Differential Pulse Position Modulation (DPPM)
Differential Pulse Position Modulation (DPPM) is an advanced, high-efficiency variant of PPM where the position of each pulse is determined relative to the previous pulse rather than a fixed clock reference.
DPPM Step-by-Step Example
Imagine we want to send the sequence of numbers: 2, 1, 0.
In DPPM, each number represents the number of empty slots to wait after the last pulse.
Step 1: Start
Initial Reference: t = 0 ms
Step 2: Send Data "2"
Wait 2 units from the previous pulse.
Pulse sent at: 0 + 2 = 2 ms
Step 3: Send Data "1"
Wait 1 unit from the previous pulse (at 2ms).
Pulse sent at: 2 + 1 = 3 ms
Step 4: Send Data "0"
Wait 0 units from the previous pulse (at 3ms).
Pulse sent at: 3 + 0 = 3.5 ms (Immediate pulse)
Summary
- Self-Synchronization: The receiver only needs to detect the distance between pulses to decode the data.
- No Dead Time: Standard PPM wastes time waiting for a "frame" to end; DPPM starts the next symbol immediately.
- Vulnerable to Errors: If one pulse is missed, the "relative" timing for all following pulses is ruined (Error Propagation).
Standard PPM vs. Differential PPM
| Feature | Standard PPM (Digital) | Differential PPM (DPPM) |
|---|---|---|
| Reference Point | Fixed Clock (Absolute) | Previous Pulse (Relative) |
| Data Rate | Constant | Variable/Faster |
| System Sync | Requires Master Clock | Self-Synchronizing |