FirstNet

What is FirstNet?

Public safety professionals need reliable communications to aid them in their lifesaving missions. Many tasks today require broadband services, i.e. when these first responders need to access data-intensive applications, search databases, or share pictures and videos. There is a clear need for rugged, easy-to-use devices designed to meet public safety requirements, as well as provide advanced features and services that enhance their ability to do their jobs.

To address the broadband requirements of first responders, the Middle Class Tax Relief and Job Creation Act of 2012 created the First Responder Network Authority (FirstNet) and allocated a total of 20 MHz of bandwidth in the 700 MHz band. It also provided $7 billion US dollars to FirstNet to build, operate, and maintain the first high-speed, nationwide wireless broadband network dedicated to public safety. The FirstNet network will be an interoperable platform used for emergency and daily public safety communications using commercial wireless LTE technology. LTE technology enables extremely high-speed data communications that are not possible with existing narrowband public safety technologies such as P25, TETRA and DMR. LTE technology is capable of efficiently delivering high-bandwidth mobile data that allows mobile devices to stream video or transfer large amounts of data quickly. The enhanced data services of LTE are some of the main driving forces that makes it attractive for public safety.

Firstnet Technology - Lower and Upper 700 MHz Bands
Lower and Upper 700 MHz Bands

In 2017, AT&T was selected by FirstNet to build and manage the first broadband network dedicated to America’s police, firefighters, and emergency medical services (EMS). The FirstNet network will cover all 50 states, 5 U.S. territories, and the District of Columbia, including rural communities and tribal lands in those states and territories.

The Challenges of Adding FirstNet

In order to better understand the concerns and challenges of adding the FirstNet network, we need to have a basic review of what PIM (passive intermodulation) is and how it is created. PIM is quite simply an undesired intermodulation distortion (IM) product that results from devices which are non-linear (e.g., think of when a mixer or diode is operated in a region where the response is not linear). Typical IM products occur at odd multiples of the fundamental frequency (i.e. IM3, IM5, IM7, etc.), however, as the IM product number increases the level of the IM signal decreases (so in most cases we are rarely concerned about IM7 or higher since those levels typically fall below the noise level of the radio systems receiver and don’t cause any problems for normal operation of the system). Intermodulation products are usually produced by active devices such as diodes and mixers, yet PIM can also occur with passive devices such as antennas, cables, and so on if they are behaving non-linearly. In fact, any time 2 or more dissimilar metals are in contact with each other, that point of contact (known as the junction) can, and often does, become non-linear in behavior. When stimulated by RF signals being transmitted in relatively close proximity, these junctions will generate their own intermodulation distortion product(s) that are then radiated back into the environment in an uncontrolled fashion. Since these devices are not active, the intermodulation distortion product(s) they create are what we call PIM. Internal PIM would occur within the radio system (transmitter, cables, polyphasers, diplexers, antennas, etc.), while external PIM would occur beyond the radio system and is where the majority of PIM is expected to occur with the deployment of FirstNet.

FirstNet public safety LTE in the United States will occupy two 10-MHz-wide blocks of spectrum at 758 MHz to 768 MHz and at 788 MHz to 798 MHz (Band 14). These frequency bands lie adjacent to existing, already in-use public safety narrowband spectrum for LMR at 769 MHz to 775 MHz and at 799 MHz to 805 MHz. Existing commercial wireless bands owned by AT&T, Verizon, and T-Mobile are also located next to and near the FirstNet bands as well. Although all of these LMR and LTE systems operating at the frequency bands above can coexist with good engineering design practices and careful frequency planning, interference issues are still a major concern as the interaction between all of these systems in the 700 MHz band may create passive intermodulation (PIM) products. The uplinks of all these systems can be highly affected by PIM interference generated by existing LTE cellular downlinks used by the major wireless carriers. This problem will only get worse when FirstNet is added to the mix as the interaction of the existing downlinks with the new FirstNet downlink will add even more sources of interference to this band. All carriers operating networks in the 700 MHz band will need to ensure high-quality installations with special care taken to reduce or prevent PIM signal levels from degrading overall network performance, whether it be to their own networks or to other carriers’ networks including FirstNet.

PIM Scenarios

Let’s take a closer look at the potential PIM the FirstNet public safety wireless broadband network could introduce to other networks. When looking at the effects of PIM in LTE networks, what we see are LTE subcarriers that come together to form the “wedding cake of PIM”.

Firstnet Weddingcake
“Wedding Cake of PIM” in LTE Networks

The new FirstNet downlink generates IM5 on Verizon's uplink and IM7 on its own uplink. IM5 products are typically high enough to cause major interference issues. While non-linearity has to be very bad for IM7 to cause interference, high noise has been seen in many sectors because of it. The AT&T 700 MHz band has the same level of concern as Verizon’s, where the downlink (Band 17) generates IM7 in the AT&T uplink but also IM9 in the Verizon and T-Mobile uplinks. The T-Mobile 700 MHz band actually has less PIM concerns than the AT&T and Verizon 700 MHz bands, though their downlink still generates IM11 in their uplink and IM7 in the AT&T uplink.

Now there are new concerns to consider. For AT&T’s Band 29, a downlink-only used for LTE service generates IM3 in their uplink as well as IM5 in T-Mobile’s uplink.

FirstNet Technology - AT&T Band 29 PIM Generation
AT&T Band 29 PIM Generation

When AT&T adds the FirstNet spectrum, this will create a higher magnitude PIM for everyone. FirstNet alone will generate IM5 in Verizon’s uplink, while we see that both FirstNet and AT&T generates IM3 in all uplinks – AT&T, FirstNet, Verizon, and T-Mobile.

FirstNet PIM Generation

Testing of the FirstNet Deployment

As many of the PIM issues that will arise are likely to be external, PIM testing will become of paramount importance in order to mitigate potential issues. While a more in depth look at PIM and PIM testing techniques can be found on the Anritsu PIM Technologies Page, it is important to have the proper equipment to conduct the necessary testing of these new FirstNet networks. Anritsu offers a complete suite of AT&T approved PIM hunting and testing equipment along with training and certification that can help ensure PIM interference is not introduced.

PIM Master™ MW82119B PIM Analyzer

Anritsu’s PIM Master MW82119B analyzer is a battery-operated, high-power PIM testing solution supporting major wireless standards in use around the world (including the LTE 700 MHz upper and lower). PIM interference occurs when normally linear components (connectors, cable assemblies, filters, antennas) generate spurious signals when subjected to the downlink signals at a cell site. This specialized test instrument is able to measure system linearity and identify fault locations both inside the cable system and beyond the antenna using Distance-to-PIM. It also features optional Site Master™ cable and antenna analyzer functionality. This powerful combination provides the ability to measure return loss, VSWR, cable loss, and Distance-to-Fault with a single test instrument.

PIM Hunter™ Probe

Sometimes even if an antenna system has low internal PIM levels, external sources of PIM can wreak havoc on network performance. Loose metal-to-metal contact is the typical cause of external PIM. Some easily identifiable sources include air conditioning equipment on the rooftop near the system, overlapping layers of metal flashing, sheet metal cable trays, sheet metal roof vents, and rusty metal objects. Some sources are not as obvious, for example, loose metal-to-metal connections hidden by roofing materials or loose cable hangers behind the antenna.

Anritsu is the only manufacturer that provides a complete set of external PIM Hunting solutions, which combine a spectrum analyzer, PIM Master MW82119B, a PIM Hunter probe, and a bandpass filter (P/N 2000-1926-R) that was specifically designed to address FirstNet and AT&T PIM issues in Bands 13 and 14 (776 MHz to 806 MHz frequency range).

Once PIM has been identified with the PIM Master MW82119B using DTP to find the distance, the PIM Hunter probe is then able to pinpoint the exact location of the PIM source. The PIM Hunter probe operates with the Spectrum Master™ MS2720T high-performance handheld spectrum analyzer or the BTS Master™ MT 8220T high-performance base station analyzer in burst detect sweep mode and with a bandpass filter. The PIM Hunter probe can also be used with the Spectrum Master MS2712E/MS2713E spectrum analyzers, Cell Master™ MT8212E/MT8213E base station analyzers, and Site Master S332E/S362E cable and antenna analyzers with custom settings and a bandpass filter.

BTS Master™ MT8220T base station analyzer

The BTS Master MT8220T is a high-performance, handheld base station analyzer for the installation and maintenance of wireless networks. It is the only all-in-one, touchscreen, handheld tool that combines: cable and line sweeping; wireless measurements for all cellular standards; ultra-sensitive spectrum analysis with ultra-fast Burst Detect™ mode; sophisticated interference tracking; and a vector signal generator for receiver testing and CPRI RF measurements. Its power and patented PIM-over-CPRI measurement now enables technicians to accelerate PIM identification by using live traffic to determine if there is PIM before or after the antenna. No site turn down time. No climbing a tower.

LMR Master™ S412E – One instrument to test LMR and LTE

LMR and LTE are very different technologies that require different tools for testing and maintenance. Supporting two separate networks can become challenging both in terms of personnel and test equipment requirements. Measurements often require a number of different tools, all of which must be carried into the field. There is a need for a portable instrument that is capable of testing both broadband LTE and narrowband LMR.

Anritsu’s LMR Master S412E land mobile radio modulation analyzer combines many of the tools needed to install, maintain, and certify LTE and LMR systems into a single instrument with a single user interface. The LMR Master S412E can deal with both the complexity of testing LTE networks and mapping bit error rate (BER) and modulation fidelity of LMR networks. It provides technicians and engineers with a tool to verify with confidence that these networks will work reliably and as expected.

Site Master S332E and S362E Cable and Antenna Analyzers

The Site Master S332E and S362E instruments combine a cable and antenna analyzer that can be configured to include either a(n): 4 or 6 GHz spectrum analyzer; 2-port transmission measurement with built-in 32V bias tee; interference analyzer with spectrogram displays (including RSSI, signal strength meter, and interference mapping); channel scanner; high-accuracy power meter; and optionally available GPS receiver for time and location stamping. With a CPRI LTE RF option, it is also capable of monitoring/troubleshooting a site’s RRH LTE uplink. Fully compatible with Anritsu’s PIM Hunter™ probe, field technicians will be able to quickly and efficiently sweep an area to identify and locate PIM. With the PIM Alert, technicians can easily be notified when PIM is present.

Spectrum Master™ MS2720T Spectrum Analyzer

The Spectrum Master MS2720T is the highest performing handheld spectrum analyzer that offers frequency coverage from 9 kHz to 43 GHz and best-in-class performance for dynamic range, DANL, phase noise, and sweep speed. This solution features over 30 analyzers in one to meet virtually every measurement need. Burst detect captures emitters as narrow as 200 microseconds the first time, every time. It also offers an LTE (FDD and TDD) measurement option up to 20 MHz plus support for carrier aggregation. Fully compatible with Anritsu’s PIM Hunter™ probe, field technicians will be able to quickly and efficiently sweep an area to identify and locate PIM.

External PIM Hunting Certification

Anritsu’s new External PIM Hunting Certification is one of the only instructor-led training course offered today that are AT&T-approved. This 1-day course teaches wireless professionals how to precisely locate and mitigate external PIM at cell sites. The course uses a combination of theory and practical, hands-on exercises to build confidence in real-world simulations. For more information, visit the External PIM Hunting Certification page.

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