eng Science and Education Publishing American Journal of Electrical and Electronic Engineering 2328-7357 2019-09-20 7 4 94 98 10.12691/ajeee-7-4-2 AJEEE2019742 article Arun Agarwal arunagrawal@soa.ac.in 1 Kabita Agarwal 2 Gourav Misra 3 Subrat Kumar Panda 4 Department of ECE, Institute of Technical Education & Research (ITER), Siksha‘O’ Anusandhan Deemed to be University, Khandagiri Square, Bhubaneswar-751030, Odisha, India Department of Electronics and Instrumentation Engineering, College of Engineering & Technology (CET), Ghatikia, Kalinga Nagar, Bhubaneswar-751003, Odisha, India School of Electronic Engineering, Dublin City University,?Glasnevin, Dublin 9, Ireland Department of ECE, Gandhi Institute For Technology (GIFT), Gramadiha, PO-Gangapada, Bhubaneswar-752054, Odisha, India This paper reviews the concept of coherent optical systems as future digital optical communication systems having capacity of achieving 100Gbps speed. While the transmission capacity increases in wavelength-division multiplexed (WDM) system, coherent technologies have been in large interest in recent years. The interest lies in finding methods of increasing the bandwidth demand with multilevel modulation formats based on coherent technologies. We also discuss about requirements and challenges in implementation of respective digital receivers and associated signal processing. http://pubs.sciepub.com/ajeee/7/4/2/ajeee-7-4-2.pdf optical systems WDM coherent bandwidth signal processing digital receivers