18 - Availability in microwave radio communication: the robustness of wireless systems

As time goes by, everything has developed, various aspects have used enchanced technology that is easy to understand and use,the field of telecommunications also affected, ease of exchanging information, data transmission, and much more, it can happen with the presence of electromagnetic waves, which are waves that can propagate in a vacuum with physical media or non-physical media, and can carry power. With electromagnetic waves consisting of magnetic and electric fields, these waves can be utilized in the field of telecommunications.

Electromagnetic waves are divided into several specters, based on frequency and wavelength, these waves are divided into 7, from the lowest frequency to the highest frequency, starting from radi waves, microwaves, infrared rays, visible light. Ultra violet light, x rays, and gamma rays. Electromagnetic waves used in the field of telecommunications are radio waves, this is because radio waves have a wide frequency, with each frequency range having its own function in radio communication, radio waves can also cover a wide distance of up to several km, and the speed is also fast enough for data transmission.

Despite the advantages of microwave radio communication, challenges arise in radio communication, the frequency used in microwave radio communication ranges from 1GHz to 300GHz, which is a high frequency, as a result the propagation wave will be vulnerable to obstacles, it can also be exposed to interference and fading. 

In microwave radio communication there are wave propagation mechanisms between the transmitter side to the receiver side, one of which is Line Of sight (LOS), wave propagation at LOS is a method where the transmitter side sends a signal straight towards the receiver side following the line of sight. This mechanism occurs when the transmitter and receiver are directly opposite without any physical obstruction between the transmitter and receiver. 

Due to the high-frequency characteristics of microwaves that are sensitive to physical obstacles and atmospheric conditions, system availability is an important aspect that needs to be considered in its design. In reality, not all signals can be received perfectly all the time. Disturbances such as rain attenuation, fading effects, and interference can cause a decrease in service quality or even a temporary break in communication. 

In fact, electromagnetic waves, especially microwaves, have become the backbone in providing a variety of important telecommunication services, ranging from people-to-people communications, data networks, to emergency, government and military communication systems. All of these services rely heavily on the availability of reliable communication signals.

Therefore, in the development of microwave radio communication systems, special attention is required to the extent to which the system can maintain service continuity under various conditions. This brings us to an important concept in communication systems, namely availability, which is a key indicator of the reliability and availability of a wireless communication system.

However, achieving a high level of availability is not a simple matter. There are various technical challenges to overcome, especially since microwave communications are highly susceptible to external interference. Factors such as the distance between the transmitter and receiver, extreme weather conditions such as heavy rain or dense fog, geographical contours, as well as interference from other communication systems can all affect signal stability. 

This article aims to take a closer look at the concept of availability: from its basic definition, aspects that affect availability, and mechanisms that ensure availability.

Definition of Availability

What is availability in radio communication? And why is it important for communication?, here's the explanation

Availability means the ability of a system to provide services continuously without any significant interruption, in the field of telecommunications availability is a measure by which a telecommunications system provides network services to users continuously, and no interruption in the system that require repair by technicians or others.

Mathematically, availability is the probability that a system or device can operate when needed, without experiencing interference due to failure or the time required for repair. The availability value is generally expressed as a percentage of operational time to the total time available, and is one of the important indicators in system performance evaluation, including in the context of microwave radio communications.

There are two general classifications of availability, namely inherent availability and achieved availability,

Inherent availability

This availability only considers the time needed to make repairs (corrective maintenance time) and does not include scheduled maintenance time (preventive maintenance). Inherent Availability is usually used in the system design stage to estimate the basic reliability of the device. The general formula is:

A_i=MTBF/(MTBF+MTTR)

With:

Ai =  avalability

MTBF (Mean Time Between Failures)= mean time between failures, and

MTTR (Mean Time To Repair)= average time to repair a failure.

MTTR (Mean Time To Repair) in the above equation only considers corrective downtime. This method does not include other downtime caused by preventive maintenance, logistical constraints, or administrative delays. One of the main purposes of this metric is to understand the impact of downtime that needs to be anticipated from the system or machine design stage.

Achieved Availability

Achieved availability is similar to inherent availability, but with an important difference: achieved availability takes into account downtime due to preventive maintenance, as well as various other delays related to logistics and administration It is represented by the following equation:

A_i=MTTBM/(MTTB+M)

With:

Ai =  availability

MTBM (Mean Time Between Maintenance) = includes the time between all types of maintenance (including preventive).

M (Mean Maintenance Time) = average time for all types of maintenance (corrective + preventive).

This type of availability measurement is more relevant to the maintenance team, as its value not only reflects technical reliability, but can also be improved through optimizing the preventive maintenance schedule and increasing efficiency in maintenance actions.

Once a formula is used to calculate availibility, there are levels within this availability that are represented using the number nine in percentages. This representation is used as a shorthand form to describe the percentage of time the system operates without interruption. The more nines listed, the higher the system reliability level. The following is an explanation of the availability level based on the number of nines:

One Nine (90%)

Indicates an availability of 90%, meaning the system is available 90% of the time in a year. Thus, there is about 10% downtime or approximately 36.5 days in one year.

Two Nines (99%)

Indicates the system has 99% availability, with downtime of approximately 3.65 days per year. This is usually considered the minimum acceptable limit for many common services.

Three Nines (99.9%)

Often referred to as "three nines", indicates the system only experiences about 8.76 hours of downtime per year. This level is quite high and is usually the minimum target for service providers.

Four Nines (99.99%)

With an availability of 99.99%, system downtime is only about 52.56 minutes per year. This level is mostly aimed at critical applications, such as medical services or real-time industrial systems.

Five Nines (99.999%)

Known as "five nines", this is a very high level of availability, with downtime of only about 5.26 minutes per year. This level is required for services that must not be interrupted at all, such as emergency systems, financial services, or major data centers.

Achieving an availability level of 99.999%, known as the "five nines", requires a multidimensional approach that includes comprehensive and integrated system planning, design, and management. The percentage of availability is governed by a service-level agreement (SLA), which is the level of service a customer expects from a supplier, specifies the metrics used to measure that service, and the remedies or penalties, if the service level is not achieved.

After knowing what availability is, the formula used to calculate availability, and the classification of levels of availability, next are the things that affect availability.

Things that affect availability

In a radio communication availability system, there are two things that affect its availability, namely equipment availability or reliability and path availability,

Equipment availability/reability

Equipment availability in radio communication systems refers to the level of availability and reliability of the hardware and software used to send, receive or amplify radio signals. This includes major components such as transmitters, receivers, antennas, repeaters, power supplies, amplifiers, UPS, backup batteries, and controllers. Equipment availability expresses what percentage of the time in a period (usually one year) these devices function normally and can perform their duties without interruption.

Some factors that affect equipment availability are:

1. Device Quality and Lifespan

Devices with low quality tend to have higher failure rates and shorter lifespans. Over time, electronic components degrade, such as decreased sensitivity in receivers or output power decay in transmitters. Long lifespan also increases the risk of sudden failure, which can lead to sudden downtime.

2. Frequency and Effectiveness of Maintenance

Infrequent or improper maintenance leads to dust accumulation, corrosion, or loose connectors that can degrade performance. Conversely, good and regular maintenance, both preventive and corrective, is able to detect potential damage before it becomes an actual failure. Schedules that are too infrequent or too frequent (to the point of disrupting operations) can also have a negative impact.

3. Power System Disruptions (Power Supply)

Unstable power supplies, sudden outages, or noise in the power lines can damage electronic components or cause system restarts. Devices such as UPS, inverters, and generators must be in good condition to support the main device during power interruptions.

4. System Redundancy

The existence of system redundancy significantly increases equipment availability. For example, in a 1+1 configuration, if one transmitter unit fails, the backup unit comes online immediately to replace it. Without redundancy, any immediate failure means full downtime.

5. Software Performance (Firmware and Control Software)

Although not a physical component, software (such as firmware and configuration software) crashes or errors can cause the device to malfunction. For example, a boot failure due to a firmware error can render the device unable to power on, even if it is not physically damaged.

Path availability

Path availability in radio communication systems refers to the availability of a signal propagation path between two communication points (usually a transmitter point and a receiver point) that can be used effectively without significant interference within a certain period of time. Unlike equipment availability, which is related to hardware conditions, path availability focuses on the quality and reliability of the radio wave propagation path, which is strongly influenced by atmospheric conditions, terrain characteristics, and the technical configuration of the communication system.

Factors that can affect path availability are:

1. Fading (Signal Weakening)

Fading is a phenomenon of decreased received signal power due to multipath propagation, which is a condition in which the signal reaches the receiver through various reflection paths with time and phase differences. The phase difference between these propagation paths can produce destructive interference that causes sharp fluctuations in the amplitude of the received signal. The result of this fading is an increased chance of interference or even disconnection of communication, so mitigation strategies such as diversity techniques are needed.

In Terrestrial Radio Relay System trajectories, selective fading (occurring in some frequency bands) or non-selective fading (occurring in all frequency bands) can occur.

Selective fading, a condition in which the various frequency components of a signal experience non-uniform fading, each frequency component can experience different attenuation and phase changes, resulting in distortion of the original signal shape. This phenomenon usually occurs due to the difference in arrival times between multipath signals at the receiver. When this time difference is large enough compared to the symbol duration, the channel response will vary across the entire frequency bandwidth of the signal, causing frequency-selective fading.

Non-Selective Fading, is a type of signal attenuation where all frequency components of the transmitted signal experience uniform amplitude attenuation. In this condition, although the overall signal power may fluctuate, the spectral shape of the signal remains unchanged. This type of fading usually occurs when the channel coherence bandwidth is larger than the signal bandwidth, so the entire signal is affected uniformly. It is common in communication channels with very small delay spreads, where the difference in arrival times between multipath signal components is insignificant to the symbol duration. Therefore, the change in amplitude of the received signal tends to be consistent across its frequency spectrum.

2. Line of Sight (LoS) and Terrain Topography

The existence of an obstacle-free Line of Sight path is an important prerequisite for effective microwave communication. The communication path should ideally be in a clear LoS condition, meaning that there are no physical obstacles between the transmitter and receiver. The presence of objects such as tall buildings, trees, hills, or other infrastructure can cause unwanted signal reflection, diffraction, or spreading, which directly reduces the received signal power and decreases path availability.

In addition, topographical aspects such as ground elevation, curvature of the earth, and terrain contours also affect transmission path design. For long communication distances, the curvature of the earth can be a limiting factor for LoS, requiring the placement of antennas with a certain height to maintain direct visibility between two communication points. Therefore, careful topographic analysis and network planning are necessary to ensure the integrity of the communication path.

3. First Fresnel Zone Clearance

The First Fresnel Zone is the elliptical area around the straight line between the transmitter and receiver, which represents the most critical radio signal propagation path. For the signal to reach the receiver with maximum power, the first Fresnel zone must be relatively free of obstructions. Obstructions within this zone, even if they are not directly on the LoS line, can cause significant diffraction and increase signal attenuation.

In general, it is required that at least 60% of the radius of the first Fresnel zone should be obstacle-free to maintain high path availability. Obstacles such as towers, vegetation, or local topography within this zone can have a destructive effect on the signal, leading to degradation of the overall system performance. Therefore, in the planning stage of a communication network, Fresnel zone calculation and obstacle evaluation become a crucial part of the technical feasibility study.

To ensure overall availability in a radio communication system, it is necessary to implement various technical mechanisms that mitigate potential interference from both the device side and the signal propagation path.

System Redundancy

In radio communication systems, system redundancy refers to the strategy of duplicating various important system components, such as transmitters, receivers, power sources, antennas, and communication lines, with the aim of maintaining service continuity. When one of the main components is damaged or fails, the prepared backup component can immediately replace its function. This replacement process can take place automatically. With this redundancy mechanism, disruptions to communication services can be minimized or even prevented.

Providing Fading Margin

Fading margin is an additional signal power reserve that is intentionally prepared in the design of radio communication systems to anticipate signal attenuation due to non-ideal propagation conditions, such as heavy rain, fog, or other atmospheric disturbances. This margin is usually calculated in decibels (dB), and represents the difference between the minimum signal power required by the receiver to function properly, and the signal power it is normally designed to receive. With a fading margin, the system has some tolerance if the signal degrades due to bad weather or multipath effects, so that the signal remains above the receiver's sensitivity threshold.

Installing an Adaptive Equalizer

An adaptive equalizer is a component or algorithm in a radio communication system designed to correct signal distortion that occurs during the transmission process, especially when the signal is affected by complex communication path conditions. These distortions can arise due to reflections from nearby objects, interference from other signals, changes in atmospheric conditions, or environmental dynamics that change over time. This equalizer is adaptive because it is able to adjust automatically to changes in communication channel characteristics without the need for manual adjustment. Adaptive equalizers can be implemented either through hardware, such as in a radio chipset or modem, or in software form through digital signal processing algorithms that work in real-time.

Conclusion 

Overall, availability is an important indicator that shows the level of reliability of a communication system in providing uninterrupted services within a certain period of time. The calculation involves the Mean Time Between Failure (MTBF) and Mean Time To Repair (MTTR) factors, which describe the balance between the frequency of breakdowns and the speed of recovery. The two main aspects that affect availability are equipment availability, which depends on the reliability of system hardware and software, and path availability, which is affected by signal propagation conditions in the real environment. To ensure a high level of availability, various mechanisms can be applied, such as the implementation of system redundancy, the provision of fading margins, and the use of adaptive equalizers. The combination of these strategies allows the communication system to continue operating reliably, even in the face of technical and environmental disturbances.

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