Probabilistically shaped polar-coded MIMO-FSO communication systems with spatially correlated fading.

In this paper, the probabilistically shaped polar-coded multiple-input multiple-output free-space optical (MIMO-FSO) communication system with or without spatially correlated (SC) fading is investigated to improve transmission performance. The designed shaping-polar encoder can flexibly generate three typical shapes of distribution via shaping bits and be decoded in the conventional method. The achievable information rate (AIR) of MIMO-FSO systems with or without SC fading is evaluated to determine the number of shaping bits for the shaping-polar encoder. The non-pairwise distributions are demonstrated to be more suitable for turbulence channels than other distributions. The results show that the AIR of the shaped 4 × 4 systems even exceeds that of the uniform 4 × 5 systems in the low signal-to-noise ratio regions over strong turbulence channels. In terms of bit error rate performance, more than 15 dB shaping gains can be achieved by the shaped 4 × 4 systems compared to the uniform single-input single-output polar-coded systems. In addition, the shaped 4 × 4 systems outperform the uniform ones ranging from 1 dB to 1.9 dB over different atmospheric turbulence channels with or without SC fading, comparable to the uniform MIMO systems with one more physical receiver.

Real-time IBFD transmission system based on adaptive optical self-interference cancellation using the hybrid criteria regular triangle algorithm.

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.

Polar coded probabilistic amplitude shaping for the free space optical atmospheric turbulence channel.

In this paper, the polar coded probabilistic amplitude shaping (PC-PAS) is investigated in a free space optical (FSO) communication system to combat the fading induced by turbulence. The achievable rate of multiple level coding (MLC) and bit-interleaved coded modulation (BICM) schemes with different distributions are studied in turbulence channels, which proves that the non-uniform distribution can achieve larger achievable rates than the uniform distribution in the FSO turbulence channel. And the PC-PAS techniques based on MLC and BICM are both investigated. For MLC-based PC-PAS, the dynamically frozen bits scheme is adopted and the modification to the labeling rule is proposed to label the non-negative constellation points. For the BICM-based PC-PAS, the exchange scheme is proposed to combine the polar codes and PAS technique. The Block error rate (BLER) is evaluated by the Monte Carlo simulation method. From the results, both the MLC-based and the BICM-based PC-PAS can improve the performance compared to the uniform distribution. And the PC-PAS based on MLC outperforms the PC-PAS based on BICM in the same turbulence condition.

Neural network decoder of polar codes with tanh-based modified LLR over FSO turbulence channel.

The deep learning-based decoder of polar codes is investigated over free space optical (FSO) turbulence channel for the first time. The feedforward neural networks (NN) are adopted to establish the decoder and some custom layers are designed to train the NN decoder over the turbulence channel. The tanh-based modified log-likelihood ratio (LLR) is proposed as the input of NN decoder, which has faster convergence and better bit error rate (BER) performance compared with the standard LLR input. The simulation results show that the BER performance of NN decoder with tanh-based modified LLR is close to the conventional successive cancellation list (SCL) decoder over the turbulence channel, which verifies that the NN decoder with tanh-based modified LLR can learn the encoding rule of polar codes and the characteristics of turbulence channel. Furthermore, the turbulence-stability is investigated and the trained NN decoder in a fixed turbulence condition also has stable performance in other turbulence conditions.

Adaptive over-the-air RF self-interference cancellation using a signal-of-interest driven regular triangle algorithm.

An optically-enabled radio frquency (RF) self-interference cancellation system is demonstrated for over-the-air in-band full duplex transmission, based on a signal-of-interest (SOI) driven regular triangle algorithm. Since the goal of a self-interference cancellation system is to retrieve the SOI that is masked by the in-band interference signal, using the SOI quality as the driven parameter for optimizing the self-interference cancellation performance is a natural and effective way to allow the system to adapt to changes and obtain the best cancellation performance. Since regular triangle algorithm has short iteration time, bursts of pseudo-random binary sequence would be used between real data transmission for optimizing the self-interference cancellation performance. The adaptive regular triangle algorithm optimizes the cancellation setting such that the in-band interference can be cancelled to a minimum, i.e., down to the noise floor. During the over-the-air experiment, 22 dB of cancellation depth is obtained over a 300 MHz bandwidth at 18.35 GHz without the need of digital self-interference cancellation.

Adaptive optical self-interference cancellation for in-band full-duplex systems using regular triangle algorithm.

In this paper, we propose an adaptive optical self-interference cancellation using regular triangle algorithm for in-band full-duplex systems. By using this algorithm, the manual adjustment of the tunable optical time delay line and attenuator is replaced with the adaptive program to change the delay and attenuation for achieving optimal cancellation point. The adjustment process is simplified as a convex function problem. We choose to attain the optimal cancellation point by directly and continuously sampling the power of the signal after cancellation and in turn adjust the time delay and attenuation according to the algorithm. In this way, the two paths in the self-interference cancellation system are precisely and automatically matched. By using our proposed algorithm, the interference signal over 300-MHz wideband is diminished to the noise floor, attaining 20-25 dB cancellation depth adaptively. Compared with other existing algorithms in both the experiment and simulation, our proposed regular triangle algorithm reaches the optimal point faster with 10-30% less number of samples from the near start region, and lowers 40-60% less number of samples from the farther start region.

Performance investigation of the polar coded FSO communication system over turbulence channel.

In this paper, for the first time, to the best of our knowledge, polar codes are introduced and experimentally implemented in a free space optical (FSO) communication system to combat atmospheric turbulence induced fading. By analyzing the characteristics of the turbulence channel, a method of evaluating the channel state information for polar decoding is proposed that can achieve good trade-off between the performance and the computational complexity of this polar coded system. To verify our scheme, an intensity modulation direct detection FSO communication experimental platform with a turbulence chamber is established. For the weak turbulence condition, comparing with the low-density parity check codes, the experimental results show that our proposed scheme has stronger error correcting capacity and lower computational complexity in combating the turbulence induced fading. Moreover, for moderate and strong turbulence conditions, the gamma-gamma turbulence model is adopted for constructing the Monte Carlo simulation. The results of the experiment and simulation both show that our proposed scheme can effectively combat atmospheric turbulence induced fading with a relatively low computational complexity in a wide range of turbulence conditions.

High performance and cost effective CO-OFDM system aided by polar code.

A novel polar coded coherent optical orthogonal frequency division multiplexing (CO-OFDM) system is proposed and demonstrated through experiment for the first time. The principle of a polar coded CO-OFDM signal is illustrated theoretically and the suitable polar decoding method is discussed. Results show that the polar coded CO-OFDM signal achieves a net coding gain (NCG) of more than 10 dB at bit error rate (BER) of 10-3 over 25-Gb/s 480-km transmission in comparison with conventional CO-OFDM. Also, compared to the 25-Gb/s low-density parity-check (LDPC) coded CO-OFDM 160-km system, the polar code provides a NCG of 0.88 dB @BER = 10-3. Moreover, the polar code can relieve the laser linewidth requirement massively to get a more cost-effective CO-OFDM system.