which modulation is the best [help]

hello
which digital modulation is the best choice for transmitting digital control signals via a wireless link between two points? QPSK? FSK? other modulations?

Continuous phase modulation is commonly used in wireless modems. In contrast with other coherent digital phase modulation techniques where the carrier phase abruptly resets to zero at the start of every symbol (e.g., M-PSK), with CPM the carrier phase is modulated in a continuous manner. For instance, with QPSK the carrier instantaneously jumps from a sine to cosine (i.e., a 90 degree phase shift) whenever one of the two message bits of the current symbol differs from the two message bits of the previous symbol. this discontinuity requires a relatively large percentage of the power to occur outside of the intended band (e.g., high fractional out of band power), leading to poor spectral efficiency. Therefore, CPM is attractive because phase continuity yield high spectral efficiency, and the constant envelope yields excellent power efficiency. Here drawback can be the high implementation complexity required for an optimal receiver.

It depends upon your situation. OQPSK will have the best noise performance but if your required data rate is so high that the spectrum bandwidth is beyond a legal limit you will have to go to more spectrally efficient modulations which in turn require a higher SNR.

Another issue is complexity of transmitter/receiver. For example OFDM is state of the art in short-range high-data rate communication, but is very complex to implement. QPSK at the other hand can be done much easier, reducing the cost of the system. FSK is even simpler (just modulate the VCO), which is why many wireless control systems use FSK and different transceiver ICs can be found.

As allways, many tradeoffs are involved ...