Class A vs. Class B
Bi-Directional Amplifiers may either be broadband (Class B), band selective (Class B) or narrowband (Class A). Note that this is an FCC classification and has no connection to fire alarm wiring classifications of class A vs. class B wiring circuits.
What's the Difference?
- FCC Definition per rule 47CFR90.219: "A signal booster designed to retransmit signals on one or more specific channels. A signal booster is deemed to be a Class A signal booster if none of its passbands exceed 75 kHz."
- Maximum Passband is 75KHz. Intended to amplify no more than one channel at one time.
- Mainly intended for outdoor use (to extend outdoor coverage).
- Mobile use allowed.
- Introduces signal delay (aka "group delay) of over 50 microseconds resulting in signal distortion and interference in signal overlap areas.
- Higher power consumption, higher heat dissipation = lower efficiency.
- Higher complexity, requires digital signal processing.
- Class A signal boosters are less common and only being used and required in few jurisdictions.
- FCC Definition per rule 47CFR90.219: "A signal booster designed to retransmit any signals within a wide frequency band. A signal booster is deemed to be a Class B signal booster if it has a passband that exceeds 75 kHz."
- Maximum Passband is more than 75KHz. It can amplify more than one channel at the same time. It can either be broadband or band-selective.
- Indoor use only (improve in-building coverage).
- Fixed Use only.
- Very low to negligible signal delay (less than 2 us). Does not introduce signal distortion and interference in signal overlap areas.
- Lower power consumption, less heat dissipation = higher efficiency.
- Less complex design, relies on tried and tested analog amplifiers.
- Class B signal boosters are most common type: Class B signal boosters have been successfully used in many jurisdictions throughout the US. City of Boston Fire Department is one of the early adopters of BDA system requirement. There are currently over 500 active class B signal boosters operating in the city of Boston for BFD.
Class A vs. Class B — Myth vs. Facts
MYTH #1: Class A Signal Boosters are better because they only amplify the select channels, thus eliminating unwanted interference to the BDA.
FACT: 800MHz and 700MHz public safety bands are dedicated for public safety use only. There are no other licensed users other than public safety in those frequency bands. Therefore, there is no need to eliminate other channels because there are no “unwanted channels” in the public safety frequency band. All public safety radio users typically need reliable in-building coverage.
Interoperability between public safety agencies is very important. Making sure that police can talk to fire and that Mutual Aid agencies can reliably communicate inside buildings is very important. Using Class B signal boosters ensures that radios will work for all public safety users, regardless of what frequency channels they use within the 700MHz and 800MHz public safety frequency bands. With Class A signal boosters, all channels must be known and enabled ahead of time, which takes pre-planning and takes long time to update all BDAs in the jurisdiction. It usually makes more sense to amplify the entire public safety band, rather than just a specific, select number of channels within the public safety band.
Another very important consideration is that class A signal boosters have a limited number of channels that they can amplify and it is usually 32 or less. If the AHJ requires additional channels to be covered later, this will not be possible.
The effect of additional “unwanted” channels within the passband of a class B broadband signal booster is not as extreme as imagined in real systems. The most severe implementations with urban areas may have as many as 20 other public safety channel carriers within the passband. When the system is designed for many channels up front the system will perform reliably. The impact on per channel power can be minimal in good designs. For example, if an urban system is designed for 20 potential channels, it would take another 20 channels to reduce the power per channel by 3 dB.
MYTH #2: Class B Signal Boosters can oscillate and produce harmful interference, and that is not the case with Class A Signal Boosters.
FACT: BDA oscillation can happen with either class A or class B signal boosters equally, there is no difference based on the class of the signal booster. What does makes a difference is the Oscillation Prevention Function of the signal booster. For example: all RSI, GW-FCI, Honeywell and Notifier class B signal boosters have built-in oscillation prevention circuits. This feature is a requirement of UL2524 standard as well. This ensure that in case of abnormal operation of the BDA system BDA will self-adjust the amplifier gain to stop the oscillation and to prevent interference with public safety radio systems.
MYTH #3: Class A Signal Boosters are less prone to intermodulation interference. Class B Signal Boosters can produce intermodulation interference because they allow multiple carriers.
FACT: Intermodulation interference can equally happen with either class A or class B signal boosters, all it takes is two or more RF carriers. What does make a big difference is the quality of the design and the immunity of the LNA (low noise amplifier) stage to high RF signals. Our signal boosters are designed to be used in high RF environments such as high-rise rooftops in major metropolitan areas and they far exceed the FCC specified maximum spurious emissions requirements. FCC rules and equipment certification requirements are there to ensure that signal boosters do not cause any harmful interference during normal operation.
MYTH #4: Class A Signal Boosters have a set power and gain per channel, eliminating near-far effect.
FACT: Near-far effect happens equally with class A and class B signal boosters. This is because automatic gain control circuit in the LNA stage (which is not channel-selective) has to reduce the amount of gain to prevent the channel module from being “overloaded”. In other words, signal attenuation in case of near-by signal source happens before the channel selective amplifier and therefore it equally affects class A and class B signal boosters.
MYTH #5: Class A Signal Boosters do not create interference.
FACT: Probably the least understood, yet the biggest shortcoming of class A signal boosters is the fact that they distort the radio signal by delaying it by more than 50 microseconds, which degrades the signal in the overlap areas and creates harmful interference.
As an example (please see the illustration below), if a firefighter is standing in front of a building, he may receive both the direct signal coming from the radio repeater site as well as a delayed signal coming from inside the building. He is standing in a “signal overlap area”. The signal that his radio receives will be distorted since the two incoming signals are no longer the same – one is “real time” and the other is 50us delayed. To the firefighter, this transmission will sound as if two people were trying to talk at the same time, or if it is a P25 digital system the transmission will be broken up or non-existent.
This means that class A BDA may cause signal interference even outside the building.
Worst yet, the same problem happens other way around: the transmission from the firefighter’s radio is received both directly by the repeater and through the BDA. So incoming transmission is now distorted too and other units may not be able to receive the firefighter’s transmission.
This is a very serious problem and a shortcoming that is not very well understood by BDA system integrators or AHJs. Interference caused by class A signal boosters is very difficult to diagnose, especially with P25 digital systems.
Class B BDAs do not cause this problem because they do not introduce any significant signal delay or distortion.