Multiplexers in Mobile Handsets with LTE- Advanced Carrier Aggregation Uli B. Koelle Avago Technologies, San Jose, Calif. n many parts of the world, smart- To increase the bit rate, CA means that up to phones have become an integral part five CCs can be aggregated, either within the I of everyday life, and many users rely on same band or across different frequency bands. their handsets as an easy way for going online. The number of downlink CCs must always In the U.S., it is reported that in early 2011, equal or be greater than the number of uplink one in three Americans owned a smartphone. CCs. Since most of the wireless traffic demand As of 2015, that number has almost doubled.1 is presently in the downlink (DL) direction, Projections indicate that in 2016 smartphone only one uplink (UL) CC is used, limiting car- penetration will exceed 80 percent in the U.S.2 rier aggregation to the DL direction. This continuous growth of smartphone popu- All smartphones on the market today already larity, and the expansion of its user base feeds include the capability to transmit and receive the need for increased wireless data traffic, on different frequency bands, for roaming pur- which has to be met with an increase in avail- poses. However, enabling simultaneous opera- able bandwidth. The 3GPP standards body for tion of two separate LTE bands for CA puts mobile broadband specifies the spectrum of additional constraints on the phone’s hardware bands that can be used in wireless communi- components and data traffic management. In cations. Since the wireless spectrum is already particular, inter-band CA with two frequency crowded, and new chunks of wireless spectrum division duplex (FDD) LTE bands means that are hard to come by, 3GPP’s LTE-Advanced each transmit band now has two equally valid specifications identify carrier aggregation (CA) receive bands associated with it. as one means to increase bandwidth over exist- ing LTE frequency bands.3 MULTIPLEXERS With LTE, data traffic is aggregated in com- Smartphones combine a lot of functional- ponent carriers (CC) which are blocks of fre- ity into a relatively small package, and the size quency spectrum 1.4, 5, 10 or 20 MHz wide. of any component inside the phone is limited. Reprinted with permission of MICROWAVE JOURNAL® from the November 2015 supplement. ©2015 Horizon House Publications, Inc. 5G andIoT Antenna GPS Series Port 1 Port 2 PCS Rx Antenna LC Matching Cell Rx Shunt Elements PCS Tx –55 dB B2 B2 Cell Tx Tx Rx (a) GND –55 dB B4 B4 s Fig. 1 Quintuplexer comprising five Fs Fp Tx Rx –55 dB filters with dissimilar passband frequencies. ͦ S21 ͦ ͦ Zseries ͦ –55 dB The RF front-end in a handset com- (a) prises all components between the Fs Fp antenna and the baseband/transceiv- ͦ Zshunt ͦ er chips, such as frequency filtering devices, switches, power amplifiers, LNAs and a number of matching and Frequency routing elements. For any FDD band, (b) a duplexer comprises two RF filters which ensure that the uplink transmit s Fig. 2 One stage half-ladder filter topology (a) with corresponding resonator signal (Tx) does not interfere with the impedances (Zseries, Zshunt) and filter downlink reception (Rx). Integrating response (b). Practical implementations of multiple non-overlapping filter bands this basic filter topology use multiple stages. into a single module (multiplexer) can reduce component count and the size mum at the Tx and Rx frequencies. (b) of the phone’s RF front-end, as well The filters inside the CA multiplexer as simplify and accelerate the inte- must provide high Tx/Rx isolation, not s Fig. 3 Band 2/4 quadplexer block diagram (a) and evaluation board (b). The gration into the phone. Introduced only within a single FDD band but dotted lines show high Tx/Rx isolation paths several years ago, multiplexers have also across the different FDD LTE needed for carrier aggregation. become a standard way to optimize, bands used in CA. miniaturize and simplify the filtering Driven by size and performance needs of multi-band phones, requir- requirements, essentially all RF filter- ing only a single antenna to cover mul- ing devices in the handset incorporate tiple bands without a switch between surface acoustic wave (SAW) or bulk the antenna and the frequency filters.4 acoustic wave (BAW) resonator filter DIP As an example, the functional block technologies. Both SAW and BAW diagram of a quintuplexer is shown in technologies utilize piezoelectric res- Figure 1. Since CA requires different onators as the basic building block and bands to be on at the same time (i.e., are well established, mature technolo- no switching between bands), a multi- gies for RF filtering. For the filter de- FSM MMPA plexer is a convenient implementation signer, noteworthy SAW or BAW reso- to meet CA filtering needs. nator properties are acoustic coupling Although common in earlier wire- (kt2) and Q-factor. Combining resona- LPF less generations, the RF front-end tors to create a filter, Figure 2a illus- uad B2/4 architecture of LTE phones no lon- trates a 1-stage half-ladder filter topol- Q ger incorporates inter-stage filters be- ogy, a basic element for many filters. tween the transceiver and the PA in This configuration forms a passband RFIC0 the transmit path. All FDD frequen- when the series resonator has a higher RFIC1 cy filtering is now done by the duplex- resonance frequency than the shunt er or multiplexer, which puts stringent resonator (see Figure 2b). Based on performance requirements on the this basic topology, any practical filter filters. Any spurious signal from the cascades multiple half-ladder stages to s Fig. 4 Band 2/4 quadplexer on the PCB PA toward the antenna needs to be provide sufficient degrees of freedom of the RF front-end module. rejected in the Rx path to not degrade to meet any realistic in-band and out- phone sensitivity. This Tx/Rx isolation of-band filtering requirements. By do- zeros outside its passband. requirement in the duplexer specifica- ing so, the multi-stage filter arranges Combining two filters into a du- tion is typically pegged at 55 dB mini- its poles within the passband, and the plexer is the simplest form of a mul- 5G andIoT Increasing the number of filters TABLE 1 connected to the same antenna node BAND 2/4 QuaDPLEXER PERFORMANCE leads to increased insertion loss in Tx Insertion Rx Insertion Tx Band Rx Band Tx 2f Tx 3f each filter passband. This is unavoid- (dB) 0 0 Loss Loss Isolation Isolation Rejection Rejection able since the open circuit condition is never perfect or lossless (refer to the Band 2 2.0 2.9 58 59 37 38 first multiplexer design constraint not- Band 4 2.0 2.0 61 60 41 14 ed above). Elaborating on this point: to optimize multiplexer performance, 4 (1.7/2.1 GHz AWS) quadplexer (see it is not only helpful to reap low loss TABLE 2 Figure 3). Bands 2 and 4 are common and high Q resonance performance BAND 2/4 QuaDPLEXER CroSS-BAND North American FDD bands, and of the resonator building blocks for ISolation since carrier aggregation is already good in-band filter performance, it is Tx Band Rx Band available in some locations,7 there also helpful to garner low loss resona- (dB) Isolation Isolation is a tangible product need for such a tor performance at off-resonance fre- Band 2 Tx 60 (B2) 65 (B4) quadplexer module. CA-compatibility quencies. With low loss off-resonance to Band underscores the value proposition performance, the open filter matching 4 Rx of the Band 2/4 quadplexer module; condition at the antenna port can be Band 4 Tx 62 (B4) 61 (B2) however, the engineering is challeng- implemented with minimum parasitic to Band ing for multiple reasons: loss and does not drain signal off the 2 Rx Band 2 Duplexer: The filter band in-band performance of any passband. gap (Tx/Rx) is relatively narrow, ap- Using available SAW or BAW resonator tiplexer. When combining RF filters proximately 1 percent of the operat- filter technologies, part of the Band 2/4 into a CA-compatible multiplexer, ing frequency between 1910 and 1930 quadplexer optimization process is there are two essential design con- MHz. Low insertion loss at both filter minimizing the losses of all four filters straints. First, all filters in the mul- passband corners (Band 2 Tx high at all four passband frequencies. tiplexer must be matched at the channel and Rx low channel) requires CA-compatible quadplexers for common antenna node. For any of a sharp roll-off for both Tx and Rx fil- the Band 2/4 combination have been the passband frequencies, only the ters. This is non-trivial, even without demonstrated by multiple suppli- respective filter is terminated at the other filters multiplexed to the same ers, and some are available for sale. antenna port’s impedance Zant, to antenna node. Figure 4 shows how the quadplexer minimize RF reflections between the Band 4 Duplexer: The filter pass- would typically be integrated with the antenna and the multiplexer in the bands with low insertion loss are rela- other RF components in the front- phone; all other filters must appear tively easy to achieve, since the pass- end of a smartphone. Measured per- as open circuits, such that there is band width is narrow and the Tx/Rx formance of an Avago Band 2/4 quad- no signal leakage into any other fil- band gap is large. However, maintain- plexer are shown in Figure 5 and the ter path.