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Pulse-position modulation

From Wikipedia, the free encyclopedia

Pulse-position modulation (PPM) is a form of signalmodulation 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 thetransmitted bit rate is M/T bits per second. It is primarilyuseful for optical communications systems, where theretends to be little or no multipath interference.

Synchronization

One of the key difficulties of implementing this technique is that the receiver must be properlysynchronized to align the local clock with the beginning of each symbol. Therefore, it is oftenimplemented differentially as differential pulse-position modulation, where by each pulse position

is encoded relative to the previous , such that the receiver must only measure the difference in thearrival 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, insteadof affecting all successive measurements.

Sensitivity to multipath interference

Aside from the issues regarding receiver synchronization, the key disadvantage of PPM is that it isinherently 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), thepresence of one or more echoes can make it extremely difficult, if not impossible, to accuratelydetermine the correct pulse position corresponding to the transmitted pulse.

Non-coherent detection

One of the principal advantages of PPM is that it is an M-ary modulation technique that can beimplemented 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 communicationssystems, where coherent phase modulation and detection are difficult and extremely expensive. Theonly other common M-ary non-coherent modulation technique is M-ary Frequency Shift Keying (M-

FSK), which is the frequency-domain dual to PPM.

PPM vs. M-FSK

Modulation techniques

Analog modulation

AM · SSB · QAM · FM · PM · SM

Digital modulation

FSK · ASK · OOK · PSK · QAMMSK · CPM ·  PPM · TCM · OFDM

Spread spectrum

CSS · DSSS · FHSS · THSS

See also: Demodulation, modemContents

1 Synchronization

2 Sensitivity to multipath interference 3 Non-coherent detection 4 PPM vs. M-FSK 5 Applications for RF Communications 6 See also

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PPM and M-FSK systems with the same bandwidth, average power, and transmission rate of M/Tbits per second have identical performance in an AWGN (Additive White Gaussian Noise) channel.However, their performance differs greatly when comparing frequency-selective and frequency-flatfading channels. Whereas frequency-selective fading produces echoes that are highly disruptive forany of the M time-shifts used to encode PPM data, it selectively disrupts only some of the Mpossible 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 heavilyimpaired by fading.

Optical communications systems (even wireless ones) tend to have weak multipath distortions, andPPM is a viable modulation scheme in many such applications.

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 inthese scenarios. One common application with these channel characteristics, first used in the early1960s, is the radio control of model aircraft, boats and cars. PPM is employed in these systems, withthe 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 theelectronics required to decode the signal are extremely simple, which leads to small, light-weightreceiver/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 tothe motor position - the receiver is merely required to demultiplex the separate channels and feed thepulses to each servo.

More sophisticated R/C systems are now often based on pulse-code modulation, which is morecomplex but offers greater flexibility and reliability.

Pulse position modulation is also used for communication to the ISO 15693 contactless Smart cardas well as the HF implementation of the EPC Class 1 protocol for RFID tags.

See also

Pulse-amplitude modulation

Pulse-code modulation Pulse-density modulation Pulse-width modulation Ultra wideband

Retrieved from "http://en.wikipedia.org/wiki/Pulse-position_modulation"Categories: Radio modulation modes

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