Ultrasonic index modulation can effectively improve the spectral efficiency and energy efficiency of intra-body communication systems. This letter proposes a codeword-based auto-correlation (C-AC) receiver for ultrasonic code-indexed modulation (UCIM) and ultrasonic time-code-indexed modulation (UTCIM) transmitters, employing Hadamard codewords. The core idea is to utilize the auto-correlation properties of the Hadamard codewords to demodulate the code index bits, where the demodulation of each code index bit is based on the sum of the dot products of every two sub-sequences, dispensing with precise synchronization and channel state information. The theoretical bit error rate (BER) is derived and verified by extensive Monte Carlo simulations over intra-body fading channels, revealing that the proposed non-coherent low-complexity C-AC receiver can achieve lower BER compared with the conventional transmitted-reference receiver for both the UCIM and UTCIM transmitters.

Ultrasonic intra-body communication (IBC) demands high data rates and high reliability to achieve revolutionary medical and healthcare applications. This paper proposes an ultrasonic multidimensional index modulation (U-MIM) technique for IBCs. The core idea is to utilize ultrasonic pulses to transmit modulation bits whilst jointly utilizing indices in time and code domains to transmit index bits without adding extra pulses, significantly improving the data rate. Then, a maximum likelihood (ML) receiver is investigated to reveal the optimal bit-error rate (BER) performance. To address the high computational complexity of the ML receiver, a low-complexity ML (L-ML) receiver is proposed by employing maximum-minimum selection and joint estimation of index bits and modulation bits to reduce the number of template signals. Furthermore, this paper analyzes the theoretical BER, spectral efficiency (SE), and computational complexity of the proposed L-ML receiver considering the proposed U-MIM and the existing ultrasonic index modulation (UsIM) and UsIM with spread spectrum (UsIM-SS) systems. Theoretical and simulation results demonstrate that U-MIM achieves higher SE than UsIM-SS and lower BER than UsIM. Moreover, U-MIM with the L-ML receiver can attain the same BER as UsIM-SS but with significantly reduced complexity. Compared with traditional ML receivers, the L-ML receiver can reduce the complexity by up to twelve orders of magnitude for both U-MIM and UsIM-SS, while maintaining equivalent BER performance. The joint use of U-MIM and L-ML can provide a promising solution for versatile IBC applications requiring high data rate, high reliability, and low complexity.

This paper investigates the physical layer security (PLS) issue of unmanned aerial vehicle (UAV) aided multiple-input multiple-output non-orthogonal multiple access (MIMO-NOMA) networks with randomly distributed passive eavesdroppers (Eves). Considering Nakagami-m fading, we propose a novel secure communication protocol that integrates transmit antenna selection (TAS) and maximum ratio combining (MRC) diversity technology. Specifically, to tackle the challenges of low spectrum efficiency and PLS performance, we propose two TAS solutions: TAS-max UN and TAS-max UF, the first one aims to enhance the performance of UN and the other one focuses on UF. To mitigate the impact of passive eavesdropping, a secure protected zone is established around the UAV to limit the Eve’s ability. Accordingly, we derive the closed-form expressions for the ergodic secrecy rate to evaluate the impact of spatial randomness. Then, the accuracy of the derived expressions is verified through Monte-Carlo simulations.To further support the theoretical analysis, asymptotic expressions under the high-SNR regime are derived, which offer valuable insights into secrecy rate trends and model convergence. Moreover, we propose a three-dimensional UAV deployment optimization framework that adopts a hybrid approach combining grid-based evaluation and Genetic Algorithm refinement, which improves ESR performance while significantly reducing computational complexity. In addition, a Simulated Annealing based power allocation scheme is introduced to optimize the power coefficient aF, achieving enhanced secrecy rate with improved search efficiency and adaptability. Extensive simulation results confirm that the proposed TAS/MRC framework, together with the secure protected zone, consistently outperforms conventional OMA and MRT schemes in terms of secrecy rate and robustness. The impact of key system parameters, including power allocation, UAV altitude, antenna configuration, and Eve density, is also thoroughly analyzed.

In this work, we investigate the physical layer security (PLS) of ambient backscatter communication non-orthogonal multiple access (AmBC-NOMA) networks where non-colluding eavesdroppers (Eves) are randomly distributed. In the proposed system, a base station (BS) transmits a superimposed signal to a typical NOMA user pair, while a backscatter device (BD) simultaneously transmits its unique signal by reflecting and modulating the BS’s signal. Meanwhile, Eves passively attempt to wiretap the ongoing transmissions. Notably, the number and locations of Eves are unknown, posing a substantial security threat to the system. To address this challenge, the BS injects artificial noise (AN) to mislead the Eves, and a protected zone is employed to create an Eve-exclusion area around the BS. Theoretical expressions for outage probability (OP) and intercept probability (IP) are provided to evaluate the system’s reliability-security trade-off. Asymptotic behavior at high signal-to-noise ratio (SNR) is further explored, including the derivation of diversity orders for the OP. Numerical results validate the analytical findings through extensive simulations, demonstrating that both the AN injection and protected zone can effectively enhance PLS. Furthermore, analysis and insights of different key parameters, including transmit SNR, reflection efficiency at the BD, power allocation coefficient, power fraction allocated to desired signal, Eve-exclusion area radius, Eve distribution density, and backscattered AN cancellation efficiency, on OP and IP are also provided.

To cope with the explosive traffic growth expected in next-generation wireless networks, it is necessary to design next-generation multiple access techniques that can provide higher spectral efficiency as well as larger-scale connectivity. As a promising candidate, power-domain non-orthogonal multiple access (NOMA) has been widely studied. In conventional power-domain NOMA, multiple users are multiplexed in the same time and frequency band with differentpresetpower levels, which, however, may limit the spectral efficiency under practical finite alphabet inputs. Inspired by the concept of spatial modulation, we propose to solve this problem by encoding extra information bits into the power levels, and exploiting different signal constellations to help the receiver distinguish between them. To convey this idea, termed power selection (PS)-NOMA, clearly, we consider a simple downlink two-user NOMA system with finite input constellations. Assuming maximum-likelihood detection, we derive closed-form approximate bit error rate (BER) expressions for both users. Moreover, the two-user achievable rate region is also characterized. Simulation results verify the analysis and show that the proposed PS-NOMA can outperform conventional NOMA in terms of BER and achievable rate.

As a typical usage scenario for the next-generation mobile communication network, massive Internet of Things (mIoT) is requested to provide high machine-type communication device (MTCD) density service. Nonorthogonal multiple access (NOMA) and mobile-edge computing (MEC) can further enhance the performance of mIoT. Furthermore, to cope with the energy consumption constraint of MTCD, energy harvesting (EH) can be leveraged. In this article, considering the social trusts of MTCDs, we propose an MEC offloading scheme for cellular Internet of Things networks with massive NOMA-aided EH MTCD and several road side units with edge servers randomly distributed in a macrocell. We aim to maximize the total sum rate of the network by jointly considering the processing mode selection, device clustering, subchannel allocation, and power allocation while satisfying the power, energy, latency, and quality of service requirements. To this end, we prove the NP-hardness of the considered optimization problem and decompose it into three subproblems, which can be solved by an iterative algorithm. Numerical results demonstrate the superior performance of the proposed scheme.

Nowadays, intelligent transportation industry is becoming a hot spot in Internet of vehicles (IoV). However, owing to the existence of numerous intelligent terminals, communication security becomes a pressing problem. On the other hand, pervasive edge computing (PEC), as a pivotal technology, can significantly improve the performance of the system compared to the traditional cloud computing. In this article, we propose a nonorthogonal multiple access (NOMA)-based PEC power allocation framework in IoV, aiming at minimizing the system latency in the presence of eavesdroppers. Besides, queuing models, imperfect channel state information, and vehicles' speeds are all considered. Since the formulated problem is complicated, we consider its lower bound and derive the suboptimal closed-form expressions of the power allocation coefficients. Furthermore, a Frank-and-Wold algorithm is proposed to achieve the optimum total power. Simulation results illustrate the superior performance of the proposed NOMA scheme.

In this article, we propose a downlink non-orthogonal multiple access (NOMA) based coordinated direct and relay system with one cell-center user and multiple cell-edge users, where a decode-and-forward (DF) relay bridges the connection between the base station and the cell-edge users. Both full-duplex (FD) and half-duplex (HD) protocols are considered for the relay. We assume that the performance of the cell-edge users is subjected to the relay, and the cancellation of the mutual interference between the relay and cell-center user is imperfect. Both the exact analytical expression of outage probability and an approximate expression of the ergodic sum rate at high signal-to-noise ratio (SNR) are derived. Numerical results demonstrate that: 1) the FD relaying NOMA system outperforms the HD relaying NOMA system at low SNR, but the situation is exactly the opposite at high SNR; 2) the mutual interference can cause a larger performance gap than the self-interference at the relay; 3) the power allocation coefficients for the cell-center user and relay can affect the performance more significantly than those for cell-edge users.11This article was presented in part at the IEEE International Workshop on Signal Processing Advances in Wireless Communications 2019 [1].

In this paper, we propose a coordinated direct and relay transmission scheme based on non-orthogonal multiple access (NOMA), where the relay and cell- center user operate in full-duplex mode to help enhance the quality of service of the cell-edge user as well as ensure the secure transmission. In the proposed system, the base station directly serves the cell-center user and relay while communicating with the cell-edge user through the relay and cell-center user. Taking advantage of NOMA, the cell-center user and relay are able to cancel the mutual interference between themselves. Moreover, they can also cooperatively apply the artificial noise scheme to interfere any potential eavesdropper without impairing the legitimate cell-edge user. Exact and closed-form expressions for the outage probability, achievable rate, and achievable secure rate are derived. Numerical results prove that the proposed system outperforms the existing counterpart in the low signal-to-noise ratio region, and ensures secure communications with appropriate power allocation.