| 455 / | HD | TELECOMMUNICATIONS | 
| 39 | DF | TRANSMITTER AND RECEIVER AT SEPARATE STATIONS {14} | 
| 500 | DF | .~ Plural transmitters or receivers (i.e., more than two stations) {4} | 
| 501 | DF | .~.~ Noise, distortion, or singing reduction {2} | 
| 504 | DF | .~.~.~ Fading compensation {2} | 
| 506 |  | .~.~.~.~ Rayleigh or multipath fading |