Adaptive Modulation and Coding Using Signal to Noise Ratio Switching Threshold

Orthogonal frequency division multiplexing (OFDM) is one of the key enabling technologies for fourth generation (4G) wireless system. It offers high data rate transmission with high spectral efficiency, immunity to multipath fading and simple implementation using fast Fourier transform (FFT). However inefficient utilization of the channel will result when OFDM system is designed for worst-case channel conditions. Thus adaptive transmission scheme that can be adjusted to channel conditions is one of the techniques to improve the performance of OFDM systems. In this thesis, three types of subband adaptive transmission scheme namely adaptive modulation (AM), adaptive coding (AC) and adaptive modulation and coding (AMC) based on SNR switching threshold are investigated. The performances of these systems are evaluated using an efficient adaptation algorithm. The efficient adaptation algorithm is based on the average value of the SNR of the subcarriers in the subband. First the performance of adaptive modulation using quadrature amplitude modulation (QAM) and phase shift keying (PSK) system are evaluated. The results obtained showed that a significant improvements in terms of bit error rate (BER), spectral efficiency and throughput can be achieved. To further enhance the system, convolutional coding is employed. However convolutional coding causes the maximum throughput to be limited. To solve this problem adaptive coding schemes which provides another area of flexibility is investigated. Finally the combination of adaptive modulation and adaptive coding is examined. Simulations results have shown that the performance of adaptive transmission schemes are superior compared to fixed (nonadaptive) transmission schemes. A high throughput performance can be achieved without sacrificing the BER. The performance comparisons of the proposed system with the conservative system showed that the proposed scheme is able to meet the BER target of 10-3 with a slightly better throughput performance around 0.3 Bps/Hz

Scattering regimes for underwater optical wireless communications using Monte Carlo simulation

Optical wireless communications has shown tremendous potential for underwater applications as it can provide higher bandwidth and better security compared to acoustic technologies. In this paper,  an investigation on scattering regimes for underwater links using Monte Carlo simulation has been presented.While the focus of this paper is on diffuse links, the simulation results of collimated links is also provided for comparison purpose. Three types of water namely clear, coastal and turbid water are being used in the simulation. It is shown that the effect of scattering on the path loss cannot be accurately modeled by the existing channel model; ie. Beers-Lambert (BL) law.  It has been shown that  the distance at which the unscattered light drops to zero can be used to estimate the transition point for the scattering regimes in case of diffuse links. The transition point for diffuse links in coastal water and turbid water can be estimated to be around 22 m and 4 m respectively. Further analysis on the scattering order probability at different scattering regimes illustrates how scattering is affected by beam size, water turbidity and distance. From the frequency response plot, it is estimated that the bandwidth of several order of GHz can be achieved when the links are operating in the minimal scattering region and will reduce to several hundreds of MHz when the link is operating in multiple scattering region

Modelling and characterisation of short range underwater optical wireless communication channels

This thesis studies the modelling and characterisation of underwater optical wireless communication links, particularly short-range diffuse links, by using numerical Monte Carlo (MC) simulation. MC simulation provides a flexible, intuitive and accurate modelling of the underwater channel, which is severely affected by absorption and scattering processes. In diffuse Underwater Optical Wireless Communication (UOWC) links, scattering is expected to have a larger impact on communication link performance due to the wider beam divergence compared to collimated beams. Thus, this thesis will investigate the characterisation of path loss, spatial, temporal and angular dispersions of diffuse links in various types of water. Firstly, a detailed investigation on the path loss performance of diffuse beam in three types of water is presented. This includes the study on the contribution of unscattered and scattered components of light to the total received power and how they are attenuated. From the percentage of unscattered light that contributed to the total power reception, the distance at which the unscattered component drops to zero can be estimated. This distance is used to predict the transition point from minimal scattering to multiple scattering regime for diffuse beams in coastal and turbid water. In addition to this, the spatial dispersion effect is also studied at off-axis locations. To further understand the behaviour of scattering in diffuse links, the scattering order probability is evaluated for various beam sizes in various types of water. Currently, this kind of information cannot be obtained either analytically or experimentally. The information on the scattering order is used as the parameter to classify the links into three scattering regimes, namely minimal, intermediate and multiple scattering regimes. Further investigations into the transition regimes are conducted by investigating the impulse response and frequency response performance for temporal dispersion effects. From the impulse response and frequency response analysis, the bandwidth that can be supported by the channel can be predicted, which provides some insight into the potential and limits of the links. In addition to temporal dispersion, the angular dispersion performance is also evaluated. It is shown through the angle of arrival (AOA) distribution that diffuse beams exhibit significant angular dispersions, implying that a large receiver field of view (FOV) is needed for optimum power performance. The information on the AOA distribution is then used to study the impact of receiver FOV on the bandwidth. Finally, the effect of aperture on the power received and scattering order histogram is evaluated. As a conclusion, the numerical results presented in this thesis will provide an improved understanding of the effect of scattering on path loss, spatial, temporal and angular dispersions along with their relationships with each other

On The Performance Of Normalized Gain Difference Power Allocation For Mimonoma-Based VLC

In this study, we build a 4×4 multiple-input multiple-output (MIMO) nonorthogonal multiple access (NOMA) based visible light communication (VLC) system. The main goal of this study is to utilize the power consumption among the indoor VLC system by using normalized gain difference power allocation (NGDPA) to ensure the same achievable sum rate even for users on the system borders. The proposed system is simulated to serve 2, 3, and up to 4 users. The obtained results were compared with 4×4 MIMO using orthogonal frequency division multiple access (OFDMA) without applying NGDPA. It was found that the proposed system shows an almost constant achievable sum rate for all users regardless of their current position within the system coverage area. The results show that users on the system edge can share up to 97.32% of the maximum achievable sum rate. The relation between normalized offset and sum rate gain for 2, 3, and 4 users’ scenarios is also discussed. The findings reveal that the system using NGDPA scheme can utilize more sum rate gain with increasing the number of users. Through this study, it can be deduced that the 4×4 MIMO-NOMA-VLC system with NGDPA scheme has been performance

On The Performance Of Normalized Gain Difference Power Allocation For Mimonoma-Based VLC

In this study, we build a 4×4 multiple-input multiple-output (MIMO) nonorthogonal multiple access (NOMA) based visible light communication (VLC) system. The main goal of this study is to utilize the power consumption among the indoor VLC system by using normalized gain difference power allocation (NGDPA) to ensure the same achievable sum rate even for users on the system borders. The proposed system is simulated to serve 2, 3, and up to 4 users. The obtained results were compared with 4×4 MIMO using orthogonal frequency division multiple access (OFDMA) without applying NGDPA. It was found that the proposed system shows an almost constant achievable sum rate for all users regardless of their current position within the system coverage area. The results show that users on the system edge can share up to 97.32% of the maximum achievable sum rate. The relation between normalized offset and sum rate gain for 2, 3, and 4 users’ scenarios is also discussed. The findings reveal that the system using NGDPA scheme can utilize more sum rate gain with increasing the number of users. Through this study, it can be deduced that the 4×4 MIMO-NOMA-VLC system with NGDPA scheme has been performance

Impact of receiver field of view on underwater optical wireless communications

This paper discusses the effect of receiver field-of-view (FOV) on the power distribution and bandwidth performance of short-range diffuse line-of-sight (LOS) links. Monte Carlo simulation is used to investigate the performance of the links for the on-axis and off-axis scenarios in clear, coastal and turbid water. In both clear and coastal water, the receiver FOV has little influence on the on-axis power reception but has a significant effect on the off-axis power reception where there is an approximately 30 dB gain (off-axis) when the receiver FOV is increased from 10° to 180°. However, in turbid water receiver FOV significantly affects the power received both on-axis and off-axis, with gains of 15-18 dB for the same change in receiver FOV. In terms of bandwidth performance, the FOV only affects the on-axis bandwidth in clear and coastal water but not in turbid water

Recent advances in underwater optical wireless communications

Optical wireless communications (OWC) are being considered for use under water because sea water exhibits a window of reduced absorption in the visible spectrum, particularly between 400–550nm. Recent technology has demonstrated the ability to support mid-range links (<200m) and at high bandwidths (<1Gbps) in clear oceans. The present paper outlines the governing transmission characteristics and reviews current experimental research in underwater OWC, highlighting the importance of the local chlorophyll concentration, particulate concentration and the resultant wavelength selection. Ideal wavelengths are found to be from 430nm, which represents a deep blue colour, to beyond 550nm in areas where the chlorophyll concentration is high

Experimental study of light wave propagation for underwater Optical Wireless Communication (UOWC)

Underwater Optical Wireless Communication (UOWC) is identified as a promising technology because it offers higher bandwidth than acoustics and radio frequency techniques. This paper investigates the performance of light wave propagation for UOWC through experimental approach. An experimental set-up is developed consists of a transmitter, receiver and a glass chamber to emulate the water channel. Three types of water including clear, sea and cloudy are used to investigate their interaction with the light emitted by light emitting diode (LED) and laser diode. The geometrical loss (GL) analysis shows the white LED suffered a severe GL (GL<<1) as the transmission link increases due to the wide viewing angle while green and yellow LED obtained an equal GL due to the same size of viewing angle. However, red laser does not experience any GL. Therefore, the received power by red laser is 35% higher than by green LED. The analysis deduces that the estimated attenuation coefficient c had an increase of 15% and 55% for green LED and red laser respectively when the UOWC medium changed from clear water to sea water. This study contributes to identify the potential and limitations of different parameter design in order to optimize UOWC performance