Enabling Efficient, Robust, and Scalable Wireless Multi-Hop Networks: A Cross-Layer Approach Exploiting Cooperative Diversity

The practical performance in terms of throughput, robustness, and scalability of traditional Wireless Multihop Networks (WMNs) is limited. The key problem is that such networks do not allow for advanced physical layers, which typically require (a) spatial diversity via multiple antennas, (b) timely Channel State Information (CSI) feedback, and (c) a central instance that coordinates nodes. We propose Corridor-based Routing to address these issues. Our approach widens traditional hop-by-hop paths to span multiple nodes at each hop, and thus provide spatial diversity. As a result, at each hop, a group of transmitters cooperates at the physical layer to forward data to a group of receivers. We call two subsequent groups of nodes a stage. Since all nodes participating in data forwarding at a certain hop are part of the same fully connected stage, corridors only require one-hop CSI feedback. Further, each stage operates independently. Thus, Corridor-based Routing does not require a network-wide central instance, and is scalable. We design a protocol that builds end-to-end corridors. As expected, this incurs more overhead than finding a traditional WMN path. However, if the resulting corridor provides throughput gains, the overhead compensates after a certain number of transmitted packets. We adapt two physical layers to the aforementioned stage topology, namely, Orthogonal Frequency-Division Multiple Access (OFDMA), and Interference Alignment (IA). In OFDMA, we allocate each subchannel to a link of the current stage which provides good channel conditions. As a result, we avoid deep fades, which enables OFDMA to transmit data robustly in scenarios in which traditional schemes cannot operate. Moreover, it achieves higher throughputs than such schemes. To minimize the transmission time at each stage, we present an allocation mechanism that takes into account both the CSI, and the amount of data that each transmitter needs to transmit. Further, we address practical issues and implement our scheme on software-defined radios. We achieve roughly 30% average throughput gain compared to a WMN not using corridors. We analyze OFDMA in theory, simulation, and practice. Our results match in all three domains. Further, we design a physical layer for corridor stages based on IA in the frequency domain. Our practical experiments show that IA often performs poorly because the decoding process augments noise. We find that the augmentation factor depends only on the channel coefficients of the subchannels that IA uses. We design a mechanism to determine which transmitters should transmit to which receivers on which subchannels to minimize noise. Since the number of possible combinations is very large, we use heuristics that reduce the search space significantly. Based on this design, we present the first practical frequency IA system. Our results show that our approach avoids noise augmentation efficiently, and thus operates robustly. We observe that IA is most suitable for stages with specific CSI and traffic conditions. In such scenarios, the throughput gain compared to a WMN not using corridors is 25% on average, and 150% in the best case. Finally, we design a decision engine which estimates the performance of both OFDMA and IA for a given stage, and chooses the one which achieves the highest throughput. We evaluate corridors with up to five stages, and achieve roughly 20% average throughput gain. We conclude that switching among physical layers to adapt to the particular CSI and traffic conditions of each stage is crucial for efficient and robust operation

Application-Aware Cross-Layer Framework for Wireless Multihop Networks

Current and future mobile and social communications require a rethinking in the development of wireless communication. Optimizing the radio transmission method is not going to scale with the ever increasing user demands. The future internet requires a wireless communication network which can adapt seamlessly to changing environments and service requirements. Especially, service requirements driven by user demand and expanding user device diversity raise a key challenge with respect to content distribution. In this work, research is conducted to improve wireless communication by considering four main aspects: The first aspect is to build a multi layer solution, instead of a conventional single layer solution to achieve higher throughput gains. Here, the physical layer, the medium access layer and the network layer are studied together to utilize capabilities across all these three layers. Thus, a unified graph model is formulated to adapt available mechanisms on the lower three layers in a joint manner. The second aspect is to envision a wireless multihop network which can scale with the increasing number of mobile devices. On the one hand, the number of mobile devices is ever increasing and so is the density of mobile devices in any given network. On the other hand, the requirements and capabilities of mobile devices are becoming more diverse and hence the heterogeneity in a wireless network is growing. This leads to the conclusion that a wireless multihop network is more future proof compared to a wireless network composed only of several base stations. Therefore, the research is focused on wireless multihop scenarios where multiple wireless devices form the network and communication between them occurs over multiple hops. The third aspect is to incorporate different requirements of applications and capabilities of applications. The plethora of applications used in wireless networks come with different sets of requirements, e.g. bandwidth, and capabilities, e.g. adaption of the video quality. Taking into account these requirements and capabilities in addition to a multi layer solution can further increase the performance. In this work, the requirements and capabilities of adaptive video streaming are integrated into an application-aware cross-layer framework. More precisely, scalable video coding and dynamic adaptive streaming over HTTP are integrated into the aforementioned framework. The novel application-aware cross-layer framework adapts network support structures at the network layer, performs resource allocation at the medium access layer, switches between communication types at the physical layer and takes into account the capabilities and requirements of applications, e.g. adaptive video-streaming, at the application layer. The fourth aspect is to utilize aggregation of distributed content, where content is cached over the whole network and can than be aggregated to be consumed by users in the network. Recent research shows promising gains achievable when content is cached at mobile devices, but mostly for single hop wireless networks. Hence, the impact of mobile content caching where popular content is cached and aggregated over multiple devices in a network is investigated in this work. In more detail, a content delivery framework which jointly exploits content already cached at mobile devices as well as switching between mechanisms at the physical layer and the network layer in order to optimally deliver the content to all destinations under changing network conditions is proposed

Radio Resource Management for Cellular Networks Enhanced by Inter-User Communication

The importance of radio resource management will be more and more emphasized in future wireless communication systems. For fair penetration of wireless services and for improved local services, inter-user communication has been receiving wide attention as it opens up various possibilities for user cooperation. The capability of inter-user communication imposes higher demands on radio resource management as additional considerations are needed. The demands for intelligent management of radio resources is also emphasized by the sparsity of radio resources. As the available spectral resources are assessed as under-utilized, much effort is devoted to developing advanced resource management methods for improving the spectral usage efficiency. The research of this thesis has contributed to the radio resource management for cellular networks enhanced by inter-user communication. Recognizing that inter-user communication can be used for message relaying or for direct communication purposes, two use cases are considered that leverage the synergy of users: cooperative relay selection and Device-to-Device (D2D) communication. We identify the importance of stochastic geometry consideration on cellular users for evaluating system performance in cooperative networking. We develop an algorithm for efficiently selecting cooperative users to maximize an End-to-End (e2e) performance metric. We analyze the optimal resource sharing problem between D2D communication and infrastructure-supported communication. We study the impact of imperfect Channel State Information (CSI) on the performance of systems with inter-user communication. Simulation results show that the performance of users with unfavorable propagation conditions can be improved with cooperative communication in a multi-cell cellular environment, at the expense of radio resources. Further, our results show that the selection of multiple cooperative users is beneficial in cases where the candidate cooperative users are spatially distributed. For resource sharing between the D2D and infrastructure-supported communication, our results show that the proposed resource sharing scheme enables higher intra-cell resource reuse without blocking the infrastructure-supported communication

Improving the Performance of Wireless LANs

This book quantifies the key factors of WLAN performance and describes methods for improvement. It provides theoretical background and empirical results for the optimum planning and deployment of indoor WLAN systems, explaining the fundamentals while supplying guidelines for design, modeling, and performance evaluation. It discusses environmental effects on WLAN systems, protocol redesign for routing and MAC, and traffic distribution; examines emerging and future network technologies; and includes radio propagation and site measurements, simulations for various network design scenarios, numerous illustrations, practical examples, and learning aids

D3.2 First performance results for multi -node/multi -antenna transmission technologies

This deliverable describes the current results of the multi-node/multi-antenna technologies investigated within METIS and analyses the interactions within and outside Work Package 3. Furthermore, it identifies the most promising technologies based on the current state of obtained results. This document provides a brief overview of the results in its first part. The second part, namely the Appendix, further details the results, describes the simulation alignment efforts conducted in the Work Package and the interaction of the Test Cases. The results described here show that the investigations conducted in Work Package 3 are maturing resulting in valuable innovative solutions for future 5G systems.Fantini. R.; Santos, A.; De Carvalho, E.; Rajatheva, N.; Popovski, P.; Baracca, P.; Aziz, D.... (2014). D3.2 First performance results for multi -node/multi -antenna transmission technologies. http://hdl.handle.net/10251/7675

Busy burst technology applied to OFDMA–TDD systems

The most significant bottleneck in wireless communication systems is an ever-increasing disproportion between the bandwidth demand and the available spectrum. A major challenge in the field of wireless communications is to maximise the spatial reuse of resources whilst avoiding detrimental co-channel interference (CCI). To this end, frequency planning and centralised coordination approaches are widely used in wireless networks. However, the networks for the next generation of wireless communications are often envisioned to be decentralised, randomly distributed in space, hierarchical and support heterogeneous traffic and service types. Fixed frequency allocation would not cater for the heterogeneous demands and centralised resource allocation would be cumbersome and require a lot of signalling. Decentralised radio resource allocation based on locally available information is considered the key. In this context, the busy burst (BB) signalling concept is identified as a potential mechanism for decentralised interference management in future generation networks. Interference aware allocation of time-frequency slots (chunks) is accomplished by letting receivers transmit a BB in a time-multiplexed mini-slot, upon successful reception of data. Exploiting channel reciprocity of the time division duplex (TDD) mode, the transmitters avoid reusing the chunks where the received BB power is above a pre-determined threshold so as to limit the CCI caused towards the reserved chunks to a threshold value. In this thesis, the performance of BB signalling mechanism in orthogonal frequency division multiple access - time division duplexing (OFDMA-TDD) systems is evaluated by means of system level simulations in networks operating in ad hoc and cellular scenarios. Comparisons are made against the state-of-the-art centralised CCI avoidance and mitigation methods, viz. frequency planning, fractional frequency reuse, and antenna array with switched grid of beams, as well as decentralised methods such as the carrier sense multiple access method that attempt to avoid CCI by avoiding transmission on chunks deemed busy. The results demonstrate that with an appropriate choice of threshold parameter, BB-based techniques outperform all of the above state-of-the-art methods. Moreover, it is demonstrated that by adjusting the BB-specific threshold parameter, the system throughput can be traded off for improving throughput for links with worse channel condition, both in the ad hoc and cellular scenario. Moreover, by utilising a variable BB power that allows a receiver to signal the maximum CCI it can tolerate, it is shown that a more favourable trade-off between total system throughput and link throughput can be made. Furthermore, by performing link adaptation, it is demonstrated that the spatial reuse and the energy efficiency can be traded off by adjusting the threshold parameter. Although the BB signalling mechanism is shown to be effective in avoiding detrimental CCI, it cannot mitigate CCI by itself. On the other hand, multiple antenna techniques such as adaptive beamforming or switched beam approaches allow CCI to be mitigated but suffer from hidden node problems. The final contribution of this thesis is that by combining the BB signalling mechanism with multiple antenna techniques, it is demonstrated that the hybrid approach enhances spatial reusability of resources whilst avoiding detrimental CCI. In summary, this thesis has demonstrated that BB provides a flexible radio resource mechanism that is suitable for future generation networks

Collaborative Broadcast in O(log log n) Rounds

We consider the multihop broadcasting problem for $n$ nodes placed uniformly at random in a disk and investigate the number of hops required to transmit a signal from the central node to all other nodes under three communication models: Unit-Disk-Graph (UDG), Signal-to-Noise-Ratio (SNR), and the wave superposition model of multiple input/multiple output (MIMO). In the MIMO model, informed nodes cooperate to produce a stronger superposed signal. We do not consider the problem of transmitting a full message nor do we consider interference. In each round, the informed senders try to deliver to other nodes the required signal strength such that the received signal can be distinguished from the noise. We assume sufficiently high node density $\rho= \Omega(\log n)$. In the unit-disk graph model, broadcasting needs $O(\sqrt{n/\rho})$ rounds. In the other models, we use an Expanding Disk Broadcasting Algorithm, where in a round only triggered nodes within a certain distance from the initiator node contribute to the broadcasting operation. This algorithm achieves a broadcast in only $O(\frac{\log n}{\log \rho})$ rounds in the SNR-model. Adapted to the MIMO model, it broadcasts within $O(\log \log n - \log \log \rho)$ rounds. All bounds are asymptotically tight and hold with high probability, i.e. $1- n^{-O(1)}$.Comment: extended abstract accepted for ALGOSENSORS 201

Pattern Diversity Characterization of Reconfigurable Antenna Arrays for Next Generation Wireless Systems

The use of multi-antenna technology in wireless radio communications has attracted tremendous attention due to its potential to increase data rates without requiring additional bandwidth and transmission power. This has been driven by the burgeoning demand for high data rates and the need for instantaneous and ubiquitous access to information. It is therefore no surprise that current and future generation wireless standards such as LTE and WiMAX have adopted the use of adaptive multi-antenna systems also known as adaptive Multiple Input and Multiple Output (MIMO) as their de facto transmission technology. In this thesis work, we focus on the design of a smart wireless antenna system, and the study of relevant techniques that enable us to reap the benefits of their deployment in small wireless devices with MIMO capability. Specifically, we employ a new class of adaptive antenna systems known as Reconfigurable Antenna Systems (RAS) for portable devices. These antennas are capable of dynamically changing their electrical and radiation characteristics to suit the conditions of the wireless channel. The changing radiation patterns lead to pattern diversity gains that improve system performance. This is in contrast to conventional non-reconfigurable arrays which depend on signal processing techniques such as antenna grouping and beamforming to achieve performance gains. However, despite the demonstrable system-level performance benefits of RAS in adaptive MIMO, few of these antennas have been adopted and integrated in state-of-the-art wireless standards. Their usage has been partly inhibited by the prohibitive costs of implementation and operation in a real wireless infrastructure. As part of this thesis research effort we attempt to integrate these new antennas into a cost-effective real wireless MIMO testbed for use in current generation technologies. The solution integration is carried-out through the use of readily available software-defined radio frameworks. We first design, analyze and characterize the pattern diversity in RAS antenna arrays that resonate at frequencies suitable for 4G applications. We then study the benefits of pattern diversity obtained from RAS arrays over conventional space diversity approaches such as antenna grouping and beamforming. This dissertation also presents low-complexity adaptive physical layer models and algorithms to exploit the benefits of RAS array integration in MIMO wireless systems. We implement these algorithms in software-defined radio frameworks, experimentally test, and benchmark them against other established approaches in literature. And finally, integrate and test these RAS array design prototypes as part of the MIMO wireless system that leverages a state-of-the-art wireless base station and mobile terminals.Ph.D., Electrical Engineering -- Drexel University, 201