Indoor Optical Wireless Channel Research Articles

Combined Deterministic and Modified Monte Carlo Method for Calculating Impulse Responses of Indoor Optical Wireless Channels

Because of the importance of calculating an accurate impulse response of an indoor optical wireless room environment that includes sufficient orders of multipath reflections from reflecting surfaces of the room, several different algorithms exist to solve the problem of calculating impulse responses accurately and in a reasonable amount of computing time. Deterministic approaches that divide the reflecting surfaces into small elements give the best accuracy but they require high computing time. Modified Monte Carlo methods provide a very fast approach of calculating impulse responses but the calculated impulse responses contain variance or, in other words, they are not as temporally smooth when compared to deterministic approaches. In this paper, we have taken a combined approach where we calculate the contribution of first reflections to the total impulse response by a deterministic method, and then the contributions of second and rest of the reflections to the total impulse response are calculated by the Modified Monte Carlo method. This carries the advantage of both of these approaches. Moreover, the algorithm can be easily implemented for parallel computation similar to the Modified Monte Carlo method to utilize the full power of modern multicore computer processors.

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.

Multi-user indoor optical wireless communication system channel control using a genetic algorithm

A genetic algorithm controlled multispot transmitter is demonstrated that is capable of optimising the received power distribution for randomly aligned single element receivers in multiple fully diffuse optical wireless communications systems with multiple mobile users. Using a genetic algorithm to control the intensity of individual diffusion spots, system deployment environment changes, user movement and user alignment can be compensating for, with negligible impact on the bandwidth and root mean square delay spread. It is shown that the dynamic range, referenced against the peak received power, can be reduced up to 27% for empty environments and up to 26% when the users are moving. Furthermore, the effect of user movement, that can perturb the channel up to 8%, can be reduced to within 5% of the optimised case. Compared to alternative bespoke designs that are capable of mitigating optical wireless channel drawbacks, this method provides the possibility of cost-effectiveness for mass-produced receivers in applications where end-user friendliness and mobility are paramount.

Comparison of Monte Carlo ray-tracing and photon-tracing methods for calculation of the impulse response on indoor wireless optical channels

We present a comparison between the modified Monte Carlo algorithm (MMCA) and a recently proposed ray-tracing algorithm named as photon-tracing algorithm. Both methods are compared exhaustively according to error analysis and computational costs. We show that the new photon-tracing method offers a solution with a slightly greater error but requiring from considerable less computing time. Moreover, from a practical point of view, the solutions obtained with both algorithms are approximately equivalent, demonstrating the goodness of the new photon-tracing method.

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.

Effective Denoising and Adaptive Equalization of Indoor Optical Wireless Channel With Artificial Light Using the Discrete Wavelet Transform and Artificial Neural Network

Indoor diffuse optical wireless (OW) communication systems performance is limited due to a number of effects; interference from natural and artificial light sources and multipath induced intersymbol interference (ISI). Artificial light interference (ALI) is a periodic signal with a spectrum profile extending up to the MHz range. It is the dominant source of performance degradation at low data rates, which can be removed using a high-pass filter (HPF). On the other hand, ISI is more severe at high data rates and an equalizing filter is incorporated at the receiver to compensate for the ISI. This paper provides the simulation results for a discrete wavelet transform (DWT)-artificial neural network (ANN)-based receiver architecture for on-and-off keying (OOK) non-return-to-zero (NRZ) scheme for a diffuse indoor OW link in the presence of ALI and ISI. ANN is adopted for classification acting as an efficient equalizer compared to the traditional equalizers. The ALI is effectively reduced by proper selection of the DWT coefficients resulting in improved receiver performance compared to the digital HPF. The simulated bit error rate (BER) performance of proposed DWT-ANN receiver structure for a diffuse indoor OW link operating at a data range of 10-200 Mbps is presented and discussed. The results are compared with performance of a diffuse link with an HPF-equalizer, ALI with/without filtering, and a line-of-sight (LOS) without filtering. We show that the DWT-ANN display a lower power requirement when compared to the receiver with an HPF-equalizer over a full range of delay spread in presence of ALI. However, as expected compared to the ideal LOS link the power penalty is higher reaching to 6 dB at 200 Mbps data rate.

Adaptive code-division multiple-access system for communications over indoor wireless optical channels based on random optical codes

The authors have designed an adaptive optical codes-based system for communications over the indoor wireless optical channel when large numbers of users access to the channel simultaneously. This system uses a code-division multiple access (CDMA) scheme based on the named random optical codes (ROC). The authors present the characteristics of this kind of optical codes and several results about its performance over noisy indoor wireless optical channels. Finally, the authors describe a CDMA system which adapts its performance to the number of users which are accessing simultaneously to the channel. The proposed system is able to maintain a target bit error rate adapting its data throughput independently of the number of simultaneous users in the optical environment, whenever certain conditions are accomplished.

Bit error performance of diffuse indoor optical wireless channel pulse position modulation system employing artificial neural networks for channel equalisation

The bit-error rate (BER) performance of a pulse position modulation (PPM) scheme for non-line-of-sight indoor optical links employing channel equalisation based on the artificial neural network (ANN) is reported. Channel equalisation is achieved by training a multilayer perceptrons ANN. A comparative study of the unequalised ‘soft’ decision decoding and the ‘hard’ decision decoding along with the neural equalised ‘soft’ decision decoding is presented for different bit resolutions for optical channels with different delay spread. We show that the unequalised ‘hard’ decision decoding performs the worst for all values of normalised delayed spread, becoming impractical beyond a normalised delayed spread of 0.6. However, ‘soft’ decision decoding with/without equalisation displays relatively improved performance for all values of the delay spread. The study shows that for a highly diffuse channel, the signal-to-noise ratio requirement to achieve a BER of 10−5 for the ANN-based equaliser is ∼10 dB lower compared with the unequalised ‘soft’ decoding for 16-PPM at a data rate of 155 Mbps. Our results indicate that for all range of delay spread, neural network equalisation is an effective tool of mitigating the inter-symbol interference.

Modified Ceiling Bounce Model for Computing Path Loss and Delay Spread in Indoor Optical Wireless Systems

This paper proposes modifications to the tradional Ceiling Bounce Model and uses it to characterize diffuse indoor optical wireless channel by analyzing the effect of transceiver position on signal propagation properties. The modified approach uses a combination of the tradional ceiling bounce method and a statistical approach. The effects of different transmitter-receiver separations and height of the ceiling on path loss and delay spread are studied in detail.

실내 광 무선 통신 특성 해석을 위한 포톤 모델링 방법

본 논문에서는 실내 환경에서 가시 광선을 이용한 무선 망 설계에 필요한 가시광의 특성 예측을 위해 포톤 모델을 이용하는 방법을 제시한다. 광선을 이용하는 경우, 높은 주파수와 짧은 파장을 갖고 있으므로 광선 추적법을 이용하는 것이 일반적이나, 환경이 복잡하고 복수의 반사, 투과뿐만 아니라 물체 표면의 거친 정도 등의 재질을 고려한 난반사까지 고려해야 하는 경우, 매우 많은 시간이 걸리는 단점이 있다. 이러한 단점을 해결하기 위해 본 논문에서는 광선 추적법을 보완하여 광선의 세기를 포톤의 밀도로 근사하여 계산 정밀도와 계산 시간을 타협하는 방식을 제시한다. In this paper, an analysis method for indoor optical wireless channel properties based on photon model is presented for characterization of communication environment. In contrast to radio waves, optical waves have very short wave-lengths and very high frequencies, so that material properties become important. Channel models including diffuse reflections and absorption effects due to material surface textures make conventional electromagnetic wave analysis methods based on ray tracing consume enormous time. To overcome these problems, an analysis method using photon model is presented that approximates light intensity by a density of photons. The photon model ensures that simulation time is within a predictable limit.

Multi-user adaptive orthogonal frequency-division multiplexing system for indoor wireless optical communications

An adaptive orthogonal frequency-division multiplexing (OFDM) system is proposed for multi-user communications over indoor wireless optical channels. The designed system uses multi-user least-squares detection techniques applied to space-division multiple access and OFDM schemes, in conjunction with angle-diversity reception. The system, which does not present an excessive increase in complexity with respect to the previous schemes, can support high bit rates for multiple users, beyond 100 Mbits/s. It also mitigates the channel fluctuations induced when either the space distribution or the number of emitters and receivers varies. The performance of the new proposed scheme is compared with that of a non-adaptive multi-user system and an adaptive single-user system, both described in the previous works, when they face similar environmental situations. The obtained results show a significant enhancement with respect to both the previous multi-user system and the adaptive single-user one, since the new scheme allows adaptively managing the system throughput on a multi-user environment.

Adaptive OFDM system for communications over the indoor wireless optical channel

The authors propose an adaptive orthogonal frequency division multiplexing (OFDM) system for communications over the indoor wireless diffuse optical channel. This channel can be characterised as short-term stationary, with severe attenuation and multipath-induced penalty, and high dependence on the space distribution of emitters and receivers. We have chosen OFDM systems because of their capability of supporting high data rates without channel equalisation. They also mitigate the quality of service fluctuations induced when space distribution of emitters and receivers varies. The performance of the new proposed scheme is compared with that of an adaptive system described in a previous work. The obtained results show that a significant increase of the system throughput is attained over noisy wireless optical channels.

On the design of bandwidth efficient signalling for indoor wireless optical channels

AbstractIt is well known that indoor wireless optical channels are limited not only in transmitted optical power, but also in signalling bandwidth. This bandwidth constraint arises due to multipath dispersion in indoor settings as well as due to response time limitations of optoelectronic components.This paper presents an overview of theoretical and practical issues in the design of signalling for bandwidth constrained intensity modulated, direct detection wireless optical channels. A brief overview of the salient qualities of the wireless optical channel are presented to highlight the amplitude constraints which arise. A survey of modulation design is then presented which includes the review of a general technique to represent optical intensity modulation in a signal space and to construct optical intensity lattice codes. Results on the channel capacity of indoor wireless optical channels are surveyed and particular emphasis is placed on recently derived asymptotically exact bounds. The use of multiple emitters and receivers in wireless optical channels is also presented and particular emphasis is placed on techniques which exploit spatial dimensions to improve spectral performance. The paper concludes with some remarks regarding the status of the research area and suggestions for future work. Copyright © 2005 John Wiley & Sons, Ltd.

Statistical impulse response models for indoor optical wireless channels

AbstractThe indoor optical wireless channel is significantly different than the radio channel. Statistical propagation models developed for the radio channel, which characterize path loss, shadowing, and multipath fading, do not apply. In this paper, we investigate statistical modelling for the indoor optical wireless channel through the examination of the characteristics of a large set of channel impulse responses. The channel responses are generated using an estimation method based on geometrical modelling of indoor environments together with an iterative technique for calculating multiple reflections. We confirm prior studies showing that channels with line‐of‐sight paths must be modelled separately from those fully diffuse channels with no such path. We show that the distribution of the channel gain in dB for the LOS component follows a modified gamma distribution, and the channel gain in dB for LOS channels including all reflections follows a modified Rayleigh distribution for most transmitter–receiver distances. Similar results are described for distributions of channel gains of diffuse channels and for distributions of rms delay spreads. Finally, we describe a method for generating a statistically realistic impulse response given any transmitter–receiver separation in an indoor environment. Copyright © 2005 John Wiley & Sons, Ltd.

Error Analysis of the Simulated Impulse Response on Indoor Wireless Optical Channels Using a Monte Carlo-Based Ray-Tracing Algorithm

This work describes a method to determine the error in a Monte Carlo-based ray-tracing algorithm used to compute the impulse response on indoor wireless optical channels. The algorithm, which accounts for multiple reflections of any order on irregularly shaped furnished rooms with diffuse and specular reflectors, allows for their analysis. Equations that estimate algorithm-produced error are given. We also report several simulation results concerning the error estimation which verify the reliability of the equations.

OFDM over indoor wireless optical channel

In this paper, the application of orthogonal frequency division multiplexing (OFDM) over an indoor wireless optical channel is investigated. Previous studies have demonstrated that it can be characterised as almost stationary but with a severe penalty induced by multipath propagation. It is also highly dependent on the spatial distribution of emitters and receivers. OFDM systems have been chosen owing to their capability of supporting high data rates without channel equalisation. They also mitigate the effect of quality fluctuations of the channel induced when the spatial distribution of emitters and receivers varies. We present several results about system performance. Finally, an adaptive OFDM system is proposed to increase data throughput. The obtained results show that a significant throughput is attained over noisy wireless optical channels.

Error Analysis of the Simulated Impulse Response on Indoor Wireless Optical Channels Using a Monte Carlo-Based Ray-Tracing Algorithm

Error Analysis of the Simulated Impulse Response on Indoor Wireless Optical Channels Using a Monte Carlo-Based Ray-Tracing Algorithm

Efficient simulation of the impulse response of the indoor wireless optical channel

The use of wireless local area networks (WLANs) is experiencing a significant growth. Infrared technology has an extensive bandwidth of free use, and has the potential to be the support of high-baud rate WLANs. For that, it is necessary to understand the propagation characteristics of the indoor optical channel. This paper presents a computationally efficient algorithm for the simulation of indoor optical channels, considering multiple reflections of the emitted signal. The models used to approximate the emitter pattern, the propagation environment and the receiver pattern are described. Two new procedures (called time-delay agglutination and time and space indexed tables) are introduced to increase the efficiency of the simulation of the impulse response of the indoor wireless optical channel. The structure of the algorithm is presented and the approaches used are discussed. The new algorithm was implemented in a simulation package, named SCOPE, which is very efficient and allows the evaluation of the main parameters of indoor optical channels considering multiple reflections of the emitted signal. Results obtained for several channels are compared with experimental and published values, and good agreement is verified. Copyright © 2000 John Wiley & Sons, Ltd.

Indoor propagation measurements at infrared frequencies for wireless local area networks applications

In a combination tutorial and research paper, propagation aspects of transmission at infrared (IR) frequencies for wireless in-building communications are explored. The tutorial section of the paper presents basic principles of propagation at IR, a comparison with indoor radio propagation, and the derivation of the channel's baseband model. The research aspect of the paper reports on the results of recent frequency response measurements at eight different sites in a university building. A major result shows that the indoor wireless optical channel is very dynamic, with great variations in the channel's characteristics for data collected in different rooms, in different locations within the same room, and for different orientations of the optical receiver at the same location of the same room. Numerical values of the channel's relative path loss and 3 dB bandwidth, along with frequency response plots covering a wide range of conditions, are presented and discussed. Finally, on the basis of the results of measurements, schemes for improving the performance of future wireless in-building optical transceivers are proposed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Simulation of multipath impulse response for indoor wireless optical channels

A recursive method for evaluating the impulse response of an indoor free-space optical channel with Lambertian reflectors is presented. The method, which accounts for multiple reflections of any order, enables accurate analysis of the effects of multipath dispersion on high-speed indoor optical communication systems. A simple algorithm for computer implementation of the technique and computer simulation results for both line-of-sight and diffuse transmitter configurations are also presented. In both cases, it is shown that reflections of multiple order are a significant source of intersymbol interference. Experimental measurements of optical multipath, which help verify the accuracy of the simulations, are discussed. >