The Impact of AGC on Cyclic Prefix Length for OFDM Systems

Abstract

In orthogonal frequency division multiplexing (OFDM) systems, a number of OFDM symbols form one subframe and consecutive subframes may be allocated to different users. Automatic gain control (AGC) is commonly employed in OFDM systems because the power of receiving signals can vary over a large dynamic range. After measuring the power of cyclic prefix (CP) of the first OFDM symbol in a subframe, AGC adjusts the gain before receiving the useful portion (UP). Otherwise, there is inter-carrier interference (ICI). CP length is usually chosen equal to the delay spread of the multipath channel. After passing through the channel, CP of the first OFDM symbol in a subframe contains the power of the last OFDM symbol in the prior subframe. When these two subframes are allocated to distinct users with different transmit powers, AGC gain is inaccurate and system performance is degraded. To solve this problem, we extend the CP of the first OFDM symbol in a subframe to include an AGC portion (AP). On the assumption that transmitted time domain signal is circularly symmetric complex Gaussian random variable, the required length of AP is also derived. Simulation results show that the proposed scheme has a gain of 0.0dB ~ 6.2dB at BLER of 0.1 over the prior scheme with 0.34% overhead.