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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. \ud \ud 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. \ud \ud 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. \ud \ud 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.

Owing to the limited cell size of eNodeB (eNB), the relay node has emerged as an attractive solution for the long-term evolution (LTE) system. The nonlinear limit of the alternative method to multipleinput and multiple-output (MIMO) based on frequency division multiplexing (FDM) for orthogonal FDM (OFDM) is analysed over varying transmission spans. In this reported work, it is shown that the degradation pattern over the linear, intermixing and nonlinear propagation regions is consistent for the 2 and the 2.6 GHz bands. The proposed bands experienced a linear increase in the error vector magnitude (EVM) for both the linear and the nonlinear regions proportional to the increasing transmission spans. In addition, an optical launch power between -2 and 2 dBm achieved a significantly lower EVM than the LTE limit of 8% for the 10-60 km spans. © The Institution of Engineering and Technology 2014.

This paper reports experimental results for the performance of an free space optical (FSO) communication link employing different modulation schemes under the influence of the atmospheric scintillation. A dedicated experimental atmospheric simulation chamber has been developed where weak and medium turbulence can be generated and its effect on the FSO link is investigated. The experimental data obtained is compared to the theoretical prediction. The paper also shows that the effect on the data transmission performance depends on the position of turbulence source positioned within the chamber.

In this paper, we present simulation results for the bit error rate (BER) performance and the fading penalty of a BPSK - subcarrier intensity modulation (BPSK-SIM) free-space optical (FSO) communication link in a log-normal atmospheric turbulence model. The results obtained are based on the Monte-Carlo simulation. Multiple subcarrier modulation schemes offer increased system throughput and require no knowledge of the channel fading in deciding what symbol has been received. In an atmospheric channel with a fading strength σ² of 0.1 obtaining a BER of 10E-6 using a 2-subcarrier system will require a signal-to-noise (SNR) of 23.1 dB. The required SNR increases with the fading strength and at a BER of 10E-9 the fading penalty due to the atmospheric turbulence is ~ 41 dB for σ²=0.9. The comparative studies of the OOK and BPSK-SIM schemes showed that for similar electrical SNR, BPSK-SIM offered improved performance across all range of turbulence variance.

The analysis and simulation for convolutional coded dual header pulse interval modulation (CC-DH-PIM) scheme using a rate 1/2 convolutional code with the constraint length of 3 is presented. Decoding is implemented using the Viterbi algorithm with a hard decision. Mathematical analysis for the slot error rate (SER) upper bounds is presented and results are compared with the simulated data for a number of different modulation techniques. The authors show that the coded DH-PIM outperforms the pulse position modulation (PPM) scheme and offers > 4 dB code gain at the SER of 10 -4 compared to the standard DH-PIM. Results presented show that the CC-DH-PIM with a higher constraint length of 7 offers a code gain of 2 dB at SER of 10 -5 compared to the CC-DH-PIM with a constraint length of 3. However, in CC-DH-PIM the improvement in the error performance is achieved at the cost of reduced transmission throughput compared to the standard DH-PIM.

SUMMARY In this paper we introduce the dual-header pulse interval modulation (DH-PIM) technique employing a simple retransmission coupled with a majority decision detection scheme at the receiver. We analytically investigate the slot error rate (SER) performance and compare results with simulated data for the symbol retransmissions rates of three, four and five, showing a good agreement. We demonstrate that the proposed scheme significantly reduces the SER compared with the standard single symbol transmission system, with retransmission rate of five offering the highest code gain of � 5dB. Copyright # 2007 John Wiley & Sons, Ltd.

Artificial neural network (ANN) finds its application in communication engineering in a diverse areas such as channel equalization, channel modelling, error control code because of its capability of nonlinear processing, adaptability, and parallel processing. On the other hand, the wavelet transform (WT) with both the time and the frequency resolution provides the exact representation of signal in both domains. Applying these modern tools to signal processing in channel equalization and digital signal detection provide an enhanced performance compared to the traditional tools. In this paper, the slot error rate (SER) performance of digital pulse interval modulation (DPIM) in diffuse indoor optical wireless links employing equalization using discrete WT (DWT) and ANN is reported. The simulation results show that a performance gain of ~8 dB is achieved by utilizing the new approach compared to the traditional approach based on the finite impulse response (FIR) digital filters.

A new method of chaotic communication system based on digitization of the chaotic signal is proposed. The digitally encoded chaotic signal is transmitted through a practical channel and recovered at the receiver. Simulation results show that despite being errors in the received bit sequence, the observer is able to synchronise and recover message signal with high accuracy provided bit error rate is < 10-4. The proposed method is compatible with the existing digital communication system and hence could be implemented in real scenario with no or little modification in the existing infrastructure.

Artificial neural network (ANN) can be an attractive alternative for adaptive equalization especially while channel is nonlinear or non-stationary. Pulse position modulation (PPM) requires the least average optical power compared to other modulation schemes in line-of-sight links but suffer severely in diffused links. The performance of PPM in a diffused channel can be improved by using different equalization techniques. In this work equalization using ANN is proposed and studied. The ANN equalized PPM shows promising results and its performance is comparable to the traditional equalization techniques. The performance can further be enhanced by using 'soft' decision decoding and the simulation results show a 2 dB gain in signal-to-noise.

This paper presents a comparative study of two equalizers, the adaptive linear and the neural equalizer for indoor optical wireless (OW) links. Three modulation schemes representative of a range of modulation techniques widely adopted in indoor OW systems are chosen to investigate the effectiveness of the proposed equalizers. It is shown that on-off keying (OOK) modulation technique is least affected by the multipath induced intersymbol interference (ISI), while the pulse position modulation (PPM) scheme is the most severely affected. In case of highly diffused channel, the equalized digital pulse interval modulation (DPIM) outperforms the equalized PPM and there is little or no gain in using PPM compared to OOK.

This paper presents an experimental demonstration of a visible light communications link with an light emitting diode and a low-bandwidth organic photodetector as transmitter and receiver, respectively, that achieves sub 4 Mbits/s speeds. An artificial neural network (ANN) equalizer is required in order to achieve such high data rates because of the influence of intersymbol interference. The digital modulation formats tested in this paper are nonreturn-to-zero on–off keying (OOK), and fourth-order pulse position modulation (4-PPM). Without equalization, data rates of 200 and 300 kbits/s can be achieved for 4-PPM and OOK, respectively. With ANN equalization, data rates of 2.8 and 3.75 Mbits/s can be achieved for the first time for OOK and 4-PPM, respectively.

This paper reports the experimental wavelet denoising techniques carried out for the first time for a number of modulation schemes for indoor optical wireless communications in the presence of fluorescent light interference. The experimental results are verified using computer simulations, clearly illustrating the advantage of the wavelet denoising technique in comparison to the high pass filtering for all baseband modulation schemes.

Binary polarisation shift keying (BPOLSK) has been proposed to mitigate the atmospheric turbulence-induced fading in free space optical (FSO) communication systems. In this study, the Q-factors obtained for the BPOLSK-FSO system are verified in conjunction with theoretical results to confirm the validity of the proposed scheme. The analytical bit error rate (BER) for the BPOLSK and non-return-to-zero on–off keying (NRZ-OOK) schemes are presented. The authors show that the BPOLSK scheme with direct detection offers improved BER performances compared to NRZ-OOK in the presence of weak turbulence, which is inferred from the experimental Q-factor and theoretical BER. For a turbulence variance σ21 of 0.003 and the transmitted optical power of −16.8 dBm, values for Q-factor are ∼11 and ∼8.5 for BPOLSK and NRZ-OOK schemes, respectively. The authors show that the predicted signal-to-noise ratio (SNR) for BPOLSK and NRZ-OOK schemes are ∼13.5 and ∼15 dB, respectively, for a BER of 10−6 and σ21 of 0.01. When σ21 increases to 0.1, ∼8 dB lower values of SNR is required for BPOLSK compared with NRZ-OOK.

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.

In this paper, analytical and simulation results for the bit error rate (BER) performance and fading penalty of a coherent optical binary polarization shift keying (2PolSK) heterodyne system adopted for a free space optical (FSO) communication link modeled as the log-normal and the negative exponential atmospheric turbulence channels are presented. The conditional and unconditional BER expressions are derived, demonstrating the comprehensive similarity between the 2PolSK and the binary frequency shift keying (2FSK) schemes with regards to the system sensitivity. The power penalty due to the non-ideal polarization beam splitter (PBS) is also analyzed. The receiver sensitivity employing 2PolSK is compared with other modulation schemes in the presence of turbulence and the phase noise. The results show that 2PolSK offers improved signal-to-noise ratio (SNR) performance compared to the binary amplitude shift keying (2ASK).

In this article we studies the impact of dynamic vehicular traffic density on the signal-to-noise-ratio and the associated bit-error-rate (BER) performance of vehicle-to-vehicle visible light communication (V2V-VLC) systems. The article uses traffic data from the M42 and M6 motorways in the U.K. to investigate the probability of coexistence of other vehicles in the adjacent lanes, which induce interference and act as potential reflectors. The results show that the probability of coexistence of other vehicles in the adjacent lanes is lane-independent and it increases during the rush hours to 90%, while it decays to less than 10% during the off-peak and early morning hours. The intervehicular distance and the BER performance vary widely between different lanes and different periods of the day. The results also show that the BER performance of V2V-VLC system with non-line-of-sight (NLOS) component and with LOS component are comparable at rush hours. However, high BER values are predicted during the off-peak hours for NLOS components of the channel.

This paper proposes a simple method to determine the laser beam wandering and scattering in free space optics. The main application of this method is to determine the dimensions of a device required to compensate for beam wandering and scattering due to the air turbulences in FSO communications. The method uses color image analysis for automatic determination of the shape of the laser spot, which provides information about the spot area and the maximum amplitude of beam wandering. Results show that when the spot is photographed on a non-colored paper screen, the method is able to determine the laser spot shape accurately. Moreover, the method is robust to the variation of the screen background illumination.

A fast Ethernet 100BASE-TX FSO link performance is evaluated within a dedicated indoor atmospheric chamber performing BER of

The optical power penalty (OPP) due to the artificial light interferences (ALIs) can be significantly high in an indoor optical wireless communication (OWC) channel making such link practically infeasible. A discrete wavelet transform (DWT) is an effective technique in reducing the ALI effects. The DWT has the advantage over the high pass filtering (HPF) to reduce ALI in terms of complexity and performance. In this paper, a comprehensive study of the DWT based denoising for the on-off keying (OOK), pulse position modulation (PPM) and digital pulse interval modulation (DPIM) is provided. The OPPs due to ALIs and DWT based denoising for these modulation techniques are presented.

The field of visible light communications (VLC) has gained significant interest over the last decade, in both fibre and free-space embodiments. In fibre systems, the availability of low cost plastic optical fibre (POF) that is compatible with visible data communications has been a key enabler. In free-space applications, the availability of hundreds of THz of the unregulated spectrum makes VLC attractive for wireless communications. This paper provides an overview of the recent developments in VLC systems based on gallium nitride (GaN) light-emitting diodes (LEDs), covering aspects from sources to systems. The state-of-the-art technology enabling bandwidth of GaN LEDs in the range of >400 MHz is explored. Furthermore, advances in key technologies, including advanced modulation, equalisation, and multiplexing that have enabled free-space VLC data rates beyond 10 Gb/s are also outlined.

Comparing with the existing incandescent, light-emitting diodes (LEDs) offer higher power efficiency, higher brightness, longer lifetime, and have a fast dynamic response in the order of a few megahertz. LEDs are recently expected to be utilised for the next generation indoor optical wireless communication (OWC) system. In this paper, we present a mathematical design model as well as a practical measurement for an indoor diffuse cellular visible light communication (VLC) system. It operates at a date rate of 5 Mb/s using the on-off keying non-return-to-zero (OOK-NRZ) modulation format. Using commercially available luminit holographic light shaping diffusers (LSD), we show that the achieved distributions of received power and the Q-factor are more uniform. The range and coverage area of the cellular link are therefore significantly extended.