Semiha Tedik Basaran

OFDMA-based network-coded cooperation: design and implementation using software-defined radio nodes

Benefits of network coding towards enhancing communication quality, both in terms of robustness or data transmission rates, make it a significant candidate as a future networking technology. Conventionally, network coding is mostly used in wired infrastructures, where transmission errors between nodes are negligible. Capturing the provided benefits of network coding via straightforward extension from wired networks to wireless networks is not trivial. In addition to the challenges introduced through the wireless channel impairments, we can also capture the spatial diversity gain provided by the broadcast nature of the wireless channels. In this work, we design and implement a network-coded cooperation (NCC) system that operates in real time through the use of software-defined radio (SDR) nodes for the first time in the literature. We specifically target wireless networks. Our system is based on orthogonal frequency division multiple access (OFDMA) that provides a practical means to enable high transmission rates through the use of narrowband subcarriers. The developed testbed is composed of three source nodes, a relay node and two destination nodes. The transmission of the proposed NCC-OFDMA system is completed in two phases; the broadcast and the relaying phases. Multiplexing of source nodes’ signals is achieved through OFDMA technique. In the broadcast phase, an OFDMA signal is transmitted to relay and destination nodes. In the relaying phase, the relay node first detects the OFDMA signal, generates network-coded symbols, and then transmits these symbols to destination nodes. At the end of these two phases, the destination nodes determine the source nodes’ signals by using network decoders. The destination nodes make use of both the uncoded and network-coded symbols, which are received in broadcast and relaying phases, respectively. Destination nodes then perform network decoding. Through real-time bit error rate and error vector magnitude measurements, we show that the NCC-OFDMA system can significantly improve the communication quality and robustness, while enabling data transmission between multiple users, as known from theoretical analyses. Some features of this implemented NCC-OFDMA system have the potential to be included in 5G standards, due to the improved radio resource usage efficiency.

A Tutorial on Network Coded Cooperation

Network Coding (NC) can significantly help increase the efficient use of network resources. However, directly applying NC to wireless networks is not an effective design solution. Wireless channels are prone to bit errors due to fading, possibly generating erroneous decisions at the relay nodes while applying NC. Furthermore, the broadcast nature of wireless channels may provide a direct communication link between source and destination nodes. In order to combat fading channel impairments, and also to exploit the inherent spatial diversity provided through the direct link, network coded cooperation (NCC) techniques should be incorporated to the NC systems for wireless networks. This tutorial provides an overview of the state-of-the-art in the NCC systems, combining the powerful tools of NC and cooperative communication, in order to increase data transmission rates and robustness of multiple source wireless networks.

Joint Subcarrier and Power Allocation in OFDMA Systems for Outage Minimization

The joint assignment of subcarriers and limited transmission power is addressed as a combinatorial optimization problem in orthogonal frequency division multiple access networks. To ensure fairness among users while jointly minimizing the total transmit power, the minimization of the number of outage subcarriers is considered as the objective function. The proposed scheme enables distinct rate requirement for each user. Optimal and reduced complexity algorithms, based on the application of Hungarian method to randomly weighted complete bipartite graphs, that reduce the outage probability according to the benchmark works while providing considerable power savings are proposed.

Joint power and subcarrier allocation in OFDMA systems

In this work, a method that evaluates both subcarrier and power allocations together for the users of OFDMA systems is proposed. In practical usage, the allocation to users is implemented from existing subcarrier group considering the transmission power of users is limited. The aim of this study is the realization of subcarrier allocation without exceeding power constraint for each user. In order to make subcarrier allocation fair between users, R2EHK algorithm has been performed. User outage probability has been utilized so as to measure the performance of proposed method. Owing to the proposed method, transmission power gain has been achieved without compromising on the outage probability performance.

Implementation of Network Coding in Wireless Systems

In this chapter, we target to give extensive performance analyses about application of network coding (NC) in wireless systems, referred to as network coded cooperation (NCC), brings both diversity and multiplexing gains. We use the diversity-multiplexing trade-off (DMT) to determine performance bounds of NCC systems. Within the scope of this study, NCC is integrated with orthogonal frequency division multiple access (OFDMA) and the corresponding system model is characterized by specifically focusing on frequency diversity gain. DMT expressions of the NCC-OFDMA system is given. A real-time implementation of the NCC-OFDMA system is presented by creating a testbed NI USRP-2921, NI PXIe-5644R, NI PXI-6683H software defined radio (SDR) modules and LabVIEW software. Obtained real-time performance measurements are essential to demonstrate the practical advantages or disadvantages of the usage of the NCC-OFDMA system. Overall, we aim to present a detailed overview of the fundamental performance bounds of NCC and its extension to the practical applicability of NCC in wireless networks.

Error Performance Analysis of Random Network Coded Cooperation Systems

This paper presents a framework for computing successful decoding probability of random network coding (RNC) in wireless networks. As cooperation emerges due to the naturally occurring broadcasting in wireless links, the application of RNC in wireless networks enables random network coded cooperation (RNCC). The theoretical successful decoding probability of RNCC systems is derived by obtaining the ratio of the full rank and the rank deficient matrices. The full rank condition of the global encoding matrix indicates the successful decoding of source symbols. The results of a single relay along with a relay selection scheme are also investigated. The validity of the presented theoretical expressions is demonstrated through identical simulation results. An implementation scenario is also presented to demonstrate the practical usage effectiveness of RNC in real-time applications, by using software-defined radio nodes.

Full-duplex transmission: A software defined radio implementation

The number of 5G studies are increasing rapidly and various solutions that aim improved communication quality are proposed. One method that is distinguished from others is the full-duplex communication. In this study, an OFDM based full-duplex communication system is implemented in real-time by using software defined radios. In literature, there are similar studies. However, this study has an unique place due to its practical and efficient digital solution and real-time implementation. With this study, a high performing system model is demonstrated and a prototype that could provide insights for future 5G studies is implemented.

A testbed for image transmission over a network coded cooperation system

Through the use of smarter relay nodes, network coding has a significant potential to address the resource scarcity problems that will be encountered in wireless networks. In this paper, we investigate the performance of the proposed orthogonal frequency division multiple access (OFDMA) based network coded cooperation (NCC) system for the first time in the literature, with image transmission scenario by using software defined radio nodes. We make use of the broadcast nature of the wireless channel and incorporate direct source destination links in the reception process to improve the performance by using NCC architecture. Promising results are obtained that strengthen NCCs potential for usage in near future applications.

Network coded cooperation in delay tolerant networks

Implementation of network coding is expanding to a diverse set of networks due to the flexible design provided through its use, enabling a tradeoff between robustness and spectral efficiency. In this paper, we investigate the application of network coding in delay tolerant networks (DTN) through the use of software defined radio (SDR) nodes. We compare the error performance of the implemented NCC system with a decode and forward (DF) cooperation scheme. Our results demonstrate that the NCC is a reliable option for DTNs, providing comparable error rates with DF and at the same time improving spectral and energy efficiency of the target network.

Different numbers of subcarrier allocation in OFDMA systems via random bipartite graphs

In this study, the problem of optimum subcarrier allocation in different numbers to users in OFDMA systems is discussed. OFDMA systems can be modelled by random bipartite graphs in scope of graph theory. Optimum subcarrier allocation with fixed numbers can be done by using maximum matching algorithms with providing fairness between users. In this study, Multi-R2EHK algorithm which provides subcarrier allocation in different numbers is proposed in accordance with requirements of different users. The results obtained with proposed algorithm shows that optimum subcarrier allocation according to user needs can be done by sacrificing some amount of performance. Besides, maximum frequency diversity and fairness between users also are provided.