Application Note

Measuring PIM on the AT&T FirstNet

Network with the Anritsu 2-Port

PIM Master

™

Introduction

In 2017, AT&T won a multibillion dollar contract from the United Sates Government to build and maintain a

communications network for first responders, including police, firefighters, and emergency medical services.

The purpose of the FirstNet service is to provide a modern voice and data network that allows seamless

communications between services, and does not become overloaded during large scale emergency

situations. FirstNet users will have priority network access during times of peak demand using dedicated

FirstNet handsets. The FirstNet network uses standard LTE bandwidths and protocols in a new block of

frequency spectrum.

AT&T’s commercial LTE network is located in band 17, and the downlink occupies 734 MHz to 746 MHz. The

band allocated for the FirstNet downlink is 758 MHz to 768 MHz and the uplink is 788 MHz to 798 MHz.

These spectrum allocations lie adjacent to other commercial LTE networks from Verizon and T-Mobile as

seen in figure 1 below.

Figure 1. LTE 700 MHz Band

Lower and Upper 700 MHz Bands

In addition to the new bands allocated to FirstNet, PIM products from the legacy AT&T LTE 700 MHz band

17 can fall into adjacent bands, normally IM5 and IM7, as shown in figure 3.

Figure 3. AT&T Band 17

Adding the FirstNet network into the already crowded frequency plan around 700 MHz creates significant

challenges for network performance. The uplinks of all cellular networks are very vulnerable to passive

intermodulation (PIM) products. PIM occurs when RF signal components generate two or more signals

that pass through any non-linear device that acts as a diode and generates intermodulation products.

LTE 700 MHz band networks typically have 10 MHz modulation bandwidth and intermodulation products

from LTE transmitters have even wider bandwidths. The most notable 3rd order (IM3) and 5th order (IM5)

PIM products can have 30 to 50 MHz bandwidth. Figure 2 below highlights how PIM products from the

FirstNet 700 MHz band 14 can fall into the bands of other users.

Figure 2. FirstNet Band 14