Adjacent channel and SFN operationAdjacent channel and SFN operation

Adjacent channel and SFN operation

Planning for digital television and datacasting services has made use of adjacent channel operation (either between two adjacent digital services, or between a digital service and an adjacent analog service) on the assumptions that: the adjacent channel transmissions are effectively co‑sited; and the ratio of adjacent channel ERP levels is constrained to be within an appropriate range.

When applying the guidelines related to adjacent channel interference, account should be taken of variable factors affecting the relative levels of received adjacent channel transmissions, such as differences in transmitter radiation patterns (in both the horizontal and vertical planes), frequency dependency of receiving antenna system performance and differences in the actual transmission sites. Until better information is available, a value of 5 dB has been applied to the basic protection ratio values.

Where additional transmission sites are proposed, it may prove difficult to satisfy the necessary adjacent channel protection requirements in cases where transmitters are not effectively co‑sited, and/or in cases where widely different ERP levels are used by the adjacent channel services.

Recognising this constraint, guideline 96 has been written to ensure that digital services operating in accordance with the DCP are protected.

In the particular case of potential adjacent channel interference between two digital services where one, or both, of the proposed digital services is not included in a DCP, it may be possible to satisfy the adjacent channel interference conditions through negotiation between the operators of the adjacent channel services. The required end result is that, in each area where adjacent channel interference could arise, the transmitters providing the adjacent channel services should be effectively co‑sited and their relative ERP levels should be co‑ordinated to ensure that the conditions of Part 6 of the Technical Planning Guidelines covering adjacent channel interference are satisfied.

Where digital licensees wish to locate transmitters at sites that are significantly separated from planned LAP, DCP or DRCP sites, where the effective co‑siting assumption does not hold[4], viewers may face a situation where they are trying to receive a distant, low level signal in the presence of a high level interfering signal from a nearby transmitter. This places a much greater onus on the digital licensee at the non‑DCP site to carefully plan and engineer the site to ensure that the required protection ratios are achieved and interference to nearby receivers is avoided. This is most important when a digital service is adjacent to a wanted analog service. However, other potential interference mechanisms also need to be considered.

The interference mechanism of next greatest concern is image channel interference from a digital service to analog television reception. Image channel interference is an artefact of superheterodyne television receiver design. It may occur if a high level signal exists on a channel that is 10 or 11 channels above the wanted channel. The susceptibility of receivers to image channel interference is dependent on the design of the radio frequency input stages of the receiver. Available information on wanted‑to‑unwanted image channel protection ratios is that a digital image channel causing interference to an analog television service will usually present a higher interference risk than a first adjacent channel digital‑to‑digital case.