In my research work, I came across the concept in the paper
Y. Hu, P. Wang, Z. Lin, M. Ding and Y. -C. Liang, "Machine Learning Based Signal Detection for Ambient Backscatter Communications," 2019 IEEE International Conference on Communications (ICC), Shanghai, China, 2019, pp. 1-6, doi: 10.1109/ICC.2019.8761796.,
eq. (5) and (6), comparing to reference [2] of that paper, G. Yang, Q. Zhang and Y. Liang, "Cooperative ambient backscatter communications for green internet-of-things", IEEE Internet of Things Journal, vol. 5, no. 2, pp. 1116-1130, April 2018.
"modulation alphabet set of QPSK signal" i.e., $s(n) \in \mathcal{A}_s$, where $n = 1,2,...,256$, $s(n)$ is QPSK signal and $\mathcal{A}_s$ is modulation alphabet set.
I am not getting what does this modulation alphabet set mean.
Any help in this regard will be highly appreciated.