Showing papers by "Matti Latva-aho published in 2011"

Resource Allocation for Cross-Layer Utility Maximization in Wireless Networks

Abstract: The cross-layer utility maximization problem, which is subject to stability constraints for a multicommodity wireless network where all links share the same number of orthogonal channels, is considered in this paper. We assume a time-slotted network, where the channel gains randomly change from one slot to another. The optimal cross-layer network control policy can be decomposed into the folloing three subproblems: 1) flow control; 2) next-hop routing and in -node scheduling; and 3) power and rate control, which is also known as resource allocation (RA). These subproblems span the layers from the physical layer to the transport layer. In every time slot, a network controller decides the amount of each commodity data admitted to the network layer, schedules different commodities over the network's links, and controls the power and rate allocated to every link in every channel. To fully exploit the available multichannel diversity, we consider the general case, where multiple links can be activated in the same channel during the same time slot, and the interference is controlled solely through power and rate control. Unfortunately, the RA subproblem is not yet amendable to a convex formulation, and in fact, it is NP-hard. The main contribution of this paper is to develop efficient RA algorithms for multicommodity multichannel wireless networks by applying complementary geometric programming and homotopy methods to analyze the quantitative impact of gains that can be achieved at the network layer in terms of end-to-end rates and network congestion by incorporating different RA algorithms. Although the global optimality of the solution cannot be guaranteed, the numerical results show that the proposed algorithms perform close to the (exponentially complex) optimal solution methods. Moreover, they efficiently exploit the available multichannel diversity, which provides significant gains at the network layer in terms of end-to-end rates and network congestion. In addition, the assessment of the improvement in performance due to the use of multiuser detectors at the receivers is provided.

Weighted Sum-Rate Maximization for MISO Downlink Cellular Networks via Branch and Bound

Abstract: The problem of weighted sum-rate maximization (WSRMax) in multicell downlink multi-input single-output (MISO) systems is considered. The problem is known to be NP-hard. We propose a solution method, based on branch and bound technique, which solves globally the nonconvex WSRMax problem with an optimality certificate. Specifically, the algorithm computes a sequence of asymptotically tight upper and lower bounds and it terminates when the difference between the upper and lower bound is smaller than a pre-specified tolerance. Novel bounding techniques via conic optimization are introduced and their efficiency is demonstrated via numerical simulations. The proposed method can be used to provide performance benchmarks by back-substituting it into many existing network design problems which relies on solving WSRMax problem. The method proposed here is not restricted to WSRMax; it can also be used to maximize any system performance metric that can be expressed as a Lipschitz continuous and increasing function of signal-to-interference-plus-noise ratio.

Coalition formation games for femtocell interference management: A recursive core approach

Abstract: Overlaying low-power, low-cost, femtocells, over existing wireless networks has recently emerged as a means to significantly improve the coverage and performance of next-generation wireless networks. While most existing literature focuses on spectrum sharing and interference management among non-cooperative femtocells, in this paper, we propose a novel cooperative model that enables the femtocells to improve their performance by sharing spectral resources, minimizing the number of collisions, and maximizing the spatial reuse. We model the femtocell spectrum sharing problem as a coalitional game in partition form and we propose a distributed algorithm for coalition formation. Using the proposed algorithm, the femtocells can take autonomous decisions to cooperate and self-organize into a network partition composed of disjoint femtocell coalitions and that constitutes a stable partition which lies in the recursive core of the considered game. Whenever a coalition forms, the femtocells inside this coalition can cooperatively pool the occupied spectral resources. Additionally, the members of any given coalition jointly schedule their transmissions in order to avoid collisions, in a distributed way. Simulation results show that the proposed coalition formation algorithm yields a performance advantage, in terms of the average payoff (rate) per femtocell reaching up to 380% relative to the non-cooperative case.

Decentralized Coordinated Downlink Beamforming via Primal Decomposition

Abstract: This letter considers the design problem of coordinated downlink minimum power beamforming for a multiuser multi-cell network, where each multi-antenna base station (BS) serves multiple single antenna users. We propose a decentralized primal decomposition based algorithm where limited amount of information is exchanged between coupled BSs at each iteration. Algorithm converges to the globally optimal solution for a static scenario. Unlike most of the previous decentralized methods, a feasible set of beamformers is guaranteed at each iteration even when the exchanged backhaul information is outdated. Consequently, the proposed approach naturally lends itself to realistic time-correlated fading scenarios.

An Efficient Channel Block Diagonalization Method for Generalized Zero Forcing Assisted MIMO Broadcasting Systems

Abstract: Generalized Zero Forcing (GZF) is an extension of the zero forcing beamforming method to deal with downlink multiuser MIMO communications when users have more than one receive antenna. In literature, Singular Value Decomposition (SVD) based channel Block Diagonalization (BD) is proposed to eliminate multiuser interference for GZF, but it is computationally inefficient. In this letter, we propose to generate a GZF precoder by using the product of a channel pseudo inverse matrix and a block diagonal matrix with an appropriate form, whose optimality for solving weighted sum rate maximization and weighted sum power minimization problems is proven. Later by using the proposed precoder a novel BD method with significantly lower complexity is presented. The complexity analysis shows that our proposal is computationally more efficient than SVD-BD.

Multicell MISO Downlink Weighted Sum-Rate Maximization: A Distributed Approach

Abstract: We develop an easy to implement distributed method for weighted sum-rate maximization (WSRMax) problem in a multicell multiple antenna downlink system. Unlike the recently proposed minimum weighted mean-squared error based algorithms, where at each iteration all mobile terminals needs to estimate the covariance matrices of their received signals, compute and feedback over the air certain parameters to the base stations (BS), our algorithm operates without any user terminal assistance. It requires only BS to BS signalling via reliable backhaul links (e.g., fiber, microwave links) and all required computation is performed at the BSs. The algorithm is based on primal decomposition and subgradient methods, where the original nonconvex problem is split into a master problem and a number of subproblems (one for each BS). A novel sequential convex approximation strategy is proposed to address the nonconvex master problem. In the case of subproblems, we adopt an existing iterative approach based on second-order cone programming and geometric programming. The subproblems are coordinated to find a (possibly suboptimal) solution to the master problem. Subproblems can be solved by BSs in a fully asynchronous manner, though the coordination between subproblems should be synchronous. Numerical results are provided to see the behavior of the algorithm under different degrees of BS coordination. They show that the proposed algorithm yields a good tradeoff between the implementation-level simplicity and the performance.

Interference management in self-organized femtocell networks: The BeFEMTO approach

Abstract: Recently, femtocell technology has gained significant interest in the wireless communication community, as a potential solution for satisfying the rapid increase in demand for wireless access, improving the poor indoor coverage, and offloading the macrocell network. Operational deployment of uncoordinated femtocells sets out new technological challenges, among which are cross-tier interference between the macro and femtocells, and co-tier interference among femtocells, in the same spectrum band. The Broadband Evolved Femto networks (BeFEMTO) project addresses these technical challenges aiming at developing advanced femtocell technologies based on LTE-A, enabling a cost-efficient provisioning of ubiquitous broadband services. The BeFEMTO project also focuses on novel concepts such as self-organizing femtocell networks, in which the goal is to develop and evaluate solutions to limit the interference impact to end-users'. In this paper, we first give an overview of the BeFEMTO project followed by preliminary results based on recent development of distributed algorithms in context aware learning mechanisms. Next, the performance assessment of self-organizing radio resource management algorithms and interference mitigating techniques for macro-femtocell coexistence is given.

Overlay/Underlay Spectrum Sharing for Multi-Operator Environment in Cognitive Radio Networks

Abstract: A new system model is introduced for spectrum sharing in a multi-operator environment. The infrastructure based cognitive radio network operates in underlay/overlay (hybrid) transmission mode sharing the spectrum of licensed users. Primary user networks (PUNs) are benefitted by the relaying capability of the cognitive radio base station so that their service degraded users due to low coverage, can enhance the performance. The cognitive radio network (CRN) also helps itself by transmitting its own data along with the primary data on the same band, while agreeing to maintain the signal to interference plus noise ratios of primary users' at a predetermined level. CRN operates in an underlay mode with an imposed interference threshold. In addition, maximizing the secondary user capacity by strategically changing the available frequencies from time slot to time slot is discussed with the introduction of two primary data relaying schemes (asynchronous / synchronous). The objective of the secondary user capacity maximization is reformulated into a linear programming problem with interference cancellation techniques. Power allocation for all the active users and the capacity of the secondary user is analyzed in both of the schemes

Optimal Power Allocation for PNC Relay Based Communications in Cognitive Radio

Abstract: In this paper, we consider an optimal power allocation scheme for a physical layer network coding relay based secondary user (SU) communication in cognitive radio networks. SUs are located on two different primary user (PU) coverage areas and we introduce an energy and spectrally efficient SU communication scheme. The sum rate of relay based two way communications for SUs is maximized under a total power constraint and interference power threshold (IPT) constraints to PUs. The optimal power allocation schemes are illustrated for different cases of SUs and relay powers. Numerical results confirm that the power allocation scheme provides the optimal solution for the achievable sum rate and the IPT constraints have an effect on the sum rate variation with the total available power.

Maximising transmission capacity of ad hoc networks via transmission system design

Abstract: The trade-off involving capacity and interference in wireless networks, using the metric transmission capacity, is investigated. Particularly, it is shown that this metric appropriately captures the effects of this trade-off, and can be used to select the combination of modulation scheme/coding scheme that maximises the transmission efficiency of a wireless ad hoc network modelled as a homogeneous Poisson point process.

Space-Frequency Scheduling in TDD Based LTE-Advanced MIMO-OFDMA Systems

Abstract: Space-frequency resource allocation and multiuser beamforming are crucial techniques to obtain high spectral efficiency requirements of IMT-A technologies. In this paper, we consider system level evaluation results of downlink TDD based LTE-A MIMO-OFDMA networks in ITU-R specific IMT-A evaluation environments. Efficient multiuser MIMO beamforming with and without intra-site coordinated multipoint (CoMP) and proportional fair space-frequency scheduling algorithms are particularly investigated. The system level results indicate that MU-MIMO and CoMP are needed in urban micro (UMi) environment to achieve the ITU-R specific spectral efficiency requirements. In ITU specific indoor scenario, MU-MIMO system is able to provide very high spectral efficiency because intercell interference is not limiting performance so strongly. The considered MU-MIMO CoMP seems to work also in rural macro environment with high mobility users and outdated CSI.

Interference Management in Femtocell Networks Using Distributed Opportunistic Cooperation

Abstract: Femtocells are envisioned to be deployed in indoor environments in order to improve both radio coverage and spectrum efficiency. This paper focuses on the self-organization of indoor femtocells, which includes mechanisms of cooperation. In this context, we propose a solution to automatically form cooperating groups among severely interfered femtocells in order to avoid interference. A hybrid access policy is proposed and compared to the closed and open policies. Results show that, as stated in the Braess Paradox, pervasive cooperation may be detrimental, when available resources are highly contended. Conversely, in particular cases, a marginal selfish behavior of femtocells can be preferable.

Effects of Antenna Correlation in MIMO Two Hop AF Relay Network over Rayleigh-Rician Channels

Abstract: In this letter, we investigate the performance of multiple input multiple output (MIMO) amplify-and-forward (AF) relay network over Rayleigh-Rician asymmetric channel with antenna correlation. We consider multiple correlated antennas at all nodes and general correlation matrices are assumed with arbitrary eigenvalue distribution. Specifically we derive the outage probability and the average symbol error rate (SER) for end-to-end signal-to-noise ratio (SNR). Moreover, simple asymptotic results are derived in high SNR in order to obtain the diversity gain variation. Finally, Monte Carlo simulations verify the accuracy of the analytical results.

Energy-efficient scheduling and power control for multicast data

Abstract: We consider the problem of multicasting from a single source to a set of destinations. We assume that the source can either reach the destinations directly or forward its traffic through a set of relays. Due to the non-linear attenuation of the signal with distance, employing the relays can help to improve the signal quality at the destinations. Meanwhile, relays also consume energy for retransmission of the received information and the rate of communication can be decreased due to the multi-hop transmission. Under the performance objective of maximizing the common amount of information (number of bits) that the source sends to all destinations per Joule of the total energy spent, we wish to identify whether direct transmission from the source to the destinations is preferable as opposed to multi-hop forwarding through the relays. In the latter case, we also identify a) which subset of the relays should be activated, b) for how long, and c) the respective destinations that each relay has to serve. Finally, we provide a set of numerical results to support our analysis.

Coordination Mechanisms for Stand-Alone Femtocells in Self-Organizing Deployments

Abstract: We investigate coordination mechanisms for controlling the co-channel interference generated by stand-alone femtocells in two-tier coexistence scenarios. Stochastic geometry is used to model network deployment scenarios, while the cumulants concept is utilized to characterize the probability distribution of the aggregate interference at a tagged user. The rationale for using coordination mechanisms is to opportunistically reuse resources without compromising ongoing transmissions on overlay macrocells, while still guaranteeing Quality of Service in both tiers. Results have shown that the analytical framework matches fairly well with numerical results obtained with Monte Carlo simulations. Yet coordination mechanisms improve performance of overlay macrocell network by substantially diminishing co-channel interference.

Performance of OSTBC MIMO amplify-and-forward two-hop relay system with asymmetric channels

Abstract: We present performance analysis of multiple input multiple output (MIMO) amplify-and-forward (AF) two-hop relay system having asymmetric independent channels of Nakagami-m fading and Rician fading. By considering channel state information (CSI)-assisted relaying, we derive outage probability, average symbol error rate (SER) and ergodic capacity. Further we provide high SNR results to understand the depth of system performance and diversity gain. Finally, we present Monte Carlo simulations to verify our analytical results.

Dual Hop MIMO OSTBC Communication over Rayleigh-Rician Channel

Abstract: This paper investigates orthogonal space-time block coded transmission for a multiple-input multiple-output (MIMO) channel with non-coherent amplify-and-forward relaying in a situation where the source-relay and relay-destination channels undergo Rayleigh and Rician fading respectively. Fading coefficients of the Rician channel are independent but not necessarily identically distributed. We derive exact expressions for the moment generating function, first and second moments of the instantaneous signal-to-noise ratio at the destination to statistically characterize the system behavior. We then analyze the system performance by deriving new analytical expressions for the bit error rate and amount of fading. These performance metrics reveal that strong line of sight components between relay-destination link always limit the performance promised by MIMO scattering environment when both nodes have multiple antennas.

On a Distributed Cognitive MAC Protocol for IEEE 802.11s Wireless Mesh Networks

Abstract: A distributed frequency agile medium access control (MAC) extension to the IEEE 802.11s for the next generation wireless mesh networks is proposed. The introduced protocol enhancements are capable of concurrent deployment of existing frequency opportunities in order to coordinate simultaneous data transmissions. The root concept is mainly based on the deployment of well-known ISM frequency bands, where the legacy 802.11-based wireless equipments operate, as the common control channel in order to establish contemporaneous transmissions. We apply the aforementioned key concept to the IEEE 802.11s common channel framework to attain two important goals: To improve the channel utilization using the concept of cognitive radio, and to lower the access delay. Through extensive event-driven simulations, taking into account primary user appearance in non-ISM frequency bands, performance of the proposed MAC enhancement is evaluated showing its higher efficiency compared to the existing solutions, in addition to its better wireless medium management.

Decentralized coordinated downlink beamforming for cognitive radio networks

Abstract: In this paper, we consider a decentralized downlink beamformer design problem in a multiantenna cognitive radio network where multiple primary and secondary transmitters serve multiple single antenna primary and secondary users, respectively The design goal is to minimize the total transmitted power across the secondary transmitters subject to the constraints of the received SINR at each secondary user and the received interference power at each primary userWe propose two alternative decentralized approaches where iterative beamformer designs are based on primal and dual decomposition methods Beamformers are calculated locally relying on the limited backhaul message passing between secondary transmitters Since the original problem is convex, both decentralized approaches converge to the globally optimal solution for static users' channels Moreover, a feasible set of beamformers are guaranteed at each iteration The proposed beamformer designs apply directly to the case of hierarchical femtocell networks where multiple macro- and femtocells co-exist Convergence behavior of the algorithms is demonstrated through simulations In addition, sum power performance of the optimal decentralized beamforming is compared with zero-forcing beamforming in a quasi-static flat fading scenario

Effect of antenna correlation on the performance of MIMO multi-user dual hop relay network

Abstract: We analyze the performance of MIMO multi-user dual hop relay network in the presence of antenna correlation. Orthogonal space time block coding (OSTBC) is used as the transmission scheme. By assuming multiple correlated antennas in all nodes, we derive the exact closed form solution for outage probability, average symbol error rate (SER) and ergodic capacity. Moreover, the performance of the system in high SNR is considered to obtain simple asymptotic results and investigate the diversity gain. All the analytical expressions are verified with Monte Carlo simulations.

Performance Analysis of Two-Way Relay System with Antenna Correlation

Abstract: This paper investigates the performance of amplify-and-forward (AF) two-way relay system with antenna correlation. Overall outage probability (OOP), average symbol error rate (SER) and ergodic capacity are analyzed. By considering general correlation structure with arbitrary eigenvalue distribution, we derive a tight lower bound for OOP and use it to evaluate average SER and ergodic capacity. Moreover, the system is studied in high signal-to-noise ratio (SNR) to obtain the insightful behavior of the performance and diversity gain. Finally Monte Carlo simulations are conducted to verify the accuracy of the results.

Evaluating the Information Efficiency of Multi-Hop Networks with Carrier Sensing Capability

Abstract: In this contribution, we consider the performance of the CSMA MAC protocol in multi-hop ad hoc networks in terms of "aggregate multi-hop information efficiency". Our model consists of a wireless network where transmitter nodes are distributed according to a homogeneous 2-D Poisson point process, and packets are generated following a Poisson distribution. Each packet is forwarded to its destination a fixed distance away from the source through an arbitrary number of hops. Approximate analytical expressions are derived for the outage probability of CSMA in its various incarnations, considering different values for the sensing threshold (related to the backoff decision) and the required communication threshold (which determines a correct packet reception). The aggregate multi-hop information efficiency is evaluated as a function of the transmission density, the communication rate, the maximum number of permitted backoffs and retransmissions, and the number of hops. Our results indicate the existence of optimal operating points for achieving maximal efficiency, and a basis is thus established for the optimization of the system parameters in order to improve the performance of multi-hop ad hoc networks.

Performance Evaluation of Adaptive MIMO-OFDM Systems with Limited Feedback Using Measurement Based Channel Models

Abstract: Performance evaluation of enhanced link adaptation method for MIMO-OFDM systems with limited feedback using measurement based channels and a stochastic channel model is presented in this paper. In particular, impact of practical channel estimation and feedback errors on link performance is analyzed. An adaptive spatial and modulation scheme selection process is based on the effective signal-to-interference-plus-noise ratio (ESINR). Mutual information effective SINR mapping method is applied for calculating ESINR values due to its capability of accurate estimation of the error rate performance of large variety of MIMO channels. Numerical results show that simple adaptive systems that switch between diversity/multiplexing or beamforming/multiplexing schemes obtain relatively good performance in realistic 2 × 2 MIMO channels. It is also shown that the imperfect channel estimation and feedback errors can have significant impact on the link performance. Furthermore, it is noticed that using the stochastic channel model in performance simulations can give rather pessimistic results compared to true measurement data.

Analysis of multi-band UWB distributed beaconing over fading channels

Abstract: In the past ten years, ultra wideband (UWB) technology has attracted great attention from academia and industry to enable a large number of applications. Typically, UWB was candidate to deliver home connectivity, sensor networks and body area networks. As UWB enabled devices operate in unlicensed mode, standardization efforts have been carried out in different parts of the globe to regulate their use in different contexts. ECMA-368 is an industry led standard for high data rate UWB communications. One of the key managing features of this standard is the so called distributed beaconing. In this paper, we analyze the distributed beaconing mechanism of ECMA-368. We focus on the transient phase in which newcomer devices attempt to join a network by accessing the standard defined beacon period. The joining procedure with which devices access the network is modeled by means of an embedded absorbing Markov chain. The relevant issue of modeling the miss-detection of beacon frames transmitted over fading channels is also assessed. Performance evaluation of distributed beaconing is given in terms of energy consumption. The required overhead for network setup is also analyzed.

Bandwidth Resource Competition in Cooperative Cognitive Wireless Communications

Abstract: In this paper, we analyze the behavior of Cooperative Cognitive Wireless nodes for the bandwidth resource competition using the ecological models of mutualism. We transform our cooperative cognitive wireless communication (CCWC) system as mutually interacting species. The two types of species are primary and secondary users. The Primary users have under-utilized licensed bandwidth to be shared with the secondary users. On the other hand, secondary users are willing to behave as possible relays for cognitive primary users, in exchange of shared licensed bandwidth. So, instead of pricing model, the proposed system is analyzed with the traditional barter system (exchange of commodities). We analyzed our proposed system based on three ecological models. Our first model is based on the coalitional form of Lotka–Volterra equations, where predator-prey role of mobile nodes are replaced with cooperation as in symbiosis. The second model deals with the population dynamism of strategically interacting partner for conflicting bandwidth resource. The third model adds the dynamism for the varying amount of bandwidth to be shared among the individual mobile users. For each type, a mathematical model is derived and then simulated for the equilibrium position analysis. The obtained results show that sharing of bandwidth resource is essential for the existence of CCWC systems.

Weighted sum-rate maximization in singlecast and multicast wireless networks - Global optimum via branch and bound

Abstract: We consider the problem of weighted sum-rate maximization (WSRMax) in wireless networks. This problem is known to be NP-hard and it plays a central role in resource allocation, link scheduling or in finding achievable rate regions for both singlecast and multicast networks. We propose a solution method, based on the branch and bound technique, which solves globally the WSRMax problem with an optimality certificate. Efficient bounding techniques are introduced as well.

Optimal Relay Selection for Energy-Efficient Multicast

Abstract: We consider multicast transmission from a single source to multiple destinations We assume that the source cannot reach the destinations directly, but must forward its traffic through a set of assisting relay nodes The performance objective under consideration is to maximize the common amount of information (number of bits) that the source delivers to all destinations per joule of the total energy spent Our aim is to obtain a policy that identifies: (a) which subset of the relays should be activated, (b) for how long, and (c) the respective destinations that each relay has to serve We consider centralized policies with exact knowledge of the channel conditions In the special case of networks employing at most two relays, we show that for any fixed assignment of destinations to relays the problem of maximizing the number of bits per joule by choosing the duration that each relay should be activated can be formulated as a convex optimization problem Unfortunately, the problem of assigning destinations to relays is combinatorially complex Thus, in the sequel we present a method with reduced complexity that exploits the knowledge of the underlying channel conditions to perform this assignment Finally, we provide a set of numerical results to illustrate the optimal relay selection and assignment of destinations to relays corresponding to different channel conditions

Decentralized multi-cell beamforming coordination for multiuser MISO systems

Abstract: In this paper, we consider a coordinated downlink beamforming problem for a multi-cell network where each multiantenna base station (BS) serves multiple single antenna users The optimization objective is to minimize the total transmitted power across coordinated BSs while guaranteeing user specific SINR targets We propose a decentralized beamformer design algorithm which is based on primal decomposition method and uplink-downlink duality Original optimization problem is decomposed via primal decomposition into two levels, ie, BS specific subproblems managed by a network-level master problem Master problem is solved at each BS using a subgradient method, and it requires a limited amount of backhaul information exchange between BSs The main contribution of the paper is an uplink-downlink duality based beamformer design for solving each BS specific subproblem Due to properties of uplink-downlink duality, convex optimization tools are not required in the proposed approach Since the original problem is convex, global convergence of the algorithm is guaranteed and optimal beamformers are achieved across BSs Moreover, feasible beamformers, which satisfy the SINR targets, are guaranteed at each iteration during the convergence process Convergence behavior of the proposed approach is studied through simulations

An Energy-Efficiency Comparison of RLNC and ARQ in the Presence of FEC

Abstract: We consider the problem of multicasting from a single source to multiple destinations over an erasure channel model. We are interested in energy-efficient communication. Our performance metric is the number of bits that the source delivers successfully to all destinations per joule of the overall energy spent. We compare the performance of Random Linear Network Coding (RLNC) and Automatic Repeat reQuest (ARQ) with respect to the above performance metric in the presence and absence of Forward Error Correction (FEC). Our numerical results illustrate that RLNC is more energy-efficient than ARQ when the links are highly unreliable. However, as the reliability protection provided to the links through FEC increases the energy-efficiency performance of RLNC deteriorates and it becomes suboptimal compared to ARQ methods.