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Option C: The UE performs ZIF 12 up-conversion of each CC before combining and feeding into asingle PA.Option D: The UE employs multiple RF chains and multiple PAs after which the high-powersignals are combined and fed into a single antenna. PA coupling at the UE can be challenging forthis option.Note that other options are also possible. One example is combining the output of two IFFTs in the digitaldomain before the D/A conversion.7.3 RF FRONT END IMPACTSIn a single carrier FDD (non-CA) scenario, a duplexer ensures that the transmission on the uplink doesnot interfere with the reception on the downlink. In a dual-band CA, say comprising of bands A and B,operators would need to ensure that the UL transmission on band A does not interfere with the DLreception on that band. Operators also need to ensure that it does not interfere with DL reception on bandB.Similar considerations apply to the UL transmission on band B. Even if one were to design a duplexer foreach of the bands that ensures that neither DL band is affected, directly connecting two duplexerstogether can affect each other’s filter characteristic, thereby losing the isolation that is needed to operateat reference sensitivity. Therefore, the general solution to this problem is a “quadplexer, ”except possiblyin some cases with large frequency separation between the two bands, where a separate filter “diplexer”can be inserted between the antenna and the two individual band-specific duplexers. Such a quadplexerwould need to be specially developed for each band combination and is likely to be significantly morecomplex and expensive compared to a duplexer.We now consider a three-band case, such as three carriers on bands A, B and C. By the samearguments as for the dual-band case, a transmission on any band should not interfere with the receptionon any other. Because the duplex filters are not tunable, the filtering solution has to be effectivelydesigned to handle the case when all three bands are in use simultaneously. A “hexaplexer” would benecessary to support such tri-band communication. Similarly to the quadplexer case, when there is largefrequency separation between at least one band and the other two bands, a separate filter “diplexer” canbe inserted between the antenna and groups of individual duplexers. So for example, two diplexers andthree band-specific duplexers may be used, or one diplexer and a combination of a quadplexer and aduplexer may be used.Note that the above problem in the three-band case is not simplified even if we enforce a constraint thatonly two bands of the three bands are active at any given time. A UE that is designed to operate on bandcombinations A+B and A+C can’t be automatically assumed to operate on band combination B+C. As asimple example, let band A be in the 1.9 GHz and let band B be lower 700 MHz and band C be upper 700MHz. A UE could be designed with a filter to ensure sufficient isolation between the 700 MHz and 1.9GHz carriers, but that filter cannot ensure isolation between the upper and lower 700MHz carriers. That isbecause the DL of the lower 700 MHz band (Band 12 or 17) and the DL of the upper 700 MHz band(Band 13) are adjacent to each other. Therefore, some signal splitting approach, as described below,12 Zero Intermediate Frequency (ZIF) up-conversion means no Intermediate stage of RF conversion between baseband and RFband. In the old day the baseband signal is modulated to IF signal first, then to RF signal before emitting in the Uplink. In thedownlink, RF signal is demodulated to IF signal first then to baseband signal. All the mobile RF chipset do ZIF today.4G Americas LTE Carrier Aggregation October 2014 41