A new technology available to public safety in-building systems integrators overcomes some of the most vexing challenges of modern multi-band, multi-agency coverage systems. The receiver system from Safe-Com Wireless introduces unique, advanced front-end receiver design to perform clean, high-dynamic-range sub-band tunable filtering especially applicable to multiband systems and interlaced frequencies.
The crucial role of police, fire, emergency and security services’ land mobile radio communications is obvious. The fact that most incidents happen indoors makes the use of those communications within buildings critical.
Enter the public safety in-building coverage industry: a pairing of public safety equipment manufactures and systems integration firms.
The first group provides the bidirectional amplifiers (BDA) and fiber-optic distributed antenna system (DAS) equipment. The second designs, integrates and maintains the system on site.
And although I place the birth of this important industry on 9/11, 18 years ago, when the horrendous mass killings at the New York World Trade Center exposed the national shortcoming of in-building public safety communications, we are still in our early stages as an industry.
Evidence of this is found in the challenges that still trouble many of us. For example, how does one properly address the integration of multiple agencies and multiple radio-frequency (RF) bands, many with challenging RF spectrum situations, including frequency interlacing and close-in transmit-receive (TX-RX) frequencies.
The interlacing of radio channels that can generate radio interference mainly occurs in the VHF and UHF bands (see Figure 1).
One of the primary causes of interlacing is the combination of multiple agencies’ channels onto a single DAS. Frequencies in the UHF bands, and sometimes VHF bands, were assigned with adequate separation of uplink and downlink frequencies. Adequate separation allows duplexers to be built with a typical 3 MHz to 5 MHz separation at a relatively low cost. The separation was important because it allowed current cavity filter technology enough guard band to filter out the high-powered downlink from leaking into the uplink. When the transmitter signal leaks into the receiver, the result is receiver desensitization. Receiver desensitization causes radio signals that originate at the distant edge of the coverage area to be lost in the elevated noise floor created by the TX noise leaking into the low-noise receiver. With 3 MHz to 5 MHz of separation built in, all was good in the land of mobile radio base stations and single-agency repeaters. Right?
All that falls apart when multiple agencies come together on one bidirectional amplifier system, as required by today’s multiple-agency in-building coverage needs. Fifty years ago, when the frequency system was developed, no one anticipated that this interlacing problem would occur.
The current solution is expensive, highly customized and physically large and requires long lead times because of its custom nature.
The old way requires custom-built cavity filters called triplexers and quadplexers. These are the larger, bulkier and pricier relatives of duplexers. They achieve the filtering using legacy cavity filter technology. It works, but it comes with problems.
A Better Solution
A better approach uses technology found in every base station and two-way radio in the world —and now uniquely implemented for the coverage industry: receivers with electronic filtering and front-end low-noise technology that can handle strong signals and can filter close-in frequencies to cleanly separate single or banded frequencies from others without interference.
The key attributes of the technology are that it is compact and electronic, tunable and modular, it has an adaptable standard design and its cost is low.
How the BDA Works
Figure 2 reveals the power of the new BDA filtering system. In the complex frequency environment that interlacing represents, ideally one would want multiple sub-band filters to be placed around every set of received frequencies. That’s exactly what the new front end BDA design does. Each sub-band filter is a plug-in card as seen in Photo 1. Each plug-in card can be set for its desired bandwidth and tuned by the user in the field to any center frequency desired. The cut-off of the receiver is extremely sharp — as much as 50 dB rejection in 120 kHz, so there are few signals that cannot be managed well with this advanced BDA design.
The block diagram in Figure 3 shows where filters are employed: right at the front end, where they are needed most. They replace the single broadband low-noise amplifier (LNA) typically found in legacy BDA systems with multiple parallel sub-band LNA modules that handle the advanced filtering required for complex interlacing found in many UHF and VHF systems.
The result is a reasonably priced high-performance BDA that can handle the toughest jobs.
Close-in TX and RX Frequencies
A comparable issue occurs with some single-agency radio frequency pairs that also greatly benefit from the BDA system. In an interlaced frequency environment, uplink and downlink frequencies appear close to each other because multiple agency frequencies were combined into a single system. There also however exist channel pairs, especially in the VHF bands, with a separation of a few hundreds of kilohertz. These systems are never easy to handle because the duplexing challenge can only be solved by large, expensive duplexers.
I recall a project with VHF frequencies so close together that we needed to rent a hydraulic winch on wheels to lift and maneuver the refrigerator-sized duplexer required. Those days are over. The front end of the new system combines channel pairs (uplink and downlink) onto one antenna and ensures proper filtering of the transmit frequency to avoid receiver desensitization. This approach can also avoid separate transmit and receive DAS antenna cable systems, further reducing install costs.
Receiver desensitization occurs in both directions with a BDA — on the donor side and on the service (in-building) side. Thus, two large and expensive duplexers are eliminated with the use of new BDA filtering design.
A fiber DAS is an even greater beneficiary of the system. In that case, every remote unit will save by using the new receivers.
Bulky duplexers and multiplexers are eliminated and replaced with small, tunable, plug-in cards.
Digital DAS Gorilla in the Room
At Safe-Com Wireless, we made a conscious decision on the design approach to our new public safety BDA and fiber DAS product line. We chose the highest dynamic range, lowest noise and lowest intermodulation interference design approach with proven technology. Therefore, what we did not use was digital signal processing of RF signals. I know: Digital is the current rage. It is shiny and new. But here is a dark, dirty secret: Digital signal processing is horrible when used to digitize RF signals. Here are a few points:
· It generates massive spurious emissions because of heavy digitizing and processing.
· It introduces interference with some digital voice technologies.
· It has a low spurious-free dynamic range — even 16-bit systems.
· It cannot handle high RF power inputs.
· It cannot handle a broad range of frequencies in one frequency band.
A smart set of engineers reviewed all the technology choices available, and the best choice turned out to be the analog RF superheterodyne design approach. That has become the Multi-Band Flex BDA and Fiber DAS System. It has the widest available dynamic range and a simple, reliable and proven architecture. The design is modular and highly configurable to handle the wide range of challenges that public safety communications presents.
The new BDA and fiber DAS system permits unencumbered integration of multiple bands, multiple frequencies and multiple technologies in a BDA or fiber DAS for public safety that complies with National Electrical Manufacturers Association (NEMA) and National Fire Protection Association (NFPA) requirements. Unique combinations that the system can handle include VHF, UHF, 700 MHz, 800 MHz, 900 MHz (including paging and two-way radio), federal VHF and UHF, the FirstNet network, duplex or simplex operation and bidirectional or unidirectional amplifier operation.
Henry Wojtunik is founder and chief technology officer of Safe-Com Wireless, a designer and manufacturer of advanced BDA and fiber DAS technology for public safety communications systems. Visit www.safe-comwireless.com.