Providing quality communications under adversarial electronic attacks, e.g., broadband jamming attacks, is a challenging task. Unlike state-of-the-art approaches which treat jamming signals as destructive interference, this paper presents a novel active anti-jamming (AAJ) scheme for a jammed channel to enhance the communication quality between a transmitter node (TN) and receiver node (RN), where the TN actively exploits the jamming signal as a carrier to send messages. Specifically, the TN is equipped with a programmable-gain amplifier, which is capable of re-modulating the jamming signals for jamming modulation. Considering four typical jamming types, we derive both the bit error rates (BER) and the corresponding optimal detection thresholds of the AAJ scheme. The asymptotic performances of the AAJ scheme are discussed under the high jamming-to-noise ratio (JNR) and sampling rate cases. Our analysis shows that there exists a BER floor for sufficiently large JNR. Simulation results indicate that the proposed AAJ scheme allows the TN to communicate with the RN reliably even under extremely strong and/or broadband jamming. Additionally, we investigate the channel capacity of the proposed AAJ scheme and show that the channel capacity of the AAJ scheme outperforms that of the direct transmission when the JNR is relatively high.

This article studies the energy efficiency (EE) of a three-node decode-and-forward (DF) relay channel, where the relay operates in full-duplex (FD) or half-duplex (HD) mode. In particular, for the FD mode, both the residual self-interference (RSI) and the extra circuit power consumption introduced by the self-interference cancellation (SIC) at the relay node are considered and modeled as linear functions over the relay transmission power. Under this setup, the EE maximization problems for the considered FD and HD modes subject to the total transmission power and spectral efficiency (SE) constraints are formulated as max–min problems, whose optimal power allocation is computed by fractional programming. Next, the EEs for the FD, HD, and direct transmission (DT) modes are compared under different channel conditions, and a hybrid mode selection scheme, i.e., selecting the one with the maximum EE among the above three modes, is proposed. Besides, we also prove that both the EE and the SE monotonically increase with the total transmission power under a given condition. Finally, numerical results show that the maximum EE of the FD relaying exceeds those of the HD and DT ones when the required SE is relatively high. Furthermore, the proposed hybrid mode selection scheme can provide up to 192.4% higher EE compared with the DT mode.

Non-orthogonal multiple access (NOMA) and millimeter wave (mmWave) are two key enabling technologies for the fifth-generation (5G) mobile networks and beyond. In this paper, we consider uplink communications with a hybrid beamforming structure and focus on improving the spectral efficiency (SE) and energy efficiency (EE) of mmWave multiple-input multiple-output (MIMO)-NOMA systems with enhanced user grouping and power allocation. It is noted that the optimization of the SE/EE is a challenging task due to the non-linear programming nature of the corresponding problem involving user grouping, beam selection, and power allocation. Our idea is to decompose the overall optimization problem into a mixed integer problem comprised of user grouping and beam selection only, followed by a continuous problem involving power allocation and digital beamforming design. Exploiting the directionality property of mmWave channels, we first propose a novel initial agglomerative nesting (AGNES) based user grouping algorithm by taking advantage of the channel correlations. To avoid the prohibitively high complexity of the brute-force search approach and to address the overlapping beam problem, we propose two suboptimal low-complexity user grouping and beam selection schemes, the two-stage direct AGNES (D-AGNES) scheme and the joint successive AGNES (S-AGNES) scheme. We also introduce the quadratic transform (QT) to recast the non-convex power allocation optimization problem into a convex one subject to a minimum required data rate of each user. The continuous problem is solved by iteratively optimizing the power and the digital beamforming. Extensive simulation results have shown that our proposed mmWave-NOMA design outperforms the conventional orthogonal multiple access (OMA) scenario and the state-of-art NOMA schemes.

Non-orthogonal multiple access (NOMA) and millimeter wave (mmWave) transmission are two key enabling technologies for the fifth-generation (5G) mobile networks and beyond. In this paper, we consider mmWave NOMA systems with max-min fairness constraints. On the one hand, existing beamforming designs aiming at maximizing the spectral efficiency are unsuitable for the NOMA systems with fairness considered in this paper. On the other hand, previous work on mmWave NOMA mostly depends on full knowledge of channel state information (CSI) which is extremely difficult to obtain accurately in mmWave communication systems. To address this problem, we propose a heuristic hybrid beamforming design based on a statistical CSI (SCSI) user grouping strategy. An analog beamforming scheme is first proposed to mitigate inter-cluster interference in a first stage. Then two digital beamforming designs are proposed to further suppress the interference based on SCSI. One is the widely used zero forcing approach and the other is derived from the signal-to-leakage-plus-noise ratio metric extended from orthogonal multiple access systems. The effective gains fed back from the users are used for power allocation. We introduce the quadratic transform method and bisection approach to reformulate this complex problem so as to render it solvable. Simulation results show that our proposed algorithms outperform existing algorithms in term of user fairness.

Phase Modulation on the Hypersphere (PMH) is a power efficient modulation scheme for the load-modulated multiple-input multiple-output (MIMO) transmitters with central power amplifiers (CPA). However, it is difficult to obtain the precise channel state information (CSI), and the traditional optimal maximum likelihood (ML) detection scheme incurs high complexity which increases exponentially with the number of transmitting antennas and the number of bits carried per antenna in the PMH modulation. To detect the PMH signals without knowing the prior CSI, we first propose a signal detection scheme, termed as the hypersphere clustering scheme based on the expectation maximization (EM) algorithm with maximum likelihood detection (HEM-ML). By leveraging machine learning, the proposed detection scheme can accurately obtain information of the channel from a few of the received symbols with little resource cost and achieve comparable detection results as that of the optimal ML detector. To further reduce the computational complexity in the ML detection in HEM-ML, we also propose the second signal detection scheme, termed as the hypersphere clustering scheme based on the EM algorithm with KD-tree detection (HEM-KD). The CSI obtained from the EM algorithm is used to build a spatial KD-tree receiver codebook and the signal detection problem can be transformed into a nearest neighbor search (NNS) problem. The detection complexity of HEM-KD is significantly reduced without any detection performance loss as compared to HEM-ML. Extensive simulation results verify the effectiveness of our proposed detection schemes.

In this paper, we propose two novel user grouping strategies for the non-orthogonal multiple access (NOMA) technique based hybrid millimeter wave (mmWave) systems. In NOMA technology, user grouping is the first step to reduce the inter-group interference and will impact the design of hybrid beamforming. In contrast to some previous user grouping strategies that allocate equal or close number of users in each group, our proposed user grouping algorithms allow different number of users per group, which enables the users to get better beam gain. We propose two user grouping algorithms, the strong user dependent (SUD) user grouping algorithm and the Agglomerative Nesting (AGNES) clustering user grouping algorithm, in which a user is grouped to the beam with the largest beam gain. In the first algorithm, the transmitter selects the strong users as the beam centroids and allocates the rest weak users to the corresponding beam groups with the largest beam gain. The second algorithm directly forms groups based on the AGNES clustering algorithm without the predefined beam centroids. Simulation results demonstrate the effectiveness of our proposed user grouping strategies.

In this paper, we propose a new Laurent decomposition based low complexity detection scheme for M-ary continuous phase modulation (CPM) signals. By introducing per survivor processing (PSP) algorithm in the demodulation of Laurent-based CPM signals, the proposed scheme simplifies the trellis structure, which reduces the detection complexity of CPM signals. Numerical results reveal that the proposed algorithm can significantly reduce the detection complexity at the price of slight bit-error-rate (BER) disgrace as compared with the conventional detection schemes.

In this study, the authors investigate the diversity‐multiplexing tradeoff (DMT) of a wireless system in which user equipments (UEs) are equipped with only one antenna while the destination has multiple antennas. Each UE is assisted by M half‐duplex cooperative users serving as relay nodes. They combine the opportunistic relaying scheme and the cooperative spatial multiplexing (C‐SM) scheme to produce an opportunistic C‐SM (OC‐SM) scheme. The fraction of time allocated to the source–relay link and the relay–destination link is treated as a design parameter to minimise the outage probability of the system. At high signal‐to‐noise ratio regime, the optimal time allocated factor is obtained in a closed‐form expression through DMT analysis. Based on the DMT result of the proposed OC‐SM scheme, an adaptive OC‐SM (AOC‐SM) approach in which the system can switch between different C‐SM models adaptively based on the rate transmitted is proposed. To reduce the complexity of the relay nodes, an AOC‐SM scheme with relay selection (SAOC‐SM) in which a constant number of relay nodes is selected from the set of relay nodes that have successfully decoded the source message is investigated. The DMT results of the SAOC‐SM scheme and the AOC‐SM scheme are obtained and compared.

In this paper, we investigated the cooperative sensing in a heterogeneous cognitive radio (CR) network scenario, where each secondary user (SU) may be equipped with different number of receive antennas and have different signal processing capacity, e.g., sampling rate. By considering the discrepancy in sensing reliability of different SUs, we extended the existing researches to propose an optimal cooperative sensing (OCS) scheme based on normalized energy detection (NED) by considering reporting errors, i.e., the fusion center (FC) may receive erroneous results from some SUs due to channel fading. It is derived that the proposed OCS scheme is the linear combination of modified local detection statistics. The optimal combining coefficient is just a function of the numbers of the antennas and the received signal samples, the signal to noise ratio (SNR) and the variance of the reporting errors at each SU. Meanwhile, the performance of the proposed OCS scheme, the well-known equal gain combination (EGC) method and the maximum normalized energy (MNE) detector with reporting errors is analytically derived, respectively. Furthermore, to avoid the prior information of each SU's SNR and the variance of the reporting errors, and simplify the decision-making as well as threshold setting, a sub-optimal cooperative spectrum sensing (SOCS) is proposed. Numerical results show that the proposed OCS and SOCS schemes all perform much better than the existing EGC and MNE methods.

A low‐complexity channel estimation algorithm is proposed to improve performance of two high frequency standardised waveforms: MIL‐STD‐188‐110B Appendix C standard and STANAG 4285. In the proposed estimation algorithm, the channel impulse response is modelled as a linear‐time function within a frame and the least square criterion is adopted. With the estimated CIRs, the authors study and compare bit‐error‐rate (BER) performance and complexity of three turbo equalisation (TEQ) schemes, including the minimum mean‐square error (MMSE), the MMSE with decision feedback equalisation (DFE) (MMSE‐DFE) and the soft‐feedback interference‐cancellation (SFIC) turbo receivers. The authors then propose a hybrid TEQ scheme that uses the MMSE‐DFE equaliser for the first iteration and the SFIC equaliser for the rest. Simulation results show that the proposed channel estimation algorithm performs well and the TEQ schemes realise a significant performance improvement as compared with a non‐iterative receiver. Moreover, the proposed hybrid TEQ scheme achieves the best trade‐off between BER and complexity for the MIL‐STD‐188‐110B Appendix C standard, whereas the SFIC‐TEQ scheme is optimal for the STANAG 4285 standard.

Iterative equalization techniques are effective in mitigating inter-symbol interference (ISI). In this paper, we first study and compare three low-complexity adaptive-channel-estimation-based turbo equalization (ACE-based TEQ) algorithms over high-frequency (HF) waveform channels based on the STANAG 4285 standard. These reduced-complexity equalization algorithms include the minimum mean-square error (MMSE), the MMSE decision feedback equalization (MMSE-DFE) and the soft feedback interference cancellation (SFIC). To address the issue of the HF channel time-variation, an explicit adaptive channel estimator is used to determine the unknown channel coefficients. In addition, we have also proposed a hybrid ACE TEQ algorithm. Numerical results show that the ACE-SFIC-based TEQ scheme is the best in terms of trade-off between bit-error rate (BER) and implementation complexity for the STANAG 4285 military waveforms.

In this paper, we propose an orthogonal space-time block coding (STBC) for continuous phase modulation (CPM) over frequency-selective channels. The new method maintains the constant envelope and the phase continuity of the CPM waveforms by inserting three blocks of data-dependent symbols in addition to a cyclic prefix (CP). At the transmitter, the data blocks are precoded and transmitted in two antennas. And at the receiver, by applying the Laurent decomposition of CPM signals, only linear processing is needed to decode the CPM signal and the decoding complexity is much lower than traditional schemes. Simulation results corroborate that the proposed scheme achieves near-optimum error performances.