Pulse Position Modulation (PPM) is a highly efficient signal modulation technique. Depending on the application, it is implemented using one of three primary methods: 1. Analog PPM (continuous shift), 2. Digital/M-ary PPM (discrete slots), and 3. Differential PPM (relative timing).
Digital Pulse Position Modulation (PPM) is a type of signal modulation in which M message bits are encoded by transmitting a single pulse within one of 2á´¹ possible time positions within a fixed time frame. This process is repeated every T seconds, resulting in a data rate of M/T bits per second.
Pulse Position Modulation Example (Analog Method)
PPM is a form of analog modulation where the position of each pulse is varied according to the amplitude of the sampled modulating signal, while the amplitude and width of the pulses remain constant. This means only the timing (position) of the pulse carries the information.
PPM is commonly used in optical and wireless communications, especially where multipath interference is minimal or needs to be reduced. Because the information is carried in timing, it's more robust in some noisy environments compared to other modulation schemes.
Although PPM can be used for analog signal modulation, it is also used in digital communications where each pulse position represents a symbol or bit pattern. However, it is not ideal for transmitting complex data files, as it is generally used for simple or low-data-rate signaling.
Example: Pulse Position Modulation (PPM) of Sinusoidal Signal
We have a sinusoidal signal:
x(t) = A sin(2Ï€ f t)And we want to modulate this signal using Pulse Position Modulation (PPM).
Step-by-Step Example
1. Sinusoidal Signal
Let’s define a sinusoidal signal as:
x(t) = 5 sin(2Ï€ ⋅ 1 ⋅ t)This is a sine wave with:
- Amplitude: A = 5
- Frequency: f = 1 Hz
2. Pulse Position Modulation Concept
We’ll now encode information into the timing of pulses. To ensure robustness and avoid pulse overlapping, we use a small modulation index (k = 0.01).
If the amplitude is large, the pulse will be slightly delayed from its reference time. If it’s small, the pulse will stay closer to the reference time.
3. Calculating the Pulse Positions
We compute the values of x(t) and the corresponding pulse positions:
Time Sample 1: t = 0
x(0) = 5 sin(0) = 0Pulse at reference time t = 0.
Time Sample 2: t = 0.1
x(0.1) = 5 sin(0.2Ï€) ≈ 2.939
Shift = 0.01 × 2.939 = 0.0294
t = 0.1 + 0.0294 = 0.1294 secondsTime Sample 3: t = 0.2
x(0.2) = 5 sin(0.4Ï€) ≈ 4.7555
Shift = 0.01 × 4.7555 = 0.0475
t = 0.2 + 0.0475 = 0.2475 secondsTime Sample 4: t = 0.3
x(0.3) = 5 sin(0.6Ï€) ≈ 4.045
Shift = 0.01 × 4.045 = 0.0404
t = 0.3 + 0.0404 = 0.3404 secondsTime Sample 5: t = 0.4
x(0.4) = 5 sin(0.8Ï€) ≈ 2.939
Shift = 0.01 × 2.939 = 0.0294
t = 0.4 + 0.0294 = 0.4294 secondsTime Sample 6: t = 0.5
x(0.5) = 5 sin(Ï€) = 0
Pulse at reference time t = 0.54. Summary of Pulse Timing
Pulse positions based on sinusoidal amplitude (ensuring no overlap):
| Sample Time (t) | Amplitude x(t) | Pulse Position (tpulse) |
|---|---|---|
| 0.0 | 0.000 | 0.0000 |
| 0.1 | 2.939 | 0.1294 |
| 0.2 | 4.755 | 0.2475 |
| 0.3 | 4.045 | 0.3404 |
| 0.4 | 2.939 | 0.4294 |
| 0.5 | 0.000 | 0.5000 |
Demodulation of PPM Signal
The noise-corrupted PPM waveform is received by the PPM demodulator circuit. A pulse generator produces fixed-duration pulses from the incoming signal and applies them to the reset (R) input of an SR flip-flop. Simultaneously, a synchronized reference pulse train (recovered via a Phase-Locked Loop) is applied to the set (S) input. The SR flip-flop generates a PWM waveform, which is then passed through a Low Pass Filter (LPF) to recover the original message.
Effect of Noise on Pulse Position Modulation
Since in a PPM system the transmitted information is contained in the relative positions of the modulated pulses, the presence of additive noise affects the performance by falsifying the perceived pulse timing. Immunity to noise can be improved by making the pulse rise rapidly, reducing the time window for noise interference.
In theory, noise would have no effect if pulses were perfectly rectangular (requiring infinite bandwidth). In practice, finite rise times mean some degradation is inevitable. Similar to continuous-wave modulation, PPM performance can be evaluated using the output signal-to-noise ratio (SNR). The figure of merit compares output SNR to channel SNR, showing that performance improves with bandwidth but suffers a threshold effect when SNR drops too low.
Try Interactive Online Simulation (web-based)
We have developed web-based interactive simulations to help you better understand Pulse Position Modulation and other complex concepts in wireless communication systems.
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PAM, PWM, & PPM Online Simulators