-hoc networks

Alaeddine El Fawal

Emerging pervasive wireless networks, pocket switched networks, Internet of things, vehicular networks and even sensor networks present very challenging communication circumstances. They might involve up to several hundreds of wireless devices with mobility and intermittent connectivity. Centralized coordination in such networks is practically unfeasible. We deal with these challenge using two potential technologies: WIFI and Ultra Wide Band (UWB) Impulse Radio (IR) for medium and short communication range, respectively. Our main goal is to improve the communication performance and to make these networks sustainable in the absence of a centralized coordination. With WIFI, the goal is to design an environment-oblivious data dissemination protocol that holds in highly dynamic unpredictable wireless ad-hoc networks. To this end, we propose a complete design for a scope limited, multi-hop broadcast middleware, which is adapted to the variability of the ad-hoc environment and works in unlimited ad-hoc networks such as a crowd in a city, or car passengers in a busy highway system. We address practical problems posed by: the impossibility of setting the TTL correctly at all times, the poor performance of multiple access protocols in broadcast mode, flow control when there is no acknowledgment and scheduling of multiple concurrent broadcasts. Our design, called "Self Limiting Epidemic Forwarding" (SLEF), automatically adapts its behavior from single hop MAC layer broadcast to epidemic forwarding when the environment changes from being extremely dense to sparse, sporadically connected. A main feature of SLEF is a non-classical manipulation of the TTL field, which combines the usual decrement-when-sending to many very small decrements when receiving. Then, we identify vulnerabilities that are specific to epidemic forwarding. We address broadcast applications over wireless ad-hoc networks. Epidemic forwarding employs several mechanisms such as forwarding factor control and spread control, and each of them can be implemented using alternative methods. Thus, the existence of vulnerabilities is highly dependent on the methods used. We examine the links between them. We classify vulnerabilities into two categories: malicious and rational. We examine the effect of the attacks according to the number of attackers and the different network settings such as density, mobility and congestion. We show that malicious attacks are hard to achieve and their effects are scenario-dependent. In contrast, rational attackers always obtain a significant benefit. The evaluation is carried out using detailed realistic simulations over networks with up to 1000 nodes. We consider static scenarios, as well as vehicular networks. In order to validate our simulation results, we build a solid and widely adaptable experimental testbed for wireless networks. It is composed of 57 mobile wireless nodes equipped with WIFI interface. The adopted platform is OpenWrt, a Linux-like firmware, which makes the testbed robust and easily configurable. With UWB IR, the main problem we deal with is the presence of uncontrolled interference. Indeed, similarly to Code Division Multiple Access (CDMA) systems, signal acquisition with UWB IR signaling requires power control in the presence of interferers, which is very expensive in an uncoordinated system. We solve this problem through a cross-layer optimization: We propose a new signal acquisition method that is independent of the received signal power and we adapt the MAC layer accordingly. Our signal acquisition method is designed to solve the IUI (Inter-User Interference) that occurs in some ad-hoc networks where concurrent transmissions are allowed with heterogeneous power levels. In such scenarios, the conventional detection method, which is based on correlating the received IR signal with a Template Pulse Train (TPT), does not always perform well. The complexity of our proposal is similar to that of the conventional method. We evaluate its performance with the Line Of Sight (LOS) and the Non-LOS (NLOS) office indoor-channel models proposed by the IEEE P802.15.4a study group and find that the improvement is significant. We also investigate the particular case where the concurrent transmissions have the same time-hopping code, and we show that it does not result in collision, such scenarios appear in ad-hoc networks that employ a common code for control or broadcast purposes. At the MAC level, we focus only on one component of a MAC layer, which is the sleeping mode that could be added to any MAC layer proposal adequate to UWB IR. We are motivated by the low power consumption constraint required by the potential applications. We identify the design elements that should be taken into account for an optimal design for a sleeping protocol for UWB-IR such as the possibility of transmitting concurrently without collision and the power consumption model of the hardware behind which is completely different than with the narrow-band signaling. Then, we design two sleeping protocols for centralized and decentralized ad-hoc networks, respectively. We evaluate their performance analytically with the adopted metric being the average life-time of the wireless nodes.

EPFL2009

Approximation for Problems in Multi-User Information Theory

Soheil Mohajerzefreh

The main goal in network information theory is to identify fundamental limits of communication over networks, and design solutions which perform close to such limits. After several decades of effort, many important problems still do not have a characterization of achievable performance in terms of a finite dimensional description. Given this discouraging state of affairs, a natural question to ask is whether there are systematic approaches to make progress on these open questions. Recently, there has been significant progress on several open questions by seeking a (provably) approximate characterization for these open questions. The main goal of approximation in network information theory is to obtain a universal approximation gap between the achievable and the optimal performance. This approach consists of four ingredients: simplify the model, obtain optimal solution for the simplified model, translate this optimal scheme and outer bounds back to the original model, and finally bound the gap between what can be achieved using the obtained technique and the outer bound. Using such an approach, recent progress has been made in several problems such as the Gaussian interference channel, Gaussian relay networks, etc. In this thesis, we demonstrate that this approach is not only successful in problems of transmission over noisy networks, but gives the first approximation for a network data compression problem. We use this methodology to (approximately) resolve problems that have been open for several decades. Not only do we give theoretical characterization, but we also develop new coding schemes that are required to satisfy this approximate optimality property. These ideas could give insights into efficient design of future network communication systems. This thesis is split into two main parts. The first part deals with the approximation in lossy network data compression. Here, a lossy data compression problem is approximated by a lossless counterpart problem, where all the bits in the binary expansion of the source above the required distortion have to be losslessly delivered to the destination. In particular, we study the multiple description (MD) problem, based on the multi-level diversity (MLD) coding problem. The symmetric version of the MLD problem is well-studied, and we can directly use it to approximate the symmetric MD problem. We formulate the asymmetric multi-level diversity problem, and solve it for three-description case. The optimal solution for this problem, which will be later used to approximate the asymmetric multiple description problem, is based on jointly compressing of independent sources. In both symmetric and asymmetric cases, we derive inner and outer bounds for the achievable rate region, which together with the gap analysis, provide an approximate solution for the problem. In particular, we resolve the symmetric Gaussian MD problem, which has been open for three decades, to within 1 bit. In the second part, we initiate a study of a Gaussian relay-interference network, in which relay (helper) nodes are to facilitate competing information flows over a wireless network. We focus on a two-stage relay-interference network where there are weak cross-links, causing the networks to behave like a chain of Z Gaussian channels. For these Gaussian ZZ and ZS networks, we establish an approximate characterization of the rate region. The outer bounds to the capacity region are established using genie-aided techniques that yield bounds sharper than the traditional cut-set outer bounds. For the inner bound of the ZZ network, we propose a new interference management scheme, termed interference neutralization, which is implemented using structured lattice codes. This technique allows for over-the-air interference removal, without the transmitters having complete access to the interfering signals. We use insights gained from an exact characterization of the corresponding linear deterministic version of the problem, in order to study the Gaussian network. We resolve the Gaussian relay-interference network to within 2 bits. The new interference management technique (interference neutralization) shows the use of structured lattice codes in the problem. We also consider communication from a source to a destination over a wireless network with the help of a set of authenticated relays, and presence of an adversarial jammer who wishes to disturb communication. We focus on a special diamond network, and show that use of interference suppression (nulling) is crucial to approach the capacity of the network. The exact capacity characterization for the deterministic network, along with an approximate characterization (to within 4 bits) for the Gaussian network is provided. The common theme that binds the diverse network communication problems in this thesis is that of approximate characterization, when exact resolutions are difficult. The approach of focusing on the deterministic/lossless problems underlying the noisy/lossy network communication problems has allowed us to develop new techniques to study these questions. These new techniques might be of independent interest in other network information theory problems.

EPFL2010