Automotive Millimeter-wave (mmWave) radar is becoming an essential modality for autonomous vehicles to enable all-weather perception, especially when LiDAR and camera fail in foggy, rainy, or snowy conditions. It is expected that the mutual interference among multiple radars becomes a critical issue in dense traffic scenarios, which can severely degrade the radar performance and lead to accidents. Despite extensive interference mitigation techniques, none can meet the less valid signal distortion while high robustness requirements for automotive radar perception in dense traffic scenarios. To overcome this predicament, we propose mmMic, a novel multiple mutual interference mitigation system that can accurately separate interference and recover valid signals to maintain the reliability of the radar measurements. The key insight is to design an interference estimator that can accurately localize the interference signal according to its linear frequency modulation features in the time-frequency (TF) domain. In addition, mmMic also fully exploits undisturbed valid signal information within an extended time-frequency domain to reconstruct the damaged signal. Our experiments on a real testbed show that mmMic can improve SINR to interference-free levels from multiple radars, achieving an average SINR improvement of 17% compared to the best-performing baseline.

In this paper, we investigate the frequency domain phase noise estimation in millimeter wave filter-bank multicarrier with offset quadrature amplitude (mmWave FBMC-OQAM) systems. In the frequency domain, the primary impairment introduced by phase noise is the common phase error (CPE), whose estimation performance is significantly affected by the inter-carrier interference (ICI) and inter-symbol interference (ISI) experienced by OQAM symbols. To address the above problem, we propose novel frequency domain phase noise estimation and pilot symbol design methods to mitigate the impact of ICI and ISI. Firstly, we quantify the interferences affecting each pilot symbol and design appropriate weights to mitigate the impact of ICI and ISI on the phase noise estimation. Then, through analysis, we observe that the ICI and ISI originate from the interaction between the imaginary intrinsic interferences and the phase noise terms. Accordingly, we propose a pilot symbol design method by eliminating the primary imaginary intrinsic interferences, thereby reducing the ICI and ISI. Furthermore, we analyze the mean squared error (MSE) lower bound and the computational complexity. Simulation results demonstrate that the proposed phase noise estimation method outperforms the traditional method and the proposed pilot structure further improves the accuracy of the phase noise estimation.

A new sidelobe suppression method is proposed for filter bank multi-carrier systems using offset quadrature amplitude modulation (FBMC-OQAM), where we jointly optimize the prototype filter and the symbol distribution to minimize the normalized stopband energy of FBMC-OQAM signals. Firstly, we investigate the relationship between both the signal sidelobe and the prototype filter as well as the symbol distribution by deriving the general power spectral density (PSD) expression of the FBMC-OQAM signals of each subcarrier. Then, we investigate the impact of the prototype filter and the symbol distribution on the symbol reconstruction by deriving the inter-symbol interference (ISI) and inter-carrier interference (ICI) expressions of FBMC-OQAM systems. Based on the PSD and ISI/ICI expressions derived, we formulate and solve the joint prototype filter and symbol distribution optimization problem. Our simulations demonstrated that the proposed joint prototype filter and symbol distribution optimization method achieves lower normalized stopband energy and better sidelobe suppression of the first sidelobe than the single-parameter based prototype filter optimization methods.

The Internet of Things (IoT) is envisioned to connect everything, spanning from terrestrial to nonterrestrial terminals, where reliable communication is expected to be allowed in both time-invariant and time-variant wireless channels. Since classic orthogonal frequency-division multiplexing (OFDM) modulation, which has been widely used in both the fourth-generation (4G) and the fifth-generation (5G) cellular systems, is sensitive to high Doppler effect, it is challenging to satisfy the ever-growing demands of future IoT. To circumvent this issue, the orthogonal time–frequency space (OTFS) scheme is proposed, which modulates the information bits in both the delay and the Doppler domains, and exhibits beneficial advantages in both static and high-mobility wireless channel scenarios. In this article, we present a comprehensive overview of OTFS for IoT, including the current transceiver design, the potential benefits, the challenge issues, as well as future design guidelines.

In this paper, we propose a symbol encryption method based on the complex-valued symbol multiplication and design the encryption symbol placement for offset quadrature amplitude modulation based filter bank multicarrier (OQAM/FBMC) systems. Specifically, we firstly analyze the encryption and decryption effects of complex-valued symbol multiplication on the original OQAM symbols. Then, by utilizing the intrinsic interference characteristics of prototype filters, a symbol encryption method and the placement design are proposed to simultaneously achieve the encryption of all OQAM symbols and low complexity decryption. Finally, the symbol error rate (SER), average signal transmit power, computational overhead and power spectral density (PSD) are analyzed. Simulation results are consistent with the theoretical analysis and demonstrate that the proposed method outperforms the traditional method based on imaginary-valued encryption symbol insertion in terms of the SERs of both legitimate and illegitimate receivers.

In this paper, we investigate the two-hop active relaying for dynamic magnetic induction based underwater wireless sensor networks (MI-UWSNs). Specifically, we firstly propose the two-hop active relaying schemes with unidirectional (UD) and tri-directional (TD) active relays, respectively, by considering the angular misalignment in practical underwater MI environments. Then, the statistical properties of the received signal-to-noise ratio (SNR) for the proposed two-hop UD and TD active relaying schemes are rigorously analyzed and the corresponding closed-form expressions of the probability density functions are derived according to the distribution of the angular misalignment. Based on the statistical SNRs, we develop the analytical expressions of the ergodic achievable rate and the average bit-to-error rate for the two-hop UD and TD active relaying schemes employing the amplify-and-forward and decode-and-forward strategies. Extensive simulation results validate the effectiveness of our theoretical analyses and demonstrate that the proposed two-hop TD active relaying scheme performs the best among all comparative schemes.

In this paper, we focus our attention on the mitigation of beam squint for reconfigurable intelligent surface (RIS) aided wideband millimeter wave (mmWave) communications. Due to the intrinsic passive property, the phase shifts of all elements in RIS should be the same for all frequencies. However, in the wideband scenario, beam squint induced distinct path phases require designing different phase shifts for different frequencies. The above irreconcilable contradiction will dramatically affect the system performance, considering the RIS usually consists of enormous elements and the bandwidth of wideband mmWave communications may be up to several GHz. Therefore, we propose some novel phase shift design schemes for mitigating the effect of beam squint for both line-of-sight (LoS) and non-Los (NLoS) scenarios. Specifically, for the LoS scenario, we firstly derive the optimal phase shift for each frequency and obtain the common phase shift by maximizing the upper bound of achievable rate. Then, for the NLoS scenario, a mean channel covariance matrix (MCCM) based scheme is proposed by fully exploiting the correlations between both the paths and the subcarriers. Our extensive numerical experiments confirm the effectiveness of the proposed phase shift design schemes.

In this paper, we propose a distribution line fitting (DLF) based channel estimation method for offset quadrature amplitude modulation based filter-bank multicarrier (OQAM/FBMC) systems. Specifically, we firstly analyze the distribution characteristics of demodulated OQAM symbols at the receiver, and derive an intrinsic interference free relation between the real and imaginary parts of demodulated OQAM symbols. Then, based on the above derivation, we design a pilot structure of arbitrary columns of pilot symbols without guard symbols between pilot symbols and data symbols. Finally, the distribution lines can be fitted by utilizing the distribution characteristics of demodulated OQAM symbols and the channel frequency response can be estimated. Simulation results demonstrate that the proposed method with one column of pilot symbols outperforms the interference approximation method (IAM) in high signal-to-noise (SNR) regions and outperforms the pair of pilots (POP) method in all SNR regions in terms of bit error ratio (BER), and the proposed method with two columns of pilot symbols outperforms both IAM and POP in all SNR regions in terms of both BER and complexity.

Hybrid Transmit Precoding (TPC) is one of the most compelling solutions for millimeter wave (mmWave) multiple-input multiple output (MIMO) systems. However, most attention has been focused on narrow-band scenarios. Hence, we dedicate our efforts to the design of hybrid TPC for wideband mmWave MIMO systems, where the beam squint dramatically affects the system performance. We firstly show that the channel matrices of the different subcarriers possess distinct subspaces in case of high bandwidths, hence traditional hybrid TPC schemes suffer from an eroded performance. Therefore, we propose novel hybrid TPC schemes exploiting the full channel state information (CSI), which project all frequencies to the central frequency and construct the common analog TPC matrix for all subcarriers. Moreover, we propose several low-complexity array-vector based hybrid TPC schemes. The high-complexity manifold optimization based hybrid TPC method and the fully digital TPC operating with and without considering beam squint are provided as benchmarks. Our extensive numerical simulations show that the proposed hybrid TPC schemes are capable of achieving similar performance to the excessive-complexity fully digital TPC, when the bandwidth tends to 0.5 GHz and always outperform the traditional hybrid TPC schemes.

In this paper, we propose a generalized space-time block coded spatial modulation (GSTBC-SM) scheme for open-loop massive multiple-input and multiple-output (MIMO) downlink communication systems. Specifically, we firstly partition the information bits into multiple groups with each group modulated by the spatial modulation (SM), where the SM symbols are invoked for orthogonal STBC (OSTBC) and quasi-orthogonal STBC (Q-OSTBC) structures. Then, message passing (MP) and block minimum mean square equalization (B-MMSE) detectors are designed for our GSTBC-SM systems, to achieve near-optimal performance with significantly reduced complexity in massive MIMO configurations. Finally, we derive the theoretical average bit error probability (ABEP) of the proposed scheme. The main contribution is that the propose scheme achieves high transmission rate and diversity gain even with small number of radio frequency (RF) chains at the transmitter. Simulation results verify the theoretical derivations and show that the proposed GSTBC-SM scheme provides near 20 dB gain over the conventional GSTBC scheme under massive MIMO configurations.

In this paper, we firstly derive two approximations of the achievable uplink rate with the perfect/imperfect channel state information (CSI) in cell-free massive multi-input multi-output (MIMO) systems, and all these approximations are not only in the simple, but also converge into the classical bounds achieved in conventional massive MIMO systems where the base-station (BS) antennas are co-located. It is worth noting that the obtained two approximations with perfect CSI could be regarded as the special cases of the obtained two approximations with imperfect CSI when the pilot sequence power becomes infinite, respectively. Moreover, the theory analysis shows that all obtained approximations with perfect/imperfect CSI have an asymptotic lower bound α/2 log2L thanks to the extra distance diversity offered by massively distributed antennas, where L is the number of BS antennas and the path-loss factor α > 2, except for the free space environment. Obviously, these results indicate that the cell-free massive MIMO system has huge potential of spectral efficiency than the conventional massive MIMO system with the asymptotically tight bound log2L.

Delay diversity (DD) is a low-complexity and flexible transmit diversity technology for coded offset quadrature amplitude modulation based filter bank multicarrier (OQAM/FBMC) transmission systems. However, the introduction of delay parameters in DD-OQAM/FBMC systems leads to increased channel frequency selectivity, which makes the channel estimation problem more complicated. In this paper, we analyze and develop preamble-based channel estimation methods in DD-OQAM/FBMC systems from the perspective of frequency-domain, transform-domain, and time-domain aspects. Besides, channel estimators that can obtain the minimum channel estimation mean square error under least squares (LS) and linear minimum mean square error (LMMSE) criteria are derived according to different levels of channel knowledge. Specifically, the proposed frequency-domain method estimates each channel frequency response individually based on a simplified frequency-domain channel model. The transform-domain method first estimates the equivalent single-input-single-output channel observed at the receiver assuming subchannel flatness, and then uses a window function in the time domain to separate the channels of different transmit antennas. The time-domain method is constructed based on an accurate time-domain channel model to directly estimate the channel impulse response without making any assumption about the subchannel flatness. A series of simulation experiments is conducted to verify the effectivity of these channel estimation methods for DD-OQAM/FBMC systems.

Generalized index modulation (GIM) which implicitly conveys information by the activated indices is a promising technique for next-generation wireless networks. Due to the prohibitive challenge of bit-to-index combination (IC) mapping optimization, conventional GIM system obtains the bit-to-IC mapping table randomly, which may suffer from some performance loss. To circumvent this issue, we propose a low-complexity graph theory assisted bit-to-IC gray coding for GIM systems by minimizing the average hamming distance (HD) between any two ICs having one different value. Specifically, we decompose and transform the optimization problem into two subproblems using the graph theory, i.e., 1) Select an IC set whose corresponding graph has the minimum degree; 2) Design a bit-to-IC mapping principle to minimize the weight of the selected graph. Low-complexity algorithms are developed to solve the subproblems with a significant reduced complexity. Both simulation and theoretical results are shown that the GIM systems with our proposed mapping table are capable of providing significant performance gains over the conventional counterparts without the need for any additional feedback-link and without extra computational complexity. It is also shown that the proposed bit-to-IC mapping table is straightforward for any GIM systems over generalized fading channels.

This paper focuses on the design of low complexity hybrid analog/digital precoding and diversity combining in millimeter wave (mmWave) Internet of Things (IoT) systems. First, by exploiting the sparseness property of the mmWave in the angular domain, we propose a spatial lobes division (SLD) to group the total paths of the mmWave channel into several spatial lobes (SLs), where the paths in each SLs form a low-rank subchannel. Second, based on the SLD operation, we propose a low complexity hybrid precoding scheme, named hybrid precoding based on SLD (HYP-SLD). Specifically, for each low-rank subchannel, we formulate the hybrid precoding design as a sparse reconstruction problem and separately maximizes the spectral efficiency. Finally, we further propose a maximum ratio combining-based diversity combining scheme, named HYP-SLD-MRC, to improve the bit error rate (BER) performance of mmWave IoT systems. Simulation results demonstrate that, the proposed HYP-SLD scheme significantly reduces the complexity of the classic orthogonal matching pursuit scheme. Moreover, the proposed HYP-SLD-MRC scheme achieves great improvement in BER performance compared with the fully digital precoding scheme.

In this paper, we propose two non-uniform quantization (NUQ) codebook-based hybrid precoding schemes for two main hybrid precoding implementations, i.e., the full-connected structure and the sub-connected structure, to reduce the feedback overhead in millimeter wave single user multiple-input multiple-output systems. Specifically, we firstly group the angles of the arrive/departures (AOAs/AODs) of the scattering paths into several spatial lobes by exploiting the sparseness property of the millimeter wave in the angular domain, which divides the total angular domain into effective spatial lobes' coverage angles and ineffective coverage angles. Then, we map the quantization bits non-uniformly to different coverage angles and construct NUQ codebooks, where high numbers of quantization bits are employed for the effective coverage angles to quantize AoAs/AoDs and zero quantization bit is employed for ineffective coverage angles. Finally, two low-complexity hybrid analog/digital precoding schemes are proposed, which utilize the NUQ codebooks. Simulation results demonstrate that the proposed two NUQ codebook-based hybrid precoding schemes achieve near-optimal spectral efficiencies and show the superiority in reducing the feedback overhead compared with the uniform quantization codebook-based works.

Hybrid precoding is essential for millimeter wave (mmWave) multiple-input multiple output (MIMO) systems due to its inherent advantage of a high gain, whilst alleviating the high cost of hardware. However, most of the existing literature considered either the narrowband or wideband single-user mmWave MIMO scenarios. Hence in this paper we focus our attention on the more challenging design of hybrid Transmit Precoding (TPC) for wideband multiuser mmWave MIMO systems by exploiting the long-term channel's covariance matrix and the angle of departure (AoD) information. Specifically, in the analog TPC designed, firstly, the analog TPC matrix having an infinite angular resolution is constructed based on the channel's covariance matrix. Then, we also propose a non-uniformly spaced quantization codebook based analog TPC having finite angular resolution. Furthermore, a phase compensation operation is carried out to alleviate the effect of beam squint. As for the design of the digital TPC, a two-stage scheme is proposed to cancel the inter-user interference and to attain multiplexing gains. We study the effects of various parameters on the achievable sum rate and demonstrate with the aid of our simulation results that the proposed hybrid TPC is capable of achieving a similar performance to the excessive-complexity fully digital TPC.

In this paper, a novel covariance-based channel feedback mechanism is investigated for frequency division duplexing (FDD) massive multi-input multi-output (MIMO) systems. The concept capitalizes on the notion of user statistical separability which was hinted in several prior works in the massive antenna regime but has not fully exploited so far. We propose a hybrid statistical-instantaneous feedback mechanism where the users are separated into two classes of feedback design based on their channel covariance. Under the hybrid framework, each user either operates on a statistical feedback mode or quantized instantaneous channel feedback mode. The key challenge lies in the design of a covariance-aware classification algorithm which can handle the complex mutual interactions among all users. The classification is derived from rate bound principles and a precoding method is also devised under the mixed statistical and instantaneous feedback model. Simulations are performed to validate our analytical results and illustrate the sum rate advantages of the proposed feedback scheme under a global feedback overhead constraint.

In this paper, we consider the uplink channel parameter estimation problem in the presence of pilot contamination for massive multiple-input-multiple-output (MIMO) systems. We propose a parallel factor (PARAFAC)-based estimation scheme, which exploits the low-rank property of massive MIMO channels caused by the finite scattering in a physical environment. Specifically, we first parameterize the channel in terms of three parameters, i.e., fading coefficients, directions of arrival (DOAs), and delays; thereby, the channel is characterized via three equivalent PARAFAC models. Then, the proposed PARAFAC-based scheme is developed, which jointly estimates these three channel parameters using an alternating least squares (ALS) algorithm. Therein, we certify the identifiability of the three channel parameters of the PARAFAC models to mitigate the pilot contamination and state the convergence of the ALS algorithm, which guarantees that the three channel parameters can be uniquely determined with the proposed scheme. Moreover, to further reduce the computational complexity, two advanced schemes are proposed by antenna selection and reducing the estimation frequency of DOAs and delays, respectively. Simulation results show that the proposed schemes can achieve both low computational complexities and close to optimal Cramer-Rao Bound performance.

In this survey paper, we focus on the offset quadrature amplitude modulation-based orthogonal frequency division multiplexing (OQAM-OFDM)-based wireless communications in future Internet of Things (IoT). First, we present the OQAM-OFDM-based wireless communication system model for IoT and show its superiority compared with OFDM. Then, we give a survey and analyze key technologies including the prototype filter design, peak to average power ratio reduction, tail reduction, channel estimation, and multiple-input multipleoutput combination in OQAM-OFDM-based wireless communication systems for IoT. Extensive simulations are conducted to verify the spectral superiority of OQAM-OFDM and compare the corresponding performances of OQAM-OFDM with OFDM in the above key technology areas.

In this paper, we propose a preamble optimization method for the frequency-division multiplexing (FDM)-structured preamble in multiple-input multiple-output systems employing offset quadrature amplitude modulation-based filter bank multicarrier. Specifically, we formulate an optimization problem based on the periodic preamble structure to minimize the mean square error (MSE) of the channel estimation. For two transmit antennas, we find the relationship between preambles and intrinsic interferences from neighboring symbols to achieve the minimum MSE, and derive the optimal closed-form solution. For more than two transmit antennas, we convert the original optimization problem into a quadratically constrained quadratic program and obtain the suboptimal solution by relaxing the nonconvex constraint. The simulation results demonstrate that, in terms of MSE and bit error rate performances, the proposed method outperforms the conventional FDM method at all signal-to-noise ratio (SNR) regimes and outperforms the interference approximation method-complex method at low-to-medium SNR regimes with lower preamble overhead.

Abstract In this paper, we propose an interference mitigation method to suppress the downlink interference in multi‐macrocell/femtocell networks, and analytically evaluate the interference mitigation and average rate performances. Specifically, the proposed interference mitigation method consists of three steps: frequency partitioning, cell partitioning, and sub‐band allocation. In the frequency partitioning step, the whole downlink frequency band is divided into nine non‐overlapping sub‐bands. In the cell partitioning step, each macrocell is divided into four macrocell regions and three femtocell regions for macrocells' and femtocells' communications, respectively. In the sub‐band allocation step, each macrocell or femtocell region is allocated a sub‐band to guarantee that any two neighboring macrocell/femtocell regions use different sub‐bands. Conducted simulation results show that the proposed method is effective in mitigating the downlink interference and improving the average downlink per‐channel rate in multi‐macrocell/femtocell networks. In summary, the major contribution of the proposed interference mitigation method is that the downlink interference can be mitigated without cooperation between macrocells and femtocells, while the full frequency utilization of the macrocell is achieved. Copyright © 2016 John Wiley & Sons, Ltd.

Offset quadrature amplitude modulation-based orthogonal frequency division multiplexing (OFDM) systems cannot be directly combined with the Alamouti code because of the intrinsic imaginary interference. In this paper, we propose a block-wise space-frequency block coding (SFBC) scheme and a block-wise space-time block coding (STBC) scheme for offset quadrature amplitude modulation-based OFDM systems, which achieve bit error rate performances that are close to OFDM systems. The proposed schemes satisfy the orthogonality condition of the Alamouti code in the complex field with guard band/intervals. To improve the spectral efficiency of the block-wise SFBC scheme, we also consider the case without the guard band. It is observed that only the two innermost subcarriers do not satisfy the complex orthogonality condition when the guard band is removed. Then, a simple equalization scheme is proposed to independently equalize the two innermost subcarriers. Simulation results show that the block-wise SFBC scheme works well under channels with mild-to-moderate frequency selectivity, and the block-wise (STBC ) scheme suffers less than 1 dB loss under severe frequency selective channels at the bit error rate of 10 − 3, when only a simple one tap zero-forcing equalizer is employed. Copyright © 2016 John Wiley & Sons, Ltd.

Abstract In this paper, a massive multiple input multiple output downlink scenario is considered where the number of users varies in a large dynamic range. An adaptive joint precoding and pre‐equalization with reduced complexity is proposed. Specifically, the successive over‐relaxation method is employed in the pre‐equalization process to avoid the high‐dimensional channel matrix inversion, and a reduced‐length feedback filter is proposed to reduce the computational complexity of the precoding. Moreover, an adaptive transceiver structure is proposed to switch on/off the precoding process so that multiple users can be accommodated with the least cost of the computational complexity. Simulation results show that, compared with the traditional scheme, the proposed adaptive joint precoding and pre‐equalization can save about 90% of the computational complexity. Copyright © 2016 John Wiley & Sons, Ltd.

Femtocells are attractive candidates in the future cellular system such as long-term evolution (LTE). However, the interference introduced by femtocells may hinder the improved system performance in LTE networks. In this paper, we consider the challenging problem of interference management in LTE networks with femtocells, propose two frequency reuse patterns and a novel interference avoidance method with the frequency resources of the macrocell reused by femtocells. The key idea of the interference avoidance method is to identify the macrocell user equipments (MUEs) which are near femtocells with the help of spectrum sensing performed by femtocells and avoid MUEs and nearby femtocells using the same frequency resources through spectrum scheduling. Moreover, we conduct simulations to verify that our proposed method is successful in avoiding the potential interference and improving the network performance.