Simulatenous PMD Compensation and Chromatic Dispersion Compensation Using LDPC Coded OFDM

• US 20090228766A1
• Filed: 03/06/2008
• Published: 09/10/2009
• Est. Priority Date: 03/06/2008
• Status: Active Grant

• Current Assignee: NEC Corporation
• Original Assignee: NEC Laboratories America Inc (NEC Corporation)
• Inventors: Djordjevic, Ivan, Xu, Lei, Wang, Ting, Cvijetic, Milorad
• Granted Patent: US 8,234,549 B2
• Time in Patent Office: Days
• US Class Current: 714/780
• CPC Class Codes: H03M 13/1102 Codes on graphs and decoding on graphs, e.g. low-density parity check [LDPC] codes
• CPC Class Codes: H03M 13/1111 Soft-decision decoding, e.g. by means of message passing or belief propagation algorithms
• CPC Class Codes: H03M 13/2957 Turbo codes and decoding
• CPC Class Codes: H03M 13/6325 Error control coding in combination with demodulation
• CPC Class Codes: H04B 10/2513 due to chromatic dispersion
• CPC Class Codes: H04B 10/2569 due to polarisation mode dispersion [PMD]
• CPC Class Codes: H04B 10/697 Arrangements for reducing noise and distortion

Abstract

A method includes estimating quadrature amplitude modulated QAM symbols in an LDPC encoded OFDM signal for transmission, performing channel estimation by training sequence to determine channel coefficients in reception of the LDPC encoded OFDM signal; and obtaining channel information detection and decoding of the LDPC encoded signal.

Claims

1. A method comprising the steps of:

estimating quadrature amplitude modulated QAM symbols in an LDPC encoded OFDM signal for transmission;

performing channel estimation to determine channel coefficients in reception of the LDPC encoded OFDM signal; and

obtaining channel information detection and decoding of the LDPC encoded signal.

2. The method of claim 1, further comprising the step of estimating bit reliabilities of a demapping of the received encoded OFDM signal.

3. The method of claim 1, wherein the estimated QAM symbols are related to the channel distortion introduced by PMD, phase shift of the QAM symbols due to chromatic dispersion and self-phase modulation.

4. The method of claim 1, wherein the step of obtaining channel information comprises determining bit log-likelihood ratios for LDPC decoding.

5. The method of claim 1, wherein the step of obtaining channel information comprises determining bit log-likelihood ratios for LDPC decoding and calculating extrinsic log-likelihood ratios.

6. The method of claim 1, wherein the step of obtaining channel information comprises i) determining bit log-likelihood ratios for LDPC decoding and ii) calculating extrinsic log-likelihood ratios till one of a parity check equation is satisfied and a predetermined number of iterations of i) and ii) have occurred.

7. The method of claim 6, comprising the step of using LDPC decoder extrinsic information for symbols log-likelihood ratios determination when one of a parity check equation is unsatisfied and a predetermined number of iterations of i) and ii) have not occurred.

8. The method of claim 1, wherein a λ

-configuration, denoted as t-(λ

,k,{0,1, . . . , λ

}), is a collection of k-subsets (blocks) of a ν

-set V such that every t-subset of V is contained in at most λ

of the blocks and let λ

₁- and λ

₂-configurations, denoted as t-(ν

₁,k₁,{0,1, . . . , λ

₁}) and t-(ν

₂,k₂,{0,1, . . . , λ

₂}), be given, denote the number of blocks in λ

₁- and λ

₂-configurations by b₁ and b₂, respectively; and

the corresponding set of blocks by B₁ and B₂, respectively, the ν

₁ν

₂-elements in the product configuration are ordered pair of elements (e₁,e₂), with e₁ belonging to B₁ and e₂ belonging to B₂, b=└

b₁/k₁┘

·

└

b₂/k₂┘

blocks of a product configuration are obtained as follows, with └

┘

denoted the largest integer less than or equal to the enclosed quantity, let the blocks from B₁ be grouped into └

b₁/k₁┘

classes of k₁ blocks each, the first block in a product configuration is obtained as a k₁k₂-block of ordered pair of elements ((α

₁,β

₁), . . . , (α

₁,β

_k2), . . . , (α

_k1,β

₁), . . . , (α

_k1,β

_k2)), with 60_i (i=1, . . . , k₁) belonging to the first block of B₁, and B_i (i=1, . . . , k₂) belonging to the first block in B₂ and the LDPC encoding comprises a first class code rate obtained by taking one block from the first class in B₁ at the time, and one block in B₂ out of the first k₂ blocks, and creating a product block.

9. The method of claim 8, wherein the LDPC encoding comprises a second class code rate obtained by observing next k₁ blocks in B₁, and next k₂ blocks in B₂ and repeating the procedure until all blocks in either B₁ or B₂ are exploited, or b blocks are generated, the blocks already used in creating the previous product blocks are excluded from further consideration and the product of P configurations can be defined iteratively by taking the product of two configurations in each step.

10. The method of claim 1, wherein the estimated QAM symbols are determined comprise the relationship

${\hat{X}}_{i,k}=(h_i^{*}/{}^{}$

h i 

2 ) 



- jθ

i 



- jφ

k 

Y i , k 

, ,

where h_i is channel distortion introduced by PMD and chromatic dispersion, and θ

₁ is the phase shift of i-th sub-carrier due to chromatic dispersion. φ

_k represents the OFDM symbol phase noise due to SPM and RF down-converter, and can be eliminated by pilot-aided channel estimation.

11. A method comprising the steps of:

performing channel estimation to determine channel coefficients in a receiver of a fiber optic communication system using LDPC coded OFDM;

estimating QAM symbols in a transmitter of the communication system; and

obtaining channel information for detection and decoding in the communication system.

12. The method of claim 11, further comprising the step of estimating bit reliabilities of a demapping of the received encoded OFDM signal.

13. The method of claim 11, wherein the estimated QAM symbols are related to the channel distortion introduced by PMD, phase shift of the QAM symbols due to chromatic dispersion and self-phase modulation.

14. The method of claim 11, wherein the step of obtaining channel information comprises determining bit log-likelihood ratios for LDPC decoding.

15. The method of claim 11, wherein the step of obtaining channel information comprises determining bit log-likelihood ratios for LDPC decoding and calculating extrinsic log-likelihood ratios.

16. The method of claim 11, wherein the step of obtaining channel information comprises i) determining bit log-likelihood ratios for LDPC decoding and ii) calculating extrinsic log-likelihood ratios till one of a parity check equation is satisfied and a predetermined number of iterations of i) and ii) have occurred.

17. The method of claim 16, comprising the step of using LDPC decoder extrinsic information for symbols log-likelihood ratios determination when one of a parity check equation is unsatisfied and a predetermined number of iterations of i) and ii) have not occurred.