Comparing radio frequency (RF) to optical wireless communication (OWC) techniques, the benefits 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 significant aspects in this context. On top of that, unlimited bandwidth is frequently mentioned as an important feature – but 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. A strong argument for a bandlimited approach is also that powerful algorithms for parameter estimation and synchronization normally used in RF receivers might become applicable to OWC systems as well.
Focusing on optical intensity modulation, a unipolar signal design is indispensable in terms of modulation scheme and pulse shaping. In the open literature, it is suggested to achieve this by M-ary PAM constellations and a squared raised cosine (SRC) function with the additional benefit that the Nyquist criterion is satisfied as well.
Not only for RF, but also for OWC solutions the most important transmission parameters must be successfully recovered by powerful algorithms. Otherwise, subsequent receiver stages, like symbol detection and error correction units, cannot be operated reliably. Of course, in case of optical intensity links, carrier frequency and phase need not be considered, whereas synchronization of the symbol timing as well as estimation of the channel state are of paramount importance. Using a PAM scheme and SRC for pulse shaping, appropriate algorithms for symbol timing recovery and SNR estimation are developed and analyzed in this OptiPuSh project.