Optical Wireless Communications Using Signal Space Diversity with Spatial Modulation

A signal space diversity (SSD) scheme was proposed to be incorporated with spatial modulation (SM) in an intensity-modulation/direct-detection-based multiple-input-single-output (MISO) indoor optical wireless communication (OWC) system to improve bit-error-rate (BER) performance and system throughput. SSD was realized via signal constellation rotation and diversity interleaving using different channel gains to improve the BER. With SM incorporated, the MISO-OWC system throughput increased. Theoretical BER expressions of the SSD scheme were established for the first time by investigating the distance of neighboring constellation symbols upon maximum-likelihood detection. Such BER expressions were further verified by numerical results. The results showed that, except for the slightly-lower-accuracy performance brought by comparable distances of neighboring constellation symbols in cases of low signal-to-noise ratios, these BER expressions were accurate in most scenarios. Moreover, theoretical investigations of channel gain distributions were performed at different signal constellation rotation angles to show the capability of the SSD scheme to improve the BER. The results showed that a significantly improved BER by two orders of magnitude could be achieved using a reasonably high channel-gain ratio and a larger constellation rotation angle. The SSD-SM scheme provides a promising option to achieve transmitter diversity with an enhanced throughput in high-speed indoor OWC systems.

New Signal Constellation Pairs for the ZTM-OFDM-IM System

In this paper, we investigate new signal constellation pairs for mapping active subcarriers of the zero-padded trimode orthogonal frequency division multiplexing with index modulation (ZTM-OFDM-IM) systems. In the presented system, one of a constellation pair is the same as the one used in the previous work, and the other is a constellation larger than the one used in the previous work. It increases the minimum Euclidean distance between the subblocks of the ZTM-OFDM-IM system with new constellation pairs under the constraint of the same spectral efficiency. Computer simulation in AWGN and frequency-selective fading channels shows that the new ZTM-OFDM-IM system has a much lower bit error rate than OFDM-IM and dual-mode OFDM-IM and slightly outperforms the system with conventional constellation pairs. Since the proposed constellation pairs prove the error performance improvement of the system, it is considered that a further study on generalized design of the constellation pair for the ZTM-OFDM-IM system is necessary in the future.

QPSK-Modulation Modem Invariant to the Rotation of the Signal Constellation Plane

In order to increase the efficiency of dedicated frequency channels, i.e. to increase the specific data transfer rate, multipositional quadrature phase shift keying (QPSK, aka 4-PSK) should be used. The problems with QPSK signal demodulation is a rotation of the signal constellation plane by an angle multiple of 90° and a slow response of the carrier oscillation recovery scheme. The study considers the existing methods for eliminating the phase ambiguity of the recovered carrier frequency in typical QPSK modems, and identifies the shortcoming of a low-speed response oscillation recovery circuit. The authors propose a QPSK demodulator circuit with a fast adjustment of the reference oscillator, which is due to the fact that no loop filter is used in the feedback and that a digital calculator of the required phase shift is used. An algorithm for the frame synchronization restoration with the simultaneous elimination of the phase ambiguity multiple of 90° was also developed using synthesized binary sequences with an ideal non-periodic autocorrelation function (NACF) at even shifts that do not have the rotary symmetry property. The phase ambiguity elimination algorithm proposed in the article can be used as an alternative to standard modems with differential coding.