ummarizes the most important challenges to complexity resulting from supporting multiple carri- ers simultaneously. The following subsections give a few examples of key challenges for EN-DC operation. Single-carrier operation (1UL-1DL) key challenges are inherent to all FDD systems in which the UE's own receiver, due to the close proximity of its own Tx carrier in the frequency domain, may be subject to MSD due to the following: 1. Tx noise not sufficiently rejected by duplexer bandpass filters as seen in the example of n8 bandwidth extension (Section 12.3.3.1) 2. Tx signal leakage, which, through receiver mixer nonlinearity and self-mixing, can generate an in-band IMD2 product Other challenges are deliberately omitted in this section. In downlink CA (1UL-xDL), intra-band non-contiguous CA is a special case where MSD can be aggravated due to an even shorter gap separating either the PCC or SCC from the UE's own Tx chain. In the case of inter-band CA, additional MSD may occur if one of the transmit carrier's harmonics directly hit one of the SCCs. One well-known example is that of LTE-CA between Band 12 and Band 4/66, where the third harmonic of Band 12 may completely overlap the Band 4 receive channel. Any components in the transmit path are potential sources of harmonics, starting with the transceiver driver amplifier and limited transceiver pin to pin isolation, filters, Transmit band Receive band Tx noise leaking in Rx band Tx carrier leakage 1 UL -1 DL at Receiver input Primary Secondary Secondary Transmit band Receive band Receive band Receive band Tx harmonics 1 UL -X DL falling onto SCC Rx Tx harmonics (H2,H3..) Primary Secondary Secondary Transmit band Receive band Receive band Receive band Tx IMD products overlapping Rx band 2 UL -x DL Tx harmonics (H2,H3..) Figure 12.16 From single UL/single DL, to dual UL intra-band multiple DL operation. 12.5 Dual Uplink Performance Challenges for NSA Operation antenna switches, antenna tuners, with the PA being the largest contributor. It is no surprise that nearly 1