ENHANCING MICROWAVE LINK AVAILABILITY USING DIVERSITY
Abstract
Information exchange and data speed are very important. In the microwave link design, there is an obstacle in the form of hills that act as a barrier between sites. The obstacle in the design causes diffraction loss so that the availability results that are in accordance with the target are not obtained.
Achieving 99% availability on microwave links is a key target for every service operator. A decrease in microwave link availability is caused by several factors, including equipment failure, propagation loss, interference, tsunamis, earthquakes, and human error. By implementing techniques such as space diversity, route diversity, polarization diversity, hot standby, and repeater planning, we can reduce the Bit Error Rate and increase overall availability in accordance with the operator's Service Level Agreement.
In mountainous regions, multi path fading and rain fading are the primary causes of signal loss. However, for links crossing the sea, rain fading is the dominant factor. In this study, we evaluated the impact of polarization diversity and space diversity on microwave link availability. We conclude that with polarization diversity, vertical polarization yields better results compared to horizontal polarization, but vertical space diversity provides the best results across all combinations.
Typically, availability is expressed as a percentage of the time a system is operational. The "Five-Nines" availability target of 99% has long been a standard for marketing and is considered a primary goal for core networks. availability is equivalent to only 5 minutes of downtime per year. Table 1 explains the relationship between availability percentage and annual downtime duration. It is worth noting that to increase availability from 99% to 99%, the downtime must be reduced from 52 minutes to just 5 minutes per year.
1. Introduction
"Microwave link availability can be defined as the ability of a functional unit to be in a state of readiness to perform a specified function under given conditions at a given instant of time or during a given time interval, assuming that external resources are available." Simply put: "Availability is the percentage of time a network provides service compared to the time the service should have been provided." The time when service is unavailable is referred to as downtime.
Theoretical availability calculations are performed at the planning stage by segmenting supporting units such as hardware, software, physical connections, and power supply. Mean Time Between Failure (MTBF) information is typically provided by equipment manufacturers. For network components that lack this data, such as power sources, statistical data or estimations are used. Estimations of each component's recovery time, known as Mean Time To Repair (MTTR), are also required. Availability is calculated using the formula:
Availability=MTBF|MTTR+MTBF
High availability is crucial for network service providers. Availability statistics are often used as an attraction for customers. By increasing network availability, the overall quality of the network also improves. Currently, most operators include availability guarantees in their SLAs. Therefore, improving availability is a primary objective for operators.
The main causes of decreased microwave link availability include equipment failure, propagation loss (including multi path fading, rain fading), location failure, interference, and human error. Of all these factors, propagation loss and interference are the most controllable. Therefore, methods such as repeater planning, route diversity, space diversity, polarization diversity, hot standby, or a combination of several techniques are applied to reduce the BER to less than 10−6 bit/s and increase Received Signal Level (RSL) to improve availability.
In this study, we investigate the effects of Space Diversity and Polarization Diversity on microwave link availability and select the most suitable technique to reduce the BER to less than 10−6 bit/s, thereby increasing RSL and achieving 99% availability as targeted by the ITU.
2. Main Factors Affecting Microwave Link Availability
2.1 Reflection
Reflection occurs when electromagnetic waves strike objects whose dimensions are much larger than their wavelength, such as the ground surface, buildings, or walls. Multi path reflections can cause signal degradation or lead to areas without coverage.
2.2 Refraction
Refraction is the bending of waves as they pass through a medium with continuously varying density. Some waves will be bent or reflected, which can cause serious problems for long-distance communication.
2.3 Rain Fading
The performance of MW links at 15/18/23 GHz with hop lengths of 10–15 km is significantly more affected by rain compared to shorter hops of 5–8 km. Larger raindrops are more elongated horizontally, causing horizontal polarization to experience greater attenuation than vertical polarization. The higher the frequency, the shorter the wavelength, leading to more severe interference when the wavelength approaches the size of water molecules.
2.4 Multi path Fading
The atmospheric refractive index varies depending on temperature, humidity, and pressure. These variations alter the direction of electromagnetic waves, causing propagation through multiple paths. Interference between these paths results in constructive and destructive combinations, leading to signal fluctuations.
2.5 Interference
Nearby base stations with the same frequency can cause interference, both co-channel and adjacent channel interference, depending on the quality of the transmitter and receiver filters.
3. Diversity Technique For Availability Improvement
3.1 Diversity Space
This technique uses multiple antennas at the transmitter and receiver to select or combine the best signal. The main challenges are cost, size, and power consumption, making this technique more commonly applied at base stations.
3.2 Polarization Diversity
The same information is transmitted simultaneously in horizontal and vertical polarization. Studies show that this technique provides significant improvement compared to receivers without diversity.
3.3 Repeater Planning
At high frequencies, rain fading worsens with increasing link length. Therefore, repeater planning is necessary to maintain line-of-sight, especially if there are obstacles.
3.4 Hot Standby
By adding a ready-to-use backup receiver, recovery time is almost zero. However, this increases costs and is only suitable for critical links because the system becomes more redundant.
4. Conclusion
Here's a detailed and comprehensive English translation of the "Conclusion" section (Number 5) of your study, ensuring clarity, precision, and strong academic tone.
This study has conducted an in-depth evaluation of the impact of implementing polarization diversity and space diversity as fading mitigation techniques to enhance microwave link availability. This comprehensive analysis aimed to identify the most effective solutions in the context of improving telecommunication network reliability
4.1 Performance of Polarization Diversity
In the context of polarization diversity, where the same information is transmitted simultaneously via horizontal and vertical polarizations,
Vertical Polarization: Links utilizing vertical polarization demonstrated an availability level of 99%. This superior performance is likely due to the characteristic of vertical waves being less affected by rain fading compared to horizontal polarization, especially at higher microwave frequencies. Raindrops, which tend to flatten horizontally as they fall, exhibit a lower attenuation effect on vertically polarized waves.
Horizontal Polarization: Conversely, links employing horizontal polarization achieved an availability of 99%. This decrease in availability can be attributed to the greater interaction between horizontally polarized waves and raindrops, resulting in more substantial signal attenuation.
5. Future Scope
Further studies need to be conducted to evaluate the impact of using repeaters and hot standby, or a combination of other techniques, to achieve the 99% availability target as set by the ITU.
5.1 Evaluating the Impact of Repeaters
Implementing signal repeaters forms a cornerstone of effective microwave network design, particularly crucial for extended link distances or when navigating difficult geographical features. In the realm of high-frequency transmissions e.g., 15 GHz and above, where phenomena like rain-induced signal degradation rain fading and multi-path interference multi path fading become pronounced challenges, repeaters are indispensable.
Extending Reach and Bypassing Obstacles: Repeaters allow signals to transcend typical line-of-sight (LOS) limitations, effectively circumnavigating physical impediments such as mountainous terrain or densely built urban areas. They achieve this by segmenting a single, long transmission path into several shorter segments, each ensuring a clear LOS.
Enhancing Signal Integrity: Strategic placement of repeaters enables the re-amplification of signals weakened by propagation losses before they reach their ultimate destination. This action significantly boosts the Received Signal Level (RSL), thereby decreasing the Bit Error Rate (BER) and ensuring a higher standard of transmission quality. Modern digital repeaters go a step further, reconstructing the signal to eliminate accumulated noise and distortion from preceding segments.
Mitigating Fading Effects: By dividing a lengthy transmission path into shorter "hops," repeaters considerably reduce the cumulative impact of both rain fading and multi path fading. Given that fading is a primary contributor to communication downtime linked to propagation, its effective mitigation directly translates to enhanced link uptime.
5.2 Evaluating the Impact of Hot Standby
A hot standby system represents a robust redundancy mechanism where a backup unit operates either in parallel with the primary system or is maintained in a state of immediate readiness. Should the primary unit experience an operational failure, the hot standby unit is configured for instantaneous takeover. This capability stands out as one of the most powerful methods for dramatically cutting down the Mean Time To Repair (MTTR), a critical metric in calculating system availability.
Rapid Service Restoration: With a hot standby configuration, the period required to reinstate service following an equipment malfunction (e.g., a primary receiver or transmitter failure) is virtually negligible. The backup system is either already engaged or can be activated within mere milliseconds.
Hardware Failure Resilience: While various diversity techniques primarily address signal propagation challenges, hot standby specifically targets and mitigates the impact of hardware malfunctions, which are notable causes of service interruptions.
Elevated System Reliability: Implementing an N+1 redundancy scheme (for instance, one backup unit supporting N active units) provides a substantial boost to the overall reliability of the system.
5.3 Combining Techniques for Optimal Availability
Given the intricate interplay of factors influencing microwave link availability, it's highly improbable that relying on a single technical approach will suffice to achieve the ambitious “Five Nines” availability target. Consequently, a hybrid methodology, which involves the integration of multiple complementary techniques, offers considerable promise for future research and development.
Synergistic Advantages: Future investigations could explore how combining techniques like spatial diversity with a hot standby system can lead to superior overall availability. This pairing would tackle both propagation issues and equipment failures simultaneously.
Comprehensive Optimization Strategies: Research can delve into various integrated configurations, The objective would be to pinpoint the most effective and resource-efficient hybrid solutions that meticulously balance gains in availability against increased complexity and associated costs.
Holistic Network Resilience: Such a multi faceted strategy would comprehensively address diverse sources of downtime, encompassing propagation anomalies, equipment malfunctions, and even human induced errors, thereby cultivating a remarkably robust and consistently available microwave communication network.
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