Energy harvesting (EH) cognitive relays are an exciting solution to the problem of inefficient use of spectrum while achieving green communications and spatial diversity. In a spectrum sharing scenario, we investigate the performance of a cognitive relay network, where a secondary source communicates with its destination over Nakagami-m channels via decode-and-forward EH relays while maintaining the outage probability of the primary user below a predefined threshold. Specifically, we derive a closed-form expression for the secondary outage probability and show that it is a function of the probability of an EH relay having sufficient energy for relaying, which in turn, depends on the energy harvesting and consumption rates of the EH relay and the primary outage probability threshold. We also show that relaxing the primary outage constraint may not always benefit the cognitive EH relay network due to the limitations imposed on the relay's transmit power by the energy constraint. * The authors are with the

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Energy harvesting (EH) cognitive relays are an exciting solution to the problem of inefficient use of spectrum while achieving green communications and spatial diversity. In a spectrum sharing scenario, we investigate the performance of a cognitive relay network, where a secondary source communicates with its destination over Nakagami-m channels via decode-and-forward EH relays while maintaining the outage probability of the primary user below a predefined threshold. Specifically, we derive a closed-form expression for the secondary outage probability and show that it is a function of the probability of an EH relay having sufficient energy for relaying, which in turn, depends on the energy harvesting and consumption rates of the EH relay and the primary outage probability threshold. We also show that relaxing the primary outage constraint may not always benefit the cognitive EH relay network due to the limitations imposed on the relay's transmit power by the energy constraint. I. INTRODUCTIONWith the ever-increasing demand for wireless services along with a need for green communications, spectral efficiency and energy efficiency have become important criteria in the design of future wireless systems. Energy harvesting (EH) cognitive radio [1]-[5] is a promising solution to improve the spectrum utilization; in particular, spectral efficiency is improved by spectrum sharing, while achieving self-sustaining green communications. In cognitive radio, a secondary user (SU) may share the spectrum with a primary user (PU) provided that the interference from it to PU remains below a given threshold [6].The use of cooperative relays in cognitive radio has gained significant attention as they have the potential to improve the coverage and reliability of SU's transmission while sharing the spectrum with PU [6]- [16]. However, the relays may have limited battery reserves, and recharging or replacing the battery frequently may be inconvenient. This invokes the need for an external power source to keep relays active in the network. The EH relays can overcome such energy shortage while exploiting the spatial diversity [17]- [21]. As to the EH relays in cognitive radio, [22], [23] consider an EH secondary relay which helps relaying the secondary data, and perform the secondary outage analysis for Rayleigh fading channels under the interference constraint at the primary receiver; while in [24], cooperative communication via multiple EH relays is considered.In this paper, we consider the case where SU uses the best relay from multiple EH relays for its own transmission over Nakagamim channels, given that PU's outage probability remains below a given threshold−we characterize the interference to PU by its outage probability. For EH relays, the optimal use of available energy is crucial. Low transmission power to conserve energy

Abstract:We study a class of two-transmitter two-receiver dual-band Gaussian interference channels (GIC) which operates over the conventional microwave and the unconventional millimeter-wave (mm-wave) bands. This study is motivated by future 5G networks where additional spectrum in the mm-wave band complements transmission in the incumbent microwave band. The mm-wave band has a key modeling feature: due to severe path loss and relatively small wavelength, a transmitter must employ highly directional antenna arrays to reach its desired receiver. This feature causes the mm-wave channels to become highly directional, and thus can be used by a transmitter to transmit to its designated receiver or the other receiver. We consider two classes of such channels, where the underlying GIC in the microwave band has weak and strong interference, and obtain sufficient channel conditions under which the capacity is characterized. Moreover, we assess the impact of the additional mm-wave band spectrum on the performance, by characterizing the transmit power allocation for the direct and cross channels that maximizes the sum-rate of this dual-band channel. The solution reveals conditions under which different power allocations, such as allocating the power budget only to direct or only to cross channels, or sharing it among them, becomes optimal.