An Experimental Demonstration of Rate-Adaptation Using Shaped Polar Codes for Flexible Optical Networks

Abstract

In this paper, we propose a polar-coded transmission system with adjustable data rate for coherent optical transmission employing quadrature amplitude modulation (QAM). The proposed system is based on a many-to-one constellation shaping method for achieving a range of data rates with arbitrarily small rate steps. An implicitly punctured polar-coded modulation system is then designed by combining polar coded bit-interleaved coded modulation system (BICM) with many-to-one labellings. The scheme is experimentally evaluated in a wavelength division multiplexed (WDM) system with five carriers modulated at 16 GBaud with polarization multiplexed (PM) 256QAM. Data rates ranging from 121 Gbps to 182 Gbps per carrier are experimentally demonstrated and the system can be directly extended to achieve higher data rates. Synchronization and equalization of PM-256QAM received symbols is performed with a pilot rate of 5%. The experimental results show 1.2 dB of shaping gain over the conventionally punctured polar codes in the optical back-to-back scenario. The maximum transmission system reach is increased by 200 km to 400 km w.r.t the conventionally punctured polar codes. It is shown that the rate adaptation does not require a change of modulation format and/or underlying forward error-correction (FEC) code. Finally, the performance of all 5 WDM channels is validated for the optimal input power.

Probabilistically Shaped Rate-Adaptive Polar-Coded 256-QAM WDM Optical Transmission System

“…It can be seen that the punctured polar codes perform about 1.2 dB worse than the non-punctured polar codes at η = 5 bits/symbol for the fixed BER. This puncturing loss can be offseted by the combination of polar codes and many-to-one mappings as proposed in [25]. The shaping gains with the proposed scheme in this work are improved to about 1.7 dB at fixed BER compared to the punctured polar codes.…”

“…The pre-set bits are carefully placed in the mapping function so as to eliminate the effect of the ambiguities caused by the many-to-one mappings. The pre-set bits are known at the receiver, which leads to higher shaping gains than reported in our previous work [25]. In this work, we show by simulations and experimental results that the shaping with pre-set bits offsets the puncturing loss as well as providing shaping gains compared to the non-punctured polar codes.…”

“…We studied the performance of 256 QAM with non-punctured polar codes, the proposed scheme with pre-set bits and punctured polar codes. Punctured polar codes are used as a benchmark since they are the standard approach to rate-adaptivity, which, however, incurs performance loss [25]. Various input data rates can be achieved by controlling the FEC code rate.…”

“…In puncturing, the log-likelihood ratios (LLRs) of punctured bits, called incapable bits, are set to zero at the decoder, while in shortening the LLR of shortened bits, called over-capable bits, are set to infinitely high values at decoder. However, both methods come at the cost of increased bit-error rate (BER) [25] due to the incapable or over-capable bits in the decoder. As a part of our previous work [25], we experimentally demonstrated how many-to-one PS could be used to offset the performance loss related to puncturing of polar codes.…”

“…However, both methods come at the cost of increased bit-error rate (BER) [25] due to the incapable or over-capable bits in the decoder. As a part of our previous work [25], we experimentally demonstrated how many-to-one PS could be used to offset the performance loss related to puncturing of polar codes.…”