Pulse Position Modulation (PPM)

Pulse Position Modulation (PPM) is a form of signal modulation in which the amplitude of a message signal is encoded by varying the position of a narrow, fixed-amplitude pulse within a specific time slot. Unlike Pulse Amplitude Modulation (PAM) or Pulse Width Modulation (PWM), both the amplitude and the width of the pulses in PPM remain constant. The information lies entirely in the timing of the pulse.

For each sample of the message signal, a single pulse is transmitted. A high amplitude in the message signal might correspond to a pulse appearing late in its time slot, while a low amplitude might correspond to a pulse appearing early.

PPM Generation

A classic way to generate PPM is by first creating a Pulse Width Modulation (PWM) signal from the original message signal. In the PWM signal, the width of each pulse corresponds to the message amplitude. The PPM signal is then derived by using the varying trailing edge of each PWM pulse to trigger the start of a new, constant-width PPM pulse. This effectively converts the information from pulse width into pulse position.

In a digital system, it is more efficient to calculate the pulse position directly without creating an intermediate PWM signal. For each sample of the message signal, the position of the pulse within its time frame is computed.

First, the allowable range for the pulse's start position is determined by the frame size minus the pulse width. The normalized amplitude of the message sample is then scaled by this range to find the exact offset:

\( \text{pulsePosition} = \text{round}(\text{normalizedAmplitude} \times (\text{samplesPerFrame} - \text{pulseWidth})) \)

Then, this position is used to place the pulse into the correct location in the final output signal array:

\( \begin{aligned} \text{frameStart} &= \text{sampleIndex} \times \text{samplesPerFrame} \\ \text{pulseStartIndex} &= \text{frameStart} + \text{pulsePosition} \end{aligned} \)

PPM Demodulation

Demodulation is the process of recovering the original analog signal from the PPM signal. This can be seen as the reverse of the generation process.

1. PPM to PWM Conversion: The first step is to convert the PPM signal back into a PWM signal. This can be done using a flip-flop circuit.
   • A clock pulse, which marks the beginning of each time slot, is used to set the flip-flop's output to high.
   • The incoming PPM pulse is then used to reset the flip-flop's output to low.
   • The result is a pulse whose width is determined by the time difference between the reference clock pulse and the PPM pulse—effectively recreating the PWM signal.
2. Low-Pass Filtering: The reconstructed PWM signal is then passed through a low-pass filter. The filter averages the PWM signal, and its output is a voltage proportional to the pulse width, which in turn is proportional to the original message signal's amplitude.

Key Characteristics

• Constant Power: Since the pulses have constant amplitude and width, the transmitter can be simpler and more power-efficient, as it only needs to operate in an 'on' or 'off' state.
• Synchronization: PPM requires precise synchronization between the transmitter and receiver so that the receiver knows the start time of each time slot to measure the pulse's position against. This can add complexity to the system.
• Bandwidth: PPM generally requires more bandwidth than PAM and PWM because it needs to accurately resolve the timing of the narrow pulses.

Advantages and Disadvantages

Advantages

• High Noise Immunity: PPM has excellent noise immunity. Since the information is in the pulse's timing, amplitude noise (which is the most common type) has little effect, as long as the receiver can still detect the presence of the pulse. This makes it superior to PAM and PWM in noisy environments.
• Power Efficiency: The transmitter sends constant-power pulses, allowing it to operate at peak efficiency. This is a significant advantage in power-limited systems like battery-powered devices or deep-space communication.

Disadvantages

• Synchronization Requirement: The need for a reliable synchronization signal between the transmitter and receiver can be a significant challenge.
• High Bandwidth Requirement: The use of narrow pulses requires a larger transmission bandwidth compared to other pulse modulation techniques.

Applications of PPM

Due to its noise immunity and power efficiency, PPM is used in various applications:

• Optical Communications: Widely used in fiber-optic and free-space optical communication systems where transmitter power is limited.
• Radio Control (RC) Systems: Used for controlling hobbyist aircraft, cars, and drones. The position of the pulse corresponds to the position of a control stick or switch.
• Wireless Communication: Employed in certain types of radio frequency (RF) links and sensor networks where low power consumption is critical.