Pulse-position modulation
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| Modulation techniques |
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| Analog modulation |
| AM · SSB · FM · PM · QAM · SM |
| Digital modulation |
| OOK · FSK · ASK · PSK · QAM MSK · CPM · PPM · TCM · OFDM |
| Spread spectrum |
| FHSS · DSSS |
Pulse-position modulation is a form of signal modulation in which M message bits are encoded by transmitting a single pulse in one of 2M possible time-shifts. This is repeated every T seconds, such that the transmitted bit rate is M/T bits per second. It is primarily useful for optical communications systems, where there tends to be little or no multipath interference.
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[edit] Synchronization
One of the key difficulties of implementing this technique is that the receiver must be properly synchronized to align the local clock with the beginning of each symbol. Therefore, it is often implemented differentially as Differential Pulse-position modulation, where by each pulse position is encoded relative to the previous pulse, such that the receiver must only measure the difference in the arrival time of successive pulses. It is possible to limit the propagation of errors to adjacent symbols, so that an error in measuring the differential delay of one pulse will affect only two symbols, instead of effecting all successive measurements.
[edit] Sensitivity to Multipath Interference
Aside from the issues regarding receiver synchronization, the key disadvantage of PPM is that it is inherently sensitive to multipath interference that arises in channels with frequency-selective fading, whereby the receiver's signal contains one or more echoes of each transmitted pulse. Since the information is encoded in the time of arrival (either differentially, or relative to a common clock), the presence of one or more echoes can make it extremely difficult, if not impossible, to accurately determine the correct pulse position corresponding to the transmitted pulse.
[edit] Non-coherent Detection
One of the principle advantages of Pulse Position Modulation is that it is an M-ary modulation technique that can be implemented non-coherently, such that the receiver does not need to use a Phase-locked loop (PLL) to track the phase of the carrier. This makes it a suitable candidate for optical communications systems, where coherent phase modulation and detection are difficult and extremely expensive. The only other common M-ary non-coherent modulation technique is M-ary Frequency Shift Keying, which is the frequency-domain dual to PPM.
[edit] PPM vs. M-FSK
PPM and M-FSK systems with the same bandwidth, average power, and transmission rate of M/T bits per second have identical performance in an AWGN (Additive White Gaussian Noise) channel. However, their performance differs greatly when comparing frequency-selective and frequency-flat fading channels. Whereas frequency-selective fading produces echoes that are highly disruptive for any of the M time-shifts used to encode PPM data, it selectively disrupts only some of the M possible frequency-shifts used to encode data for M-FSK. Conversely, frequency-flat fading is more disruptive for M-FSK than PPM, as all M of the possible frequency-shifts are impaired by fading, while the short duration of the PPM pulse means that only a few of the M time-shifts are heavily impaired by fading.
Optical communications systems (even wireless ones) tend to have weak multipath distortions, and PPM is a viable modulation scheme in many such applications.
[edit] Applications for RF Communications
Narrowband RF (Radio Frequency) channels with low power and long wavelengths (i.e., low frequency) are affected primarily by flat fading, and PPM is better suited than M-FSK to be used in these scenarios. One common application with these channel characteristics is the radio control of model aircraft, boats and cars. PPM is employed in these systems, with the position of each pulse representing the angular position of an analogue control on the transmitter, or possible states of a binary switch. The number of pulses per frame gives the number of controllable channels available. The advantage of using PPM for this type of application is that the electronics required to decode the signal are extremely simple, which leads to small, light-weight receiver/decoder units. (Model aircraft require parts that are as lightweight as possible). Servos made for model radio control include some of the electronics required to convert the pulse to the motor position - the receiver is merely required to demultiplex the separate channels and feed the pulses to each servo.
More sophisticated R/C systems are now often based on pulse-code modulation, which is more complex but offers greater flexibility and reliability.
Pulse position modulation is also used for communication to the ISO 15693 contactless Smart card as well as the HF implementation of the EPC Class 1 protocol for RFID tags.
