Tactical Wireless Communications and Networks: Design Concepts and Challenges

2.3 Non-Coherent Demodulation

If you operate a high-end oscillator at 200 MHz, it can shift by 1 Hz (per second, which is very small theoretically) leading to a phase error of 2π radians. Techniques for phase locking have been developed where the demodulator estimates the phase of the carrier signal. However, these techniques are at times ineffective and may need frequent phase locking periods (with channels that transmit in bursts such as time division multiple access—TDMA—this can make the channel very inefficient).

What makes non-coherent demodulation more important in tactical wireless communications is node mobility, which causes the phase of the carrier signal to be shifted; this is known as the Doppler shift. Doppler shift occurs in addition to the oscillator issues mentioned above.

So far, we have been addressing signal detection based on known references of the orthogonal bases functions (cosω_ct and sinω_ct in the case of PSK and QAM). This ideal case assumes that the demodulator is phase-synchronized to the received signal phase while given the amplitudes of the signals to be detected (known as coherent detection). When the received signal reference phase is distorted (imagine that the reference axis of the received signal is randomly shifted) the demodulation problem becomes more complex. The demodulator now has to use non-coherent demodulation, which assumes that the phase angle is a random variable with a mean around the transmitted phase (known phase).

The key objective ...