Encoding and decoding communications traffic in a pulse amplitude modulation format and optical apparatus for same

Patent metadata

Field	Value
Publication number	US-11677475-B2
Application number	US-202017603247-A
Country	US
Kind code	B2
Filing date	Mar 12, 2020
Priority date	Apr 12, 2019
Publication date	Jun 13, 2023
Grant date	Jun 13, 2023

Abstract

A method (100) of encoding communications traffic bits onto an optical carrier signal in a pulse amplitude modulation, PAM, format. The method comprises: receiving (102) bits to be transmitted; receiving (104) an optical carrier signal comprising optical pulses having an amplitude and respective phases; performing (106) PAM of the optical pulses to encode at least one respective bit in one of a pre-set plurality of amplitudes of a said optical pulse; and performing (108) phase modulation of the optical pulses to encode at least one further respective bit in a phase difference between a said optical pulse and a consecutive optical pulse.

First claim

The invention claimed is:

1. A method of encoding communications traffic bits onto an optical carrier signal in a pulse amplitude modulation format, the method comprising:
receiving communications traffic bits to be transmitted;
receiving an optical carrier signal comprising optical pulses having an amplitude and respective phases;
performing pulse amplitude modulation of the optical pulses to encode at least one respective bit in one of a pre-set plurality of amplitudes of a said optical pulse; and
performing phase modulation of the optical pulses to encode at least one further respective bit in a phase difference between a said optical pulse and a consecutive optical pulse.

2. The method of claim 1 , comprising encoding a respective plurality of bits on a pre-set plurality, M, of amplitudes comprising positive and negative amplitudes of a plurality, M/2, of amplitude levels.

3. The method of claim 2 , comprising encoding a respective plurality, m=log 2 M−1, of bits on a pre-set plurality, M, of amplitudes.

4. The method of claim 1 , comprising encoding a respective plurality, N−1, of bits in a phase difference between a said optical pulse and the consecutive optical pulse, wherein the optical pulses have one of a plurality, N, of phases as a result of the phase modulation.

5. The method of claim 1 , wherein the optical pulses are one of Root Raised Cosine shaped pulses and Gaussian shaped pulses.

6. A method of decoding communications traffic bits carried on an optical carrier signal, the method comprising:
receiving an optical signal comprising optical pulses having respective amplitudes and respective phases, and having a symbol period, T;
directly detecting the optical pulses to generate a corresponding electrical received signal;
oversampling the electrical received signal at a rate of two samples per symbol period to obtain even samples indicative of respective amplitudes of the pulses and odd samples indicative of respective phase differences between consecutive pulses; and
decoding the even samples to obtain at least one even bit for each even sample and decoding the odd samples to obtain at least one odd bit for each odd sample, the even bits and the odd bits being interleaved according to the respective samples.

7. The method of claim 6 , wherein sampling the electrical received signal comprises sampling at times, kT, corresponding to the central instants of the optical pulses to obtain the even samples and sampling at times,

k
⁢
T

+

T
2

,

corresponding to maximum interference between consecutive pulses to obtain the odd samples.

8. The method of claim 6 , wherein the odd samples are indicative of corresponding amplitudes of the electrical received signal and there is a direct correlation between the amplitude of the electrical received signal and the phase difference between the respective consecutive pulses.

9. The method of claim 8 , wherein the odd samples amplitudes are proportional to the square modulus of the cosine phase difference between respective consecutive pulses.

10. An optical transmitter comprising:
an optical source configured to generate an optical carrier signal comprising optical pulses having an amplitude and respective phases;
an optical modulator configured to receive the optical pulses and to perform pulse amplitude modulation and phase modulation on the optical pulses; and
an encoding circuit configured to receive communications traffic bits to be transmitted and to:
map at least one respective bit to one of a pre-set plurality of optical amplitudes;
map at least one further respective bit to a phase difference between consecutive optical pulses; and
generate and transmit a respective drive signal to the optical modulator, the optical modulator, in response to the drive signal, being configured to perform at least one of pulse amplitude modulation and phase modulation to thereby encode the bits onto respective optical pulses.

11. The optical transmitter of claim 10 , wherein the encoding circuit is configured to map a respective plurality of bits to a pre-set plurality, M, of amplitudes comprising positive and negative amplitudes of a plurality, M/2, of amplitude levels.

12. The optical transmitter of claim 11 , wherein the encoding circuit is configured to map a respective plurality, m=log 2 M−1, of bits on a pre-set plurality, M, of amplitudes.

13. The optical transmitter of claim 10 , wherein the optical modulator is configured to phase modulate the optical pulses to have one of a plurality, N, of phases and the encoding circuit is configured to map a respective plurality, N−1, of bits in a respective phase difference between consecutive optical pulses.

14. The optical transmitter of claim 10 , wherein the optical pulses are one of Root Raised Cosine shaped pulses and Gaussian shaped pulses.

15. An optical receiver comprising:
a photodetector configured to:
receive an optical signal comprising optical pulses having respective amplitudes and respective phases, and having a symbol period, T; and
directly detect the optical pulses to generate a corresponding electrical received signal;

a sampling circuit configured to oversample the electrical received signal at a rate of two samples per symbol period to obtain even samples indicative of respective amplitudes of the pulses and odd samples indicative of respective phase differences between consecutive pulses; and
a decoding circuit configured to decode the even samples to obtain at least one even bit for each even sample and decode the odd samples to obtain at least one odd bit for each odd sample, the even bits and the odd bits being interleaved according to the respective samples.

16. The optical receiver of claim 15 , wherein the sampling circuit is configured to sample the electrical received signal at times, kT, corresponding to the central instants of the optical pulses to obtain the even samples and at times,

k
⁢
T

+

T
2

,

corresponding to maximum interference between consecutive pulses to obtain the odd samples.

17. The optical receiver of claim 15 , wherein the odd samples are indicative of corresponding amplitudes of the electrical received signal and there is a direct correlation between the amplitude of the electrical received signal and the phase difference between the respective consecutive pulses.

18. The optical receiver of claim 17 , wherein the odd samples' amplitudes are proportional to the square modulus of the cosine phase difference between respective consecutive pulses.

Assignees

• Ericsson Telefon Ab L M

Inventors

• Catuogno Tommaso
• Forestieri Enrico
• Poti Luca
• Fresi Francesco
• Cavaliere Fabio
• Secondini Marco

Classifications

• H04B10/524 Primary

  Pulse modulation · CPC title

• H04B10/69

  Electrical arrangements in the receiver · CPC title

• H04B10/695

  Arrangements for optimizing the decision element in the receiver, e.g. by using automatic threshold control · CPC title

• H04B10/5561 Primary

  Digital phase modulation · CPC title

• H04B10/541

  Digital intensity or amplitude modulation · CPC title