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Posts
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portfolio
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publications
Distributed User Association and Resource Allocation Algorithms for Three Tier HetNets
Published in IEEE Transactions on Wireless Communications, 2020
In this article, we consider joint optimization of user association and resource allocation in three tier HetNets. We formulate the objective of minimizing the resources required to clear a given set of files, as a linear program. We show that the optimal user association is determined by a rate-biasing rule, where a bias value is associated with each BS. We show that each rate-bias value crucially only takes values from a finite set which we characterize. We present a complete analytical solution along with new structural results.
Recommended citation: S. Gopalam, S. V. Hanly and P. Whiting, "Distributed User Association and Resource Allocation Algorithms for Three Tier HetNets," in IEEE Transactions on Wireless Communications, vol. 19, no. 12, pp. 7913-7926, Dec. 2020. https://ieeexplore.ieee.org/abstract/document/9205657
Distributed and Local Scheduling Algorithms for mmWave Integrated Access and Backhaul
Published in IEEE/ACM Transactions on Networking, 2022
We consider the stability region of a mmWave integrated access and backhaul (IAB) network with stochastic arrivals and time-varying link rates. In the scheduling of links, we consider a limit on the number of RF chains, and the half-duplex constraint which occurs due to the wireless backhaul links. We characterize the stability region, and propose a back-pressure policy for the IAB network under the RF chains and half-duplex constraints.
Recommended citation: S. Gopalam, S. V. Hanly and P. Whiting, "Distributed and Local Scheduling Algorithms for mmWave Integrated Access and Backhaul," in IEEE/ACM Transactions on Networking, vol. 30, no. 4, pp. 1749-1764, Aug. 2022. https://ieeexplore.ieee.org/abstract/document/9737293
Closed Form Approximations for UAV Line-of-Sight Probability in Urban Environments
Published in IEEE Access, 2023
This paper presents a new approach to estimate the probability of line-of-sight (LoS) for unmanned aerial vehicle (UAV) communications.
Recommended citation: I. Mohammed, S. Gopalam, I. B. Collings and S. V. Hanly, "Closed Form Approximations for UAV Line-of-Sight Probability in Urban Environments," in IEEE Access, vol. 11, pp. 40162-40174, 2023.' https://ieeexplore.ieee.org/abstract/document/10103694
Distributed Resource Allocation and Flow Control Algorithms for mmWave IAB Networks
Published in IEEE/ACM Transactions on Networking, 2023
This paper presents a new distributed slot reservation frame-work for joint resource allocation and flow control in mmWave IAB networks. We derive the Dynamic Slot Reservation (DSR) algorithm from a novel approach to solve a minimum clearing time linear program in a completely distributed manner.
Recommended citation: S. Gopalam, S. V. Hanly and P. Whiting, "Distributed Resource Allocation and Flow Control Algorithms for mmWave IAB Networks," in IEEE/ACM Transactions on Networking, vol. 31, no. 6, pp. 3175-3190, Dec. 2023. https://ieeexplore.ieee.org/abstract/document/10137766
Zak-OTFS Implementation via Time and Frequency Windowing
Published in IEEE Transactions on Communications, 2024
This paper presents an efficient practical Zak-OTFS modulation implementation using time and frequency windowing methods. We present two general classes of delay-Doppler (DD) twisted convolution (TC) filters (Type-1 and Type-2), and show that they can be realized by time and frequency windowing functions. We then propose practical methods to generate time domain Zak-OTFS signals, for actual transmission, using the windowing functions. For Type-1, the signals are generated using an interpolation filter. For Type-2, they are generated using a form of precoded OFDM. We show that this allows a wide variety of pulse shapes to be implemented in practice for Zak-OTFS modulation. This was not previously possible. We also show that the Type-2 signals are more spectrally efficient than their Type-1 counterparts. Finally, we compare the channel predictability of the two implementations.
Recommended citation: S. Gopalam, I. B. Collings, S. V. Hanly, H. Inaltekin, S. R. B. Pillai and P. Whiting, "Zak-OTFS Implementation via Time and Frequency Windowing," in IEEE Transactions on Communications, doi: 10.1109/TCOMM.2024.3366403. https://ieeexplore.ieee.org/document/10436682
A New Micro-Subcarrier OFDM Based Waveform for Delay Doppler Domain Communication
Published in IEEE Access, 2024
This paper presents a new OFDM based modulation scheme for communication in doubly dispersive channels. We call this Delay-Doppler OFDM (DD-OFDM). Our waveform has the same sparse-channel benefits as orthogonal-time-frequency-space (OTFS) modulation, while offering advantages in terms of simpler channel estimation, lower symbol error rate and lower out-of-band (OOB) emissions. We propose the DD-OFDM modulation scheme by introducing precoding across frames of frequency subcarriers. We show that the resulting waveform has different data carrying basis functions compared to OTFS modulation. We present the DD-OFDM receiver and derive the base-band model equations. We show that the base-band model for DD-OFDM leads to a simple and accurate channel estimation algorithm in non-integer fractional Doppler channels.
Recommended citation: S. Gopalam, S. B. Pillai, P. Whiting, H. Inaltekin, I. B. Collings and S. V. Hanly, "A New Micro-Subcarrier OFDM-Based Waveform for Delay Doppler Domain Communication," in IEEE Access, vol. 12, pp. 57879-57894, 2024, doi: 10.1109/ACCESS.2024.3390682. https://ieeexplore.ieee.org/abstract/document/10504818
Joint Beam Allocation and Scheduling for mmWave Cellular Networks
Published in IEEE Transactions on Information Theory, 2024
This paper provides capacity results for multi-user mm-wave hybrid-beamforming, and presents optimal joint beam allocation and user scheduling algorithms. We characterize the downlink capacity of a practical system with quantized analog beamforming code-books under the constraint that users cannot be scheduled at the same time if they are closer together than a beam width in angle. We show that the capacity region is determined by a small number of linear inequality constraints. We also present capacity-achieving scheduling algorithms that provide beam allocations guaranteeing that user rate requirements are met within each resource block. In particular, we propose “sand-filling” algorithms that are provably optimal and which have linear complexity.
Recommended citation: S. Gopalam, I. B. Collings, S. V. Hanly and H. Inaltekin, "Joint Beam Allocation and Scheduling for mmWave Cellular Networks," in IEEE Transactions on Information Theory, doi: 10.1109/TIT.2024.3399760. https://ieeexplore.ieee.org/abstract/document/10529272
Optimal Zak-OTFS Receiver and Its Relation to the Radar Matched Filter
Published in IEEE Open Journal of the Communications Society, 2024
This paper presents optimal receiver implementations for Zak-OTFS modulation. Zak-OTFS has a receiver structure that includes a twisted convolution filter followed by delay-Doppler domain sampling. We first show that this receiver is equivalent to a correlation demodulator where the receive pulses are determined by the choice of the receive delay-Doppler domain twisted convolution (TC) filter. We formulate the notion of a matched TC filter as the receive TC filter that maximizes the SNR in an additive white Gaussian noise (AWGN) channel, for a given transmit TC filter. We show that the matched TC filter formulation is crucial for understanding noise processes in the delay-Doppler domain. More generally, for a doubly dispersive channel, we define a receive TC filter that is matched to the twisted convolution of the channel with the transmit TC filter. We show that this receive TC filter, sampled at the delay-Doppler grid points is the optimal Zak-OTFS receiver that recovers sufficient statistics for maximum likelihood detection of the data symbols. We first present an implementation of this optimal receiver for a general sparse doubly dispersive channel which requires radar matched filter processing and involves computing ambiguity functions. We then present a second implementation that uses a receive TC filter that is matched to the transmit TC filter (not the channel) and only requires time and frequency windowing. We show that these two approaches converge when the window supports are large relative to the fundamental periods of the delay-Doppler grid. We also show that the second approach has an interpretation as a rake receiver operating in the delay-Doppler domain.
Recommended citation: S. Gopalam, H. Inaltekin, I. B. Collings and S. V. Hanly, "Optimal Zak-OTFS Receiver and Its Relation to the Radar Matched Filter," in IEEE Open Journal of the Communications Society, doi: 10.1109/OJCOMS.2024.3416467. https://ieeexplore.ieee.org/document/10564129
talks
Greedy Scheme for Optimal Resource Allocation in HetNets with Wireless Backhaul
Published:
Abstract: We formulate a linear programming problem to find the minimum clearing time in HetNets. Although this program is NP hard in general, we consider particular topologies that arise in HetNets, including a two cell HetNet and a linear chain of HetNets, both with wireless backhaul, and we provide an efficient, greedy algorithm that provably solves the minimum clearing time problem for these networks. We show how this algorithm can be applied to jointly optimize the ABS time across multiple macros in a HetNet, and we demonstrate capacity gains of the algorithm, compared to standard approaches to Inter Cell Interference Coordination. This paper provides insight into how to manage interference in presence of more than one macro, and how to efficiently operate wireless backhaul in HetNets.
Scheduling in Millimeter-wave Integrated Access and Backhaul Networks
Published:
This is a talk I had given during my research visit to India, that was funded by the AIRS fellowship. Prof. Sibiraj Pillai was my host for this trip.
Scheduling in Millimeter-wave Integrated Access and Backhaul Networks
Published:
This is a talk I had given during my research visit to India, that was funded by the AIRS fellowship. Prof. Sibiraj Pillai was my host for this trip.
Slot reservation framework for joint flow control and resource allocation in mmWave IAB networks
Published:
Abstract: The challenge of cross-layer control in wireless networks is to optimally control flow rates in conjunction with scheduling links. This is an NP Hard problem in general due to the hardness of the scheduling component. It was recently shown that maximum weighted schedule can be computed efficiently in mmWave integrated access and backhaul (IAB) networks due to the tree structure. Hence in our recent work, we have considered the problem of joint resource allocation and flow control for mmWave IAB networks. Our new approach to scheduling is based around solving a minimum clearing time linear program. We develop the Static Slot Reservation (SSR) algorithm which can be described as a book ahead slot reservation system that is completely distributed and solves the minimum clearing time linear program. We develop our approach further in the Dynamic Slot Reservation (DSR) algorithm, by jointly controlling flow rates and slot reservation in a dynamic IAB network with stochastic UE arrivals and departures. The proposed DSR framework leads to uniform end-to-end delays for flows and can also be implemented to provide end-to-end QoS guarantees. In this talk, we provide an overview of our new distributed slot reservation framework.
This talk is based on joint work with Prof. Philip Whiting and Prof. Stephen Hanly.
Zak-OTFS Implementation using Time and Frequency Windowing.
Published:
Abstract: Zak-OTFS is a new delay-Doppler modulation technique which uses Zak domain basis functions for signaling, and requires a new filtering technique for pulse shaping called delay-Doppler domain twisted convolution (TC) filtering. In this talk, we show how to implement these filters in the time domain and generate the corresponding time domain waveforms. We introduce two general classes of TC filters (Type-1 and Type-2), and show that they can be realized by time and frequency windowing. We then propose practical methods to generate time domain signals for these TC filters. For Type-1, the signals are generated using an interpolation filter. For Type-2, they are generated using a form of precoded OFDM. We show that the Type-2 signals are more spectrally efficient than their Type-1 counterparts.
Short Message Success Rate for LEO Satellite IoT Data Harvesting
Published:
Abstract: This paper analyses the data message success rate for Internet of Things (IoT) sensing devices communicating over Low Earth Orbit (LEO) satellite links. We present an analytical framework for optimizing multi-objective multi-packet reception on the uplink. We present an analytical result for the probability of message success for a given ground terminal, and present an analytical result for the overall probability of message success, averaged across all terminals.
Minimizing Clearing Time in mmWave Networks with Overlapping Coverage
Published:
Abstract: This paper considers millimeter-wave (mmWave) networks with hybrid beamforming communications, where base stations have a limited number of radio frequency (RF) chains. The base stations have overlapping coverage to overcome blockage issues in both downlink and uplink transmission. We propose a user association (UA) scheme that minimizes the time required for clearing data traffic of users in the coverage area. We formulate the UA problem as a time allocation problem, allocating time to user-base station links. We provide an innovative two-stage approach to solve this problem. Stage one optimizes a time fraction allocation for user-base station links. Then these time fractions are distributed across the RF chains at each base station using a fully distributed algorithm. Stage two then schedules the user-base station links, provably solving the UA minimum clearing time problem. We then characterize the achievability of any set of target user rates. Numerical results show that our proposed UA scheme achieves significantly reduced clearing times in comparison to baseline schemes.
teaching
Teaching experience 1
Undergraduate course, University 1, Department, 2014
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Teaching experience 2
Workshop, University 1, Department, 2015
This is a description of a teaching experience. You can use markdown like any other post.