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When to use 5W BDA vs. 2W Fiber Remote Units
With Comba Telecom’s 5W CriticalPoint Public Safety BDA now available, we decided to take a look at the use cases of when to use a 5W BDA and when to stick with a fiber DAS.
With Comba Telecom’s 5W CriticalPoint Public Safety BDA now available, we need to look more closely at the use cases of when to use a 5W BDA and when to stick with a fiber DAS. We have already gone through campus applications (https://combausa.com/en/tech-briefs/why-a-fiber-das-is-better-than-individual-bdas-for-campus-applications) and looked at why we should use a fiber DAS when a single building requires more than just a single BDA (https://combausa.com/en/tech-briefs/choosing-between-fiber-das-and-individual-bdas-for-large-building-applications). Comba only had a 2W BDA and a 2W Fiber Remote unit to offer when the large building application was written. Now that Comba has a 5W BDA and still has a 2W Fiber Remote, we will look at when to use one 5W BDA vs. two 2W Fiber Remotes.
Starting with the most important question, let us first look at cost. This answer is simple: A 5W BDA and battery backup costs less than a fiber DAS with 2 remotes and appropriate battery backup.
|
5W BDA |
Fiber DAS |
||
|
Item |
Quantity |
Item |
Quantity |
|
5W BDA |
1 |
Master Unit |
1 |
|
100 AH BBU |
1 |
Remote Unit |
2 |
|
|
|
55 AH BBU |
1 |
|
|
|
100 AH BBU |
1 |
|
|
|
Fiber + Enclosures |
1 |
|
Total Cost |
$ |
Total Cost |
$$ |
The cost for a fiber DAS, which includes the master unit, remote units, battery backups, and fiber is approximately twice the cost of a single 5W BDA and battery backup. Please contact Comba for exact pricing, but the point to be made here is that we want to install a 5W BDA when possible. We will keep this in mind for each of the following scenarios. Comba disclaimer: the following scenarios are talking about achieving a -95dBm downlink coverage level only. When designing, keep in mind all other requirements for your jurisdiction, including DAQ, dominance over native signal, and uplink signal strength requirements. These requirements may change how you need to design the system.
Before diving too deep, let us quickly review the equipment we are comparing and what type of building we are working with. Comba’s 5W BDA has an output power of 5W, or 37dBm, per band. Comba’s Public Safety Fiber DAS has 2W remote units, with an output power of 2W, or 33dBm, per band. When deploying 2 remote units, you have a total composite output power of 4W, or 36dBm per band. This means that a fiber DAS with 2 remotes has less total RF output power, however, a fiber DAS with 2 remotes will typically cover more area. The typical building where we would compare when to use a 5W BDA or a fiber DAS is going to be 400,000 to 900,000 square feet (this depends on building density, channel count, local requirements, constructability, and more). In these scenarios, two fiber remotes would be located anywhere from 100 to 400 feet away from each other. At 100 feet, ½” coaxial cable has about 2.25dB of loss at 700 and 800MHz frequencies, so you lose the extra power to cable loss.
For this example, we will look at a 40-story tall building, with approximately equal sized floors. The total square footage is about 800,000 square feet, with about 20,000 square feet per floor.
In the 5W BDA only scenario, we place the BDA on the 21st floor, and cover 20 floors up and 20 floors down. There are a few challenges to this design that could cause problems. First, the donor antenna cable run will be over 200 feet in the 5W BDA scenario. A link budget must be done to ensure you can get full output power and an appropriate signal strength back to the donor tower. Second, there must be a location to install the BDA at or near the 21st floor, and this location must comply with local code. Finally, the building must be constructed so you can have a continuous riser from the first to the top floor for service antennas, without a long riser run between floors.
The design calls for a minimum of a 40 foot antenna radius at -95 dBm, which, based on building density, means the input power to each antenna must be greater than -15.6 dBm. Note: All calculations are estimates done for the purpose of demonstration – your requirements may differ.
In the perfect scenario, we place the 5W BDA on the 20th or 21st floor, or put the 2W fiber remotes on the 10th and 30th floors. With a stacked riser closet, the 5W BDA has a total passive loss of about 38.7 dB to the service antennas. In the 2W fiber remote scenario, there is about 30.5 dB of loss to each antenna.
|
|
5W BDA, Ideal Installation |
2W Fiber Remotes, Ideal Installation |
|
Output Power, Total (dBm) |
37 |
33 |
|
Number of Channels |
15 |
15 |
|
Power per Channel (dBm) |
25.2 |
21.2 |
|
Loss to Antenna (dB) |
38.7 |
30.5 |
|
Antenna Output Power (dBm) |
-13.5 |
-9.3 |
|
Coverage Radius @ -95dBm, 3.7 VPLE with 8dB margin |
45 |
59 |
Both cases will work to cover the full building, which is great – you can take your pick of which system to install, and based on cost, the best choice here is going to be the 5W BDA.
Now we will look at non-ideal scenarios for equipment location. A common situation is that the BDA is installed on the very bottom or very top floor, based on space restrictions. A donor antenna cable run of over 500 feet is not ideal, so we will assume in the worst case, the BDA will go on the top floor. In a similar scenario, the worst case for fiber will be the top and bottom floors for equipment locations.
|
|
5W BDA, Top Floor Installation |
2W Fiber Remotes, Top & Bottom |
|
Output Power, Total (dBm) |
37 |
33 |
|
Number of Channels |
15 |
15 |
|
Power per Channel (dBm) |
25.2 |
21.2 |
|
Loss to Antenna (dB) |
43.9 |
34.2 |
|
Antenna Output Power (dBm) |
-18.7 |
-13.0 |
|
Coverage Radius @ -95dBm, 3.7 VPLE with 8dB margin |
33 |
47 |
While the fiber system will still cover the building, the 5W BDA is no longer able to provide adequate coverage, so we would have to use a fiber system in this building. Always be sure that you confirm the equipment mounting location before starting a design or submitting a bid – it can change the required equipment dramatically!
For the next scenario to compare, we will look at a warehouse style building. This building will be one level tall and a 634,400 square feet (1,220 feet long by 520 feet wide). In this building, we will have tall shelving units with walking aisles between them. Once again, we must make the disclaimer that every building is different, so bid based on your building and local requirements.
Since we have already stated that a 5W BDA will be a less expensive install, we first design based on a 5W BDA. The BDA is centrally located, with directional antennas on the outsides of the building and omni antennas in the middle. Running the predictions in your RF propagation software, we have plenty of signal strength and will pass our -95dBm requirement with margin to spare:
If we are bidding this project, however, we cannot assume that the BDA will be able to be located in the middle of the building. What happens with this same design, but instead of putting the BDA in the middle of the building, we must put it in the corner, near where the FACP is located?
In the “worst case BDA location” situation, the building fails. The reason is simple – the long cable run adds extra loss, leading to a system you can’t balance and lower overall antenna output power. In the first scenario, with the BDA in the middle of the building, the antenna ERP/channel ranges from -5.66 dBm to -1.87 dBm. Using the exact same cable pathways, the antenna ERP/channel with the BDA in the corner of the building ranges from -18.20 to -8.09 dBm. We are losing anywhere from about 6 to 13 dBm output power at each antenna location.
To fix this issue, we can replace the BDA with a fiber DAS. We will still put the master unit in the required corner location, but now we can run fiber to any point in the building, so we pick two convenient locations and place the fiber remotes in the center, one at the top and one at the bottom. Running predictions, we can see that we improve in signal strength with 2 fiber remotes as compared to one 5W BDA:
To summarize, if you have a building that a 5W BDA will cover if conveniently placed, you can substitute (2) 2W fiber remotes and cover the building as well. The more cost-effective solution is the 5W BDA, however, you may not have the option of placing the BDA where you want and designing the building so it works, and a good replacement is a 2 fiber remote solution.
Watch Matt Lunny's Video Tech Talk on Uplink Strength, Gain & Noise are affected by a buildings size and distance to the tower.
Comba Telecom is seeking an Assistant Product Manager to assist with managing the entire Public Safety product line. This position offers the opportunity to grasp the wireless communication technologies, stay abreastof wireless industry trends, dealing with business and technical challenges involving the products, team working experiences with various departments.
Responsibilities:
- Assist with management of current products
- Assist in new product development projects, work with various departments to ensure
- Actively collect information of industry standards and product requirements from customers and other internal teams; regularly conduct assessment of competitors and competitive products.
- Participate in defining and driving new product specification and features.
- Participate in defining and driving short-term and long-term product roadmaps.
Requirements:
Please send your resume to: Email: This email address is being protected from spambots. You need JavaScript enabled to view it. or Fax: (408) 526-0181.
Please quote our reference number (OLM/NA) on your email subject.
Comba Telecom is an Equal Opportunity Employer.
5G is finally here. We’ll, not quite, but Apple does own the mobile device market share in the United States at 60% as shown in the chart below.
Source: StatCounter Global Stats - OS Market Share
In reality, 5G has been around for over a year with all the major networks currently building out their network infrastructure to meet the 5G technology and demand that is to come. Samsung has been shipping their 5G devices for a few months and now that Apple will be shipping theirs, that should help drastically move the needle to get subscribers on the 5G bandwagon.
In fact, I personally own an iPhone 7 and now that the iPhone 12 is out, I look forward to upgrading. So, IMHO, I believe the demand will be there for the 5G network generation upgrade. I hope Operators can speed up their buildouts, even during this pandemic. In Apple’s iPhone launch, Verizon’s Chairman and CEO, Hans Vestberg, noted that they are building their 5G Ultra Wide Band network, to provide ultra low-latency coverage to cover 60 cities by the end of 2020.
But no matter the Operator, 5G speeds will be nowhere near the touted 100x faster than 4G or latencies to 1ms. The reality is that new 5G device owners will not likely see these speeds for a while. To quickly understand why, Verizon, AT&T, T-Mobile and Sprint are building their networks utilizing a mix of low (< 1GHz), mid (1GHz – 6GHz) and high (24GHz – 40GHz) band frequencies to supply 5G bandwidth. But each 5G band is only capable of delivering at a maximum amount of bandwidth. For example, a low band (600-700MHz) tower can cover hundreds of square miles with 5G service that ranges in speed from 30 to 250 megabits per second (Mbps). A mid band (2.5/6GHz) tower may cover a several-mile radius with 5G that currently ranges from 100 to 900Mbps. Finally, a high band (millimeter wave/24-39GHz) tower could cover a one-mile or lower radius while delivering roughly 1-3Gbps speeds. Each of these tiers will improve in performance over time. Utilizing a mix of these bands to deliver 5G speeds to the customer will ultimately be the most cost effective in the long run.
But in time, the speeds for 5G will deliver on the promise as was the case when the Operators switched from 3G to 4G over a decade ago.
This section is meant to detail design considerations for your public safety system. It is not a comprehensive guide to design, only a guide for what to expect and consider when doing your design.
Comba has released multiple tech briefs in the past regarding preplanning public safety communication systems and code for rebroadcasting signals, and they should both be read for more information on planning your public safety design:
Each jurisdiction throughout the country will have different requirements for ERRC Systems. The local code should describe minimum building size requirements for testing, failure criteria, frequencies required, battery backup requirements, system acceptance testing, and more. Codes are constantly changing and even if a national code is adopted, your local jurisdiction may make their own amendments to the national code.
IFC Section 510 and NFPA 1221 are the two most referenced codes when referring to in-building radio communication requirements. The IFC may be replaced with a state fire code. These are updated every 3 years and it usually takes time for the jurisdiction to adopt and enforce new code, so the AHJ will always be the best reference for what is actually required. In many cases, the AHJ will have their own code (ordinance) that incorporates excerpts from IFC Section 510, or NFPA 1221, or both - along with their own local electrical and building code requirements. And some cases, the local ordinance will incorporate excerpts from previous/older versions of IFC or NFPA guidelines.
There have also been cases where an integrator complied with the current published version of the local code – only to find out the hard way during the ATP that the AHJ had modified the code, was now enforcing the modified version – but had not yet made it publicly available in the current published version. So even though the system was fully compliant with the published code, the systems were failed by the AHJ until system changes were made.
Another example of undocumented changes – we have seen situations where the local code discloses the frequencies/channels to be covered. Then at ATP, it is discovered that the AHJ had added or changed channels since the published code document was created and had not yet been added to the published code. Again, the AHJ failed the ATP until the changes were made – and this can sometimes cos the integrator and/or building owner LOTS of money.
You must know your jurisdictional requirements for a proper design. It is highly encouraged that you obtain the published version of the local code, then talk with the AHJ to make certain that this is what they expect compliance with – or find out what has changed, so you can design accordingly. Verify the required coverage areas, verify the channels to be covered, verify the electrical requirements. Then you can be assured that your design is 100% compliant with what the AHJ will require.
The layout of the building is the next thing you must be familiar with before starting the design. Between nearby satellite imagery, floor plans, reflected ceiling plans, and building elevations, there should always be enough information to have a constructible design. The key parts of our public safety DAS are the donor antenna, BDA, and service antennas.
Your donor antenna should, if possible, have line of sight to the donor tower. Always consider the surrounding buildings, any geographical obstacles, and even the roof construction before choosing a final location for a donor antenna. If possible, see if you can find out about any expected new construction (i.e. a big building across the street) that could potentially ruin your clear line of sight to the tower. A cable pathway should be considered such that it complies with local codes and the cable length works with your link budget. Make sure that the cable is properly grounded.
Ensure a proper room is chosen to house the BDA, battery backup (if required), and any other required system components. Depending on building size, the BDA might need to be strategically located in the center of the building to avoid requiring a higher power BDA or a fiber system, or the building may be small enough where the BDA location doesn’t matter at all. In the room you choose for the BDA, you could need some or all of the following:
· A cable pathway to the donor antenna
· Cable pathways to all service antennas
· AC power for either the battery backup or BDA
· A cable pathway to the FACP or annunciator panel for alarming
· Proper earth grounding
· Adequate mounting space and equipment clearance
· Required signage signifying there is a signal booster in the room
· An EPO switch
A good design will contain an exhaustive list of requirements and adhere to all of them.
Service antennas should be located strategically, usually in public spaces (for example, in a residential complex, antennas would be located in common areas such as hallways rather than in private units). Antenna density should be considered and will be discussed further below. Cable pathways must be feasible – for example, try to avoid pathways that require coring through concrete walls if there is an alternate going through wood framing. Some pathways will require survivability, whether this means conduit or 2-hour rated spaces – again, make sure you understand the local code and show on the design which cable pathways will require special consideration.
For a much deeper explanation into designing with Class A vs. Class B, read this tech brief: https://www.combausa.com/en/tech-briefs/class-a-vs-class-b
For design purposes, the key takeaway from the tech brief are that a Class A BDA can be designed with a lower antenna density than a Class B BDA with similar results, ending up with a lower overall cost. It also incorporates channelized gain control, resulting in a lower noise impact on the donor tower.
In iBwave, there are two common ways of modeling the signal strength: Fast Ray Tracing and Variable Path Loss Exponent. Fast Ray Tracing propagation modeling is a ray tracing algorithm that is used to predict in building signals. It considers the actual materials of the building and requires that the user creates a full 3D model of the building. Variable Path Loss Exponent uses a multiplier on the free space path loss equation to approximate the walls of the building without having to model the building entirely. With iBwave, there are ways to accurately and inaccurately use both of these propagation methods.
If using fast ray tracing, creating an accurate 3D model is important for accurate predictions. If using variable path loss exponent, it is important to know that these will not be perfect and you should build in a design margin to make up for unexpected extra losses (the author of this paper uses a 10dB design margin).
Sample 3D Building in iBwave with which you would use Fast Ray Tracing as your prediction model.
Sample floor plan of the same building in iBwave. Note that different areas of this floor are assigned different area types for separate variable path loss exponents.
Neither propagation model will ever give you completely accurate results (unless you use fast ray tracing and use model tuning) and you will need to take into account the time it takes to build an accurate 3D model versus the possible mistakes you could make with a variable path loss exponent.
1. Come to the jobsite prepared with all tools you need to be successful
2. Design the system for constructability
3. Know your jurisdiction - code requirements (published and very new); know the contact people if you have questions
4. Ensure adequate off-air signal
5. Ensure adequate isolation
6. Be ready to troubleshoot
7. DO YOUR LINK BUDGET
Downloads: Commissioning a BDA - A Walkthrough Guide Part II