An adaptive polar coded probabilistic shaping scheme for free-space optical (FSO) communication is proposed and demonstrated to improve transmission performance. The input distribution can be adaptively adjusted by maximizing the achievable rate under different FSO turbulence channels. Furthermore, the transmission rate is less than the achievable rate to guarantee reliable transmission. In different FSO turbulent channels, the feasible regions of the optimization problem are explored with the aid of the exponential distribution, and the complexity of the single optimization is decreased to ( ) . The systematic polar codes are adopted to combat the fading, and a sparse-dense encoder for polar codes is designed to maintain the target distribution. The achievable rate and block error rate (BLER) performance of this adaptive probabilistic shaping scheme are evaluated. The results indicate that the adaptive nonuniform distribution generated by the proposed scheme outperforms the uniform distribution under the same channel conditions. The code rate of 0.8 is more suitable than 0.9 in coping with various turbulence channels. The proposed scheme can obtain a range of 0.50~0.90dB shaping gains over the uniform distribution from weak to moderate turbulence channels for both the achievable rate and the BLER performance at 1.5 bits per channel use.
A wideband multipath self-interference cancellation (SIC) system employing both dual-drive Mach–Zehnder modulator-based analog SIC and least mean square (LMS) algorithm-based pre-adaptive filter digital SIC is proposed and demonstrated for the cancellation of multipath self-interference (SI) and facilitation of in-band full-duplex (IBFD) orthogonal frequency-division multiplexing (OFDM) signal transmission. The multipath effect is an unavoidable challenge in SIC due to the dynamic and unpredictable properties in each path, as well as the need for separate matching components for compensating for each path. In this Letter, an LMS algorithm-based adaptive filter is used as a pre-equalizer to adapt and generate the matching signal to the closest approximate of the multipath SI signal. The adaptation is based on the minimization of the error signal generated from the matching signal and multipath SI signal in the LMS algorithm. With the introduction of the LMS adaptive filter to the analog SIC, an additional 9 dB cancellation improvement is obtained, resulting in a total of 32 dB cancellation depth over a cancellation bandwidth of 2.7 GHz at a center frequency of 1.65 GHz. To the best of our knowledge, the achieved performance is by far the widest cancellation bandwidth in a multipath SIC system, which is essential in a large bandwidth and high data rate transmission system. With the help of the proposed LMS adaptive filter digital SIC assisted analog SIC located at the remote node, power-efficient IBFD transmission of an OFDM signal through a 25 km fiber is experimentally demonstrated with a 6 dB bit error rate and 8% error vector magnitude improvements.
In this Letter, we first propose and demonstrate a real-time in-band full duplex (IBFD) transmission system based on adaptive optical self-interference cancellation (OSIC). The field programmable gate array (FPGA) is used for high-speed and real-time orthogonal frequency-division multiplexing (OFDM) transmission. The hybrid criteria regular triangle (RT) algorithm is first proposed to combine signal power and the bit error rate (BER) together as the objective function to realize the adaptive control process. With this algorithm, the real-time adaptive OSIC system is able to converge and fully recover the signal of interest (SOI) within 12 sampling times, which is by far the fastest, to the best of our knowledge, convergence under the real-time transmission scenario. Experiments show that the system can achieve 28 dB cancellation depth across 0–1.45 GHz wideband, and 40 dB cancellation depth at 900 MHz, 2.4 GHz, and 5 GHz, which is the best cancellation performance in current real-time adaptive OSIC schemes and shows the potential of our system in different commercial applications.
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