The FCC proposes for unlicensed transmitters operating in the TV bands (Channels 4-51), a transmitter power output limit of 1 watt in the white channel for fixed facilities and 100 mW for portable facilities. Note that no modulation scheme is specified nor prohibited.
Converted to ERP, (effective radiated power), this power limit is 2.4 W, neglecting the loss in the coax from the transmitter to the antenna.
By using active transmitting antennas (where the final amplifier is physically at the transmitting antenna), line loss can be neglected, so the ERP can be up to 2.4 W.
The FCC requires transmitter power output to be backed off if the antenna gain exceeds 6 dB. In this column, I will report results of my theoretical study where the ERP per transmitter is 2.4 W. Whether the final amplifier is indoors or mounted on the transmitting antenna, its cost should be about the same, so I believe most of these digital citizen-band rigs will radiate about 2.4 W average power. Their peak power will depend upon the choice of modulation schemes.
We can assume that these digital CB rigs will employ digital modulation, so their output spectrum is noise-like. I have analyzed the total noise power that may be received in a white channel in a typical suburban community where single-family homes are on quarter-acre lots (typically 100-by-100 feet).
Over the shortest path of about 100 feet, one 2.4 W ERP source delivers -20 dBm. Four would deliver -14 dBm of noise in the white channel. My model assumes that every home has one of these digital CB rigs, and that they all operate on the same white channel.
WORST-CASE SCENARIO
This model gives a worst-case value for the noise power in this white channel. Many of these white channels will be adjacent to a DTV channel in the same community. These digital CB rigs may create adjacent channel interference (ACI) in DTV receivers by either of two mechanisms:
IM3 (third-order intermodulation) products generated in the DTV receiver due to overloading by the strong DC noise on one or even both adjacent channels.
Sideband splatter into the DTV channel from the digital CB rigs operating on adjacent channels to the DTV channel.
The FCC proposes the sideband splatter in any 100 KHz sub-band outside of the digital CB channel shall be at least -20 dB below the digital CB signal power in a 100 KHz portion of the occupied channel. My interpretation of this for digital modulation schemes such as COFDM and CDMA is that the noise in adjacent channels is to be at least 20 dB with respect to the power radiated within the digital CB channel.
My calculations show that the noise in the white channel approaches -6.65 dBm, which I believe will overload many DTV receivers, blocking reception. Several papers by myself and by Dr. Oded Bendov recently published in "Transactions on Broadcasting," by the IEEE in Vol 51, No. 1, March 2005, indicate that the IM3 produced in the mixer by adjacent digital CB signals below -6.65 dBm can be expected unless the receiver's third-order intercept power is above +15 dBm, as this column has previously proposed.
If the noise power in the digital CB channel is -6.65 dBm, sideband splatter from these transmitters may approach -26.6 dBm average power. Given the noise floor in the DTV channel, which may approach -26.6 dBm, the minimum usable DTV signal power received must exceed -10 dBm. Ask yourself where is your -10 dBm contour or where is your 120 dB uV/m field strength contour?
As a reality check, perhaps only 40 percent of homes will have a digital CB rig. Not all of these rigs will operate simultaneously, but in primetime, many will. Not all will use the same white channel but many will. This is why my worst-case calculation of -6.65 dBm for ERP of 2.4 W may be reasonable. It may be as good a worst-case number as can be proposed. What do you think? Or should we accept a lower coverage than 100 percent?
In conclusion, if the federal government wants to use DTV channels for the emergency alert system, this channel-sharing scheme would severely limit the usefulness of DTV channels in the event of an actual emergency. Remember that on 9/11, cell phones in New York were useless when the telephone lines failed and everyone tried to use their cell phones. What about using their unlicensed wireless transmitters on Channels 4 through 51 in an emergency when the phone lines are out?
In an actual emergency, will your emergency generator be able to support your maximum authorized ERP or will you have to operate at reduced power? If reduced power operation is indicated, to what extent would you have to reduce your ERP? I would like to hear from my readers on this topic as it may affect EAS over DTV channels, especially if these digital CB rigs increase the noise level in or even near your channel.
If you're interested in the details of my calculation, send me an e-mail and I forward them to you. These calculations extend to 1 mile from the target site as beyond that distance over a line-of-sight path, the noise level does not increase significantly.
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