The rollout of 3G networks across the country is old news with the recent announcements by Verizon, AT&T, and others to rapidly deploy 700 MHz 4G LTE this year throughout the USA. These high speed networks are slated to grow with ten’s of thousands of new towers and antennas, and to employ novel digital modulation schemes. This raises the concern of noise ingress from nearby LTE base stations or handsets to Cable TV (CATV) headends, plant equipment, and subscriber equipment.

As one CATV operator stated at a recent Society of Cable Television Engineers (SCTE) meeting, “We are starting to hear more about noise from cellular/LTE base stations.”

This problem is a challenge for CATV since there has been virtually no “high density RF” at frequencies below 800 MHz, where most cable TV services have traditionally been transported.  All cellular, PCS, and WiMAX are at frequencies above this range. So, in addition to traditional noise sources, a cable operator must now be wary of 700 MHz band LTE installations near its headends and within its coaxial cable plant area. CATV infrastructure that is susceptible to this new noise ingress source includes broken/worn coax, connectors, set-top boxes, pedestals, DSLAM equipment as well as head-end components.

Figure 1: Channel Plans for LTE and Existing Cable TV Sub-band
Figure 1: Channel Plans for LTE and Existing Cable TV Sub-band

Figure 1 shows a typical LTE channel plan superimposed on the CATV sub-band. It is clear how interference can occur when the 700 MHz LTE frequencies ingress the cable plant on CATV channels. Since the 700 MHz modulation is digital, it will show up as an increase in noise floor in the CATV program or data channel as depicted in Figure 2.  This may drive error rates beyond a tolerable level and cause the collapse of digital programming.

Figure 2: LTE can add broadband noise to CATV channels
Figure 2: LTE can add broadband noise to CATV channels

The most obvious vulnerability of the cable system is its coaxial plant, particularly the connections. The common F – connector used in CATV coaxial plant typically has a shielding effectiveness (SE) of greater than 95 dB at 300 MHz; however, the SE drops to 70 dB at 1000 MHz. It has been estimated that a properly made up and terminated coax system can well withstand fields of 1000 mv/m without objectionable ingress. When cables that end in open barrels such as un-terminated outlets and head-end entries, the interference threshold typically drops from 1000 mV/m to 300 mV/m with some cases seen below 100 mV/m.

The electric field from an LTE base station with 1000 Watts ERP will have a worse case intensity of approximately 1 V/m at 500 feet, enough to easily induce unwanted noise into susceptible cable plant components in its vicinity. In a collocated or shared headend site with an LTE base station, interference fields can easily reach a level to overcome the shielding of headend equipment. In a subscriber environment, an LTE Smartphone operating in the 700 MHz band with 20 dBm of ERP (3 dB below maximum) will have a field strength of approximately 1 V/m at 5 feet; enough to ingress bad connections, to say nothing of penetrating holes and poor shielding in cable modems or set-top boxes. Hence, the potential for co-channel interference to CATV from LTE base stations and mobile terminals is real.

AT LBA, we are currently working with LTE carriers and CATV operators to help quantify and resolve these and other electromagnetic interference issues. Our expertise includes resolution of RF interference, site zoning support, and RF safety. In our next blog on the subject, we will suggest some steps which may be taken to secure headend – cell site compatibility.

Please contact LBA Chief Technical Officer Dr. Chris Horne, PE at chris.horne@lbagroup.com for further information.

About The Author

LBA Group, Inc. has 50 years of experience in providing RF asset solutions and risk management for industrial and telecommunications infrastructure assets. The company is comprised of LBA Technology, a leading manufacturer and integrator of radio frequency systems, lightning protection, and EMC equipment for broadcast, industrial, and government users worldwide; the professional engineering consultancy Lawrence Behr Associates, and LBA University, providing on-site and online professional training. The companies are based in Greenville, N.C., USA.

11 Comments

  1. I contend that it is also contaminating the 540-1710kHz MF band.

    We’re receiving numerous complaints from in-car listeners since 4G lit-up here in Northern California (SF Bay Area).

  2. Hi, it seems really odd that LTE would interfere with in-car listening! That sounds a bit like what we HAM operators are accused of every now and then…but it is normally something else interfering.

    I am to, to say the least, on both side here being both a strong representative of an LTE operator but also a Ham radio operator. I would really like to know if there are measurements (in real life) made that has proven that LTE interfere with other services. Any experience or customer cases in US that shows that LTE is the cause?

    From my understanding the German operators have had no problems or complaints at all with their introduction of LTE on the DigitalDivided band in Germany. In the Nordics and Baltics we are just launching LTE800 and it is of course important to know what we are up against and what to to about it.

    Nothing is stopping the LTE train but of course we should make sure that we can solve any problems related to it.

    73

    /SM0NDH

  3. Guys,
    This is very good information and we have seen this in some metro areas. Us the Rohde & Schwarz PR100 receiver with directional antenna to pinpoint the source of the interference using integrated GPS tool.

  4. LTE base station transmitting at 1000 Watts ERP? I wouldnt like to be near that! The effects on Cable TV would be the least of my concerns. Perhaps re-runing your analysis on a more accurate 40 Watts is in order.

    I totally get the issue with handset though. It would be nice to hear what ACR/ACLR filtering is in place or what the CableTV rejection is given the seperation with LTE uplink bands. I am guessing the user will need to be at the cell edge (i.e. transmitting on full power) as well as sat on top of the CableTV box for a sizeable issue.

    Also, we are concerned with the impact of interference degrading the customers experience, but what about the sum of interference being cabled back from many customers premises to the central office (or switching location)? What are these filters like? Could this affect CableTV users with no LTE terminals nearby?

    Very interesting topic!

  5. Actually, 1000 watts eirp is not to far off. The FCC permits 2000 watts eirp. A 20 watt transmitter with an 18 db gain antenna gives 1260 watts. Its easy to get transmitter power and effective radiated power confused. Still, it’s low power compared to the eirp of UHF-TV stations which can reach 16 million watts! Even a common Class A FM station uses 10,000 watts total eirp. If you are going to work around any of these antennas, you will want RF awareness training. You can get that at http://www.lbagroup.com/associates/rftraining.php.

  6. LTE can knock out cable TV which is bad news for anyone who likes to talk and watch at the same time. The study confirms suggestions last year that LTE deployments could interfere with cable-TV. Ofcom’s research found this could indeed happen if the LTE handset is exactly the same frequency, and operating near full power within a metre of the modem or set-top box. The result the result can be broken-up TV pictures and loss of bandwidth: unless Virgin Media decides to beef up their set-top box line.

  7. Well, even with all the new telecom technology that has come along, HF radio (3 – 30 MHz, roughly) is still alive and in daily use, but in ways perhaps not entirely foreseen back in the pre- and early post-World War II era. And in ways perhaps not known to the younger generation, who has never lived in a world without instant IP-based communications.

  8. Chuck,

    I wonder if you are still getting reports of interference to AM? Could you elaborate a bit on the form the interference takes and how you point it back to the 4G emitter? It would also be useful to know the frequency band of that operation and whether it is LTE or WiMax. Thanks,

    LB

    @Chuck Bullett, CSRE

  9. Comcast Motorola cable box 4G interference in DC
    Old post, but just wanted to share an experience. A friend recently switched to a 4G phone and whenever he came over my cable (many channels) would pixelate and freeze or go black. This never does this any other time. He had his phone out and was texting and I noticed that simultaneously the cable would malfunction. We moved the to the next room and texted with no issue but within 10 feet of the cable box any text message would cause disruption.

  10. @Alan Fahrner – Yes, it was our government’s fault that you were using a wireless microphone system in the 700MHz band, which was never legal for that use in the first place. Did you buy that wireless microphone from someone in China? Maybe you should have researched the legal frequencies that were available for use by citizens before you made your purchase!

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