Non-line-of-sight (NLOS) Propagation in Mobile Ad-hoc Networks

An Introduction to NLOS Propagation

In wireless communications, understanding non-line-of-sight (NLOS) propagation is essential to creating and deploying robust Mobile Ad-hoc Networks (MANET). Unlike line-of-sight (LOS) propagation which operates without impairments, NLOS propagation refers to the transmission of communication signals in scenarios where a direct and unobstructed path between the transmitter and receiver is not present. 

NLOS environments are characterized by impairments that range from unknown obstacles and terrain features to even distant environmental objects that can attenuate, distort or scatter the radio’s communication signal. In dense, urban environments, NLOS challenges are abundant – buildings, structures and moving vehicles can create obstacles, reflections, or multi-path environments that alter a signal’s trajectory and its integrity.  

NLOS propagation plays an essential role in modern wireless and MANET radio communications. In MANET networks every radio acts as a transmitter, receiver, and communications relay.  This enables each radio link and the entire mesh network to automatically route data along the optimal path to reach its destination.  By using radios as relays, MANET networks adapt to their environment, enabling signals to traverse NLOS obstacles, expanding their network coverage area or distance and enabling connectivity in complex environments.  

For communication network operators and RF engineers, understanding the dynamics and capabilities of NLOS propagation is essential to architecting wireless networks that can overcome environmental challenges, and provide the vital communication link for mission-critical applications. 

This edition of the Waveform Blog provides insights into the challenges of NLOS, and the techniques and solutions available to enhance MANET radio communications. 

Factors Affecting NLOS Propagation

With NLOS propagation, various factors come into play, each influencing the reliability and performance of wireless communications. To gain a better understanding, we first need to dissect contributing NLOS factors and their ramifications in MANET radio mesh networks

Fresnel Zone

An elliptical region between the transmitter and receiver, the Fresnel Zone plays a pivotal role in NLOS propagation. Obstacles within this zone, such as buildings or terrain features can obstruct the signal’s path, causing signal loss. In densely populated urban environments, where structures permeate the landscape, the Fresnel Zone’s integrity is often compromised, posing significant challenges to communication reliability.  As a rule of thumb, Fresnel Zone obstructions are typically quantified as a 6dB loss in signal – that equates to achieving half the distance of signal range in free space. 

Environmental Obstructions

Buildings, vegetation, trees and even moving vehicles can disrupt signal propagation and create signal reflectors that bounce radio waves off their surfaces. This can lead to multi-path effects that scatter and attenuate the signal, making it challenging to maintain consistent communications. 

Earth and Atmosphere 

The earth’s curvature and atmospheric conditions can introduce signal refraction and bending, affecting the trajectory of radio waves. This bending can lead to unexpected signal paths that impact NLOS propagation much like direct line-of-sight (LOS) obstructions. In long-range communications across rugged terrain, the earth’s curvature becomes a critical factor to consider.    

Together, these factors collectively contribute to the degradation of signals in NLOS scenarios, affecting communication reliability, data rates and coverage area. Network and RF engineers designing and optimizing wireless networks must meticulously account for these variables to ensure robust communication connectivity.

NLOS Propagation Solutions

Coded Orthogonal Frequency-Division Multiplexing (COFDM)

COFDM is a digital multiplexing technique where the waveform is split into multiple subcarriers with data sent on each carrier. This enables multiple bits of data to be transmitted in parallel across multiple streams with information redundancy. In NLOS environments characterized by reflections, data becomes extremely frequency-selective – with different levels of signal attenuation and data loss. Some frequencies experience little to no loss, while others will experience heightened signal attenuation. As one of the key characteristics of Silvus MN-MIMO waveform, COFDM counteracts the effects of frequency selectivity on data loss, enabling the data to be recovered because it has been redundantly coded onto other subcarriers. 

Multiple-Input, Multiple-Output (MIMO) Antenna Techniques 

MIMO operates on a simple yet powerful principle – employing multiple antennas at both the transmitter and receiver to improve the range, throughput, and robustness of wireless communications. When a signal travels through a dynamic NLOS environment, MIMO equips it with resilience. It splits the signal into multiple streams transmitted each over distinct antennas – and at the receiving end reconstructs it to become a stronger, more robust version. 

MIMO-based waveform technology, such as Silvus’ proprietary MN-MIMO waveform leverages these advanced MIMO antenna techniques to counter the adverse effects of NLOS, enabling signals to outmaneuver tactical RF challenges.

Eigen-Beamforming 

Unlike traditional high-gain antennas or amplifiers, Eigen-Beamforming adapts dynamically, irrespective of antenna orientation or line-of-sight obstacles. By adjusting the phase and amplitude of the transmitted signal to leverage the optimal eigen modes of the channel, Eigen-Beamforming ensures that the optimal signal is always sent to the receiver, even in the presence of high multi-path environments in an NLOS channel. Additionally, because Eigen-Beamforming is done on each subcarrier independently, it can be used in conjunction with other techniques such as COFDM to increase throughput even across frequency-selective channels.  Therefore, because it’s not adversely affected by scattering objects or reflections, Eigen-Beamforming thrives in complex, and cluttered NLOS environments, with the ability to create up to 4x effective transmit power and range. 

Space-Time Coding 

As its name implies, Space-Time Coding takes the digitized signal from the radio and adds redundancy to it in both space and time.  Space refers to the physical distance between the antennas on a MIMO radio, allowing for added diversity and robustness. In addition, the bits of the digitized signal are also coded in time, with redundant bits sent into the channel at different time intervals to counteract any time-based variations in the NLOS environment. 

Spatial Multiplexing 

Utilizing multiple antennas operating at an identical center frequency, Spatial Multiplexing enables the transmission of multiple unique information streams from these different antennas. A communications network comprised of MIMO antennas at both the transmitter and receiver side, can then decode these streams individually and thus increase the amount of data flowing through a fixed channel bandwidth. Because it has the ability to increase the spectral efficiency (bits per second per Hz of the channel) of these data streams, Spatial Multiplexing is a core MIMO technique in delivering unsurpassed data rates in mobile, multi-path and NLOS environments. 

NLOS Propagation – Application Use Case Scenario

Consider a military or public safety team navigating a complex urban environment. Through the use of MANET radios and MIMO waveform technology, these teams can experience seamless communications between ground personnel, vehicles, and their command centers, even when buildings or other environmental conditions obstruct their line-of-sight. Adaptive MANET mesh networks, MIMO antenna techniques, COFDM and Eigen-Beamforming work together to enable NLOS propagation, enabling signals to adapt dynamically to their environment. Space-Time Coding enables data robustness and Spatial Multiplexing helps to maximize data rates – ensuring mission-critical information isn’t lost as it flows through the communications network. 

The result: strong NLOS propagation with clear and robust high-fidelity video, voice, and IP data connectivity without the need for a dedicated infrastructure.

Silvus Technologies – Empowering Connectivity In NLOS Environments

As the world’s leading developer of advanced MANET radio systems, powered by Silvus’ proprietary MN-MIMO waveform, Silvus Technologies is reshaping mesh network technology for mission-critical applications – on the ground, in the air and at sea. 

Our StreamCaster MANET radios and MN-MIMO waveform are purpose-built to outmaneuver Tactical RF challenges in NLOS environments with class-leading throughput, range, EW resiliency and scalability. On a mission to solve the toughest communication problems on the planet, Silvus continues to push the limits of tactical communications delivering next-generation capabilities to Defense, Public Safety and Commercial customers around the world.  

At Silvus, we never stop innovating comms technology for the tactical edge.

Want to Learn More?

Check out the case study on TX Eigen-Beamforming or Contact Us today to learn how Silvus can help you solve your most difficult communication challenges.

 

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