Skip to Main Content
Adaptive modulation of subcarriers for orthogonal frequency division multiplexing (OFDM) has been shown to improve system performance significantly. As a prerequisite, accurate channel state information (CSI) must be available at the transmitter. For time varying, noisy channels, however, this is difficult to achieve in practical systems. In this paper, we consider optimal subcarrier assignment under incomplete CSI. Soft estimates and the Cramer-Rao bound are used to analyze the effect of CSI errors. As an important control parameter, the frame length is coming in. We investigate analytically its influence on the performance of adaptive modulation in OFDM systems for the rate and power optimization problems. Simulations demonstrate that the frame length may not be neglected as an important overall system performance parameter.