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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Pulse-position modulation</span></span>
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</style><table class="sidebar nomobile nowraplinks skin-invert"><tbody><tr><th class="sidebar-title" style="background-color: #bdb"><a href="Passband" title="Passband">Passband</a> <a href="Signal_modulation" title="Signal modulation">modulation</a></th></tr><tr><td class="sidebar-image"><span typeof="mw:File"></span></td></tr><tr><th class="sidebar-heading" style="background-color: #cfc;">
<a href="Signal_modulation#Analog_modulation_methods" title="Signal modulation">Analog modulation</a></th></tr><tr><td class="sidebar-content hlist" style="color: black;">
<ul><li><a href="Amplitude_modulation" title="Amplitude modulation">AM</a>
<ul><li><a href="Space_modulation" title="Space modulation">SM</a></li>
<li><a href="Single-sideband_modulation" title="Single-sideband modulation">SSB</a></li></ul></li>
<li><a href="Angle_modulation" title="Angle modulation">Angle modulation</a>
<ul><li><a href="Frequency_modulation" title="Frequency modulation">FM</a></li>
<li><a href="Phase_modulation" title="Phase modulation">PM</a></li></ul></li>
<li><a href="Quadrature_amplitude_modulation" title="Quadrature amplitude modulation">QAM</a></li></ul></td>
</tr><tr><th class="sidebar-heading" style="background-color: #cfc;">
<a href="Signal_modulation#Digital_modulation_methods" title="Signal modulation">Digital modulation</a></th></tr><tr><td class="sidebar-content hlist" style="color: black;">
<ul><li><a href="Amplitude-shift_keying" title="Amplitude-shift keying">ASK</a></li>
<li><a href="Amplitude_and_phase-shift_keying" title="Amplitude and phase-shift keying">APSK</a></li>
<li><a href="Continuous_phase_modulation" title="Continuous phase modulation">CPM</a></li>
<li><a href="Frequency-shift_keying" title="Frequency-shift keying">FSK</a></li>
<li><a href="Multiple_frequency-shift_keying" title="Multiple frequency-shift keying">MFSK</a></li>
<li><a href="Minimum-shift_keying" title="Minimum-shift keying">MSK</a></li>
<li><a href="On%E2%80%93off_keying" title="On–off keying">OOK</a></li>
<li><a href="Phase-shift_keying" title="Phase-shift keying">PSK</a></li>
<li><a href="Quadrature_amplitude_modulation" title="Quadrature amplitude modulation">QAM</a></li>
<li><a href="Single-carrier_FDMA" title="Single-carrier FDMA">SC-FDE</a></li>
<li><a href="Trellis_coded_modulation" title="Trellis coded modulation">TCM</a></li>
<li><a href="TC-PAM" title="TC-PAM">TC-PAM</a></li>
<li><a href="Wavelet_modulation" title="Wavelet modulation">WDM</a></li></ul></td>
</tr><tr><th class="sidebar-heading" style="background-color: #cfc;">
<a href="Hierarchical_modulation" title="Hierarchical modulation">Hierarchical modulation</a></th></tr><tr><td class="sidebar-content hlist" style="color: black;">
<ul><li><a href="Quadrature_amplitude_modulation" title="Quadrature amplitude modulation">QAM</a></li>
<li><a href="Wavelet_modulation" title="Wavelet modulation">WDM</a></li></ul></td>
</tr><tr><th class="sidebar-heading" style="background-color: #cfc;">
<a href="Spread_spectrum" title="Spread spectrum">Spread spectrum</a></th></tr><tr><td class="sidebar-content hlist" style="color: black;">
<ul><li><a href="Chirp_spread_spectrum" title="Chirp spread spectrum">CSS</a></li>
<li><a href="Direct-sequence_spread_spectrum" title="Direct-sequence spread spectrum">DSSS</a></li>
<li><a href="Frequency-hopping_spread_spectrum" title="Frequency-hopping spread spectrum">FHSS</a></li>
<li><a href="Time-hopping" title="Time-hopping">THSS</a></li></ul></td>
</tr><tr><th class="sidebar-heading" style="background-color: #cfc;">
See also</th></tr><tr><td class="sidebar-content hlist" style="color: black;">
<ul><li>Capacity-approaching codes</li>
<li><a href="Demodulation" title="Demodulation">Demodulation</a></li>
<li><a href="Line_code" title="Line code">Line coding</a></li>
<li><a href="Modem" title="Modem">Modem</a></li>
<li><a href="Angle_modulation" title="Angle modulation">AnM</a></li>
<li><a href="Polar_modulation" title="Polar modulation">PoM</a></li>
<li><a href="Pulse-amplitude_modulation" title="Pulse-amplitude modulation">PAM</a></li>
<li><a href="Pulse-code_modulation" title="Pulse-code modulation">PCM</a></li>
<li><a href="Pulse-density_modulation" title="Pulse-density modulation">PDM</a></li>
<li><a href="Pulse-width_modulation" title="Pulse-width modulation">PWM</a></li>
<li><a href="Delta-sigma_modulation" title="Delta-sigma modulation">ΔΣM</a></li>
<li><a href="Orthogonal_frequency-division_multiplexing" title="Orthogonal frequency-division multiplexing">OFDM</a></li>
<li><a href="Frequency-division_multiplexing" title="Frequency-division multiplexing">FDM</a></li>
<li><a href="Multiplexing" title="Multiplexing">Multiplexing</a></li></ul></td>
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<p><b>Pulse-position modulation</b> (<b>PPM</b>) is a form of signal <a href="Modulation" class="mw-redirect" title="Modulation">modulation</a> in which <i>M</i> message bits are encoded by transmitting a single pulse in one of <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle 2^{M}}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<msup>
<mn>2</mn>
<mrow class="MJX-TeXAtom-ORD">
<mi>M</mi>
</mrow>
</msup>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle 2^{M}}</annotation>
</semantics>
</math></span><img src="./b3937e25d3d60b85daf3a3396486d29a839a2418.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:3.122ex; height:2.676ex;" alt="{\displaystyle 2^{M}}" loading="lazy"></span> possible required time shifts.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> This is repeated every <i>T</i> seconds, such that the transmitted bit rate is <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle M/T}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mi>M</mi>
<mrow class="MJX-TeXAtom-ORD">
<mo>/</mo>
</mrow>
<mi>T</mi>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle M/T}</annotation>
</semantics>
</math></span><img src="./3a9d678d31d9e88de313e21ddec4a5c656e29e33.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:5.241ex; height:2.843ex;" alt="{\displaystyle M/T}" loading="lazy"></span> bits per second. It is primarily useful for <a href="Optical_communication" title="Optical communication">optical communications</a> systems, which tend to have little or no <a href="Multipath_propagation" title="Multipath propagation">multipath</a> interference.
</p>
<meta property="mw:PageProp/toc">
<div class="mw-heading mw-heading2"><h2 id="History">History</h2></div>
<p>An ancient use of pulse-position modulation was the <a href="Hydraulic_telegraph#Greek_hydraulic_semaphore_system" title="Hydraulic telegraph">Greek hydraulic semaphore system</a> invented by Aeneas Stymphalus around 350 B.C. that used the <a href="Water_clock" title="Water clock">water clock</a> principle to time signals.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> In this system, the draining of water acts as the timing device, and torches are used to signal the pulses. The system used identical water-filled containers whose drain could be turned on and off, and a float with a rod marked with various predetermined codes that represented military messages. The operators would place the containers on hills so they could be seen from each other at a distance. To send a message, the operators would use torches to signal the beginning and ending of the draining of the water, and the marking on the rod attached to the float would indicate the message.
</p><p>In modern times, pulse-position modulation has origins in <a href="Telegraphy" title="Telegraphy">telegraph</a> <a href="Time-division_multiplexing" title="Time-division multiplexing">time-division multiplexing</a>, which dates back to 1853, and evolved alongside <a href="Pulse-code_modulation" title="Pulse-code modulation">pulse-code modulation</a> and <a href="Pulse-width_modulation" title="Pulse-width modulation">pulse-width modulation</a>.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> In the early 1960s, Don Mathers and Doug Spreng of <a href="NASA" title="NASA">NASA</a> invented pulse-position modulation used in <a href="Radio-control" class="mw-redirect" title="Radio-control">radio-control</a> (R/C) systems. PPM is currently being used in <a href="Fiber-optic_communication" title="Fiber-optic communication">fiber-optic communications</a>, deep-space communications, and continues to be used in R/C systems.
</p>
<div class="mw-heading mw-heading2"><h2 id="Synchronization">Synchronization</h2></div>
<p>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 <i>differential pulse-position modulation</i>, whereby each pulse position is encoded relative to the previous, such that the receiver must only measure the difference in the <a href="Arrival_time" class="mw-redirect" title="Arrival time">arrival time</a> 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 affecting all successive measurements.
</p>
<div class="mw-heading mw-heading2"><h2 id="Sensitivity_to_multipath_interference">Sensitivity to multipath interference</h2></div>
<p>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 <a href="Time_of_arrival" title="Time of arrival">time of arrival</a> (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.
Multipath in Pulse Position Modulation systems can be easily mitigated by using the same techniques that are used in Radar systems that rely totally on synchronization and time of arrival of the received pulse to obtain their range position in the presence of echoes.
</p>
<div class="mw-heading mw-heading2"><h2 id="Non-coherent_detection">Non-coherent detection</h2></div>
<p>One of the principal advantages of PPM is that it is an <i>M</i>-ary modulation technique that can be implemented non-coherently, such that the receiver does not need to use a <a href="Phase-locked_loop" title="Phase-locked loop">phase-locked loop</a> (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 <i>M</i>-ary non-coherent modulation technique is <a href="Multiple_frequency-shift_keying" title="Multiple frequency-shift keying"><i>M</i>-ary frequency-shift keying</a> (M-FSK), which is the frequency-domain dual to PPM.
</p>
<div class="mw-heading mw-heading2"><h2 id="PPM_vs._M-FSK">PPM vs. M-FSK</h2></div>
<p>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 <i>additive white Gaussian noise</i> (AWGN) channel. However, their performance differs greatly when comparing <a href="Fading" title="Fading">frequency-selective and frequency-flat fading</a> 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. On the other hand, 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.
</p><p>Optical communications systems tend to have weak multipath distortions, and PPM is a viable modulation scheme in many such applications.
</p>
<div class="mw-heading mw-heading2"><h2 id="Applications_for_RF_communications">Applications for RF communications</h2></div>
<p>Narrowband RF (radio frequency) channels with low power and long wavelengths (i.e., low frequency) are affected primarily by <a href="Fading#Flat_versus_frequency-selective_fading" title="Fading">flat fading</a>, and PPM is better suited than M-FSK to be used in these scenarios. One common application with these channel characteristics, first used in the early 1960s with top-end <a href="High_frequency" title="High frequency">HF</a> (as low as 27 MHz) frequencies into the low-end <a href="Very_high_frequency" title="Very high frequency">VHF</a> band frequencies (30 MHz to 75 MHz for RC use depending on location), is the <a href="Radio_control" title="Radio control">radio control</a> of <a href="Model_aircraft" title="Model aircraft">model aircraft</a>, boats and cars, originally known as "digital proportional" radio control. 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). <a href="Servomechanism" title="Servomechanism">Servos</a> made for model radio control include some of the electronics required to convert the pulse to the motor position – the receiver is required to first extract the information from the received radio signal through its <a href="Intermediate_frequency" title="Intermediate frequency">intermediate frequency</a> section, then <a href="Multiplexing" title="Multiplexing">demultiplex</a> the separate channels from the serial stream, and feed the control pulses to each servo.
</p>
<div class="mw-heading mw-heading2"><h2 id="PPM_encoding_for_radio_control">PPM encoding for radio control</h2></div>
<p>A complete PPM frame is about 22.5 ms (can vary between implementation), and signal low state is always 0.3 ms. It begins with a start frame (high state for more than 2 ms). Each channel (up to 8) is encoded by the time of the high state plus the lower state. (PPM high state + 0.3 = servo <a href="Pulse-width_modulation" title="Pulse-width modulation">PWM</a> pulse width).
</p><p>More sophisticated radio control systems are now often based on <a href="Pulse-code_modulation" title="Pulse-code modulation">pulse-code modulation</a>, which is more complex but offers greater flexibility and reliability. The advent of 2.4 GHz band <a href="Frequency-hopping_spread_spectrum" title="Frequency-hopping spread spectrum">FHSS</a> radio-control systems in the early 21st century changed this further.
</p><p>Pulse-position modulation is also used for communication with the <a href="ISO/IEC_15693" title="ISO/IEC 15693">ISO/IEC 15693</a> <a href="Contactless_smart_card" title="Contactless smart card">contactless smart card</a>, as well as in the <a href="High_frequency" title="High frequency">HF</a> implementation of the <a href="Electronic_Product_Code" title="Electronic Product Code">Electronic Product Code</a> (EPC) Class 1 protocol for <a href="RFID" class="mw-redirect" title="RFID">RFID</a> tags.
</p>
<div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2></div>
<ul><li><a href="Pulse-amplitude_modulation" title="Pulse-amplitude modulation">Pulse-amplitude modulation</a></li>
<li><a href="Pulse-code_modulation" title="Pulse-code modulation">Pulse-code modulation</a></li>
<li><a href="Pulse-density_modulation" title="Pulse-density modulation">Pulse-density modulation</a></li>
<li><a href="Pulse-width_modulation" title="Pulse-width modulation">Pulse-width modulation</a></li>
<li><a href="Ultra_wideband" class="mw-redirect" title="Ultra wideband">Ultra wideband</a></li></ul>
<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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