Sec.  74.903  Interference.

    (a) Since interference in this service will occur only when an 
unfavorable desired-to-undesired signal ratio exists at the antenna 
input terminals of the affected receiver, the directive properties of 
receiving antennas can be used to minimize the hazard of such 
interference. Interference may also be controlled through the use of 
directive transmitting antennas, geometric arrangement of transmitters 
and receivers, and the use of the minimum power required to provide the 
needed service. Harmful interference will be considered present when the 
reference receiving antenna is oriented to receive the maximum desired 
signal, and a free space calculation determines that the desired-to-
undesired signal ratio is less than the value specified for the 
respective channel under consideration.
    (1) Cochannel interference is defined as the ratio of the desired 
signal to the undesired signal, at the output of a reference receiving 
antenna oriented to receive the maximum desired signal level. Harmful 
interference will be considered present when a calculation using a 
terrain sensitive signal propagation model determines that this ratio is 
less than 45 dB (or the appropriate value for bandwidths other than 6 
MHz.)
    (2) Adjacent channel interference is defined as the ratio of the 
desired signal to undesired signal present in an adjacent channel, at 
the output of a reference receiving antenna oriented to receive the 
maximum desired signal level.
    (i) Harmful interference will be considered present when a 
calculation using a terrain sensitive signal propagation model 
determines that this ratio is less than 0 dB (or the appropriate value 
for bandwidths other than 6 MHz.)

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    (ii) In the alternative, harmful interference will be considered 
present for an ITFS station constructed before May 26, 1983, when a 
calculation using a terrain sensitive signal propagation model 
determines that this ratio is less than 10 dB (or the appropriate value 
for bandwidths other than 6 MHz), unless:
    (A) The individual receive site under consideration has been 
subsequently upgraded with up-to-date reception equipment, in which case 
the ratio shall be less than 0 dB. Absent information presented to the 
contrary, however, the Commission will assume that reception equipment 
installation occurred simultaneously with original station equipment; or
    (B) The license for an ITFS station is conditioned on the proffer to 
the affected ITFS station licensee of equipment capable of providing a 
ratio of 0 dB or more at no expense to the ITFS station licensee, and 
also conditioned, if necessary, on the proffer of installation of such 
equipment; and there has been no showing by the affected ITFS station 
licensee demonstrating good cause and that the proposed equipment will 
not provide a ratio of 0 dB or more, or that installation of such 
equipment, at no expense to the ITFS station licensee, is not possible 
or has not been proffered.
    (3) For purposes of this section and except as set forth in Sec.  
74.939 regarding the protection of response station hubs, all 
interference calculations involving receive antenna performance shall 
use the reference antenna characteristics shown in Figure I, Sec.  
74.937(a) or, in the alternative, utilize the actual pattern 
characteristics of the antenna in use at the receive site under study. 
If the actual receive antenna pattern is utilized, the applicant must 
submit complete details including manufacturer, model number(s), co-
polar and cross-polar gain patterns, and other pertinent data.
    (4) If an application can demonstrate that the installation of a 
receiving antenna at an existing licensee's site with characteristics 
superior to those of the standard antenna (or, alternatively, the 
appropriate existing antenna in use at the site) will permit the 
applicant to provide service without interference to the existing 
licensee, the application will be considered grantable with the 
condition that the applicant bears all costs of upgrading the existing 
licensee's reception equipment at that site(s). Such a showing should 
include interference calculations for both the existing or reference 
antenna and the proposed antenna. The manufacturer, model number(s), co-
polar and cross-polar gain patterns of the replacement antenna should be 
supplied as well as an accurate assessment of the expected reimbursement 
costs.
    (5) No receive site more than 35 miles from the transmitter shall be 
entitled to interference protection.
    (6) Notwithstanding the above, main, booster and response stations 
shall use the following formulas, as applicable, for determining 
compliance with: (1) Radiated field contour limits where bandwidths 
other than 6 MHz are employed at stations utilizing digital emissions; 
and (2) Cochannel and adjacent channel D/U ratios where the bandwidths 
in use at the interfering and protected stations are unequal and both 
stations are utilizing digital modulation or one station is utilizing 
digital modulation and the other station is utilizing either 6 MHz NTSC 
analog modulation or 125 kHz analog modulation (I channels only).
    (i) Contour limit: -73 dBW/m2 + 10 log(X/6) dBW/m \2\, 
where X is the bandwidth in MHz of the digital channel.
    (ii) Co-channel D/U: 45 dB + 10 log(X1/X2) dB, 
where X1 is the bandwidth in MHz of the protected channel and 
X2 is the bandwidth in MHz of the interfering channel.
    (iii) Adjacent channel D/U: 0 dB + 10 log(X1/
X2), where X1 is the bandwidth in MHz of the 
protected channel and X2 is the bandwidth in MHz of the 
interfering channel.
    (b) All applicants for instructional television fixed stations are 
expected to take full advantage of such directive antenna techniques to 
prevent interference to the reception of any existing or previously-
proposed operational fixed, multipoint distribution, international 
control or instructional television fixed station at authorized 
receiving locations. Therefore, all applications for new or major 
changes must

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include an analysis of potential interference to all existing and 
previously-proposed stations in accordance with paragraph (a) of this 
section. An applicant for a new instructional television fixed station 
must include the following technical information with the application:
    (1) An analysis of the potential for harmful interference with the 
receive sites registered as of September 17, 1998, and with the 
protected service area, of any authorized or previously-proposed 
cochannel station if:
    (i) The proposed transmitting antenna has an unobstructed electrical 
path to receive site(s) and/or the protected service area of any other 
station that utilizes, or would utilize, the same frequency; or
    (ii) The proposed transmitter is within 80.5 km (50 miles) of the 
coordinates of any such station.
    (2) An analysis of the potential for harmful adjacent channel 
interference with the receive sites registered as of September 17, 1998, 
and with the protected service area, of any authorized or previously-
proposed station if the proposed transmitter is within 80.5 km (50 
miles) of the coordinates of any station that utilizes, or would 
utilize, an adjacent channel frequency.
    (3) An analysis concerning possible adverse impact upon Mexican and 
Canadian communications if the station's transmitting antenna is to be 
located within 80.5 km (50 miles) of the border.
    (4) In lieu of the interference analyses required by paragraphs 
(b)(1) and (2) of this section, an applicant may submit (a) statement(s) 
from the affected cochannel or adjacent channel licensee(s) that any 
resulting interference is acceptable.
    (5) Specific rules relating to response station hubs, booster 
stations, and 125 kHz channels are set forth in Sec. Sec.  21.909, 
21.913, 21.949, 74.939, 74.949 and 74.985. To the extent those specific 
rules are inconsistent with any rules set forth above, those specific 
rules shall control.
    (c) Existing licensees and prospective applicants, including those 
who lease or propose to lease excess capacity pursuant to Sec.  
74.931(c) or (d), are expected to cooperate fully and in good faith in 
attempting to resolve problems of potential interference before bringing 
the matter to the attention of the Commission.
    (d) Each authorized or previously-proposed applicant, or licensee 
must be protected from harmful electrical interference at each of its 
receive sites registered previously as of September 17, 1998, and within 
a protected service area as defined at Sec.  21.902(d) of this chapter 
and in accordance with the reference receive antenna characteristics 
specified at Sec.  21.902(f) of this chapter. An ITFS entity which did 
not receive protected service area protection prior to September 17, 
1998 shall be accorded such protection by a cochannel or adjacent 
channel applicant for a new station or station modification, including a 
booster station, response station or response station hub, where the 
applicant is required to prepare an analysis, study or demonstration of 
the potential for harmful interference. An ITFS entity receiving 
interference protection provided by this section will continue to 
receive such protection if it elects to swap channels with another ITFS 
or MDS station as specified in Sec.  74.902(f).

[28 2003 FR 13731 , Dec. 14, 1963, as amended at  50 FR 26758 , June 28, 1985; 
 51 FR 9799 , Mar. 21, 1986;  58 FR 44951 , Aug. 25, 1993;  60 FR 20246 , Apr. 
25, 1995;  60 FR 57368 , Nov. 15, 1995;  63 FR 65114 , Nov. 25, 1998;  64 FR 63737 , Nov. 22, 1999;  65 FR 46620 , July 31, 2000]