transmission is easier to implement with FHSS than with DSSS. The key idea behind AFH is to use only the "good" frequencies and avoid the "bad" ones—those experiencing frequency selective fading, those on which a third party is trying to communicate, or those being actively jammed. Therefore, AFH should be complemented by a mechanism for detecting good and bad channels. But if the radio frequency interference is itself dynamic, then AFH's strategy of "bad channel removal" may not work well. For example, if there are several colocated frequency-hopping networks (as Bluetooth Piconet), they are mutually interfering and AFH's strategy fails to avoid this interference. The problem of dynamic interference, gradual reduction of available hopping channels and backward compatibility with legacy Bluetooth devices was resolved in version 1.2 of the Bluetooth Standard (2003). Such a situation can often happen in the scenarios that use unlicensed spectrum. In addition, dynamic radio frequency interference is expected to occur in the scenarios related to cognitive radio, where the networks and the devices should exhibit frequency-agile operation. Chirp modulation can be seen as a form of frequency-hopping that simply scans through the available frequencies in consecutive order to communicate. Frequency hopping can be superimposed on other modulations or waveforms to enhance the system performance. See also Dynamic frequency hopping List of multiple discoveries Maximum length sequence Orthogonal frequency-division multiplexing Radio frequency sweep Notes References Bibliography Władysław Kozaczuk, Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War Two, edited and translated by Christopher Kasparek, Frederick, MD, University Publications of America, 1984, . Multiplexing Quantized radio modulation modes Radio frequency propagation Radio resource management Military radio systems ja:スペクトラム拡散#周波数ホッピング