Wednesday, 22 July 2020 18:03

Commissioning a BDA - A Walkthrough Guide Part I

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Part 1: Preparation – What to Bring to the Job Site

The very first step in commissioning a BDA is to make sure you come to the job site prepared. This walkthrough is assuming that the system is installed and ready to be powered up. Although it is usually installed with a battery backup, the setup and commissioning of the battery backup will be skipped in this walkthrough – this will be done in a future walkthrough (although this list of required tools includes what is needed to ensure the BBU is functioning properly as well).

This is not an exhaustive list of requirements for your toolkit, but they will get you through most of your systems with no problems:


You should not show up to the jobsite missing any of this equipment (except maybe the radio). The commissioning procedure to follow will use all parts from this list. However, sometimes you do need a bit more. The other equipment I keep in my toolkit includes:

·       PPE: Depending on the building, I make sure I have all required PPE to enter. PPE includes a hard hat, protective goggles, hard toe workboots, and a safety vest

·       Multimeter: For troubleshooting power and alarm issues.

·       Multibit Screwdriver: I have all the standard bits and an electronics screwdriver set.

·       Crescent Wrench and Adjustable Pliers: I also have a 5/16 wrench for SMA connectors.

·       Termination Loads: Just in case.

·       Fiber One-Click Cleaner: For when you are working on fiber systems – this is easy to carry around.

Part 2: What to do Prior to Going on Site

To make sure you are prepared once you get to the job site, there are some steps to take prior to deployment to ensure you are successful. This includes verifying the link budget, tower locations, system design, and system expectations.

Step 1: Verify Donor Tower Location

Verify with the AHJ, communications office, or other person having authority to be able to quickly check that the donor antenna is pointed to the correct tower.

Step 2: Create an Expected Off-Air Link Budget

Save time onsite and create your expected link budget before going to the job site. Figure out the distance to the donor tower, the ERP of the tower, the expected received signal, and the expected BDA input (and use this to compare on site). Calculate expected gain values so you can figure out what your minimum isolation will need to be. If this is all done in Excel, you can enter your measured values once you get onsite and have it immediately tell you the new BDA settings based on your measured values.

Step 3: Study the Design

Verify that the coverage of the design appears to meet the jurisdiction requirements. Make sure all areas of the building are covered (or if preliminary data shows only partial coverage is necessary, make sure all necessary areas are covered). Know where the head end is, how the cable is run, and where all antennas are. Figure out what your expected DAS signal will be in the head end (this is a quick thing to check once you finally turn the system on, so knowing the expectation can validate your system). Look for and mark any critical areas on the floor plans, so you will know where to check for required coverage areas.

Completing these steps will improve the process of commissioning the BDA and shorten the time spent troubleshooting onsite.

Part 3: Validate the Install Upon Arriving to the Job Site

If possible, the first thing I do upon arrival to the jobsite is validate the installation. If you can meet with the lead installer to have them show you the system, this is even better because they will have the as-builts and be able to tell you about any changes that were required upon install. These steps don’t need to be done in any particular order, but they should all be completed. From the rooftop down, I typically check the following before ever logging in to the BDA:

·       Donor Antenna Installation

o   Is the donor antenna securely mounted?

o   Is the azimuth correct?

o   Is this the antenna that was on the permit set?

o   Does it have line of site to the donor tower?

o   Is the connector weatherproofed?

o   Are the mast and antenna both grounded?

o   Was outdoor rated cable used?

o   Is there a weatherhead or other waterproof entrance into the building?

·       Lightning Protection

o   Is there a lightning protector installed?

o   Does it pass or block DC, as required per install?

o   Is it grounded properly?

·       Donor Antenna Cable Run

o   Is this cable run in conduit, as required?

o   Is this cable run in a rated enclosure, as required? (typically 2-hour for donor cable)

o   Is this cable run riser/plenum rated, as required?

o   Is there a jumper into the BDA? Note: although this is not required, running a ½” rigid coaxial cable into a BDA makes for a more difficult time disconnecting/reconnecting the cable. I always prefer to have a 3’ flexible jumper at the end to make the connection into the donor terminal on the BDA.

·       BDA

o   Is the BDA properly mounted?

o   Is it in a rated room, as required?

o   Is there power to the BDA?

o   Has all alarming been connected?

o   Is the BDA grounded?

o   Is there proper clearance in front of and below the BDA?

o   Is there an EPO switch for the BDA, if required?

o   Are all accessories still with it? (user manual and keys are the two main accessories typically left behind after install)

·       BBU

o   Is the BBU properly mounted?

o   Have the batteries been installed correctly?

o   Is it in a rated room, as required?

o   Is there power to the BBU?

o   Is the power to the BBU in conduit on a dedicated circuit?

o   Has all alarming been connected?

o   Is the BBU grounded?

o   Is there proper clearance for the BBU?

o   Is there an EPO switch for the BBU, if required?

·       Service Antennas and Passive Infrastructure

o   Are all cables, splitters, and antennas installed in the correct locations?

o   Are all cables and splitters in rated enclosures, as required?

o   Are all components connected properly?

This is not an exhaustive list of questions, but it will ensure you have inspected the installation as needed. Depending on your jurisdiction, this list can be edited to match the local requirements.

Part 4: Commissioning the BDA

Step 1: Measure Off-Air Signal

Measure the off-air signal at the input to the BDA. Plug your spectrum analyzer into the donor cable input to the BDA and record the control channel signal strength. This should be close to what you expected based on your calculated link budget. Update the numbers from your link budget with this new, actual value. Calculate your actual gain required for full output and the required isolation to achieve this gain.

Note: Signal Strength = -60 dBm Off Air

System has 8 Channels. Composite power = -60 dBm + 10*log(8) = -51 dBm off air signal, composite

For 33dBm BDA, Gain = 33dBm  – (-51dBm) = 84 dB Gain Required.


Widen the span on your spectrum analyzer to look at the wideband input to the BDA. Ensure that there are no strong interferers that may affect the downlink ALC. Be wary of adjacent cellular signals – if there is an adjacent cellular signal that is more than 10dB stronger than the public safety signal it could impact the downlink performance of the BDA. In this case, consider cellular rejection filters, such as the Comba FP-78-IN1.

Note the cellular signal on the right side of the screenshot. Our public safety signal is well above the cellular signal strength so we will not need any additional filtering here.

Step 2: Test Isolation

Although automatic isolation testing is included in all Comba’s BDAs, this walkthrough is going to include manual testing as part of commissioning. This section may be skipped, but no BDA guide is complete without it.

The image below shows the general setup for testing isolation. From the previous step, leave your spectrum analyzer plugged into the donor antenna line. Plug a signal generator into the service antenna line. On your spectrum analyzer, find a clean frequency (a frequency without a noise or signal on it). Set your signal generator to that frequency at the maximum output power (note: do not transmit above 30dBm into the service line – most field signal generators have a max output of 10-20 dBm). Depending on your jurisdiction’s requirements, you may need to measure at certain frequencies or find a low, middle, and high frequency to measure.

Your total isolation is the transmit power minus the receive power. For example, if you output +20 dBm from your signal generator and measure -95dBm on your spectrum analyzer, the isolation is 20 dBm – (-95 dBm) = 115 dB. 

It is good practice to measure isolation while injecting into both the donor line and the service line (to simulate uplink and downlink isolation). When performing these tests, you should use appropriate signals (when injecting into the donor line, use uplink frequencies, and when injecting into the service line, use downlink frequencies). It is important to note that you should not inject a signal into the donor line for an extended period of time or on a frequency that is licensed and being used in the area – this could cause noise on someone’s radio system.

Step 3: Perform Software Setup of the BDA

Now it is time to log in to the BDA and set it up.

Log in to the BDA, making sure you are using an incognito or private browsing window. The IP address of Comba BDAs is, the username is admin, and the password is admin.

Navigate to the devices page and begin your frequency setup. For a Class A BDA, type in each channel for the system. For a Class B BDA, type in the frequency range for the system. For Class B systems, either enter frequencies based on jurisdiction requirements or find the lowest frequency and round down to the nearest 200 kHz and find the highest frequency and round up to the nearest 200 kHz. For example, if your frequency range is 855.1625 through 857.725 MHz, you would set up the BDA as 855.0 through 857.8 MHz. Turn the RF switches for the channels or sub-bands on, but leave the overall RF switch off.

Go through the BDA commissioning guide and follow the steps. The BDA isolation in the commissioning guide should be close to the isolation you measured previously. When doing the channel setup in the commissioning guide, you should expect to see similar numbers to what was measured with your spectrum analyzer and calculated in the link budget. See the BDA user manual for more information on the BDA commissioning guide.

Sample Isolation Test Results – Note: Minimum Isolation for 84 dB of gain is 84+20 = 104dB

Once complete with the commissioning guide, ensure your RF services are all turned on. Set the downlink gain to what was measured with your link budget. Look into the appendix for how to do a proper link budget. At this stage, I recommend leaving the uplink gain set to the lowest setting while doing downlink testing, then coming back to set up uplink once completed.

Channel Setup of the BDA

Unplug the service line from the BDA and connect your spectrum analyzer to the MT port on the BDA (make sure you use attenuators as necessary). Check that your downlink output from the BDA is what you would expect based on the input and the gain (note: Output = Input + Gain). This will be your Downlink BDA Output. You can also verify that the wideband downlink output does not have any additional spurious emissions that could affect your system.

Downlink BDA Output. Note: -60 dBm input + 84 dB Gain = 24 dBm Output

Step 4: Antenna Verification Testing

We are going to break downlink testing into two categories: antenna verification testing and final grid testing. In between the two, we will be setting the uplink gain.

For antenna verification testing, you will walk underneath each antenna and make over the air measurements of the signal strength of your system at about 5 feet away from each antenna. Verify that the signal strength you are seeing is as expected (your iBwave or Ranplan design should have an expected output from each antenna, and free space path loss at 5 feet of distance is about 35dB for 800MHz and 30dB for 470MHz. If your antenna output is -10 dBm per channel, you would expect to see -45 dBm at 5 feet away on 800MHz or -40 dBm at 5 feet away on 470MHz).

Use this test as a validation of the installation. If one of the measurements is significantly higher or lower than expected, you may have a bad cable or a reversed coupler.

Record the maximum value that you see at 5 feet away from an antenna. This will be your Downlink Maximum Measured Signal.

While doing the antenna validation test, go to the expected worst signal coverage areas as well (this can usually be found by looking at the prediction heat maps or by studying the building and finding the spot furthest away from an antenna or with the most walls between it and an antenna). Record the lowest signal that you measure – this will be your Downlink Minimum Measured Signal.

Step 5: Uplink Settings

Unless you have someone at the donor tower measuring the uplink received signal strength, configuring the uplink is going to be based on calculations only. We want these calculations to be as accurate as possible. Below is a sample uplink gain calculation table:


In this table, the Downlink BDA Output was measured at the MT port of the BDA in Step 3 and the Measured Signal are the  Downlink Maximum Measured Signal and Downlink Minimum Measured Signal that were measured over the air in Step 4.

DAS Loss is calculated with the equation DAS Loss = Downlink BDA Output – Measured Signal. Note that the minimum measured signal corresponds to the maximum DAS loss, while the maximum measured signal corresponds to the minimum DAS loss.

We start the uplink section with downlink measurements because we assume that the reverse link budget is the same as the forward link budget for passive losses. At this point, I will concede that it is more accurate to have a radio in hand and make actual measurements or use a signal generator set to the same power level as a radio to make the measurements. In practice, most system integrators do not have these tools, so we must work with what we have.

The Mobile Radio Output is the power output from the handheld radio. You can either look up the brand that is used in the jurisdiction, ask the AHJ, or ask the communications department for this information.

The Expected UL BDA Input = Mobile Radio Output – DAS Loss. To ensure the best performance from the BDA, it is recommended that your maximum UL BDA input is not greater than -30dBm. If it is, and your downlink cannot be attenuated, you may use external attenuators inside the BDA to reduce the UL input power only.

Side note: Let us quickly go over design philosophy while talking about the Expected UL BDA Input. When designing a Public Safety DAS, the usual goal is to ensure the system coverage level is greater than -95dBm. In the example above, convenient numbers were used to make this look easy to do. What if, instead of -45 dBm as our maximum measured signal and -85 dBm as our minimum, we had -35 dBm and -95 dBm, respectively. That puts our expected UL BDA input at -21 dBm maximum and -81 dBm minimum. Based on the maximum of -21 dBm, I would want to put a 10dB attenuator on the UL side of the BDA, but based on the -81 dBm minimum, if my gain is set to 90 dB without adding this additional 10dB attenuator, the signal is reaching the donor site at -101 dBm. What can make this situation worse is having two first responders in the building, one at the minimum measured signal location and one at the maximum measured signal location, trying to communicate on different talk groups. Please read about the near-far problem for more information. Although a BDA can help mitigate this issue, the best way to ensure proper functionality of the system is to make sure you have a good design to begin with, that incorporates a sufficient number of antennas.

The BDA Gain is next on the table, but this is usually entered last. This will be the setting we will enter into the web GUI.

The Uplink BDA Output = UL BDA Input + BDA Gain. The minimum is usually simply Minimum Input + Gain, but the maximum is usually ALC limited (which is perfectly fine and a normal function of the BDA). Based on your ALC Setting (Mode 1 is total power shared by number of active channels, Modes 2 and 3 are total power shared by number of programmed channels), the Maximum UL BDA Output is either going to be based on the spec sheet maximum output, the spec sheet maximum output split equally between the number of channels, or for rare cases, the maximum UL BDA Output will just be the UL BDA Input + BDA Gain.

The Path Loss to Tower can either be calculated using the distance to the tower and transmission line losses, or if the ERP of the downlink signal at the tower is known, can be calculated by Path Loss to Tower = Tower ERP Output - Downlink BDA Input.

Check with your jurisdiction on requirements for the Tower Received Signal Strength. This is calculated with the formula Tower Received Signal Strength = Uplink BDA Output – Path Loss to Tower. It is recommended that you enter the jurisdictional requirements here, then find out what gain is needed at the BDA to achieve these requirements.

Once calculated, we can now program the UL gain setting into the BDA.

BDA Settings after setting UL Gain.

Note: The UL noise floor has been ignored for these calculations so far. Look for a future update that expands on how to calculate the thermal noise floor at the donor tower. 

Step 6: Final Grid Testing

Now that we have both UL and DL set as needed, we can do the final grid testing. Split your floors into 20 grids (or the appropriate amount as required by your jurisdiction) and make signal level measurements in each grid. If you have a radio, perform a DAQ test in each grid. Otherwise, walk with the AHJ to ensure their radio works in all places they deem appropriate to test (again, this will vary greatly by jurisdiction).

Sample 20-Grid Test

Once everything passes, it is recommended to go back to the BDA and export the BDA RF settings and include this in your grid testing report. This will make it easy to validate settings during future annual inspections.



View Part II here:

Downloads: Commissioning a BDA - A Walkthrough Guide


Wednesday, 24 June 2020 11:41

Comba Insider: Issue #31

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Comba Insider: Issue #30

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Comba Insider: Issue #29

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Comba Insider: Issue #28

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Comba Insider: Issue #27

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Comba Insider: Issue #26

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  • YEAR 2


When designing a public safety DAS for a single large building, sometimes you need more than just a single BDA to provide the coverage needed. While there is a choice between placing multiple BDAs into the same building or using a fiber DAS, the answer is clear that a fiber DAS is a much better choice.u need more than just a single BDA to provide the coverage needed. While there is a choice between placing multiple BDAs into the same building or using a fiber DAS, the answer is clear that a fiber DAS is a much better choice.


Multiple independent BDAs will be ignored in this paper – for an in depth discussion, please read the previous campus fiber DAS paper, which is more relevant to why multiple independent BDAs are not a good idea – especially within the same building, where they have more possibilities to cause interference with each other:

Installation Simplicity

We will start by looking into the logistics of an installation:


Fiber DAS

Cascaded BDAs

Split Donor

Donor Antenna

Single Donor

Single Donor

Single Donors

RF Transport

Fiber from MU to each RU

Coax from BDA to BDA

Coax from donor to each BDA


Can be done at a single point or at each RU

Must be done at each BDA

Must be done at each BDA


Single point of control for the entire system

Controlled individually at each BDA

Controlled individually at each BDA

Installing a fiber DAS is the most similar situation here to having a single BDA – with a fiber DAS, there is a single donor antenna into a master unit. Fiber is used for essentially lossless RF transport between the master and the remote units. The single point that would need dry contact alarming is at the master unit, but if the jurisdiction requires, it can also be done at each remote unit individually. The system integrator would perform all commissioning activities at the master unit, so any changes that occur at the master would get pushed to the remote units.

In a cascaded BDA system, there is still a single donor antenna, but now there is coaxial cable that must be pulled between each BDA. At each BDA output, signal is tapped off the main feed and transported to the next BDA. Care must be taken to ensure you meet the link budget as designed. Any RF changes must be made and double checked on each BDA in the system – as the name implies, if you make a change, it has a cascaded affect, so the user will need to be constantly going between IDF closets to take measurements and make adjustments. In the cascaded BDA scenario, alarming must be done at each BDA individually, so each BDA will require a full set of dry contact alarms – which also necessitates multiple long runs of alarm cables back to the FACP.

In a split donor antenna system, there is a splitter at the donor antenna with an individual coaxial cable run to each BDA. When designing for a large building, this might mean that an individual coaxial cable run from the donor antenna could be hundreds or thousands of feet long – either requiring 7/8” cable or making signal strength requirements unachievable. Similar to the cascaded BDA system, alarming must be done at each BDA individually. However, now each BDA will act almost as an independent system, so while the system integrator must go to each BDA and commission it separately, there is not a cascaded affect when making changes to just one of the BDAs.

Cascaded BDA Solution (and why to not use this!)

With a cascaded BDA design, we have an obvious problem that should prohibit us from ever using this approach: cascaded noise. The figure below shows a sample cascaded BDA system with 3 BDAs. At the output of each BDA is a 30dB directional coupler, with the coupled port going on to the next BDA down the line. At the input to the next BDA there would be an attenuator of appropriate value, based on cable length and link budget. 


Let us look at what the uplink noise would be in this example:

Assuming a perfect setup and that UL noise rise = UL gain + noise figure, after 3 BDAs we go from a -47 dBm noise power output to a -35 dBm noise power output. This is a 12dB rise that makes our signal to noise ratio 12dB worse (in addition to reaching the tower at a 12dB higher noise floor).

This is a perfect scenario that assumes that noise power is related to the BDA gain – in many digital BDAs, noise power is related to maximum BDA gain. Let us look at this again assuming noise power is related to maximum gain rather than set UL gain. In this scenario, the gain is set to 80dB while the maximum BDA gain is 90db:

With all other assumptions the same, we see that the noise power out of the last BDA in line is -5 dBm, although our uplink signal power remains the same. This noise power rise is only 15dB weaker than our “low” signal strength output, which will almost certainly raise the noise floor at the donor site.

In reality, the uplink noise rise of a cascaded system will probably fall somewhere in between these two examples (the noise figure is not truly summed in this fashion – look up “Friis Formulas for Noise” for a deeper understanding), but even a small miscalculation in gain or loss can dramatically raise the noise floor (and then get cascaded through multiple BDAs).

In addition to the noise rise in a cascaded BDA system, time delay must also be studied. Let us assume we are in a jurisdiction that requires a Class A BDA and that each filter adds 20 microseconds of delay. Through 3 BDAs, that will add 60 total microseconds of delay, which is more than some radio systems can handle. Remember that this is 60 microseconds on uplink and 60 microseconds on downlink – this means the total roundtrip delay for this system is 120 microseconds. It depends on the digital system that is in place but many trunked systems would not be able to handle this amount of delay.

You would also have the issue of standing equidistant from antennas on 2 separate BDAs – if you have an equal power input to BDA3 and BDA2, then transmitting back to the antenna would give you the exact same signal strength twice, offset by 20 microseconds. This may lead to time delay interference (see for more information on TDI).

Overall, it is highly recommended to not use cascaded BDAs in a single large building because a simple mistake that could be overlooked on a single BDA system can turn into a large noise rise through multiple uplink amplifiers.

Split Donor Antenna System

An alternative to the cascaded BDA solution is using a single donor antenna and a passive splitter and splitting the donor signal to multiple BDAs. This solution is much better than cascading BDAs because you no longer have in-line amplifiers, but it still has many flaws.

Donor antenna isolation is problem number one with this solution. For a split donor antenna system, you have multiple BDAs running off a single donor antenna – you will need to manually test for isolation because a BDA on its own cannot detect oscillation between itself and another BDA’s service antennas. The isolation test will need to be done at the donor antenna through all the BDAs (so you must use a low power signal tuned to a passband frequency of the BDAs). BDAs have oscillation protection and shutdown, but this is typically programmed for a single BDA oscillating. If two separate BDAs are contributing to the oscillation, they may not detect a noise rise as oscillation and could continue transmitting as noise.

A second issue with a split donor antenna system is the simple problem of cable length – when a building is large enough, you may be running many hundreds or thousands of feet of coaxial cable between the donor antenna and the furthest BDA. Presuming approximately 2 dB of loss per hundred feet of coax this can add far too much loss to your link budget to enable an acceptable design and a working system.

Fiber DAS

For a single building that is too large for one BDA to cover, it is recommended to use a fiber DAS. A single master unit with a short run for the donor antenna ensures adequate off-air signal strength. Low loss fiber optic runs to the remote units ensure that no building is too large to cover. Flexible alarming ensures you can meet AHJ requirements for dry contact alarms. Uplink signal strength (and therefore noise) comes from a single output port on the master unit.            

All changes to the system may be done directly from the master unit, so frequency settings, commissioning, isolation testing, and software upgrades are all done at one location.

A fiber DAS does have an additive noise floor based on the number of remote units, so the design needs to take this into account. However, the noise is only through the master unit, so one can easily measure and mitigate the noise produced from the system. With the simplicity of ensuring network compliance and meeting design criteria, the clear solution for public safety coverage in a large building is a fiber DAS.


Downloads: Choosing Between Fiber DAS and Individual BDAs for Larger Building Applications