Analysis of the Impact of AGC on Cyclic Prefix Length for OFDM Systems

In some orthogonal frequency division multiplexing (OFDM) systems, a number of OFDM symbols form one subframe and consecutive subframes may be allocated to different users. The power estimation and the gain adjustment need to be carried out within the cyclic prefix (CP) of the first OFDM symbol in a suhframe. After passing through the multipath channel, the CP of the first OFDM symbol in a suhframe contains a portion of the power of the last OFDM symbol in the prior subframe. When these two subframes are allocated to distinct users with different received powers, the gain adjustment is inaccurate and the system performance is degraded. To combat the inter-user interference at the boundary of subframes, the CP of the first OFDM symbol in a subframe is proposed to include an adjustment portion (AP). The overhead of the AP is amortized over the OFDM symbols in a subframe and is negligible when the number of OFDM symbols in a subframe is large. A model is given to calculate the optimal length of the AP in this paper. The model uses an information theory to formulate the tradeoff between the accuracy of power measurement and the overhead of the AP.