Abstract
In this paper, we suggest to maximize the instantaneous throughput of primary and secondary networks of cognitive radio networks by optimizing packet length. The Instantaneous Signal to Interference plus Noise Ratio (ISINR) is measured at secondary destination and send it back to secondary source to select the Optimal Packet Length (OPL). Besides, Primary Receiver PR measures the ISINR and send it back to primary transmitter PT to adapt packet length. The proposed protocol consists to maximize the instantaneous throughput and allows higher throughput than maximizing the average throughput.