Consequently, P25 Phase 2’s HDQPSK (Harmonized Differential Quadrature Phase Shift Keying) modulation on the downlink achieves 6000 symbols/sec inside a 12.5kHz channel, while P25 Phase 1’s C4FM modulation only achieves 4800 symbols/sec in the same bandwidth.Īnother trade-off is transmitter efficiency. So PSK-based modulations can achieve a greater data rate (bit per second) inside a given channel bandwidth. However, because the transition between symbols involves only a frequency shift (amplitude stays constant), a simple, non-linear FM transmitter can be successful. In general, for a given symbol rate, LSM is spectrally cleaner (less spectrum overflowing in to neighboring channels), but requires linearized transmitters because the transition between symbols involves both the amplitude and phase of the signal changing.įor the same symbol rate, C4FM is less ‘clean’ and produces more energy in adjacent channels. The C4FM eye diagram shows that after filtering, the ‘eye’ is only around 40% open, and spectrally, the signal is much less ‘clean’ than LSM. Transitions between symbols involve changes in frequency, so the resulting signal is constant amplitude. Symbol rate is 4800 symbols/sec, giving a data rate of 9600bits/sec. The four symbols (again named +3, +1, -1, and -3) each conveys two bits of data. What is C4FM?Ĭontinuous Four Frequency Modulation (C4FM) is a type of FSK, so symbols are separated in frequency. Spectrally, we have a much cleaner signal that can now be transmitted. After filtering, the transitions now appear curved and the ‘eye’ has closed slightly, to about 80% open. The signal cannot be transmitted in this form, so filtering of the I and Q outputs is necessary to remove unwanted spectral content. At the output of the IQ modulator (before filtering) the transitions between symbols appear as straight lines and the eye is 100% open, but spectrally, there would be many sidebands. The LSM diagrams on the right show IQ, eye diagrams and spectrum before and after filtering. Transitions between consecutive symbols involve changes in amplitude and phase, so the resulting signal is non-constant amplitude. There are four symbols (+3, +1, -1, and -3) so each symbol conveys two bits of data. LSM is a type of differential QPSK, meaning the symbols are separated by differences in phase between one symbol and the next. We will describe them, cover their relative advantages and disadvantages, then use an example simulcast system to show a practical comparison. To make it ‘real’, we will compare two actual modulation schemes: Linear Simulcast Modulation (LSM) – a type of PSK, and Continuous Four Frequency Modulation (C4FM) – a type of FSK. Tait Principal Engineer Ian Graham is often asked about their relative advantages and disadvantages, so in this article he sets out to explore the subject. Most digital radio modulation types are based on either Phase Shift Keying (PSK) or Frequency Shift Keying (FSK).
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