357 research outputs found
Mitigation of Side-Effect Modulation in Optical OFDM VLC Systems
Side-effect modulation (SEM) has the potential to be a significant source of
interference in future visible light communication (VLC) systems. SEM is a
variation in the intensity of the light emitted by a luminaire and is usually a
side-effect caused by the power supply used to drive the luminaires. For LED
luminaires powered by a switched mode power supply, the SEM can be at much
higher frequencies than that emitted by conventional incandescent or
fluorescent lighting. It has been shown that the SEM caused by commercially
available LED luminaires is often periodic and of low power. In this paper, we
investigate the impact of typical forms of SEM on the performance of optical
OFDM VLC systems; both ACO-OFDM and DCO-OFDM are considered. Our results show
that even low levels of SEM power can significantly degrade the bit-error-rate
performance. To solve this problem, an SEM mitigation scheme is described. The
mitigation scheme is decision-directed and is based on estimating and
subtracting the fundamental component of the SEM from the received signal. We
describe two forms of the algorithm; one uses blind estimation while the other
uses pilot-assisted estimation based on a training sequence. Decision errors,
resulting in decision noise, limit the performance of the blind estimator even
when estimation is based on very long signals. However, the pilot system can
achieve more accurate estimations, thus better performance. Results are first
presented for typical SEM waveforms for the case where the fundamental
frequency of the SEM is known. The algorithms are then extended to include a
frequency estimation step and the mitigation algorithm is shown also to be
effective in this case
Spectrum-Efficient Triple-Layer Hybrid Optical OFDM for IM/DD-Based Optical Wireless Communications
In this paper, a triple-layer hybrid optical orthogonal frequency division multiplexing
(THO-OFDM) for intensity modulation with direct detection (IM/DD) systems with a high spectral efficiency is proposed. We combine N-point asymmetrically clipped optical orthogonal frequency division
multiplexing (ACO-OFDM), N/2-point ACO-OFDM, and N/2-point pulse amplitude modulated discrete
multitoned (PAM-DMT) in a single frame for simultaneous transmission. The time- and frequency-domain
demodulation methods are introduced by fully exploiting the special structure of the proposed THO-OFDM.
Theoretical analysis show that, the proposed THO-OFDM can reach the spectral efficiency limit of the
conventional layered ACO-OFDM (LACO-OFDM). Simulation results demonstrate that, the time-domain
receiver offers improved bit error rate (BER) performance compared with the frequency-domain with ∼40%
reduced computation complexity when using 512 subcarriers. Furthermore, we show a 3 dB improvement
in the peak-to-average power ratio (PAPR) compared with LACO-OFDM for the same three layers
Superposition coded modulation with peak-power limitation
We apply clipping to superposition coded modulation (SCM) systems to reduce the peak-to-average power ratio (PAPR) of the transmitted signal. The impact on performance is investigated by evaluating the mutual information driven by the induced peak-power-limited input signals. It is shown that the rate loss is marginal for moderate clipping thresholds if optimal encoding/decoding is used. This fact is confirmed in examples where capacityapproaching component codes are used together with the maximum a posteriori probability (MAP) detection. In order to reduce the detection complexity of SCM with a large number of layers, we develop a suboptimal soft compensation (SC) method that is combined with soft-input soft-output (SISO) decoding algorithms in an iterative manner. A variety of simulation results for additive white Gaussian noise (AWGN) and fading channels are presented. It is shown that with the proposed method, the effect of clipping can be efficiently compensated and a good tradeoff between PAPR and bit-error rate (BER) can be achieved. Comparisons with other coded modulation schemes demonstrate that SCM offers significant advantages for high-rate transmissions over fading channels
Iterative receiver for hybrid asymmetrically clipped optical OFDM
This paper proposes an iterative receiver to enhance the performance of hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (HACO-OFDM) in optical wireless communication systems. In HACO-OFDM scheme, asymmetrically clipped optical OFDM (ACO-OFDM) and pulse-amplitude-modulated discrete multitone (PAM-DMT) signals are transmitted simultaneously, which is more spectrally efficient compared with ACO-OFDM and PAM-DMT. However, the existing HACO-OFDM receiver directly recovers the signals in the frequency domain, which could not eliminate the interference thoroughly between ACO-OFDM and PAM-DMT signals and limits its performance. In our proposed receiver, the ACO-OFDM and PAM-DMT signals are detected in the frequency domain and regenerated in the time domain. After that, they are subtracted from the received signals iteratively. Thus, ACO-OFDM and PAM-DMT signals can be distinguished. By taking advantage of the signal symmetry properties of ACO-OFDM and PAM-DMT in the time domain, pairwise clipping is utilized to further reduce the effect of noise and estimation error, resulting in improved performance. In addition, unequal power allocation is proposed to guarantee that ACO-OFDM and PAM-DMT signals have similar performance in HACO-OFDM systems. Simulation results show that the proposed method provides significant signal-to-noise ratio gain over the conventional receiver for both equal and unequal power allocations at the cost of slightly increased complexity
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