Using multi-level pulse amplitude modulation with probabilistic shaping

• US 10,461,972 B2
• Filed: 10/30/2017
• Issued: 10/29/2019
• Est. Priority Date: 10/30/2017
• Status: Active Grant

• Current Assignee: ZTE Corporation
• Original Assignee: ZTE Corporation
• Inventors: Yu, Jianjun, Chien, Hung-Chang
• Primary Examiner(s): Tran, Dzung D
• Application Number:
  US15/798,305
• Publication Number:
  US 20190132164A1
• Time in Patent Office: 729 Days
• CPC Class Codes: H04B 10/5161 Combination of different modulation schemes
• CPC Class Codes: H04B 10/541 Digital intensity or amplitude modulation
• CPC Class Codes: H04B 10/60 Receivers
• CPC Class Codes: H04B 10/69 Electrical arrangements in the receiver
• CPC Class Codes: H04L 2025/03636 Algorithms using least mean square [LMS]
• CPC Class Codes: H04L 2025/03726 Switching between algorithms
• CPC Class Codes: H04L 25/03019 adaptive, i.e. capable of adjustment during data reception
• CPC Class Codes: H04L 25/0305 using blind adaptation
• CPC Class Codes: H04L 25/4917 using multilevel codes
• CPC Class Codes: H04L 27/01 Equalisers baseband equalizers at the transmitter end H04L25/03343; in analogue transmission systems H04B3/04, H04B7/005
• CPC Class Codes: H04L 27/06 Demodulator circuits; Receiver circuits
• CPC Class Codes: H04L 27/3433 using an underlying square constellation

Abstract

An optical receiver apparatus includes a photo detector that detects amplitude modified optical signals transmitted through an optical transmission channel and mapped to constellation points and converts the optical signal into electrical signals, a sampling circuit that samples the electrical signals and generates digital signals, and a signal processing circuit that has a two-step equalization algorithm converging actual constellation points into desired constellation points. The two-step equalization algorithm includes a first equalization algorithm that uses multiple modulus in performing an error signal calculation, and a second equalization algorithm that includes a filtering procedure that compares actual constellation points with desired constellation points and an adjustment procedure that iteratively adjusts parameters of the filtering procedure in accordance with the comparison result.

Claims

1. An optical receiver apparatus, comprising:

a photo detector that detects amplitude modified optical signals transmitted through an optical transmission channel and mapped to constellation points and converts the optical signal into electrical signals;

a sampling circuit that samples the electrical signals and generates digital signals; and

a signal processing circuit that has a two-step equalization algorithm converging actual constellation points into desired constellation points, the two-step equalization algorithm comprising;

a first equalization algorithm that uses multiple modulus in performing an error signal calculation; and

a second equalization algorithm that includes a filtering procedure that compares actual constellation points in an intermediate signal resulting from the first equalization algorithm with desired constellation points and an adjustment procedure that iteratively adjusts parameters of the filtering procedure in accordance with the comparison result,

wherein the first equalization algorithm includes a cascaded multi-modulus algorithm that utilizes at least two reference circles in a cascaded way, and wherein the at least two reference circles include reference circles that have reference radiuses corresponding to the highest and lowest amplitudes of the amplitude modified optical signal.

2. The apparatus of claim 1, wherein the amplitude modified optical signals include optical signals that are modulated using a 16-level pulse amplitude modulation scheme with probabilistic shaping.

3. The apparatus of claim 1, wherein the first equalization algorithm includes a cascaded multi-modulus algorithm that converges errors in the constellation points to a range of correction of the second equalization algorithm.

4. The apparatus of claim 1, wherein the second equalization algorithm includes a decision directed least mean square algorithm that further reduces the errors.

5. An optical communication system, comprising:

a light source that generates a light wave as an optical carrier wave, at a transmitter side;

a modulation signal generator that processes information bits to be transferred in a form of an optical multi-level pulse amplitude modulation signal using a probabilistic shaping scheme, at the transmitter side;

an optical signal receiving circuit that detects the optical multi-level pulse amplitude modulation signal mapped to constellation points and converts the optical signal into electrical signals, at a receiver side; and

a signal processing circuit that has a two-step equalization algorithm converging actual constellation points into desired constellation points, at the receiver side, the two-step equalization algorithm comprising;

a first equalization algorithm that uses multiple modulus in performing an error signal calculation; and

a second equalization algorithm that includes a filtering procedure that compares actual constellation points in an intermediate signal resulting from the first equalization algorithm with desired constellation points and an adjustment procedure that iteratively adjusts parameters of the filtering procedure in accordance with the comparison result,

wherein the first equalization algorithm includes a cascaded multi-modulus algorithm that utilizes at least two reference circles in a cascaded way, and wherein the at least two reference circles include reference circles that have reference radiuses corresponding to the highest and lowest amplitudes of the amplitude modified optical signal.

6. The system of claim 5, wherein the light source includes an external cavity laser to input an optical carrier signal to the modulation signal generator.

7. The system of claim 5, wherein the modulation signal generator includes a Mach-Zehnder modulator (MZM).

8. The system of claim 5, wherein the optical multi-level pulse amplitude modulation signal has 16 or higher amplitude levels.

9. The system of claim 5, wherein the optical signal receiving circuit includes a photo detector that detects the optical signal and converts the optical signal into electrical analog signals.

10. The system of claim 9, wherein the optical signal receiving circuit includes a downsampling circuit that generates the electrical signals by downsampling from transmission baud rate to a baseband baud rate.

11. The system of claim 5, wherein the first equalization algorithm includes a cascaded multi-modulus algorithm that converges errors in the constellation points to an acceptable range of the second equalization algorithm.

12. The system of claim 5, wherein the second equalization algorithm includes a decision directed least mean square algorithm that further reduces the errors.

13. A method of recovering information bits from a probabilistically shaped (PS), multi-level pulse amplitude modulation (PAM) optical signals, comprising:

receiving the PS multi-level PAM optical signal mapped to constellation points;

converting the optical signal into an electrical signal; and

processing the converted electrical signal using a two-step equalization algorithm converging actual constellation points into desired constellation points, at the receiver side, the two-step equalization algorithm comprising;

a first equalization algorithm that uses multiple modulus in performing an error signal calculation; and

a second equalization algorithm that includes a filtering procedure that compares actual constellation points in an intermediate signal resulting from the first equalization algorithm with desired constellation points and an adjustment procedure that iteratively adjusts parameters of the filtering procedure in accordance with the comparison result,

wherein the first equalization algorithm includes a cascaded multi-modulus algorithm that utilizes at least two reference circles in a cascaded way, and wherein the at least two reference circles include reference circles that have reference radiuses corresponding to the highest and lowest amplitudes of the amplitude modified optical signal.

14. The method of claim 13, wherein the two-step equalization algorithm includes a decision-directed least mean square (DD-LMS) algorithm having a filtering procedure that compares actual constellation points with desired constellation points and an adjustment procedure that iteratively adjusts parameters of the filtering procedure in accordance with the comparison result.

15. The method of claim 13, further comprising obtaining a bit-error ratio (BER) and an achievable information rate (AIR) after performing the two-step equalization algorithm.

16. The method of claim 15, further comprising, after performing the two-step equalization algorithm, performing a decision procedure based on the results of the two-step equalization algorithm.

17. The method of claim 13, further comprising performing a down sampling before performing the equalization algorithms.