Optical Communications in Light Signals - Essay Example

Introduction Introduction 1. Optical Communications Optical communication represents the transfer of information in the form of light signals. There are transmitters that transfer the desired communication message and this communication takes place through optical fibres. The major elements necessary to allow this communication include a modulator & demodulator, a transmitter & receiver for transmitting and receiving the communicated message, and a channel for the transfer of the data (Optical Communication, 2014). The information or signal that is passed through optical communications can take place in both analog and digital formats. With this the signal is converted into a form which is compatible with the system. For analog form, A/D converter is used for the transmission of the signal, and for digital form, such a block is not required if data already exists in the digital form (Bagad and Dhotre, 2009, p.4-26). Particular coding schemes are used in optical communications, such as that include PCM, PPM, Digital PPM, Multiple PPM, PIM, Dicode PPM. PCM – PCM or pulse-code modulation is a method based on digital encoding and are effectively used in optical fibre communications. The baseband video signal in the system is quantified and sampled. There is a series of bits that encodes the sample. The light is put on and off using binary signals. The bandwidth is required to be enhanced over the baseband method for its use. This is a good choice for optical communications, particularly when laser diode has to be utilized owing to presence of inherent nonlinearity in the source of the optics (Bandyopadhyay, 2014, p.314). PPM – PPM or pulse position modulation is used for optical communications where the code scheme involves a bit time divided into two slots. Data can either be 1 or 0. If it is 1, the pulse gets transmitted in the first bit time slot. If the data is 0, then the pulse gets generated in the second slot of the bit time. Expansion of PPM is also possible and proves to be efficient for optical communications (Alexander, 1997, p.228). Digital PPM – PPM can also be used digital transmission of information based on optical communications. It is also referred to as differential pulse position modulation. It is a variation of the PPM coding, which can transmit data irrespective of the presence of time. If delays occur in the transmission, they take reference from falling edge of the pulse that was earlier transmitted (Lazaridis, 2011). Multiple PPM and Dicode PPM – Dicode PPM and Multiple PPM are considered as the most efficient bandwidths for optical communications. The best forms of sensitivities are offered by these PPM without causing the large bandwidth of the system to increase. However multiple PPM can be highly time consuming but provides a noble problem solution in optical communications (Nikolaidis, 2008). PIM – PIM or pulse interval modulation forms a part of anisochronous PTM techniques, the coding schemes of which have a fixed structure for their symbol. PTM or pulse time modulation represents a radio transmission where modulation of the carrier results in production of several pulses allowing transmission of the amplitude and signal pitches (Pulse-time modulation, 2014). Thus there are variations in the length of the symbol and it can be measured from the content or information that is carried by the symbol. Thus it enables effective transmission of the signals in optical communications (Herceg, Svedek and Matic, 2010). 1.2. Error Correction Digital electronic data storage systems are all the more used in the recent times where errors can also take place and hence error correction methods are also essential. A most widely developed mechanism includes error detection and correction coding theory. Detection and recovery is essential for digital systems, hence the mechanisms being developed. The codes are varied depending on the capabilities of corrections required, the efficiency of the codes, and the level of complexity associated with the coding and decoding methods (Schrimpf and Fleetwood, 2004, p.60). The first class of linear block codes that were developed for error correction purposes include the Hamming codes. Digital communications and data storage systems have widely made use of these Hamming codes owing to their availability of variations. Single correction and double bit detection was possible to be provided by the Hamming codes. While error correction methods are devised, it has to be kept note of that the processes of coding are significantly complex in nature (Schrimpf and Fleetwood, 2004, p.61). Reed Solomon error correction codes are used for the purpose of correcting errors in digital systems. The Reed Solomon codes mainly target the errors that occur in optical recording systems (McDaniel and Victora, 1995, p.816). Reed Solomon Codes represent BCH codes, their length requiring larger sizes of the field. But the codes have significant importance, in error correction as well. These codes are used by compact disc players effectively for error correction needs (Betten et al, 2006, p.249). Based on blocks, the Reed Solomon codes are used in several applications that include digital communications and data storages. The following are some of the most effective applications of Reed Solomon codes (Reed-Solomon Codes, 1998): Devices for storing information – these generally devices such as tapes, compact disks, DVDs, barcodes, and other such devices Communication links or devices that are mobile or wireless such as mobile phones, microwave, and so on Devices and networks involved in satellite data transmissions 1.3. Design Automation Design Automation (DA) is the method by which steps in the design of integrated circuits can be automated. VHDL represents very high speed integrated circuit describing a hardware description language – HDL. The purpose of creating VHDL was to make hardware designs portable for use and VHDL could be used as a stimulator and developed for this purpose. VHDL is a tool for design-automation. Model digital systems can take advantage of this tool at their different levels of generalisation and substantiation of functionality. Test data that is generated can also be verified using the design automation tool. Based on such verification, the hardware can be installed in the systems (Kaur, 2011, pp.6-7). Another effective design automation tool is the FPGA or field programmable gate array design automation. Embedded processors are offered by manufacturers of FPGA which can be customized for interfacing with the logic fabric of FPGA. The two types of FPGA embedded processors are a soft embedded processor and a hard embedded processor. The soft processors have features of configuration including caches and registered sizes of files, blocks of RAM/ROM, and necessary instructions for customization. Availability of these processors is in the form of description language. And the hard processors are embedded on the FPGA in a dedicated form of hardware (Alpert, Mehta and Sapatnekar, 2008, p.953). References Alexander, S.B. (1997). Optical Communication Receiver Design. Bellingham: SPIE Press. Alpert, C.J., Mehta, D.P. and S.S. Sapatnekar (2008). Handbook of Algorithms for Physical Design Automation. Florida: CRC Press. Bagad, V.S. and I.A. Dhotre (2009). Data Communication Systems. India: Technical Publications. Bandyopadhyay, M.N. (2014). OPTICAL COMMUNICATION AND NETWORKS (Google eBook). India: PHI Learning Pvt. Ltd. Betten, A. et al (2006). Error-Correcting Linear Codes: Classification by Isometry and Applications (Google eBook). Berlin: Springer Science & Business Media. Herceg, M., Svedek, T. and T. Matic (2010) Pulse Interval Modulation for Ultra-High Speed IR-UWB Communications Systems. EURASIP Journal on Advances in Signal Processing. Kaur, G. (2011). VHDL: Basics to Programming. India: Pearson Education India. Lazaridis, G. (2011). Pulse Position Modulation and Differential PPM. Pcbheaven. [Online]. Retrieved on 26 September 2014 from: http://www.pcbheaven.com/wikipages/Pulse_Position_Modulation/ McDaniel, T.W. and R. Victora (1995). Handbook of Magneto-Optical Data Recording: Materials, Subsystems, Techniques (Google eBook). Amsterdam: Elsevier. Nikolaidis, K. (2008). An Investigation of an Optical Multiple Pulse Position Modulation Link over a Dispersive Optical Channel. Doctoral thesis, University of Huddersfield. Optical Communication (2014). Techopedia. [Online]. Retrieved on 26 September 2014 from: http://www.techopedia.com/definition/24942/optical-communication Pulse-time modulation (2014). Reference. [Online]. Retrieved on 29 September 2014 from: http://dictionary.reference.com/browse/pulse-time+modulation Reed-Solomon Codes (1998). CMU. [Online]. Retrieved on 26 September 2014 from: http://www.cs.cmu.edu/~guyb/realworld/reedsolomon/reed_solomon_codes.html Schrimpf, R.D. and D.M. Fleetwood (2004). Radiation Effects and Soft Errors in Integrated Circuits and Electronic Devices. Singapore: World Scientific. Read More