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

Adaptive relaying protocol multiple-input multiple-output orthogonal frequency division multiplexing systems

In wireless broadband communications, orthogonal frequency division multiplexing (OFDM) has been adopted as a promising technique to mitigate multi-path fading and provide high spectral efficiency. In addition, cooperative communication can explore spatial diversity where several users or nodes share their resources and cooperate through distributed transmission. The concatenation of the OFDM technique with relaying systems can enhance the overall performance in terms of spectral efficiency and improve robustness against the detrimental effects of fading. Hybrid relay selection is proposed to overcome the drawbacks of conventional forwarding schemes. However, exciting hybrid relay protocols may suffer some limitations when used for transmission over frequency-selective channels. The combination of cooperative protocols with OFDM systems has been extensively utilized in current wireless networks, and have become a promising solution for future high data rate broadband communication systems including 3D video transmission. This thesis covers two areas of high data rate networks. In the first part, several techniques using cooperative OFDM systems are presented including relay selection, space time block codes, resource allocation and adaptive bit and power allocation to introduce diversity. Four (4) selective OFDM relaying schemes are studied over wireless networks; selective OFDM; selective OFDMA; selective block OFDM and selective unequal block OFDM. The closed-form expression of these schemes is derived. By exploiting the broadcast nature, it is demonstrated that spatial diversity can be improved. The upper bound of outage probability for the protocols is derived. A new strategy for hybrid relay selection is proposed to improve the system performance by removing the sub-carriers that experience deep fading. The per subcarrier basis selection is considered with respect to the predefined threshold signal-to-noise ratio. The closed-form expressions of the proposed protocol in terms of bit error probability and outage probability are derived and compared with conventional hybrid relay selection. Adaptive bit and power allocation is also discussed to improve the system performance. Distributed space frequency coding applied to hybrid relay selection to obtain full spatial and full data rate transmission is explored. Two strategies, single cluster and multiple clusters, are considered for the Alamouti code at the destination by using a hybrid relay protocol. The power allocation with and without sub-carrier pairing is also investigated to mitigate the effect of multipath error propagation in frequency-selective channels. The second part of this thesis investigates the application of cooperative OFDM systems to high data rate transmission. Recently, there has been growing attention paid to 3D video transmission over broadband wireless channels. Two strategies for relay selection hybrid relay selection and first best second best are proposed to implement unequal error protection in the physical layer over error prone channels. The closed-form expressions of bit error probability and outage probability for both strategies are examined. The peak signal-to-noise ratio is presented to show the quality of reconstruction of the left and right views

Towards reliable communication in LTE-A connected heterogeneous machine to machine network

Machine to machine (M2M) communication is an emerging technology that enables heterogeneous devices to communicate with each other without human intervention and thus forming so-called Internet of Things (IoTs). Wireless cellular networks (WCNs) play a significant role in the successful deployment of M2M communication. Specially the ongoing massive deployment of long term evolution advanced (LTE-A) makes it possible to establish machine type communication (MTC) in most urban and remote areas, and by using LTE-A backhaul network, a seamless network communication is being established between MTC-devices and-applications. However, the extensive network coverage does not ensure a successful implementation of M2M communication in the LTE-A, and therefore there are still some challenges. Energy efficient reliable transmission is perhaps the most compelling demand for various M2M applications. Among the factors affecting reliability of M2M communication are the high endto-end delay and high bit error rate. The objective of the thesis is to provide reliable M2M communication in LTE-A network. In this aim, to alleviate the signalling congestion on air interface and efficient data aggregation we consider a cluster based architecture where the MTC devices are grouped into number of clusters and traffics are forwarded through some special nodes called cluster heads (CHs) to the base station (BS) using single or multi-hop transmissions. In many deployment scenarios, some machines are allowed to move and change their location in the deployment area with very low mobility. In practice, the performance of data transmission often degrades with the increase of distance between neighboring CHs. CH needs to be reselected in such cases. However, frequent re-selection of CHs results in counter effect on routing and reconfiguration of resource allocation associated with CH-dependent protocols. In addition, the link quality between a CH-CH and CH-BS are very often affected by various dynamic environmental factors such as heat and humidity, obstacles and RF interferences. Since CH aggregates the traffic from all cluster members, failure of the CH means that the full cluster will fail. Many solutions have been proposed to combat with error prone wireless channel such as automatic repeat request (ARQ) and multipath routing. Though the above mentioned techniques improve the communication reliability but intervene the communication efficiency. In the former scheme, the transmitter retransmits the whole packet even though the part of the packet has been received correctly and in the later one, the receiver may receive the same information from multiple paths; thus both techniques are bandwidth and energy inefficient. In addition, with retransmission, overall end to end delay may exceed the maximum allowable delay budget. Based on the aforementioned observations, we identify CH-to-CH channel is one of the bottlenecks to provide reliable communication in cluster based multihop M2M network and present a full solution to support fountain coded cooperative communications. Our solution covers many aspects from relay selection to cooperative formation to meet the user’s QoS requirements. In the first part of the thesis, we first design a rateless-coded-incremental-relay selection (RCIRS) algorithm based on greedy techniques to guarantee the required data rate with a minimum cost. After that, we develop fountain coded cooperative communication protocols to facilitate the data transmission between two neighbor CHs. In the second part, we propose joint network and fountain coding schemes for reliable communication. Through coupling channel coding and network coding simultaneously in the physical layer, joint network and fountain coding schemes efficiently exploit the redundancy of both codes and effectively combat the detrimental effect of fading conditions in wireless channels. In the proposed scheme, after correctly decoding the information from different sources, a relay node applies network and fountain coding on the received signals and then transmits to the destination in a single transmission. Therefore, the proposed schemes exploit the diversity and coding gain to improve the system performance. In the third part, we focus on the reliable uplink transmission between CHs and BS where CHs transmit to BS directly or with the help of the LTE-A relay nodes (RN). We investigate both type-I and type-II enhanced LTE-A networks and propose a set of joint network and fountain coding schemes to enhance the link robustness. Finally, the proposed solutions are evaluated through extensive numerical simulations and the numerical results are presented to provide a comparison with the related works found in the literature

Network Coding for Multihop Wireless Networks: Joint Random Linear Network Coding and Forward Error Correction with Interleaving for Multihop Wireless Networks

Optimising the throughput performance for wireless networks is one of the challenging tasks in the objectives of communication engineering, since wireless channels are prone to errors due to path losses, random noise, and fading phenomena. The transmission errors will be worse in a multihop scenario due to its accumulative effects. Network Coding (NC) is an elegant technique to improve the throughput performance of a communication network. There is the fact that the bit error rates over one modulation symbol of 16- and higher order- Quadrature Amplitude Modulation (QAM) scheme follow a certain pattern. The Scattered Random Network Coding (SRNC) system was proposed in the literature to exploit the error pattern of 16-QAM by using bit-scattering to improve the throughput of multihop network to which is being applied the Random Linear Network Coding (RLNC). This thesis aims to improve further the SRNC system by using Forward Error Correction (FEC) code; the proposed system is called Joint RLNC and FEC with interleaving. The first proposed system (System-I) uses Convolutional Code (CC) FEC. The performances analysis of System-I with various CC rates of 1/2, 1/3, 1/4, 1/6, and 1/8 was carried out using the developed simulation tools in MATLAB and compared to two benchmark systems: SRNC system (System-II) and RLNC system (System- III). The second proposed system (System-IV) uses Reed-Solomon (RS) FEC code. Performance evaluation of System IV was carried out and compared to three systems; System-I with 1/2 CC rate, System-II, and System-III. All simulations were carried out over three possible channel environments: 1) AWGN channel, 2) a Rayleigh fading channel, and 3) a Rician fading channel, where both fading channels are in series with the AWGN channel. The simulation results show that the proposed system improves the SRNC system. How much improvement gain can be achieved depends on the FEC type used and the channel environment.Indonesian Government and the University of Bradfor

Optimum Design of Spectral Efficient Green Wireless Communications

This dissertation focuses on the optimum design of spectral efficient green wireless communications. Energy efficiency (EE), which is defined as the inverse of average energy required to successfully deliver one information bit from a source to its destination, and spectral efficiency (SE), which is defined as the average data rate per unit bandwidth, are two fundamental performance metrics of wireless communication systems. We study the optimum designs of a wide range of practical wireless communication systems that can either maximize EE, or SE, or achieve a balanced tradeoff between the two metrics. There are three objectives in this dissertation. First, an accurate frame error rate (FER) expression is developed for practical coded wireless communication systems operating in quasi-static Rayleigh fading channels. The new FER expression enables the accurate modeling of EE and SE for various wireless communication systems. Second, the optimum designs of automatic repeat request (ARQ) and hybrid ARQ (HARQ) systems are performed to by using the EE and SE as design metrics. Specifically, a new metric of normalized EE, which is defined as the EE normalized by the SE, is proposed to achieve a balanced tradeoff between the EE and SE. Third, a robust frequency-domain on-off accumulative transmission (OOAT) scheme has been developed to achieve collision-tolerant media access control (CT-MAC) in a wireless network. The proposed frequency domain OOAT scheme can improve the SE and EE by allowing multiple users to transmit simultaneously over the same frequency bands, and the signal collisions at the receiver can be resolved by using signal processing techniques in the physical layer

Cross-layer schemes for performance optimization in wireless networks

Wireless networks are undergoing rapid progress and inspiring numerous applications. As the application of wireless networks becomes broader, they are expected to not only provide ubiquitous connectivity, but also support end users with certain service guarantees. End-to-end delay is an important Quality of Service (QoS) metric in multihop wireless networks. This dissertation addresses how to minimize end-to-end delay through joint optimization of network layer routing and link layer scheduling. Two cross-layer schemes, a loosely coupled cross-layer scheme and a tightly coupled cross-layer scheme, are proposed. The two cross-layer schemes involve interference modeling in multihop wireless networks with omnidirectional antenna. In addition, based on the interference model, multicast schedules are optimized to minimize the total end-to-end delay. Throughput is another important QoS metric in wireless networks. This dissertation addresses how to leverage the spatial multiplexing function of MIMO links to improve wireless network throughput. Wireless interference modeling of a half-duplex MIMO node is presented. Based on the interference model, routing, spatial multiplexing, and scheduling are jointly considered in one optimization model. The throughput optimization problem is first addressed in constant bit rate networks and then in variable bit rate networks. In a variable data rate network, transmitters can use adaptive coding and modulation schemes to change their data rates so that the data rates are supported by the Signal to Noise and Interference Ratio (SINR). The problem of achieving maximum throughput in a millimeter-wave wireless personal area network is studied --Abstract, page iv

Dynamic control of wireless networks with confidential communications

Future wireless communication systems are rapidly transforming to satisfy everincreasing and varying mobile user demands. Cross-layer networking protocols have the potential to play a crucial role in this transformation by jointly addressing the requirements of user applications together with the time-varying nature of wireless networking. As wireless communications becoming an integral and crucial part of our daily lives with many of our personal data is being shared via wireless transmissions, the issue of keeping personal transactions confidential is at the forefront of any network design. Wireless communications is especially prone to attacks due to its broadcast nature. The conventional cryptographical methods can only guarantee secrecy with the assumption that it is computationally prohibitive for the eavesdroppers to decode the messages. On the other hand, information-theoretical secrecy as defined by Shannon in his seminal work has the potential to provide perfect secrecy regardless of the computational power of the eavesdropper. Recent studies has shown that information-theoretical secrecy is possible over noisy wireless channels. In this thesis, we aim to design simple yet provably optimal cross-layer algorithms taking into account information-theoretical secrecy as a Quality of Service (QoS) requirement. Our work has the potential to improve our understanding the interplay between the secrecy and networking protocols. In most of this thesis, we consider a wireless cellular architecture, where all nodes participate in communication with a base station. When a node is transmitting a confidential messages, other legitimate nodes are considered as eavesdroppers, i.e., all eavesdroppers are internal. We characterize the region of achievable open and confidential data rate pairs for a single and then a multi-node scenario. We define the notion of confidential opportunistic scheduler, which schedules a node that has the largest instantaneous confidential information rate, with respect to the best eavesdropper node, which has the largest mean cross-channel rate. Having defined the operational limits of the system, we then develop dynamic joint scheduling and flow control algorithms when perfect and imperfect channel state information (CSI) is available. The developed algorithms are simple index policies, in which scheduling and flow control decisions are given in each time instant independently. In real networks, instantaneous CSI is usually unavailable due to computational and communication overheads associated with obtaining this information. Hence, we generalize our model for the case where only the distributions of direct- and crosschannel CSI are available at the transmitter. In order to provide end-to-end reliability, Hybrid Automatic Retransmission reQuest (HARQ) is employed. The challenge of using HARQ is that the dynamic control policies proposed in the preceding chapter are no longer optimal, since the decisions at each time instant are no longer independent. This is mainly due to the potential of re-transmitting a variant of the same message successively until it is decoded at the base station. We solve this critical issue by proposing a novel queuing model, in which the messages transmitted the same number of times previously are stored in the same queue with scheduler selecting a head-of-line message from these queues. We prove that with this novel queuing model, the dynamic control algorithms can still be optimal. We then shift our attention to providing confidentiality in multi-hop wireless networks, where there are multiple source-destination pairs communicating confidential messages, to be kept confidential from the intermediate nodes. For this case, we propose a novel end-to-end encoding scheme, where the confidential information is encoded into one very long message. The encoded message is then divided into multiple packets, to be combined at the ultimate destination for recovery, and being sent over different paths so that each intermediate node only has partial view of the whole message. Based on the proposed end-to-end encoding scheme, we develop two different dynamic policies when the encoded message is finite and asymptotically large, respectively. When the encoded message has finite length, our proposed policy chooses the encoding rates for each message, based on the instantaneous channel state information, queue states and secrecy requirements. Also, the nodes keep account of the information leaked to intermediate nodes as well the information reaching the destination in order to provide confidentiality and reliability. We demonstrate via simulations that our policy has a performance asymptotically approaching that of the optimal policy with increasing length of the encoded message. All preceding work assumes that the nodes are altruistic and/or well-behaved, i.e., they cooperatively participate into the communication of the confidential messages. In the final chapter of the thesis, we investigate the case with non-altruistic nodes, where non-altruistic nodes provide a jamming service to nodes with confidential communication needs and receiving in turn the right to access to the channel. We develop optimal resource allocation and power control algorithms maximizing the aggregate utility of both nodes with confidential communication needs as well as the nodes providing jamming service

Interference Management Techniques for Cellular Wireless Communication Systems

The growing demand for higher capacity wireless networks can be met by increasing the frequency bandwidth, spectral efficiency, and base station density. Flexible spectrum access, multiantenna, and multicarrier techniques are key enablers in satisfying the demand. In addition, automation of tasks related to network planning, optimization, interference management, and maintenance are needed in order to ensure cost-efficiency. Effective, dynamic, and automated interference management tailored for bursty and local data traffic plays a central role in the task. Adjacent channel interference (ACI) management is an enabler for flexible spectrum use and uncoordinated network deployments. In this thesis the impact of ACI in local area time division duplex (TDD) cellular systems is demonstrated. A method is proposed where the transmitters optimize their transmitted spectral shape on-line, such that constraints on ACI induced by power amplifier non-linearity are met. The proposed method increases the fairness among spectrum sharing transceivers when ACI is a limiting factor. A novel interference-aware scheduling technique is proposed and analyzed. The technique manages co-channel interference (CCI) in a decentralized fashion, relying on beacon messages sent by data receivers. It is demonstrated that the proposed technique is an enabler for fair spectrum sharing among operators, independent adaptation of uplink/downlink switching points in TDD networks, and it provides overall more fair and spectrally efficient wireless access. Especially, the technique is able to improve the cell-edge throughput tremendously. New services are emerging that generate local traffic among the users in addition to the data traffic between the users and the network. Such device-to-device (D2D) traffic is effectively served by direct transmissions. The thesis demonstrates the possibilities for allowing such direct D2D transmissions on a shared band together with the cellular communication. It is shown that interference management is needed in order to facilitate reliable and efficient shared band operation. For this purpose, three methods are proposed that provide interference aware power control, interference aware multiuser and multiband resource allocation, and interference avoiding spatial precoding. It is shown that enabling direct transmission itself provides most of the gains in system capacity, while the interference management schemes are more important in promoting fairness and reliability.Langattomien tietoliikenneverkkojen käyttö kasvaa erittäin nopeasti mobiilien internet-palvelujen ja älykkäiden päätelaitteiden suosion myötä. Järjestelmien tiedonsiirtokapasiteettiä voidaan lisätä kasvattamalla kaistanleveyttä, spektritehokkuutta ja tukiasemaverkon tiheyttä. Kehityksen mahdollistaa mm. joustava taajuuksien käyttö ja moniantenni- ja monikantoaaltotekniikat. Lisäksi radioverkkojen suunnitteluun, optimointiin, ylläpitoon ja interferenssinhallintaan liittyvien tehtävien automatisoinnilla voidaan pienentää verkko-operaattoreiden kustannuksia. Tässä hetkellisen ja paikallisen tietoliikenteen tehokas, dynaaminen ja automatisoitu interferenssinhallinta on keskeisessä asemassa. Viereisen kanavan interferenssin hallinta mahdollistaa osaltaan joustavan spektrinkäytön ja koordinoimattoman verkkojen asennuksen. Väitöskirjassa on analysoitu viereisen kanavan interferenssin vaikutusta aikajakoiseen dupleksilähetykseen perustuvien paikallisten radioverkkojen toimintaan. Lisäksi väitöskirjassa on kehitetty menetelmä, jolla voidaan hallita interferenssiä reaaliaikaisesti. Menetelmä maksimoi lähetetyn signaalin spektritehokkuuden siten, että tehovahvistimen epälineaarisuuden aiheuttama viereisen kanavan interferenssi on rajoitettu. Väitöskirjassa on kehitetty ja analysoitu uudenlainen interferenssitietoinen lähetysten ajoitustekniikka. Tekniikka hallitsee reaaliaikaisesti ja hajautetusti saman kanavan interferenssiä vastaanottimien lähettämien majakkasignaalien avulla. Esitetyt simulaatiot osoittavat, että tämä mahdollistaa operaattoreiden välisen taajuuskaistojen jaon, ja alas- ja yloslinkkien aikajaon joustavan säädön. Tämän lisäksi on mahdollista saavuttaa korkeampi yleinen spektritehokkuus. Erityisesti tiedonsiirtonopeus solujen reunoille kasvaa esitetyn tekniikan avulla huomattavasti. Uudenlaiset tietoliikennepalvelut lisäävät laitteidenvälisen paikallisen tietoliikenteen määrää. Spektrinkäytön kannalta tämä liikenne on tehokkainta lähettää suoraan laitteesta toiseen. Väitöskirjassa on tutkittu joustavaa spektrinkäyttöä suorien laitteidenvälisten lähetysten ja soluverkon välillä. Interferenssin hallinta takaa luotettavan ja tehokkaan spektrin yhteiskäytön. Tätä varten väitöskirjassa on kehitetty kolme menetelmää, jotka perustuvat tehonsäätöön, lähetysten ajoitukseen ja moniantennilähetykseen