Rate Adaptive Hierarchical Modulation-Assisted Two-User Opportunistic Scheduling

We propose and study a two-best user opportunistic scheduling scheme using fixed power discrete rate adaptive hierarchical constellations (known also as embedded, multi- resolution, or asymmetrical constellations). According to the classical opportunistic scheduling, the scheduler transmits information to the selected first best user in each transmission slot in order to maximize the spectral efficiency. Our newly proposed scheme transmits information not only to the first best user but also to the second best user if the channel qualities of the selected two users support. As such the frequency of channel access of the users is increased without any degradation of overall spectral efficiency compared to the classical single best user scheduling. The key idea is to rely on hierarchical constellations to transmit information to two best users simultaneously. Specifically, according to the channel quality of the first best user, the alphabet size of the square quadrature amplitude modulation (QAM) is selected in order to maximize the spectral efficiency. However, if the first best user's channel quality is good enough to decode the information transmitted in the second hierarchy of the selected size hierarchical QAM constellation while the second best user's channel quality is capable of decoding the information transmitted in the first hierarchy, information is transmitted to both the selected first and second best users using the hierarchical QAM constellation. Otherwise, information is transmitted only to the first best user using the selected size uniform QAM constellation. We present expressions for the information transmission probability and the overall average spectral efficiency [bits/sec/Hz] defined as the overall average transmitted data rate per unit bandwidth for a specified number of users, a specified average carrier-to-noise ratio (CNR), and bit error rate (BER). Numerical results for a fading environment where the channel variation among the users is identically and independently distributed (i.i.d.) are presented. These results show that, in such i.i.d. fading environment, our newly proposed fixed power discrete rate adaptive hierarchical modulation- assisted two-best user scheduling provides more frequent access to the information (almost double) without any average spectral efficiency degradation compared to the classical single best user opportunistic scheduling.