Multi-Antenna Towards Inband Shift Keying

TL;DR

The paper addresses the challenge of maintaining reliable links in mmWave/terahertz bands where beamforming overhead and alignment can hinder rapid connectivity. It proposes MA-TISK, a multi-carrier, multi-antenna scheme that uses repetition-coded GMSK with differential precoding to confine the spectrum while preserving a constant envelope, enabling coherent demodulation via windowed FFT without beam alignment. The method optimizes subcarrier spectra through per-subcarrier phase vectors, uses flexible pulse shaping, and demonstrates favorable spectral confinement and a high SIR (around $34.4$ dB) for a 160 Mbit/s aggregate rate across 64 subcarriers. This approach offers a practical, hardware-friendly path to robust, high-bandwidth links in dense mmWave/THz deployments, especially for IoT use cases, with performance improving as the number of antennas increases; the analysis also connects to fundamental capacity considerations, noting that at low SNR the rate approximates $R obreak o obreak rac{ ext{SNR}}{ obreak ext{ln} 2}$ and that MA-TISK can leverage this regime through repetition coding.

Abstract

We propose a new continuous phase frequency shift keying that is particularly suited for multi-antenna communications when the link budget is critical and beam alignment is problematic. It combines the constant envelope of frequency modulation with low-rate repetition coding in order to compensate for the absence of transmit beamforming. Although it is a frequency modulation, its transmit signal shows close to rectangular spectral shape. Similar to GSM's Gaussian minimum shift keying, it can be well approximated by linear modulation, when combined with differential precoding. This allows for easy coherent demodulation by means of a windowed fast Fourier transform.

Figures (6)

• Figure 1: Short-term spectrum during the frequency-correction burst of GSM frank:90. In the figure, ${\rm f}_{\rm T}$ denotes the carrier frequency.
• Figure 2: Spectrum of GSM in bold red line. Spectrum for quaternary GMSK in bold blue line. For comparison the asymmetric spectra with repetition codes of rate $\frac{1}{32}$ are shown in thin lines repeating phase shifts of $\frac{\pi}{2}$ and $\frac{3\pi}{4}$, respectively.
• Figure 3: Spectra of quaternary GMSK with repetition coding of rate $\frac{1}{32}$ and phase constellation $\{\pm \frac{\pi}{2}, \pm \frac{3\pi}{2}\}$. In the leftmost and rightmost curve, the phase shifts $-\frac{3\pi}{2}$ and $\frac{3\pi}{2}$ are repeated, respectively. Same for the inner curves and phase shifts of $\pm\frac{\pi}{2}$.
• Figure 4: Instantaneous frequencies of quaternary GMSK with repetition coding of rate $\frac{1}{32}$ and random QPSK data. In the lower and upper curve, the phase shifts $-\frac{3\pi}{2}$ and $\frac{3\pi}{2}$ are repeated, respectively. Same for the inner curves and phase shifts of $\pm\frac{\pi}{2}$.
• Figure 5: Instantaneous frequencies of 64 subcarriers with Gaussian filtered rectangular pulses.