@article{deshpande_castellanos_khosravirad_du_viswanathan_heath jr_2023, title={A Wideband Generalization of the Near-Field Region for Extremely Large Phased-Arrays}, volume={12}, ISSN={["2162-2345"]}, DOI={10.1109/LWC.2022.3233011}, abstractNote={The narrowband and far-field assumption in conventional wireless system design leads to a mismatch with the optimal beamforming required for wideband and near-field systems. This discrepancy is exacerbated for larger apertures and bandwidths. To characterize the behavior of near-field and wideband systems, we derive the beamforming gain expression achieved by a frequency-flat phased array designed for plane-wave propagation. To determine the far-field to near-field boundary for a wideband system, we propose a frequency-selective distance metric. The proposed far-field threshold increases for frequencies away from the center frequency. The analysis results in a fundamental upper bound on the product of the array aperture and the system bandwidth. We present numerical results to illustrate how the gain threshold affects the maximum usable bandwidth for the n260 and n261 5G NR bands.}, number={3}, journal={IEEE WIRELESS COMMUNICATIONS LETTERS}, author={Deshpande, Nitish and Castellanos, Miguel R. R. and Khosravirad, Saeed R. R. and Du, Jinfeng and Viswanathan, Harish and Heath Jr, Robert W. W.}, year={2023}, month={Mar}, pages={515–519} }
 @article{deshpande_castellanos_heath_2022, title={Nonuniform true time delay precoding in wideband MISO systems}, ISSN={["1058-6393"]}, DOI={10.1109/IEEECONF56349.2022.10051948}, abstractNote={We focus on true time delay (TTD) precoding which is a reduced hardware complexity alternative to fully-digital frequency selective beamforming in wide band systems. It is challenging to select the delays for TTD beamforming architectures for a channel with multi-path and near-field effects because of the nonuniform delay variation across the array. We design the TTD beamformer by maximizing the total energy in the filtered channel response. Based on the idea of temporal focusing, we reformulate the TTD constraint by including a sparsity regularizer penalty term in the objective. We then propose a procedure to compute the delays using the precoder response obtained after solving a sparse eigenvalue problem for maximizing the total energy. Incorporating sparsity in the problem formulation enables us to obtain a response that closely resembles the TTD precoder response in time domain. The proposed approach outperforms the time-reversal precoder based TTD design and the phase-shifter implementation in terms of the wideband mutual information.}, journal={2022 56TH ASILOMAR CONFERENCE ON SIGNALS, SYSTEMS, AND COMPUTERS}, author={Deshpande, Nitish Vikas and Castellanos, Miguel R. and Heath, Robert W., Jr.}, year={2022}, pages={1233–1237} }
 @article{deshpande_dey_amudala_budhiraja_2022, title={Spatially-Correlated IRS-Aided Multiuser FD mMIMO Systems: Analysis and Optimization}, volume={70}, ISSN={["1558-0857"]}, DOI={10.1109/TCOMM.2022.3161007}, abstractNote={We consider a two-way full-duplex (FD) system where a massive multi-input-multi-output FD base station (BS) communicates with multiple FD users via an intelligent reflecting surface (IRS). We derive a closed-form network spectral efficiency lower bound by considering spatial correlation at the BS and IRS. We demonstrate the importance of modelling spatial correlation by simplifying this lower bound for uncorrelated channels to show that the IRS capability to modify the wireless medium now is significantly impeded. This lower bound, which is a function of only the long-term channel statistics, is also used to maximize the non-concave global energy efficiency metric by optimally allocating the transmit powers, and by designing the IRS phases. We derive closed-form updates for optimizing the transmit powers using Lagrangian dual, Quadratic, and Dinkelbach’s transforms. We then optimize the IRS phases by using the projected gradient ascent algorithm. We numerically show that increasing the number of IRS elements can help a FD mMIMO BS outperform its half-duplex counterpart, which otherwise under-performs due to its limited ability to cancel various FD interferences.}, number={6}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Deshpande, Nitish Vikas and Dey, Sauradeep and Amudala, Dheeraj Naidu and Budhiraja, Rohit}, year={2022}, month={Jun}, pages={3879–3896} }
 @article{deshpande_dey_amudala_sharma_budhiraja_2021, title={Analysis of Statistical CSI-based Optimized Phase-Shift IRS-aided FD mMIMO System}, ISSN={["2576-6813"]}, DOI={10.1109/GLOBECOM46510.2021.9685692}, abstractNote={We consider a multi-user system where a massive multi-input-multi-output (mMIMO) full-duplex (FD) base-station (BS) communicates with multiple FD users via an intelligent reflecting surface (IRS). We derive novel uplink and downlink spectral efficiency (SE) lower bound expressions when the BS estimates composite BS-IRS-user channels. The lower bounds are derived considering spatially-correlated IRS and user channels, and require only statistical channel state information (CSI). We propose a projected gradient ascent based IRS phase optimization algorithm, which also uses only statistical CSI, and enables the system to achieve a higher SE in the presence of the loop and inter-user interferences. We analytically investigate the dependence of SE on the IRS phase for spatially correlated channels considered herein. We show that the proposed analysis can help in increasing the SE by appropriate IRS placement.}, journal={2021 IEEE GLOBAL COMMUNICATIONS CONFERENCE (GLOBECOM)}, author={Deshpande, Nitish and Dey, Sauradeep and Amudala, Dheeraj Naidu and Sharma, Ekant and Budhiraja, Rohit}, year={2021} }