With the rapid growth of demand for ever-increasing data rate, spectrum resources have become more and more scarce. As a promising technique to increase the efficiency of the spectrum utilization, cognitive radio (CR) technique has the great potential to meet such a requirement by allowing un-licensed users to coexist in licensed bands. In conventional CR systems, the spectrum sensing is performed at the beginning of each time slot before the data transmission. This unfortunately results in two major problems: 1) transmission time reduction due to sensing, and 2) sensing accuracy impairment due to data transmission. To tackle these problems, in this paper we present a new design paradigm for future CR by exploring the full-duplex (FD) techniques to achieve the simultaneous spectrum sensing and data transmission. With FD radios equipped at the secondary users (SUs), SUs can simultaneously sense and access the vacant spectrum, and thus, significantly improve sensing performances and meanwhile increase data transmission efficiency. The aim of this article is to transform the promising conceptual framework into the practical wireless network design by addressing a diverse set of challenges such as protocol design and theoretical analysis. Several application scenarios with FD enabled CR are elaborated, and key open research directions and novel algorithms in these systems are discussed.

Device-to-Device (D2D) communication, which enables direct communication between nearby mobile devices, is an attractive add-on component to improve spectrum efficiency and user experience by reusing licensed cellular spectrum in 5G system. In this paper, we propose to enable D2D communication in unlicensed spectrum (D2D-U) as an underlay of the uplink LTE network for further booming the network capacity. A sensing-based protocol is designed to support the unlicensed channel access for both LTE and D2D users. We further investigate the subchannel allocation problem to maximize the sum rate of LTE and D2D users while taking into account their interference to the existing Wi-Fi systems. Specifically, we formulate the subchannel allocation as a many-to-many matching problem with externalities, and develop an iterative user-subchannel swap algorithm. Analytical and simulation results show that the proposed D2D-U scheme can significantly improve the system sum-rate.Part of this work has been accepted by Proc. IEEE ICC 2017 [1]. AP-like method, D2D-U requires assist and control from the central BS. With the involvement of BS, D2D users can work as an underlay of LTE system in both licensed and unlicensed spectra.As aforementioned, the major challenges of implementing D2D-U are (1) the opportunistic feature of unlicensed channel access due to current 802.11 mechanism adopted by Wi-Fi systems;and (2) the interference management issue among the three types of systems, i.e., the access and transmission of D2D-U users do not cause significant interference to the existing Wi-Fi system as well as the LTE-U system. To cope with the first challenge and be compatible with current LTE standards 1 , we design a duty cycle based protocol [25]-[27], in which the BS schedules transmissions according to the data demand. To tackle the second challenge, unlike the work in [28] which only maximizes the total sum-rate, we investigate the subchannel allocation problem to leverage the maximization of the sum-rate of LTE-U and D2D-U users and the protection of Wi-Fi performance. This subchannel allocation problem is originally a mixedinteger non-linear programming (MINLP) problem, which is generally NP-hard. For this reason, we reformulate it as a many-to-many game with externalities [29]- [33], and solve it with low computational complexity by designing an iterative user-subchannel swap-matching algorithm.The major contributions of this paper are summarized as follows.• We propose a feasible duty cycle based protocol for the LTE-U and D2D-U users to utilize the unlicensed spectrum.• An approximated model is elaborated to evaluate the interference to Wi-Fi networks introduced by LTE-U and D2D-U users.• We investigate the subchannel allocation problem by a many-to-many matching game with externality, and analyze its stability, convergence, complexity, and optimality.The rest of the paper is organized as follows. In Section II, we first introduce the system model for the coexistence among LTE, D2D, and Wi-Fi users, as well as their PHY/MAC fea...

We report on the experimental observation of a bidirectional fiber soliton laser passively mode-locked by single-wall carbon nanotubes. Two stable pulse trains in opposite directions are delivered simultaneously from the ring cavity. The counterpropagating pulses have different central wavelengths, pulse durations, and repetition rates. By adjusting the fiber birefringence and cavity length, the central wavelengths of two solitons can be the same or different. Experimental observations and analyses demonstrate that the different operating wavelengths result in the unequal repetition rates of two pulses. These unique features may be attributed to the cavity asymmetry and fiber birefringence.

Abstract-In traditional cognitive radio networks, secondary users (SUs) typically access the spectrum of primary users (PUs) by a two-stage "listen-before-talk" (LBT) protocol, i.e., SUs sense the spectrum holes in the first stage before transmit in the second stage. In this paper, we propose a novel "listenand-talk" (LAT) protocol with the help of the full-duplex (FD) technique that allows SUs to simultaneously sense and access the vacant spectrum. Analysis of sensing performance and SU's throughput are given for the proposed LAT protocol. And we find that due to self-interference caused by FD, increasing transmitting power of SUs does not always benefit to SU's throughput, which implies the existence of a power-throughput tradeoff. Besides, though the LAT protocol suffers from self-interference, it allows longer transmission time, while the performance of the traditional LBT protocol is limited by channel spatial correction and relatively shorter transmission period. To this end, we also present an adaptive scheme to improve SUs' throughput by switching between the LAT and LBT protocols. Numerical results are provided to verify the proposed methods and the theoretical results.