![]() They concluded that the bit error ratio (BER) was vastly improved by employing NLMS based equalization. Sirvi and Tharani in 2016 tested NLMS equalization. They concluded that their system reached the highest data rate ever (until 2014) by using a single commercially available RGB-LED in a VLC system. ![]() By using this hybrid system, they reached improvements in performance. in 2014 proposed a hybrid time-frequency adaptive equalization algorithm, which combines frequency domain equalization (FDE) and decision-directed least mean squares (DD-LMS). Their results showed that an adaptive equalization scheme for indoor high-speed VLC systems provides benefits and is feasible. They compared RLS algorithm with the modified cascaded multi-modulus algorithm (M-CMMA) and concluded that the RLS algorithm can outperform it by a Q factor of 1 dB. in 2015 used RLS based adaptive equalization for RGB-LED based wavelength division multiplexing (WDM) VLC system. Modern microwave links can use up to 4096-QAM however, the signal quality requirements increase significantly for modulation formats with a large number of states. ![]() However, by increasing the number of symbols transmitted during a period of time, the length of said symbols is shortened therefore, any delayed pulse that arrives outside the set interval causes significant distortion of the transmitted data. Ĭurrently, the requirements for quality of RF and VLC systems are slowly increasing, and even higher data rates are desired. Error correction codes can be also used to alleviate ISI. ![]() Adaptive equalization is one of the ways to overcome intersymbol interference see. Intersymbol interference is a situation when one symbol (or state) interferes with other symbols, thus increasing the error rate and leading to the lower reliability of the whole platform. It can be also caused by the position of LEDs or the bit interval. ISI is a major concern in visible light communication, as the medium (light) can be reflected by various surfaces. Īs a result of signals being delayed differently (due to multipath propagation), so-called intersymbol interference (ISI) occurs on the receiving side. The right column presents examples of receiving solutions, spanning from conventional PIN photodetectors to pricier avalanche photo diodes (APDs), advanced CMOS cameras, and inexpensive alternatives in solar panels. The center column represents examples of various channel impairments, which influence the transmitted signal. The left side represents various transmitting sources (represented by physical solutions), which are often explored by scientists. Examples cover all scenarios from vehicle-to-everything (V2X) to machine-to-machine communication (M2M).įigure 1 represents various scenarios of visible light communication systems. The whole platform manages to improve measured data simply by making changes to the software side of the testing prototype.Įxamples of various impairments in visible light communication systems. The experiments also show that the simulated results are very similar to real measurements, thus proving the validity of the chosen approach. Based on measurements, QR-RLS provides the best results, improving measured values by up to 10%. Three FSDE algorithms were tested: least mean squares (LMS), normalized least mean squares (NLMS), and QR decomposition based RLS (QR-RLS). The whole platform is based on modified National Instruments software defined radios (NI SDR) and a commercially available Philips light source, often used in Czech government institutions. A highly modular platform is introduced the whole experiment is simulated in software and then thoroughly explored and analyzed during practical measurements in the laboratory, simulating real-world situations. This paper focuses on a channel feed-forward software defined equalization (FSDE) of visible light communication (VLC) multistate quadrature amplitude modulation (M-QAM) based system, implemented in the LabVIEW programming environment.
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