System and method for generating PPM waveforms

1. A method, performed by a system, for generating Pulse Position Modulated (PPM) waveforms in a lidar, the method comprising:

a) creating a modulation pool, based on a maximum nominal pulse repetition frequency (PRF), wherein a number of elements in the modulation pool is given by a ratio of a maximum possible modulation of the system to a minimum possible modulation of the system;

b) eliminating bad modulation levels from the modulation pool to generate a good modulation pool, wherein eliminating bad modulation levels comprises eliminating modulation values that cause baud collisions, and wherein baud collisions are determined by which modulation values are applied at which code element position;

c) selecting a modulation level from the good modulation pool to generate a pulse position modulated (PPM) code element;

d) repeating steps b and c N times to generate an N-element PPM code, wherein N is an integer greater than 1;

e) selecting a PRF less than the maximum nominal PRF;

f) generating a PPM waveform by applying the N-element PPM code to the selected PRF, wherein the PPM code is independent of the selected PRF; and

g) transmitting the PPM waveform by the lidar toward a target to determine a range to the target.

2. The method of claim 1 , wherein the modulation level is selected using a selection function.

3. The method of claim 2 , wherein the selection function selects a modulation level via random sampling of the good modulation pool.

4. The method of claim 2 , wherein the selection function computes a next PPM code such that a new modulation level maximizes a minimum distance between its own position and position of modulation levels that have been eliminated from the modulation pool.

5. The method of claim 2 , wherein the selection function is optimized to generate longer PPM codes.

6. The method of claim 2 , wherein the selection function chooses a new modulation level from the good modulation pool such that the new modulation level maximizes the minimum distance between its own position and position of modulation levels that have been removed from the modulation pool.

7. The method of claim 1 , wherein the modulation values that cause baud collisions, j bad , for (N+1)-th code element is computed by:

j bad =j N+1-(i-k) +j i −j k for i<k<N

where i and k are indices to any ith and kth code elements.

8. A lidar for generating Pulse Position Modulated (PPM) waveforms to determine a range to a target comprising:

a processor for creating a modulation pool, based on a maximum nominal pulse repetition frequency (PRF), wherein a number of elements in the modulation pool is given by a ratio of a maximum possible modulation of the system to a minimum possible modulation of the system; eliminating bad modulation levels from the modulation pool to generate a good modulation pool, wherein eliminating bad modulation levels comprises eliminating modulation values that cause baud collisions, and wherein baud collisions are determined by which modulation values are applied at which code element position; and selecting a modulation level from the good modulation pool to generate a pulse position modulated (PPM) code element, wherein the processor

repeats eliminating bad modulation levels and selecting a modulation level N times to generate an N-element PPM code, wherein N is an integer greater than 1; selects a PRF less than the maximum nominal PRF; and generates a PPM waveform by applying the N-element PPM code to the selected PRF, wherein the PPM code is independent of the selected PRF;

a transmitter for transmitting the PPM waveform by the lidar toward the target; and

a receiver for receiving a waveform reflected from the target to determine the range to the target.

9. The lidar of claim 8 , wherein the processor selects the modulation level using a selection function.

10. The lidar of claim 9 , wherein the selection function selects a modulation level via random sampling of the good modulation pool.

11. The lidar of claim 9 , wherein the selection function computes a next PPM code such that a new modulation level maximizes a minimum distance between its own position and position of modulation levels that have been eliminated from the modulation pool.

12. The lidar of claim 9 , wherein the processor optimizes the selection function to generate longer PPM codes.

13. The lidar of claim 9 , wherein the selection function chooses a new modulation level from the good modulation pool such that the new modulation level maximizes the minimum distance between its own position and position of modulation levels that have been removed from the modulation pool.

14. The lidar of claim 8 , wherein the modulation values that cause baud collisions, j bad , for (N+1)-th code element is computed by:

j bad j N+1-(i-k) +j i −j k for i<k<N

where i and k are indices to any ith and kth code elements.