Power spectrum analysis of the cross/intermodulation in the nonlinear LNA

This study deals with the effects of the cross/intermodulation (CM/IM) on the communication performance when both Tx (transmit) leakage and external single tone interference pass through LNA in code division multiple access (CDMA)/frequency division duplex (FDD) transceiver. The CM/IM powers happen because of the nonlinearity of the LNA and these get work as interference to the desired signal. The CM/IM interference powers are analyzed with the frequency change of external single tone interference in Rx (receive) band. The relationship between these interference powers, input third-order intercept point (IIP₃), and the dependence of the allowable noise figure (NF _allowable) on the IIP₃ are presented in this study. If IIP₃ is large enough for CM/IM interference power spectrum slightly to overlap with the Rx band, the interference power in Rx band gets smaller but NF_allowable increases as IIP₃ is increased. Thus, the bit-error-rate (BER) performance is degraded. On the other hand, when the interference power spectrum largely overlaps with the Rx band, it is important to increase the IIP ₃ to get the better BER performance since the performance improvement by the linearity growth is larger than the performance degradation caused by the increment of NF_allowable