Optical Wireless Communication Channels Research Articles

Impact Analyses of High-Order Light Reflections on Indoor Optical Wireless Channel Model and Calibration

This paper analyzes the impact of high order light reflections on indoor optical wireless communication (IOWC) channel models. Based on observing the results of computer simulations, a calibration method is proposed to reduce model errors. Channel models are generated by tracing and adding up diffuse light reflections and sequential sub-reflections along its traveling path. As computation complexity increases significantly with the number of reflection orders considered, researchers traditionally, though incorrectly, take the contribution of first a few reflection orders, most commonly three, to represent the complete channel. Discarded high-order reflections bring no significant performance difference to low-speed transmission systems; however, major contemporary IOWC research institutions focus on high-speed Gigabits per second (Gbps) communications and the model errors resulting from discarded high-order reflections are no longer negligible. This is where the importance of our proposed method lies. root-mean-square (RMS) delay-spread, for instance, is severely underestimated by neglecting higher-order reflections. We simulate an IOWC system in an ordinary 6 m × 6 m × 3 m room and calculate the contributions of each order of reflections at 841 locations. It shows the RMS delay-spread estimation using the first three orders underestimates the true value by 15.3% on the average and by at most 26.6% as maximum. To limit error within half a symbol period, 1 Gbps and 10 Gbps systems tolerate underestimations up to 13.7% and 1.4%, respectively. These must be achieved by applying first five and nine orders. To maintain the computation efficiency of low-order reflection models and improve their accuracies, we propose a statistical calibration method. It reduces average model error of first three reflection orders from 15.7% to 4.3%. The numbers of orders required by 1 and 10 Gps systems are individually reduced to 3 and 7.

Adaptive differential amplitude pulse‐position modulation technique for optical wireless communication channels based on fuzzy logic

Optical wireless communication has the potential for extremely high data rates of up to tens of Gb/s. In this study, a hybrid modulation technique named adaptive differential amplitude pulse-position modulation (DAPPM) is proposed to improve channel immunity by utilising optimised modulation to channel. The unit transmission rate, channel capacity, peak-to-average power ratio, bandwidth requirement and power requirement of the DAPPM were determined and compared with other modulation schemes such as on–off key, pulse-amplitude modulation, pulse-position modulation (PPM), differential PPM and multilevel digital pulse interval modulation. The simulation results show that DAPPM gives better bandwidth and power efficiency depending on the number of amplitude level (A) and the maximum length (L) of a symbol. In addition, the fuzzy logic module is developed to assist the adaptation process of DAPPM. Mamdani fuzzy logic method is used in which the decisions made by the system will be approaching to what would be decided by the user in the real world.

Performance of Non-Adaptive and Adaptive Subcarrier Intensity Modulations in Gamma-Gamma Turbulence

We investigate a variable-rate, constant-power adaptive subcarrier intensity modulation employing M-ary phase shift keying and rectangular quadrature amplitude modulation for optical wireless communication over the Gamma-Gamma turbulence channels. The adaptive schemes offer efficient utilization of optical wireless communication channel capacity by adapting the modulation order according to the received signal-to-noise ratio and a pre-defined target bit-error rate requirement. Novel closed-form series solutions are presented for the achievable spectral efficiency, average bit-error rate, and outage probability using a series expansion approach of the modified Bessel function. In addition, asymptotic bit-error rate and outage probability analyses are presented. Our asymptotic bit-error rate analysis shows that the diversity order of both non-adaptive and adaptive systems depends only on the smaller channel parameter of the Gamma-Gamma turbulence. Numerical results demonstrate high accuracy of our series solutions with finite number of terms and improved spectral efficiency achieved by the adaptive systems without increasing the transmitter power or sacrificing bit-error rate requirements.

Mitigation of Optical Background Noise in Light-Emitting Diode (LED) Optical Wireless Communication Systems

One challenge faced by the in-home light-emitting diode (LED) optical wireless communication is the optical noises. Here, we first experimentally characterize the effect of optical background noise to the performance of the LED optical wireless communication channel. We demonstrate using Manchester coding for the LED to mitigate the optical noise. No adaptive monitoring, feedback, or optical filtering is required. The theoretical and numerical analysis of Manchester decoding process to mitigate the optical background noise is provided. Our experimental result shows that Manchester coding can significantly eliminate optical noise generated by the AC-LED operated at <; 500 kHz and fluorescent light.

Improvement of the Transmission Bandwidth for Indoor Optical Wireless Communication Systems Using a Diffused Gaussian Beam

The multipath dispersion in a non-directed line-of-sight (LOS) indoor optical wireless communications (OWC) channel limits the transmission bandwidth. In this study, we improve the transmission bandwidth by employing a TEM_{00} mode laser beam and an alignment-free light shaping diffuser (LSD). We derive a new expression for the radiation field distribution of the diffused Gaussian beam (DGB) and its multipath impulse response. The transmission bandwidth is determined based on the numerical simulation of the root mean square (RMS) delay spread in a room. Results show that indoor OWC systems employing DGB offers significantly higher transmission bandwidth compared to the Lambertian beam (LB) for a wide divergence angle.

The Performance of a Dual Header Pulse Interval Modulation in the Presence of Artificial Light Interferences in an Indoor Optical Wireless Communications Channel with Wavelet Denoising

Problem statement: Similar to other baseband modulation schemes, the performance of the Dual Header Pulse Interval Modulation (DH-PIM) is adversely affected by Artificial Light Interferences (ALI) in an indoor Optical Wireless Communications (OWC) channel. Approach: The Discrete Wavelet Transform (DWT) based denoising is studied for reducing the effect of the ALI. Computer simulation is carried out to demonstrate the effectiveness of the proposed system. The Baseline Wander (BLW) affects for a range of bit resolutions is also analyzed. Results: The normalized optical power requirement in the presence of ALI is high in the range of 9-15 dB depending upon the bit resolution. A significant reduction in the optical power penalty is observed with the DWT denoising scheme. Conclusion: The DWT based denoising scheme is effective in reducing the consequence of the ALI. The optical power penalty decreases with an increase in the data rate and the bit resolution, which is due to reduction of the DC values and low frequency spectrum.

Optimising the performance of digital pulse interval modulation with guard slots for diffuse indoor optical wireless links

The intersymbol interference (ISI) in a diffuse indoor optical wireless communication channel limits the maximum data rate for error-free transmission. In this study, simpler decoding approaches are suggested for digital pulse interval modulation (DPIM) with guard slot(s) (GSs) to reduce the effect of ISI. The unique patterns in DPIM symbols with GSs are exploited to carry out optimum decoding without utilising an equaliser at the receiver. The simplest decoding approaches would be to find an optimum threshold level that offers the least power penalty. However, performance can be enhanced by employing a hybrid decoding scheme in which the amplitudes of adjacent slots are compared in order to carry out the slot decision. The decoding algorithm that maximises the likelihood of ‘one’ can offer the optimum performance. These decoding approaches are simpler to implement and offer a relatively high gain (>6 dB) at a moderately dispersive channel.

Underwater Wireless Optical Communication Channel Modeling and Performance Evaluation using Vector Radiative Transfer Theory

This paper presents the modeling of an underwater wireless optical communication channel using the vector radiative transfer theory. The vector radiative transfer equation captures the multiple scattering nature of natural water, and also includes the polarization behavior of light. Light propagation in an underwater environment encounters scattering effect creating dispersion which introduces inter-symbol-interference to the data communication. The attenuation effect further reduces the signal to noise ratio. Both scattering and absorption have adverse effects on underwater data communication. Using a channel model based on radiative transfer theory, we can quantify the scattering effect as a function of distance and bit rate by numerical Monte Carlo simulations. We also investigate the polarization behavior of light in the underwater environment, showing the significance of the cross-polarization component when the light encounters more scattering.

Tunable Pulse Amplitude and Position Modulation Technique for Reliable Optical Wireless Communication Channels

Modulation techniques have attracted increasing attention in optical wireless communications. Basic schemes such as on off keying (OOK), pulse amplitude modulation (PAM) and pulse position modulation (PPM) have been validated as suitable for the optical wireless channel. This paper starts from the analysis of these three modulation schemes in terms of their power and bandwidth requirements. As a result, a new tunable hybrid modulation technique is proposed. The proposed modulation scheme takes the real time channel conditions into account, which is different from other schemes. By employing amplitude and position modulation selectively, a guaranteed system performance can be secured, without compromising power and bandwidth efficiency. This is also a new approach to realize reliable optical wireless links.

Effects of specular surfaces of finite thickness on wireless optical communication channel response

The finite thickness of glass surfaces, frequently used in office environments, must be taken into account in optical channel characterisation. A reflection coefficient that considers this effect is presented and then integrated into an optical channel simulation tool that processes both diffuse and specular contributions.