FirstPrevious

BPR-4 - Application Procedures and Rules for Television Broadcasting Undertakings

Appendix 10 - Zones One and Two Canada and U.S.A.

Top of Page

Appendix 11 - Television/Land Mobile Mutual Interference Analysis (Provisional)

Purpose

The purpose of this appendix is to define the factors and to provide a method for evaluating mutual interference between land mobile services and TV services on channel 7, 14 and 69 operating in adjacent frequency bands. It is to be noted that the present analysis does not deal with interference from multiple sources.

A. Interference to Land Mobile

Interference to land mobile base station receivers may occur when they operate near the site of a TV transmitter and on frequencies close to the TV channel (within 4 MHz). Such interference may be eliminated or reduced by increasing the physical separation of the stations.

The following are the types of interferences and possible remedies:

1. Out-of-channel radiation from the television undertaking. This type of interference is produced intrinsically and with respect to the ambient "noise floor":
   1. out-of-channel radiation by the television transmitter which can intrinsically create interference to LM. Normal operation usually results in video components¹
   2. LM usable sensitivity at VHF and UHF frequencies which is determined by internal noise and man-made noise. The sensitivity of a typical receiver is such that an input signal of 0.5 μV (-114 dBm, assuming a receiver input impedance of 50 O) would result in a 12 dB SINAD ratio at the output. To minimize the potential for interference to land mobile receivers, out-of-channel television emissions at the specific LM carrier frequency and at the input of land mobile should ideally not exceed -120 dBm (assuming a signal to interference ratio of 6 dB at that level).
   While the above numbers apparently prohibit co-existence of LM and adjacent band television services except at great distances, statistical, technical and subjective factors have, in reality, proven that separation distances much lower than those implied by the theoretical numbers above can provide satisfactory results. In fact, a minimum separation distance² of 35 km is deemed to be sufficient for full power television undertakings. The minimum separation distance may be decreased by 3 km per dB of reduced ERP value to a minimum separation distance of 15 km. In real interference situations, the actual values that could be attributed to the above factors and in the footnote no. 2 may differ from those assumptions. Additional factors such as TV antenna pattern, the degree of additional TV transmitter filtering and/or other rejections in the LM system may provide shorter separation distances. The ambivalent nature of all these factors does not provide a sound foundation on which distance separation requirements could be based. The Department will therefore consider individual agreements between the TV applicant and the affected LM operators as an alternatives to minimum separation distances.
2. Desensitization of the base station receiver. Interference can be minimized by installing cavity filters at the LM receiver to provide some rejection of the adjacent visual or aural carriers except where the frequency is too close.
3. Intermodulation products. Intermodulation products result from the mixing of two or more undesired signals at the input of the receiver. This may be eliminated by employing appropriate filters at the land mobile receiver to reduce the magnitude of the TV signal contributing to the intermodulation product. If the intermodulation products are caused by mixing inside the receiver, upgrading the receiving equipment or better shielding may also alleviate the problem.

B. Interference to Television

Figures 1 to 3 reflect measured values of protection ratio versus frequency separation for various channels.

In the case of channels 7 and 14, if a land mobile transmitter of relatively high power is operating on a frequency near 2.33 MHz below the edge of the channels, a mixing process takes place producing severe color interference. For other frequencies in the lower 4 MHz adjacent band of channels 7 and 14 or in the entire upper adjacent 4 MHz band of channel 69, interference to television generally takes the form of distortion of the luminance information.

The potential for interference in the second, third and fourth adjacent TV channel is negligible.

LM assignments in the frequency band beyond 4 MHz from the edge of television channels can operate within the television service contour and provide the required protection. Within 4 MHz, however, full protection to television service from LM base station operations may be provided either through co-siting the TV transmitting antenna with the related adjacent channel LM base station transmitter, or by selecting the television site in such a way to keep the LM base station outside the proposed service contour (Grade B) where the protection ratios are met. These options may prove to be impractical in some cases due to location.

Co-siting TV and LM transmitters will result in a uniform signal differential inside the service contour of the television undertaking. The best approach to co-siting is to locate both the TV and LM transmitting antennas on the same tower or supporting structure. In such cases, similar antenna patterns are not needed as long as the TV/LM effective radiated power (ERP) ratio meets or exceeds that implied in Table 1. For best results, the LM receiver(s) should be kept at some distance rather than at the transmitting antenna site.

When not co-located, LM base station transmitters inside a television service contour will establish pockets of interference the size of which varies depending on the frequency separation and the strength of the television signal. As the separation distance between the two stations increases, the field strength of the TV signal decreases enlarging progressively the size of the area of interference. For all such cases, the applicant shall indicate the presence of interference zones from the related LM assignments on the coverage map. The interfering field strength of the land mobile station may be calculated using the propagation curves of Figure 4.

Viewing tests were conducted in 1989 (refer to report BTRB-7 entitled Protection to Television from Land Mobile Stations Operating Adjacent to Television Channels 7, 13, 14 and 69) to determine the susceptibility of television receivers to adjacent band LM interference. LM signals in the frequency bands adjacent to channels 7, 14 and 69 were applied to the receiver terminals and the levels were adjusted for "Just Perceptible" interference. The viewing conditions for the assessment of picture quality were in accordance with the CCIR Recommendation 500-3. The TV signal at the receiver terminal was adjusted to -25, -35 and -55 dBm (dB above 1 mW).

The average protection or rejection ratios obtained at the laboratory are based on non-varying desired and undesired input voltages at the receiver. In practice, it is necessary to define the percentage of time as well as the percentage of locations for which the desired quality of service is met.

The picture quality for coverage inside the Grade A (or Grade B) contour is defined as a picture of acceptable quality for at least 70% (50% for the Grade B) of receiver locations and 90% of the time. If F(50,50) propagation curves are used inside these contours, a time probability factor (T) and a location probability factor (L) have to be used to account for signal fading and terrain irregularities respectively. Consequently, protection to television reception from LM base station transmitters is derived from the following formula:

F_u = F_d - PR + AD - H(L) - H(T)     (1)

PR is the average protection ratio in dB obtained from the receiver measurements (see Figures 1 to 3 in this appendix);

AD is the combined cross-polarization and directional discrimination of the TV receiving antenna against LM signals in dB (AD ≈ 15 dB when LM base station outside the Grade B, otherwise, AD = 0 dB);

H(L) is the adjustment made in dB with respect to the percentage of locations where the desired field strength level will be above the calculated value;

H(T) is the adjustment made in dB with respect to the percentage of time when the desired field strength level will be above the calculated value.

At VHF frequencies (channel 7)

At UHF frequencies (channels 14 and 69)

Near the Grade B Contour

The protection ratios of Figures 1 to 3 are average values obtained at a TV receiver input level of -55 dBm. This level of signal represents viewing conditions close to the edge of the protected contour.

Co-located (same tower) TV and LM Transmitters

For TV receiving locations near the transmitting site, the television signal levels will be very high and therefore outdoor receiving antennas are not normally used. The reduced antenna size (rabbit ears) and its reduced height above ground limit the actual level. Measurements have indicated that the TV signal into the receiver using an indoor antenna inside the Grade A contour is usually between -25 and -35 dBm. In addition, it is assumed that the indoor TV receiving antenna provides no cross-polarization discrimination and indeed very little directional discrimination against vertically polarized LM transmissions, hence:

AD = 0 dB

Table 1 gives the average values of the protection ratio for television. These are based on a receiver input level of -35 dBm (inside the Grade A contour). The ratios for -25 dBm are somewhat similar since they do not exhibit significant dependence on the level of the desired television signal.

Top of Page

Footnotes

¹ In the worst case, components as high as the below figures can exist for a few seconds at a time:
-40 dB @ fv - 4.5 MHz (aural image, 3.25 ± 0.01 MHz below channel edge)
-40 dB @ fv + 9.0 MHz (visual image, 4.25 ± 0.01 MHz above channel edge)
-60 dB @ fv - 3.58 MHz (chroma image, 2.33 ± 0.01 MHz below channel edge)
-37 dB ( worst case sporadic video components) down to 3 MHz below and up to 4 MHz above channel edge.
Emissions at a specific frequency could, from a particular contributor, be reduced by a further 20 to 30 dB at the television transmitter by the use of special filters.

² The separation distance is based on out-of-channel (within 4 MHz from the edge of channel) TV emissions that are suppressed by 60 dB (referenced to peak envelope visual power), a receive LM antenna gain of 5 dB (10 dB for UHF) and a cross-polarization discrimination of 20 dB.

FirstPrevious