Abstract
A number of inherently unipolar orthogonal frequency division multiplexing (OFDM) modulation schemes have been introduced recently in an attempt to improve the energy efficiency of OFDM-based intensity modulation and direct detection (IM/DD) systems. All such algorithms, including asymmetrically clipped optical OFDM (ACO-OFDM), pulse-amplitude-modulated discrete multitone modulation (PAM-DMT) and unipolar orthogonal frequency division multiplexing (U-OFDM), experience an inherent loss in spectral efficiency caused by the restrictions imposed on the OFDM frame structure required for the generation of a unipolar signal. The current paper presents a modified modulation approach, termed enhanced U-OFDM (eU-OFDM), which compensates the spectral efficiency loss in U-OFDM. At the same time, it still allows for the generation of an inherently unipolar modulation signal that achieves better performance in terms of both electrical power and optical power dissipation compared to the conventional state-of-the-art technique direct current (DC)-biased optical OFDM (DCO-OFDM). To the best of the authors' knowledge, the current work also presents the first experimental proof-of-concept demonstration of both U-OFDM and eU-OFDM, and clearly demonstrates the significant energy advantages that these two schemes can introduce in an optical wireless communications (OWC) system.
Highlights
D ATA throughput in wireless communication networks is increasing exponentially
This section investigates the performance of eU-orthogonal frequency division multiplexing (OFDM) in the context of a linear additive white Gaussian noise (AWGN) channel
Clipping of the modulation signal from below, is relevant to all devices. It cannot be avoided in a scheme such as DCO-OFDM due to the high peak-to-average power ratio (PAPR) of an OFDM signal which increases linearly with the number of active subcarriers in the frequency domain [20], [21]
Summary
D ATA throughput in wireless communication networks is increasing exponentially. By 2017, it is expected that traffic demands in mobile networks will be more than 11 Exabytes per month [1]. OFDM becomes a very appealing option for a modulation scheme It enables cost effective equalisation with single-tap equalisers in the frequency domain, as well as adaptive data and energy loading in different frequency regions depending on the communication channel properties. Note that all four inherently unipolar approaches achieve equivalent performance in an additive white Gaussian noise (AWGN) channel [8] In each of these four methods, the electrical power dissipation penalty relative to a bipolar OFDM signal is only 3 dB for any M-QAM constellation size. Asadzadeh et al [15] have proposed an alternative modulation method named spectrally factorized optical OFDM (SFO-OFDM) It analyses the frequency-domain signal requirements that lead to an inherently unipolar OFDM signal and attempts to generate a modified set of constellation symbols which can always fulfil these requirements.
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