Abstract
The design and advantages of a novel light emitting diode (LED) bandwidth extension technique, based on the adoption of negative impedance converters (NICs), are discussed in the context of visible light communication systems. The design principles of the proposed negative impedance converter are introduced, with analytical derivations of the impedance frequency behaviour and the circuit performance frequency limitations. A two-transistor circuit is designed to generate a negative capacitance over the range -3 to -5 nF and is experimentally demonstrated, using discrete transistors, passive elements and commercially available LED constructed on a PCB, with frequencies up to 50 MHz. NIC design considerations necessary to obtain optimum LED bandwidth extension are discussed and outlined. Measurements demonstrate advantageously the optically lossless nature of the proposed solution and that, unlike traditional passive equalization or pre-distortion based bandwidth extension techniques, substantial improvement in the LED bandwidth, of up to 400 % can be obtained without compromising the output optical power.
Highlights
V ISIBLE light communication (VLC) is an emerging access network technology that has gained momentous research interest in recent years, mainly due its advantages of wide, unlicensed bandwidth and its ability to provide the dual functionality of illumination and data communication [3], [4]
APPLICATION OF negative impedance converters (NICs) FOR light emitting diode (LED) BANDWIDTH EXTENSION we demonstrate the effect of compensating for the bandwidth-limiting LED diffusion capacitance by utilising the proposed NIC circuit to generate parallel negative capacitance
We demonstrate a novel LED bandwidth extension technique that is based on the introduction of a parallel negative capacitance to offset the bandwidth-limiting LED diffusion capacitance and extend its operating bandwidth
Summary
V ISIBLE light communication (VLC) is an emerging access network technology that has gained momentous research interest in recent years, mainly due its advantages of wide, unlicensed bandwidth and its ability to provide the dual functionality of illumination and data communication [3], [4]. More sophisticated passive equalizers in the from of T-bridge networks, configured as single stage [16] and cascaded [17], were demonstrated to extend the LED bandwidth by over an order of magnitude at the expense of substantial loss in signal power. While the use of blue filtering and analogue preequalisation have been effective in demonstrating high data rates in VLC links by extending the transmitter bandwidth, their benefits will always be limited as they often incur significant loss in optical power. Our work in [1], [2] proposed a novel optically lossless LED bandwidth extension technique based on the application of circuits known as negative impedance converters (NICs). NICs are inherently unstable due to the presence of positive feedback and they may have limited operating frequency range, caused by the non-ideal characteristics of the active devices, such as the finite input impedance and non-zero output impedance as shown in [32], [39], [41], [42]
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