Introduction
This document delivers a high-level view of best practices involved with installing and operating outdoor ePMP links.
This document covers:
- Lightning / surge protection practices
- Choosing mounting locations and installation methods
- Determining system coverage and capacity
- Adjusting antenna angles for optimal coverage
Lightning / surge protection
Structures, equipment and people must be protected against power surges (typically caused by lightning) by conducting the surge current to ground via a separate preferential solid path. The actual degree of protection required depends on local conditions and applicable local regulations.
Ground bonding and surge suppression
To adequately protect an ePMP installation, both ground bonding (intentionally connecting metallic parts to create a safe path for electricity to travel) and transient voltage surge suppression (to protect from temporary rises of voltage from lightning strikes or other sources) are required.
Warning
Electro-magnetic discharge (lightning) damage is not covered under warranty. The recommendations in this guide, when followed correctly, give the user the best protection from the harmful effects of EMD. However 100% protection is neither implied nor possible.
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Note International and national standards take precedence over the requirements in this article |
Installation examples
Examples of ePMP Access Point and ePMP Subscriber Module installations are shown below. In this case, the ground bonding is achieved at the Access Point by connecting a grounding conductor from a known good ground to the device thumbscrew (thereby routing harmful surges away from sensitive electronics).
Both the Access Point and Subscriber Module are installed with transient surge suppression by way of the co-located surge suppressor.
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Cambium Networks subscriber modules utilize an outdoor rated unshielded twisted pair (UTP) cable. In order to comply with the NEC, installers must provide a listed antenna discharge unit surge suppressor for each conductor of the cable. This surge suppressor should be located outside the building as near as practicable to the entrance of the conductors to the building. In addition, the surge suppressor unit shall not be located near combustible material.
Installation guidelines
Selecting sites for network elements
The APs must be positioned
- with hardware that the wind and ambient vibrations cannot flex or move.
- where a tower or rooftop is available or can be erected.
- where a grounding system is available.
- with lightning arrestors to transport lightning strikes away from equipment.
- at a proper height:
- higher than the tallest points of objects immediately around them (such as trees, buildings, and tower legs).
- at least 2 feet (0.6 meters) below the tallest point on the tower, pole, or roof (for lightning protection).
- away from high-RF energy sites (such as AM or FM stations, high-powered antennas, and live AM radio towers).
- in line-of-sight paths
- to the SMs and BH.
- that will not be obstructed by trees as they grow or structures that are later built.
Installing to avoid direct lightning strikes
Use the rolling sphere method to determine where it is safe to mount equipment. An imaginary sphere, typically 50 meters in radius, is rolled over the structure. Where the sphere rests against the ground and a strike termination device (such as a finial or ground bar), all the space under the sphere is considered to be in the zone of protection (Zone B). Similarly, where the sphere rests on two finials, the space under the sphere is considered to be in the zone of protection.
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Assess locations on masts, towers and buildings to determine if the location is in Zone A or Zone B: · Zone A: In this zone a direct lightning strike is possible. Do not mount equipment in this zone. · Zone B: In this zone, direct EMD (lightning) effects are still possible, but mounting in this zone significantly reduces the possibility of a direct strike. Mount equipment in this zone. |
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Warning
Do not mount equipment in Zone A which can put the equipment, structures and life at risk.
Surveying Sites
Factors to survey at potential sites include
- what pre-existing wireless equipment exists at the site. (Perform spectrum analysis.)
- whether available mounting positions exist near the lowest elevation that satisfies line of site, coverage, and other link criteria.
- whether you will always have the right to decide who climbs the tower to install and maintain your equipment, and whether that person or company can climb at any hour of any day.
- whether you will have collaborative rights and veto power to prevent interference to your equipment from wireless equipment that is installed at the site in the future.
- whether a pre-existing grounding system (path to Protective Earth ) exists, and what is required to establish a path to it.
- who is permitted to run any indoor lengths of cable.
Assuring the Essentials
In the 2.4 GHz and 5 GHz frequency band ranges, an unobstructed line of sight (LOS) must exist and be maintainable between the radios that are involved in each link.
Line of Sight (LOS) Link
In these ranges, a line of sight link is both
- an unobstructed straight line from radio to radio.
- an unobstructed zone surrounding that straight line.
Fresnel Zone Clearance
An unobstructed line of sight is important, but is not the only determinant of adequate placement. Even where the path has a clear line of sight, obstructions such as terrain, vegetation, metal roofs, or cars may penetrate the Fresnel zone and cause signal loss.
Calculating the Aim Angles
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b |
angle of elevation |
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B |
vertical difference in elevation |
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A |
horizontal distance between modules |
The proper angle of tilt can be calculated as a factor of both the difference in elevation and the distance that the link spans. Even in this case, a plumb line and a protractor can be helpful to ensure the proper tilt. This tilt is typically minimal.
The number of degrees to offset (from vertical) the mounting hardware leg of the support tube is equal to the angle of elevation from the lower module to the higher module (B in the example provided above).
Determining ePMP system coverage and capacity
The ePMP Capacity Planner (available with documentation at https://support.cambiumnetworks.com/files/epmp/#r2) offers functionality to determine the expected performances in terms of distances of an ePMP Series system operating in line-of-sight (LOS), near line-of-sight (nLOS) or non line-of-sight (NLOS) propagation condition according to the configuration of several system parameters like channel bandwidth and antenna selection.
Coverage and capacity
In the COVERAGE AND CAPACITY section of the ePMP Capacity Planner tool user interface the results of the link budget (or, link capability) calculation are shown for the SM with antenna type selected in the SM antenna for displaying data field.
This section shows the expected range, the expected maximum speed of the downlink and uplink, the sector capacity (the capacity of the AP and all of the associated subscribers), the latency, and a plot summarizing coverage and capacity results.
The Potential DL/UL Range columns represent the maximum distance at which the radio link can operate with the selected configuration. Values of range in red indicate that the potential range is larger than the maximum range set in the Max range field in the SYSTEM CONFIGURATION section.
The Max DL/UL/Total Throughput is the Downlink/Uplink/Total capacity of the sector assuming all the registered SMs are operating at that modulation.
The DL/UL/Total Capacity is the Downlink/Uplink/Total capacity of the sector, taking into account the percentage of users using each modulation, under the assumption that the users are evenly distributed in the covered area and that they all generate the same amount of traffic. This calculation also includes the percentage of SMs estimated to support a single stream MCS. The capacity of the sector is calculated as a proportion of the peak capacity taking into account the AP antenna pattern and the fact that the users are evenly distributed in the covered area.
The covered area is limited by the Max range field set in the SYSTEM CONFIGURATION section.
The DL/UL/Total scheduling latency (in ms) shows the time needed at the AP to schedule all the SMs connected in the sector, and it depends on the number of SMs set in the SYSTEM CONFIGURATION section. This is less than the total latency the system experiences, which depends on additional factors.
The plot in the COVERAGE AND CAPACITY section shows the range of communication that can be achieved with each modulation level in the downlink, up to the maximum range set in the Max range field in the SYSTEM CONFIGURATION section.
The plot in the COVERAGE AND CAPACITY section shows the range of communication that can be achieved with each modulation level, up to the maximum range set in the Max range field in the SYSTEM CONFIGURATION section. If some modulation levels cover an area outside the Max range field, those modulation levels are not used. The legend of the plot indicates which modulation levels are not used, together with the peak DL/UL/Total throughput for each used modulation level.