Fast channel estimation and equalization scheme for offset-QAM OFDM systems.

Jian Zhao,Paul Townsend

doi:10.1364/oe.27.000714

Abstract

We show that the orthogonality between signal and intrinsic imaginary interference (IMI) in offset quadrature amplitude modulation (offset-QAM) orthogonal frequency division multiplexing (OFDM) can still be maintained under a certain condition even when the timing slots of different subcarriers are misaligned. We show that the phase and velocity differences over subcarriers induced by fiber dispersion satisfy this condition. Based on this, we propose a fast channel estimation and equalization scheme without prior channel information including coarse dispersion. We investigate the proposed scheme in a 40-Gbit/s offset-16QAM OFDM experiment and 240-Gbit/s polarization-division-multiplexed offset-16QAM OFDM simulations, both over 1200-km single-mode fiber. It is shown that the proposed scheme gives better performance, reduced overhead for the training sequence, and/or lower complexity than other schemes. We also compare offset-QAM OFDM with the proposed scheme and conventional OFDM, and show that in addition to the elimination of cyclic prefix overhead, offset-QAM OFDM gives better performance and longer transmission reach for a moderate/small number of subcarriers.

Highlights

Offset quadrature amplitude modulation orthogonal frequency division multiplexing (OFDM) has exhibited advantages in coherent super-channels, uplinks in passive optical networks, long-haul transmission, direct-detection systems, and radio-overfiber systems [1,2,3,4,5,6]
We propose a fast channel estimation and equalization scheme without prior channel information including coarse dispersion
We show that the orthogonality of offset-QAM OFDM can be maintained even when any subcarrier has any time delay, provided that the phase difference between that subcarrier and its adjacent subcarriers is equal to a value determined by their timing difference

Summary

Introduction

Offset quadrature amplitude modulation (offset-QAM) orthogonal frequency division multiplexing (OFDM) has exhibited advantages in coherent super-channels, uplinks in passive optical networks, long-haul transmission, direct-detection systems, and radio-overfiber systems [1,2,3,4,5,6]. In [11], the IMI was adaptively estimated and compensated by multi-tap equalizers These methods are only suitable in cases where the distortion on each subcarrier is negligible and the maximal delay difference between subcarriers is small with respect to the symbol period. Conventional offset-QAM OFDM assumes that the pulses of all subcarriers are aligned in time and there is a π/2 phase difference between adjacent subcarriers. The scheme is applicable for both short and long distances as well as different numbers of subcarriers, even in cases where the channel delay exceeds the OFDM symbol period and the distortion on the pulse of each subcarrier is not negligible. We will show that fiber dispersion satisfies Eq (3), and so orthogonalty can be maintained it is not required to have prior knowledge on the amount of dispersion

Channel effect on offset-QAM OFDM

Proposed channel estimation and equalization scheme

Findings

Summary