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The 4.9 GHz Band, RF bits and pieces

by: Pat Amodio, Chief of Radio Frequency Engineering, ERIC, PSHSB

April 8, 2011 - 12:53 PM

In February, the FCC held a workshop on the 4.9 GHz band, spectrum dedicated to Public Safety for Broadband use. This 50 megahertz of spectrum in the 4.9 GHz band (4940–4990 MHz) is the largest spectrum allocation for public safety broadband services. The workshop consisted of two panels.  The first, which I had the pleasure to moderate, discussed how the 4.9 GHz band is currently used by public safety.   What did we learn from this workshop? Let me offer the perspective for a Radio Frequency (RF) Engineer.

The first panel had excellent state and local representation and included panelists from the Brookline (MA) Police Department; the Virginia Department of Transportation (VDOT); the Missouri Department of Public Safety; and the Los Angeles County Sheriff’s Department, California.

Scott Wilderfrom Brookline Police Department explained that their wireless network allows officers to access all public safety databases from their vehicle mounted laptops. Officers can, among other things, upload reports accessing criminal databases and download video and images of missing and wanted persons. The antenna is installed on the roof of the police cruiser. The network equipment and antennas are mounted on light poles within Brookline – ideal for propagating a signal to and from police vehicles.

Bill Brown from VDOT, described one of their uses of 4.9 GHz using an aerial view of Interstate 64 running east and west. Bill explained that VDOT was unable to install a T1 line to get Internet use for the Travelers on the eastbound side due to likely construction problems and costs.  Instead, VDOT used the 4.9 GHz band to solve this problem. Although not an ideal application because it is not line of sight and there are trees obstructing the link, it is never-the-less a short distance (less than a mile). So, instead of spending $70,000 to establish an internet service on the eastbound side with a T1, use of 4.9 GHz permitted VDOT to establish a link within 3 weeks --- 20 times the bandwidth of the T1 for less than $5,000 in equipment costs. You don’t need an RF planning tool to figure out that is suitable and cost effective.

Stephen Devine from Missouri Department of Public Safety mentioned that it is important that when utilizing the 4.9 GHz band, you know the propagation characteristics while making sure of the applications used in the band. He explained that 4.9 GHz is not practical for mobile use, long-term or high traffic use. I agree that there are propagation scenarios (such as going through buildings) and less than optimal applications (surfing the web in an automobile), that impede the use of 4.9 GHz.

Lt. Mark Wilkinsfrom Los Angeles County Sheriff’s Department said within the County of Los Angeles, there are approximately 10 public safety agencies that have licensed the use of 4.9 GHz for applications such as, Downlink Video, Video Surveillance and Backhaul operations, with five entities that currently use 4.9 GHz.  He showed a system diagram of how they currently use 2.4 GHz, which will basically be the same for 4.9 GHz. This appears to be a good workable design for 4.9 GHz, taking into account that the distance of the links in the diagram average around 10 miles, are high sites and also offer line of sight propagation.

Finally, the panel also included two perspectives from the vendor community with Mark Jules of Avrio RMS Group and Martin Levetin of Strix Systems.  Mark Jules explained that the 4.9 GHz band is most useful in areas like cities, universities, ports and stadiums. He suggested the band be accessible for special events, for example, inaugurations, the Democratic National Convention, G-20, and the Super Bowl. Quick installation especially with the events like the Super Bowl and G-20 is a big plus to the system.

Martin Levetin, of Strix Systems is a wireless mesh vendor, gave real-world case study examples where the 4.9 GHz band is used. These included fixed and mobile data by Public Works, Police, Fire, Video surveillance of selected neighborhoods and assets, CAD system, anti-virus updates, software application updates, field reporting upload. He described the different situations, the business cases, how they set up the network, and the deployment hurdles the Company had overcome. Martin responded to a question that I posed to Mark Jules. Where would you use 4.9 GHz and where would you use 700 MHz? Answer - For a system in Australia this particular network has 109 cameras deployed in a 10 square mile area. That is about 10 cameras per square mile. He explained that because 4.9 GHz has more bandwidth than a 700 MHz system, if video is a heavy player; 4.9 GHz would be the preferable choice. I agree with Martin, if you tried to deploy this (109 cameras) on 700 MHz, I think you would run out of steam quickly.

Given the unique RF propagations characteristics in the band that create some technical challenges (4.9 GHz does not work well in buildings), there are advantages!  Overall, the workshop highlighted some very interesting and positive capabilities of the 4.9 GHz band, such as quick installation, larger bandwidth and better video capabilities.

Updated: April 12, 2011 - 12:09 PM
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