Modulation Schemes

Nos dias de hoje, o uso de satelites e essencial na vida quotidiana e afeta quase todos os campos de atividade da sociedade. Na Defesa, as comunicacoes por satelite tem um papel preponderante: a possibilidade de obter informacao sobre as atividades do inimigo e sobre o teatro de operacoes em tempo real, e vital para o sucesso de qualquer missao. A maioria das ligacoes intersatelites, e entre os satelites e a estacao de base, usa a banda de radiofrequencia (RF). As comunicacoes oticas sao uma tecnologia emergente que oferece, entre outras vantagens, maior largura de banda, que e necessaria para poder  transmitir um volume crescente de dados. Este facto representa uma mais-valia significativa para as operacoes militares. O futuro das comunicacoes por satelite esta pois, dependente dos desenvolvimentos nos sistemas de comunicacao oticos. Neste artigo, o diagrama de blocos e as principais caracteristicas destes sistemas serao apresentadas e analisadas.

This paper considers the design methodology and the modulation of the quasi Z-source modular multilevel converter (qZS-MMC) with half bridge sub-modules and evaluates its performance in voltage boosting mode for medium voltage applications. The qZS-MMC consists of two quasi Z-source networks inserted between the two terminals of the DC input source and the DC-link terminals of a modular multilevel converter (MMC), which allows the generation of an output voltage larger than the input DC voltage. Two modulation schemes have been analysed based on a mathematical derivation for the converter internal voltages, currents, and stored energy. The quasi Z-source circuit is proven to provide the qZS-MMC with half bridge sub-modules to deal with DC-faults. The experimental results validate the performance of the proposed modulation schemes and the DC-fault blocking capability of the qZS MMC. Finally, the losses of the qZS-MMC is compared against a standard MMC using full bridge sub-modules that can also provide DC fault capability. The range in which the qZS-MMC is more efficient has been identified. Furthermore, the qZS-MMC can provide a significant reduction in number of semiconductor power devices with the same performance.

Nos dias de hoje, o uso de satelites e essencial na vida quotidiana e afeta quase todos os campos de atividade da sociedade. Na Defesa, as comunicacoes por satelite tem um papel preponderante: a possibilidade de obter informacao sobre as atividades do inimigo e sobre o teatro de operacoes em tempo real, e vital para o sucesso de qualquer missao. A maioria das ligacoes intersatelites, e entre os satelites e a estacao de base, usa a banda de radiofrequencia (RF). As comunicacoes oticas sao uma tecnologia emergente que oferece, entre outras vantagens, maior largura de banda, que e necessaria para poder  transmitir um volume crescente de dados. Este facto representa uma mais-valia significativa para as operacoes militares. O futuro das comunicacoes por satelite esta pois, dependente dos desenvolvimentos nos sistemas de comunicacao oticos. Neste artigo, o diagrama de blocos e as principais caracteristicas destes sistemas serao apresentadas e analisadas.

I. INTRODUCTION Satellite communication systems have evolved significantly over the last decade and currently are a key element in modern communication systems. With the growth on demand for services, particularly for mobile communication services, television and Internet, these communication systems are in continuous progress. There for, substantial investments are being made, at governmental and particular level, to respond to the ongoing developments in this area [1]. In Defense, satellite communications play a key role. On the military aspect, the ability to obtain information in time about the enemy and the theater of operations, is vital to fulfil the missions. In this kind of activities, the use of satellites allows navigation of the military forces and the achievement of images for the reconnaissance of the territory. It also provides communications, essential to coordination between forces on the battlefield, as well as to obtain updated weather information. One of the great challenges of this type of communication systems is the implementation of optical components, to establish laser links with satellites. So, the need of increasingly larger bandwidths becomes essential the development of optical communication systems, in ground and space. Although some optical communication systems are being used, its development is still at an early stage, so the large scale use is not yet a reality [1], [2], [3].

This paper presents a test bed for wireless optical LANS. This test bed is flexible, supporting multiple implementation choices. The test bed currently covers all physical layer issues, from FEC coders and LED drivers to front-ends, clock recovery circuits, Viterbi decoders, and sectored receivers. These are implemented with multiple technologies, with DSPs and FPGAs for digital functions, and ASICs and discrete electronics for the analogue electronics. Furthermore, the test bed provides a semi-controlled real life test environment, allowing for close comparison between theoretical and practical results.

This paper examines advanced modulation coding schemes for an optical transceiver systems–based optical wireless communication (OWC) channel model. These modulation techniquesinclude On-Off keying and return to zero (RZ)/non–return to zero (NRZ) coding. The signal power level against time and frequency spectral variations are measured. The max. Q factor and min. bit error rate (BER) are estimated and clarified for each modulation code scheme by using an optisystem simulation model. Transmission bit rates of up to 40 Gb/s can be achieved for possible distances up to 500 km with acceptable Q factor. The received power and max. Q factor are measured and clarified with OWC distance variations. The On-Off keying modulation code scheme resulted in better performance than the other modulation code schemes did.

This paper deals with the design and implementation aspects of high data rate digital demodulators. The Existing remote sensing satellites support data rates of several hundred mega bits per second. The future trend is towards giga bit rate transmission. This necessitates for demodulators of the ground receive system to process faster and handle the ever-rising data throughput more efficiently. Different Satellites use different modulation schemes with variable data rates. In order to cater to the Multi mission /Multisatellite data reception requirements of a ground station, it is necessary to have greater flexibility and programmability features embedded in the design of demodulators. The demodulation techniques for Binary / Quadrature Phase shift Keying (BPSK/ QPSK) are well established and understood when implemented with analog circuits. The BPSK/ QPSK can be demodulated by different techniques such as squaring loop, Costas loop and others in analog domain. The Costas loop technique is adopted for developing the digital demodulator because unlike in Square Loop technique, in this the carrier recovery and data demodulation can be done simultaneously with simple blocks level design. The high data rate digital demodulator performs IF amplification, filtering and analog to digital conversion of the received IF signal followed by a Digital demodulator. The basic design strategy includes a configurable data rate BPSK/ QPSK demodulation with COSTAS loop circuitry utilizing the flexibility of FPGA implementation. The basic design considering a sampling clock from local clock oscillator operating at 125 MHz and 70 MHz carrier down converted to 30 MHz for 8 Mbps data rate (For BPSK). Later the sampling clock is increased to 250 MHz and the carrier is direct 70 MHz with data rate 42.4456 Mbps (For QPSK). The Performance of the Demodulator is evaluated using MATLAB simulation tools. The development has done with ISE implementation tools. The purpose of this paper is to evaluate the new technology by implementing a BPSK/QPSK demodulator on an ADC-FPGA board. A mathematical algorithm was developed and implemented with ISE tools for digital demodulator design. The input test signals from Modulation Simulators and signal generators have been interfaced to the FPGA board through Analog-To-Digital Converter (ADC). The recovered carrier output and I, Q demodulated data patterns have been verified through ISE tools and Wave Vision Software for FFT analysis.

Free-Space Optical communications (FSO) propagated over a clear atmosphere suffers from irradiance fluctuation caused by small but random atmospheric temperature fluctuations. This results in decreased Signal-to-Noise Ratio (SNR) and consequently impaired performance. In this paper, the Bit Error Rate (BER) performance of FSO communication systems employing Binary Phase Shift Keying (BPSK) modulation is derived. To evaluate the system error performance in weak turbulence regime, the Probability Density Function (pdf) of the received irradiance after traversing the atmosphere is modeled using the log normal distribution. The effects of pointing errors on the performance of a FSO communication system are studied

The performance of wireless infrared transmission systems for indoor use is severely impaired by the noise and interference induced by natural and artificial ambient light. In order to combat the effects of ambient light on the system performance, both optical filtering and electrical filtering is usually adopted. However, even when resorting to these techniques, the optical power penalty imposed by the interference may be very large. In particular, the interference produced by fluorescent lamps driven by electronic ballasts imposes very large optical power penalties on systems operating at data rates up to a few tens of Mbps. In this paper, a different technique to overcome the penalty induced by artificial light interference is analysed. This technique explores the different optical spectra of the transmitted signal and the artificial light and the characteristics of optical filtering to cancel the interfering signal. Some aspects of its implementation are also discussed. The results obtained with this approach are shown to be much better than those obtained through electrical high-pass filtering. (*)

This document proposes and studies a novel receiver structure for indoor optical wireless communication systems that exploits the directional nature in both signal and noise propagation through the use diversity techniques. In this document we concentrate on the ability of the proposed sectored receiver in combating the ambient noise. Significant optical power gains in both diffuse and quasi-diffuse systems are demonstrated. The optical gains are seen to increase with the relative weight of the directional noise within the cell, with the sharpness of the directional noise source beam width and in environments where there are noise sources positioned outside the cell. Also the SNR of a sectored receiver is seen to be much less sensitive to the position and beam width of the noise sources than the SNR of a non-sectored receiver allowing for more universal transceiver designs. In addition a sectored receiver can be implemented with only a moderate increase in complexity. This work is being carried out as part of the ESPRIT.6892 POWER (Portable Workstation for Education in Europe) project commissioned by the CEC. Receivers for indoor optical wireless communication systems are usually based on a single optical collector that covers on its own the entire field-of-view. This is a better configuration in environments where both the signal and the noise are isotropic. However, these conditions are rarely found in practice. An isotropic signal coverage submission page 1 University of Aveiro submission page 2 University of Aveiro 'mhm;~~;nn

Optical wireless communications offer a viable alternative to radio frequency (RF) communication for indoor use and other applications where high performance links are needed. These systems use infrared technology (IR), which has significant advantages over RF. This paper presents a review of the most significant issues related to infrared communication technology, which will enable the realization of future high performance and cost-effective indoor optical wireless systems. Several possible configurations for indoor optical wireless systems, modulation, and multi-access techniques are presented as well as their advantages and limitations discussed.

We report a large area photo-diode based homodyne detector for free-space quantum coherent communication. The detector's performance is studied in terms of detection bandwidth and electronic noise for shot-noise limited quantum signal detection. Using large area photo-diodes increases signal collection efficiency from turbulent atmospheric channels, in comparison with typical fibre based free-space homodyne detectors. Under identical atmospheric turbulence and receiver aperture conditions, our homodyne detector based on 1mm diameter photo-diode experiences 0dB loss due to turbulence while a 10um fibre based detector experiences 13.5dB of signal loss over a 700km free-space link, at 90 degree elevation angle.

This paper presents a test bed for wireless optical LANS. This test bed is flexible, supporting multiple implementation choices. The test bed currently covers all physical layer issues, from FEC coders and LED drivers to front-ends, clock recovery circuits, Viterbi decoders, and sectored receivers. These are implemented with multiple technologies, with DSPs and FPGAs for digital functions, and ASICs and discrete electronics for the analogue electronics. Furthermore, the test bed provides a semi-controlled real life test environment, allowing for close comparison between theoretical and practical results.

The key challenge in free space optical (FSO) communications is combating the turbulence induced fading. As the channel fading in FSO is quasi-static, the transmission parameters such as the code rates, transmit power and modulation schemes can be modified with respect to the channel state information transmitted via the feedback path. As a result, adaptive channel coding is considered as one of the practical approaches to improve the FSO link performance. In this paper we investigate the FSO system with polar codes and analyse its performance by determining the optimum code-rate required to achieve a bit error rate of 10-9 under weak turbulence. We show that, using Monte-Carlo simulations for the scintillation indices of 0.12 and 0.2, the successive cancellation list (SCL) decoder offers coding gains of 2.5 and 0.3 dB, respectively as compared with SC decoder, and for scintillation index of 0.31, the SC decoder offers a coding gain of 2.5 dB compared to SCL decoder for the code rate.

In a multi-rate wireless network, a node can dynamically adjust its link transmission rate by switching between different modulation schemes. In the current IEEE802.11a/b/g standards, this rate adjustment is defined for unicast traffic only. In this paper, we consider a wireless mesh network where a node can dynamically adjust its link layer multicast rates to its neighbors, and address the problem of realizing low-latency network-wide broadcast in such a mesh. We first show that the multi-rate broadcast problem is significantly different from the single-rate case. We will then present an algorithm for achieving low latency broadcast in a multi-rate mesh which exploits both the wireless multicast advantage and the multi-rate nature of the network. Simulations based on current IEEE802.11 parameters show that multi-rate multicast can reduce broadcast latency by 3-5 times compared with using the lowest rate alone. In addition, we show the significance of the product of transmission rate and transmission coverage area in designing multi-rate wireless mesh networks for broadcast.

The main degrading factor of infrared communication systems is the shot noise due to ambient light. Due to the directional nature of both signal and noise, the signal-to-noise ratio (SNR) at the receiver can vary significantly. In order to minimise these SNR fluctuation effects, this paper discusses some advanced techniques for the design of non-directed wireless infrared communication systems that can produce interesting performance improvements. The first technique, angle diversity, makes use of the directional nature of ...

As per the current scenario there is lot of power shortages in all over the world. Renewable energy sources are the promising energy source among the existing resources of energy out of which solar PV is most popular due to its significant advantages over other type of renewable energy resources. Controlling the output of solar PV system is a key aspect. The main theme of this paper is to present physical modelling and simulation of solar PV system and DC-DC boost converter in SIMSCAPE library of MATLAB .the advantage of using SIMSCAPE library is that it models the system physically and the results obtained from it will be considering all the physical effect. in this paper the output of solar cell has been interfaced with the boost converter. Once the system can be modelled in SIMCAPE it can be easily implemented on hardware. So we can easily implement for real time applications.

The main degrading factor of infrared communication systems is the shot noise due to ambient light. Due to the directional nature of both signal and noise, the signal-to-noise ratio (SNR) at the receiver can vary significantly. In order to minimise these SNR fluctuation effects, this paper discusses some advanced techniques for the design of non-directed wireless infrared communication systems that can produce interesting performance improvements. The first technique, angle diversity, makes use of the directional nature of ...

Following a brief introduction reviewing various laser systems suitable for ISL, a typical link budget for an optical ISL with a Nd laser is calculated. Laser diode pumped Nd lasers are described, and several host materials are considered. Modulation formats and typical receiver performances for direct detection are examined in order to compute the minimum transmitted power required for the proposed link. Results show that laser diode pumped Nd laser systems are competitive for optical ISL.