On-board DBF Array Antennas with Quick-Response SDMA and Interference Suppression for HAPS

High Altitude Platform Station (HAPS) system is one of the recent wireless technologies that exploit many promising advantages from satellite system as well as from terrestrial system. Long Term Evolution (LTE) is a 4G wireless broadband technology developed by Third Generation Partnership Project (3GPP), an industry trade group. 3GPP has been centered on enhancing LTE radio standards to improve the performance of wireless communications link, receive diversity has been proposed by the 3GPP within mode 1 of transmission in the LTE standard. In this paper, we focus on the HAPS-LTE downlink system performance evaluation with optimum receive spatial diversity and has channel side information known at the receiver (CSIR) for different scenarios. Simulation results show that, HAPS channel characteristics such as elevation angle, user mobility, and channel bandwidth greatly affect the performance of the proposed system. It is also shown that, maximal-ratio combining (MRC) diversity algorithm is able to improve the system performance in bad channel conditions.

This article proposes a novel 5G Internet of Things (IoT) networks with High Altitude Platform Station (HAPS) in the sky. In this proposed method, the 5G HAPS that consists of airplane and uniform rectangular array (URA) can achieve reliable connections between ground gateway (GW) and user equipments (UEs), at the same time GW requires the relative position information of the HAPS. HAPS is susceptible to various factors in the sky, resulting in system capacity reduction. Correspondingly, we propose a direction of arrival (DoA) prediction method based on the latest Long Short-Term Memory (LSTM) model to solve the above problems. Simulation results show that our approach makes a significant improvement to the 5G Internet of Things networks with High Altitude Platform Station.

Space-air-ground integrated networks (SAGINs) are attracting increasing attention for extending wireless communication coverage. Furthermore, high-altitude platform stations (HAPSs) can provide low-latency internet access to areas where terrestrial networks do not operate as unmanned flying base stations. However, as more frequency bands are used by wireless communication systems in terrestrial and non-terrestrial networks, it is necessary to make effective use of the finite frequency bands for multiple systems. Therefore, this study focuses on frequency sharing between the uplink from HAPS to satellite and the downlink from HAPS to ground users for efficient use of finite frequency bands in a SAGIN. In such frequency sharing, the back lobe toward the rear of the HAPS antenna for uplink communication causes interference when receiving the downlink signal from HAPS. Therefore, the zero-forcing (ZF) method is applied to the directivity control of the HAPS-mounted uplink antenna, which helps direct the nulls with significantly low antenna gain toward ground users. However, the coverage of the nulls due to the above control alone is extremely insignificant, which cannot sufficiently guarantee interference suppression for diverse user distributions. Therefore, this study proposes a HAPS-based null broadening approach with directivity control according to the user distribution for interference suppression in a SAGIN. The effectiveness of our proposal is evaluated by comparing it with the existing covariance matrix tapper(CMT)-based null broadening, ZF-based nulling, and no nulling for the HAPS-mounted uplink antenna using numerical analysis.

This paper discusses a high altitude relay system (HARS) of command and telemetry links for UAVs deployed at lower altitude in wide area. We propose and design a multiple access scheme in space, frequency, and time domains to use the same frequency band for the user and feeder links by employing a multi-beam digital beamforming (DBF) antenna on board the high altitude platform station (HAPS).

Space division multiple access (SDMA) is a multiple access scheme for mobile communications that improves system capacity. We developed the SDMA base station (BS) and carried out several tests. We describe the configuration of the SDMA BS and the results of the tests. The result of the indoor measurement indicates that the SDMA BS has sufficient interference suppression performance. The results of the field test indicate that the traffic capacity is several times greater than existing systems and interference emitted from the SDMA BS is decreased. Therefore, it is confirmed that the SDMA BS achieves substantial improvements in spectrum efficiency without any degradation.

A new digital beamforming approach for SDMA using spreading sequence array weights

The high cost and power consumption of digital beamforming, as a result of the high number of RF chains, has overshadowed its performance on multi-antenna wireless powered communication networks (WPCNs). This setback forced researchers to low-cost alternatives such as hybrid beamforming, which decreases the number of expensive RF chains by utilizing cheaper phase shifters. This cost-cutting, however, comes with reduced control over beamforming weights and compromise performance. To circumvent this deficiency, scheduling of energy harvesting (SEH), utilizing the degree of freedom in the time domain, has been proposed. In SEH, the downlink slot is subdivided into multiple variable-length subslots with different beamforming weights. In this paper, we examine the effect of SEH on the optimization of minimum length scheduling for space division multiple access (SDMA) uplink transmission compared to time division multiple access (TDMA) uplink transmission. Via simulations, we demonstrate that SDMA benefits more from the additional degree of freedom provided by the usage of SEH for any number of nodes. However, SDMA yields inferior delay performance compared to TDMA as the number of nodes increases, which restricts the application of SDMA with SEH, making it impractical.

FOMP algorithm for Direction of Arrival estimation

This paper presents an experimental evaluation of a novel and innovative sequential beamspace (SBS) smart antenna system. The system is capable of accurate direction of arrival (DOA) estimation in beamspace and efficient element space beamforming. Moreover, the robust functionality of such a system includes high resolution radio frequency (RF) emitter DOA estimation and beamforming in a noisy environment in the presence of strong interference. The beamspace DOA estimation accuracy, beamforming pattern, and output signal quality have been thoroughly studied and quantified. The algorithms have been tailored to utilize an 8 element uniform circular array (UCA) and an 8 channel analog beamformer (BF) operating at 5.8 GHz to gather lab-based experimental results. The previously completed simulations in [1], and the experimental results herein show that the proposed system can achieve good performance once it is properly synchronized using a time delay correction filter. In addition, a significant decrease in hardware is realized when operating in beamspace versus element space.

This paper presents a brief overview of narrowband direction of arrival (DOA) estimation algorithms and techniques. A comprehensive study is carried out in this paper to investigate and evaluate the performance of variety of algorithms for DOA estimation. Two categories of DOA estimation algorithms are considered for discussion which are Classical methods and Subspace based techniques. Classical methods include Sum-and-Delay method and Capon’s Minimum Variance Distortionless Response (MVDR) while Subspace based techniques are multiple signal classification (MUSIC) and The Minimum Norm Technique. Also ESPIRIT technique is evaluated. Inefficiencies are pointed out and solutions are suggested to overcome these shortfalls. Simulation results shows that the MUSIC algorithm is able to better represent the DOAs of signals with more prominent peaks. The Min-Norm algorithm also identifies the DOAs of signals similar to the MUSIC algorithm, but produces spurious peaks at other locations. The MVDR method identifies the DOAs of signals, but the locations are not represented by sharp peaks, due to spectral leakage. The classical beamformer also produces several spurious peaks. MUSIC show higher accuracy and resolution than the other algorithms. It should be noted that MUSIC is more applicable because it can be used for different array geometries.

For ultralow-power Internet of Things, we consider a monostatic multiantenna backscatter communication network (MBCN) supporting massive devices through nonorthogonal multiple access (NOMA) over space division multiple access (SDMA). For the network, we optimize the transmit beamforming (BF) for signal excitation of a reader, reflection coefficients of devices for backscattering and energy harvesting, and receive BF for information decoding at the reader toward the maximum fairness in data collection. We first show that the optimization problem for the MBCN without successive interference cancellation (SIC) has a similarity with the problem considered for a wireless power communication network but the conventional algorithm is not applicable to the MBCN with SIC. Thus, we propose a new alternating optimization algorithm that is applicable to both problems with and without SIC at a lower complexity. Simulation results show that the proposed algorithm provides a remarkable gain over the conventional one for the MBCN with SIC while exhibiting a similar performance for the MBCN without SIC. In addition, NOMA/SDMA enhanced by the proposed algorithm increases the minimum system throughput as well as the total energy harvested in the network by utilizing limited resources more effectively for a larger number of devices.

Aim: The aim of this research is to increase the system's capacity rate of the system using the novel Modified ESPRIT Multi Signal Classification (Modified ESPRIT-MUSIC) algorithm in comparison with Estimation of Signal Parameters via Rotational Invariance Technique (ESPRIT). In this paper, a reliable novel Modified ESPRIT Multi Signal Classification (Modified ESPRIT-MUSIC) direction of arrival (DoA) estimation technique employed to reduce interference in a Digital beamforming Multi-Input Multi-Output Non-Orthogonal Multiple Access (MIMO NOMA) system over Rayleigh fading channel. Materials and Methods: The Modified ESPRIT-MUSIC algorithm is implemented in Digital beamforming at lower signal to noise ratio (SNR) levels. The DoA estimation technique is used in the MIMO NOMA system to decrease interference and increase sum rate. The addition of the estimation process with an additional estimation algorithm gives accurate user detection. The performance of this proposed algorithm is compared with the existing ESPRIT algorithm for the same antenna parameters. Group 1 is the ESPRIT algorithm, and Group 2 is the Modified ESPRIT-MUSIC algorithm. The sample size for each group is 20. The total sample size is 40. The pretest power simulation is 80% with a significance of 0.15. Results: The performance analysis of the proposed system is calculated in terms of capacity vs SNR. The increase in capacity of the proposed system by accurate user detection through beamforming is possible because of the combined ESPRIT-MUSIC algorithm. The mean capacity value for the proposed novel system is 13 bps/Hz at 10 dB and 26 bps/Hz at 20 dB SNR, and the mean capacity value for ESPRIT algorithm is 12 bps/Hz at 10 dB and 23 bps/Hz at 20 dB SNR. Conclusion: From the simulation results and independent sample tests, the novel Modified-ESPRIT MUSIC algorithm detects signals significantly better than the ESPRIT algorithm and increases the system's capacity.

DOA (Direction of Arrival) estimation is a major problem in array signal processing applications. Recently, compressive sensing algorithms, including convex relaxation algorithms and greedy algorithms, have been recognized as a kind of novel DOA estimation algorithm. However, the success of these algorithms is limited by the RIP (Restricted Isometry Property) condition or the mutual coherence of measurement matrix. In the DOA estimation problem, the columns of measurement matrix are steering vectors corresponding to different DOAs. Thus, it violates the mutual coherence condition. The situation gets worse when there are two sources from two adjacent DOAs. In this paper, an algorithm based on OMP (Orthogonal Matching Pursuit), called ILS-OMP (Iterative Local Searching-Orthogonal Matching Pursuit), is proposed to improve DOA resolution by Iterative Local Searching. Firstly, the conventional OMP algorithm is used to obtain initial estimated DOAs. Then, in each iteration, a local searching process for every estimated DOA is utilized to find a new DOA in a given DOA set to further decrease the residual. Additionally, the estimated DOAs are updated by substituting the initial DOA with the new one. The simulation results demonstrate the advantages of the proposed algorithm.

To achieve high-resolution and wide-swath (HRWS) imaging, a space-time waveform-encoding (STWE) spaceborne synthetic aperture radar (SAR) system is adopted. In rugged terrain, the beam-pointing mismatch problem will appear when the traditional digital beamforming (DBF) technique is used to separate the received echoes. This problem leads to decreasing the received echo’s gain, deteriorating the quality of the image product and making the interpretation of SAR image difficult. To address this problem, an advanced echo separation scheme for STWE spaceborne SAR based on the DBF and blind source separation (BSS) is put forward in this paper. In the scheme, the echoes are transmitted within the adjacent pulse repetition intervals to simulate multiple beams, and the scattered echoes are received by the sixteen-channel antennas in elevation simultaneously. In post-processing, a detailed flow is adopted. In the method, the DBF is firstly performed on received echoes. Due to the error caused by terrain undulation, the degree of echo separation is not enough. Then, the BSS is performed on the multiple echoes obtained after the DBF processing. Finally, the highly separated echo signal can be obtained. In this scheme, there is no need to perform the direction of arrival (DOA) estimation before the DBF processing, which saves valuable computing resources. In addition, to verify the proposed scheme, point target and distributed target simulations based on the 16-channel data of an elevation X-band DBF-SAR system are carried out. The results of point targets indicate that the residual echo caused by rough terrain can be reduced by more than 14 dB using the proposed scheme. The proposed scheme can be directly implemented into existing SAR systems; thus, it does not increase the complexity of the system design. The scheme has the potential to be applied to future spaceborne SAR missions.

We present a multi-microphone multi-speaker direction of arrival (DOA) tracking algorithm. In the proposed algorithm, the DOA values are discretized to a set of candidate DOAs. Accordingly, and following the W-disjoint orthogonality (WDO) property of the speech signal, each time-frequency (TF) bin in the short-time Fourier transform (STFT) domain is associated with a single DOA candidate. The conditional probability of each TF observation given its corresponding DOA association, is modeled as a multivariate complex-Gaussian distribution, with the power spectral density (PSD) of each source an unknown parameter. By applying the Fisher-Neyman factorization, it can be shown that this conditional probability is proportional to the signal-to-noise ratio (SNR) at the outputs of minimum variance distortionless response (MVDR)-beamformers (BFs), directed towards all candidate DOAs. We model these observations as either a frequency-wise parallel Hidden Markov Model (HMM) or as a coupled HMM with coupling between adjacent frequency bins. The posterior probability of these associations is inferred by applying an extended FB (FB) algorithm, and the actual DOAs can be inferred from this posterior. An experimental study demonstrates the benefits of the proposed algorithm using both a simulated dataset and real recordings drawn from the acoustic source localization and tracking (LOCATA) dataset.