BE Radio January/February 1997 Contract Engineering Coexsiting with Cellular By William Fawcett ------------------------------------------------------------------------ William Fawcett is president of Mountain Valley Broadcast Service, Inc., a broadcast engineering firm in Harrisonburg, VA. ------------------------------------------------------------------------ Cellular telephones have had the most profound effect on the communications industry of any new technology in the last 20 years. Ask most tower contractors today and you're likely to find that 80% of their business is related to cellular --- and more recently, personal communications systems (PCS) --- installations. For the broadcaster, the cellular industry has dramatically changed the tower rental business. Many cellular operators prefer to build their own towers. This is good for broadcasters because it may offer them more suitable locations to find rental space for their own facilities. Conversely, in locales where there are restrictions on additional towers, cellular operators may turn to broadcasters to rent space on their existing towers as cellular operations expand. The availability of emergency power, a common feature at broadcast and cellular sites, makes either facility even more attractive. Cellular operators make good tenants. Because of the large capital investment required, most cellular operators have stable business operations, and are used to paying their bills on time. Although each area typically has only two cellular operators, the emerging PCS industry may support perhaps six operators in a specific region. Considering that a fully built-out PCS system will require a site every two miles, the demand for tower space is simply staggering. Tower rental is an alternate revenue stream that every broadcaster should fully investigate. Some caveats Contract law is an exacting field. If you aren't experienced in this area, you must find someone who is. Everything and anything can (and should) be negotiated. In a tower lease contract, there are usually 25 to 35 different points that need to be addressed. Obtain samples from other broadcasters involved in leasing, and find out from them what works and what doesn't. One important aspect to cover is shut-down procedures. With the new RFR regulations, it's often not possible to work on a tower while a broadcaster remains on the air. Specify under what circumstances and when you will be willing to shut down to allow the cellular company to perform on-the-tower maintenance. You have more say over this if you control the site, but this point is often overlooked by broadcasters renting space on someone else's tower. AM broadcasters must allow for additional expense to re- tune and bridge their tower after a cellular installation. The expense of multiple iso-couplers on a series-fed tower can be great; a better solution might be conversion to a shunt-fed unipole. For a stable operation, be certain that all coax is bonded to the tower every 20 feet or so. With all of the additional expenses involved, it's likely that there won't be too many cellular operations on AM towers. As an aside, if a cellular (or other) tower sprouts up near your AM directional array, you may be able to require them to de-tune their tower and commission a directional proof to certify that they have not distorted your pattern. Protect your investment. Interference is another factor. Again, the controlling party has the upper hand, and will usually specify how interference to their operation will be handled. Cellular interference Seldom, if ever, would a cellular system interfere with a radio broadcast operation. Cellular telephones operate in the 800MHz band, specifically 824 to 849MHz and 869 to 894MHz. The "band plan" for the cellular service is detailed in 47 CFR 22.902 and is illustrated in Figure 1. To understand this plan, some terminology must be defined. In each service area, the cellular band is divided up into two blocks of 416 channel pairs each. Half go to the "A- system," with the remainder to the "B-system." The B- system operator is often spoken of as the wireline carrier, meaning the B-operator may also be involved in landline telephone service (such as GTE or a Regional Bell Operating Company [RBOC]). The A-operator will generally be a wireless-only firm (like Cellular One), although there are exceptions to this rule. Each channel pair consists of a mobile (subscriber) transmit frequency and a base (cell-site) transmit frequency. The base frequency is always 45MHz above the mobile frequency. Naturally, the cell site "listens" on the mobile's transmit frequency and vice-versa. Twenty-one channel pairs in each system are designated as control pairs, also known as "paging frequencies" (although they have nothing to do with your pocket pager). These channels are defined in 47 CFR 22.902(c) and they are in the center of each frequency segment. The control channels handle the initial "handshaking" with the mobile units, set the mobile's power level based on received signal strength, assign a voice channel to an active unit, and control the handing-off functions as a mobile moves from cell to cell. If you're looking at cellular channels on a spectrum analyzer, the control channels are the ones that are always active. You can use these control channels' frequencies to identify which cellular site you are "looking at." Besides the low likelihood of interference with AM or FM broadcast transmission, it would also be unusual for cellular service to interfere with properly maintained 950MHz aural STL systems, either. Because cellular frequencies are above the FM band, harmonics will also not be bothersome to radio broadcasting. The reverse situation is not so simple, however. It's possible for an FM broadcast transmitter to cause severe interference to a large block of cellular channels. The 9th or 10th harmonic of a broadcast transmitter may fall within the cellular band. Keep in mind that the FM broadcast transmitter is a wideband signal, and that wide bandwidth becomes even wider when multiplied 10 times. Practically speaking, an FM station's harmonic can span more than 45 cellular channels. Frequencies between 91.7MHz and 94.3MHz may create harmonics that fall within the mobile portion of the cellular band. Frequencies between 96.7MHz and 99.3MHz can fall within the base portion. The remainder of the FM frequencies by themselves don't have harmonic relationships with cellular frequencies, although intermodulation products are always a possibility. Even a harmonic that is 80dB down can be problematic, especially in the mobile portion of the band. This is because the cell site is listening in the mobile segment for weak signals emanating from hand-held pocket phones. A 100kW broadcast station with a 9th harmonic attenuated a full 80dB might be putting out a signal equivalent to 10mW. That's not much, but if your transmitter is located near a cell site, it's enough to cause troublesome interference. Solving problems Some FM broadcasters may not meet the specific requirements of 47 CFR 73.317. Using older equipment, they operate under the "grandfather" clause of 73.317(a). However, that clause states that they must clean up their act in cases of harmful interference. Interference to cellular service is one of those cases. In one severe case, an FM broadcaster using only a quarter-wave stub was found interfering with two cell sites, one of which was eight miles away. In all, 50 cellular channels were affected. The installation of a modern low-pass filter (at the broadcaster's expense) brought instant relief. Because of the profound effects of FM-band harmonics on cellular service, it's extremely important to verify harmonic levels all the way to 1GHz during your periodic RF proofs, especially if your frequency is one that has a harmonic relationship with the cellular band. A 6- to 9- element yagi cut for the cellular band will allow you to get above the noise floor of your spectrum analyzer, and with triangulation, will allow you to identify the source of the offending FM harmonic signals. A list of control (and voice) channels assigned to each cellular site is a useful diagnostic tool. Your local cellular company should be willing to supply you with a list for such coordination and interference-mitigation purposes. The emerging PCS technology in the 2GHz band, should present no interference problems to radio broadcasters at all. It's possible that a PCS second harmonic could cause problems with your C-band downlink, but no instances of this happening have been reported. Cellular is here to stay, and on balance it seems beneficial to the broadcaster. Astute engineers will make it their business to become familiar with cellular technology, not only to protect their broadcast interests, but to build a possible diversification in the services they can provide. Many cellular operators are inexperienced in troubleshooting interference, and antenna sweeps are another service in great demand. There's going to be plenty of work out there for the versatile RF engineer. Figure 1. Spectrum used by cellular operations.