Sujan Rajbhandari

Performance Analysis of Ethernet/Fast-Ethernet Free Space Optical Communications in a Controlled Weak Turbulence Condition

The paper experimentally investigates the performance of the Ethernet and Fast-Ethernet free space optical (FSO) communications link under the influence of the weak atmospheric turbulence. A dedicated experimental indoor atmospheric chamber is used to generate and control the turbulence conditions that affect the FSO link performance. The chamber is calibrated and the measured data are verified with the theoretical prediction. We also demonstrate methods to control the turbulence levels and determine the equivalence between the indoor and outdoor FSO links. It is also observed that the connectivity of Ethernet and Fast-Ethernet links are highly sensitive to atmospheric conditions.

Exploiting Equalization Techniques for Improving Data Rates in Organic Optoelectronic Devices for Visible Light Communications

This paper presents the use of equalization techniques in visible light communication (VLC) systems in order to increase the data rate. Here we investigate two VLC links a silicon (Si) light emitting diode (LED) and an organic photodetector (OPD), and an organic LED (OLED) plus an Si photodetector (PD), together with three equalization schemes of an RC high pass equalizer, a fractionally spaced zero-forcing equalizer (ZF) and an artificial neural network (ANN). In addition we utilize a pre-distortion scheme to enhance the performance of the digital equalizers. For both systems the bit rate achieved are 750 kb/s from a raw bandwidth (BW) of 30 kHz and 550 kb/s from a raw BW of 93 kHz.

Coherent Heterodyne Multilevel Polarization Shift Keying With Spatial Diversity in a Free-Space Optical Turbulence Channel

This paper proposes a coherent multilevel polarization shift keying (MPOLSK) modulation scheme using the spatial diversity detection in a free-space optical (FSO) turbulence channel. The symbol error probability (SEP) analysis for the proposed MPOLSK system under various turbulence regimes has been carried out. The power penalty caused by the phase tracking error has been investigated. For instance, the error floors of 1.3×10-5 and 3.4×10-3 are observed for 8POLSK with the phase tracking variances of σΔ = 0.3 and 0.5, respectively. The maximum ratio combining technique has been applied in the MPOLSK receiver in order to improve the SEP performance. To achieve a SEP of 10-9, the spatial diversity gains required for a 16POLSK-FSO employing four detectors are ~5.8, ~15.8, and ~13.7 dB in weak, moderate, and strong turbulence regimes, respectively.

Ethernet FSO Communications Link Performance Study Under a Controlled Fog Environment

In this letter the performance of a free space optical (FSO) communication link in the presence of fog is experimentally investigated in an indoor environment. A dedicated indoor atmospheric chamber, replicating the outdoor environment, is being used to evaluate the FSO link performance under the fog condition. Theoretical analysis supported by the experimental evaluation has been carried out for the intensity modulation/direct detection on-off keying non-return-to-zero (OOK-NRZ), OOK return-to-zero (OOK-RZ) and four pulse position modulation (4-PPM) modulation formats at the Ethernet baseline data-rate. The results shown indicate that the 4-PPM signalling scheme is the most robust to the fog attenuation due to its high peak-to-average power ratio. Moreover, results also show the potential of the indoor atmospheric chamber to replicate outdoor atmospheric effects (such as fog) and be able to quickly characterise and assess the FSO system under a controlled environment.

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.

A synopsis of modulation techniques for wireless infrared communication

A number of modulation techniques have been proposed and thoroughly analysed in literature for optical wireless communication systems. Each modulation technique has its unique attractive features as well as its challenges. Some are very simple to implement and bandwidth efficient like the on-off keying (OOK). Pulse interval modulation (PIM) techniques are reputed for their inherent synchronisation pulse, subcarrier modulation offers increased throughput, resilience to the inter-symbol interference (ISI) and immunity against the fluorescent-light noise near DC, while pulse position modulation (PPM) provides the unparalleled power efficiency in line of sight (LOS) links but the performance degrades severely in dispersed communication channel. There has been an enormous work on the analysis of these and many more modulation techniques under different channel and environmental conditions; we however present here a concise synopsis of the mostly reported wireless infrared modulation techniques.

Experimental investigation of polarisation modulated free space optical communication with direct detection in a turbulence channel

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.

Performance of BPSK Subcarrier Intensity Modulation Free-Space Optical Communications using a Log-normal Atmospheric Turbulence Model

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.

Performance analysis for 180° receiver in visible light communications

The paper presents the concept, design and analysis for a visible light communications receiver which has a full 180° perception angle thus offering the guard against blocking and enhancing the system mobility. The proposed receiver includes seven sub-receivers with the proposed geometrical shape investigated and analyzed in MATLAB® for evaluating the received power, RMS delay spread and corresponding signal to noise ratio. The results show that the receiver is fully mobile within the test area and can handle data rates far greater than that offered by commercially available LEDs for BER ≥ 10-6.

Analysis of fog and smoke attenuation in a free space optical communication link under controlled laboratory conditions

In this paper, the attenuation of free space optical (FSO) communication systems operating at selected wavelengths of 0.83 μm, 1.31 μm and 1.55 μm in the controlled laboratory based fog and smoke environments are compared. The fog and smoke are generated and controlled homogeneously along the indoor atmospheric chamber of length 6 m. The experimental results from the selected wavelengths are compared for the continuous spectrum range (0.6 μm <; λ <; 1.55 μm) attenuation. The results show that, unlike the fog attenuation, the resultant smoke attenuation linearly decreases from the visible towards the near infrared (NIR) range from dense to light smoke conditions.