47 CFR § 74.791 - Call signs.
(a) Low power stations. Call signs for low power stations will be made up of a prefix consisting of the initial letter K or W followed by the channel number assigned to the station and two additional letters and a suffix consisting of the letters -D.
(b) Television translator stations. Call signs for digital television translator stations will be made up of a prefix consisting of the initial letter K or W followed by the channel number assigned to the station and two additional letters and a suffix consisting of the letter -D.
(c) Low power television stations and Class A television stations. Low power television and Class A television stations may be assigned a call sign with a four-letter prefix pursuant to § 73.3550 of this chapter. Low power stations with four-letter prefixes will be assigned the suffix -LD and digital Class A stations with four-letter prefixes will be assigned the suffix -CD.
(d) Call sign protocol. The use of the initial letter generally will follow the pattern used in the broadcast service, i.e., stations west of the Mississippi River will be assigned an initial letter K and those east, the letter W. The two letter combinations following the channel number will be assigned in order, and requests for the assignment of the particular combinations of letters will not be considered. The channel number designator for Channels 2 through 9 will be incorporated in the call sign as a 2-digit number, i.e., 02, 03, etc., so as to avoid similarities with call signs assigned to amateur radio stations. In the event that the two letter combination following the channel numbers reaches ZZ, the next subsequent call sign shall have three letters, beginning with AAA.