Future vehicle-to-vehicle (V2V) services provide various applications that require ultra-reliability and low latency communications, and millimeter wave (mmWave) technology becomes a candidate to meet the above requirements. However, there is a lack of existing researches on mmWave V2V channel measurements, let alone measurements in special scenarios such as vehicle obstructions and parking lots. This paper reports 41 GHz mmWave V2V channel measurement campaigns conducted in an underground parking lot with small vehicle obstructions. We adopt the time domain channel measurement method based on Golay complementary sequences to obtain received signal strength indication (RSSI) and channel impulse response (CIR) data. The channel parameters, such as extra loss, PDP (power delay profile), root-mean-square delay spread (RMS-DS) and K factor at different distance between receiver (RX) and obstruction car, are simulated and analyzed. The measurement and study results provide guidance for the design of mmWave V2V communication systems in the presence of vehicle obstructions.

Millimeter-Wave (mmWave) has emerged as a competitive solution to meet the high-data-rate requirements of future high speed train (HST) communications due to its abundant spectrum resources. However, existing research on actual measurements of mmWave channels for HST is relatively scarce, let alone at speed up to 500 km/h. This paper reports the world’s first mmWave HST channel measurement campaigns with a maglev train at a maximum speed of 500 km/h in the semi-closed station scenario. A large number of large-and small-scale fading characteristics are obtained and analyzed, such as path loss, shadow fading, root-mean-squared delay spread (RMS-DS), K factor, coherence bandwidth, and decorrelation distance. We propose an improved cluster-based Saleh-Valenzuela channel model which is more suitable for mmWave channel measurements. Numerical results and best fitting distribution results are obtained for seven key cluster parameters. The simulated and measured channels are compared to verify the accuracy of this improved model. Finally, the significance of the study is discussed, and the measurement and analysis results are compared with the existing measurement results in the train station scenario. The measurement results fill the gap of the 500 km/h mmWave HST measurements, and help to guide and design the future mmWave HST communication system.

Millimeter-wave (mmWave) technology becomes a candidate to meet the high-data-rate requirements of future vehicle-to-vehicle (V2V) communication systems. The obstructions of vehicles are particularly common in V2V communications. However, there remains a gap in existing works regarding mmWave V2V propagation with vehicle obstructions. This paper reports the mmWave V2V channel measurements and modeling with large and small vehicle obstructions in urban street and underground parking lot scenarios. The effect of the distance between the obstruction vehicle and the receiver (RX) on the mmWave channel characteristics is investigated. We employe the deterministic models (two-path, three-path and knife-edge diffraction models) considering the beam pattern of directional antennas to reveal the mmWave propagation mechanism, and propose a stochastic model that can be applied to both line-of-sight (LOS) and obstructed line-of-sight (OLOS) cases to fit the path loss. Channel non-stationarity and channel consistency are analyzed and compared. Small-scale fading characteristics, such as root-mean-square delay spread (RMS-DS), K factor and their cross-correlation, are studied in detail. The measurement and analysis results are of great help for mmWave V2V channel modeling, communication system design and performance evaluation for different scenarios and different obstruction vehicle types.

A reliable Vehicle-to-Vehicle (V2V) communication system is particularly important for intelligent transportation systems (ITS), and high-data-rate transmission is required for future V2V systems with the increase of vehicular sensors. In addition, an important V2V scenario, parking IoT, is ignored by researchers. In this paper, V2V 41GHz millimeter wave (mmWave) channel measurements are conducted in both underground and open parking IoTs for the first time. The received signal strength indication (RSSI) and channel impulse response (CIR) data are obtained by using the time domain measurement method based on Golay complementary sequences. Large- and small-scale fading characteristics, such as path loss, shadow fading, Root mean-square delay spread (RMS-DS) and Ricean K factor, are analyzed. The channel characteristics of underground parking IoT and open parking IoT are compared. The measurement and analysis results are useful for the design and channel modeling of mm Wave V2V systems in parking IoT scenario.

In space-air-ground integrated networks (SAGINs), the global energy efficiency (GEE) is a crucial metric for balancing the network throughput and energy consumption, and the maximization of GEE requires the optimizations of both user association and power allocation. Most existing methods optimize user association and power allocation separately or successively, relying on instantaneous non-local channel state information (CSI) exchanges. Nevertheless, both the separate and successive methods may fail to find the jointly optimal solution, and acquiring the instantaneous non-local CSI across the SAGINs is challenging due to the long communication distances between the access points (APs) and users. To address these issues, we leverage cloud-edge collaborations and propose an online delayed-interaction collaborative-learning independent-decision multi-agent DRL (DICLID-MADRL) algorithm. With the proposed algorithm, each AP can independently select users and configure transmit power with only local information to enhance GEE. Simulation results demonstrate that the proposed algorithm achieves a higher GEE with reduced time complexity compared to the state of the arts.

Parking lots, as one of the most significant vehicle-to-vehicle (V2V) communication scenarios, have rarely been studied for their millimeter-wave (mmWave) V2V channel characteristics. This paper presents the 41 GHz mmWave channel measurements and analysis results in open and underground parking lot scenarios for the first time. A large number of channel measurement data are obtained, including received signal strength indications (RSSIs) and channel impulse responses (CIRs) at different distances and different arrival angles for three cases. By clustering the power delay profile (PDP), cluster parameters such as cluster number, inter-cluster duration, cluster arrival rate and decay factor are obtained. The channel non-stationarity and consistency are revealed and discussed by various methods and from different perspectives. Large- and small-scale fading characteristics parameters, such as path loss, shadow fading, root-mean-square delay spread (RMS-DS), angle spread (AS), fade depth, and Ricean K factor are analyzed and discussed. The mmWave V2V channel characteristics of three cases are compared and analyzed. The measurement and modeling results fill the gaps of mmWave V2V channel measurements in open and underground parking lot scenarios and provide valuable suggestions for the design and modeling of mmWave V2V communication systems.

Train stations, as one of the most significant buildings on a high-speed railway (HSR), have been rarely studied for their millimeter-wave (mmWave) propagation characteristics. This paper presents the first 41 GHz mmWave band channel measurements on a HSR closed station at ultra-high speeds (up to 288 km/h). Multi-dimensional channel measurement data at different azimuth angles and speeds from inside to outside the station are obtained. It is found that the received signal strength indication (RSSI) experiences a significant attenuation caused by the influence of the station gate. In combination with the Saleh-Valenzuela (SV) model, an improved time delay clustering algorithm is used to cluster the power delay profile (PDP). According to the clustering results, parameters such as inter-cluster duration, cluster arrival rate, and intra-cluster delay spread are obtained. Most of the large- and small-scale fading characteristics, including path loss, shadow fading, PDP, angle spread, small-scale fading characteristic quantity, small-scale amplitude fading, and Rician K factor are investigated. The measurement and modeling results fill the gaps of channel measurements in closed HSR station scenarios and provide valuable suggestions for the design and modelling of HSR station communication systems.

Covert communication is crucial in privacy protection for users of wireless networks, and efforts have been made to improve the system throughput subject to a constraint on a warden's detection error probability. However, most existing research is based on an unclear relation between system throughput and covertness, due to the use of the Kullback-Leibler divergence. To solve this problem, in this paper, we explore the exact throughput-covertness relation by investigating the likelihood variation distance. An algorithm is established to find the minimum channel uses required for the transmission of a given amount of information and a certain covertness constraint. The rigorous proof of the convergence of the algorithm is also given. Our results show that an overly strict constraint on covertness would lead to a large sacrifice of system throughput, and it is not cost-effective to increase the number of channel uses to improve covertness if only a small amount of information is to be transmitted. These results could be very helpful for system designers in achieving a desirable tradeoff between system throughput and covertness.

Physical layer security can supplement upper-layer cryptographic algorithms and provide additional protection of the confidential information. The secrecy capacity of physical layer security can be further improved in massive MIMO systems through high-gain beamforming. However, previous research is mainly based on the plane-wave assumption. In massive MIMO systems with physically large arrays, it is possible that legitimate users are located in the Fresnel zone of the radiation field, which leads to spherical wavefronts. In this paper, we investigate physical layer security in the spherical-wave channel. A novel secure transmission scheme that is robust against cooperative eavesdropping is proposed. As shown by simulation results, the proposed scheme, which can be applied in the conventional plane-wave channel as well, can achieve distance-domain security in the spherical-wave channel, and thus is a potential candidate for location-based secure communications in the near future.

Differential spatial modulation (DSM) is a multiple input multiple output (MIMO) wireless transmission technique that achieves additional data bit transmission by utilizing antenna activation indexes. Combined with amplitude phase shift keying (APSK) constellations, the spectrum efficiency of DSM system can be further improved. However, the theoretically optimal maximum likelihood (ML) detection suffers from a high complexity that grows exponentially with the number of transmit antennas (TAs). Besides, the imperfect channel state information in fast fading channels leads to a severe deterioration of the detection performance. To address these problems, we propose a novel transmission scheme named reordered amplitude phase shift keying aided differential spatial modulation (RAPSK-DSM) and its low-complexity decision-feedback differential detection (DFDD) based detection algorithm. The proposed RAPSK-DSM scheme provides a better solution for addressing the error propagation problem and its detector utilizes the preceding channel gain vectors from previous detection results to mitigate the performance loss caused by time-selective channel fading. Furthermore, by decomposing and pre-computing the detection process, the complexity of proposed detector is effectively reduced due to the avoidance of redundant computations. Numerical analyses and simulation results show that the proposed RAPSK-DSM system is able to achieve substantial bit error rate (BER) performance improvement under fast fading channel conditions at the expense of low complexity.

With the increased demand for unmanned driving technology and big-data transmission between vehicles, millimeter-wave (mmWave) technology, due to its characteristics of large bandwidth and low latency, is considered to be the key technology in future vehicular communication systems. Different from traditional cellular communication, the vehicular communication environment has the characteristics of long distance and high moving speed. However, the existing communication channel tests mostly select low-speed and small-range communication scenarios for testing. The test results are insufficient to provide good data support for the existing vehicular communication research; therefore, in this paper, we carry out a large number of channel measurements in mmWave vehicle-to-infrastructure (V2I) long-distance communication scenarios in the 41 GHz band. We study the received signal strength (RSS) in detail and find that the vibration features of RSS can be best modeled by the modified two-path model considering road roughness. Based on the obtained RSS, a novel close-in (CI) model considering the effect of the transmitter (TX) and receiver (RX) antenna heights (CI-TRH model) is developed. As for the channel characteristics, the distribution of the root-mean-square (RMS) delay spread is analyzed. We also extend the two-section exponential power delay profile (PDP) model to a more general form so that the distance-dependent features of the mmWave channel can be better modeled. Furthermore, the variation in both RMS delay spread and PDP shape parameters with TX-RX distance is analyzed. Analysis results show that TX and RX antenna heights have an effect on large-scale fading. Our modified two-path model, CI-TRH model, and two-section exponential PDP model are proved to be effective.

The high mobility of future high-speed trains (up to 500 km/h) poses great challenges on the design of dynamic beamforming (BF) algorithms for millimeter-wave (mmWave) communications in high-speed rail (HSR) networks. Thanks to the linear structure of HSR cellular networks, the track of the train is almost predictable. Thus, the fixed BF, i.e., highly directional antennas with fixed pointing directions, can be employed as a low-cost solution to replace dynamic BF. However, the performance gap between dynamic BF and fixed BF should be evaluated to justify the use of fixed BF in HSR mmWave communications. In this paper, empirical studies are conducted based on the raw data obtained from extensive measurement campaigns. Two classic environments in railway traffic are considered: the traditional train station and the HSR tunnel. Analyses of the measurement results, including the received signal strength, power delay profile, root-mean-squared delay spread, and channel non-stationarity are presented. Then, based on the measured data, we generalize the widely-used close-in (CI) free-space path loss (PL) model so that the generalized model can characterize the PL in the HSR tunnel with a higher accuracy. Finally, the performance gap between perfect dynamic BF and fixed BF is evaluated based on the generalized model and the measured data. Our results show that the average throughput of dynamic BF is only 4% higher than that of fixed BF in the HSR tunnel, but 21% higher in the train station when severe beam misalignment is present.

Amplitude phase shift keying (APSK) aided differential spatial modulation (APSK-DSM) is a multiple-input-multiple-output wireless transmission technique which not only has the desirable features of differential spatial modulation (DSM) but also has a higher spectrum efficiency than DSM. However, the error propagation problem and the violation of the quasi-static channel assumption in conventional APSK-DSM design will cause significant performance loss, especially in undesirable channel conditions. Besides, the maximum-likelihood detection based on the traditional block-by-block search has a complexity that grows exponentially with the block size, which becomes intractable in systems with large transmit antenna numbers. To overcome those drawbacks, we propose a novel transmission scheme named the absolute amplitude differential phase spatial modulation (AADP-SM) in this paper. AADP-SM is able to alleviate the error propagation problem and achieve a near-coherent performance by invoking multiple previous blocks in the current detection. By taking into account the channel fading rate, AADP-SM is also more robust in fast-fading channels than APSK-DSM. A novel non-coherent detection algorithm is proposed for AADP-SM to reduce the exponential detection complexity to a polynomial order. Finally, we show through simulation that AADP-SM is tolerable to imperfect information about the channel statistics.