Not include DC for CSI IM measurement

DC subcarrier has a significantly higher energy compared to other CSI IM
resource elements. This causes the average interference + noise
estimation to be higher than CSI-RS signal power making the CQI 0. With
a third-party gnb that schedules PDSCH based on CQI, the UE never
receives DL DCI once it starts sending CSI reports.

openair1/PHY/NR_UE_TRANSPORT/csi_rx.c

@@ -782,6 +782,8 @@ static void nr_csi_im_power_estimation(const PHY_VARS_NR_UE *ue,
           LOG_I(NR_PHY, "(ant_rx %i, sc %i) real %i, imag %i\n", ant_rx, sc, rx_signal[sc].r, rx_signal[sc].i);
 #endif
 
+          if (sc == 0) // skip DC for noise power estimation
+            continue;
           sum_re += rx_signal[sc].r;
           sum_im += rx_signal[sc].i;
           sum2_re += rx_signal[sc].r * rx_signal[sc].r;