Dimitrov, S.; Sinanovic, S.; and Haas, H., “Signal Shaping and Modulation for Optical Wireless Communication”, IEEE Journal of Lightwave Technology, (to appear)
The data transmission in optical wireless communication (OWC) systems is achieved through intensity modulation and direct detection (IM/DD). The information-carrying signal modulates the intensity of a light emitting diode (LED) at the transmitter, and it is detected by a photodiode (PD) at the receiver. For this purpose, the signal needs to be real-valued and positive. Suitable candidates for data modulation are the single-carrier pulse modulation schemes such as multi-level pulse position modulation (M-PPM) and multi-level pulse amplitude modulation (M-PAM) [1,2]. In addition, multi-carrier modulation such as multi-level quadrature amplitude modulation with orthogonal frequency division multiplexing (M-QAM OFDM) can be used to achieve high data rates. In order to ensure that the time domain signal is real-valued, Hermitian symmetry within the OFDM frame is applied. Unipolarity is generally achieved through direct-current (DC) biasing, e.g. DC-biased optical OFDM (DCO-OFDM) . An alternative approach employs a structure on the OFDM frame which allows the zero-level time domain signal clipping without any loss of information, yet reducing the spectral efficiency by 50%, e.g. asymmetrically clipped optical OFDM (ACO-OFDM) .
In a realistic communication scenario, the signal is passed through an LED with a non-linear transfer characteristic. Furthermore, a linear optical wireless channel with a finite frequency response attenuates the signal, and additive white Gaussian noise (AWGN) is added at the receiver. The LED characteristic can be linearized through predistortion between positive minimum and maximum levels of radiated optical power. A consistent framework for optical-to-electrical conversion enables the comparison between the single-carrier and multi-carrier modulation formats in terms of spectral efficiency for a given signal-to-noise ratio (SNR). While the uniform single-carrier pulsed signals fit within the dynamic range of the transmitter, the Gaussian OFDM signals require optimum pre-clipping. The systems apply different approaches to minimize the channel effect on the transmitted signal. In practical implementations, single-carrier transmission employs a linear feed-forward equalizer (FFE) or a non-linear decision feedback equalizer (DFE) with zero forcing (ZF) or minimum mean squared error (MMSE) criteria. OFDM benefits from a cyclic prefix (CP) extension of the time domain signal, and through bit and power leading at the transmitter side the computational effort is reduced to single-tap linear FFE with ZF or MMSE criteria.
For a practical dynamic range of the transmitter front-end, the systems are compared in a novel fashion in terms of electrical SNR requirement and spectral efficiency in the dispersive optical wireless channel. In visible light communication (VLC) systems, the additional DC bias power required to create a non-negative signal can serve a complementary functionality, such as illumination. Therefore, it can be excluded in the calculation of the electrical SNR. In infrared (IR) communication, however, the DC power is generally constrained by eye-safety regulations, and it is included in the calculation of the electrical SNR. When the additional DC bias power is neglected, DCO-OFDM and PAM show the greatest spectral efficiency for a flat fading channel in the SNR region above 6.8 dB. However, since optical OFDM with bit and power loading suffers a lower SNR penalty than PAM with DFE as the signal bandwidth exceeds the coherence bandwidth of the dispersive optical wireless channel, DCO-OFDM demonstrates a superior spectral efficiency. When the DC bias power is counted towards the electrical signal power, DCO-OFDM and ACO-OFDM suffer a greater SNR penalty due to the DC bias as compared to PAM and PPM, respectively. However, the presented optimum signal shaping framework enables O-OFDM to greatly reduce this penalty and minimize the gap to single-carrier transmission within 2 dB in the flat fading channel. When the signal bandwidth exceeds the channel coherence bandwidth, DCO-OFDM outperforms PAM with FFE, and it approaches the spectral efficiency of the more computationally intensive PAM with DFE, while ACO-OFDM outperforms PPM with FFE and DFE.
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