Isn't Hybrid ARQ Sufficient?

In practical systems, reliable communication is often accomplished by coding at different network layers. We question the necessity of this approach and examine when it can be beneficial. Through conceptually simple probabilistic models (based on coin tossing), we argue that multicast scenarios and protocol restrictions may make concatenated multi-layer coding preferable to physical layer coding alone, which is mostly not the case in point-to-point communications.Comment: Paper presented at Allerton Conference 201

Diversity analysis, code design, and tight error rate lower bound for binary joint network-channel coding

Joint network-channel codes (JNCC) can improve the performance of communication in wireless networks, by combining, at the physical layer, the channel codes and the network code as an overall error-correcting code. JNCC is increasingly proposed as an alternative to a standard layered construction, such as the OSI-model. The main performance metrics for JNCCs are scalability to larger networks and error rate. The diversity order is one of the most important parameters determining the error rate. The literature on JNCC is growing, but a rigorous diversity analysis is lacking, mainly because of the many degrees of freedom in wireless networks, which makes it very hard to prove general statements on the diversity order. In this article, we consider a network with slowly varying fading point-to-point links, where all sources also act as relay and additional non-source relays may be present. We propose a general structure for JNCCs to be applied in such network. In the relay phase, each relay transmits a linear transform of a set of source codewords. Our main contributions are the proposition of an upper and lower bound on the diversity order, a scalable code design and a new lower bound on the word error rate to assess the performance of the network code. The lower bound on the diversity order is only valid for JNCCs where the relays transform only two source codewords. We then validate this analysis with an example which compares the JNCC performance to that of a standard layered construction. Our numerical results suggest that as networks grow, it is difficult to perform significantly better than a standard layered construction, both on a fundamental level, expressed by the outage probability, as on a practical level, expressed by the word error rate

Wireless Inter-Session Network Coding - An Approach Using Virtual Multicasts

This paper addresses the problem of inter-session network coding to maximize throughput for multiple communication sessions in wireless networks. We introduce virtual multicast connections which can extract packets from original sessions and code them together. Random linear network codes can be used for these virtual multicasts. The problem can be stated as a flow-based convex optimization problem with side constraints. The proposed formulation provides a rate region which is at least as large as the region without inter-session network coding. We show the benefits of our technique for several scenarios by means of simulation.United States. Defense Advanced Research Projects Agency (Subcontract 18870740-37362-C

ECJ judges read the morning papers. Explaining the turnaround of European citizenship jurisprudence

Recent jurisprudence of the European Court of Justice (ECJ) marks a striking shift towards a more restrictive interpretation of EU citizens’ rights. The Court's turnaround is not only highly relevant for practical debates about ‘Social Europe’ or ‘welfare migration’, but also enlightening from a more general, theoretical viewpoint. Several recent studies on the ECJ have argued that the Court is largely constrained by member state governments’ threats of legislative override and non-compliance. We show that an additional mechanism is necessary to explain the Court's turnaround on citizenship. While the ECJ extended EU citizens’ rights even against strong opposition by member state governments, its recent shift reflects changes in the broader political context, i.e., the politicization of free movement in the European Union (EU). The article theorises Court responsiveness to politicization and demonstrates empirically, how the Court's jurisprudence corresponds with changing public debates about EU citizenship

The Throughput of Slotted Aloha with Diversity

In this paper, a simple variation of classical Slotted Aloha is introduced and analyzed. The enhancement relies on adding multiple receivers that gather different observations of the packets transmitted by a user population in one slot. For each observation, the packets transmitted in one slot are assumed to be subject to independent on-off fading, so that each of them is either completely faded, and then does not bring any power or interference at the receiver, or it arrives unfaded, and then may or may not, collide with other unfaded transmissions. With this model, a novel type of diversity is introduced to the conventional SA scheme, leading to relevant throughput gains already for moderate number of receivers. The analytical framework that we introduce allows to derive closed-form expression of both throughput and packet loss rate an arbitrary number of receivers, providing interesting hints on the key trade-offs that characterize the system. We then focus on the problem of having receivers forward the full set of collected packets to a final gateway using the minimum possible amount of resources, i.e., avoiding delivery of duplicate packets, without allowing any exchange of information among them. We derive what is the minimum amount of resources needed and propose a scheme based on random linear network coding that achieves asymptotically this bound without the need for the receivers to coordinate among them