International Journal of Optoelectronic Engineering
2012; 2(4): 1825
doi: 10.5923/j.ijoe.20120204.01
T. H. Lotz , W. SauerGreff , R. Urbansky
University of Kaiserslautern, Department of Electrical and Computer Engineering, Kaiserslautern, 67663, Germany
Correspondence to: T. H. Lotz , University of Kaiserslautern, Department of Electrical and Computer Engineering, Kaiserslautern, 67663, Germany.
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Copyright © 2012 Scientific & Academic Publishing. All Rights Reserved.
Achieving high spectral efficiency in optical transmissions has recently attracted much attention, aiming to satisfy the ever increasing demand for high data rates in optical fiber communications. Therefore, strong Forward Error Correction (FEC) coding in combination with multilevel modulation schemes is mandatory to approach the channel capacity of the transmission link. In this paper we give design rules on the joint optimization of coding and signal constellations under practical considerations. We give tradeoffs between spectral efficiency and hardware complexity, by comparing coding schemes using capacity achieving constellations with bitinterleaved coded modulation and iterative decoding (BICMID) against applying conventional square quadrature amplitude modulation (QAM) constellations but employing powerful low complexity lowdensity paritycheck (LDPC) codes. Both schemes are suitable for optical single carrier (SC) and optical orthogonal frequencydivision multiplexing (OFDM) transmission systems, where we consider the latter one in this paper, due to wellstudied equalization techniques in wireless communications. We numerically study the performance of different coded modulation formats in optical OFDM transmission, showing that for a fiber optical transmission link of 960 km reach the net spectral efficiency can be increased by ≈0.4 bit/s/Hz to 8.61 bit/s/Hz at a post FEC BER of <10^{15} by using coded optimized constellations instead of coded 64QAM.In this paper we propose a high spectral efficient coded modulation scheme for implementation in future optical communication systems operating at data rates beyond 400 Gb/s. In detail, we adapt the “Turbo Principle” to BICMID[8] and combine it with a highrate outer algebraic code to obtain a postFEC BER <10^{15}, which is a typical demand in optical transponders. Furthermore we give simple design principles for the design of BICMID based on the extrinsic information transfer (EXIT) chart analysis[9]. The optical channel is considered to be weaklynonlinear. Therefore the proposed techniques are also applicable for singlecarrier transmission; however we consider OFDM since it appears to be more appropriate for the high order modulation formats and efficient equalization algorithms that are well established in wireless communications.
Keywords: Coherent Communications, Optical OFDM, BitInterleaved Coded Modulation and Iterative BICMID, Spectral Efficiency, Capacity Approaching Constellations (IPM)
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Figure 1. The 64IPM signal constellation 
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Figure 2. Channel capacities for different IPM and square QAM modulations 
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Figure 3. The optical OFDM communication system 
Figure 4. Capacities of different modulation formats after 960 km SSMF transmission 
Figure 5. Proposed concatenated iterative softdemapping and decoding scheme 
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Figure 6. EXIT chart for the designed 64IPM signal constellation and RSC code at a fiber perchannel launch power of 7 dBm after 960 km SSMF transmission 
Figure 7. BER vs. perchannel input power at the decoder output of the designed coded 64IPM scheme after 960 km SSMF transmission 
Figure 8. EXIT chart for the designed 64QAM signal constellation and LDPC code at a fiber perchannel launch power of 7 dBm after 960 km SSMF transmission 
Figure 9. BER vs. perchannel input power at the decoder output of the designed coded 64QAM scheme after 960 km SSMF transmission 
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