Robust channel quality indicator reporting for multi-carrier and multi-user systems

This paper considers the best-M channel-quality-indicator (CQI) reporting scheme for multi-carrier and multi-user systems. The perfect knowledge of CQIs is crucial for efficient resource allocation. Using outdated (which occurs due to feedback delay) and imperfectly estimated CQIs for resource allocation can severely degrade the system performance. The conventional approach of dealing with the imperfections in CQIs at the base station (BS) is not efficient for multi-carrier systems as the BS has a small number of CQIs and therefore, does not know the distribution of the imperfections for all subcarriers. Approaching this problem more realistically, we model the CQI variations as a discrete-time linear dynamic system where the distribution of the imperfections is unknown, and propose a novel best-M scheme that accounts for feedback delay and imperfect CQI estimation at the CQI reporting level. Instead of reporting the estimated CQIs (i.e., the conventional approach), in our scheme, each user reports so-called adapted CQIs computed by using an H∞H∞-controller based approach. Unlike the conventional approaches, the H∞H∞-controller does not require information on the probability distribution of the imperfections, and it guarantees robust performance in scenarios where the conventional approaches fail. The impact of feedback delay and imperfect CQI estimation on resource allocation is taken into account in the computation of adapted CQIs. That is adapted CQIs are computed such that if they are used for resource allocation, the deviation between the allocated rate by the BS and the actual channel rate is reduced. Results show that our scheme outperforms the schemes used in the current wireless standards.