Scoping Out CA CA(Carrier Aggregation) is a technique used to combine multiple Long ‐ Term Evolution (LTE) component carriers (CCs) across the available spec­ trum to[1] :   Support wider bandwidth signals   Increase data rates   Improve network performance As of today, up to five CCs can be allocated for 100 MHz of bandwidth per user[1].

Scoping Out CA Mobile carriers can use CA to increase performance on their networks, as shown below[1] : 100% improvement in physical layer For example, AT&T’s median download speed in Chicago increased from Mbps to Mbps when AT&T started using CA there[1].

Exploring Data Rate Evolution Carriers will use CA technology to combine spectrum in low ‐, mid ‐, and high ‐ band frequencies to boost speed and capacity, and the modem class are shown as below[1]:   Any bandwidth above 20 MHz requires at least two ‐ CC CA   Any bandwidth above 40 MHz requires at least three ‐ CC CA   Any bandwidth above 60 MHz requires at least four ‐ CC CA

Exploring Data Rate Evolution With CA, Downlink data rate evolution is shown as below[1]:

Relating FDD and TDD to CA Each CC in FDD or TDD can have a bandwidth of 1.4, 3, 5, 10, 15, or 20 MHz[1,2]. For FDD, the CC number in DL should be larger than or equal to which in UL[1,2]. For TDD, the CC number in DL should be equal to which in UL[1,2]. 3GPP defined FDD ‐ TDD aggregation in Release 12, which allows either FDD or TDD as the primary cell. FDD ‐ TDD aggregation can provide an attractive combination of low ‐ band FDD for good coverage and high ‐ band TDD with more spectrum for higher data rates[1].

Relating FDD and TDD to CA But data rate differences arise depending on whether a carrier is using FDD ‐ LTE or TDD ‐ LTE[1]. As shown above, obviously, both in DL and UL, the data rate in FDD is always higher than TDD in all bandwidth[1].

Joining Adjacent CCs in the Same Band The simplest CA deployment scenario, intra ‐ band contiguous CA, aggregates multiple adjacent CCs in a single operating band[1]. Unfortunately, aggregating contiguous CCs is not always possible. However, as new spectrum bands (such as 3.5 GHz and 600 MHz) are allocated in the future, intra ‐ band contiguous CA may become more common[1].

Bringing Separate CCs Together in the Same Band As shown below, Intra ‐ band non ‐ contiguous CA is a common deployment scenario, aggregating multiple separated CCs in a single operating band[1].

Combining Multiple CCs in Different Bands Inter ‐ band CA, shown in below, aggregates multiple CCs in different operating bands (the CCs aggregated in each band can be contiguous or non ‐ contiguous)[1].

Understanding Downlink Challenges Downlink CA challenges include[1]:   Downlink sensitivity   Harmonic generation   Desense challenges in CA RF radio design

Downlink sensitivity In a non ‐ CA, single carrier FDD (frequency division duplex) scenario, an RF duplexer ensures that transmissions on the uplink do not interfere with reception on the downlink[1]. Connecting two duplexer paths can affect the filter characteristic of both duplexers, thereby causing you to lose transmit and receive path isolation required to operate at system sensitivity[1,2].

Downlink sensitivity Therefore, for CA case[2] :   Single Antenna : to insert diplexer (LB/HB combination) or phase shifter(LB/LB, HB/HB combination).   Multiple Antennas : No diplexer or phase shifter.

Downlink sensitivity In addition, for CA case, you need quadplexer if you want[2] :   Single Antenna   No diplexer, no phase shifter

Harmonic generation When the harmonic of a transmit signal falls in the receive band of a paired CA band, the sensitivity is degraded because the harmonic level in that band is high enough to prevent the desired signal from being detected[1].

Harmonic generation Thus, for LB/MB(HB) combination CA, harmonics filtering is required[1]. As shown above, in addition to inherent duplexer(comprising TX BPF) and diplexer(comprising LPF), we have to insert an additional LPF in LB path[1].

Desense challenges in RF design For CA case, multiband RF radio signals can interfere with each other because of[1]:   Insufficient in-band isolation   Insufficient cross-band isolation   Both of insufficient in-band and cross-band isolation

Desense challenges in RF design As mentioned above, insufficient in-band isolation, known as TX leakage, which is due to:   Bad duplexer TX-to-RX isolation(at least 50 dB)   Bad duplexer layout   The impedance seen from duplexer antenna port is NOT 50 Ohm.

Desense challenges in RF design As mentioned above, insufficient cross-band isolation is due to[1]:   Poor isolation between antennas   Poor isolation between PCB layout traces   Poor isolation between ASM(Antenna Switch Module) ports

Desense challenges in RF design As mentioned above, for (B17/B4) combination CA case, B17 third harmonics may interfere B4 received signal due to poor isolation between antennas, as shown below[1]:

Desense challenges in RF design As mentioned above, for (B17/B4) combination CA case, B17 third harmonics may interfere B4 received signal due to poor isolation between the two ANT port traces[1].

Desense challenges in RF design As mentioned above, for (B17/B4) combination CA case, B17 third harmonics may interfere B4 received signal due to poor isolation between ASM ports[1]. B17 3f0

Desense challenges in RF design Thus, for (LB/MB) or (LB/HB) combination of CA, the LB/MB/HB primary and diversity switches should be independent to mitigate LB harmonics desense issue[3].

Desense challenges in RF design Conversely, an ASM or DSM comprising all LB/MB/HB ports is not suitable for (LB/MB) or (LB/HB) combination of CA[4].

Intra-Band Uplink Challenges Intra ‐ band uplink CA signals use more bandwidth and have higher peak ‐ to ‐ average power ratios (PAPRs) than standard LTE signals because more subcarriers lead to higher PAPRs[1,2]. Also, numerous possible configurations of resource blocks (RBs) exist in multiple component carriers (CCs) where signals could mix and create spurious out ‐ of ‐ band problems[1].

Intra-Band Uplink Challenges As an example, two contiguous 20 MHz CCs using all 200 RBs would be allowed to back off the maximum power by 2dB. For the same two 20MHz CCs with 50 RBs allocated in each CC — positioned so there are 100 adjacent RBs — the transmitter would have 1dB MPR. This situation occurs because the 100 adjacent RBs can’t create as many out ‐ of ‐ band problems as in the 200 RB scenario[1].

Intra-Band Uplink Challenges As mentioned above, Intra ‐ band, uplink CA signals have higher peaks, more signal bandwidth, and new RB configurations. High linearity of PA is required even though MPR is implemented[1]. ACLR, intermodulation products of non ‐ contiguous RBs, spurious emissions, noise, and sensitivity must be considered[1]. For example, a PA transmitting two 20 MHz CCs with 2 dB back ‐ off requires more linearity than a 20 MHz 100 RB FDD waveform at 1 dB back ‐ off to achieve the same ACLR, even without considering memory effects related to the wider bandwidth[1].

Recognizing Inter ‐ Band Uplink Challenges Inter ‐ band uplink CA combines transmit signals from different bands. The maximum total power transmitted from a mobile device is NOT increased in these cases, so for two transmit bands, each band carries half the power of a normal transmission, or 3 dB less than a non ‐ CA signal[1]. Unlike Intra-Band UL CA, because CCs are in different bands, there will not be high PAPRs issue. And the transmit power is reduced for each, the PA linearity isn’t an issue[1].

Recognizing Inter ‐ Band Uplink Challenges Nevertheless, other front ‐ end components, like switches, have to deal with high ‐ level signals from different bands that can mix and create intermodulation products, which can interfere with one of the active cellular receivers. Thus, to manage these signals, switches must have very high linearity[1].

Reference [1] Carrier Aggregation Fundamentals For Dummies, Qorvo [2] LTE Carrier Aggregation Technology Development and Deployment Worldwide [3] SDR660 + QLN10xx + QLN Qualcomm® RF360™ with QPA5460, QPA88xx, and QPA4340 Global ET Configuration Design Example Schematic, Qualcomm [4] SKY Tx-Rx Front-End Module for Quad-Band GSM / GPRS / EDGE w/ 14 Linear TRx Switch Ports, Dual-Band TD-SCDMA, and TDD LTE Band 39, SKYWORKS