Flip Orthogonal Frequency Division Multiplexing Research Articles

Transmission Techniques for Multi User MIMO VLC Systems Using Flip-OFDM

Flip Orthogonal Frequency Division Multiplexing (flip-OFDM) is one of the unipolar forms of OFDM used in Visible Light Communication (VLC) systems. This paper analyzed the multi user VLC systems to transfer flip-OFDM based signal using different form of Multiple Input Multiple Output (MIMO) transmission techniques. These transmission techniques are Repetitive Coding (RC), Spatial Modulation (SM), generalized spatial modulation (Generalized SM) and spatial multiplexing (SMP). Multi User Interference (MUI) at the transmitter has been bypassed using precoding process of Block Diagonalization (BD). The performance analysis of proposed system is carried out with the help of simulation. A comparative study for RC, SM, generalized SM and SMP for multi user VLC system is performed with regard to Bit Error Rate (BER) performance. These transmission techniques are compared with and without the use of flip-OFDM. Furthermore, we also analyzed the Peak-to-Average Power Ratio (PAPR) of flip-OFDM signal. Simulation results showed that the generalized SM outperforms RC, SMP, SM approximately by 9dB, 5dB, 2dB Signal-to-Noise RATIO (SNR), respectively without flip-OFDM and, 8.5dB, 7dB, 3.3dB SNR respectively with flip-OFDM to obtain a BER of 10−6. We also analyzed the MIMO transmission techniques with regards to transmission efficiency. It is found that SMP offers better transmission efficiency with lower modulation order. To obtain the spectral efficiency of 8 bits per channel use (bpcu), RC, SM, generalized SM and SMP needs modulation order M = 256, 64, 8 and 4, respectively. Hence SMP is power efficient.

Performance optimisation of indoor SVD‐based MIMO‐OFDM optical wireless communication systems

The performance of multiple-input and multiple-output (MIMO) flip-orthogonal frequency division multiplexing (flip-OFDM) for indoor optical wireless communications (OWCs) is studied here. To improve the system performance in terms of the bit error rate (BER), the singular value decomposition (SVD)-based MIMO technique is applied together with the bit loading technique. The authors, especially, focus on optimising the BER performance of SVD-based MIMO-OFDM OWC systems under the impact of the OFDM symbol design, the configuration of MIMO transmitters, the required data rates, the available bandwidth, and the angular diversity based on tilting receiver photo detectors (PDs). The system BER expressions are analytically derived and verified to be accurate through comparison with simulation results. The authors find that the 9 × 4-MIMO system is the optimal configuration in terms of a large coverage of low BER regions and low computational complexity for the 5 m × 5 m room. In addition, polar angle tilting of PDs can effectively improve the BER performance. Finally, in case of light dimming control with a low total transmit optical power, the system can benefit from a high multiplexing gain with a large tilting polar angle, as indicated by improved condition numbers of MIMO channel matrices.

BER Performance Comparison of DCO-OFDM and Convolutional Coded DCO-OFDM in IM/DD Systems

Optical wireless communication (OWC) is an alternative technology to meet the demands of the exponentially-growing high data rate applications run by broadband users. The implementation of single carrier modulation techniques in OWC is an age old technology, but for the last few years research is focused towards the multicarrier modulation techniques in OWC. In OWC, information is carried using intensity modulation and retrieved using direct detection. To perform intensity modulation, the baseband signal should be a unipolar signal. To obtain a unipolar signal, various techniques such as DC-biased orthogonal frequency division multiplexing (DCO-OFDM), Asymmetrically clipped orthogonal frequency division multiplexing (ACO-OFDM), Flip orthogonal frequency division multiplexing (Flip-OFDM) and Unipolar orthogonal frequency division multiplexing (U-OFDM) techniques are reported in the literature. Though the DCO-OFDM is spectrally efficient compared to other techniques it requires more power to achieve the targeted BER. In this article, a convolutional Coded DCO-OFDM (CDCO-OFDM) has been introduced by applying channel coding. A convolutional encoder and a hard-decision Viterbi decoder are considered in CDCO-OFDM. It has been observed that CDCO-OFDM requires less transmitted power than DCO-OFDM to attain the targeted BER. The performance of DCO-OFDM and CDCO-OFDM is evaluated for 4, 16, 64 QAM- 7dB and 13dB bias, in the presence of the additive white Gaussian noise (AWGN) channel. It is observed that CDCO requires less power to transmit than DCO-OFDM for the BER 10-4.

Efficient coding techniques for flip-OFDM in IM/DD systems

ABSTRACTOptical wireless communication (OWC) technology is one of the most prominent alternative schemes for radio frequency (RF) communications. Flip-Orthogonal Frequency Division Multiplexing (Flip-OFDM) is a type of indoor OWC that transmits the positive and the absolute values of the negative parts of the signal in two consecutive OFDM symbols. The Flip-OFDM system exhibits better bit error rate (BER) performance than that of the DC-biased optical OFDM (DCO-OFDM). However, in Flip-OFDM, the recombination of the positive and negative sub-frames doubles the noise power, which causes distortion and degrades the BER performance. In this paper, a method to mitigate the noise effect using Convolutional and Turbo coders with Additive White Gaussian Noise (AWGN) and ceiling bounce channel models is proposed. Simulation results show that the Convolutional and Turbo coded Flip-OFDM system gives a better BER performance compared to that of the uncoded Flip-OFDM system. In addition, the Turbo coded Flip-OFDM system has a better BER performance than that of the Convolutional coded Flip-OFDM system.

PAPR reduction for Flip-OFDM in IM/DD systems via PTS technique

ABSTRACTOptical wireless communication (OWC) has recently gained much attention for addressing the increasing demand in the global broadband access market. Flip-orthogonal frequency division multiplexing (Flip-OFDM) is a type of OFDM that can be implemented in indoor OWC. Due to the ability of Flip-OFDM to keep the same bit error rate performance in asymmetrically clipped optical OFDM (ACO-OFDM) while saving 50% of the receiver hardware complexity, it can be used as an alternative to ACO-OFDM. However, the main drawback that can reduce the performance of the Flip-OFDM is the high peak-to-average power ratio (PAPR). So, in this article, partial transmit sequence (PTS) is applied to reduce the PAPR of Flip-OFDM over ceiling bounce model. Simulation results show that the proposed PTS-based Flip-OFDM achieves significantly lower PAPR values compared to Flip-OFDM.

Low-complexity peak-to-average power ratio reduction scheme for flip-orthogonal frequency division multiplexing visible light communication system based on μ -law mapping

An orthogonal frequency division multiplexing (OFDM) technique called flipped OFDM (flip-OFDM) is apposite for a visible light communication system that needs the transmitted signal to be real and positive. Flip-OFDM uses two consecutive OFDM subframes to transmit the positive and negative parts of the signal. However, peak-to-average power ratio (PAPR) for flip-OFDM is increased tremendously due to the low value of total average power that arises from many zero values in both the positive and flipped frames. We first analyze the performance of flip-OFDM and perform a comparison with the conventional DC-biased OFDM (DCO-OFDM); then we propose a flip-OFDM scheme combined with μ-law mapping to reduce the high PAPR. The simulation results show that the PAPR of the system is reduced about 17.2 and 5.9 dB when compared with the normal flip-OFDM and DCO-OFDM signals, respectively.

Experimental Evaluation of Modulation Formats’ Performance in Diffuse UV Channels

The fundamental properties of free space optical channels in the ultraviolet (UV) spectral range between 200 and 280 nm are the enhanced scattering and the solar blind operation. Exploiting these properties, point-to-point links in a non-line-of-sight (NLOS) regime were experimentally tested and the performance of pulse position modulation of fourth order (4-PPM) and flip–orthogonal frequency division multiplexing (Flip-OFDM) modulation formats was evaluated. At the transmitting side, light-emitting diodes (LEDs) emitting at 265 nm were used. At the receiving side, a photo-multiplier tube was combined with an optical filter. Distances up to 20 m were covered and kilobits/s rates were applied. The performance was also evaluated after modifying artificially the atmospheric conditions by the operation of a fog machine. In short, 4-PPM outperformed Flip-OFDM, whereas a medium that offers enriched scattering due to fog may boost the performance of short-range NLOS links in the UV spectral range.

Polarity Information Coded Flip-OFDM for Intensity Modulated Systems

A polarity-information-coded flip orthogonal frequency division multiplexing is proposed for intensity modulation/direct detection optical communications in this letter. In the proposed scheme, the modulated signals in the frequency domain are not constrained to have Hermitian symmetry. The real and imaginary parts of the time-domain complex signals are separated, and the polarities of the real and imaginary parts are jointly encoded and modulated. The transmit strategy and the receive algorithm of the proposed scheme are analyzed in detail. The major advantage of the proposed scheme is that its spectral and optical power efficiencies are higher than existing schemes, which is validated in simulation.