Dr. Timothy Kurzweg
Detection of stale channel state information in free space optical interconnects using space-time block coding. Free space optical interconnect (FSOI) systems are a promising solution to interconnect bottlenecks in high speed systems. To overcome some sources of diminished FSOI performance caused by close proximity of multiple optical channels, multiple-input multiple-output (MIMO) systems implementing encoding schemes such as space-time block coding (STBC) have been developed. These schemes utilize information pertaining to the optical channel to reconstruct transmitted data. The STBC system is dependent on accurate channel state information (CSI) for optimal system performance, yet sacrifices bandwidth efficiency to obtain accurate CSI. As a result of dynamic changes in optical channels, a system in operation will need to update the CSI. Dynamic changes in the optical channel of interest occur at low frequencies, may occur only intermittently, and without periodicity. Therefore validation of CSI during operation is a necessary tool to ensure FSOI systems operate efficiently, at the expensive of bandwidth efficiency.
In this thesis, a method of detecting out-of-specification system performance caused by stale channel CSI is demonstrated. This thesis develops, for the first time, a method to utilize likelihood values to determine stale optical channels. To refresh stale CSI, additional training sequences are periodically transmitted, reducing bandwidth efficiency. Selectively refreshing CSI only when CSI is known to be stale provides a mechanism to minimize bandwidth efficiency lost due to transmission of training sequences. The average likelihood values can be used to ensure system operation within bit error rate (BER) thresholds. Systems exceeding average likelihood target values indicate the failure of the FSOI to maintain a minimum BER. This method provides a mechanism to utilize training sequences only when needed to avoid lost bandwidth due to over training.
