Downlink Signal Research Articles

Characterizing turbulence profile layers through celestial single-source observations.

Future spacecraft missions aim to communicate with the Earth using near-infrared lasers. The possible bit rate of free-space optical communication (FSOC) is orders of magnitude greater when compared to current radio frequency transmissions. The challenge of ground-space FSOC is that atmospheric turbulence perturbs optical wavefront propagation. These wavefront aberrations can be measured using a Shack-Hartmann wavefront sensor (SHWFS). A ground-based adaptive optics (AO) system can mitigate these aberrations along the optical path by translating wavefront measurements into deformable mirror commands. However, errors result from atmospheric turbulence continuously evolving, and there are unavoidable delays during AO wavefront correction. The length of an acceptable delay is referred to as the coherence time-a parameter dependent on the strength of turbulence profile layers and their corresponding wind-driven velocity. This study introduces a novel technique, to the best of our knowledge, for using SHWFS single-source observations, e.g., the downlink signal from a geostationary satellite, to measure the strength and velocity of turbulence profile layers. This work builds upon previous research and demonstrates that single-source observations can disentangle turbulence profile layers through studying the cross-covariance of temporally offset SHWFS centroid measurements. Simulated data are used to verify that the technique can recover the coherence time. The expected and measured results have a correlation coefficient of 0.95.

A full duplex FSO transmission system using RZ-4PAM downlink signals with 67% duty cycle and AMI uplink signals

To improve the transceiver quality of Free Space Optical (FSO) system and ensure that user access is feasible the network, a high-speed full-duplex 100 meters FSO transmission scheme using 67% duty cycle RZ-4PAM downlink and alternative mark inversion (AMI) uplink signals is designed system. In the downlink, one PAM sequence generator and one Mary pulse generator (MPG) module are used to generate 67% RZ-4PAM optical signals at 10Gbit/s through a Mach-Zehnder intensity modulator (MZM). The upstream AMI signals generator by the pseudo-random binary sequence (PRBS) generator and the precoding process. We have measured optical spectrums, time-domain waveforms and eye diagrams before and after signals transmission by system simulation. The transceiver performance of the downstream RZ-4PAM with 67% duty cycle and upstream AMI optical signals the in the 100m FSO transmission system is analyzed.

Embedding the Localization and Imaging Functions in Mobile Systems: An Airport Surveillance Use Case

Driven by the extended applications and scarce spectrum resources, integrating the radio sensing functions into the future mobile system has been a consensus between the stakeholders. This paper demonstrates the feasibility of joint localization and imaging functions by exploiting the reference signal in the radio frame defined in the mobile system. Essentially, the subspace-based algorithms are adopted to jointly estimate the angle and distance and to enable the localization function for uplink and downlink signals and the derivation of theoretical performance boundaries. The vector antenna and virtual array concepts are introduced in the downlink scenario to enhance the angle estimation resolution. The uplink sounding reference signal is exploited to enable the imaging function as analog to synthetic-aperture radar (SAR). The joint localization and imaging performances are verified with a realistic ray-tracing channel based on the 3D ground and buildings model of Muret airport in France. The simulation results show that the reference signal can provide acceptable localization accuracy and target-distinguishing capability by adopting a virtual array and joint time-spatial smoothing in downlink and uplink, respectively. The joint localization and imaging results prove that the future mobile network is potentially a viable infrastructure to provide economic surveillance solutions for airports, particularly for secondary airports that are not well equipped with dedicated surveillance systems.

A RoF-FSO communication system using 10Gbit/s polsk 4-PAM downstream signals

In this paper, the performance of a 60 GHz radio over fiber free space optics (RoF-FSO) communication system with 10Gbit/s 4-ary pulse amplitude modulation (4-PAM) polarization shift-keying (Polsk) downlink signal is proposed and designed. In the receiver, the 60GHz millimeter-wave with 4-PAM downstream signals is generated by the beating of two continuous light waves, and the high frequency signal is demodulated by self-mixing zero difference detection method. Meanwhile, the optical spectra, radio frequency (RF) spectrum, eye-diagram are measured, and the transmission performance of the used RoF-FSO system are analyzed.

Research on long distance submarine optical fiber transmission system using MSK downlink signals

The commonly used submarine optical fiber communication system is expected to provide long-distance reliable signals transmission. We present a novel optical fiber submarine system using minimum shift keying (MSK) encoded downstream signals. 10Gb/s MSK coded signals are successfully transmitted over the 100km submarine single-mode fiber (SMF).The optical spectrums, eye-diagrams and bit error rate (BER) before and after transmission of MSK signals in the system were measured and analyzed. The results show that, compared with conventional access methods, this scheme can provide high-speed, stable and secure data communication.

Evaluation Of Signal Strength And Quality Of A Ku-Band Satellite Downlink During Raining Season In Guinea Savanna Region Of Nigeria

This study presents the evaluation of signal strength and quality of a ku-band satellite downlink signals during raining season in Guinea Savanna region of Nigeria. Rain rate data and ku-band signal over Jos were retrieved from the Nigeria Meteorological Agency (NIMET). Rain rate data was measured using Davis Vantage Vue weather station while microwave signal were obtained using spectrum analyzer. The measured data were analyzed using Microsoft office excel. Rainfall is broadly classified into two types: stratiform (drizzle: ≤5mm/hr and widespread: >5≤25mm/hr) and convection (shower: >25≤50mm/hr and thunderstorm: >50mm/hr). Results were calculated based on ITU-Recommendations P.618-12 and P.838. The results obtained revealed that the effect of rain on signal strength and quality on a ku-band link depend on the rain type and duration of rainfall. The effect of drizzle (≤5mm/hr) on the ku-band link is not significant regardless the duration of rainfall. The results also shows that severe signal losses above 60dB were obtained under shower (>25≤50mm/hr) and thunderstorm (>50mm/hr). Also, widespread (>5≤25mm/hr) that prevailed for a longer period causes severe signal losses. This is to say that lower rain rate of about 25mm/hr that prevailed for a very long time have significant effect on Kuband link. The results of this study will serve as crucial design parameters for communication systems engineers for providing fade margins and developing accurate fade mitigation techniques (FMTs) for the Guinea Savanna region of Nigeria. It was observed from this study that the experimental studies within the Ku-band frequency are getting over exhausted. Therefore, experimental studied in the region of higher band like K and Ka are highly recommended.

Impact of 3D Antenna Radiation Pattern on Heterogeneous Cellular Networks

Heterogeneous cellular networks (HCNs) play a significant role in 5G networks, but have mainly been modeled and analyzed on a two-dimensional (2D) plane. In this paper, we model a three-dimensional (3D) two-tier HCN consisting of macro-cells and small-cells via a stochastic geometry approach, where the base stations (BSs) in each tier are assigned an appropriate antenna downtilt to enhance the downlink signal and suppress the inter-cell interference. Based on the 3D HCN model, we derive the downlink spatially-averaged coverage probability and area spectral efficiency (ASE) as functions of the BS antenna downtilts, BS heights, BS densities and cell-association bias. To facilitate fast numerical evaluation, we propose accurate approximations for the integral parts in the coverage probability expression. We then obtain the optimal BS antenna downtilts for both tiers that maximize the downlink spatially-averaged coverage probability by using partial derivative and the bisection method. Analytical and simulation results show that for given BS heights, densities of small-cell BSs (SBSs) and cell-association biases of SBSs, our optimized BS antenna downtilts of both tiers can significantly enhance the downlink spatially-averaged coverage probability and ASE in comparison with the fixed BS antenna downtilts network. Useful insights into the 3D deployment of HCNs considering BS antenna downtilts are obtained based on the analytical and simulation results.

Surveying on MIMO Technology for Future Wireless Communication

Massive MIMO is an extension of traditional MIMO with the exception that the BSs in massive MIMO are equipped with large number of antennas, usually hundred or more. This large number of antennas provide several positive advantages towards wireless communication with respect to increasing volume of data traffic. Each antenna is capable of serving multiple users simultaneously leading to reduction in power consumption as well as data rate amplification. Additionally, narrow and more focused beams are pointed to individual user devices located at the cell edge thereby upgrading of downlink signal quality. Using massive MIMO technique also increases reliability of the links, reduces noise effects, and mitigates and interference. With increasing number of users gets service, the throughput of the system also increases.

Enhanced performance of a bidirectional radio over fiber system employing l1-regularization -based sparse second-order volterra non-linear equalizer

A simple bidirectional radio over fiber (RoF) design based on orthogonal frequency division multiplexing (OFDM)-based millimeter wave (mm-wave) signal generation and transmission with compensation of linear and non-linear impairment with the aid of sparse second-order Volterra non-linear equalizer (S2-VNLE) is proposed. At the central station (CS), an optical 60 GHz OFDM-based mm-wave downlink signal is generated by a 16-QAM downlink radio frequency (RF) signal. The generated OFDM-based downlink mm-wave signal is transmitted to the base station (BS) via bidirectional single-mode fiber (SMF) of length 75 km. At BS, the downlink signal is photo detected and transmitted via an antenna to the user end. The 16-QAM uplink RF signal, obtained via a receiving antenna from the user end, is modulated on the unused carrier of the downlink mm-wave signal. The modulated uplink signal is transmitted to the CS through the same bidirectional fiber. The carrier reuse and fiber reuse reduce the system complexity. However, the simultaneous signal co-propagation may induce linear and non-linear impairment, further degrading the system performance and has to be compensated. The compensation of linear and non-linear impairment is done by full Volterra non-linear equalization (FVNLE). However, pruning the insignificant Volterra filter coefficient enables a reduction in computational complexity. The sparse second-order Volterra non-linear equalization (S2-VNLE) is obtained by l1-regularization to cut off the kernels with a moderate contribution. The weights of the tap coefficient are updated by the RLS algorithm. The transmission performance of compensated bidirectional RoF system is investigated. The performance analysis of the bidirectional RoF system shows improved efficiency, having performance closer to FVNLE with reduced system and computational complexity.

Interference Management for the Coexistence of DTTV and LTE Systems within the Proposed Digital Dividend Band in Nigeria

This paper seeks to present the Interference Management for the Coexistence of DTTV and LTE Systems within the proposed digital dividend band in Nigeria. The study focused on LTE Down-link (DL) signal from the nearest cell site interfering with the Digital Terrestrial Television (DTTV) fixed outdoor receiving antenna in Port Harcourt, Nigeria. The qualitative signal analysis of the DTTV systems is essential as DTTV system cannot start to operate in the newly formed frequency band without the evaluation of the possible harmful influence of the coexisting systems. This research work investigated the Compatibility of the two systems and the Probability of interference of Channel 17 (490MHz) and Channel 51 (693MHz) when DTTV and LTE systems coexist within the proposed Digital Dividend band. A test-bed approach method was adopted for the generation of the required simulation data. Star Time transmitting Station in Port Harcourt and Smile LTE 4G Communication LTE Base Station (eNBs) Network also in Port Harcourt were adopted as the Victim Link Transmitter (VLT) and Interfering Link Transmitter (ILT) respectively. Data was obtained, analyzed, and evaluated. It was observed from the simulation result that the probability of interference is a function of the separation distance between ILT and VLR. The Compatibility analysis result shows that the resulting C/I is above the protection criteria (19dB), that is there’s a minimal rate of interference. Hence, the interference issue can be managed when the two systems coexist in700MHz band. It was also established that DTTV channel 51 suffers more interference when compared with DTTV channel 17 for the same separation distance. The study recommended the minimum protection distance approach (Interference Avoidance method) as the interference management techniques when DTTV and LTE systems coexist in the proposed digital dividend (700MHz) band in Nigeria.

Mixed Numerology Interference Recognition Approach for 5G NR

The coexistence of multiple numerologies can cause interference and degrade system performance in 5G devices. In this letter, a convolutional neural networks (CNN)-based mixed numerology interference (MNI) recognition approach is devised to resolve the interference for user equipment (UE) to receive new radio (NR) downlink signal. The results are learned from the frequency domain magnitude and phase waveform, for the best precision and efficiency. We use as little sample data as possible to reduce memory/buffer costs and to alleviate the computing resources. Simulations are performed to verify the viability on various signal-noise ratio (SNR), channel condition, modulation, etc. It is shown that the accuracy can reach 97% or higher at varying SNR and fading channels. Furthermore, a dynamic filtering/windowing (DFW) scheme is proposed. The link level simulation results show that the scheme can improve the attainable Signal to Interference plus Noise Ratio (SINR) by 3~6dB for the indicated cases.

An improved approach for testing gravitational redshift via satellite-based three frequency links combination

We propose a novel approach for testing the gravitational redshift based on frequency signals transmitted between a spacecraft and a ground station. The main idea is integrating one uplink signal from ground to spacecraft and two downlink signals from spacecraft to ground. Based on the integration and certain correction models, the gravitational shift of the signals between spacecraft and ground station can be detected at high precision level. The gravitational redshift effect can be used to validate the Einstein equivalence principle (EEP), which can be tested at about 10 - 6 ∼ 10 - 8 levels in different cases for a short period (less than a month) if the stability of on-board atomic clock reach 10 - 17 /day. Compared to the scheme of Gravity Probe-A (GP-A) experiment conducted in 1976, in the new approach, any on-board signal transponders is not required, and the frequency values of the three links can be arbitrary. Since the hardware requirement is decreased, a number of spacecrafts can be candidates for a gravitational redshift experiment provided that they are able to emit two different frequency signals and receive a frequency signal from the ground.

Empirical Low-Altitude Air-to-Ground Spatial Channel Characterization for Cellular Networks Connectivity

Cellular-connected unmanned aerial vehicles (UAVs) have recently attracted a surge of interest in both academia and industry. Understanding the air-to-ground (A2G) propagation channels is essential to enable reliable and/or high-throughput communications for UAVs and protect the ground user equipments (UEs). In this contribution, a recently conducted measurement campaign for the A2G channels is introduced. A uniform circular array (UCA) with 16 antenna elements was employed to collect the downlink signals of two different Long Term Evolution (LTE) networks, at the heights of 0-40m in three different, namely rural, urban and industrial scenarios. The channel impulse responses (CIRs) have been extracted from the received data, and the spatial/angular parameters of the multipath components in individual channels were estimated according to a high-resolution-parameter estimation (HRPE) principle. Based on the HRPE results, clusters of multipath components were further identified. Finally, comprehensive spatial channel characteristics were investigated in the composite and cluster levels at different heights in the three scenarios.

Improved Doppler Shift Estimation Algorithm for Down-Link Signals of Space-Based AIS

As an enhanced system for marine monitoring and autonomous navigation, the space-based automatic identification system (AIS) has attracted extensive attention and become a hot topic for research. However, it encounters the problem of Doppler shift stemmed from the relative motion between satellites and ships, which leads to performance degradation. To circumvent this issue, in this correspondence, we propose an improved Doppler shift estimation algorithm for down-link signals of space-based AIS, utilizing the calculation of the autocorrelation and ratio on the Rice factor. Specifically, the addressed method is robust to the non-Gaussian noise. Further, the suggested approach has low complexity compared with the existing algorithm. Finally, numerical simulations, such as the standard deviation of the estimated Doppler shift versus signal-to-noise ratio, Rice factor and non-Gaussian noise, and the complexity comparisons, are carried out to validate the theoretical analysis, and demonstrate the superior performance of the proposed estimation approach.

Performance Analysis of Hybrid Cellular and Bidirectional Device-to-Device Cooperative NOMA Communication Systems

In this paper, we consider a coexisting network of cellular transmission and device-to-device (D2D) communication, in which two users need to receive downlink signals from base station (BS), meanwhile, they want to exchange information with each other. For such a network, a hybrid cellular and bidirectional D2D cooperative non-orthogonal multiple access (HCBD-CNOMA) scheme is proposed. To characterize the performance of our proposed scheme, we derive the closed-form ergodic rate and outage probability by considering both perfect and imperfect successive interference cancellation (SIC). To acquire more insights, their corresponding asymptotic expressions are obtained as well. Moreover, considering the fact that the nodes in Internet of Things (IoT) network are usually powered by battery and thus their power is limited, we further consider the scenario that two users are energy-constraint. In this scenario, users first need to harvest energy from the transmission of BS based on time splitting (TS) or power splitting (PS) energy harvesting (EH) protocol and then use these energy to accomplish subsequent communications. Similarly, the exact ergodic rate and outage probability as well as their asymptotic expressions for this scenario are analyzed. Simulation results validate the accuracy of analytical results, and reveal the performance gain of our schemes over the benchmark schemes.

Opportunistic In-Flight INS Alignment Using LEO Satellites and a Rotatory IMU Platform

With the emergence of numerous low Earth orbit (LEO) satellite constellations such as Iridium-Next, Globalstar, Orbcomm, Starlink, and OneWeb, the idea of considering their downlink signals as a source of pseudorange and pseudorange rate measurements has become incredibly attractive to the community. LEO satellites could be a reliable alternative for environments or situations in which the global navigation satellite system (GNSS) is blocked or inaccessible. In this article, we present a novel in-flight alignment method for a strapdown inertial navigation system (SINS) using Doppler shift measurements obtained from single or multi-constellation LEO satellites and a rotation technique applied on the inertial measurement unit (IMU). Firstly, a regular Doppler positioning algorithm based on the extended Kalman filter (EKF) calculates states of the receiver. This system is considered as a slave block. In parallel, a master INS estimates the position, velocity, and attitude of the system. Secondly, the linearized state space model of the INS errors is formulated. The alignment model accounts for obtaining the errors of the INS by a Kalman filter. The measurements of this system are the difference in the outputs from the master and slave systems. Thirdly, as the observability rank of the system is not sufficient for estimating all the parameters, a discrete dual-axis IMU rotation sequence was simulated. By increasing the observability rank of the system, all the states were estimated. Two experiments were performed with different overhead satellites and numbers of constellations: one for a ground vehicle and another for a small flight vehicle. Finally, the results showed a significant improvement compared to stand-alone INS and the regular Doppler positioning method. The error of the ground test reached around 26 m. This error for the flight test was demonstrated in different time intervals from the starting point of the trajectory. The proposed method showed a 180% accuracy improvement compared to the Doppler positioning method for up to 4.5 min after blocking the GNSS.

A novel bidirectional RoF link with compensated SBS and RB for 16-QAM OFDM based mm-wave downlink and uplink vector signal generation and transmission on a single fiber

A novel bidirectional RoF link with compensated SBS and RB for 60 GHz OFDM-based 16-QAM downlink and uplink vector signal generation and co-propagation over a single fiber is proposed. At the central station (CS), 60 GHz optical mm-wave is generated and modulated in the optical single-sideband modulation (OSSB) technique by the 16-QAM OFDM downlink vector signal. At the base station (BS), the 60 GHz downlink signal is detected by a high-speed photodetector (PD). The 60 GHz electrical mm-wave signal is transmitted to the user end by an antenna. The uplink carrier is extracted from the unmodulated sideband of the mm-wave downlink vector signal. This reduces the complexity of BS and makes it free from the laser source. The 16-QAM OFDM-based mm-wave uplink signal is received from the user end through an antenna and down-converted to a 10 GHz uplink vector signal. The uplink signal modulates the extracted blank carrier in the OSSB technique. The uplink and downlink vector signals are co-propagated through the standard single bidirectional fiber. Although the carrier reuse over a single fiber reduces the fiber capacity by half, it generates non-linear backscattering such as Rayleigh Backscattering (RB) and Stimulated Brillouin Scattering (SBS). A theoretical analysis is made on the backscattered power and its effect on the fiber transmission. The compensation of SBS by maintaining the SBS threshold and mitigation of RB is carried out by appropriate modulation technique. In addition, downlink and uplink signal with different data rate is transmitted through the compensated bidirectional RoF link, and the transmission performance is analyzed. Based on the analysis, the theoretical model for simultaneous downlink and uplink signal transmission over a single fiber is proposed and verified by the software simulation. The compensated bidirectional RoF link shows up a better performance up to 75 km with negligible power penalty.

Performance Analysis of Simultaneous Collision-Free Duplexing Method in Point-to-Multipoint Environment

In this letter, we have studied how to use simultaneous collision-free duplexing (SCD) in point-to-multipoint environment. SCD is a method exchanging uplink and downlink signals simultaneously using the same resource for each link and has the advantages in that it does not need complicated self-interference cancellation unlike full duplexing. In addition, SCD is different from dynamic time division duplexing (D-TDD) in which interlink interference can be completely avoided. Therefore, to analyze the effect of interlink interference on SCD, we have derived the upper bound of link capacity and the required value of interlink isolation level for better SCD performance than D-TDD. We have verified the analyzed isolation level and confirmed that SCD has good performance even in practical point-to-multipoint environment through simulation.

A new method for resolving phase ambiguity in radio interferometry using Earth rotation synthesis

As a key technique in deep space navigation, radio interferometry can be used to determine the accurate location of a spacecraft in the plane-of-sky by measuring its signal propagation time delay between two remote stations. To improve the measurement accuracy, differential phase delay without phase ambiguity is usually desired. Aiming at the difficulties of resolving phase ambiguity with few stations and narrowband downlink signals, a new method is proposed in this work by taking advantage of the Earth rotation. The high accurate differential phase delay between the spacecraft and a calibrator can be achieved not only in the in-beam observation mode but also in the out-of-beam observation mode. In this paper we firstly built the model of phase ambiguity resolution. Then, main measurement errors of the model are analyzed, which is followed by tests and validations of the model and method using the tracking data of the Cassini mission and Chang’E-3 mission. The results show that the phase ambiguities can be correctly resolved to generate a 10-picosecond level accuracy differential phase delay. Angular measurement accuracy of the Cassini reaches the milli-arc-second level, and the relative position accuracy between the Chang’E-3 rover and lander reaches the meter level.

Quasi-Distributed Antenna Selection for Spectral Efficiency Maximization in Subarray Switching XL-MIMO Systems

In this paper, we consider the downlink (DL) of a zero-forcing (ZF) precoded extra-large scale massive MIMO (XL-MIMO) system. The base-station (BS) operates with limited number of radio-frequency (RF) transceivers due to high cost, power consumption and interconnection bandwidth associated to the fully digital implementation. The BS, which is implemented with a subarray switching architecture, selects groups of active antennas inside each subarray to transmit the DL signal. This work proposes efficient resource allocation (RA) procedures to perform joint antenna selection (AS) and power allocation (PA) to maximize the DL spectral efficiency (SE) of an XL-MIMO system operating under different loading settings. Two metaheuristic RA procedures based on the genetic algorithm (GA) are assessed and compared in terms of performance, coordination data size and computational complexity. One algorithm is based on a quasi-distributed methodology while the other is based on the conventional centralized processing. Numerical results demonstrate that the quasi-distributed GA-based procedure results in a suitable trade-off between performance, complexity and exchanged coordination data. At the same time, it outperforms the centralized procedures with appropriate system operation settings.