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Capacity-achieving and Flicker-free FEC coding scheme for Dimmable Visible Light Communication Based on Polar Codes
Junbin Fang, Zhen Che, Xiaolong Yu, Zhe Chen, Zoe L. Jiang, Siu-Ming Yiu, Kui Ren, Xiaoqing Tan
TL;DR
Dimmable VLC needs FEC that preserves reliable communication while supporting dimming and preventing flicker without costly auxiliary coding. This paper proposes a polar-code FEC scheme with integrated lighting support, reporting balanced codewords, short runs, higher coding efficiency, and strong error-correction performance. The experiments also show low-complexity operation and coding gains over RS and LDPC baselines under specified dimming ratios.
Problem
Conventional FEC codes do not inherently support dimming and flicker mitigation in VLC, so auxiliary coding reduces efficiency and increases structural complexity.
Method
The proposed scheme encodes VLC information with polar codes, uses compensation symbols for dimming control, and applies interleaving before OOK transmission.
Results
The scheme provides balanced codewords, short runs, about twofold higher coding efficiency than other schemes, and capacity-achieving error correction with low-complexity encoding and decoding.
Takeaways & Limitations
The proposed FEC design supports dimmable, flicker-free VLC while simplifying the system structure and retaining high coding efficiency.
Abstract
from arXiv · showhide
Visible light communication (VLC) could provide short-range optical wireless communication together with illumination using LED lighting. However, conventional forward error correction (FEC) codes for reliable communication do not have the features for dimming support and flicker mitigation which are required in VLC for the main functionality of lighting. Therefore, auxiliary coding techniques are usually needed, which eventually reduce the coding efficiency and increase the complexity. In this paper, a polar codes-based FEC coding scheme for dimmable VLC is proposed to increase the coding efficiency and simplify the coding structure. Experimental results show that the proposed scheme has the following advantages: 1) equal probability of 1's and 0's in codewords, which is inherently supporting 50% dimming balance; 2) short run length property (about 90% bits have runs shorter than 5) which can avoid flickers and additional run-length limited line coding; 3) higher coding efficiency about twofold than that of other coding schemes; 4) capacity achieving error correction performance with low-complexity encoding and decoding, which is about 3 dB higher coding gain than that of RS(64,32) in IEEE standard for dimming ratio 50% and about 1 dB higher coding gain than that of LDPC codes for dimming ratio 25% (or 75%).
I. INTRODUCTION
Dimmable VLC must combine reliable communication with illumination requirements, but conventional FEC schemes often need auxiliary coding that lowers efficiency and increases complexity. The paper proposes polar-code FEC intended to provide dimming support, flicker mitigation, efficient coding, and low-complexity decoding.
- VLC simultaneously provides short-range wireless communication and LED illumination, making lighting behavior part of the communication design.
- FEC coding for VLC must address both dimming support and flicker mitigation because OOK symbols control LED brightness.
- Conventional FEC codes may create unbalanced 1s and 0s or long runs, requiring compensation symbols, scrambling, puncturing, or RLL coding.
- These auxiliary procedures reduce coding efficiency and transmission rate while complicating the VLC structure and adding latency.
- The proposed polar-code scheme targets capacity-achieving error correction with low-complexity encoding and decoding while integrating dimming and flicker mitigation.
II. RELATED WORKS
Prior dimmable-VLC coding schemes address illumination constraints through compensation, puncturing, scrambling, or RLL mechanisms, but these additions impose efficiency, complexity, or performance costs. The related-work comparison motivates a more integrated coding approach.
- Dimmable VLC requires FEC together with mechanisms that control average illumination and prevent perceptible brightness changes.
- IEEE-oriented schemes use RS or concatenated RS-convolutional codes with compensation symbols for dimming and RLL codes for flicker mitigation.
- Modified RM codes guarantee exact 50% DC balance but have code rates scaling as O(log2(N)/N) and inferior error-correction performance to iterative codes.
- Turbo-based schemes require puncturing, scrambling, and compensation-symbol insertion, producing high overhead and requiring symbol flipping above a 1/2 dimming ratio.
- LDPC-based schemes combine puncturing, compensation symbols, and RLL coding, potentially yielding very low overall rates and increased decoding cost and latency.
- Existing dimmable-VLC FEC schemes share low coding efficiency and complicated coding structures, as summarized in the comparison overview.
III. DIMMABLE VLC SYSTEM WITH POLAR CODES
The proposed system encodes information with polar codes, adds compensation symbols for the target dimming ratio, and interleaves the resulting frame before OOK optical transmission. At the receiver, demodulation and deinterleaving precede polar decoding.
- The polar encoder maps a k-bit information vector u to an N-bit codeword x.
- A dimming compensator inserts Ncs compensation symbols, producing a transmitted frame of length Nframe = N + Ncs.
- The FEC code rate is Rc = k/N, while overall coding efficiency is ηoverall = k/Nframe.
- An interleaver permutes codeword and compensation-symbol bits before OOK modulation to mitigate burst errors caused by channel fading.
- After VLC-channel transmission and photodiode reception, OOK demodulation and deinterleaving feed the polar decoder, which estimates the transmitted message.
B. FEC coding Scheme based on Polar Codes
Polar coding uses channel polarization to separate reliable and unreliable coordinate channels, assigning information and frozen bits accordingly. This achieves rates approaching channel capacity with O(NlogN) encoding and decoding complexity.
- Channel Polarization: Polar codes recursively combine and split copies of a symmetric binary memoryless channel into coordinate channels with capacities tending toward 0 or 1.
- Channel Polarization: Information bits occupy coordinate channels with capacity near 1, while preset frozen bits occupy channels with capacity near 0.
- Channel Polarization: The polar code rate can approach channel capacity as Rpolar = Ninfo/N = I(W).Ninfo counts near-capacity-one channels, and Nfrozen counts near-capacity-zero channels.
2) Polar Encoder:
The polar encoder forms codewords by multiplying the input vector by a structured generator matrix, with information and frozen bits selected through an information set and its complement.
- Polar Encoder: The generator matrix is GN = BNF ⊗n, where F is the basic polar transform, n = log2(N), and BN is bit reversal.
- Polar Encoder: For an arbitrary information set A, the codeword combines information-bit and frozen-bit contributions through XOR.
- Polar Encoder: A and Ac denote the information and frozen sets, while uA and uAc contain the corresponding bits.
- Polar Encoder: Encoding complexity is O(NlogN), including O(N) bit reversal and O(NlogN) computation for the recursive transform.
3) Polar Decoder:
The polar decoder estimates the transmitted input from the channel output, known frozen bits, and recursive likelihood calculations. Its successive-cancellation structure has O(NlogN) complexity.
- Polar Decoder: The decoder estimates the input vector using the received channel output, the known information set, and preset frozen bits.
- Polar Decoder: Known frozen bits are restored directly and then assist estimation of the information bits.
- Polar Decoder: Successive-cancellation decoding computes likelihood ratios recursively and decides bits sequentially from index 1 through N.
A. Code Weight Balance for Dimming Support
Experiments evaluate polar codeword weights across rates 1/4, 1/2, and 3/4. Most codewords have nearly equal numbers of zeros and ones, supporting balanced optical intensity around 50%.
- Code Weight Balance for Dimming Support: The experiment analyzes 10,000 randomly generated 1024-bit polar codewords at rates 1/4, 1/2, and 3/4 using weight histograms.
- Code Weight Balance for Dimming Support: 86.4% of codewords have weights between 488 and 536, while no codeword weighs less than 448 or more than 560.
- Code Weight Balance for Dimming Support: The mean code weight is 512 with standard deviation 16, or about 1.6% of the 1024-bit codeword length.
- Code Weight Balance for Dimming Support: At approximately 200 Hz, the averaged dimming fluctuates around 50±1.6% over 5.12 ms, described as negligible relative to 50 ± 50% on-off pulses.
IV. EXPERIMENTAL RESULTS AND DISCUSSION
Experiments show that polar codewords balance 0s and 1s across code rates, while eliminating auxiliary balance coding and supporting flexible dimming. These properties simplify VLC design and improve coding efficiency.
- Polar codewords provide equal probabilities of 0s and 1s, corresponding to balanced ON and OFF symbols for 50% light intensity.This permits dimming by inserting ON or OFF compensation symbols.
- Weight balance, scrambling, DC-balance line coding, and code puncturing are unnecessary for dimming support.Removing these auxiliary techniques simplifies the proposed VLC system design.
B. Short Run Length Property for Flicker Mitigation
Polar codewords mostly contain short runs, helping mitigate flicker in VLC. This property also removes the need for RLL line coding and can increase spectral efficiency.
- Short run lengths help limit brightness changes that could become perceptible flicker during VLC transmission.Run length reflects light intensity over a shorter period, so it must be controlled for flicker mitigation.
- About 90% of bits in 10,000 tested 1024-bit polar codewords belong to runs shorter than 5.The maximum observed run length was 20, occurring twice across the 10,000 tests.
- The short-run property eliminates RLL line coding and can increase spectral efficiency by removing that low-efficiency coding stage.
C. Overall Coding Efficiency
The proposed polar-code scheme maintains coding efficiency across dimming ratios while providing strong error-correction performance for dimmable VLC.
- Efficiency comparison: ηoverall = 0.5 at 50% dimming, about twice higher than the LDPC codes-based scheme.At other dimming ratios, the proposed scheme with Rc = 0.5 retains about two-fold coding efficiency relative to LDPC.
- Efficiency comparison: The proposed scheme supports arbitrary code rates, unlike RM codes whose rate is fixed by log2(N)/N.This flexibility is described as more practical and higher than the fixed RM code rate.
- Error-correction performance: For 50% dimming, polar codes reach near error-free BER below 10^-5 at SNRrx values of 2.4 dB, 4.9 dB, and 8.2 dB for code rates 0.25, 0.5, and 0.75.The experiment simulated 10,000 randomly generated 1024-bit codewords per code rate and SNR value over an AWGN channel.
- Error-correction performance: For dimming ratio 25% or 75%, the proposed scheme achieves about 1 dB higher coding gain than LDPC codes.The 75% dimming performance is shown in Figure 6.
V. CONCLUSION
Traditional FEC schemes for dimmable VLC require auxiliary procedures for lighting functions, reducing efficiency and complicating system design. The proposed polar-code scheme addresses these constraints with balanced codewords, short runs, higher efficiency, and strong error correction.
- V. CONCLUSION: Traditional FEC schemes require modification to support dimming and flicker mitigation in VLC illumination systems.Their auxiliary coding procedures contribute to low coding efficiency and complicated coding structure.
- V. CONCLUSION: The proposed polar-code scheme provides code weight balance, short run lengths, higher coding efficiency, and better error-correction performance.These properties support a simplified dimmable VLC system design with capacity-achieving error correction.