The following is a list of modulation schemes used in FSO and satcom along with a descriptor of each scheme and examples of practical applications.
Analog modulation deals only with changing the variables of the sinusoidal carrier wave of a sinusoidal signal. It is mainly reserved for teaching purposes, classical applications, and academic examples. While analog modulation is still used in radio transmission, most modern links use digital modulation for greater data throughput.
- Amplitude Modulation (AM)
- Amplitude modulation is a technique in which the amplitude of the carrier signal is varied in order to send information. AM is used mainly for radio communication due to high spectral inefficiency and low amount of available transmission frequencies.
- Frequency Modulation (FM)
- Frequency modulation is a technique in which the frequency of the complex part of the carrier signal is varied in order to send information. FM is famous for being the primary way to conduct radio and musical broadcasting due to high SNR and resistance to interference.
- Phase Modulation (PM)
- Phase modulation is a technique in which the phase of the complex part of the carrier signal envelope is varied in order to send information. PM has significant importance in digital encoding, digital waveform generation, and sound production for distortion and synthetic effects.
- Pulse Amplitude Modulation (PAM)
- In PAM, the information in a signal is encoded into a series of evenly spaced Dirac delta functions (also called impulse functions, signal pulses, or unit functions). The amplitude of these Dirac delta functions form the envelope of the original signal and are then modulated by a carrier signal. PAM is used primarily in digital data signal modulation but it is now less common since pulse position modulation is more popular.
- Quadrature Amplitude Modulation (QAM)
- Quadrature amplitude modulation uses multiple orthogonal sinusoidal carrier signals to send information by changing the amplitude of the carrier wave. Since the signals are orthogonal, the receiver is able to easily separate the transmitted signals and receive both carrier’s information. See the QAM main page for more.
Digital modulation is similar to analog modulation with one distinct change: the informational signal is digital rather than analog. The carrier wave is still sinusoidal.
- Amplitude-Shift Keying (ASK)
- ASK is a type of modulation that sends digital data in the form of the amplitude of a carrier wave rather than having information modulated inside the carrier wave itself. When the carrier wave has a large amplitude, a “1” is received. When the carrier wave has a small or zero amplitude, a “0” is received. In some systems, the exact amplitude can be used to transmit multiple bits. For example, maximum amplitude would be “11”, 75% max amplitude would be “10”, 50% max amplitude would be “01”, and 25% max amplitude would be “00”. As the system transmits more variables, i.e., information other than 0’s and 1’s, the amplitude can be modulated as previously described, giving the system a higher data rate and more flexibility.
- Bit-Interleaved Coded Modulation (BICM/BITM)
- In BICM, digital information is encoded into bits and then scrambled (or interleaved) to prevent against a continuous string of errors occurring in the received data stream. For example, if a patch of hot air moves into and then out of the beam path once quickly, the signal before and after the hot air will be transmitted normally, but the signal inside the hot air will be riddled with many errors. By scrambling the bits around before transmission and reassembling them at the receiver end, large strings of bit errors will be spread over a long stream of data, making them easy to correct to the proper bits. BICM is used in systems where a large amount of continuous errors can be expected to occur, such as in areas of hot and cold air.
- Differential Amplitude Pulse-Position Modulation (DAPPM)
- DAPPM is a combination of PAM and DPPM. Like PAM, the information in the signal is coded onto evenly-spaced Dirac delta functions. From here, the carrier wave is transmitted via pulse within set time slots in a certain window of time just like DPPM, and the information received is received relative to the pulses that preceded it.
- Dual Header Pulse Interval Modulation (DHPIM)
- In DHPIM, like PIM, there are two things to keep track of: intra-signal timing (frame window) and signal spacing (time window). However, unlike in PIM, in DHPIM the frame window length can be varied. Instead of a receiver having a set number of frame windows within the time window, there are a variable amount of frame windows determined by the amount of information that must be sent and each new frame is denoted by a starting pulse, called a header, like the Starting Code for a Morse code signal. On the receiver end, the frame window length is determined by counting the number of time slots between each header, where each time slot may or may not have a pulse in it that carries information. DHPIM eliminates unused time slots in the time window, increasing throughput, and requires less bandwidth than PIM or PPM. DHPIM is used primarily in optical wireless communications. 
- Differential Pulse-Position Modulation (DPPM)
- DPPM is the same as PPM with one key difference. Instead of having the transmitter’s and receiver’s clocks synced to each other, the receiver merely looks for pulse timings relative to previously received pulse timings. DPPM is used primarily in free-space optical communications.
- Differential Phase-Shift Keying (DPSK)
- DPSK is the same as PSK with the chief difference being that the receiver does not need to store the reference signal to demodulate. The DPSK modulation scheme looks only at the phase shift with respect to the wave cycle that preceded it. For example, a sine wave with a 0 degree phase transmits the symbol “0″. The wave has phase-shifted +90 degrees; it transmits the symbol “1”. The wave is held at this phase for one period: the phase does not change, 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 is shifted forward by +90 degrees; the phase has changed again, so the transmitted symbol is “1”.
- Frequency-Shift Keying (FSK)
- FSK is a scheme by which the carrier wave transmits information by having its frequency changed. Most systems will have two discrete frequencies between which the system will oscillate, with the faster frequency transmitting a “1” and the slower frequency transmitting a “0”. This set-up is called binary frequency-shift keying (BPSK). Like other modulation schemes, if more complicated information with more variables needs to be transmitted, then more frequencies will be used, increasing data rates and flexibility.
- M-ary Phase-Shift Keying (MPSK)
- MPSK is a type of PSK that focuses on using an M number of phase positions to transmit binary code. The binary signals are usually visualized on a signal space constellation map. As the number of phase positions M goes up, the number of transmittable signals also increases. For example, in 2PSK also called BPSK, only 2 phase positions are used, so only 1 and 0 are transmittable. In 4PSK also called QPSK, there are 4 phase positions, so 01, 10, 00, and 11 are transmittable. The number of transmittable bits increases with the number of phase position combinations in a 2^M fashion. That is, if a system needs to transmit 4 bits per phase, it will need 2^4 = 16 phase positions (16PSK). If a system needs to transmit 8 bits per phase, it will need 2^8 = 256 phase positions (256PSK). Usually 8PSK is the highest a system will go due to the increase in error rate and the availability of better modulation schemes beyond that level. MPSK was used in early space communications and is used currently in wireless RF standards, low-cost transmitters, and satellite broadcasting.
- Multiple Sub-carrier Intensity Modulation (MSIM)
- MSIM is a combination of SIM and multiple sub-carrier modulation (MSM). Like SIM, a premodulated RF signal is modulated onto the intensity profile of an optical signal. With MSIM, many premodulated RF signals at different frequencies are modulated onto the optical signal for transmission, enabling high-speed information transfer. MSIM is studied for its applications in optical wireless communications. 
- Offset Quadrature Phase-Shift Keying (OQPSK)
- OQPSK, also called staggered quadrature phase-shift keying, is a type of PSK in which the phase shifts are locked to 90 degrees only. This locks the signal into a maximum of four possible transmission codes (three 90 degree phase shifts plus the 0 degree starting phase), giving the code the “quadrature” nomenclature. The lower phase shift when compared to BPSK increases system SNR and reduces the overall error rate of the system. 
- On-Off Keying (OOK)
- OOK is a type of amplitude-shift keying in which information is transmitted via the presence or absence of a signal. The simplest example is using a flashlight to send binary code to a neighbor across the street, with “1” being when the light is on and “0” being when the light is off. OOK is used primarily to send Morse code over RF channels.
- Overlapping Pulse-Position Modulation (OPPM)
- OPPM is similar to PPM with the difference that all bits must be continuous in a time window. For example, in a 1-second window where pulses are 1 millisecond long, if 250 pulses are sent in the time window, all pulses must be back-to-back. In PPM, the 250 pulses may be scattered anywhere in the time window, but in OPPM they must all occur one after the other. This decreases the complexity of signals received and makes it easier to demodulate information at the receiver. OPPM is used primarily in optical wireless communications. 
- Phase-Shift Keying (PSK)
- PSK is a scheme that transmits bits of information by changing the phase of a constant frequency reference wave. For example, a sine wave with a phase of 0 degrees will be received as “1” and a sine wave that has been shifted 45 degrees will be received as “0”. A signal can be phase shifted between two set degrees, called binary or bipolar phase-shift keying (BPSK) where only 0 or 1 is transmitted, or it can be phase shifted between four degrees, called quadrature phase-shift keying (QPSK) where 00, 01, 10, or 11 can be transmitted. A signal can also have both its amplitude and phase shifted, called asymmetric phase shift keying (APSK), leading to an exponential increase in transmittable symbols and greatly increasing data throughput. The challenge of PSK is that the demodulator at the receiver interface must keep track of the constant frequency wave so that the receiver knows what to output from the data stream. This pitfall is avoided by using DPSK.
- Pulse-Code Modulation (PCM)
- PCM is a digital sampling scheme which samples a signal’s amplitude at regular intervals and can only sample a discrete number of amplitudes. The interval at which a signal is sampled is the sampling rate and the number of discrete amplitudes sampled is called the bit depth. Most commonly, in any computerized audio file, there will be a descriptor of “bit rate” which details how many bits per second are sampled in the audio file (the sampling rate). The higher the bit rate, the closer the audio is to a live version of the song in terms of strict playback. Bit depth determines the resolution of the signal, that is, how good the signal quality is in terms of point-to-point signal contrast. Common bit depths include 16 bit for CD audio and 24 bit for Blu-Ray and DVD audio, but can go as high as 64 bit. PCM is used to digitally represent analog signals and is found in integrated analog-to-digital (ADC) circuits.
- Pulse Interval Modulation (PIM)
- PIM is similar to PPM but uses the physical timing between received bits as a method of sending information rather than using the signal itself. For example, a time delay of 1 nanosecond between two received signals means “00”, 2 nanoseconds means “01”, 3 nanoseconds means “10”, and 4 nanoseconds means “11”. This information is encoded on the digital signal which is modulated on to a carrier wave which is then sent to a receiver for processing. PIM differs from PPM in that there is no timing window that changes how an information stream is processed. Instead, the beginning of each time window is signaled by a very short pulse, similar to the Starting Signal code in Morse code. PIM is used primarily in free-space optical communications.
- Pulse-Position Modulation (PPM)
- A PPM system uses two principals to send information: intra-signal timing and signal spacing. In intra-signal timing, or the timing window, say that there is a window of time in which a signal can be sent, i.e., assume a signal can only be sent in a 1 second window before the transmitter turns off for another 3 seconds. Within this 1 second window, called the frame window, the signal with all information encoded on it is sent via a pulse that is 1 millisecond long, and depending on where that millisecond pulse falls within the 1 second window and how many pulses are sent, the receiver interprets the received signal to mean different things. If the transmitter is turned off for 2 seconds instead of 3 seconds and another pulse is sent in a new window, the receiver interprets this information differently as well. It is important to have the clocks of the transmitter and receiver perfectly synced up since information can be misinterpreted if the clocks are not perfectly synced. PPM is used primarily in optical communications.
- Pulse Time Modulation (PTM)
- PTM is a class of modulation techniques that focuses on modulating the information from a signal on to a carrier wave in the time domain. PWM, PPM, and DPPM are examples of PTM schemes.
- Pulse Width Modulation/Pulse Duration Modulation (PWM/PDM)
- PWM is a scheme that enables a user to control how long a signal is sent in a set time frame. PWM is used to control average signal power and is closely related to the duty cycle of a pulsed system.
- Sub-carrier Binary Phase-Shift Keying (SC-BPSK)
- Binary Phase-Shift Keying (BPSK) is the simplest form of PSK in which a signal is only phase-shifted between two degrees to send either a 1 or 0 bit. In SC-BPSK, an RF signal which has been modulated using BPSK is modulated on to an optical carrier wave for wireless communications, making the RF wave a sub-carrier wave.
- Sub-carrier Quadrature Phase-Shift Keying (SC-QPSK)
- Quadrature Phase-Shift Keying (QPSK) is a form of PSK in which the signal is phase-shifted between four different degrees to send 00, 01, 10, or 11 bit combinations. In SC-QPSK, an RF signal which has been modulated using QPSK is modulated on to an optical carrier wave for wireless communications.
- Sub-carrier Intensity Modulation (SIM)
- SIM is a technique by which a modulated RF signal is modulated onto the intensity of an optical signal. By performing amplitude modulation on the optical signal, the RF sub-carrier is also modulated, thus giving the technique its name. SIM is used in wireless visible light communications technologies.
- Trellis-Coded Modulation (TCM)
- In TCM, a signal is convolved and modulated in the same step. This scheme is a combination of convolutional coding (also called trellis coding) and modulation. For a more detailed explanation of TCM, visit the TCM main page.
Below are miscellaneous modulation schemes or related technologies which do not fit exclusively into a strictly digital or analog modulation hierarchy.
- Direct-Sequence Spread Spectrum (DSSS)
- DSSS is a type of signal protection which works by spreading the spectrum of a message signal using a pseudorandom bit modulation sequence. The bit scheme is composed of very short-duration bits, causing the bandwidth of the signal to be larger than the signal itself and securing it against signal interference. Most GPS systems use DSSS since it is efficiently structured for sending ranging signals and satellite coordinates to end users. 
- Frequency-Hopping Spread Spectrum (FHSS)
- FHSS is a pseudo-modulation scheme that changes the frequency of a signal’s carrier wave multiple times among a chosen set of frequencies in a pre-specified pattern. FHSS prevents frequency-selective interference from destroying the signal’s information. It also makes the signal difficult to eavesdrop since the frequency-hopping pattern is not known to the eavesdropper. FHSS is used in some wireless optical systems as a means of securing the signals. 
- Orthogonal Frequency Division Multiplexing (OFDM), aka multi-carrier modulation
- OFDM is a type of multiplexing where all signals multiplexed are orthogonal to each other, preventing signal cross-talk. OFDM is used in both the RF and optical domains, with one such application being coherent orthogonal optical frequency division multiplexing (CO-OFDM).