Non-Concatenated FEC Codes for Ultra-High Speed Optical Transport Networks

Abstract

This paper presents a non-concatenated forward error correction (FEC) code suitable for applications in 100Gb/s optical transport networks (OTN). A typical requirement in this application is a net coding gain (NCG) >;10 dB at a bit error rate (BER) of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> with an overhead (OH) of ~20%. As discussed in [1], non-concatenated codes are the ultimate frontier in terms of performance for OTN applications, because of their superior performance, lower latency, and lower overhead than concatenated codes. However, a major stumbling block for the use of these codes has been the existence of BER floors at levels significantly higher than the required 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> (typically 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> ). In this paper we present a new coding scheme based on a low density parity check (LDPC) code with an expected net coding gain of 11.30dB at 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> , 20% OH, and a block size of 24576 bits. This represents a significant improvement over the previous state of the art [2], based on a concatenated code with a block size of 74844 bits and 20.5% OH. The code is designed to minimize the BER floor while simultaneously reducing the memory requirements and the interconnection complexity of the iterative decoder [3]. Experimental results obtained with an FPGA-based hardware emulator demonstrate an NCG of 10.70 dB at a BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-13</sup> and no error floors. These experimental results are extrapolated to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> using importance sampling techniques, resulting in the expected performance stated above. Moreover, we find that fixed-point implementation is the main cause of error floors below 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-13</sup> Based on this finding, we introduce a new low complexity postprocessing technique to push BER floors down to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> .