This paper proposes the establishment of a simultaneous cognitive radio communication based on a subdistribution of power made over unselected subchannels which were discarded by the primary user through an initial optimal power allotment. The aim of this work is to show the possibility of introducing an opportunistic communication into a licensed transmission where the total power constraint is shared. The analysis of the proposed transmission scheme was performed by considering 128 and 2048 independent subchannels affected by Rayleigh fading, over 10,000 channel realizations, and three different signal-to-noise ratios (8？ , 16？ , and 24？ ). From the system evaluation it was possible to find the optimal power allotment for the primary user, the subdistribution of power for the secondary user, as well as the attenuation and the capacity per subchannel for every channel realization. Moreover, the PDF and CDF of the total obtained capacities, as well as the generation of empirical capacity regions, were estimated as complementary results. 1. Introduction The radio signals propagating through the environment are associated to a specific operation frequency belonging to one of the many wireless communications systems (i.e., LTE, WiMAX, etc.) existing today, which are strictly allocated by government agencies (i.e., FCC) or international organizations (i.e., ITU) [1, 2]. However, due to the continuous growing and development of the wireless industry, the current static frequency allocation has led to a problem related with spectrum scarcity [3, 4]. Nevertheless, recent worldwide measurement studies have revealed that most of the license spectrum experiences low utilization efficiency [5–7], which means that there exists the possibility of exploiting the underutilized spectrum in an opportunistic manner. According to this, an emerging technology that is able to reliable sense the spectral environment over a wide band, detect the presence/absence of licensed users (primary users), and use the spectrum only if the communication does not interfere with primary users is defined by the term cognitive radio (CR) [8, 9]. So, the spectrum utilization can be improved by making a secondary user access into the spectrum holes or spectrum portions that in a particular location and time are not being used by a primary user. In this regard, according to the current proposals of the CR protocol, the device is constantly aware of its wireless environment in order to determine (at least in space and time) which part of the spectrum is not being occupied by making use of
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