Co-existence of Public Safety LTE and LMR

October 3, 2018 Anritsu Company

October 3, 2018

One of the hottest topics in the public safety arena is the deployment of private Long Term Evolution (LTE) high-speed wireless communications – either as a replacement (not likely) or complement (probable) to traditional Land Mobile Radio (LMR) standards such as P25 and TETRA. Industry expert consensus is that a profound transformation is underway and those of us in the LMR community need to understand the changes and their meaning.  

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The Need for LTE

Narrowband analog and digital LMR systems have been sufficient for police, fire, EMS, and other first responders for years. In today’s mission critical world, however, many situations require broadband services. Examples include when first responders need to access data-intensive applications, search databases, or share video. LTE enables such extremely high-speed data communications while current LMR technologies are severely limited in these applications. The enhanced data services of LTE are some of the main driving forces that makes it attractive for public safety.

LTE is also the technology that will allow public safety networks to meet the mandates set by Congress when it passed FirstNet in the aftermath of the September 11 attacks. LTE technology will be a cornerstone of the high-speed, nationwide wireless broadband network dedicated to public safety.

One reason LTE was selected for the public safety sector is because it has already been successfully rolled out by commercial operators. Because of this fact, the premise is that the technology is well understood and easy to install, use, and maintain.

It is important to note, though, that commercial LTE differs quite a bit from LTE systems for critical communications. Public safety LTE networks will have special features and requirements to make them much more reliable, a necessity given the environments in which they will be used. Two key factors that those responsible for public safety need to consider when planning LTE network deployment are interference and handset power.

Interference – As we stated at the beginning of this post, LTE will not supplant P25 and TETRA. Rather, it will operate in conjunction with existing LMR networks. Because these networks will often occupy the same frequency bands, interference becomes a primary concern for system designers, installers, and maintainers.  

Handset Power – LMR handsets typically transmit with 3 to 5 watts of power. Their LTE counterparts may only transmit with 1 watt, approximately. This means LMR systems will have longer range. To compensate, operators will need to install many more LTE sites spaced closer together. The result will be higher equipment and maintenance costs. Therefore, a broadband network at 700 MHz will not be able to replace LMR in many locations across the country because of RF propagation properties, and matching LTE to LMR coverage and reliability is just too cost-prohibitive.

Test Considerations

As noted earlier, LTE and LMR networks will invariably co-exist – at least for the foreseeable future. For this reason, testing is an important issue for carriers and those responsible for installing and maintaining the networks. FirstNet public safety LTE will occupy two 10-MHz-wide spectrum blocks from 758 MHz to 768 MHz and its duplex spectrum offset +30 MHz away at 788 MHz to 798 MHz. These frequency bands lie adjacent to the public safety narrowband spectrum of 769 MHz to 775 MHz and its duplex pair +30 MHz away at 799 MHz to 805 MHz.

A recent study conducted by the U.S. Department of Homeland Security (DHS) suggests that LMR and LTE systems operating at these frequencies can coexist with proper engineering design practices and careful frequency planning. Even with these proper design considerations, interference may still be a concern because the guard bands between the LTE and LMR spectra are only 1 MHz wide.

Some portions of the 700-MHz FirstNet bands may be heavily affected by passive intermodulation (PIM) interference generated by existing LTE cellular downlinks used by major wireless carriers. This means all carriers operating those networks will need to ensure high-quality installations with particular attention given to reduce or prevent PIM signal levels from de-grading overall network performance.

The Test Conundrum

LMR and LTE are vastly different technologies. LTE is much more complex, due to its variable channel bandwidths and use of both multiple-input multiple-output (MIMO) communications and orthogonal frequency-division multiple access (OFDMA) modulation to support high data rates.

Supporting two separate networks can become challenging in terms of personnel requirements and test equipment needs. Both LTE and LMR systems have to contend with multipath, fading, and other factors that degrade signal quality. Handheld test equipment that can handle the complexity of testing LTE networks and mapping bit error rate (BER), as well as the modulation fidelity of LMR networks, is critical so technicians and engineers have confidence that the public safety communications systems they install and maintain will work according to specification.  

It is typically assumed that all these measurements need to be taken with multiple instruments. This poses a few problems. The first is that it is cumbersome to carry numerous tools into the field. It also requires crews to be trained and proficient in two highly different technologies or deploy two teams — one dedicated to LTE and one to LMR.

That is not the case, however. The LMR Master™ S412E (figure 2) is the industry’s first and only battery-powered LMR handheld field analyzer that can test broadband and narrowband systems. It combines many of the tools needed to install, maintain and certify LTE and LMR systems into a single instrument with one user interface. Multi-function analyzers such as the LMR Master S412E can significantly reduce the number of tools technicians and engineers need to verify operation of wireless network infrastructure and diagnose problems in the field.

Figure 2
Figure 2

What the Future Holds

Next-generation public safety communications will more than likely pair narrowband LMR networks for voice with broadband LTE networks for high-speed data. Ensuring these networks are properly installed and maintained is vital to ensuring mission-critical public safety communications so first responders and the general public remain safe.

To learn more about public safety communications systems, visit the Anritsu Land Mobile Radio technology page.

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