Browsing by Author "Tirkkonen, Olav, Prof., Aalto University, Department of Communications and Networking, Finland"

Now showing 1 - 12 of 12

Churn-proof Wireless Caching with D2D Communication
(2018) Pääkkönen, Joonas
School of Electrical Engineering | Doctoral dissertation (article-based)
Popular data objects will be cached close to end-users in future wireless networks. Both distributed storage and inter-user communication in wireless systems alongside caching can be utilized to increase the performance of such networks compared to traditional server-client based approaches. This holds true especially when users are clustered in certain geographical areas forming local caching communities, and when user equipment can be used to store and distribute data, which decreases the backhaul load and increases overall energy-efficiency. Energy-efficiency is a key performance metric in modern wireless networks where enormous amounts of information must be transmitted between a large number of devices. The research of this thesis contributes to the study of wireless caching with inter-user communication especially in terms of energy-efficiency. The aim is to minimize the expected overall energy consumption of the cellular system. The joint use of Device-to-Device (D2D) communication and erasure coding for Distributed Storage Systems (DSS) is shown to protect cached data against mobile churn, which increases the energy-efficiency of the system. This is mainly due to the fact that erasure coding provides protection against data loss in caching communities, which in turn decreases the need of contacting remote base stations. Besides coded wireless D2D caching, the statistics of caching for maximal byte hit rates as well as inter-user communication with message forwarding have been studied in detail. The statistical approach to cache sizing takes both file popularities and file sizes into account when designing the cache so that the expected amount of traffic from the cache is maximized, thus alleviating the strain on the origin server. The multihop message forwarding technique decodes functions of transmitted messages so that the probability of a decoding failure vanishes. Numerical simulations are used to verify the theoretical calculations. Both theoretical calculations and simulation results indicate that the use of erasure coding has the potential to drastically decrease the energy consumption when appropriate coding methods are used.
Coding on Flag Manifolds for Limited Feedback MIMO Systems
(2013) Pitaval, Renaud-Alexandre
School of Electrical Engineering | Doctoral dissertation (article-based)
The efficiency of the physical layer in modern communication systems using multi-input multi-output (MIMO) techniques is largely based on the availability of channel state information (CSI) at the transmitter. In many practical systems, CSI needs to be quantized at the receiver side before transmission through a limited rate feedback channel. This is typically done using a codebook-based precoding transmission, where the receiver transmits the index of a codeword from a pre-designed codebook shared with the transmitter. To construct such codes one has to discretize complex flag manifolds. For single-user MIMO with a maximum likelihood receiver, the spaces of interest are Grassmann manifolds. With a linear receiver and network MIMO, the codebook design is related to discretization of Stiefel manifolds and more general flag manifolds. In this thesis, coding in flag manifolds is studied. In a first part, flag manifolds are defined as metric spaces corresponding to subsurfaces of hyperspheres. The choice of distance defines the geometry of the space and impacts clustering and averaging (centroid computation) in vector quantization, as well as coding theoretical packing bounds and optimum constructions. For two transmitter antenna systems, the problem reduces to designing spherical codes. A simple isomorphism enables to analytically derive closed-form codebooks with inherent low-implementation complexity. For more antennas, the concept of orbits of symmetry groups is investigated. Optimum codebooks, having desirable implementation properties as described in industry standardization, can be obtained using orbits of specific groups. For large antenna systems and base station cooperation, a product codebook strategy is also considered. Such a design requires to jointly discretize the Grassmann and Stiefel manifolds. A vector quantization algorithm for joint Grassmann-Stiefel quantization is proposed. Finally, the pertinence of flag codebook design is illustrated for a MIMO system with linear receiver.
Control Plane Functions for 5G: Spectrum Sharing and Mobility Frameworks
(2019) Hailu, Sofonias Amdemariam
School of Electrical Engineering | Doctoral dissertation (article-based)
Unlike the previous generations, the design of the 5th Generation of cellular mobile communications (5G) is driven by a large number of diverse use cases, which are often classified into three types: enhanced Mobile Broadband (eMBB), Ultra Reliable and Low Latency Communication (URLLC) and massive Machine Type Communication (mMTC). These diverse use cases brought a new set of problems such as ultra-low latency, ultra-high reliability and extremely long battery life requirements, in addition to the traditional but exponentially growing throughput requirement. As such, the control plane design plays a key role in 5G to efficiently integrate and use all concepts that address diverse use cases and even directly addressing the requirements of some of the use cases. In this thesis, we studied control plane functions for inter-operator spectrum sharing and mobility. The spectrum sharing framework includes a coordination protocol for co-primary spectrum sharing. We have shown that this can be implemented using a non-cooperative game model that requires minimal information exchange and equal favour exchanged with instantaneous reciprocity among the operators. The game is shown to have Nash equilibrium, that can be reached by playing sequential games with myopic best response strategies. Two cooperative game models are also studied for co-primary spectrum sharing. These games can be used as an upper bound for performance analysis of the non-cooperative game. Simulation results show that the performance of the non-cooperative game outperforms default spectrum usage partitions for mutual renting and resource pooling and achieves close to optimal performance in some scenarios. The mobility framework discusses a state machine with novel RRC Connected Inactive state. In contrast to the spectrum sharing framework, the mobility framework mainly addresses User Equipments (UEs) who do not have high data activity. As such, the design is driven by the objective of minimizing UE power consumption, at the same time reducing the number of messages and latency, which has an impact on the network. The characteristics of RRC Connected Inactive and the procedures required in this state are designed to this end. Simulation results show that it has significantly better performance than LTE in terms of signalling overhead, UE power consumption and control plane latency.
Enabling Energy-Efficient and Backhaul-aware Next Generation Heterogeneous Networks
(2015) Prasad, Athul
School of Electrical Engineering | Doctoral dissertation (article-based)
Heterogeneous networks have been firmly established as the direction in which next-generation cellular networks are evolving. We consider the dense deployment of small cells to provide enhanced capacity, while the macro cells provide wide area coverage. With the development of dual connectivity technology, deploying small cells on dedicated carriers has become an attractive option, with enhanced flexibility for splitting traffic within the network. The power consumption and latency requirements of the backhaul link are also gaining increasing prominence due to these factors. Backhaul link quality itself is expected to play an important role in influencing the deployment costs of next-generation 5G systems. Energy efficiency as a network design paradigm is also gaining relevance due to the increasing impact cellular networks are having on the global carbon emission footprint. For operators, improving energy efficiency has the added advantage of reducing network operation expenditures. For the end-users, avoiding unnecessary draining of device battery power would improve the user experience. In this work, we study energy efficient mechanisms for inter-frequency small cell discovery, based on mobility awareness and proximity estimation. Further, we apply generalized small cell discovery concepts in a device-to-device communication environment in order to optimize the energy consumption for device discovery. We also look at energy efficient small cell operations based on traffic characteristics and load constraints-based offloading in relation to the radio access and backhaul power consumption. In addition we study intelligent means of dist-ributing delay-critical functionalities such as Hybrid ARQ, while centralizing the computationally-intense processes in a 5G, cloud-based, centralized radio access network. Numerical evaluations done using a LTE-Advanced heterogeneous network and analytical settings indicate that significant UE and network power consumption reductions could be achieved with the considered enhancements. Using the optimized small cell operation schemes investigated in this work, reductions in network power consumption and consequent improvements in the overall energy efficiency of the network were observed. The performance of the distributed opportunistic HARQ mechanism for a centralized radio access network is compared to the optimal and static retransmission mechanisms, and the evaluated scheme is shown to perform close to the optimal mechanism, while operating with a non-ideal backhaul link.
Functions and Abstractions for Radio Access Network Softwarization
(2017) Lembo, Sergio
School of Electrical Engineering | Doctoral dissertation (article-based)
It is expected that future mobile Radio Access Networks (RANs) should offer enough degrees of freedom to be adapted quickly and, at low cost, to operate in ways that may be not conceived today. The possibility to program the network, to offer new and tailored services, will encourage a myriad of technical and business innovations. With this goal, several paradigms and network architectures that comprise RAN softwarization are being proposed. The research presented in this thesis contributes to softwarization for heterogeneous RANs. Joint development of functions and abstractions is addressed. A simplified framework for RAN softwarization is presented, consistent with the paradigms of Software Defined Networks (SDN), and other architectures proposed in the literature. The framework consists of a Central Coordinator and Controller (CCC) for logical centralized network management, plug-in Network Functions (NFs), or applications, and a RAN with Real Time Controllers (RTCs). We discuss the development, implementation and performance of two kinds of NFs. First, NFs for Inter-Cell Interference Coordination (ICIC), optimizing inter-tier resource allocation at cell and network-level, with muting in one or two tiers. Second, a NF for dynamic spectrum management in Frequency-Division Duplexing (FDD) Flexible Duplexing cells. In addition, local functions and abstractions for the RTCs are discussed. As examples, we present two methods for optimizing switching points in the context of Adaptive Modulation and Coding (AMC). As a basis for this, we discuss functions to abstract link-level details. Results from simulations with ICIC-NFs in different Heterogeneous Networks (HetNets), show that the proposed functions and abstractions are suitable to improve the performance of the network. For FDD flexible duplexing cells, the approach presented as a NF helps to determine the trade-off between downlink transmit power of a flexible duplexing cell, and distance between this cell and a victim, belonging to another operator. The functions and abstractions discussed for the RTCs, complete the framework. From the results observed, we can conclude that the adoption of a framework as the proposed, based on RAN softwarization, with suitable functions and abstractions, provides a promising solution to make the RAN flexible enough to incorporate new and tailored services.
Millimeter-Wave Communication and Mobile Relaying in 5G Cellular Networks
(2018) Deng, Junquan
School of Electrical Engineering | Doctoral dissertation (article-based)
Future fifth generation (5G) mobile communication systems should fulfill a wide range of technical requirements to cope with the explosive growth of mobile data traffic, massive number of connected devices and emergence of new services. Ultra-fast transmission speeds and consistent user experience are fundamental 5G requirements. To achieve these 5G goals, a combination of different advanced radio transmission technologies, different spectrum bands and different networking methods is necessary. Network densification using millimeter-wave (mmWave) communications, combined with massive multiple-input multiple-output (MIMO) and beamforming techniques, provides a framework to achieve throughputs in the range of Gbps. However, for the operators, the capital and operational expenditures increase significantly as the base station (BS) density increases. In this regard, a cost-effective 5G radio access network (RAN) solution is of great interest. In this thesis, Device-to-Device (D2D) relaying and low-complexity mmWave system architectures are considered to fulfill this objective. The main goal of D2D relaying is to create a new type of network connectivity to carry mobile traffic, so that user experience consistency is improved without a need of ultra-dense BS deployment. The D2D relaying connectivity is extremely useful in mmWave bands as mmWave signals can be easily blocked by various obstacles. D2D relaying should be subject to network control by a RAN controller. We consider a hierarchical control framework to perform D2D discovery, relay selection, mmWave beam selection, resource allocation and interference management. With this framework, D2D relaying in four practical network scenarios is considered, and low-complexity algorithms are developed to manage D2D relaying. In addition to D2D relaying, we also consider developing low-cost mmWave BSs for ultra-dense mmWave network deployments. A low-complexity hybrid architecture with subarrays and low-resolution phase shifters is designed, and a switch network is added into the architecture for the channel estimation purpose. A novel grid-less compressive-sensing channel estimation method based on atomic norm minimization, and a two-stage multi-user MIMO precoding scheme are proposed with this hybrid architecture. Simulations with the 3GPP mmWave channel model show that the considered architecture solution can achieve comparable spectrum efficiency as the performance given by architectures with much higher hardware costs.
Network Interference Cancelation for 5G
(2020) Zhou, Liang
School of Electrical Engineering | Doctoral dissertation (article-based)
The future fifth generation (5G) mobile communication systems are currently being developed under expectations of fulfilling various technical requirements, which include massive connectivity, high capacity, low latency and ultra-reliability. In order to achieve high capacity, operating at high frequency spectrum such as millimeter wave is considered as an appealing option, which requires more dense allocation of Transmission Reception Points (TRP). With more micro and macro TRPs deployed with small inter-site distance, cell-edge users in 5G networks may encounter strong interference from neighbor transmissions in both uplink and downlink. Successive Interference Cancelation (SIC) receivers, which are applied in 5G with Non-Orthogonal Multiple Access (NOMA), have been presented as a potential solution for heavy interference scenarios, where the performance gain can be further improved by network Interference Cancelation (IC). Motivated by the demands for improving spectral efficiency in different interference environments, this thesis addresses Radio Resource Management (RRM) optimization with network IC in specific 5G uplink and downlink scenarios. In the uplink, this thesis investigates Device-to-Device (D2D) communications. D2D pairs of transmitters and receivers share the same cellular uplink resource. Situations with and without an uplink cellular user are considered. A centralized RRM optimization algorithm is proposed where the cellular base station maximizes the network utility by adjusting all D2D and cellular users' transmission power, rate and IC configurations. Additionally, distributed RRM algorithms based on strategic games are developed. Simulation results of the centralized and the distribute algorithms show considerable gains in spectral efficiency. In the downlink, a scenario with only cellular users is considered. Each cellular user may perform SIC on one of the two strongest nearby downlink transmissions. This opens up the possibility that a cell-edge user may be served by the second nearest cell, and cancel the interference from the closest cell. This is called cell-edge inversion in this thesis. The utility of the whole network is maximized by a radio resource optimization method that is distributed among the cells. The simulation results show significant improvement of the rate of cell-edge users.
Performance Analysis and Mitigation Techniques for I/Q-Corrupted OFDM Systems
(2015) Oruthota, Udesh
School of Electrical Engineering | Doctoral dissertation (article-based)
Orthogonal Frequency Division Multiplexing (OFDM) has become a widely adopted modulation technique in modern communications systems due to its multipath resilience and low implementation complexity. The direct conversion architecture is a popular candidate for low-cost, low-power, fully integrated transceiver designs. One of the inevitable problems associated with analog signal processing in direct conversion involves the mismatches in the gain and phases of In-phase (I) and Quadrature-phase (Q) branches. Ideally, the I and Q branches of the quadrature mixer will have perfectly matched gains and are orthogonal in phase. Due to imperfect implementation of the electronics, so called I/Q imbalance emerges and creates interference between subcarriers which are symmetrically apart from the central subcarrier. With practical imbalance levels, basic transceivers fail to maintain the sufficient image rejection, which in turn can cause interference with the desired transmission. Such an I/Q distortion degrades the systems performance if left uncompensated. Moreover, the coexistence of I/Q imbalance and other analog RF imperfections with digital baseband and higher layer functionalities such as multiantenna transmission and radio resource management, reduce the probability of successful transmission. Therefore, mitigation of I/Q imbalance is an essential substance in designing and implementing modern communications systems, while meeting required performance targets and quality of service. This thesis considers techniques to compensate and mitigate I/Q imbalance, when combined with channel estimation, multiantenna transmission, transmission power control, adaptive modulation and multiuser scheduling. The awareness of the quantitative relationship between transceiver parameters and system parameters is crucial in designing and dimensioning of modern communications systems. For this purpose, analytical models to evaluate the performance of an I/Q distorted system are considered.
Physical and Control Layer Solutions for Reliable Machine-Type Random Access
(2019) Boyd, Christopher Eric
School of Electrical Engineering | Doctoral dissertation (article-based)
Machine-type communications (MTC), in both the massive (mMTC) and ultra-reliable (uMTC) variety, will facilitate many new types of applications in 5G networks. Many of these applications will have service requirements that current wireless systems cannot meet. A particular challenge is providing reliable access to the large number of uncoordinated and infrequently active devices that characterise MTC. Restrictions on latencies preclude the traditional use of re-transmissions and lengthy resource scheduling procedures for reliability in the random access channel (RACH), encouraging instead a grant-free access paradigm. In such contention-based access, medium access control (MAC) layer coding schemes that provide repetition diversity are critical for limiting packet losses. This thesis considers Combinatorial Code Designs (CCD) for achieving the targets set for ultra-reliable low-latency communications (URLLC) in the 5G RACH. In contrast to coded access schemes which use random coding, we show that uniquely pre-allocating repetition patterns to users from a code designed specifically for use with successive interference cancellation (SIC) is a better solution for URLLC. We introduce interference cancelling (IC) codes that guarantee successful reception of all simultaneously accessing users up to a given number, which are shown to be particularly robust to packet loss when user activity is low, as in MTC. Simulations demonstrate that these codes outperform ALOHA-type schemes and meet the "five nines" reliability target of URLLC in both a simple collision model and a more realistic scenario that models both the MAC and physical (PHY) layers. Fully characterising the impact of the PHY layer uplink waveform on reliability is also paramount in designing an ultra-reliable random access procedure for 5G. As such, this thesis considers measures of time-frequency localisation (TFL) for stochastic signals. Such measures are complementary to those traditionally used in prototype filter design. We derive a generalised Heisenberg measure for multi-dimensional stochastic signals, consider the specific case of Gabor systems, and discern relevant bounds. The generalised TFL measure describes how well multiplexed waveform packets are contained inside their time-frequency resources, and thus their inter-user interference potential in a multiple-access scenario. We confirm the measure's veracity by simulating an asynchronous, grant-free random access system, and comparing the performance of small packets of common waveforms. At lower user activity, where out-of-band emissions are the limiting factor to reliability, the respective performances of these waveforms is consistent with the presented theory.
Resource Allocation for 5G Systems
(2021) Singh, Bikramjit
School of Electrical Engineering | Doctoral dissertation (article-based)
The 5th generation of cellular communications is driven by three broadly classified use cases, enhanced mobile broadband, massive machine type communication, and ultra-reliable low latency communication. These new use cases bring a new set of problems such as provisioning of extremely high user rates, ultra-high reliability, ultra-low latency, extremely low jitter, and extremely long battery life, to name a few. Therefore, in this thesis, we focus on lower protocol layers of radio access networks, designing technology components enabling communication confirming new extreme requirements introduced by 5th generation systems. To increase the amount of spectrum available for an operator in a specific geographic region, we discuss inter-operator spectrum sharing. The operators employ non-cooperative game-theoretic mechanisms to enable sharing without revealing their RAN-related information, e.g., optimization targets and traffic load to each other. The operators are independent and motivated by self-interest. However, if the sharing persists over a long time, non-cooperative operators may develop a degree of cooperation to maximize their self-interest by employing repeated games. Rational operators tend to cooperate due to fear of repercussions if they focus on optimizing long-term instead of short-term gains. Such cooperation emerges from individuals' greedy decisions. Simulation results show that the spectrum sharing based on repeated games provides better network utility than default spectrum allocations in the scenarios like limited spectrum pool or mutual renting. In contrast to spectrum allocation, which focuses on the rate improvement for most users in a mobile broadband scenario, the other half of the thesis pivots to providing extremely reliable service, which induces the need to concentrate on the extremely deficient users in a service area. To improve critical communication, we consider allocation designs that boost rate availability and reliability. The availability targets a specific rate that a cell can provide at probable locations. We inspect multi-hop mechanisms for extreme availability, focusing on time scheduling weights, power allocation, and precoding weights. Inter-cell interference can limit availability at the cell edge, and thus we consider coordination among cells in limiting the interference. Furthermore, we consider allocation designs for minimum rate improvement using transmission repetitions for low latency contention-based access services. Simulation results show that the employed techniques significantly improve 5G ultra-reliable, low latency services.
Secret Key Generation for Ambient Backscatter Communication
(2023) Lietzén, Jari
School of Electrical Engineering | Doctoral dissertation (article-based)
The interest in wireless Internet of Things (IoT) devices and Ambient Intelligence has increased significantly in recent years. The security of IoT devices has become a concern, as IoT has made it possible for things and people to interact with each other anytime and any place. Therefore, sufficient protection against active eavesdropping or confusing devices with other users' devices is an essential requirement. Due to the embedded nature of these devices, they are often limited in their computational, communication and power resources. Ambient backscatter communication (AmBC) is seen as a viable solution for resource limited devices, as the wireless nodes are communicating without any active RF components. However, the interference from the ambient transmitter remains a major challenge, as the ambient signal is present at the receiver together with the backscattered signal. This thesis contributes to secure IoT device communication in an AmBC setting. The contributions are a two-way secret key agreement protocol and a backscatter device design. We developed a novel secret key agreement protocol that uses an advantage distillation method to collect secret key from error corrected parity bits. Our protocol provides complementary performance compared to protocols known in the literature. We have analysed the performance of the key agreement protocol in two different operating scenarios, in a quantum key distribution setting and in a satellite setting. The second contribution is a backscatter device design that introduces polarization conversion between the direct and scattered path signals and exploits that at the dual polarization receiver antenna to substantially decrease the interference from the ambient transmitter. We showed that in an anechoic RF chamber, our proposed set-up could achieve more than 25 dB isolation between the backscattered component, and the ambient component for narrowband signals. In this thesis, we analyse secret key generation between ambient backscatter devices where the channel between an ambient transmitter and the backscatter devices is used as a source of randomness. We show that even in non-line-of-sight channels the distance from legitimate users to an eavesdropper being larger than a few wavelengths is not alone a sufficient security guarantee. This is in contrast with previous secret key generation methods where the distance is assumed to prevent the eavesdropper from having any information about the key prior to error correction. Our simulations show that a distance based approach is too optimistic, and there is a possibility that the eavesdropper still knows a substantial part of the final key. A working solution is based on a two-way key agreement protocol, and assuming that the eavesdropper's error rates are k times that of the legitimate users, with k < 1. This method gives the legitimate users the freedom to trade off between achievable key rate and the eavesdropper's knowledge of the final key.
Towards Robust Spectrum Sensing in Cognitive Radio Networks
(2013) Wei, Lu
School of Electrical Engineering | Doctoral dissertation (article-based)
This thesis focuses on multi-antenna assisted energy based spectrum sensing. The studies leading to this thesis have been motivated by some practical issues with energy based detection. These include the noise uncertainty problem at the secondary receiver, the presence of multiple active primary users in cognitive cellular networks, the existence of unknown noise correlations and detection in the low signal-to-noise ratio regime. In this thesis, the aim is to incorporating these practical concerns into the design of spectrum sensing algorithms. To this end, we propose the use of various detectors that are suitable for different scenarios. We consider detectors derived from decision-theoretical criteriors as well as heuristic detectors. We analyze the performance of the proposed detectors by deriving their false alarm probability, detection probability and receiver operating characteristic. The main contribution of this thesis consists of the derived closed-form performance metrics. These results are obtained by utilizing tools from multivariate analysis, moment based approximations, Mellin transforms, and random matrix theory. Numerical results show that the proposed detectors have indeed resolved the concerns raised by the above practical issues. Some detectors could meet the needs of one of the practical challenges, while others are shown to be robust when several practical issues are taken into account. The use of detectors constructed with decision-theoretical considerations over the heuristically proposed ones is justified as well.