Research Interests

My research interests are in advanced optimization, control, and machine learning techniques applied to radio resource management in radio access networks. I have developed highly decentralized algorithms for radio resource managements in wireless data networks and have an extensive experience in designing algorithms and features for PHY and MAC layers, MIMO systems, scheduling, power control, and routing. This page gives a selective overview of my work by organizing papers into broad research areas. For a more complete list of publications, please visit the publications page.

Stardization of LTE-A

  • Optimal user scheduling and rate selection in REMA broacast-channel transmission.
    Soldati, A. P. Perotti, B. M. Popovic.
    Submitted to IEEE Global Communications Conference 2016.
    Abstract. Superposed transmission on the broadcast channel delivers multiple private messages simultaneously to the intended users. Superposed transmission is employed in non-orthogonal multiple access to increase the aggregate throughput and connectivity of wireless cellular systems. Rate-adaptive constellation Expansion Multiple Access (REMA, a.k.a. RA-CEMA) is a nonorthogonal multiple access scheme which exhibits performance similar to the conventional superposed transmission schemes for transmission on the degraded broadcast channel, while requiring less complex signal processing and scheduling algorithms. This paper solves the problem of user scheduling and rate allocation for REMA. We pose the problem as a weighted sumrate optimization and develop provably optimal algorithms for user selection and rate allocation with REMA broadcast-channel transmission. System-level evaluations show that our schemes outperform an LTE-A compliant orthogonal multiple access system and exhibit similar or better performance compared to other known non-orthogonal multiple access schemes.

Decomposition Methods and Cross-layer Design

  • Distributed cross-layer coordination of congestion control and resource allocation in S-TDMA wireless networks.
    P. Soldati, B. Johansson, M. Johansson.
    Wireless Networks, Vol. 14, No. 6, pp. 949-965, Dec. 2008.
    Abstract. We consider the problem of joint congestion control and resource allocation in spatial-TDMA wireless networks. The design problem is posed as a utility maximization problem subject to link rate constraints which involve both transmission scheduling and power allocation. Starting from the performance limitations of a centralized optimization based on global network information, we proceed systematically in our development of two distributed and transparent protocols that rely on local information only. In the process, we introduce a novel decomposition method for convex optimization, establish its convergence for the utility maximization problem, and demonstrate how it suggests a distributed solution based on TCP/AQM and incremental updates of the transmission schedule. We develop a two-step procedure for finding the schedule updates and suggest two schemes for distributed link scheduling and power control under realistic interference models. Although the final protocols are suboptimal, we isolate and quantify the performance losses incurred by each simplification and demonstrate strong performance in examples.
  • Mathematical Decomposition Techniques for Distributed Cross-Layer Optimization of Data Networks.
    B. Johansson, P. Soldati, M. Johansson.
    IEEE Journal on Selected Areas in Communications, Vol. 24, No. 8, pp. 1535-1547, August 2006.
    Abstract. Network performance can be increased if the traditionally separated network layers are jointly optimized. Recently, network utility maximization has emerged as a powerful framework for studying such cross-layer issues. In this paper, we review and explain three distinct techniques that can be used to engineer utility-maximizing protocols: primal, dual, and cross decomposition. The techniques suggest layered, but loosely coupled, network architectures and protocols where different resource allocation updates should be run at different time-scales. The decomposition methods are applied to the design of fully distributed protocols for two wireless network technologies: networks with orthogonal channels and network-wide resource constraints, as well as wireless networks where the physical layer uses spatial-reuse time-division multiple access. Numerical examples are included to demonstrate the power of the approach.
  • Cross-layer resource allocation model for cellular-relaying network performance evaluation.
    B. Timus, P. Soldati, D. Kim, J. Zander.
    IEEE Transactions on Vehicular Technology, Vol. 60, No. 6, pp. 2765-2776, May 2011.
    Abstract. The enhancement of cellular networks with relaying technologies is expected to bring significant technoeconomic benefits at the expense of more complex resource allocation. Suitable models for solving network dimensioning problems in cellular-relaying networks must handle radio resource allocation among hundreds of links and tackle interactions between networking layers. For this purpose, we propose a novel cross-layer resource allocation model based on average interference and ideal rate adaptation for the physical layer (PHY), time shares for the medium access layer, and fluid flows for the transport and network layers. We formulate a centralized social welfare maximization problem. When the routes are selected with an a priori algorithm, we show that the resource allocation problem admits an equivalent convex formulation. We show a numerical example for how to use the proposed framework for configuring the backhaul link in a practical relaying network. The overall problem of selecting routes and allocating time shares and link rates is nonconvex. We propose an iterative suboptimal algorithm to solve the problem based on a novel approximation of PHY. We state and prove several convergence properties of the algorithm and show that it typically outperforms routing based on signal-to-noise ratio only.

Latency-constrained scheduling

  • Modular design of jointly optimal controllers and forwarding policies for wireless control.
    B. Demirel, Z. Zou, P. Soldati, M. Johansson.
    IEEE Transactions on Automatic Control Vol. 59, No. 12, pp. 3252-3265, Dec. 2014.
    Abstract. We consider the joint design of packet forwarding policies and controllers for wireless control loops where sensor measurements are sent to the controller over an unreliable and energy-constrained multi-hop wireless network. For fixed sampling rate of the sensor, the co-design problem separates into two well-defined and independent subproblems: transmission scheduling for maximizing the deadlineconstrained reliability and optimal control under packet loss. We develop optimal and implementable solutions for these subproblems and show that the optimally co-designed system can be efficiently found. Numerical examples highlight the many trade-offs involved and demonstrate the power of our approach.
  • Performance bounds and latency-optimal scheduling for convergecast in WirelessHART networks.
    H. Zhang, P Soldati, M. Johansson.
    IEEE Transactions on Wireless Communications, Vol. 12, No. 6, pp. 2688-2696, June 2013.
    Abstract. Convergecast, in which data from a set of source devices is delivered to a single data sink, is a critical functionality in networks deployed for industrial monitoring and control. We address the latency-optimal link scheduling problem for convergecast in networks operating according to the recent WirelessHART standard. When there is no restriction on the number of channels, we present a latency-optimal scheduling policy in which each routing node is required to buffer at most one packet at any point in time. For networks with a limited number of channels, we first establish a lower bound on the number of channels for latency-optimal convergecast and a lower bound on latency for convergecast using a fixed number of channels, and then present a heuristic scheme for channel-constrained latency-optimal convergecast scheduling. Simulation results confirm that, at much modest computational cost, our heuristic scheme can construct convergecast schedules with latency close to that of the optimal schedules.
  • Energy-efficient deadline-constrained maximum reliability forwarding in lossy networks.
    Z. Zou, P. Soldati, H. Zhang, M. Johansson.
    IEEE Transactions on Wireless Communications, Vol. 11, No. 10, pp. 3474-3483, Oct. 2012.
    Abstract. This paper studies the problem of optimal forwarding for reliable and energy-efficient real-time communication over multi-hop wireless lossy networks. We impose a strict per-packet latency bound and develop forwarding policies that maximize the probability that the packet is delivered within the specified deadline minus a transmission energy cost. A solution to this problem allows to characterize the set of achievable latency-reliability pairs and to trace out the Pareto frontier between achievable deadline-constrained reliability and transmission energy cost. We develop dynamic programming-based solutions under a finite-state Markov channel model. Particular instances with Bernoulli and Gilbert-Elliot loss models that admit numerically efficient solutions are discussed and our results are demonstrated on several examples.
  • Optimal routing and scheduling of deadline-constrained traffic over lossy networks.
    P. Soldati, H. Zhang, Z. Zou, M. Johansson.
    In Proc. of the IEEE Global Communications Conference, pp. 1-6, Dec. 2010.
    Abstract. The traditionally wired automation infrastructure is quickly migrating to more flexible and scalable wireless solutions. To cope with the stringent requirements of process automation in terms of latency and reliability, the network resources must be optimized to ensure timely and reliable communication. This paper considers the joint routing and transmission scheduling problem for reliable real-time communication over lossy networks. Specifically, we impose a strict latency bound for packet delivery from source to destination, and devise optimal transmission scheduling policies that maximize the success probability of delivering the packet within the specified deadline. A solution to this problem allows to characterize the set of achievable latencies and packet reliability for a given network. We offer a complete understanding of the problem when erasure events on links are independent and follow a Bernoulli process. We consider both static and dynamic resource allocation policies, and compare them in numerical examples.

Traffic steeting

  • Multiple connectivity and spectrum access utilisation in heterogeneous small cell networks.
    G. P. Koudouridis, P. Soldati, G. Karlsson.
    Springer International Journal of Wireless Information Networks, Vol. 23, No. 1, March 2016.
    Abstract. In the context of heterogeneous and small cell networks, users will have the possibility to connect to multiple radio access (RA) carriers that will be available by a dense deployment of RA infrastructure consisting of high-power and low-power access nodes. Determining which RAs a user should be associated with and select from, for its downlink transmissions, depends on the long-term and short-term data rates that these RAs may offer to the user. In this study the multi-RA association and utilisation is decomposed into a multi-RA to user association problem that assigns multiple RAs to users, and a multi-RA selection problem that determines which of the assigned RAs should be used at any time for the user transmissions. As a solution to the first problem, we propose a distributed dual-based spectrum access scheme (DSA) that considers multi-connectivity, whilst, the second problem is solved by means of a heuristic multi-RA selection scheme that utilise different multi-radio transmit diversity (MRTD) schemes while taking into account different inter-cell interference coordination (ICIC) schemes. Our two-step approach is evaluated by means of simulations which demonstrate cell-edge user throughput performance improvements that exceed 100 % when the multi-connectivity DSA is employed. Further significant user rate and energy efficiency improvements up to 69 and 38 % respectively can be achieved when MRTD is combined with ICIC.
  • Distributed spectrum access in dense 5G networks.
    P. Soldati, G. P. Koudouridis.
    In Proc. of IEEE Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC), Sept. 2015.
    Abstract. The next generation of radio access networks is expected to offer users flexible radio access to a wide range of frequency bands and radio access technologies (RATs) in dense deployments. In this context, we investigate novel lightweight algorithms for spectrum access in ultra-dense networks. We pose the problem as a network utility maximization and apply Lagrange duality to devise distributed algorithms for users to jointly select access nodes and spectrum bands. Leveraging our theoretical framework, we further propose a heuristic scheme for spectrum access based on the expected long-term user data rate per RAT, hence on a measure of the RATs' traffic load. Numerical examples show that our schemes achieve significant throughput gains on top of the gains achievable by network densification.

Power Control and MIMO systems

  • Near optimum power control and precoding under fairness constraints in network MIMO systems.
    G. Fodor, M. Johansson, P. Soldati.
    Int. J. Digital Multimedia Broadcasting, pp. 1-17, 2010.
    Abstract. We consider the problem of setting the uplink signal-to-noise-and-interference (SINR) target and allocating transmit powers for mobile stations in multicell spatial multiplexing wireless systems. Our aim is twofold: to evaluate the potential of such mechanisms in network multiple input multiple output (MIMO) systems, and to develop scalable numerical schemes that allow real-time near-optimal resource allocation across multiple sites. We formulate two versions of the SINR target and power allocation problem: one for maximizing the sum rate subject to power constraints, and one for minimizing the total power needed to meet a sum-rate target. To evaluate the potential of our approach, we perform a semianalytical study in Mathematica using the augmented Lagrangian penalty function method. We find that the gain of the joint optimum SINR setting and power allocation may be significant depending on the degree of fairness that we impose. We develop a numerical technique, based on successive convexification, for real-time optimization of SINR targets and transmit powers. We benchmark our procedure against the globally optimal solution and demonstrate consistently strong performance in realistic network MIMO scenarios. Finally, we study the impact of near optimal precoding in a multicell MIMO environment and find that precoding helps to reduce the sum transmit power while meeting a capacity target.
  • On pilot dimensioning in multicell single input multiple output systems.
    P. Soldati, M. Johansson, G. Fodor, S. Sorrentino.
    In Proc. of the IEEE Global Communications Conference, pp. 1382-1387, Dec. 2011.
    Abstract. Resource management schemes in multicell orthogonal frequency division multiplexing (OFDM) networks typically assume perfect channel knowledge at the transmitter or employ a statistical channel estimation error. In this paper we propose a model that explicitly captures the inherent tradeoff between the power allocated to transmitting data and pilot signals in multicell single input multiple output (SIMO) systems. We first study the asymptotic behavior of the required data power to reach a predefined signal-to-noise ratio (SNR) target as a function of the employed pilot power in a single OFDM cell, then develop a distributed multicell algorithm that strives to minimize the multi-cell sum data power with respect to a predefined signal-to-interference-and-noise-ratio (SINR) target vector. The results provide new insights in dimensioning the pilot and data transmit power levels and serve as a basis for developing distributed power control schemes in multicell systems.
  • On the impact of uplink power control in network MIMO systems with MMSE and SIC receivers.
    G. Fodor, S. Sorrentino, M. Johansson, P. Soldati.
    In Proc. of the IEEE Int. Symposium on World of Wireless Mobile and Multimedia Networks (WoWMoM), pp. 1-9, June 2010.
    Abstract. Network multiple input, multiple output (MIMO) systems are built around a broadband backbone network that allows for the fast communication of channel state information (CSI) as well as user data between different base stations. Previous works have shown that multicell channel adaptive (opportunistic) power control can minimize the sum power or maximize the sum rate when the backbone is used for the exchange of CSI in network MIMO systems. In this work we investigate the gains of multicell opportunistic power control under per user fairness constraints when both CSI and user data are shared between multiple sites. We find that multicell opportunistic power control working in concert with uplink joint signal detection is an efficient means both for the capacity and the power control problems that not only minimizes sum power or maximizes overall capacity, but is also able to provide arbitrary level of fairness.

Wireless Sensor Networks

  • A Generalized Markov chain model for effective analysis of slotted IEEE 802.15.4
    P. G. Park, P. Di Marco, P. Soldati, C. Fischione, K. H. Johansson.
    In Proc. of the IEEE 6th Int. Conference on Mobile Adhoc and Sensor Systems (MASS), pp. 130-139, Oct. 2009. (Best paper award).
    Abstract. A generalized analysis of the IEEE 802.15.4 medium access control (MAC) protocol in terms of reliability, delay and energy consumption is presented. The IEEE 802.15.4 exponential backoff process is modeled through a Markov chain taking into account retry limits, acknowledgements, and unsaturated traffic. Simple and effective approximations of the reliability, delay and energy consumption under low traffic regime are proposed. It is demonstrated that the delay distribution of IEEE 802.15.4 depends mainly on MAC parameters and collision probability. In addition, the impact of MAC parameters on the performance metrics is analyzed. The analysis is more general and gives more accurate results than existing methods in the literature. Monte Carlo simulations confirm that the proposed approximations offer a satisfactory accuracy.
  • Opportunistic routing in low duty-cycle wireless sensor networks.
    E. Ghadimi, O. Landsiedel, P. Soldati, S. Duquennoy, M. Johansson.
    ACM/IEEE Transactions on Sensor Networks Vol. 10, No. 4, pp. 1-34, June 2014.
    Abstract. Opportunistic routing is widely known to have substantially better performance than unicast routing in wireless networks with lossy links. However, wireless sensor networks are usually duty cycled, that is, they frequently enter sleep states to ensure long network lifetime. This renders existing opportunistic routing schemes impractical, as they assume that nodes are always awake and can overhear other transmissions. In this article we introduce ORW, a practical opportunistic routing scheme for wireless sensor networks. ORW uses a novel opportunistic routing metric, EDC, that reflects the expected number of duty-cycled wakeups that are required to successfully deliver a packet from source to destination. We devise distributed algorithms that find the EDC-optimal forwarding and demonstrate using analytical performance models and simulations that EDC-based opportunistic routing results in significantly reduced delay and improved energy efficiency compared to traditional unicast routing. In addition, we evaluate the performance of ORW in both simulations and testbed-based experiments. Our results show that ORW reduces radio duty cycles on average by 50% (up to 90% on individual nodes) and delays by 30% to 90% when compared to the state-of-the-art.
  • Revisiting Multi-channel Communication to Mitigate Interference and Link Dynamics in Wireless Sensor Networks.
    A. Gonga, O. Landsiedel, P. Soldati, M. Johansson.
    In Proc. of the IEEE International Conference on Distributed Computing in Sensor Systems (DCOSS), pp. 186-193, May 2012.
    Abstract. Multichannel communication has been proposed as alternative to adaptive (single-channel) routing protocols for mitigating the impact of interference and link dynamics in wireless sensor networks. While several studies have advocated features of both techniques (not without running up against contradicting arguments) a comprehensive study that aligns these results is still lacking. This paper aims at filling this gap. We present an experimental test bed setup used to perform extensive measurements for both single-channel and multichannel communication. We first analyze single-channel and multichannel communication over a single-hop in terms of packet reception ratio, maximum burst loss, temporal correlation of losses, and loss correlations across channels. Results show that multichannel communication with channel hopping significantly reduces link burstiness and packet loss correlation. For multi-hop networks, multi-channel communication and adaptive routing show similar end-to-end reliability in dense topologies, while multichannel communication can outperform adaptive routing in sparse networks with bursty links.