In this article, we introduce two variants of energy efficient M-ary frequency-shift keying (FSK) for low data rate/low power Internet-of-Things (IoT) applications. Both variants, i.e., M-ary direct current (DC)-FSK and M-ary unipolar (U)-FSK are compatible with intensity-modulation and direct detection (IM-DD) implementation of visible light communication (VLC). The two techniques intrinsically differ in the manner of attaining a non-negative signal for intensity-modulation. Mary DC-FSK uses a DC-offset, while, M-ary U-FSK sequentially transmits the positive and the sign flipped negative halves of the bipolar M-ary FSK symbols. The spectral efficiencies of Mary DC-FSK and M-ary U-FSK are augmented by biorthogonal extension of frequency waveforms resulting in 2M-ary biDC-FSK and 2M-ary biU-FSK, respectively. Two optimal maximum likelihood (ML) receiver configurations with different complexities are introduced for M-ary DC-FSK/2M-ary biDC-FSK. Whereas, for M-ary U-FSK/2M-ary biU-FSK, an optimal ML and a suboptimal receiver are proposed. We appraise the performance of these methods in terms of Euclidean distance, bit error rate (BER) in additive white Gaussain noise and time dispersive channels, energy efficiency with respect to spectral efficiency and computational complexity. Simulations confirm that the proposed techniques are more energy efficient than classical M-ary pulseamplitude modulation (PAM) in an absolute sense.
In this article, we introduce intensity modulation and direct detection compatible enhanced optical-orthogonal frequency division multiplexing with index modulation (EO-OFDM-IM) schemes. These approaches augment the spectral efficiency (SE) relative to classical counterparts by enlarging the index domain information using the so-called virtual sub-carriers. The classical O-OFDM-IM schemes do not necessarily enhance the SE because of low cardinality of IM complex-valued sub-carrier set which is limited by constraints like Hermitian symmetry. The index domain extension for EO-OFDM-IM schemes is achieved by replacing the complex-valued sub-carriers (as in O-OFDM-IM) by twice real-valued virtual sub-carriers. The realization of non-negative signals is based on precepts of classical O-OFDM approaches, that are direct current (DC) O-OFDM and asymmetrically clipped (AC) O-OFDM. Thus, we refer to the EO-OFDM-IM approaches as DCEO-OFDM-IM and ACEO-OFDM-IM. We shall establish that in addition to improving SE, EO-OFDM-IM schemes provide extended granularity effectuating better SE/energy efficiency (EE) trade-off and improved bit error rate performance over classical counterparts. The EO-OFDM-IM schemes, however, are suitable for lower alphabet cardinalities of pulse-amplitude modulation making it difficult to attain high spectral efficiencies while maintaining EE. To circumvent this limitation, dual-mode (DM) counterparts, DCEO-OFDM-DM and ACEO-OFDM-DM are proposed. The numerical simulations shall demonstrate that the EO-OFDM-DM approaches are more energy and spectral efficient than classical O-OFDM-DM schemes and provide an advantageous granularity for EE/SE trade-off. Additionally, we use efficient index mapping and de-mapping algorithms based on Pascal's triangle, which allows investigating these approaches for peak SE by precluding the so-called sub-block partitioning. For peak SE, the use of optimal maximum-likelihood (ML) detector is cumbersome, therefore, we introduce two sub-optimal low-complexity detectors based on energy detection and ML criterion. INDEX TERMS Optical-orthogonal frequency-division multiplexing, index modulation, intensity modulation and direct detection, optical wireless systems.
In this letter, we propose novel dual-mode chirp spread spectrum (DM-CSS) modulation for lowpower wide-area networks for Internet-of-Things applications. DM-CSS is capable of transmitting higher number of bits relative to other counterparts, such as Long Range (LoRa) modulation. Considering the same symbol period, the number of transmitted bits in DM-CSS are augmented by: (i) simultaneously multiplexing even and odd chirp signals; (ii) using phase shifts of 0 and π radians for both even and odd chirp signals; and (iii) using either up-chirp or down-chirp signal. Relative to LoRa, the transmitted bit rate increases by up to 116.66% for the same bandwidth and spreading factor. Moreover, we present a non-coherent receiver architecture for DM-CSS. Simulation results reveal that DM-CSS is not only more spectral efficient but also more energy efficient than most classical counterparts. It is also demonstrated that DM-CSS is robust to phase and frequency offsets.
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