Physical Communication最新文献

A resource allocation method is proposed to maximize the energy efficiency for covert communication in downlink NOMA systems via reradiating on antennas. Firstly, the optimization problem of maximizing the effective covert energy efficiency of the covert user is constructed under the condition of satisfying the outage probability of the public user and the error detection probability of the warden. Then, the optimization problem is simplified, and the solution of the optimization problem is given when the power allocation coefficient of NOMA is known. Finally, the optimal solution is obtained by traversing the power allocation coefficient of NOMA and solving the optimization problem for each power allocation coefficient. The simulation results show that with the increase of the total power, the energy efficiency first increases and then tends to a constant, that is, there is an optimal value of the power allocation coefficient, which maximizes the energy efficiency.

Visible Light Communication (VLC) has emerged as a promising alternative for indoor and vehicular wireless communication, offering several advantages over traditional radio frequency (RF) technology. With the adoption of optical-orthogonal frequency division multiplexing (O-OFDM) schemes, visible light communication (VLC) has become more robust and adaptable in indoor, outdoor, vehicular, and underwear communications. Recently, an orthogonal time frequency space (OTFS) modulation technique has evolved with better performance than OFDM. The recent finding in the context of VLC shows that the OTFS technique shows remarkable advantages over conventional OFDM techniques except for the modem design complexity. This work introduces a low-complexity direct current-biased optical OTFS (DCO-OTFS) modulation based on OFDM. This paper evaluates the proposed system’s performance through simulations, providing evidence of its bit-error-rate (BER), peak-to-average power ratio (PAPR), and complexity behavior. Comparative assessments against the DCO-OFDM system are presented to understand the advantages of the low-complex DCO-OTFS system. The findings reveal that the proposed system not only provides low computational complexity in modem design but also maintains superior error performance, with a notable 10 dB signal-to-noise ratio (SNR) gain over DCO-OFDM along with a superior PAPR, making it a commendable choice for VLC applications.

Combining reconfigurable intelligent surface (RIS) and unmanned aerial vehicle (UAV) provides ubiquitous connectivity for 6G air–ground communications, effectively enhancing coverage. However, due to the ”multiplicative fading” effect, passive RIS can only offer weak capacity gain. In addition, the mobility of UAVs may lead to imperfect channel state information (CSI), making it difficult to perform accurate beamforming. To address these issues, this paper adopts active RIS to actively amplify the reflected signals to overcome the high path loss caused by ”multiplicative fading”. To adapt to the randomness of channel changes, this paper employs the soft actor–critic (SAC) algorithm, which is based on the maximum entropy strategy. This approach jointly optimizes the precoding of the base station (BS) and the beamforming of the aerial RIS (ARIS), aiming to maximize the multi-user transmission rate. Simulation results show that when active ARIS is employed, the proposed algorithm achieves similar sum-rate results in imperfect CSI and perfect CSI scenarios and realizes 71% and 74% performance improvement compared to the traditional passive RIS, respectively. Moreover, the sum-rate remains stable within a certain range when the UAV hovers at any position between the BS and the user.

Orthogonal time frequency space (OTFS) system offers a high data rate and effectively exploits full diversity compared to orthogonal frequency division multiplexing (OFDM), achieving a seamless trade-off between data rate and processing gain. However, detecting OTFS signals is challenging due to the complex conversion between delay-Doppler (DD) and time domains. In this article, we propose a stationary iteration-based approximate inversion (SIAI) technique for low-complexity detection in uplink OTFS systems.The proposed SIAI detection technique features square-order computational complexity and delivers performance close to that of a linear minimum mean square error (LMMSE) detector. Simulation results demonstrate that the SIAI technique outperforms several state-of-the-art detection methods in terms of both error performance and computational complexity. Additionally, the robustness of the SIAI technique is validated in scenarios with imperfect channel state information at the receiver.

Device-to-device communication (D2D-C) is one of the breakthrough technologies of the fifth-generation (5G) network that has attracted researchers working in data-regroup applications. It offers great advantages, such as minimum delay, base station load balancing, improved spectral efficiency, and many more. Despite these benefits, efficient resource allocation is one of the major concerns. Existing state-of-the-art solutions are based on game theory, graph theory, etc. The one-to-one Maximum matching scheme offers the best solution in graph theory and has not been explored to its potential. Also, the number of eavesdroppers in the system can affect the existing resource allocation schemes and degrade the system’s performance. This can be taken care of by implementing a D2D-C user access control mechanism, which restricts D2D-C users from having eavesdropping intentions. So, considering these issues, we propose a Fisher Jenks natural break (FJNB) optimization and kernel density estimation clustering-based resource allocation scheme with a D2D-C user access control mechanism. It improves the overall system’s total sum rate and computation time performance.

In Cognitive Radio Network (CRN), secondary users (SUs) opportunistically access the channel licensed by the Primary User (PU). Recently there has been growing interest in cooperative cognitive communication, spectrum leasing, and spectrum sharing techniques. These techniques have potential to substantially improve the system utilization of CRN as a whole. In this paper, we develop an elaborate and accurate queueing model for the cooperative channel access mechanisms in CRN. Using the proposed model we derive the queue length distribution of PU, as well as SU and the mean sojourn time of PU, as well as SU. We study the effect of mean arrival time of PU, mean arrival and service time of SU on the derived performance metrics. Through extensive simulation we validate the analytical model. Our analysis brings out the trade-off between the throughput of SU and transmission delay of the PU. Our results offer insight to the design engineer to optimally fix the system parameters of the CRN.

In this paper, we propose a novel energy-efficient resource optimization scheme in non-orthogonal multiple access (NOMA) networks with simultaneous wireless information and power transfer (SWIPT). In this system, we consider practical imperfect channel information that accounts for random channel delays and channel estimation errors. Additionally, the small cell users (SCUs) in a heterogeneous network (HetNet) can harvest energy from the small base station (SBS) signal by energy harvesting. Based on the non-linear energy harvesting (NEH) model, the problem of maximizing energy efficiency with imperfect channel state information (CSI) is formulated. Since the formulated problem is a probabilistic mixed non-convex optimization problem, a two-stage algorithm is developed to jointly optimize sub-channel matching and power allocation. In particular, the problem is first transformed into a non-probabilistic problem through the relaxation method. For the sub-channels matching problem, a low-complexity many-to-many matching algorithm is designed to achieve dynamic matching based on the preference lists. For the power allocation problem, the closed-form transmission power of SCUs can be derived by the Dinkelbach method and Lagrangian dual approach. Simulation results show that the proposed scheme can achieve higher energy efficiency than existing linear energy harvesting (LEH)-NOMA and NEH-OFDMA schemes, with improvements of $37.99\%$ and $84.69\%$, respectively.

In this article, we examine the achievable secrecy rate of a cooperative network including one source, one destination, and one untrusted amplify-and-forward relay, where randomly located adversary jammers attempt to disrupt the wireless communications. The relay has multiple antennas, while the other nodes have a single antenna. The adversary jammers are placed following a Poisson Point Process (PPP) and aim to interfere with the untrusted relay's channel. The relay employs maximal-ratio combining (MRC) to mitigate the jammers' impact and retransmits the signal to the destination using maximum ratio transmission (MRT). To prevent the relay from capturing the message, the destination injects pre-known artificial noise, known as destination-assisted cooperative jamming. We present a closed-form solution for the Ergodic secrecy rate (ESR) of the system with Rayleigh fading channels. An optimization problem for maximizing the secrecy rate is also designed, resulting in a closed-form formula for optimal power allocation (OPA) between the source and destination.

Phase distortion is one of the performance deteriorating factors in Frequency Modulated Continuous Wave (FMCW) radar systems like Radar Altimeter (RA). This signal distortion, mainly introduced by the frequency synthesis units, affects the accuracy of altitude measurements in RA. Thus, the estimation and subsequent cancellation of phase distortion in real time are indispensable for improved system performance. In this work, we have developed an algorithm for estimating and mitigating phase errors. The phase distortions on the beat signal are modeled as the sum of systematic phase errors and random phase fluctuations. Phase error is estimated from the intermediate frequency signal along the reference path by adopting quadrature demodulation and Chirp Z transform (CZT) based frequency estimation technique. With the help of a suitable cancellation filter, the effect of phase error is reduced to a significant level and the efficiency of the scheme is demonstrated using simulation results.

Carrier frequency synchronization between the transmitter and receiver is challenging due to oscillator inaccuracies and Doppler shifts, which lead to inter-carrier interference. To address this synchronization error, i.e., carrier frequency offset (CFO), we introduce a novel method for estimating the CFO in reconfigurable intelligent surfaces (RIS)-assisted orthogonal frequency division multiplexing (OFDM) systems. Additionally, we present a method to optimize reflection coefficients, as adjusting the reflection coefficient of RIS can effectively manipulate signal propagation and amplitude, thereby enhancing system performance. In the proposed method, a randomized set of reflection coefficients is applied for the first OFDM symbol, and CFO is estimated using the oversampling method within that symbol over the Rayleigh fading environment. A deterministic approach is adopted to reduce grid search complexity. Simulation results demonstrate the superior performance of the proposed CFO estimation method, particularly in high signal-to-noise ratio scenarios, compared to the existing Zadoff Chu sequence-based estimator in terms of mean square error (MSE). Subsequently, the CFO is compensated in the second OFDM symbol using the estimated CFO, and the reflection coefficient is optimized by identifying the channel tap associated with the maximum channel impulse gain technique. The optimized reflection coefficient enhances system performance in terms of bit error rate (BER) for RIS-assisted OFDM systems.