Peyman Pahlevani

Novel concepts for device-to-device communication using network coding

Device-to-device communication is currently a hot research topic within 3GPP. Even though D2D communication has been part of previous ad hoc, meshed and sensor networks proposals, the main contribution by 3GPP is that the direct communication among two devices is carried out over a dynamically assigned, licensed spectrum; thus, it is under full control of the cellular network. D2D communication creates a market potential for new services, new approaches to efficient spectrum use, and security concepts. This is especially true if D2D communication is extended to larger communication groups organized in meshed clusters. In this article, we discuss the potential and shortcomings of D2D communication as proposed today, advocating for the use of network coding as an enabling technology for enhanced security and communication efficiency using the PlayNCool and CORE protocols as key examples to deliver smarter D2D systems.

On Optimal Policies for Network-Coded Cooperation: Theory and Implementation

Network-coded cooperative communication (NC-CC) has been proposed and evaluated as a powerful technology that can provide a better quality of service in the next-generation wireless systems, e.g., D2D communications. Previous contributions have focused on performance evaluation of NC-CC scenarios rather than searching for optimal policies that can minimize the total cost of reliable packet transmission. We break from this trend by initially analyzing the optimal design of NC-CC for a wireless network with one source, two receivers, and half-duplex erasure channels. The problem is modeled as a special case of Markov decision process (MDP), which is called stochastic shortest path (SSP), and is solved for any field size, arbitrary number of packets, and arbitrary erasure probabilities of the channels. The proposed MDP solution results in an optimal transmission policy per time slot, and we use it to design near-optimal heuristics for packet transmission in a network of one source and N ≥ 2 receivers. We also present numerical results that illustrate the performance of the proposed heuristics under a variety of scenarios. To complete our analysis, our heuristics are implemented in Aalborg Universitys Raspberry Pi testbed and compared with random linear network coding (RLNC) broadcast in terms of completion time, total number of required transmissions, and percentage of delivered generations. Our measurements show that enabling cooperation only among pairs of devices can decrease the completion time by up to 4.75 times, while delivering 100% of the 10000 generations transmitted, as compared to RLNC broadcast delivering only 88% of them in our tests.

PlayNCool: Opportunistic network coding for local optimization of routing in wireless mesh networks

This paper introduces PlayNCool, an opportunistic protocol with local optimization based on network coding to increase the throughput of a wireless mesh network (WMN). PlayNCool aims to enhance current routing protocols by (i) allowing random linear network coding transmissions end-to-end, (ii) recoding at intermediate nodes, and more importantly (iii) the identification and selection of helpers for each individual link. PlayNCool is easy to implement, compatible with existing routing protocols, and relies on simple intuition and closed-form, local optimization techniques. The intuition behind our protocol is that each helper to a link in a multi-hop path reinforces that link by listening to coded packets transmitted in the link and by judiciously choosing when to start transmitting to make the data exchange faster and more efficient. This paper pays special attention to techniques to determine how much a helper should wait before springing into action based on channel conditions for the optimization of a single link, i.e., the helper will play it cool by only speaking after it has heard enough to be truly useful. These techniques constitute a key feature of PlayNCool and are applicable in large scale mesh networks. We show that PlayNCool can provide gains of more than 3× in individual links, which translates into a large end-to-end throughput improvement, and that it provides higher gains when more nodes in the network contend for the channel at the MAC layer, making it particularly relevant for dense mesh networks.

Sharing the Pi: Testbed Description and Performance Evaluation of Network Coding on the Raspberry Pi

This paper presents the design and performance evaluation of an inexpensive testbed for network coding protocols composed of Raspberry Pis. First, we show the performance of random linear network coding primitives on the Raspberry Pi in terms of processing speed and energy consumption under a variety of configuration setups. Our measurements show that processing rates of up to 230 Mbps are possible with the Raspberry Pi. Also, the energy consumption per bit can be as small as 3 nJ/bit, which is several orders of magnitude smaller than the transmission/reception energy use. Surprisingly, overclocking the Raspberry Pi from 700 MHz to 1000 MHz not only produces an increase in processing speed of up to 68 % for large generation sizes, but also provides a reduction of 64 % in the processing energy per bit for most tested scenarios. Then, we show Raspberry Pi as an inexpensive, viable, and flexible platform to deploy large research networking testbeds for the evaluation of network coding protocols. We propose key parameters and representations to evaluate protocol performance in network nodes as well as validating the testbeds statistics using the case of a one-hop broadcast with random linear network coding, which is well understood in theory.

On the need of novel Medium Access Control schemes for network coding enabled wireless mesh networks

This paper advocates for a new Medium Access Control (MAC) strategy for wireless meshed networks by identifying overload scenarios in order to provide additional channel access priority to the relay. The key behind our MAC protocol is that the relay will adjust its back off window size according to the incoming and outgoing packet ratio. We describe the new protocol as an extension to the CSMA/CA protocol and implement the protocol on our own hardware platform. By means of our own testbed, we investigate two basic network structures, namely, the two-way relay and the cross topology. It is well known that network coding will improve the throughput in such systems, but our novel medium access scheme improves the performance in the cross topology by another 66 % for network coding and 150 % for classical forwarding in theory. These gains translate in a theoretical gain of 33 % of network coding over classical forwarding when both systems implement the improved MAC. However, our measurement results show an even larger gain for network coding, namely, up to 65 % over forwarding, as it copes better with channel losses under high load scenarios.

On the Coded Packet Relay Network in the Presence of Neighbors: Benefits of Speaking in a Crowded Room

This paper studies the problem of optimal use of a relay for reducing the transmission time of data packets from a source to a destination using network coding. More importantly, we address an effect that is typically overlooked in previous studies: the presence of active transmitting nodes in the neighborhood of such devices, which is typical in wireless mesh networks. We show that in systems with a fair medium access control mechanism (MAC), the use of a relay in a crowded medium brings forth considerable and unforeseen improvements, including up to 3.5x gains in terms of throughput compared to using only the direct link in some of our examples, and a considerable extension of the operating region where using a relay is beneficial. The problem is formulated as a Markov Decision Process (MDP) and numerical results are provided comparing simple, close-to-optimal heuristics to the optimal scheme.

Network coding to enhance standard routing protocols in wireless mesh networks

This paper introduces a design and simulation of a locally optimized network coding protocol, called PlayNCool, for wireless mesh networks. PlayN-Cool is easy to implement and compatible with existing routing protocols and devices. This allows the system to gain from network coding capabilities implemented in software without the need for new hardware. PlayNCool enhances performance by (i) choosing a local helper between nodes in the path to strengthen the quality of each link, (ii) using local information to decide when and how many transmissions to allow from the helper, and (iii) using random linear network coding to increase the usefulness of each transmission from the helpers. This paper focuses on the design details needed to make the system operate in reality and evaluating performance using ns-3 in multi-hop topologies. Our results show that the PlayNCool protocol increases the end-to-end throughput by more than two-fold and up to four-fold in our settings.

Network coding for hop-by-hop communication enhancement in multi-hop networks

In our recent study, we introduced the PlayNCool protocol that increases the throughput of the wireless networks by enabling a helper node to strengthen the communication link between two neighboring nodes and using random linear network coding. This paper focuses on design and implementation advantages of the PlayNCool protocol in a real environment of wireless mesh networks. We provide a detailed protocol to implement PlayNCool that is independent from the other protocols in the current computer network stack. PlayNCool performance is evaluated using NS-3 simulations and real-life measurements using Aalborg Universitys Raspberry Pi test-bed. Our results show that selecting the best policy to activate the helper node is a key to guarantee the performance of PlayNCool protocol. We also study the effect of neighbor nodes in the performance of PlayNCool. Using a helper in presence of active neighbors is useful even if the channel from helper to destination is not better than the channel between sender and destination. PlayNCool increases the gain of end-to-end communication by two-fold or more while maintaining compatibility to standard wireless ad-hoc routing protocols.

Reliable Communication in Wireless Meshed Networks Using Network Coding

The advantages of network coding have been extensively studied in the field of wireless networks. Integrating network coding with existing IEEE 802.11 MAC layer is a challenging problem. The IEEE 802.11 MAC does not provide any reliability mechanisms for overheard packets. This paper addresses this problem and suggests different mechanisms to support reliability as part of the MAC protocol. Analytical expressions to this problem are given to qualify the performance of the modified network coding. These expressions are confirmed by numerical result. While the suggested reliability mechanisms introduce some signaling overhead, the results show that the performance is yet improved.

On the Performance of the Cache Coding Protocol

Network coding approaches typically consider an unrestricted recoding of coded packets in the relay nodes to increase performance. However, this can expose the system to pollution attacks that cannot be detected during transmission, until the receivers attempt to recover the data. To prevent these attacks while allowing for the benefits of coding in mesh networks, the cache coding protocol was proposed. This protocol only allows recoding at the relays when the relay has received enough coded packets to decode an entire generation of packets. At that point, the relay node recodes and signs the recoded packets with its own private key, allowing the system to detect and minimize the effect of pollution attacks and making the relays accountable for changes on the data. This paper analyzes the delay performance of cache coding to understand the security-performance trade-off of this scheme. We introduce an analytical model for the case of two relays in an erasure channel relying on an absorbing Markov chain and an approximate model to estimate the performance in terms of the number of transmissions before successfully decoding at the receiver. We confirm our analysis using simulation results. We show that cache coding can overcome the security issues of unrestricted recoding with only a moderate decrease in system performance.