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title: "Inter-symbol interference and silicon photomultiplier VLC receivers in ambient light."
date: 2021-03-30T09:26:11+01:00
draft: false
category: paper
excerpt: "IEEE Photonics Technology Letters: removing the optical filter from a wide field-of-view SiPM receiver reveals a new form of inter-symbol interference caused by saturation."
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::publication[3]

Published in IEEE Photonics Technology Letters.
Removing the narrow optical band-pass filter from a SiPM receiver widens its field of view, but lets in ambient light.
This paper reports what that does to the link, including a ~wave:new form~ of inter-symbol interference (ISI).

## Abstract

Results of an investigation into the performance of a wide field of view silicon photomultiplier (SiPM) receiver under ambient light are reported. During these investigations a new form of inter-symbol interference (ISI) was discovered, which can be attributed to saturation of the SiPM. This new form of ISI means that using decision feedback equalization (DFE) improves the receiver's performance at low data rates.

## Key Findings

The analysis uses the model of a SiPM's bias current under illumination set out in the [3.45 Gbps journal paper](/posts/sipm-ook-receiver-3-45-gbps-journal/).

This is the first report of what happens when the band-pass filter is removed.
Unexpectedly, DFE then made a significant difference at data rates below 400 Mbps.
Eye diagrams taken before and after removing the filter show a third level appearing in the eye.
Comparing three eye diagrams shows that this ISI arises because ambient light pushes the SiPM close to saturation.
The high peak powers of orthogonal frequency-division multiplexing (OFDM) mean the OFDM signal itself might cause the same ISI, so OFDM links to SiPMs will need a way to stop it limiting the data rate.
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