offset-QAM Orthogonal Frequency Division Multiplexing Research Articles

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

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.

Detection and Equalization of Set-Partitioned Offset-QAM OFDM in IMDD Systems

We design the detection algorithm for orthogonal frequency division multiplexing (OFDM) based on set-partitioned offset quadrature amplitude modulation (SP-offset-QAM) in adaptively-loaded intensity modulation and direct detection (IMDD) systems. The algorithm mitigates signal-signal beating interference (SSBI), improves the equalization capability of conventional one-tap equalizers, and reduces the complexity and the required length of training sequence in the decoding of multi-dimensional SP-QAM signals. We experimentally demonstrate the adaptively-loaded SP-offset-QAM OFDM over 50-km single-mode fiber to verify the proposed algorithm. It is shown that the modified equalization significantly improves the performance while the SSBI mitigation brings benefits after transmission. The proposed decoding scheme reduces the complexity comparable to that in conventional QAM. It is also shown that the SP-offset-QAM OFDM outperforms the SP-based conventional QAM and conventional offset-QAM OFDM both at back-to-back and after 50-km transmission, when the algorithm is applied to all schemes.

Equalization of Dispersion-Induced Crosstalk in Optical Offset-QAM OFDM Systems

In discrete-Fourier-transform-based offset quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM), the orthogonality between the inter-carrier interference (ICI) and the signal is broken as dispersion increases, which in turn limits the dispersion tolerance. The ICI on a subcarrier is only from its adjacent two subcarriers, but its influence depends on the frequency of the subcarrier. We find that it is the high-frequency subcarriers that limit the dispersion tolerance and thus propose to equalize these dominant subcarriers to improve the performance. Simulations of 112-Gb/s offset-4QAM and 224-Gb/s offset-16QAM OFDM are performed to verify the analysis. We also experimentally demonstrate the transmission of a 38-Gb/s offset-16QAM OFDM signal over 1200-km single-mode fiber without cyclic prefix (CP). The bit error rate (BER) is reduced from $10^{-3}$ to $10^{-4}$ at $\sim 16$ -dB optical signal to noise ratio by equalizing only the high frequency half of the subcarriers. This is in contrast to conventional OFDM which cannot realize a BER of $10^{-3}$ at 400 km when CP is not applied.

DFT-based optical offset-QAM OFDM: analytical, numerical, and experimental studies

We investigate discrete Fourier transform-based offset quadrature amplitude modulation (offset-QAM) orthogonal frequency division multiplexing (OFDM) technology. We derive a closed-form expression for the de-multiplexed signal and analyze the influence of crosstalk on implementation algorithms and system performance. It is found that channel estimation in offset-QAM OFDM is different from that in conventional OFDM (C-OFDM) due to the residual crosstalk terms and requires particular study. We propose simple and efficient channel estimation algorithms and show, in a 38-Gbit/s offset-16QAM OFDM experiment with 840-km single-mode fiber, that these algorithms can enable the system performance close to the theoretical limit. By using these algorithms, we compare this technology with C-OFDM and Nyquist FDM (N-FDM) and numerically and experimentally show that DFT-based offset-QAM OFDM can greatly enhance the net data rate for fiber transmissions compared to C-OFDM and exhibit lower complexity than N-FDM. These advantages together with the successfully developed implementation algorithms make this technology very promising for optical communication systems.

Channel estimation in DFT-based offset-QAM OFDM systems.

Offset quadrature amplitude modulation (offset-QAM) orthogonal frequency division multiplexing (OFDM) exhibits enhanced net data rates compared to conventional OFDM, and reduced complexity compared to Nyquist FDM (N-FDM). However, channel estimation in discrete-Fourier-transform (DFT) based offset-QAM OFDM is different from that in conventional OFDM and requires particular study. In this paper, we derive a closed-form expression for the demultiplexed signal in DFT-based offset-QAM systems and show that although the residual crosstalk is orthogonal to the decoded signal, its existence degrades the channel estimation performance when the conventional least-square method is applied. We propose and investigate four channel estimation algorithms for offset-QAM OFDM that vary in terms of performance, complexity, and tolerance to system parameters. It is theoretically and experimentally shown that simple channel estimation can be realized in offset-QAM OFDM with the achieved performance close to the theoretical limit. This, together with the existing advantages over conventional OFDM and N-FDM, makes this technology very promising for optical communication systems.

DFT-based offset-QAM OFDM for optical communications

We experimentally demonstrate and numerically investigate a discrete-Fourier-transform (DFT) based offset quadrature-amplitude-modulation (offset-QAM) orthogonal frequency division multiplexing (OFDM) system. We investigate the scheme using a set of square-root-raised-cosine functions and a set of super-Gaussian functions as signal spectra. It is shown that offset-QAM OFDM exhibits negligible penalty for all investigated spectra, in contrast to rectangular-function based Nyquist FDM (N-FDM) and sinc-function based conventional OFDM (C-OFDM). The required guard interval (GI) length for dispersion compensation in offset-QAM OFDM is analyzed and shown to scale with twice the subcarrier spacing rather than the full OFDM bandwidth. Experimental results show that 38-Gb/s offset-16QAM OFDM supports 600-km fiber transmission with negligible penalty in the absence of GI while a GI length of eight is required in C-OFDM. Further numerical simulations show that by avoiding the GI, 112-Gb/s polarization multiplexed offset-4QAM OFDM can achieve 23% increase in net data rate over C-OFDM under the same transmission reach. We also discuss the design of the pulse-shaping filter in the DFT-based implementation and show that when compared to N-FDM, the required memory length of the filter for pulse shaping can be reduced from 60 to 2 in offset-QAM OFDM regardless of the fiber length.

An Improved Selective Mapping Method for PAPR Reduction in OFDM/OQAM System

Orthogonal frequency division multiplex/offset QAM (OFDM/OQAM) has been proven to be a promising multi-carrier modulation (MCM) technique for the transmission of signals over multipath fading channels. However, OFDM/OQAM has also the intrinsic disadvantage of high peak-to-average-power ratio (PAPR) that should be alleviated. In this paper, a novel selective mapping (SLM) method is proposed for OFDM/OQAM system. Since the pulse shape may cover a few OFDM symbols, the basic principle of the proposed method is to apply the SLM method in the range of the most relevant OFDM symbols. Analysis and simulation results show that, compared to the existing SLM algorithms for OFDM/OQAM system, the proposed method has better PAPR performance and lower computation complexity.

Reduction of the Clipping Noise for OFDM/OQAM System

Orthogonal frequency division multiplex/offset QAM (OFDM/OQAM) has been proven to be a promising multi-carrier modulation (MCM) technique for the transmission of signals over multipath fading channels. However, OFDM/OQAM has also the intrinsic disadvantage of high peak-to-average-power ratio (PAPR) that should be alleviated. This paper focuses on the reduction of the clipping noise and out-of band radiation caused by the clipping process. The basic principle is to estimate the clipping noise and then eliminate it from the received signal. Analysis and simulation results show that, with one time iteration, the proposed method can effectively improve the bit error ratio (BER) performance.

Performance Comparison between OQAM and <i>N</i>-continuous OFDM

Orthogonal frequency division multiplex/offset QAM (OFDM/OQAM) and N-continuous OFDM are both improved multi-carrier modulation (MCM) techniques for the transmission of signals over multipath fading channels. In this paper, we aim to compare these two techniques with respect to out-of-band radiation, bit error ratio (BER) and calculation complexity. Analysis and simulation results show that, compared to the N-continuous OFDM, the OFDM/OQAM has lower out-of-band radiation, calculation consumption and similar BER performance.

Analysis and design of OFDM/OQAM systems based on filterbank theory

A discrete-time analysis of the orthogonal frequency division multiplex/offset QAM (OFDM/OQAM) multicarrier modulation technique, leading to a modulated transmultiplexer, is presented. The conditions of discrete orthogonality are established with respect to the polyphase components of the OFDM/OQAM prototype filter, which is assumed to be symmetrical and with arbitrary length. Fast implementation schemes of the OFDM/OQAM modulator and demodulator are provided, which are based on the inverse fast Fourier transform. Non-orthogonal prototypes create intersymbol and interchannel interferences (ISI and ICI) that, in the case of a distortion-free transmission, are expressed by a closed-form expression. A large set of design examples is presented for OFDM/OQAM systems with the number of subcarriers going from four up to 2048, which also allows a comparison between different approaches to get well-localized prototypes.