Unmanned aerial vehicles (UAVs) have been widely deployed as aerial relays in wireless networks. Moreover, a UAV swarm can form a virtual array and improve the transmission distance and energy efficiency of relay signals through collaborative beamforming (CB). However, due to the different propagation channels between the source node and each relay UAV, there are time delays and phase offsets among the amplify-and-forward (AF) processed signals of each UAV, resulting in CB performance deterioration. Hence, a time-phase-robust collaborative beamforming algorithm (TPR-CB) for UAV swarm-enabled AF relay systems is proposed in this paper. Firstly, a TPR-CB architecture based on the tapped delay line (TDL) is designed. Subsequently, the iterative calculation formula for the TDL weight vector is deduced by minimizing mean square error (MSE) through stochastic gradient descent (SGD). The MSE expression and feasible step size range of SGD are provided. Finally, the expressions for the average far-field beampattern and coherent gain are derived to characterize the performance of TPR-CB. The simulation results show that compared with the existing CB method, TPR-CB can improve the coherent gain and beampattern in the presence of time delay and phase offset.

In the Internet of Things (IoT), distributed idle devices can transmit cooperative jamming to confuse eaves-dropping devices, while authorized receivers utilize distributed cooperative jamming (DCJ) cancellation techniques to achieve signal-to-noise ratio gain, thereby enabling physical layer secure connectivity. However, constrained by the cost and size of IoT terminals, the crystal oscillators they employ typically exhibit low frequency stability, leading to challenges in achieving precise time-frequency alignment during DCJ cancellation. This significantly degrades the DCJ cancellation capability. To address this, the effect of time-frequency mismatch on DCJ cancellation and system sensitivity loss are analysed, and a closed-form expression for the jamming cancellation ratio with respect to the normalized time-frequency error is derived. Simulation results confirm the theoretical analysis that time-frequency mismatch causes a dramatic degradation in the performance of DCJ cancellation and results in a significant increase in the system sensitivity loss, particularly under scenarios involving large-scale jamming nodes.

To address the substantial measurement challenges in characterizing self-interference (SI) intensity for full-duplex (FD) large-scale arrays, we propose an innovative prediction method using Vision Transformer (ViT) and transfer learning. The proposed model integrates array multi-modal features and employs multi-head attention mechanisms to accurately predict SI intensity in FD array systems with limited measured data. The proposed transfer learning strategy is composed of a pre-training phase based on the spherical-wave model and a fine-tuning phase utilizing limited measured data, which further enhances the predictive performance of the proposed model. Our experimental evaluation in the upper 6 GHz (U6G) band (6675 ∼ 6875 MHz) shows that, compared to traditional methods, the proposed method achieves approximately 16% and 10% improvements in Structural Similarity Index Measure (SSIM) and the proportion of predictions with errors below 3 dB, respectively. These results confirm its effectiveness for SI intensity prediction in FD arrays and indicate its potential for supporting beam management and optimization in B5G/6G FD systems.

The future 6G network will enhance smart cities' data integration and analysis capabilities through Integrated Sensing and Communication (ISAC) technology. This paper proposes a millimeter wave (mmW) in-band full-duplex (IBFD) ISAC system architecture and explores three direction-finding techniques based on subarray splitting for its architecture. Firstly, the mutual interference problem in the IBFD ISAC system was analyzed. Using the Taylor synthesis method to suppress array sidelobes and reduce false alarm rate, combined with a two-dimensional fast Fourier transform (2D-FFT), the direction-finding accuracy is improved under an extremely low signal-to-noise ratio (SNR). Then, the performance of three direction-finding methods was compared through simulation. The simulation results showed that the sum difference beam direction-finding method has better mean square error (MSE) performance. In addition, a sum difference beam direction-finding experiment was conducted, and the direction-finding errors for moving and static targets were as low as 0.6° and 0.5°, respectively, demonstrating excellent direction-finding capability.

This paper proposes an integrable analog domain linearization architecture for power amplifiers (PAs) in multi-input multi-output (MIMO) systems, using a spectrum shaping circuit with nonlinear feedback. This architecture consists of a nonlinear distortion extracting loop and an N-order spectrum shaping circuit. The extracting loop aligns the PA input and output signals in amplitude with a 180-degree phase difference and then adds them together to extract the nonlinear distortion signal. The spectrum shaping circuit reshapes the spectrum of the extracted nonlinear distortion signal using N identical analog chains but with different group delays and attenuation parameters, and finally injects it back to the PA input to suppress the nonlinear distortion of the target frequency. Moreover, the delay in the remaining spectrum shaping channels has multiple relationships to the first channel, which can be easily integrated into the PA design stage with low complexity. Particularly, the center frequency of the nonlinearity suppression can be easily shifted by modifying the attenuation parameters in the N-order spectrum shaping circuit. We designed the proposed architecture using off-the-shelf discrete components and demonstrated its nonlinear distortion suppression performance at different target frequencies for the PA chip CMPA006002F.

Artificial noise (AN) can degrade eavesdroppers' signal-to-noise ratio (SNR) in anti-terrorism emergency communications to secure tactical data transmissions. However, system secrecy performance depends on licensed receivers' AN cancellation capabilities. Imperfect channel state information (CSI) will induce residual AN at licensed receivers, thereby degrading system secrecy performance. Unlike the previous methods of simply optimizing power allocation at the transmitter, this study proposes a joint transceiver optimization scheme to mitigate the impact of channel estimation error. Firstly, we present an ANaided anti-terrorism emergency communication model and theoretically derive the statistical properties of the channel estimation error and the closed-form expression of the achievable secrecy rate. Then, a joint optimization scheme for power allocation at the transmitter and cancellation factor at the receiver is presented to maximize the achievable secrecy rate. On this basis, the optimal values of power allocation weight and cancellation factor are derived. The simulation and analysis results prove that the proposed optimization scheme can effectively improve the system security performance in the presence of channel estimation error, and its achievable secrecy rate can be improved by about 16% compared with the power allocation alone.

The space-air-ground integrated networks (SAGNs) are promising components of the next generation communication system, and unmanned aerial vehicle (UAV) swarms are the primary infrastructures of the SAGNs aerial layer, used for relaying information. Furthermore, UAV swarms form virtual antenna arrays (VAAs) and implement collaborative beamforming (CB), which can improve the deep-space communication distance and expand the ground coverage. However, in practical engineering, due to the doppler effect and local oscillator random frequency drift, there is a frequency offset between UAVs, even after frequency synchronization, resulting in CB performance degradation. Therefore, in the presence of frequency offset, this paper investigates and presents the derivation of the average far-field beampattern, 3-dB beamwidth, and complementary cumulative distribution function (CCDF) for UAV swarm executing CB in SAGNs. The results show that the average far-field beam-pattern, peak power, 3-dB beamwidth, and CCDF, deteriorate as frequency offset intensifies. Furthermore, as the UAV swarm scale increases, the peak power attenuation is aggravated, and the decrease in CCDF is steeper. With the increase in the UAV swarm distribution range, the 3-dB beamwidth expansion is alleviated.

While 6G provides communication services for massive wireless devices, it faces huge security risks due to its open feature. Cooperative jamming techniques in physical layer security provide a viable way to ensure 6G security. By actively transmitting jamming signals to deteriorate the eavesdropping channel, while authorized receivers implement cooperative jamming cancellation (CJC) using the prior knowledge of cooperative jamming, thus securing the communication at the physical layer. In practice, incomplete matching of oscillator device characteristics will lead to the in-phase and quadrature (IQ) imbalance problem, where the quadrature branches of the up and down converted carrier signals are not perfectly orthogonal to the in-phase branches, which may degrade the CJC performance. For this reason, this paper specifically analyzes the impact of the IQ imbalance problem on the CJC performance, where a closed-form expression of the jamming cancellation ratio (JCR) with respect to amplitude imbalance and phase imbalance is derived. The simulation results show that when jamming-to-noise ratio (JNR) is 35 dB, the system can tolerate up to 1% amplitude imbalance and 1° phase imbalance with 3 dB JCR degradation. Moreover, the IQ imbalance has a noticeable effect on the reachable security rate of the communication system.

Covert communication is an important approach to ensure information security by hiding the transmission behavior. Space-domain-coding intelligent reflecting surface (SDC-IRS) can adjust the phase of the reflection signal for passive beamforming in angle domains, which is widely employed in covert communications. However, the gains by SDC-IRS vanish when the warder is proximal to the receiver in angle domains. To overcome this limitation, in this paper, the space-time-coding IRS (STC-IRS) is considered, which can adjust both the phase and the frequency of the reflection signal for passive beamforming in angle-distance domains. Specifically, system performance under STC-IRS and SDC-IRS is compared, revealing the essence that angle and distance discrimination for the receiver is achieved with STC-IRS. Further, to fully exploit STC-IRS, optimization problems are formulated to maximize the covert rate in both line-of-sight scenarios and Rician fading scenarios. To solve the above problems, penalty-based algorithms are proposed where the transmit power, the phase shift and the frequency shift at STC-IRS are optimized jointly with majorization-minimization and block successive upper bound minimization techniques. Considering more general and adverse cases, the proposed algorithms are also extended to the scenario with multiple warders. Simulation results demonstrate the superiority of the proposed scheme compared with other benchmarks. Especially, when the warder and the receiver overlap in angle domains, covert rates with STC-IRS exceed 3 bps by distance domain discrimination, whereas covert rates with SDC-IRS are less than 0.01 bps.

The artificial noise shielded frequency-hopping (AN-FH) architecture can secure wireless communications against interference and eavesdropping. However, the AN suppression performance is highly sensitive to the radio frequency (RF) impairments, and the transceiver in-phase and quadrature (IQ) imbalances and phase noise in FH systems are more severe for the wide FH bandwidth. In this study, the AN-FH transceivers under RF impairments are mathematically modeled. Subsequently, distortions for transceiver phase noise are represented by the common phase error, distortions for transceiver IQ imbalances are represented by the mirror image component, and closed-form expressions for their respective power are derived and compared with the thermal noise power. Finally, the AN suppression capability is evaluated via the AN suppression ratio (ANSR), defined as the ratio of the AN-plus-noise power before and after suppression. It is found that when only phase noise exists, shortening the channel compensation cycle can enhance ANSR and reduce ANSR degradation; when the distortions’ power is smaller than the thermal noise power, it is also recommended to enhance the transmitting power and the power ratio of AN to information signals. When only IQ imbalances exist, increasing the power ratio of AN to information signals is suggested; when the distortions’ power is smaller than the thermal noise power, enhancing the transmitting power is also suggested.

Cooperative jamming is one of the promising technologies for securing wireless communications. The cooperative jammer transmits jamming to deteriorate the eavesdropping channel, while the authorized receiver implements cooperative jamming cancellation (CJC) using the pre-stored prior information, thus achieving confidential communication at the physical layer. However, the signal transmission will introduce time delay and frequency offset, as well as zero-intermediate-frequency structures that will cause in-phase and quadrature (IQ) imbalances and local oscillator (LO) leakage, which lead to degradation of the CJC performance. In this paper, we specifically analyze the combined effect of time-frequency mismatch, IQ imbalance and LO leakage on the CJC performance by deducing the closed-form expression for the jamming cancellation ratio. A CJC architecture considering time-frequency mismatch, IQ imbalance and LO leakage is proposed, and the corresponding recursive algorithm for CJC is presented. Simulation results show that the proposed algorithm can effectively improve the CJC performance with the presence of time-frequency mismatch, IQ imbalance and LO leakage, and thus improves the security of the communication system.

Due to the popularity of unmanned aerial vehicles (UAVs), UAV detection technology has attracted considerable attention. In order to widely deploy UAV detection for preventing civil UAVs, a UAV detection method that can be used with any transceiver and operate in parallel with any communication system is proposed in this paper. UAV flights influence electromagnetic environments during signal propagation, and the proposed method detects UAVs based on the variance of higher-order cumulants. In this paper, the principle of the method is deduced and proven, and the detection performance is analysed. Explicit expressions for the detection and false alarm probabilities are obtained, and the influencing factors are investigated. Finally, the feasibility of the proposed method is verified experimentally, and the detection distance, signal-to-noise ratio and detection height are analysed. The results give the effective detection distance and show that the proposed method has superior anti-noise performance.

Full-duplex wireless communication can double the spectral efficiency by transmitting and receiving information simultaneously, but it suffers from strong self-interference (SI). Baseband SI cancellation (SIC) can estimate and subtract the SI from the receiving signal, while perfect SIC is unachievable for the existence of the intended signal. To enhance SIC capability, we propose a two-stage recursive least squares (RLS) filter architecture with low computation complexity and strong environmental robustness. In the first stage, the filter acts as a traditional RLS filter to cancel SI. In the second stage, we introduce the reconstructed intended signal as an additional input of the RLS filter, and the filter jointly estimates the SI channel and intended channel to attenuate the SI signal approaching the noise floor. Besides, to improve the robustness of the proposed method, we introduce an adjusted diagonal loading into the two-stage RLS to reduce the convergence time. The computation complexity and storage complexity are analyzed as well. Simulation results show that the proposed method has up to 10 dB SIC improvement over the traditional RLS, and achieves the same performance as the two-stage least squares (LS) method after two iterations. On the other hand, our proposed two-stage RLS filter reduces the computation complexity by about 81.6% compared to the two-stage LS method when the filter order is 16.

In single-input single-output (SISO) systems, transmitting the expected signals with artificially generated noise can effectively enhance the physical layer security (PLS). However, the traditional noise suppression at the intended receivers will increase algorithm complexity, particularly over multi-path fading channels, which poses challenges for some massive Internet-of-Things (IoT) networks that take advantage of low-cost and resource-constrained nodes. In this paper, we design a novel self-noise (SN) waveform for transmission security. The transmitter stacks the signal segments periodically, and the additive SN in the current signal period derives from the previous signal periods. Based on this, SN can be completely suppressed at intended receivers by differential operations between two adjacent signal periods. Both analysis and simulation results confirm that the proposed scheme can significantly reduce the complexity of noise suppression and its performance is not affected by multi-path channel environments. Besides, the proposed scheme can effectively worsen the signal-to-noise ratio (SNR) of eavesdroppers while having negligible impacts on intended receivers.

Full duplex (FD) communication is considered as a promising technology in the development of 5G-advanced and 6G systems as it theoretically doubles the capability of channel. However, this improvement relying on a simultaneously bidirectional manner of communication induces intrinsic self-interference (SI) demanding to be fully cancelled, which is generally intractable. This challenge is minimized in the scenario of integrated access and backhaul (IAB) networks operated in millimeter wave (mmW) band, as its transceivers are stationary and the complexity of SI is greatly reduced by beamforming technique. As a matter of fact, FD can be a pioneering technique to unlock the full potential mmW-based IAB network by releasing its suffering of the half-duplex inefficiency. This article presents the design principle and validation of a practical SI cancellation (SIC) technique in the case of high transmission power class in mmW-based IAB networks. The proposed technique sequentially eliminates SI from the spatial, RF, and digital domains that reduces the SI down to the noise floor. To validate the technique, a prototype system is developed in accordance with 5G IAB specifications and field tests are conducted. Results suggest that the designed mmW SIC transceiver significantly reduces residual-interference to noise ratio (R-INR) to 2.1 dB or less. Additionally, a system-level simulation is conducted in line with 5G IAB evaluation methodology, which explores the potential performance gain of the proposed technique in presence of cross-link interference (CLI). Results indicate that the method could yield a cell throughput gain of about 84%, compared to the current time division duplex deployment.

In recent years, cooperative jamming (CJ) is introduced as a promising method to improve the security performance in the presence of eavesdroppers. By masking the confidential signal with the help of the cooperative jammer, the signal-to-noise ratio of the eavesdropper can be selectively reduced without prior information, whereas the authorized receiver is unaffected since the CJ can be suppressed by some means. Unfortunately, with the impacts of the clock jitter caused by non-ideal crystals, the received CJ may suffer sampling clock offset, carrier frequency offset and phase noise, which deteriorates the performance of the CJ cancellation at the authorized receiver. In this paper, the impacts of clock jitter on CJ cancellation are investigated. First, the deterioration of CJ caused by sampling clock offset is modeled as inter-symbol interference (ISI) in the time domain. Then, the impacts of the sampling clock offset, carrier frequency offset and phase noise on the spectral purity of CJ are modeled as inter-frequency interference (IFI). Furthermore, the expression of the power of residual CJ (PRCJ) is derived to analyze the impacts of clock jitter on CJ cancellation. Finally, numerical results prove the theoretical analysis, indicating that the PRCJ increases as the carrier frequency offset and the 3 dB coherence bandwidth of phase noise increases, and is more sensitive to phase noise.

Artificial noise (AN) is an effective physical security measure against eavesdropping in wireless communication. In previous literatures, AN is generally analyzed in multiple transmit antenna systems and is suppressed by zero-forcing algorithm, which is implemented by using the spatial degrees of freedom. In this paper, an AN suppression model under single-input multi-output (SIMO) systems is presented, and the impact of imperfect frequency synchronization on the AN suppressed performance is analyzed. Specifically, we first analyze the AN suppression performance of single branch with imperfect frequency synchronization. Then the output signal to interference plus noise ratio (SINR) after multi-branch merging is derived. Finally, the secrecy capacity of AN based on SIMO systems is extracted on the basis of the obtained SINR. Theoretical and simulation results show that imperfect frequency synchronization reduces the SINR of each branch, and thus degrades both the output SINR and secrecy capacity in SIMO systems. Moreover, when the residual AN power is greater than noise power, no diversity gain will be achieved by multi-branch merging.

The ever-increasing roll-out of 5G integrated access and backhaul (IAB) networks in millimeter wave (mmW) band and its current hard-constraint of half duplex operation motivate the study of full duplex (FD) communication in a high power beamforming system. In an mmW IAB network using FD, the intrinsic self-interference inherits the nature of mmW high attenuation and cross link interference can be quite manageable via sharp beamforming between fixed nodes. As a matter of fact, mmW IAB could be a pioneering use case of FD in near future. In this article, a practical self-interference cancellation (SIC) transceiver design is presented targeting for “down to noise floor” SIC in the case of high transmission power class. We developed an mmW prototype testbed system in accordance with 5G IAB specifications. Our experimental results via prototype verifies the feasibility of the designed mmW SIC transceiver and confirms the potential of 2 times throughput gain from FD compared with conventional half duplex in an IAB scenario.

Cooperative jamming (CJ) can mask signal-of-interest (SOI) by additional jamming signal in the presence of eavesdroppers, and thus improves the security performance from the perspective of physical layer. In exchange, an additional CJ canceller should be designed for the destination node to restore the SOI, whose performance is possibly degraded by the nonideal hardware features. Typically, the inherent nonlinearity of power amplifier is unavoidably distorted the CJ as the transmit power increases, leading to the limit of the CJ cancellation performance. Therefore, nonlinear CJ cancellation is suggested to alleviate the impact of nonlinear CJ components on the SOI. Besides, the short-term instability of the oscillator is characterized by phase noise in the time domain, resulting in a random phase mismatch between the received CJ and the reference signal, which further degrades the nonlinear CJ cancellation performance. Inspired by the above problems, this paper explicitly analyzes the impact of phase noise on nonlinear CJ cancellation, where the expression of CJ cancellation ratio (CJCR) in the presence of phase noise and nonlinear distortion is derived, where the coefficients of the nonlinear canceller are estimated by LS estimator, and the deterioration of the spectral purity of CJ caused by phase noise is modeled as inter frequency interference (IFI) additional noise. Theoretical analyses and simulation results show that the CJCR decreases as the normalized 3 dB coherence bandwidth of phase noise increases.

Integrating full duplex and phased arrays is promising for future wireless communication systems. However, in the joint systems, the strong self-interference (SI) signals leaked from the transmit array will superpose on each receiving antenna, which puts forward strict demands for the dynamic range and linearity of the low noise amplifiers. In this work, we focus on how to cancel the superposed SI signal using an analog multi-tap filter in wideband scenarios. We model the wideband near-field SI channel for the full duplex array and analyze the optimal cancellation performance with and without device errors, respectively. The theoretical analysis is verified by simulation, which indicates that the SI signal with 200 MHz bandwidth can be canceled to the noise level by only two taps without device errors. Moreover, the step sizes of the variable attenuators and phase shifters need to be less than 0.6 dB and 4 degrees, respectively, to cancel the SI above 30 dB with two taps, and the RF canceller with more taps is more robust in the existence of device errors, that is, the better cancellation performance.

Full duplex (FD) communication is an enabling technology with simultaneous uplink and downlink transmission over the same spectrum, which could not only virtually double the spectrum but also shorten the latency of bi-directional communications. One of the key challenges to bring full duplex into reality is how to design a practically implementable self-interference cancellation (SIC). This paper promotes a practical joint-design of SIC capable of >120dB SIC gain to make in-band FD practically viable. It consists of novel integrated SIC antenna, multi-tap tunable RF SIC and non-linear digital SIC. Further, a prototype system implemented using in-band FD with 5G NR commercial level hardware components is presented, which not only verifies in-band FD is practically implementable with the proposed joint SIC, but also achieves highest SIC results, i.e. 122.5dB SIC capability with 32dBm transmit power, to our best knowledge. This result confirms the appealing potential of in-band FD and conclusively, the in-band FD communication with the developed effective SIC turns out to be a promising enabler for future business thriving.

In simultaneous transmit and receive (STAR) phased array systems, suffering from the multi-dimensional cross-coupling strong self-interference (SI), the radio frequency (RF) front-end of receive array is severely saturated. Against this issue, we present a transmit beamforming (TxBF) SI cancellation (SIC) design to construct the TxBF vector, where a robust TxBF SIC optimization problem with maximum receive power constraints and a worst-case uncertainty channel error model is formulated, the approximation solution of which is solved by the semidefinite relaxation and S-procedure. Simulation results show that our design effectively avoids the RF front-end saturation at the cost of near-minimum beamforming gain loss within a given uncertainty channel error bound.

Nonlinear self-interference (SI) cancellation is crucial for eliminating the impact of the transmitter-side nonlinearity on the comprehensive SI cancellation performance in full-duplex radios. However, in the propagation environment with rich reflection paths of signals, the number of the delayed taps required for time-domain nonlinear SI cancellation booms exponentially with the increase of the multi-path number, leading to unacceptable complexity. In this paper, a novel frequency-domain nonlinear SI canceller based on successive interference cancellation (SIC) technique is proposed for orthogonal frequency division multiplexing (OFDM) modulated full-duplex systems subjected to both transmitter nonlinearity and receiver nonlinearity. The proposed frequency-domain nonlinear SIC (FD-NSIC) cancels nonlinear components one by one via separately estimating the channel frequency response suffered by each order nonlinear component, which significantly relaxes the computational cost. Besides, to avoid the influence of the strong correlation between nonlinear components, orthogonalization is operated before SIC to white basis functions, which accelerates the convergence speed of the proposed FD-NSIC. Simulations are explicitly performed, showing that compared with the conventional frequency-domain nonlinear canceller based on recursive least squares (RLS) channel estimation, the proposed FD-NSIC is capable of canceling the SI to the noise floor with only 52% computational cost under 50 dB interference-to-noise ratio.

Network function virtualization (NFV) implements mobile edge computing (MEC) services as software appliances, and allows resources to be adaptively allocated to accommodate demand variations. Scalability and network cost (including operational cost and response latency) are key challenges. This paper presents a new game-theoretic approach to minimizing the network cost, where access points (APs) select MEC servers and routes in a decentralized manner, and unloaded routers and links are deactivated for cost saving. The key idea is that we interpret the minimization of network cost as a mixed game with a non-monotonic cost function capturing both the operational cost and response latency. We prove that the game is conditionally an ordinary potential game and converges to$\alpha $-approximate equilibriums. A closed-form expression is derived for the convergence delay. Another important aspect is that we integrate Stackelberg routing into the proposed mixed game to avoid inefficient equilibriums (with high cost or latency). We prove that the mixed game can converge faster to better equilibriums under linear response latency models. Extensive simulations corroborate the new game-theoretic approach can significantly outperform existing techniques in terms of efficiency, convergence, and scalability.

In the timing asynchronous self-interference cancellation (TASIC) of full-duplex (FD) orthogonal frequency division multiplexing (OFDM) systems, the relative delay between the signal of interest (SoI) and the self-interference (SI) may be greater than the length of the cyclic prefix (CP) due to the long distance transmission of the SoI and the access timing control, which causes a loss in the orthogonality of the SI when performing SI cancellation (SIC) in the frequency domain directly. In order to maintain the orthogonality of the SI pilots and estimate the channel response without the interference caused by the SoI, a channel estimation method is given utilizing either the complete or incomplete subcarriers. Based on it, a TASIC method is proposed to cancel the SI, where the expression of the time-domain SI is derived considering the influence of the multi-path channel and the relative delay between the SoI and the SI. The theoretical and simulation results show that the value of the residual interference (RI) power is stable and the SIC ratio (SICR) increases with the increase of the interference to signal ratio (ISR), and the proposed TASIC method can largely avoid the impairment brought by the relative delay between the SoI and the SI.

Full duplex radio is one of the most promising techniques for the fifth-generation (5G) wireless networks. It usually includes a feedback loop to capture the nonlinear distortion of the transmitted signal for linearization of power amplifiers (PAs) and nonlinear self-interference (SI) cancellation. This paper presents an FD transceiver architecture consisting of the digital predistortion (DPD), radio frequency (RF)-tapping SI cancellation, and baseband (BB)-tapping SI cancellation. In particular, the issue of high sampling rate required in the feedback loop is addressed, and a BB-tapping method, which adopts a nonlinear modeling method with a low-rate aliasing transmitted signal, is proposed to significantly reduce the required feedback sampling rate. Numerical results verify that by using the proposed BB-tapping cancellation, a sampling rate of 25 MSPS is enough for the feedback loop in the scenario of a 100 MHz source signal.

The reconfigurable intelligent surface (RIS), which consists of a large number of passive and low-cost reflecting elements, has been recognized as a revolutionary technology to enhance the performance of future wireless networks. This paper considers an RIS assisted multicast transmission, where a base station (BS) with multiple-antenna multicasts common message to multiple single-antenna mobile users (MUs) under the assistance of an RIS. An equivalent channel model for the considered multicast transmission is analyzed, and then an optimization problem for the corresponding channel capacity is formulated to obtain the optimal covariance matrix and phase shifts. In order to solve the above non-convex and non-differentiable problem, this paper first exploits the gradient descent method and alternating optimization, to approach the locally optimal solution for any number of MUs. Then, this paper considers a special case, which can obtain the global optimal solution, and shows the sufficient and necessary condition for this special case. Finally, the order growth of the maximal capacity is obtained when the numbers of the reflecting elements, the BS antennas, and the MUs go to infinity.

This paper considers a point-to-point communication system which is protected by a cooperative jammer from being eavesdropped. Previous works in this field always assume that the cooperative jamming is cancelled perfectly at the receiver, without considering any imperfections in the jamming cancellation of practical systems. In this paper, by taking into account the residual jamming caused by frequency offset at the receiver, a power allocation scheme is proposed to maximize the secrecy capacity over the additive white Gaussian noise (AWGN) channel, which is derived as an exact closed-form solution. In addition, the relationship between the optimal jamming power and the relative channel quality is analyzed under imperfect jamming cancellation. Simulation results confirm that the proposed power allocation scheme outperforms the power allocation scheme without considering any imperfections in the jamming cancellation, and illustrate that the frequency offset significantly degrade the security performance. Specially, for small frequency offset, the total power consumption increases with the frequency offset, which means the frequency offset consumes extra power to achieve the secrecy capacity target. However, for large frequency offset, the secrecy capacity may fail to meet its preset goal no matter how sufficient the power budget is. More seriously, when the frequency offset is remarkable, the secrecy capacity may drop to 0, which implies that the eavesdropper can always overhear the confidential information.

Compared with the conventional mobile edge cloud (MEC) network, the network function virtualization (NFV)-enabled MEC network provides new flexibility on the MEC service deployment. Resource wastage owing to dynamic workloads in traditional MEC networks can be overcome through adaptive resource allocation. In this paper, we investigate the resource allocation problem to minimize the operational cost (e.g., energy consumption, capital expenditure) and the average response time in the NFV-enabled MEC network. We consider the problem from the perspective of MEC service deployment, assignment, and routing among the access points (APs) and MEC servers. We propose an user-network cooperation-based algorithm with low-complexity. In the proposed algorithm, the network announces a path-switching rule (i.e., α-approximate deviation) with proportionally shared operational cost, while the APs selfishly choose their paths with the least cost accordingly. We analyze the selfish behaviors of APs with game theory. We prove existence and convergence of α-approximate equilibriums. Also, we evaluate the efficiency of the equilibriums with the price of stability (POS). Furthermore, an enhanced algorithm based on public service advertising (PSA) is proposed to improve the convergence performance and equilibriums efficiency. Through simulations, we show the superiority of the proposed algorithms over existing algorithms (e.g., BnB-SD and greedy routing) on the accuracy and convergence performance (measured by the overall path switching).

Pattern division multiple access (PDMA), which can exploit time, frequency, spatial resources or any combination of these resources, has been a promising candidate multiple access technology for the fifth generation (5G) wireless systems. Addressing the high-reliability, low-latency and massive-connectivity requirements of 5G systems is becoming one of the most promising research trends. In this paper, we combine the PDMA with the full-duplex relay and propose a cooperative PDMA (Co-PDMA) system to improve the reliability of cell-edge users with a short delay. We analyze the outage performance of the Co-PDMA system when the instantaneous channel state information could or could not be available at the transmitter. In addition, we also compare the outage performance in the cooperative and the non-cooperative PDMA system when the selection combining (SC) or the maximum ratio combining (MRC) scheme is utilized at the receiver. The numerical results show that the proposed Co-PDMA system can obviously improve the outage performance, and the MRC scheme can further improve the system reliability. Moreover, when the instantaneous channel state information is known at the transmitter, the PDMA with successive interference cancellation (SIC) decoding has a great advantage on improving reliability compared with the conventional orthogonal multiple access (OMA).

In this paper, we address the multi-user scheduling problem of the multi-user two-way full-duplex (FD) decode-and-forward relay system. According to the availability of the channel state information (CSI) and system state information (SSI), three scheduling schemes are investigated in terms of the system outage performance. Specifically, for the non-CSI case, we analyze the random scheduling scheme, which serves as a baseline for comparison. For the full CSI case, we propose the Max-Min scheduling scheme, which is theoretically proved to be sub-optimal in the low signal-to-interference-plus-noise ratio (SINR) regime but optimal in the high SINR regime. To minimize the outage probability, we propose the optimal scheduling scheme, which involves the full SSI. In addition, the exact closed-form outage probability expressions corresponding to different schemes are derived under the general independent but not identically distributed channels. The residual self-interference incurred by the FD mode is taken into account. Moreover, independent and identically distributed channels are also considered as special cases. Finally, numerical simulations are performed to corroborate the theoretical results. The results reveal that the Max-Min scheduling and optimal scheduling schemes significantly improve the outage performance compared with the random scheduling scheme.

This paper proposes a simple hybrid decode- compress- and-forward and compress-and-forward (DCF&CF) relay scheme for the coordinated multipoint (CoMP) uplink transmission. In particular, we consider a scenario that two mobile users (MUs) transmit signal to two access points (APs), which are linked to the central unit (CU) via lossless capacity-limited fronthaul. To characterize the achievable rate region of the considered system, a rate maximization problem is formulated and solved by exploiting the monotonicity and convexity of its objective function. Furthermore, maximum sum rate of the two MUs is obtained according to the maximized rate region boundary. Finally the analysis is validated by numerical results.

This paper presents the nonlinear effects of the analog self-interference (SI) canceller and a practical solution to suppress the nonlinear distortions in multiple-input multiple-output (MIMO) full-duplex wireless communication systems. Due to the inherent nonlinearities of the active radio frequency (RF) components used to tune the attenuations and the phase shifts in the analog SI cancellers, nonlinear distortions are introduced to the residual SI signal by the analog cancellation and enter the receive chain. In this paper, we build an extra feedback chain for each analog canceller to obtain the reference signal coupled from the transmitted RF signal and observe the characteristics of the canceller. Using the observations, we develop a nonlinear model including all the cancellers at each receive antenna with the presence of MIMO SI channel to estimate the nonlinear components. By subtracting the model estimates from the received signal instantaneously, the corresponding nonlinear distortions are effectively mitigated. After the nonlinearity mitigation, a linear digital cancellation scheme based on the Maximum Likelihood Estimation is applied to further reduce the residual SI signal caused by the MIMO SI channel to the noise floor. Experiments are performed on a 2x2 MIMO full-duplex testbed to validate the effectiveness of the proposed nonlinearity modeling and suppression method using Long Term Evolution signals of 20-MHz bandwidth.

In this paper, we propose a novel approach to cancel Self-interference (SI) signal in analog domain in Full-duplex (FD) communication system. This approach is based on self-mixing and called Self-Mixed Self-Interference Analog Cancellation (SM-SIAC) where three feedback loops are employed for auto-tuning. In condition that these loops converge, we analyze the cancellation performance.

Oscillator phase noise in a full duplex radio causes the mismatch between the self-interference (SI) signal and the cancelling signal, and thus degrades performance of the digital SI cancellation (SIC). In this paper, we analyze the effect of phase noise on digital SIC in wireless full duplex. We consider an OFDM-based full duplex radio corrupted by the phase noise at both the transmitter and the receiver, which are modeled as independent Wiener processes. A closed-form expression for the cancellation ability of a common digital SIC scheme is derived, in terms of the interference-to-noise ratio (INR), the SI subcarrier spacing and the oscillator's 3dB coherence bandwidth. The theoretical analysis and simulations reveal that the digital SIC ability degrades with the increase of the ratio of the oscillator's 3dB coherence bandwidth to the signal bandwidth, which determines the upper bound of the digital SIC ability.

The sampling clock accuracy of the digital predistortion (DPD) feedback path is limited by nonideal electronic components, which introduces random clock jitter and thus degrades the linearization performance. In this paper, we propose a new DPD architecture capable of estimating and mitigating the sampling clock jitter. The theoretical analysis reveals that the sampling clock jitter has a adverse effect on the signal-to-noise ratio (SNR) of the samples received and hence degrades the DPD performance. By applying the proposed reference injection method, the clock jitter can be estimated and mitigated, effectively. Simulation results demonstrate that the proposed DPD architecture can improve the normalized mean square error (NMSE) and the adjacent channel leakage ratio (ACLR) performances, compared with conventional DPD architecture when feedback sampling clock jitter is considered.

In this paper, we investigate the effect of shadowing on optimal power allocation of amplify- and-forward cooperative relaying systems. Considering the joint effects of path loss, correlated shadowing and flat Rayleigh fading, the approximate outage probability at high signal-to- noise ratio (SNR) is first derived. Then we solve the power allocation problem by minimizing the approximate outage probability subject to a total power constraint. It is shown by the analytical results that the correlation coefficients and the standard deviations of shadowing have a significant impact on the optimal power allocation. The simulation results show that the proposed power allocation scheme yields about 2dB SNR gain compared to the equal power allocation in the high SNR regime.

SUMMARY Because of the increasing energy consumption in radio access networks, a new radio access network called Cloud radio access network (C‐RAN) has attracted increasing attention. In C‐RAN, the distributed radio remote units (RRUs) are located in different sites and the placement of RRUs will influence the system performance. However, few researches investigate the optimal RRU placement from the perspective of energy efficiency, which is one of the most important characteristics of C‐RAN. In this paper, we first derive the energy efficiency of C‐RAN to achieve certain QoS requirements considering the circuitry energy consumption. Then, we obtain the optimal placement of RRUs by minimizing the consumed energy per bit. Numerical results are also presented to validate the analysis. Copyright © 2012 John Wiley & Sons, Ltd.

SUMMARY Cognitive radio is always based on spectrum sensing to cognize the physical characteristics of the wireless channel and carry out wireless communication resource scheduling. This kind of method makes it hard for cognitive radio to respond to a change in the radio environment in advance. Therefore, both the predictive ability and cognitive contents are limited. This paper proposes a new system called visual cognitive radio, which uses visual information to cognize the radio environment. Visual observation has a good predictive ability and abundant cognitive information, which enables the visual cognitive radio to react to the frequent change in the radio environment and make optimal configurations to the process of wireless communication. This paper presents a typical communication scene as an example to explain the advantages of visual cognitive radio, and also makes a preliminary analysis of its applications and challenges. Copyright © 2011 John Wiley & Sons, Ltd.

This paper focus on how to design the location of transmit antennas for Multiple-Input Multiple-Output (MIMO) space-time coding architecture with distributed transmit antennas. Considering the effects of path loss, correlated shadowing, multipath fading and white Gaussian noise, we first derive the approximate area averaged symbol error rate (AASER) in a circular cell at high signal-to-noise ratio (SNR) with M-PSK for orthogonal space-time block coded (OSTBC) systems with distributed transmit antennas. Then, we obtain the transmit antenna location by minimizing approximate AASER. Theoretical and numerical results show that the transmit antenna location depends on the cross-correlation coefficient and the standard deviation of shadow fading, and that the transmit antennas should be symmetrical with respect to the cell center.

To obtain the receive diversity of timing acquisition, the cooperative processing of multiple distributed received antennas is considered in this paper. In the flat Rayleigh channels, we assume two remote antennas at the base station for receiving the signal transmitted from the mobile station with a single antenna. By utilizing the coverage range of each remote antenna, a constraint is exploited to associate the time-delays from the mobile station to the two remote antennas. With this constraint, we propose a maximum likelihood-based cooperative timing acquisition method to improve the probability of correct acquisition for each remote antenna. Also, a lower bound of the probability of correct acquisition is derived to evaluate the performance of the proposed method. Compared to the independent timing acquisition, the analysis and simulations show that the probability of correct acquisition for each remote antenna can be improved by the cooperation of the two remote antennas.

This paper investigates the cooperative timing acquisition (CTA) for distributed antenna systems (DAS). As a preliminary study, the path loss and additive white Gaussian noise (AWGN) channel are considered without assuming multipath fading and shadowing. In the DAS coverage region, two remote antennas (RA) are assumed at base station (BS) for receiving the signal transmitted from the mobile station (MS) with a single antenna. Under the constraint that is derived from the known locations of the two RAs, a maximum-likelihood (ML)-based CTA method is proposed. By the analysis and simulation, it is shown that the probability of correct acquisition for each RA can be improved by the cooperation of the two RAs.

In this paper, an asynchronous layered space-time architecture is proposed to realize the spatial multiplexing over the Ricean multiple-input multiple-output (MIMO) channels. Based on reducing the correlation of spatial shaping pulses between the multiplexed streams, this approach can be used to solve the problem of detection in the ill-conditioned channel matrix. It is shown that at the receiver, the reduction of the correlation enables the requirement on the number of the receive antennas to be removed by the zero forcing (ZF) detector compared with the conventional synchronous layered space-time architecture. First, this paper presents how to exploit the correlation of spatial shaping pulses between the multiplexed streams. Then deriving the exact closed-form expression of error rate for the proposed scheme, this paper finds that the maximum possible diversity gain can be achieved based on the independent layered architecture by the ZF detector at the cost of limited multiplexing gain.

In this paper, an asynchronous layered space-time architecture is proposed to realize the spatial multiplexing over the Ricean multiple-input multiple-output (MIMO) channels. Based on reducing the correlation of spatial shaping pulses between the multiplexed streams, this approach can be used to solve the problem of detection in the ill-conditioned channel matrix. It is shown that at the receiver, the reduction of the correlation enables the requirement on the number of the receive antennas to be removed by the zero forcing (ZF) detector compared with the conventional synchronous layered space-time architecture. First, this paper presents how to exploit the correlation of spatial shaping pulses between the multiplexed streams. Then deriving the exact closed-form expression of error rate for the proposed scheme, this paper finds that the maximum possible diversity gain can be achieved based on the independent layered architecture by the ZF detector at the cost of limited multiplexing gain.

This paper focuses on a modified Vertical-Bell Labs Layered Space-Time (V-BLAST) system, where different delay offsets are intentionally applied to the spatially multiplexed data streams. We consider a multiuser MIMO uplink environment, where each user carries Mttransmit antennas, but is allocated only one signature sequence. Compared with the conventional V-BLAST, the modified scheme gains advantage mainly in two aspects: firstly, it is able to recover the transmitted information from only one receive antenna, whereas the conventional V- BLAST requires that Mrges Mt(Mris the number of receive antenna); secondly, the modified system could achieve the maximum possible diversity gain Mrof V-BLAST at the cost of limited multiplexing gain, whereas the diversity gain is only Mr- Mt+ 1 in conventional V-BLAST with ZF detector. The exact closed-form expression of bit error rate (BER) is derived for an Mttimes Mrmodified system with ZF strategy. And further explanation for the performance difference between the modified and conventional scheme is given from the spatial correlation point of view. Finally, we compare the performance of our proposed system with the conventional scheme by computer simulations which validate the analytical results.

In this paper, we address the problem of carrier frequency offset estimation for multiple-input multiple-output (MIMO) correlated fading channels. A maximum likelihood (ML) carrier frequency offset estimator based on the average likelihood function is proposed, which can make use of the channel correlation information by averaging the conditional likelihood function over all realizations of the channel and can exploit both transmit and receive spatial diversity as well. The Cramer-Rao bound (CRB) for the problem is also derived, and simulation results are given to illustrate the performance of the proposed estimator and compare it with the CRB. It is shown that the proposed estimator can provide satisfactory frequency offset estimates in MIMO correlated fading channels.

This paper proposed a modified vertical-Bell Labs layered space-time (V-BLAST) system, where different delay offsets are intentionally applied to the spatially multiplexed data streams in order to improve bit error rate (BER) performance. The proposed system can use only one receive antenna to recover the transmitted information by zero forcing (ZF) detection, whereas the conventional synchronous V-BLAST system requires that Mrges Mt(where Mtand Mrare the number of transmit and receive antennas, respectively). Compared with the conventional synchronous V-BLAST system, the modified system can achieve Mrth-order diversity using ZF detection. Although delay offsets in the system introduce an attenuation factor which decreases the instantaneous signal-to-noise ratio (SNR), the BER performance of the modified system with proper system parameters demonstrates a performance improvement over the conventional synchronous V-BLAST system.