| Multiple Frequency-shift Keying |
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a form of M-ary Orthogonal Modulation , where each symbol consists of one element from an alphabet of orthogonal waveforms. M, the size of the alphabet, is usually a power of two so that each symbol represents log2M bits. Like other M-ary orthogonal schemes, the required Eb/N0 Ratio for a given probability of error decreases as M increases without the need for multisymbol coherent detection. In fact, as M approaches infinity the required Eb/N0 ratio decreases asymptotically to the Shannon Limit of -1.6 dB. However this decrease is slow with increasing M, and large values are impractical because of the exponential increase in required bandwidth. Typical values in practice range from 4 to 64, and MFSK is combined with another Forward Error Correction scheme to provide additional coding gain. TYPES Defined examples of a multiple frequency-shift keying system include '' Dual-tone Multi-frequency '' (DTMF), which is used in touch tone phones and the Multi-frequency trunk signals used in Twentieth Century telephone exchanges. MFSK or "polytone" modes used for Shortwave communications:
MFSK modes used for VHF , UHF communications
SPECTRUM EFFICIENCY VERSUS OTHER HF MODES throughput]] SEE ALSO |
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