Public Safety Advocate: Radio Interference and the Network Below Other Networks

This week’s Advocate is a little late due to an out-of-office work commitment. Meanwhile, many activities that will impact public-safety communications in 2020 continue. One issue we must increasingly deal with is wireless communications interference. As the FCC and NTIA find more ways to allocate or share more spectrum to accommodate other users and groups, the ability to resolve interference problems becomes critical. There are many types of radio interference, and most interference is not intentional. This week we will look at a few types of interference and how to minimize problems. 

Next we will look at a company that has a new take on providing rural coverage, which it refers to as “the network under other networks.” I believe this approach is promising and could help extend broadband to rural America not only for businesses, farmers, and citizens, but also to extend FirstNet coverage into more rural areas.

Radio Interference

There are a number of different types of interference to existing radio systems be they TV and radio broadcast, cellular, or Land Mobile Radio (LMR). The interference types that will be covered in this Advocate include:

  1. Noise-floor increases
  2. On-channel interference from another user on the same channel
  3. Adjacent-channel interference (bleed-over-co-channel inference)
  4. Harmonic interference—2nd, 3rd and 4th order 

There are other types of interference but at this point it should be noted that while the FCC and the NTIA, which is responsible for government radio channels, have both mandated requirements for radio transmitters for the various types of radio services. There are no such requirements for radio receivers. Some of the interference types listed above can be caused by radio receivers which, for various reasons, are designed to cover a wide swath of radio spectrum and are not as immune to some types of interference as older crystal-controlled radios. 

It should also be noted that all types of radio systems operate within a specified portion of the radio spectrum. LMR radios operate on very narrow channels, TV broadcasters operate within a TV channel band that is 6 MHz wide (with digital TV, all that spectrum is no longer required), and cellular broadband systems operate within allocations that can vary from a few megahertz of spectrum to more than 20 MHz. Unlicensed spectrum such as WiFi use different amounts of spectrum depending on the type of WiFi the radio is designed to transmit. 

Let’s start with Noise Floor issues. A good definition of the noise floor is provided by everythingRF:

“The Noise Floor is the signal created from adding up all the unwanted signals within a measurement system. The noise floor consists of noise from a number of sources which includes thermal noise, atmospheric noise and noise from components used to make the measurement system.

Noise floor is an important parameter in spectrum analyzers and vector network analyzers. It determines the lowest possible signal level that these systems can measure. For example, to measure a signal that is -140 dBm, the system must have a noise floor of less than -140 dBm.”

Since most people don’t understand what -140 dBm means, there is another way to perhaps understand the implications of noise-floor issues. Testing has been conducted at the Idaho National Laboratory radio test range because it is far from most sources of radio and other noises that could raise the noise floor. Since this test range has a low noise floor, equipment can hear radio signals that are just above the noise floor. You would not be able to receive the same signal in a major metro area, because the noise floor is higher and the signal would be lost in the noise. 

Another way to look at this is that when you installed a WiFi access point in your home or office five years ago, the range was probably pretty good and data speeds were adequate. However, over time, the range of that single access point seems to have diminished and the data rates are slower. Both the loss of coverage and slower data rates are caused by a rise in the noise floor as a result of all your neighbors installing their own WiFi access points. The “cure” has been to add more access points to regain the coverage you once had. I measured the noise floor in my office and electronics workshop when we moved in about four years ago and measured it again for this column. There has been a significant increase in the noise floor and a computer search for WiFi access points tells the story. Today I can “see” many more access points than only four years ago. Since most of these access points operate on the same portion of spectrum, the noise floor has increased. The easiest ways to overcome the increase in the noise floor are to increase the transmit power (which is limited by FCC rules) or add more access points to regain the coverage you once had.

There are other ways the noise floor can be increased for a specific segment of spectrum. As mentioned, the FCC and NTIA issue radio licenses for specific uses on specific spectrum. While radio transmitters are regulated, they can emit lower amounts of radio signal outside their designated channel. This is “bleed-over,” which is described by as “a strong signal on an adjacent frequency is “bleeding over” or leaking into the channel you’re listening to and has nothing to do with overload, intermodulation or intermediate frequency (IF) images but rather insufficient receiver selectivity to reject it.” Bleed-over should come to mind when interference is suspected. In the case of the Oakland, Calif. Interference (see below), using different handhelds from different vendors helped determine that only one LMR handheld suffered from the interference and other devices from the same and other vendors were unaffected. 

The amount of radio power permitted to bleed over from one channel to the next is stringently regulated by the FCC. Since receive specifications are not mandated by the FCC, a receiver designed to work on many channels within a common portion of spectrum is more susceptible to bleed-over than a radio designed to listen to only one channel that has filters to help eliminate bleed-over. We will be seeing more and more bleed-over interference and it is past time for the FCC to mandate “smart receivers” that can be automatically “tuned” electronically for a given channel. When the radio is tuned to a different channel, the system would automatically retune the receiver to the new channel. Unfortunately, it appears that neither the FCC nor radio vendors want to implement a smart receiver. 

On-Channel Interference and Bleed-Over

There are two types of on-channel interference (not counting malicious on-purpose interference). With LMR, FCC rules make it clear that a license is not exclusive and the spectrum will be shared with others. When it comes to public-safety and business radio spectrum, the FCC requires an application for a license on a specific channel or sets of channels that must be “coordinated” with a frequency coordinator. In the non-public-safety world, for-pay coordinators can be hired to validate that the channel(s) applied for in a given area are “clean,” meaning no one in that specific locale has a license for the same channels, but others may be licensed next to the specified operating area. The same is true within the public-safety community. If a frequency might cause interference, the coordinator can require the new applicant to perform a test, and the existing license holder may request a waiver before the channel can be licensed to an organization in a different area.

FCC rules are fairly clear when it comes to on-channel interference. Since both parties (the interferer and the agency interfered with) are licensed for that channel by the FCC, the newest license holder is generally held responsible for correcting any interferences issues. 

Other types of radio systems have different rules. Cellular and broadcast services generally have an exclusive license for their systems in a given area, but there can still be interference to or from these services due to atmospheric conditions. You may have, for example, listened to an AM radio station thousands of miles away during evening hours but during the day the station was not audible. This is because radio signals are susceptible to atmospheric conditions that can affect where the signal can be heard. 

Harmonic Interference

When two or more radio channels mix, usually at the same site, the sum or difference of the signals can create what might appear to be on-channel interference and it can be extremely difficult to identify the source of harmonic interference.

For a number of years, I owned an old FAA radio communications site that had been decommissioned in the hills above San Jose, Calif. I leased space to a number of one-way paging companies that were transmitting at high power to cover a large area. I also had two-way radio customers on the site. 

One day I received a call from an LMR operator saying it was being interfered with by another customer. I packed up my spectrum analyzer and, accompanied by several hams, went to the site to begin my investigation. It took us many days of searching before we found that a rusted bolt on the tower was causing two of the transmit radio signals to mix. This combination of two frequencies generated what sounded like a signal to the receive channel of the complaining customer’s radio. We cleaned the offending bolt and the problem went away. 

I have also seen rusted barbed wire, rusted chain-link fences, and other metals mix two or more signals and cause interference on another channel or channels above or below the two channels that mixed with each other.

Interference Conclusions

The issue of interference can be confusing and there are even more types of interference in addition to those listed above that can cause performance degradation to all types of radio systems. One reason I am opposed to sharing spectrum between unlicensed and licensed users is that when there is an interference issue (and there will be issues), there is no way for the licensed user to locate the unlicensed interfering transmitter and determine who owns it and is responsible for its proper operation.

A number of interference issues can also occur with older radio equipment. Recently, interference between FirstNet and the 700-MHz public-safety LMR service was traced to older radios in the LMR fleet. Before FirstNet was approved, some licensed 700-MHz LMR spectrum users were also licensed to use the portion of the band that has since been converted to LTE broadband for FirstNet. These systems were moved out of FirstNet spectrum and relocated into the remaining LMR 700-MHz spectrum. However, older radios had been built to receive and transmit in both the current LMR 700-MHz spectrum and what is now FirstNet spectrum. These radios seem to be susceptible to what appears to be interference from FirstNet. The cure for this does not lie with fixing something in the FirstNet network. The answer is to retire these radios or rework them to remove what has become the FirstNet portion of the public-safety spectrum. This should be considered a receiver equipment problem and not an interference problem.  It has been twelve years since the FCC made this band plan change and it should be obvious that it is time to replace equipment that was built to the previous band plan.

Reports of this type of interference are not new. The Oakland, Calif. Police Department experienced a great deal of interference before the FirstNet network was turned on and it blamed a cellular radio system. The cure for that interference was to replace the radios with newer radios that are not as susceptible to cellular network bleed-over.

As more systems are licensed and the FCC continues to push for shared services including unlicensed use of licensed spectrum, we will continue to see an increase in interference. I believe technology exists to turn radio receivers into smart receivers that will automatically tune to the appropriate channel or band segment and reject other sources of interference. Since the FCC seems to be ignoring these issues and vendors aren’t willing to voluntarily do anything that might increase the cost of their devices by even a few bucks, I believe these problems will only get worse. 

 Networks Below Other Networks

Several weeks ago, I received an email from some people who suggested that since I was a proponent of rural broadband, I should find out more about what they are doing. I looked at their website, which is set up as a teaser at the moment, and decided to schedule a conference call with them. What I learned is really interesting.

The company is Space Between Enterprises (SBE) and I found its business model to be based on the needs of rural America starting with farms and expanding out to provide its network to others in rural communities. SBE began by building a network to demonstrate its Internet of Things (IoT) capabilities. The requirement for the first farm was simply to turn water values on and turn them off. As SBE expanded, it developed a way to place its networks below existing cellular networks and, in some cases, to use the cellular networks for backhaul for its own network. I have been told the company is currently working with FirstNet and other networks to prove its concept. 

Before I continue, once I learned it is using mesh WiFi networks I was somewhat skeptical. My regular readers know I have never been much of a believer in muni-WiFi systems as I never found a business model that worked, as can be evidenced by the number of muni-WiFi failures over the past ten years. However, as we continued our discussion, I came to understand that what does not pencil out in cities and even suburban communities does make sense when serving rural areas. While SBE is using WiFi, it has incorporated some proprietary technologies into the system. Even so, anyone who has WiFi-capable devices, including smartphones, can use the system.

The essence of the system is to set up multiple smaller, less expensive and easy-to-erect towers and place them so the network is truly mesh. So far, activities have been focused in California where it has been very successful starting with farm communities and expanding services to cover more of the rural areas. 

It appears from our discussion that while SBE is using cellular and some satellite backhaul, mesh WiFi, and other low-power sensor networks, as technologies evolve, the system will evolve. One of the founders has been deeply involved in the cellular and radio industry for a long time working with 1G through today’s 4G and soon 5G systems. Today, it has a 5G mobile router. I like the business model of being below cellular networks and currently providing speeds from 5 to 25 Mbps down and somewhat less up to the network. While these speeds may not impress those expecting any and all 5G systems to provide gigabit speeds and who will soon be disappointed in mid and low-band 5G, these speeds are sufficient for streaming video, gaming, text, voice, and, of course, IoT devices and applications at rates that are more affordable than current commercial cellular network pricing. 

SBE is working with FirstNet and SBE folks tell me they are considered to be a “network extender” for FirstNet. As such, they can provide their home gateways with the FirstNet (Built with AT&T) SIM for FirstNet access. 

We now have more, different types of build-out in rural areas, which means better broadband for all including first responders. These build-outs include FirstNet, rural cellular companies that have partnered with FirstNet, and now SBE with its network extender or below other networks mesh system. It appears that all this focus on providing rural broadband to those who do not currently have access will make it easier and less expensive to extend broadband services for FirstNet customers as well as organizations, companies, farms, and individual citizens that want access to broadband as soon as possible.

I plan to follow SBE’s progress as I think it has a niche that can become viable for deploying networks. A next step is to connect to fiber where it is available. Again, I was impressed with the company’s understanding of what is needed in rural areas and its ability to provide services under the umbrella of the major networks while also incorporating access to these networks.

Winding Down

This past week I was conducting drive tests in a specific county. The data we collected includes coverage for FirstNet, cellular systems, and existing LMR police, fire, and EMS systems. This data will enable the client to determine what type of coverage is available in what part of the county or city, and how cellular networks, LMR systems, and FirstNet stack up against each other.

In this set of tests, we were briefed on what local agencies consider the most important areas for coverage. The influx of people into the county for recreation and where they are located changes as the seasons change. Overall, we continue to find that LMR systems offer the best coverage but LMR is only for push-to-talk services. We do not publish data when doing this type of work since it belongs to the client and we have agreed not to make it public. However, we will continue to publish network comparisons we perform on our own and not under contract. 

This time around, in addition to Sierra Wireless MG90 mapping data that is stored in a cloud, we added a second set of data that measures signal strength in DBM as well as coverage, and we can collect up and down data speeds at various way-points. This last test is dependent on time of day and several unknowns including how many users are in the same cell sector and channel loading that varies on a near-constant basis. With all this data, we can determine where improvements are needed and identify any dead spots in LMR systems.

While most network operators, including FirstNet, are constantly checking their own and competitors’ coverage with their own or sub-contracted drive tests, we include LMR coverage as well. Sometimes we recommend using major network operators with FirstNet as prime and other networks as a data-on-demand type of connection. On this set of drive tests, we noted that a fair number of areas are covered by FirstNet Band 14, even in more rural areas of the county. 

This computes not only with FirstNet’s claims about Band 14 roll-out but with our own observations driving in the Phoenix area. I drive different routes to the same location and I come across new Band 14 sites as they are added. It is interesting to run speed tests on these new sites (with the above-stated caveats) because Band 14 up and down speeds tend to be higher than non-Band 14 speeds. This is partly because Band 14 is not yet loaded to the same level as other LTE spectrum that is used for commercial services as well as the public-safety community. But that only explains some of the differences we are seeing.

Now I am back in the office, finishing up this week’s Advocate, and looking forward to returning to my Thursday morning column delivery schedule.

Until next week…

Andrew M. Seybold
©Andrew Seybold, Inc. 2020


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