lte and lmr coverage comparisons - commdex

20 August-September 2020 MissionCritical Communications www.MCCmag.com

TThere has been a lot of discussioninvolving the potential transitionfrom an LMR network to a cellularpush-to-talk (PTT) system, but therehasn’t been much information aboutthe significant differences in technol-ogy. “The Blue Book: Wireless Com-munications for Public Agencies”will be published soon, and a selec-tion of content based on the draftbook follows. The free digital bookcovers many topics, including LMR,cellular, the First Responder NetworkAuthority (FirstNet), cybersecurity,

LTE and LMR Coverage

Comparisons

A typical radio site

Imag

es c

ourt

esy

Rya

n P

olte

rman

n

Many variables affect the coverage of an LTE network.

By Ryan Poltermann

A typical LTE site


unmanned aerial systems (UAS) andsatellite communications.

Cellphone Coverage Variables Radio trunking can be used as thepoint of comparison. As a summary,the following are aspects that willchange the performance of a cellphone: Number of cell sites. As thepower output of a cell site is typicallylower and the transfer speed is higher,a cellphone requires more sites to pro-vide the same amount of coverageexpected from radio systems. Frequency bands. Cell sitesdon’t use just one frequency band;they can use a number of frequencybands such as 700 MHz and 1.7, 1.9and 2.3 GHz. Band 14 isn’t exclusiveto public-safety use, even going backto the initial FirstNet request for pro-posals (RFP). FirstNet uses more thanjust band 14, as well. 5G frequencies,such as 24 and 39 GHz, can be evenhigher than LTE and have significantcoverage limitations. Each cellularfrequency performs differently, andwhich frequency band you’re usingcan change even at the same location.However, radio systems use one frequency band for each site, andtherefore, the site’s entire coverage isconsistent. Cell site backhaul. How muchtotal bandwidth does the site itselfhave to communicate with the maincellular infrastructure? Think of it assimilar to your home internet — how

much faster your Wi-Fi is versus thelower speed your internet providertypically offers. In cellular, however,you don’t have a guaranteed speed,and the maximum speed available toyou is being shared among all of thecellular users at the site. This willalmost always be the limiting factorwhen it comes to cellular speeds, andit will be much lower than the adver-tised speed. The maximum speed atthe site may also be further limited ifmultiple cellular sites use the samebackhaul/transport. How busy is the cell site?Because multiple antennas are used atthe cell site, performance can be dra-matically different depending onwhich antenna you’re on. Cellphone power output. Thepower output of the cellphone islower than a radio push to talk (PTT),and the power can be reduced evenfurther if the cellphone is close to auser’s body. Be careful about compar-ing power directly because the ratedpower output for a cellphone is fromthe antenna, whereas the rated poweroutput for the radio is before theantenna. There is also a significantfocus on high-power user equipment(HPUE) for band 14, although band14 HPUE was never meant for smartphones. Modulation. As the speedsincrease, there is less room for erroreither from the devices themselves orinterference from the environment.This means the range will inherentlybe less.

LTE Needs More Sites At a high level, the architecturecan be considered somewhat similarto LMR, but the site density, or howmany RF sites there are, is muchhigher and should be factored intodisaster situations. When a cell site isisolated from the network, it’s downcompletely. This will change whenisolated operations for public safety(IOPS) technology is available. Don’tforget to ask the provider about howlong the cell sites can run on backuppower. The number of sites required isboth a property of LTE’s higher fre-quencies — 600 MHz and above —along with the power output LTE usesand how it transmits.

Multiple Frequency Bands While trunked radio systems useone frequency band per site (700/800MHz being an exception), cell sitescan use many frequency bands. Thiscan cause issues with older cellphonesor phones from another carrier, whichmay not support all of the frequencybands at the site. Using many bandscan also cause inconsistent perform-ance from a user’s perspective. Here’s an example. The cell siteassigns frequency A to a user whowalks into a building without issue.The next day, the cell site assigns fre-quency B to the user and momentarilydrops the user’s coverage when theuser walks into the exact same build-ing with the cellphone switching backto frequency A upon entering. This

Radio Site AntennaSite Density LTE Site Antenna

The site density required for LTE (lightgray) is higher for the same coverage.

A typical omnidirectional antenna pattern on the left and a highly directional cellular antennapattern on the right (both viewed from above)

www.MCCmag.com MissionCritical Communications August-September 2020 23

performance inconsistency can frus-trate users and administrators as well.How does a system administrator findout what happened and why there’s anissue? Is there even a way for a sys-tem administrator to correct it? Besides the RF impacts, each fre-quency band may have different band-widths available to it. Band 14, as anexample, has 10 megahertz for talkingto the cellphone and 10 megahertz fortalking to the cell site. Assumingsome standard things, such as 2-by-2multiple input multiple output(MIMO) at 64 quadrature amplitudemodulation (QAM) for the technical,the absolute maximum speed(throughput) you would theoreticallysee is about 100 Megabits per second(Mbps). Higher frequencies tend tohave more bandwidth available, andbecause the signals don’t go as far,they can be reused more often.

Antennas For most radio sites, you’ll haveone or two transmit antennas and oneor two receive antennas. An omnidi-

rectional antenna is used at most radiosites. An omnidirectional antenna pat-tern is mostly circular because theantenna is meant to transmit in allhorizontal directions. LTE sites have more antennasarranged horizontally, three or moreper side. This is because the anten-nas are directional. This directionalLTE antenna is called a sectorantenna, and it provides coverage toonly part of an area. Each antennaessentially acts as its own site andworks with the other antennas at thesite to provide more complete cov-erage. Antennas on the same sidemay also work together to improvebandwidth to the cellular device,known as MIMO, but the site’s totalthroughput to the cellular infrastruc-ture still has to be divided between

all of the antennas and users on thesystem.

Changes with Loading Project 25 (P25) uses two potentialmethods to transmit: FDMA orTDMA. Within each modulation type,the coverage map is always the sameregardless of the number of users. Cellular coverage, however, ismore dynamic. Cellular coveragebreathes based on the number of userson a particular antenna and site, therelative distance of the users and thecapabilities of the devices. As a sim-plification, the performance can’t belooked at from a site perspective butwhich side of the site you’re on. In the first circle on Page 24, Sec-tor 1 has a lot of users, and the per-formance of the site isn’t going to be

Cellular coverage breathes based on thenumber of users on a particular antennaand site, the relative distance of the usersand the capabilities of the devices.


high. Sector 2 has a moderate numberof users, so the performance is okaybut not great. Sector 3 has only a fewusers, so it will have the highest per-formance. If users know where the site is,they might be able to improve per-formance by moving, as shown by themiddle circle above. This would justbe moving to a different antenna onthe same site instead of a differentcellular site. When a sector starts toget overloaded, the site will reducethe coverage in that sector to accom-modate the users it has closer to thesite as shown in the third image. As mentioned, this is a simplifica-tion. Real cell sites will have morethan three antennas (sectors), andmany additional factors impact cover-age and performance. Two examplesare MIMO and beamforming. All ofthese factors make understanding cov-erage and troubleshooting difficult, asit won’t be consistent and couldchange in a moment. You may wonder what happens to apublic-safety user who is in the fringesof coverage. How does that affect thecoverage breathing and handoffbetween sites? The short answer is thatit’s not clear and will come down tothe carrier’s implementation. Be sureto ask your carrier how your users arehandled in these situations.

What Device and How’s It Worn? In reality, cellular coverage maps

don’t mean much. Radio coveragemaps are modeled with a specificdevice worn in a particular way(receive on hip belt clip and transmitat head level, receive/transmit in beltswivel case) or installed in a vehiclewith a certain antenna. This isn’t thecase for cellular coverage maps,which only show where “coverage” isor isn’t, and don’t reflect actual usage. To complicate matters further, thepower output of a cellphone isreduced if it’s close to the user’s body.In contrast, radios use a constantpower output. Cellphone performanceis also dependent on how the cell siteand cellphone are sending the infor-mation (frequency and modulation),and the user can’t control this. Fastertransmission speeds offer less rangeand less consistent performance inmovement and changing environ-ments. In contrast, radio systems use aconsistent method every time for theRF site and the radio. If you get detailed cellular cover-age maps, you’ll notice that the speedadvertised on the map is extremelylow, especially when compared withthe speeds you may be expecting orwhat the carrier might even be adver-tising. Smaller numbers make thecoverage look bigger, so check if thebandwidth on the coverage maps isrealistic for your operations. Regardless of which carriersyou’re testing, test at the same timeand with the same type of devices.This will keep the playing field as

level as possible and demonstratewhere actual issues are instead of the-oretical ones. Always compare yourresults with crowdsourced coveragemaps such as Opensignal and Root-Metrics. This won’t be enough to providean informed decision, but it acts as a starting point. The Blue Book hasadditional information available forfree, and it can help guide youragency. Test as much as possible,ask questions and share informationwith other agencies. By workingtogether, you can get the best possi-ble solution. n

Ryan Poltermann is the principal investiga-

tor of National Cooperative Highway

Research Programs Project 03-129:

Essential Communications: A Guide to

Land Mobile Radio (LMR) and the author of

the “Blue Book: Wireless Communications

for Public Agencies.” With publication pend-

ing by the National Academies Press, the

book covers all facets of wireless communi-

cations and will be free to download. Polter-

mann is an innovation architect at Com-

mdex and responsible for public-safety

innovation within the company. He has

been involved in public-safety communica-

tions design and consulting for more than a

decade and has contributed to more than

half the U.S. states and six countrywide

systems. He also participated in the Public

Safety Communications Research (PSCR)

public-safety haptic and augmented reality

challenges. Email feedback to

[email protected].

Antennas play a major role in performance.Numbers indicate the sector, and wavesindicate cellular performance.

Moving just a little bit could make a huge performance difference as shown in Sectors 1 and 3 above.

Coverage breathes, and some users mightget left out. Users with exclamation marksno longer have coverage.

lte and lmr coverage comparisons - commdex

Documents