Constant Modulus Waveforms for IoT-Centric Integrated Sensing and Communications

TL;DR

This work addresses the challenge of enabling integrated sensing and communications (ISAC) for resource-constrained IoT deployments by advocating constant modulus, single-carrier waveforms with unit PAPR. It surveys standardisation activity, defines performance metrics, and analyzes multiple constant modulus signalling schemes—including fixed and variable frequency-pattern designs such as Costas codes, frequency permutations, and FSK/PSK modulations—using mean square bandwidth, the radar ambiguity function, and per-subpulse data rate and detector complexity. Through Monte Carlo evaluations, the paper delineates tradeoffs: phase-modulated Costas codes offer stable sensing with low data rates, frequency permutations improve data rate at the cost of higher receiver complexity, and FSK-based schemes yield the highest data rate with PSL controllable via phase optimization. The results inform IoT scenario mappings (e.g., BLE, LoRA, IoR) and provide guidance for energy-efficient ISAC design and potential standardisation directions in 6G-era IoT networks.

Abstract

Integrated sensing and communications (ISAC) is considered a key enabler to support application scenarios such as the Internet-of-Things (IoT) in which both communications and sensing play significant roles. Multi-carrier waveforms, such as orthogonal frequency division multiplexing (OFDM), have been considered as good candidates for ISAC due to their high communications data rate and good time bandwidth property for sensing. Nevertheless, their high peak-to-average-power-ratio (PAPR) values lead to either performance degradation or an increase in system complexity. This can make OFDM unsuitable for IoT applications with insufficient resources in terms of power, system complexity, hardware size or cost. This article provides IoT-centric constant modulus waveform designs that leverage the advantage of unit PAPR and thus are more suitable in resource-limited scenarios. More specifically, several single-carrier frequency and/or phase-modulated waveforms are considered. A comprehensive discussion on their radar sensing and communications performance is conducted based on performance metrics, including the radar ambiguity function, the bandwidth property, the data rate, and the communications receiver complexity.

Figures (5)

• Figure 1: ISAC waveforms with different PAPR values.
• Figure 2: Box plots of the normalised squared RMS bandwidth vs. data rate in bits/subpulse for the seven constant modulus ISAC signalling schemes. Note that we use perms. as the abbreviation of permutations in Fig. \ref{['fig:msbw_vs_dr']} - Fig. \ref{['fig:psl_vs_rx']}.
• Figure 3: Box plots of the normalised squared RMS bandwidth vs. the average per subpulse symbol detector complexity for the seven constant modulus ISAC signalling schemes.
• Figure 4: Box plots of the AF PSL vs. data rate in bits/subpulse for seven constant modulus ISAC signalling schemes.
• Figure 5: Box plots of the AF PSL vs. the average per subpulse symbol detector complexity for seven constant modulus ISAC signalling schemes.