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In this article, the dynamic spectrum accessing techniques-the overlay, underlay, and hybrid spectrum-sharing approaches-are exploited for the military communication. Furthermore, the importance of the energy efficiency of cognitive radio (CR) networks (CRNs) for military applications is discussed. We also emphasize security threats over the physical layer and their mitigation in CRNs. Finally, we explore why the cross-layer design of protocols is preferred for greater energy efficiency and security.

In this paper, we have explored an optimal power allocation scheme for the spectrum sharing with imperfect channel state information between the cognitive/secondary user (CU) and licensed/primary user (PU) in the Rayleigh fading environment. We have analyzed the ergodic capacity of CU link under the combination of peak transmit power and peak/average interference power constraints with or without the primary user interference. In addition to this, the outage capacity with multiple primary users’ interference is also analyzed with the error variance under the joint peak transmit power and peak interference power constraint as well as individual peak interference power constraint. Moreover, the power disbursement is also investigated to achieve the lower limit of ergodic and outage capacity. The minimum mean square channel estimation technique is used for the channel estimation between CU and PU. Further, the convex optimization method is used for the optimal power allocation.

In this paper, performance metric of orthogonal frequency division multiplexing (OFDM) system is analyzed over a composite fading channel, i.e., generalized K-fading channel. Here, OFDM system is considered which includes repetition code to enhance the wireless link performance with simplicity. Symbol error rate (SER) performance is evaluated using binary phase shift keying (BPSK) and 16-quadrature amplitude modulation (16-QAM) over generalized K-fading channel. This channel model considers Nakagami-m distribution to define multipath and gamma distribution to represent shadowing effects. Simulation results demonstrate that improved system performance can be achieved by using repetition code in severely faded environment. A comparative study of coded and uncoded system is also given in this paper. Consequently, SER performance is improved with the increase of shape parameters.

In this paper, we have explored a potential spectrum sharing technique based on the power allocation in fading environment under transmit power constraint at secondary user/cognitive user (SU) and the interference power constraint at the primary user (PU). In addition to this, we have also analyzed the outage capacity limits over the fading channel, when the partial channel state information is provided to SU by the PU in the form of mean and variance. However, the minimum mean square error (MMSE) channel estimation technique is used to estimate the channel gain of SU and PU link. The outage capacity limit is analyzed under joint transmit power and interference outage power constraints. Moreover, the effect of error variance and outage constraint over the fading channel is also analyzed with constant peak transmit power of the SU.