Adaptive modulation is a technique used to modify the modulation of a signal based on the characteristics of the channel between the transmitter and receiver. It can also be used by an end user to manually adapt a modulated signal for their own modulation purposes. When the channel is undergoing harsh conditions (for FSO, this is rain, fog, haze, etc.), the transmitter and receiver cannot communicate effectively with the current signal modulation scheme. The scheme is then adapted to adjust for non-ideal channel conditions and improve signal throughput; that adjustment is adaptive modulation.
Adaptive modulation is important in FSO since the channel between transmitter and receiver greatly controls the effectiveness of transmission. When light source modulation is fixed, it cannot “adapt” to the presence of attenuators, such as sandstorms, fog, or snow. Adaptive modulation is a tool that can be used to improve link transmission for varied channels, increasing the effectiveness of an FSO system.
This technique is used in both FSO and mobile communications, particularly in EDGE. Adaptive modulation may be used to improve power and bandwidth efficiency, but if the channel deteriorates, bandwidth efficiency is always sacrificed for a lower-order modulation scheme. Examples of adaptive modulation include:
- Adaptive coding and variable coding rate
- Variable power adaptation: varying the power of a transmitter
- Channel inversion: a transmitter adapts its power to maintain constant signal-to-interference ratio at the receiver, which is useful for multiple transmitters transmitting to the same receiver (Definition and brief explanation)
- Truncated channel inversion
- Optical sub-carrier intensity modulation: accomplished by using multiple carrier modulation in the optical domain to create many carrier frequencies and modulating the intensity of one of those frequencies (also called a sub-carrier frequency) (Descriptive video)
- Multiple sub-carrier intensity modulation: sub-carrier intensity modulation but for multiple frequencies instead of just one.
- Differential phase shift keying: strategically shifting the phase of a received signal to produce symbols that are differentiated and determined by those that precede them.
- For example, a sine wave with a 0 degree phase transmits the symbol “0”. The wave is phase-shifted +90 degrees; it now transmits the symbol “1”. The wave is held at this phase for one period: the phase has not changed, so the transmitted symbol is “0”. The wave is shifted back to a 0 degree phase, and since the new phase is different from the one before it by -90 degrees, the symbol for this period is “1”. The wave does not shift again; since the current phase is the same as the previous, the transmitted symbol is “0”.