Rate-adaptive Concatenated Forward Error Correction for High Throughput Communications

A novel flexible forward error correction (FEC) architecture for high throughput applications is proposed. We proposed a framework to design a coded modulation (CM) scheme that offers two degrees of freedom to control the rate-adaptivity, performance, and decoding complexity of the system. The concatenated FEC (CFEC) architecture comprises an inner, soft-decision (SD), and rate-adaptive polar code, serially concatenated to an outer hard-decision (HD) staircase or zipper code. The task of an inner SD code is to reduce the bit error rate (BER) below the threshold of an outer HD code. The outer code is responsible for correcting the remaining errors and bringing BER below 10−15, as needed, for example, in optical communication systems. The inner polar code in the bit-interleaved coded modulation (BICM) based concatenated coding scheme is optimized to achieve rate flexibility with nearcontinuous granularity. A method is developed to optimize the frozen set of the inner polar code according to the BER threshold of an outer HD code.
It is shown that the BICM scheme offers seamless rate-adaptivity, and better performance is achieved compared to the existing fixed rate CFEC standard. A complexity-reduced parallel decoding-based multilevel coding (MLC) architecture is developed. The inner code structure is re-designed so that a portion of bits bypass the inner code and are assigned to the high-reliability bit-levels higher-order modulation symbols. This clever design provides rate flexibility without changing the underlying code design and modulation format, and it applies to other inner SD and outer HD codes. Experimental data and simulations over the channel show that the MLC scheme has a better performance and complexity than the previous CFEC designs. Finally, an MLC-based concatenated coding scheme with fixed decoding complexity is proposed. A method is developed to optimize the multi-stage decodingbased MLC scheme that utilizes a fixed number of inner decodings while changing the data rate with two degrees of freedom. It is shown that the optimized technique can achieve fixed decoding complexity for various outer HD codes. Over the range of data rates, code designs for a complexity-optimized MLC scheme with outer staircase and zipper code are shown to utilize 75% and 73%.
fewer inner decodings, respectively, compared to the BICM scheme. Compared
to the previously existing MLC schemes, the proposed optimized scheme simultaneously requires up to 40% fewer inner decodings operations and offers up to
0.72 dB coding gain.