Feedback Solution for Blind Symbol Timing Recovery in Bandlimited Optical Intensity Channels

The reliable recovery of transmission parameters is of paramount importance not only for radio frequency, but also for optical wireless communication receivers. In the context of a bandlimited optical intensity link, achieved by a PAM scheme and squared raised cosine or squared double jump functions used for pulse shaping, feedforward algorithms for symbol timing estimation have been developed in recent papers published by the first author. This work is extended by the current contribution, where a feedback solution is discussed with no knowledge about data symbols. Such non-data-aided (blind) concepts are a perfect instrument to track smaller vari-ations of the timing error affecting the payload data. It is pro-posed that a Gardner detector is used for the recovery loop, which is investigated with respect to open-loop characteristic, jitter performance, and acquisition behavior.

I. Introduction

Comparing radio frequency (RF) to optical wireless communication (OWC) techniques, the advantages of the latter are well known: no regulatory and license issues, rather inexpensive and easy to deploy, high throughput and no problems with data security, just to mention the most signifi-cant aspects in this context [1]–[4]. In addition, unlimited bandwidth is frequently mentioned as an important feature, albeit this is for many scenarios not true in the strict sense due to inevitable hardware constraints of opto-electronic components in transmitter and receiver units [5]. A strong argument for a bandlimited approach is also that algorithms for parameter estimation and synchronization, which are indispensable for powerful RF architectures, would become applicable to OWC systems as well [6], [7]. Further reasons for bandwidth constraints are related to multipath distortions in optical indoor systems or dispersion effects in optical fiber networks [8], [9].