Digital modulation

Phase I of the P25 project uses a four-symbol FSK (4FSK) modulation. Standard FSK, covered earlier, uses two frequencies or “tones” to achieve 1-bit/Hz. However, 4FSK is a variant that uses four frequencies to provide 2-bits/Hz efficiency .14 Tem 2021

First of all, GFSK modulation is a much cleaner signal than FSK . This is because GFSK process filters out spurious to a much narrower transmission bandwidth than FSK. But in addition to that, we are now transmitting at 76.8Kbps which has a much wider bandwidth than 4.8Kbps we used before.

Quadrature Phase Shift Keying (QPSK) is a form of Phase Shift Keying in which two bits are modulated at once, selecting one of four possible carrier phase shifts (0, 90, 180, or 270 degrees) . QPSK allows the signal to carry twice as much information as ordinary PSK using the same bandwidth.

Frequency-shift keying (FSK) is a method of transmitting digital signals using discrete signals . The two binary states — logic 0 (low) and 1 (high) in a binary frequency-shift key mechanism — are each represented by an analog waveform.