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Low Complexity, Parallel Algorithms, and Scalable Architectures for Real Time Coherent Optical OFDM Systems

Pramod Udupa 1
1 CAIRN - Energy Efficient Computing ArchItectures with Embedded Reconfigurable Resources
Inria Rennes – Bretagne Atlantique , IRISA-D3 - ARCHITECTURE
Abstract : Coherent Optical-OFDM (CO-OFDM) communication system is built on most advanced techniques for detection, modulation and dispersion compensation viz., coherent detection, orthogonal multi-carrier modulation (OFDM) and electronic dispersion compensation (EDC). The re-emergence of coherent detection in optical communication systems was made possible by the advancement in very high rate digital circuits. Coherent detection (CoD) has higher sensitivity for signal detection compared to direct detection (DD) methods. It enables use of dual-polarization transmission and it preserves phase information of optical signal and passes it to electrical domain. The use of OFDM modulation provides significant flexibility and efficient use of allocated bandwidth. Due to availability of phase information in digital domain, low cost digital signal processing (DSP) processors can be used for dispersion compensation in digital domain, which makes the solution flexible and reconfigurable. But, the introduction of CO-OFDM system in place of older intensity modulation-direct detection (IM-DD) system significantly increases the cost of the system, i.e. higher number of optical components and higher amount of electronic resources are required for reception of the signal. Due to increase of resources both in optical and electronic domain, it is justifiable for only long-range transmission distances. The choice of algorithm, architecture and fixed-point optimization play a significant role in reduction of electronic resources required for realization of CO-OFDM systems. In this thesis, low-complexity algorithms and architectures for CO-OFDM systems are explored. First, low-complexity algorithms for estimation of timing and carrier frequency offset (CFO) in dispersive channel are studied. A novel low-complexity timing synchronization algorithm, which can withstand large amount of dispersive delay, is proposed and compared with previous proposals. Then, the problem of realization of low-complexity parallel architecture is studied. A generalized scalable parallel architecture, which can be used to realize any auto-correlation algorithm, is proposed. It is then extended to handle multiple parallel samples from ADC and provide outputs, which can match the input ADC rate. The scalability of the architecture for higher number of parallel outputs and different kinds of auto-correlation algorithms is explored. An algorithm-architecture approach is then applied to the entire CO-OFDM transceiver chain. At the transmitter side, radix-22 algorithm for IFFT is chosen and parallel Multipath Delay Commutator (MDC) Feed-forward (FF) architecture is designed which consumes lesser resources compared to MDC FF architectures of radix-2/4. At the receiver side, efficient algorithm for Integer CFO estimation is adopted and efficiently realized with- out the use of complex multipliers. Reduction in complexity is achieved due to efficient architectures for timing synchronization, FFT and Integer CFO estimation. Fixed-point analysis for the entire transceiver chain is done to find fixed-point sensitive blocks, which affect bit error rate (BER) significantly. The algorithms proposed are validated using optical experiments by the help of arbitrary waveform generator (AWG) at the transmitter and digital storage oscilloscope (DSO) and Matlab at the receiver. BER plots are used to show the validity of the system built. Hardware implementation of the proposed synchronization algorithm is validated using real-time FPGA platform.
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Submitted on : Thursday, January 8, 2015 - 12:04:19 PM
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  • HAL Id : tel-01099824, version 1


Pramod Udupa. Low Complexity, Parallel Algorithms, and Scalable Architectures for Real Time Coherent Optical OFDM Systems. Signal and Image processing. Université de Rennes 1, 2014. English. ⟨tel-01099824⟩



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