KEYWORDS: Light emitting diodes, Telecommunications, Visible radiation, Signal to noise ratio, Modulation, Signal attenuation, Demodulation, Data transmission, Superposition, Transmitters
This paper proposes an innovative approach to improve the adjustable bandwidth of the LED, a pre-equalization circuit is used at the end of the transmitting system in the traditional visible light communication system. However, the preequalization circuit causes a large amount of energy loss while increasing the adjustable bandwidth of the LED. To save energy on the pre-equalization circuit, we adopt an advance approach known as, non-orthogonal multiple access (NOMA) [1] in our visible light communication system (VLC). By adopting this method, different signals can be provided by different variation of power with the help of transmitter, without increasing the adjustable bandwidth of the LED. These signals are superimpose in the power domain and after this process signals are transmitted through LED. In the receiver module, serial interference cancellation (SIC) technology is adopted to demodulate these singles. Simulation results will show that our proposed system can save a large amount of energy on the pre-equalization circuit.
We study the limitation of electrical bandwidth in QPSK optical coherent receiver with decision-aided maximum likelihood (DAML) carrier phase estimation. Before signal sampling, a low-pass rectangular filter is utilized to model the bandwidth limitation of electronic devices in the receiver. On one hand the limited bandwidth can reduce the power of additive noise, on the other hand it would distort the signal since inter-symbol interference (ISI) between adjacent symbols occurs. In this paper we discuss the effects on signal amplitude and phase caused by ISI and find original distribution of QPSK constellation points will shift due to ISI. When adjacent symbol interference, that is, the ISI effect of one previous symbol, is taken into account, the distribution changes into the form of 16-QAM, while changing as 4K+1-QAM if K previous symbols are considered. The impact on the subsequent DAML algorithm and the final bit error rate (BER) calculation due to constellation points shifts are analyzed intensively. Monte-Carlo (MC) simulation results show that 0.7 to 0.9 times symbol rate is a reasonable bandwidth range for different optical signal to noise ratios (OSNRs), and optimum bandwidth is often inside this range. Results also reveal that filter-induced ISI would degrade the BER performance of DAML receiver when OSNR is over 10 dB.
KEYWORDS: Phase shift keying, Receivers, Error analysis, Signal to noise ratio, Monte Carlo methods, Interference (communication), Digital signal processing, Signal processing, Optical communications, Transmitters
In coherent optical phase-shift-keying (PSK) system, the conventional decision-aided maximum likelihood (DA-ML) phase estimator only considers constant phase noise over the entire observation interval. In this paper, we take into account the time-varying phase noise and propose a flexible DA-ML phase estimator for carrier phase estimation. The weighted coefficients are introduced to estimate carrier phase more accurately than the conventional scheme. The phase estimation error of this flexible DA-ML receiver is analyzed and compared with results from the conventional DA-ML receiver. The analytical bit error rate (BER) follows well with Monte Carlo (MC) simulations. Simulation results also show that by using the flexible DA-ML receiver in quadrature PSK (QPSK) system, bit signal-to-noise ratio (SNR) improvement over the conventional DA-ML receiver at BER of 10-3 is 0.2dB and 1.2dB with block length of 20 and 50, respectively, when the laser linewidth is 2MHz.
KEYWORDS: Light emitting diodes, Orthogonal frequency division multiplexing, Control systems, Modulation, Visible radiation, Telecommunications, Receivers, Signal detection, Signal to noise ratio, Reliability
In traditional dimming control system using pulse width modulation (PWM) combined with M-QAM OFDM scheme,
OFDM signal is only transmitted during “on” period. To guarantee the communication quality, reduction of duty cycle
will cause increased symbol rate or added LED power. This means system BER performance degradation and power
consumption. In order to solve the defects of the traditional dimming scheme, we propose a new dimming control
scheme in indoor visible light communication, which combines OFDM signal and multi-pulse position modulation
(MPPM) light pulse well with each other. By means of dividing traditional PWM pulses into MPPM pulses with the
same duty cycle, the pattern effect of MPPM pulses is utilized, which makes excess signal transmission possible.
Simulation results show that when reducing the brightness of LED the achievable symbol rate using dimming control
patterns is not higher than the traditional PWM scheme and the LED power is also reduced, which satisfies both system
reliability and energy effectiveness under constant high data rate and BER less than 10-3.
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