  The technical limitations of the intelligent reflecting surface (IRS)
(re)configurations in terms of both communication overhead and energy
efficiency must be considered when IRSs are used in cellular networks. In this
paper, we investigate the downlink time-frequency scheduling of an IRS-assisted
multi-user system in the orthogonal frequency-division multiple access (OFDMA)
framework wherein both the set of possible IRS configurations and the number of
IRS reconfigurations within a time frame are limited. We formulate the sum rate
maximization problem as a non-polynomial (NP)-complete generalized
multi-knapsack problem. A heuristic greedy algorithm for the joint IRS
configuration and time-frequency scheduling is also proposed. Numerical
simulations prove the effectiveness of our greedy solution.
