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.

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