Low Earth Orbit (LEO) satellites are increasingly being used to provide connectivity for wide area Internet of Things (IoT) sensing applications. Since distributed IoT terminals are not able to coordinate their transmissions with each other to avoid packet collisions, Quality-of-Service (QoS) depends on the transmission attempt success statistics, which are time-varying. This paper presents asymptotic analysis results that characterize the IoT terminal's transmission attempt and success processes. We show that they converge to inhomogeneous Poisson processes, in the large population regime, and characterize the timedependent intensities as functions of the terminal locations and the attempt rate scheme. We also propose three terminal attempt rate schemes that are solutions to max-min optimization problems. The performance of the proposed schemes are compared in terms of individual terminal QoS as well as the populationwide QoS distribution. Performance for various IoT applications is also presented, using our asymptotic Poisson limit results.

A document by Swaroop Gopalam . Click on the document to view its contents.

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.

In this paper, we present a new general delay-Doppler (DD) signalling framework that uses periodic discrete Weyl-Heisenberg (PD-WH) systems for data modulation and analog twisted convolution filters for pulse shaping. We show that the input-output (I/O) relationship of a modulation scheme under this framework, depends only on discrete DD channel taps and the ambiguity function of the PD-WH prototype pulse. We show that the I/O relationship of a scheme is predictable if the prototype ambiguity function has a bed-of-nails structure that is suited to the spreads of the discrete DD channel, so that DD aliasing is prevented. We propose DD modulation using a linear chirp prototype pulse and show that it has a predictable I/O relationship. It also has a slightly better symbol-error-rate performance compared to Zak-OTFS, in numerical simulation. We also develop a new composite pilot response technique for underspread DD channel measurement (for schemes with non-predictable I/O relationships) and also for overspread DD channel measurement (for schemes with predictable I/O relationships).

A document by Swaroop Gopalam . Click on the document to view its contents.

A document by Swaroop Gopalam . Click on the document to view its contents.