Symbiotic radio (SR) is a promising technology to achieve ultra-low-power transmission for the next generation wireless network. However, its enabling technique, ambient backscatter transmission, suffers from double fading and path loss that severely restricts its service range. In this work, the reconfigurable holographic surface (RHS) is introduced at the transmitter to tackle this problem. Unlike the widely studied reconfigurable intelligent surface (RIS), the RHS that uses amplitude control can offer higher integration and lower power consumption. Seeing that the treatment of the direct-link interference in the received signal is the key problem for the receiver of the backscatter devices to achieve successful signal detection, we propose two strategies. InStrategy I, the ambient data symbol is detected for canceling the direct-link interference, while inStrategy II, we design the data stream sent by the ambient transmitter and its transmission frame in collaboration with zero-forcing beamforming to alleviate the interference among all ambient users and backscatter devices. Based on the two strategies, respectively, problems are formulated to jointly optimize the transmit power, the digital beamforming, and the holographic beamforming of the RHS such that the energy efficiency of the SR network (SRN) is maximized. For each of the two problems, we develop specific hybrid beamforming algorithm that decomposes the original problem into three sub-problems. These sub-problems respectively optimize the transmit power, the digital beamforming, and the holographic beamforming that are solved in an alternative manner. Simulation results verify that the proposed strategies can well exploit the potential of the RHS in enhancing the transmission performance of the SRN.

Symbiotic radio is an energy- and spectrum-efficient transmission technology to achieve seamless connection in the next generation wireless network. To tackle the transmission distance limitation of the backscatter devices (BD) in the symbiotic radio network (SRN), in this work, reconfigurable holographic surface (RHS) is introduced to accommodate the multiple BDs by assisting their concurrent transmission. Compared to the traditional reconfigurable intelligent surface, RHS has the advantages of higher energy efficiency and integration with lower hardware cost. An optimization problem is formulated to jointly optimize the digital beamforming of the radio-frequency chains and the holographic pattern of the RHS such that the achievable sum rate of the received signal at the sink node (SN) which is used to collect the information of all the BDs is maximized under the achievable-rate constraint of the ambient mobile user. This challenging non-convex problem is transformed into semi-definite programming by successive convex approximation and sequential parametric convex approximation, which are solved in an alternating way. The effectiveness of using the RHS to assist the concurrent transmission of the multiple BDs in the SRN has been validated by the numerical results.

In this work, we investigate a spectrum-sharing network, where an Internet of Things (IoT) system and a WiFi system coexist in the unlicensed band. The IoT devices can actively transmit signal by contending for the channel with the WiFi users. Alternatively, they can also convey information by passively backscattering the ambient WiFi signal, and thus form symbiosis with the WiFi during the transmission. Since the active and the passive transmission mode have their own merits under specific circumstances, how to select one of them adaptively and efficiently with guaranteeing the normal service of the WiFi system is of high importance. To solve this problem, we first investigate the effective throughput of the IoT system from the cross-layer perspective. Then, by jointly considering the effective throughput and the power consumption of the IoT system, we formulate the transmission mode selection problem. Two solutions are given depending on whether the signaling between the two systems is available. By entailing the intersystem parameter sharing, the model-based transmission mode selection scheme is proposed, which gives the closed-form optimal solution. When the intersystem parameter sharing is absent, the model-less transmission mode selection scheme is proposed by using the deep reinforcement learning (DRL) technique. Extensive simulation results validate the theoretical analysis, and demonstrate the effectiveness of the proposed transmission mode selection schemes.

The Internet-of-Things (IoT) paradigm holds the promise to revolutionize the way we live and work through connecting various machine-type communication terminals. In this work, we investigate a symbiotic network from a cross-layer perspective, where passive IoT devices coexist in symbiosis with an ambient network that uses carrier sense multiple access with collision notifications (CSMA/CNs) MAC protocol. In the ambient network, each full-duplex mobile user (MU) aims to transmit its own packets to the common access point (AP) while receiving the signal backscattered from its associated backscatter device (BD). Considering the imperfectness of carrier sensing of the MU, two key parameters, i.e., the probability of detection and the probability of false alarm, are quantified. Then we derive the PHY-layer outage probabilities and analyze the corresponding diversity orders for both the CSMA/CN and the BD system. By incorporating the outage probabilities and the carrier sensing metrics into the MAC-layer analysis, the cross-layer outage capacities of the CSMA/CN and the BD system are derived. Simulation results demonstrate that the system performance can be improved by appropriately setting the PHY-layer parameters, such as the BD reflection coefficient α and the number of samples for carrier sensing K, as well as the MAC-layer parameters, such as the sensing duration and the initial contention window. With the BD reflection coefficient being 0.05, the outage capacity of the overall system has improved by 41.85% compared with carrier sense multiple access with collision avoidance protocol.

Symbiotic radio (SR) is a new paradigm of spectrum sharing that allows passive Internet-of-Things (IoT) device to transmit messages over the ambient RF signal. Considering that the reader needs to collect information from multiple IoT devices in practice, and the traditional multiple access schemes that require coordination among users are unaffordable for the passive devices, we propose a random code assisted multiple access scheme, in which each backscatter device (BD) independently chooses a random code to spread its signal and the incident signal is backscattered. To avoid the need of the instantaneous information, including the specific codes and the instantaneous channel state information of the backscatter links, and to optimize the transmit power and the BD reflection coefficients, we derive the asymptotic BD signal to interference plus noise ratio (SINR) by employing the large-dimension random matrix theory. We formulate the joint optimization problem to maximize the minimum asymptotic SINR among BDs with guaranteeing the primary target rate. By investigating the property of the problem, the original problem is transformed and solved by the proposed iterative-based algorithm. Extensive simulations are executed to verify our theoretical analysis and demonstrate the outperformance of the proposed algorithm.

In this paper, we study the cross-layer performance of a symbiotic system comprising passive internet-of-things (IoT) devices and the ambient system using carrier sense multiple access (CSMA) with collision notification (CSMA/CN) MAC protocol. Different from the CSMA with collision avoidance (CSMA/CA), the CSMA/CN protocol can enhance the throughput of the ambient system notably by using physical layer techniques to detect collisions. We first evaluate the physical-layer outage probabilities for the ambient and the backscatter device (BD) system respectively, and then introduce the outage probability of the ambient system into the MAC-layer performance analysis. Thereafter, the crosslayer outage capacities of the ambient system and the BD system are derived by considering the available transmission time for the BD system. We find that the overall system performance can be improved by appropriately setting the reflection coefficient of the BD and the parameters of the MAC protocol. The simulation results are provided to verify our theoretical analysis and demonstrate the system performance.

Intelligent reflecting surface (IRS), with a large number of reflective elements, is a promising technology to achieve both spectrum and energy efficient wireless communication. The IRS can reflect the incident electromagnetic wave passively and steer it to the desirable way before reaching the intended receiver by adjusting the phase shift on the reflective elements. In order to better improve communication quality, the beamforming vector at the base station (BS) and the phase shift induced by the IRS should be jointly designed carefully. However, thus far, previous works on IRS have assumed that the channel state information (CSI) is perfectly known at the BS, which is not available in the practical systems. In this paper, we study an IRS-enhanced multi-user multiple-input single-output (MISO) downlink communication system assuming imperfect CSI. An optimization problem is formulated to jointly optimize the beamforming vector at the BS and the phase shift at the IRS such that the total transmit power is minimized under the individual outage probability constraints. An algorithm based on alternating optimization (AO) and semi-definite relaxation (SDR) is proposed to solve this challenging non-convex problem. Finally, numerical results have validated the effectiveness of the proposed algorithm.

The licensed-assisted access-based long term evolution (LAA-LTE) is a promising solution to provide enhanced LTE services by sharing unlicensed bands with WiFi systems. However, the intense contention with the incumbent WiFi system makes it challenging for the LAA-LTE system to support guaranteed quality-of-service (QoS) for the users. This paper is interested in the QoS-aware LAA-LTE/WiFi coexistence system. We first propose a flexible coexistence framework using the listen-before-talk mechanism, based on which the QoS metrics of LAA-LTE and WiFi systems are quantified. Then, a joint user association and resource allocation problem is formulated, which aims to maximize the number of QoS-preferred users supported by LAA-LTE, while protecting the WiFi users. The considered optimization problem is equivalently decomposed into two subproblems, the sum-power minimization problem and the user association problem. For the first subproblem, the deep-cut ellipsoid method is adopted to optimize the LAA-LTE transmission time, subcarrier assignment, and power allocation. For the latter one, an efficient algorithm called successive user removal is proposed. The simulation results have demonstrated the effectiveness of the proposed scheme, based on which the tradeoff among different QoS metrics in the coexistence system is observed.

Visible light communication (VLC) system is a great candidate for indoor downlink access. Due to the limitation of light-emitting diode (LED)'s bandwidth, orthogonal frequency division multiplexing (OFDM) is used to enhance the transmission capacity of VLC system. Non-orthogonal multiple access (NOMA) can support multiple users in VLC system by sharing time and spectrum resources. The combination of OFDM and NOMA improves the overall channel capacity and spectral efficiency of a multi-user VLC system. In order to increase the data rate further, high order modulation formats are often applied. However, for the superposition of multiple high order modulation signals such as 16-quadrature amplitude modulation (QAM) and 64-QAM, it is hard to recover each user's signal under traditional successive interference cancellation (SIC) method, which is usually used in NOMA scheme. In this paper, we proposed a novel method to decode the superposed signals. This method is called ergodicity and comparison (EAC). By applying the EAC method, the bit error rate (BER) performance of two- user OFDM-NOMA VLC system is investigated under different power allocation conditions. In this system, the modulation formats for both the two users' signals are 4-QAM, 16-QAM and 64-QAM. Peak clipping effect on the received superposed signals is also considered. Numerical results demonstrate that the EAC method can improve significantly the BER performance of OFDM-NOMA VLC system, when high order M-QAM (M> 4) signals are applied to the two users.

License-assisted access LTE (LAA-LTE) has been proposed to deal with the intense contradiction between tremendous mobile traffic demands and crowded licensed spectrums. In this paper, we investigate the coexistence mechanism for LAA-LTE based heterogenous networks (HetNets). A joint resource allocation and network access problem is considered to maximize the normalized throughput of the unlicensed band while guaranteeing the quality-of-service requirements of incumbent WiFi users. A two-level learning-based framework is proposed to solve the problem by decomposing it into two subproblems. In the master level, a Q-learning based method is developed for the LAA-LTE system to determine the proper transmission time. In the slave one, a game-theory based learning method is adopted by each user to autonomously perform network access. Simulation results demonstrate the effectiveness of the proposed solution.

By offloading mobile traffic for macrocells, small cells can help to alleviate the pressure on conventional cellular networks from explosive data growth. On the other hand, the severe spectrum scarcity problem has prompted the research on spectrum sharing recently. In this paper, a spectrum sharing system of OFDMA small cells overlaying CDMA networks is considered, in which each OFDMA user in the small cells requests a specific transmission rate. We investigate the interferences among entities in the spectrum sharing system, from which a resource allocation problem is formulated along with the rate constraints, with the objective to meet the target rates of OFDMA users and to simultaneously protect CDMA system. This high- complexity and intractable problem is transformed into two separable problems by calculating the minimum required interference and selecting the activated users. We propose the on-demand resource allocation scheme to solve the resource allocation problem efficiently by deploying a semi-distributed algorithm. Simulation results are provided to evaluate the effectiveness and performance of the proposed scheme.

Licensed-assisted access (LAA), which conveys control signal via licensed anchor carrier and data information via both licensed and unlicensed bands, is a promising solution to enhance the throughput of wireless communications. In view of the potential impact on the incumbent services in the unlicensed band, how to design the medium access control (MAC) protocol for LAA system to make fair and friendly coexistence with its neighboring incumbent users is one of the most critical and challenging issues. In this paper, a LAA using LTE (LAA-LTE) system in the WiFi unlicensed spectrum is investigated. The listen-before-talk (LBT) protocol is designed for the LAA-LTE system. By quantifying the WiFi throughput in the coexisting system, allowable LTE transmission time is determined by considering different targets of WiFi service protection. Then, the LTE transmission time is optimized for maximizing the overall normalized channel rate contributed by both LAA-LTE and WiFi system, with the protection to the WiFi system. Our work offers guidelines of designing the LAA-LTE system, paving the way to a controllable, not only harmonious, coexistence of LAA-LTE and WiFi systems in the unlicensed spectrum.

Licensed-assisted access (LAA), which conveys data information via both licensed and unlicensed bands through spectrum aggregation, becomes a promising solution to enhance the capacity of wireless systems. In view of the potential impact on the incumbent system operating in unlicensed bands, the medium access control (MAC) protocol design for LAA system to harmonically coexist with its neighboring incumbent users is one of the most critical and challenging issues. In this paper, we consider a long-term evolution-based LAA (LAA-LTE) system operating in the WiFi unlicensed spectrum, for which the listen-before-talk-based MAC protocol is carefully designed. By quantifying the WiFi throughput and packet delay in the coexisting system, we formulate the constraints of LAA-LTE transmission time to fairly maintain WiFi services. The conditions of known and unknown network size of incumbent WiFi system are each considered separately. Then, the feasible region of LAA-LTE transmission time is determined, and the LAA-LTE protocol is optimized for maximizing the LAA-LTE throughput or the overall throughput contributed by both LAA-LTE and WiFi system. The theoretical analysis is validated via simulation, which also illustrates important observations when LAA-LTE and WiFi systems coexist. This paper offers guidelines to design the LAA-LTE system, paving the way to a controllable, not only harmonious, coexistence of LAA-LTE and WiFi systems in the unlicensed spectrum.

In this paper, we investigate a spectrum refarming system where an OFDMA system shares the licensed band of a CDMA system. Both systems share the same cell site, but each has different base station (BS) antennas. Joint resource allocation problem is formulated to optimize the CDMA receive power, OFDMA transmit power, and subcarrier assignment. Conventional interference control to protect the primary CDMA system relies on the availability of cross-channel gains (CCGs) from each secondary OFDMA transmitter to the CDMA BS receiver. However, the CCGs are difficult to be obtained due to lack of intersystem cooperation. To address this problem, we first decouple the original problem into higher and lower-level problems via primal decomposition. Then, a robust lower-level resource allocation (R-LRA) scheme, which controls interference without using CCGs is proposed, with which CDMA users can be sufficiently protected. Thereafter, an enhanced R-LRA (ER-LRA) scheme is proposed to decrease the conservation of R-LRA scheme. Assisted by the CDMA inner power control and based on the ER-LRA, efficient algorithm is designed to solve the higher-level problem. Extensions of the ER-LRA for other scenarios are also studied. Simulation results are provided to validate the proposed schemes in facilitating and improving the spectrum sharing performance.

The pilot spoofing attack is one kind of active eavesdropping activities conducted by a malicious user during the channel training phase. By transmitting the identical pilot (training) signals as those of the legal users, such an attack is able to manipulate the channel estimation outcome, which may result in a larger channel rate for the adversary but a smaller channel rate for the legitimate receiver. With the intention of detecting the pilot spoofing attack and minimizing its damages, we design a two-way training-based scheme. The effective detector exploits the intrusive component created by the adversary, followed by a secure beamforming-assisted data transmission. In addition to the solid detection performance, this scheme is also capable of obtaining the estimations of both legitimate and illegitimate channels, which allows the users to achieve secure communication in the presence of pilot spoofing attack. The detection probability is evaluated based on the derived test threshold at a given requirement on the probability of false alarming. The achievable secrecy rate is utilized to measure the security level of the data transmission. Our analysis shows that even without any pre-assumed knowledge of eavesdropper, the proposed scheme is still able to achieve the maximal secrecy rate in certain cases. Numerical results are provided to show that our scheme could achieve a high detection probability as well as secure transmission.

Conventional spectrum refarming (SR) techniques assemble the legacy services to partial licensed spectrum, and free up the remained spectrum for operating other communication systems. In this paper, we propose a new broadband SR framework in which the orthogonal-frequency division multiple access (OFDMA) system dynamically shares the code division multiple access (CDMA) spectrum, which contains multiple bands in a concurrent manner. The interference margin provided by the downlink isometric random precoded CDMA system is first derived. To protect all the CDMA users, interference constraints that regulate the OFDMA transmission are then formulated. We formulate the joint CDMA load planning and OFDMA resource allocation problem for the SR system. By applying primal decomposition, it is shown that the higher-level problem is non-convex in general. We first propose an efficient algorithm to find the myopic optimal solution by investigating the derivative property of the higher-level problem. Through enhancing the original constrains, the higher-level problem is shown to be convex and solved with another efficient algorithm. Simulation results are provided to evaluate the SR performance, illustrating the significance of the CDMA load planning on the OFDMA throughput, the advantage of the proposed SR model over conventional SR, and the protection to the CDMA system.

Conventional spectrum refarming (SR) techniques assemble the legacy services to partial licensed spectrum, and free up the remained spectrum for operating other systems. In this paper, we propose a new broadband SR framework in which the OFDMA system can concurrently share partial or all the CDMA spectrum that contains multiple bands. The interference temperature provided by the downlink isometric random precoded CDMA system is derived, and the in-band and cross-band interferences introduced by the OFDMA system to CDMA system are quantified. With the derived interference temperature and interference power, resource allocation problems of the OFDMA system under full-concurrent and partial-concurrent broadband SR are formulated and solved. Numerical results are provided to evaluate the SR performance, illustrate the advantage of the full-concurrent broadband SR, and show the significance of the CDMA system load planning on the SR performance.

Spectrum refarming (SR) refers to a radio resource management technique which supports different generations of cellular networks to operate in the same radio spectrum. In this paper, an underlay SR model is proposed for an Orthogonal Frequency Division Multiple Access (OFDMA) system to share the spectrum of a Code Division Multiple Access (CDMA) system through intelligently exploiting the interference margin provided by the CDMA system when operating with a low system load. The asymptotic signal-to-interference-plus-noise ratio (SINR) of the CDMA system is used to quantify the interference margin, which interestingly does not depend on the instantaneous information (spreading codes and channel state information) of the CDMA system, thanks to its internal power control. By using the transmit power constraints together with the derived interference margin, the uplink resource allocation problem for OFDMA system is formulated and solved through dual decomposition method. The proposed SR system only requires high level system parameters from the CDMA system, hence, the upgrading of legacy CDMA system is not needed. Simulation results have verified our theoretical analysis, and validated the effectiveness of the proposed resource allocation algorithm and its capability to protect the legacy CDMA users.

Spectrum refarming (SR) is a promising radio resource management technique which allows different generations of cellular networks to operate in the same radio spectrum. In this paper, an underlay SR model is proposed for an Orthogonal Frequency Division Multiple Access (OFDMA) system to refarm the spectrum assigned to a Code Division Multiple Access (CDMA) system through intelligently exploiting the interference margin provided by the CDMA system. We investigate the mutual effect of the two systems by evaluating the asymptotic signal-to-interference-plus-noise ratio (SINR) of CDMA users, based on which the interference margin tolerable by CDMA system is determined. With the interference margin together the transmit power constraints, the resource allocation problem of OFDMA system is formulated and solved through dual decomposition method. Simulation results have verified our theoretical analysis and validated the effectiveness of our proposed OFDMA resource allocation solution, as well as its protection to CDMA services.

In this paper, a non-coherent base station cooperation (NC-BSC) scheme is examined for the downlink OFDMA wireless cellular networks. We formulate the resource allocation in the scheme as a subcarrier allocation optimization problem with equal power allocated on each subcarrier. To balance the tradeoff between system throughput and fairness, a multi-cell scenario controllable fairness is extended from single-cell scenario mentioned in [1], based on which the controllable global fairness (GF) scheme in the NC-BSC system is proposed. Two heuristic iterative algorithms are proposed to realize the subcarrier allocation in the NC-BSC system with and without the controllable fairness in a distributed manner. The performance is compared with traditional dynamic subcarrier allocation in noncooperative multi-cell networks. Simulation results show the performance enhancement through implementation of NC-BSC and the effectiveness of multi-cell controllable fairness in the NC-BSC system.