Analysis of OFDM-based intensity modulation techniques for optical wireless communications
View/ Open
thesis files.zip (20.88Mb)
Date
01/07/2013Author
Dimitrov, Svilen Dimitrov
Metadata
Abstract
Optical wireless communication (OWC) is a promising alternative to radio frequency (RF) communication
with a significantly larger and unregulated spectrum. Impairments in the physical
layer, such as the non-linear transfer characteristic of the transmitter, the dispersive optical wireless
channel and the additive white Gaussian noise (AWGN) at the receiver, reduce the capacity
of the OWC system. Single-carrier multi-level pulse position modulation (M-PPM) and multilevel
pulse amplitude modulation (M-PAM) suffer from inter-symbol interference (ISI) in the
dispersive channel which reduces their capacity even after channel equalization. Multi-carrier
modulation such as optical orthogonal frequency division multiplexing (O-OFDM) with multilevel
quadrature amplitude modulation (M-QAM) is known to maximize the channel capacity
through bit and power loading. There are two general signal structures: bipolar Gaussian signal
with a direct current (DC) bias, i.e. DC-biased O-OFDM (DCO-OFDM), or unipolar half-
Gaussian signal, employing only the odd subcarriers, i.e. asymmetrically clipped O-OFDM
(ACO-OFDM). In this thesis, the signal distortion from the transmitter nonlinearity is minimized
through pre-distortion, optimum signal scaling and DC-biasing.
The optical front-ends impose minimum, average and maximum optical power constraints, as
well as an average electrical power constraint, on the information-carrying signals. In this thesis,
the optical signals are conditioned within these constraints through optimum signal scaling
and DC-biasing. The presented analysis of the optical-to-electrical (O/E) conversion enables
the derivation of the electrical signal-to-noise ratio (SNR) at the receiver, including or excluding
the additional DC bias power, which is translated into bit-error rate (BER) performance.
In addition, a generalized piecewise polynomial model for the non-linear transfer characteristic
of the transmitter is proposed. The non-linear distortion in O-OFDM is translated by means of
the Bussgang theorem and the central limit theorem (CLT) into attenuation of the data-carrying
subcarriers at the receiver plus zero-mean complex-valued Gaussian noise. The attenuation
factor and the variance of the non-linear distortion noise are derived in closed form, and they
are accounted towards the received electrical SNR. Through pre-distortion with the inverse of
the proposed piecewise polynomial function, the linear dynamic range of the transmitter is
maximized, reducing the non-linear distortion to double-sided signal clipping.
Finally, the OWC schemes are compared in terms of spectral efficiency and electrical SNR
requirement as the signal bandwidth exceeds the coherence bandwidth of the optical wireless
channel for a practical 10 dB linear dynamic range. Through optimum signal scaling and DCbiasing,
DCO-OFDM is found to achieve the highest spectral efficiency for a target SNR, neglecting
the additional DC bias power. When the DC bias power is counted towards the signal
power, DCO-OFDM outperforms PAM with linear equalization, approaching the performance
of the more computationally intensive PAM with non-linear equalization. In addition, the average
optical power in O-OFDM is varied over dynamic ranges of 10 dB, 20 dB and 30 dB.
When the additional DC bias power is neglected, DCO-OFDM is shown to achieve the Shannon
capacity, while ACO-OFDM exhibits a 3 dB gap which grows with higher SNR targets.
When the DC bias power is included, DCO-OFDM outperforms ACO-OFDM for the majority
of average optical power levels with the increase of the SNR target or the dynamic range.