Yazgan, Ayhan; Simsek, Cemaleddin; Albayrak, Cenk; Tugcu, Emin; Turk, Kadir; Herencsar, N.

Channel measurement for visible light communication (VLC) is theoretically and experimentally performed in a real office platform. Six armatures composed of phosphor based white LED arrays are fed by designed analog LED drivers. Measurements are taken in a 6m x 7.8m office room which is formed as 10×13 matrix with 60 cm grid distance for both simulation and experiment studies. The link distance between LED arrays and optical receiver is kept minimum 2.5 meters for each measurement. Experimental results are generally in good agreement with simulation results, which are obtained using Barry's theoretical channel model.

Simsek, Cemaleddin; Tugcu, Emin; Albayrak, Cenk; Yazgan, Ayhan; Turk, Kadir; Herencsar, N.

Visible light communication (VLC) is an emerging technology to fulfill both illumination and communication purposes considered as either an alternative or a support system for some of scenarios carried by RF communication systems. Colour shift keying (CSK) is the modulation given in IEEE 802.15.7 VLC standard which proposes Reed-Solomon as error correction code. Polar code is the first theoretically proven capacity achieving error correction code with low encoding and decoding complexities. Performance of Polar code with belief propagation decoder has not been studied under VLC CSK scheme. In this study we investigate performance of Polar code with belief propagation decoder under VLC CSK scheme and compare it with performance of Reed-Solomon code. Result show that Polar code with belief propagation decoder outperforms Reed-Solomon code by at least 1.5dB.

Simsek, Cemaleddin; Albayrak, Cenk; Türk, Kadir

Polar code has become a major milestone in information theory field in recent times. Researchers are still observing more efficient encoding and decoding structures. In this study, a new WIB based structure is proposed which reduces the computational complexity of WIB introduced as an early termination method for BP polar decoder in literature. Both proposed and WIB methods are implemented with VHDL and hardware complexities are compared via XILINX ISE synthesis tool. Most important complexity reduction came with removing unnecessary addition operations which make early termination method possible to implement only with hit-wise logical operations. Results show that along with complexity reduction proposed approach can also acquire same performance in half time compared to previous counterpart with fewer logic gates.

Albayrak, Cenk; Simsek, Cemaleddin; Türk, Kadir

In this letter, a new early termination method is proposed for Luby transform (LT) belief propagation (BP) decoder. The proposed method is decided whether BP decoder output converges to original data bits by observing only sign alterations of log-likelihood ratio (LLR) messages in BP decoder structure. Simulation results and complexity analyzes show that proposed method has low computational complexity and small average iteration amounts compared to conventional early termination methods in literature. In addition to this, the proposed method doesn't cause any performance degradation in BP decoder. The method can be easily applied to code families which can be decoded by BP algorithm such as low density parity check (LDPC) codes, polar codes and Raptor codes.

Simsek, Cemaleddin; Turk, Kadir; Herencsar, N.

Belief propagation (BP) polar code decoder is well-studied from many aspects. This study proposes a hardware optimization to improve performance of polar BP decoder by modifying both processing element (PE) and early stopping criterion (ESC). PE is optimized by using high-speed parallel-prefix Ling adder instead of carry ripple adder and WIB ESC introduced in literature is optimized by removing unnecessary adder array. FPGA implementation is made to verify the idea behind this study. The results show that throughput of polar code belief propagation decoder can be increase significantly by using proposed approach. More accurate comparison can be made by application specific chip design with same parameters. This part of the study will be held in future.

Albayrak, Cenk; Simsek, Cemaleddin; Turk, Kadir

We propose a new early termination method (ETM) with low computational complexity for Luby transform belief propagation (BP) decoder. The proposed method, which we call least reliable messages (LRM) ETM, observes only sign alterations of a small cluster in log-likelihood ratio messages passing between nodes in BP decoder. Simulation results and complexity analyzes show that LRM ETM has quite low computational complexity and small average iteration amounts without any performance degradation compared with conventional ETMs in literature. The method can be applied to code families which can be decoded by BP such as low density parity check codes, polar codes, and Raptor codes.

Simsek, Cemaleddin; Turk, Kadir

As the first theoretically proven capacity-achieving error correction code, polar codes have become a milestone in information theory field, which draw serious attention by its low complexity encoding and decoding structures. For the decoding section, studies are focused on efficient and low complex algorithms based on successive cancellation and belief-propagation (BP) decoders. To reduce computational complexity of BP-based decoders further, early stopping methods that avoid unnecessary iterations can be used. In this letter, we propose a reduced complexity early stopping method for BP-based polar code decoders. The proposed method is based on the hypothesis that observing only a small cluster of information bits polarized to the highest error probabilities is enough to detect successful decoding. Simulation results show that the proposed early stopping criterion significantly reduces the computational complexity of successful decoding detection as well as the total operation numbers of the whole decoding process compared with previous methods in the literature.