Algorithm design for scheduling and medium access control in heterogeneous mobile networks

Mención Internacional en el título de doctorThe rapid growth of wireless mobile devices has led to saturation and congestion of wireless channels – a well-known fact. In the recent years, this issue is further exacerbated by the ever-increasing demand for traffic intensed multimedia content applications, which include but are not limited to social media, news and video streaming applications. Therefore the development of highly efficient content distribution technologies is of utmost importance, specifically to cope with the scarcity and the high cost of wireless resources. To this aim, this thesis investigates the challenges and the considerations required to design efficient techniques to improve the performance of wireless networks. Since wireless signals are prone to fluctuations and mobile users are, with high likelihood, have difference channel qualities, we particularly focus on the scenarios with heterogeneous user distribution. Further, this dissertation considers two main techniques to cope with mobile users demand and the limitation of wireless resources. Firstly, we propose an opportunistic multicast scheduling to efficiently distribute or disseminate data to all users with low delay. Secondly, we exploit the Millimeter-Wave (mm-Wave) frequency band that has a high potential of meeting the high bandwidth demand. In particular, we propose a channel access mechanism and a scheduling algorithm that take into account the limitation of the high frequency band (i.e., high path loss). Multicast scheduling has emerged as one of the most promising techniques for multicast applications when multiple users require the same content from the base station. Unlike a unicast scheduler which sequentially serves the individual users, a multicast scheduler efficiently utilizes the wireless resources by simultaneously transmitting to multiple users. Precisely, it multiplies the gain in terms of the system throughput compared to unicast transmissions. In spite of the fact that multicast schedulers are more efficient than unicast schedulers, scheduling for multicast transmission is a challenging task. In particular, base station can only chose one rate to transmit to all users. While determining the rate for users with a similar instantaneous channel quality is straight forward, it is non-trivial when users have different instantaneous channel qualities, i.e., when the channel is heterogeneous. In such a scenario, on one hand, transmitting at a low rate results in low throughput. On the other hand, transmitting at a high rate causes some users to fail to receive the transmitted packet while others successfully receive it but with a rate lower than their maximum rate. The most common and simplest multicasting technique, i.e., broadcasting, transmits to all receivers using the maximum rate that is supported by the worst receiver. In recent years, opportunistic schedulers have been considered for multicasting. Opportunistic multicast schedulers maximize instantaneous throughput and transmit at a higher rate to serve only a subset of the multicast users. While broadcasting suffers from high delay for all users due to low transmission rate, the latter causes a long delay for the users with worse channel quality as they always favor users with better channel quality. To address these problems, we designed an opportunistic multicast scheduling mechanism that aims to achieve high throughput as well as low delay. Precisely, we are solving the finite horizon problem for multicasting. Our goal is that all multicast users receive the same amount of data within the shortest amount of time. Although our proposed opportunistic multicast scheduling mechanism improves the system throughput and reduces delay, a common problem in multicast scheduling is that its throughput performance is limited by the worst user in the system. To overcome this problem, transmit beamforming can be used to adjust antenna gains to the different receivers. This allows improving the SNR of the receiver with the worst channel SNR at the expense of worsening the SNR of the better channel receivers. In the first part of this thesis, two different versions of the finite horizon problem are considered: (i) opportunistic multicast scheduling and (ii) opportunistic multicast beamforming. In recent years, many researchers venture into the potential of communication over mm-Wave band as it potentially solves the existing network capacity problem. Since beamforming is capable to concentrate the transmit energy in the direction of interest, this technique is particularly beneficial to improve signal quality of the highly attenuated mm-Wave signal. Although directional beamforming in mm-Wave offers multi-gigabit-per-second data rates, directional communication severely deteriorates the channel sensing capability of a user. For instance, when a user is not within the transmission coverage or range of the communicating users, it is unable to identify the state of the channel (i.e., busy or free). As a result, this leads to a problem commonly known as the deafness problem. This calls for rethinking of the legacy medium access control and scheduling mechanisms for mm-Wave communication. Further, without omni-directional transmission, disseminating or broadcasting global information also becomes complex. To cope with these issues, we propose two techniques in the second part of this thesis. First, leveraging that recent mobile devices have multiple wireless interface, we present a dual-band solution. This solution exploits the omni-directional capable lower frequency bands (i.e., 2.4 and 5 GHz) to transmit control messages and the mm-Wave band for high speed data transmission. Second, we develop a decentralized scheduling technique which copes with the deafness problem in mm-Wave through a learning mechanism. In a nutshell, this thesis explores solutions which (i) improve the utilization of the network resources through multicasting and (ii) meet the mobile user demand with the abundant channel resources available at high frequency bands.This work has been supported by IMDEA Networks Institute.Programa Oficial de Doctorado en Ingeniería TelemáticaPresidente: Ralf Steinmetz.- Secretario: Carlos Jesús Bernardos Cano.- Vocal: Jordi Domingo Pascua

Network coding for transport protocols

With the proliferation of smart devices that require Internet connectivity anytime, anywhere, and the recent technological advances that make it possible, current networked systems will have to provide a various range of services, such as content distribution, in a wide range of settings, including wireless environments. Wireless links may experience temporary losses, however, TCP, the de facto protocol for robust unicast communications, reacts by reducing the congestion window drastically and injecting less traffic in the network. Consequently the wireless links are underutilized and the overall performance of the TCP protocol in wireless environments is poor. As content delivery (i.e. multicasting) services, such as BBC iPlayer, become popular, the network needs to support the reliable transport of the data at high rates, and with specific delay constraints. A typical approach to deliver content in a scalable way is to rely on peer-to-peer technology (used by BitTorrent, Spotify and PPLive), where users share their resources, including bandwidth, storage space, and processing power. Still, these systems suffer from the lack of incentives for resource sharing and cooperation, and this problem is exacerbated in the presence of heterogenous users, where a tit-for-tat scheme is difficult to implement. Due to the issues highlighted above, current network architectures need to be changed in order to accommodate the users¿ demands for reliable and quality communications. In other words, the emergent need for advanced modes of information transport requires revisiting and improving network components at various levels of the network stack. The innovative paradigm of network coding has been shown as a promising technique to change the design of networked systems, by providing a shift from how data flows traditionally move through the network. This shift implies that data flows are no longer kept separate, according to the ¿store-and-forward¿ model, but they are also processed and mixed in the network. By appropriately combining data by means of network coding, it is expected to obtain significant benefits in several areas of network design and architecture. In this thesis, we set out to show the benefits of including network coding into three communication paradigms, namely point-topoint communications (e.g. unicast), point-to-multipoint communications (e.g. multicast), and multipoint-to-multipoint communications (e.g. peer-to-peer networks). For the first direction, we propose a network coding-based multipath scheme and show that TCP unicast sessions are feasible in highly volatile wireless environments. For point-to-multipoint communications, we give an algorithm to optimally achieve all the rate pairs from the rate region in the case of degraded multicast over the combination network. We also propose a system for live streaming that ensures reliability and quality of service to heterogenous users, even if data transmissions occur over lossy wireless links. Finally, for multipoint-to-multipoint communications, we design a system to provide incentives for live streaming in a peer-to-peer setting, where users have subscribed to different levels of quality. Our work shows that network coding enables a reliable transport of data, even in highly volatile environments, or in delay sensitive scenarios such as live streaming, and facilitates the implementation of an efficient incentive system, even in the presence of heterogenous users. Thus, network coding can solve the challenges faced by next generation networks in order to support advanced information transport.Postprint (published version

Cognitive Communications in White Space: Opportunistic Scheduling, Spectrum Shaping and Delay Analysis

abstract: A unique feature, yet a challenge, in cognitive radio (CR) networks is the user hierarchy: secondary users (SU) wishing for data transmission must defer in the presence of active primary users (PUs), whose priority to channel access is strictly higher.Under a common thread of characterizing and improving Quality of Service (QoS) for the SUs, this dissertation is progressively organized under two main thrusts: the first thrust focuses on SU's throughput by exploiting the underlying properties of the PU spectrum to perform effective scheduling algorithms; and the second thrust aims at another important QoS performance of the SUs, namely delay, subject to the impact of PUs' activities, and proposes enhancement and control mechanisms. More specifically, in the first thrust, opportunistic spectrum scheduling for SU is first considered by jointly exploiting the memory in PU's occupancy and channel fading. In particular, the underexplored scenario where PU occupancy presents a {long} temporal memory is taken into consideration. By casting the problem as a partially observable Markov decision process, a set of {multi-tier} tradeoffs are quantified and illustrated. Next, a spectrum shaping framework is proposed by leveraging network coding as a {spectrum shaper} on the PU's traffic. Such shaping effect brings in predictability of the primary spectrum, which is utilized by the SUs to carry out adaptive channel sensing by prioritizing channel access order, and hence significantly improve their throughput. On the other hand, such predictability can make wireless channels more susceptible to jamming attacks. As a result, caution must be taken in designing wireless systems to balance the throughput and the jamming-resistant capability. The second thrust turns attention to an equally important performance metric, i.e., delay performance. Specifically, queueing delay analysis is conducted for SUs employing random access over the PU channels. Fluid approximation is taken and Poisson driven stochastic differential equations are applied to characterize the moments of the SUs' steady-state queueing delay. Then, dynamic packet generation control mechanisms are developed to meet the given delay requirements for SUs.Dissertation/ThesisPh.D. Electrical Engineering 201

Instantly Decodable Network Coding: From Point to Multi-Point to Device-to-Device Communications

The network coding paradigm enhances transmission efficiency by combining information flows and has drawn significant attention in information theory, networking, communications and data storage. Instantly decodable network coding (IDNC), a subclass of network coding, has demonstrated its ability to improve the quality of service of time critical applications thanks to its attractive properties, namely the throughput enhancement, delay reduction, simple XOR-based encoding and decoding, and small coefficient overhead. Nonetheless, for point to multi-point (PMP) networks, IDNC cannot guarantee the decoding of a specific new packet at individual devices in each transmission. Furthermore, for device-to-device (D2D) networks, the transmitting devices may possess only a subset of packets, which can be used to form coded packets. These challenges require the optimization of IDNC algorithms to be suitable for different application requirements and network configurations. In this thesis, we first study a scalable live video broadcast over a wireless PMP network, where the devices receive video packets from a base station. Such layered live video has a hard deadline and imposes a decoding order on the video layers. We design two prioritized IDNC algorithms that provide a high level of priority to the most important video layer before considering additional video layers in coding decisions. These prioritized algorithms are shown to increase the number of decoded video layers at the devices compared to the existing network coding schemes. We then study video distribution over a partially connected D2D network, where a group of devices cooperate with each other to recover their missing video content. We introduce a cooperation aware IDNC graph that defines all feasible coding and transmission conflictfree decisions. Using this graph, we propose an IDNC solution that avoids coding and transmission conflicts, and meets the hard deadline for high importance video packets. It is demonstrated that the proposed solution delivers an improved video quality to the devices compared to the video and cooperation oblivious coding schemes. We also consider a heterogeneous network wherein devices use two wireless interfaces to receive packets from the base station and another device concurrently. For such network, we are interested in applications with reliable in-order packet delivery requirements. We represent all feasible coding opportunities and conflict-free transmissions using a dual interface IDNC graph. We select a maximal independent set over the graph by considering dual interfaces of individual devices, in-order delivery requirements of packets and lossy channel conditions. This graph based solution is shown to reduce the in-order delivery delay compared to the existing network coding schemes. Finally, we consider a D2D network with a group of devices experiencing heterogeneous channel capacities. For such cooperative scenarios, we address the problem of minimizing the completion time required for recovering all missing packets at the devices using IDNC and physical layer rate adaptation. Our proposed IDNC algorithm balances between the adopted transmission rate and the number of targeted devices that can successfully receive the transmitted packet. We show that the proposed rate aware IDNC algorithm reduces the completion time compared to the rate oblivious coding scheme

Network Coding Strategies for Satellite Communications

Network coding (NC) is an important technology that allows the network services to be optimal. The main advantage of NC is to reduce the necessity for re-transmissions of packets. Satellite Communications (SatComs) are one of the potential applications that can leverage on the benefits of NC due to their challenging fading environments and high round trip times. The motivation is to take the physical layer-awareness into consideration for adapting and hence extend the NC gains. Different rate and energy efficient adaptive NC schemes for time variant channels are proposed. We compare our proposed physical layer adaptive schemes to physical layer non-adaptive NC schemes for time variant channels. The adaptation of packet transmissions is on the basis of the corresponding time-dependent erasures, and allows proposed schemes to achieve significant gains in terms of throughput, delay and energy efficiency. The proposed schemes are robust for large and small size of packets. Although, the energy per bit is affected, a similar rate and energy gains can be arise. However, the performance gains are not motivated by the packet size, but through duty cycle silence of transfer packets. In this thesis, virtual schemes are also proposed to solve an open literature problem in the NC. The objective is to find a quasi-optimal number of coded packets to multicast to a group of independent wireless receivers suffer from a different channel conditions. In particular, we propose two virtual network that allows for the representation of a group of receivers as a multicast group to be visible as one receiver and single channel. Most of the schemes are applied to LEO/MEO/GEO satellite scenarios. They demonstrate remarkable gains compared to that strategy in which the adaptation depends only on one receiver point-to-point

Achieving Quality of Service Guarantees for Delay Sensitive Applications in Wireless Networks

In the past few years, we have witnessed the continuous growth in popularity of delay-sensitive applications. Applications like live video streaming, multimedia conferencing, VoIP and online gaming account for a major part of Internet traffic these days. It is also predicted that this trend will continue in the coming years. This emphasizes the significance of developing efficient scheduling algorithms in communication networks with guaranteed low delay performance. In our work, we try to address the delay issue in some major instances of wireless communication networks. First, we study a wireless content distribution network (CDN), in which the requests for the content may have service deadlines. Our wireless CDN consists of a media vault that hosts all the content in the system and a number of local servers (base stations), each having a cache for temporarily storing a subset of the content. There are two major questions associated with this framework: (i) content caching: which content should be loaded in each cache? and (ii) wireless network scheduling: how to appropriately schedule the transmissions from wireless servers? Using ideas from queuing theory, we develop provably optimal algorithms to jointly solve the caching and scheduling problems. Next, we focus on wireless relay networks. It is well accepted that network coding can enhance the performance of these networks by exploiting the broadcast nature of the wireless medium. This improvement is usually evaluated in terms of the number of required transmissions for delivering flow packets to their destinations. In this work, we study the effect of delay on the performance of network coding by characterizing a trade-off between latency and the performance gain achieved by employing network coding. More specifically, we associate a holding cost for delaying packets before delivery and a transmission cost for each broadcast transmission made by the relay node. Using a Markov decision process (MDP) argument, we prove a simple threshold-based policy is optimal in the sense of minimum long-run average cost. Finally, we analyze delay-sensitive applications in wireless peer-to-peer (P2P) networks. We consider a hybrid network which consists of (i) an expensive base station-to-peer (B2P) network with unicast transmissions, and (ii) a free broadcast P2P network. In such a framework, we study two popular applications: (a) a content distribution application with service deadlines, and (b) a multimedia live streaming application. In both problems, we utilize random linear network coding over finite fields to simplify the coordination of the transmissions. For these applications, we provide efficient algorithms to schedule the transmissions such that some quality of service (QoS) requirements are satisfied with the minimum cost of B2P usage. The algorithms are proven to be throughput optimal for sufficiently large field sizes and perform reasonably well for finite fields

Efficient operation of coded packet networks

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. [109]-122).A fundamental problem faced in the design of almost all packet networks is that of efficient operation -- of reliably communicating given messages among nodes at minimum cost in resource usage. We present a solution to the efficient operation problem for coded packet networks, i.e., packet networks where the contents of outgoing packets are arbitrary, causal functions of the contents of received packets. Such networks are in contrast to conventional, routed packet networks, where outgoing packets are restricted to being copies of received packets and where reliability is provided by the use of retransmissions. This thesis introduces four considerations to coded packet networks: 1. efficiency, 2. the lack of synchronization in packet networks, 3. the possibility of broadcast links, and 4. packet loss. We take these considerations and give a prescription for operation that is novel and general, yet simple, useful, and extensible. We separate the efficient operation problem into two smaller problems, which we call network coding -- the problem of deciding what coding operation each node should perform given the rates at which packets are injected on each link -- and subgraph selection -- the problem of deciding those rates.(cont.) Our main contribution for the network coding problem is to give a scheme that achieves the maximum rate of a multicast connection under the given injection rates. As a consequence, the separation of network coding and subgraph selection results in no loss of optimality provided that we are constrained to only coding packets within a single connection. Our main contribution for the subgraph selection problem is to give distributed algorithms that optimally solve the single-connection problem under certain assumptions. Since the scheme we propose for network coding can easily be implemented in a distributed manner, we obtain, by combining the solutions for each of the smaller problems, a distributed approach to the efficient operation problem. We assess the performance of our solution for three problems: minimum-transmission wireless unicast, minimum-weight wireline multicast, and minimum-energy wireless multicast. We find that our solution has the potential to offer significant efficiency improvements over existing techniques in routed packet networks, particularly for multi-hop wireless networks.by Desmond S. Lun.Ph.D