Subcarrier Coding Research Articles

Optical phase conjugation with dual frame OFDM for dispersion and nonlinearity mitigation over multimode fiber

This paper presents a modification in conventional asymmetrically clipped optical OFDM (ACO-OFDM) frame for direct detection intensity modulation (DD-IM) system. The conventional ACO-OFDM for DD-IM utilizes only one-fourth of available subcarriers for information symbols and therefore the bandwidth efficiency is very low. Authors propose to retain the othwerwise discarded negative polarity symbols to increase the bandwidth efficiency and transmission power for given number of subcarriers in ACO-OFDM. Proposed dual frame OFDM consists of two similar subframes and one of them is transmitted with optical phase conjugation for dispersion and non-linearity mitigation in multimode optical fiber. Proposed system is compared with another scheme-phase conjugated sub-carrier coding (PCSC) in OFDM. At − 15 dBm received optical power, proposed dual frame shows bit error rate (BER) $$1.974*10^{-3}$$ when compared to PCSC that shows BER value of $$1.13*10^{-1}$$ . BER Performance for the proposed dual frame OFDM with QPSK and QAM-16 modulation schemes is also compared in this research work.

Dual frame OFDM with optical phase conjugation for MIMO system in multimode fiber

For nonlinearity and dispersion mitigation in optical fiber, the authors propose a dual frame OFDM transmission, based on optical phase conjugation. The proposed dual frame OFDM is modification of asymmetrically clipped OFDM (ACO-OFDM) frame. The proposed OFDM frame consist of two positive frames, first one is time compressed ACO-OFDM frame and second one is optical phase conjugated version of first frame. This increases the bandwidth efficiency and total transmitted power for the system. For electronic dispersion compensation (EDC) in conventional OFDM, signal pre-processing is applied at transmitter side where it requires channel information. In case of OFDM and multiple input multiple output (MIMO) system setup for multi mode fiber, channel becomes almost unpredictable. The proposed dual frame OFDM does not require EDC at transmitter side. For received optical power $$-12$$ dbm conventional ACO-OFDM records bit error rate (BER) $$4.48*10^{-1}$$ and proposed dual frame ACO-OFDM with OPC records BER $$2.58*10^{-4}$$ . Simulation results for variable fiber length show BER $$2.20*10^{-1}$$ for phase conjugated subcarrier coding and $$2.24*10^{-3}$$ for proposed ACO-OFDM with OPC for multi-mode fiber length 500 m. It also allows different multiframe transmission based on time and wavelength diversity.

Bandwidth efficient frame structures for flip OFDM with phase conjugated sub carriers and LDPC encoding

The paper presents two modified frame structures for flipped OFDM over optical fiber. Conventional flipped OFDM carries two different frames back to back with positive samples and negative samples with flipped polarity. Almost half of the subcarriers in each frame are loaded with zeros. Conventional flipping enhances the spectral efficiency compared to asymmetrically clipped optical OFDM (ACO-OFDM) but also increases the noise variance at receiver. Proposed flipped OFDM structures are time compressed versions of conventional flipped OFDM. This results in the final flipped OFDM with frame duration which is only half as compared to that in conventional OFDM. Simulation results show that time compression increases the bandwidth efficiency (bits/s Hz−1) along with DC electrical power. Proposed flipped OFDM frames show significant improvement in error performance over single mode fiber of 50 km length at 10 Gbps data rate and low density parity check (LDPC) codes. For nonlinearity and dispersion compensation, phase conjugated subcarrier coding (PCSC) is performed before the inverse fast Fourier transform (IFFT) calculation in OFDM generation, which further improves the fiber length reach capacity by almost 11 km and 2 km respectively in both the proposed flipped OFDM frame structures when compared to conventional flipped OFDM frame structure.

Demonstration of Phase-Conjugated Subcarrier Coding for Fiber Nonlinearity Compensation in CO-OFDM Transmission

In this paper, we demonstrate through computer simulation and experiment a novel subcarrier coding scheme combined with pre-electrical dispersion compensation (pre-EDC) for fiber nonlinearity mitigation in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. As the frequency spacing in CO-OFDM systems is usually small (tens of MHz), neighbouring subcarriers tend to experience correlated nonlinear distortions after propagation over a fiber link. As a consequence, nonlinearity mitigation can be achieved by encoding and processing neighbouring OFDM subcarriers simultaneously. Herein, we propose to adopt the concept of dual phase conjugated twin wave for CO-OFDM transmission. Simulation and experimental results show that this simple technique combined with 50% pre-EDC can effectively offer up to 1.5 and 0.8 dB performance gains in CO-OFDM systems with BPSK and QPSK modulation formats, respectively.

Spectrum sidelobe suppression for discrete Fourier transformation-based orthogonal frequency division multiplexing using adjacent subcarriers correlative coding

The large spectrum sidelobes of orthogonal frequency division multiplexing (OFDM) signal lead to the interference with the licensed user working on the adjacent band in spectrum pooling. The power spectral leakage of discrete Fourier transformation (DFT)-based OFDM systems can be divided into two parts, in-band–out-of-subband (IBOSB) spectral leakage and out-of-band (OOB) spectral leakage. A correlative coding technique is proposed to suppress the IBOSB leakage and redistribute the OOB leakage of DFT-based OFDM signal through the subcarrier coding in the frequency domain with appropriate weighting. The correlation is introduced between adjacent subcarriers and mutual cancellation of the spectrum sidelobes reshapes the spectrum of DFT-based OFDM signal. The optimum weighting factor is obtained through optimisation. Analytical results and simulation show that the proposed coding scheme significantly suppresses the IBOSB leakage in contrast with the common OFDM system and introduces least interference with the licensed user. Furthermore, the carrier–interference ratio (CIR) of OFDM system with correlative coding increases by 3 dB. At the same time, maximum likelihood sequence detection (MLSD) can mostly compensate the bit error rate (BER) degradation caused by error propagation at the expense of introducing a little computation complexity.

Performance Analysis of Hermite-Symmetric Subcarrier Coding for OFDM Systems over Fading Channels

Performance Analysis of Hermite-Symmetric Subcarrier Coding for OFDM Systems over Fading Channels

Efficient Frequency Sharing of Baseband and Subcarrier Coding UHF RFID Systems

UHF band passive RFID systems are being steadily adopted by industries because of their capability of long range automatic identification with passive tags. For an application which demands a large number of readers located in a limited geographical area, referred to as dense reader mode, interference rejection among readers is important. The coding method, baseband or subcarrier coding, in the tag-to-reader communication link results in a significant influence on the interference rejection performance. This paper examines the frequency sharing of baseband and subcarrier coding UHF RFID systems from the perspective of their transmission delay using a media access control (MAC) simulator. The validity of the numerical simulation was verified by an experiment. It is revealed that, in a mixed operation of baseband and subcarrier systems, assigning as many channels as possible to baseband system unless they do not exploit the subcarrier channels is the general principle for efficient frequency sharing. This frequency sharing principle is effective both to baseband and subcarrier coding systems. Otherwise, mixed operation fundamentally increases the transmission delay in subcarrier coding systems.

Variable LDPC coding rate performances for multicarrier systems using HD decoding

An adaptive coding scheme minimising the BER of a bit-interleaved coded modulation OFDM system performing hard-decisions (HD) after the demodulation and ideal interleaving is proposed. The subcarrier code rate levels subject to a rate constraint are varied according to perfect channel state information at the transmitter. The derived adaptive scheme uses low-density parity-check codes. A modified bit-flipping algorithm provides HD decoding and can be applied either alone or combined with a power loading scheme. It is shown by Monte Carlo simulations that, for broadband systems with Rayleigh fading, the loading procedure provides a signal-to-noise ratio gain up to 3dB for an average BER level of 10−5.