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ABCs of Carrier Aggregation

ABCs of Carrier Aggregation

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ABCs of Carrier Aggregation

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  1. Carrier Aggregation (CA) is a technique combining multiple signals into one data channel to enhance data rates in the system with optimized usage of available bandwidth. 5G NR expands this trend, utilizing CA in both FR1 and FR2 [2]. According to Shannon Theorem, the broader bandwidth enhances data rate. In LTE-Advanced, up to five CCs (component carrier) can be allocated for 100 MHz (20 MHz X 5) of bandwidth per user [1]. Operators 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

  2. Consequently, you can choose the number of CCs according to the request of the data rate. For example, 450 Mbps needs 60 MHz for aggregated downlink bandwidth, which means you need 3 CCs at least. With CA, Downlink data rate evolution is shown as below: 2x2 MIMO is introduced to LTE-Advanced as well, which can help boost the data rate. Each CC in FDD or TDD can have a bandwidth of 1.4, 3, 5, 10, 15, or 20 MHz. However, we usually choose 20 MHz since the major concept of CA is to enhance bandwidth, thereby boosting the data rate. As for the number of CCs, as shown below: FDD: TDD: 2

  3. 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]. From the transmission perspective, the low band has lower propagation loss, which causes longer transmission distance and broader coverage. From the spectrum perspective, the spectrum in high-band TDD is not so crowded, which means we can have more usable spectrum and broader bandwidth. As shown below, apparently, both in DL and UL, the data rate in FDD is always higher than TDD in all bandwidth. 3

  4. Let’s take 20 MHz bandwidth for example, FDD spectrum is also called paired spectrum, which means 20 MHz for Downlink and 20 MHz for Uplink individually. TDD spectrum is called Un-paired, which means 20 MHz is used for both Downlink and Uplink. In FDD, due to symmetric bandwidth so both Uplink and Downlink have the same throughput. In TDD, due to asymmetric and same bandwidth is shared by Uplink and Downlink in turn, so the total throughput is also shared accordingly [3]. To be brief, in FDD, a total of 40 MHz bandwidth is usable at any time. In TDD, merely 20 MHz bandwidth is usable at any time. 4

  5. 5

  6. There’re three scenarios for CA: Intraband contiguous Intraband non-contiguous Interband (MUST be non-contiguous) As mentioned earlier, up to five CCs can be combined. Five CCs whose bandwidth is 3MHz are aggregated through intraband contiguous scenario, which results in a 15 MHz wide signal [5]. 6

  7. The major challenge of downlink CA is desense. Let’s take DL 2CA (B17-B4) for example. Firstly, it’s the interband scenario since primary CC and secondary CC belong to different bands. If we choose B17 as primary CC, it means B17 has uplink and downlink signals simultaneously. Nevertheless, the secondary CC, B4, has only a downlink signal during CA operation, even though it’s FDD system. As shown above, in the DL 2CA scenario, there’re two downlink signals (PCC and SCC), but only one uplink (PCC). Consequently, three signals are in operation simultaneously: B17 TX, B17 RX, and B4 RX. 7

  8. As shown above, there’re two possible desense issues, one is B17 TX leakage, And the other one is that the 3rd harmonic product from B17 TX interferes with B4 RX. (B17 TX = 700 MHz, B17 3TX = 2100 MHz = B4 RX) 8

  9. As shown above, B17 TX would interfere with B17 RX through duplexer, and it may interfere with B4 RX through ASM. Thus, we need to enhance both in-band and cross-band isolation. The isolation is the most crucial parameter of the duplexer, which should be at least 50 dB. Take TDK B8017, a duplexer for B17, for example, which possess 53 dB isolation at least. 9

  10. Additionally, the quality of isolation depends on the layout as well. The proper layout should be as below: The impedance seen from the duplexer antenna port must be 50 Ohm, or the isolation may be degraded. 10

  11. Take the MMPA, SKY77645-11, for example. The 3rd harmonic product from B17 TX is -12 dBm approximately. Take the ASM, SKY13456-11, for example. The port-to-port isolation is 22 dB if we choose adjacent ports for B17 and B4. Take the B17 duplexer, B8017, for example. The rejection of the 3rd harmonic product from B17 is approximately 30 dB. 11

  12. Thus, we can calculate the 3rd harmonic product at B4 received band would be: -12 – 30 -22 = -64 dBm/20 MHz The thermal noise floor of received band for 20 MHz wide would be: -174 dBm/Hz + 10log(20 MHz) = -100 dBm/20 MHz The total noise floor is: Apparently, the harmonic product dominates the overall noise floor. To put it another way, the total noise floor increases 36 dB, or we can say SNR would decrease 36 dB. This situation must lead to desense issue. 12

  13. Therefore, we have to reject the harmonic product and augment the port-to-port isolation of ASM. We add a LPF in B17 TX path. Compared to BPF, LPF can have a lower loss. And, we use two individual ASMs for LB and MB to enhance the port- to-port isolation. Some ASMs incorporate LPF, such as SKY13456-11. 13

  14. However, since it combines LB and HB ASMs, port-to-port isolation is a potential issue as well. Finally, we need to three ports device to connect the ANT ports of the two individual ASMs. SPDT is not suitable since B17 and B4 operate simultaneously during CA operation. Thus, we choose diplexers. Firstly, it can reject harmonic products further, thereby avoiding interfering with B4 received signal of other UEs. Secondly, the B17 TX signal from other UE may be received by the antenna, and which may produce the harmonic product that can interfere with B4 received signal through the nonlinearity of ASM. The diplexer can reject the B17 TX signal from other UE. Of course, the modified architecture has some side effects. The main reason is the additional loss from LPF and diplexer. From both B17 and B4 TX perspective, the additional loss increases the PA post-loss, thereby aggravating other TX performances and current consumption. 14

  15. PA output power PA post-loss From the B17 and B4 RX perspective, the additional loss increases the total noise figure, thereby aggravating sensitivity. Indeed, the additional loss degrades antenna radiation efficiency as well. Let's get back to the topic of the port-to-port isolation of ASMs. We assume the 3rd harmonic product at B4 received band would be: -110 dBm/20 MHz Then, the total noise floor is Since the thermal noise floor of received band for 20 MHz wide would be 15

  16. -100 dBm/20 MHz. That is, with the 3rd harmonic product, the total noise floor merely increases 0.5 dB, which is acceptable. We assume 3rd harmonic product from PA is -12 dBm, the rejection from both duplexer and LPF is 30 dB. Therefore, to achieve -110 dBm/20 MHz level of the 3rd harmonic product, the port-to-port isolation of the ASM should be at least 40 dB. That’s why we need to use two separate ASMs for LB and MB individually. 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. 16

  17. Now that the solution is for harmonic product issues mainly, why do we still have to choose the low band such as B17 as the CA combination? As mentioned earlier, from the transmission perspective, the low band has lower propagation loss, which causes longer transmission distance and more extensive coverage. One thing is worth mentioning; the B17 TX signal may couple to B4 RX path due to poor isolation between traces. Thus, lay either LB or MB traces in the inner layers for better isolation. Otherwise, both TX and harmonic product from B17 may couple to B4 RX path simultaneously. And, B17 TX may produce harmonic product through the nonlinearity of ASM. That is, the total harmonic product level would be the summation of the two interferences. If both of them are -90 dBm, then the total harmonic product level would be -87 dBm. 17

  18. Indeed, we can also use two separate antennas for LB and MB individually, thereby eliminating the diplexer. So, with that, TX performance, current consumption, sensitivity, and antenna radiation efficiency would improve. Nevertheless, the isolation between LB and MB antennas should be large enough; otherwise, both B17 TX and harmonic products radiate to the B4 RX path as well. 18

  19. As for the antenna-to-antenna isolation, we have to consider the diversity antenna as well. The isolation between main and diversity antennas should be large enough. Otherwise, there would be 4 issues: 1. B4 TX => B4 DRX 2. B17 TX => B17 DRX 3. B17 TX => B4 DRX (B17 TX signal may produce 3rd harmonic product through the nonlinearity of DSM) 4. B17 3f0 => B4 DRX 19

  20. To put it another way, the worst case is the B4 DRX signal would be interfered by B4 TX, B17 TX, and B17 f0 simultaneously if the isolation between the primary antenna and secondary antenna is not large enough. Thus, the locations of main and diversity antennas inside a cellphone are as shown above, the primary antenna is usually on the bottom side, and the diversity antenna is usually on the top side. In addition to enhancing the antenna-to-antenna isolation, we put the primary antenna on the bottom side to mitigate SAR (Specific Absorption Rate) issue. 20

  21. Consequently, as for LB harmonic product issue, there’re usually three mechanisms: Poor port-to-port isolation of ASMs Poor trace-to-trace isolation of PCB layout Poor antenna-to-antenna isolation 21

  22. If you still want a single antenna without a diplexer, a three-port device is also necessary. You can have two choices: 1.Phase shifter 2.Quadplexer MB Signal LB Path MB Path LB Signal Let’s analyze the principle of a diplexer. LPF provides high impedance for the MB signal. Conversely, HPF provides high impedance for the LB signal. 22

  23. MB Signal LB Path MB Path LB Signal The principle of phase shifters is identical to that of diplexer. 23

  24. The quadplexer, as its name implies, is composed of two duplexers. LB RX LB TX MB TX MB RX However, a quadplexer usually has a higher loss than a single duplexer. Thus, you have to compare the loss between (duplexer + diplexer) and (quadplexer), and choose a lower scenario. 24

  25. Compared to DL CA, UL CA design is more challenging. The main challenge is the linearity of TX chain. The spectrum of TX signals is as shown below: As for the intraband contiguous scenario, we can regard it as a signal with broader bandwidth. According to the formula: The broader bandwidth brings more subcarriers, thereby resulting in higher PAPR (Peak-to-Average Ratio). And, high linearity of PA is required even though MPR (Maximum Power Reduction) is implemented. Besides, self-heating and wide bandwidth are key sources to the memory effect in PAs, which results in asymmetric ACLR. High PAPR leads to high current consumption, thereby causing the self-heating issue. Therefore, UL CA of intraband contiguous scenario would result in asymmetric ACLR due to memory effect. 25

  26. As for intraband non-contiguous scenario, the definition of duplex spacing and band gap is as shown below: FDD system has the inherent TX leakage issue, and how much it interferes with RX signal is relevant to the duplex spacing. 26

  27. However, take RX signal 1 for example, which would be corrupted by both TX signal 1 (relevant to duplex spacing) and TX signal 2 (relevant to band- gap). In particular, the situation often occurs in (low channel CC – high channel CC) combination. If the band gap is very narrow, such as B8 (band gap = 10 MHz), the issue may worsen further. This situation means that the ACLR of each CC must be low enough not to aggravate sensitivity, which puts more strict demands on PAs linearity. 27

  28. As for Inter‐band uplink CA, which 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]. That is, Non-CA : 23 dBm CA: 20 dBm ; 20 dBm Unlike Intra-Band UL CA, because CCs are in different bands, there may not be high PAPRs and RX band noise issues. Since the transmit power is reduced for each, the PA linearity isn’t an issue [1]. 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. 28

  29. Indeed, the aforementioned harmonic product issue occurs in inter-band UL CA as well. Additionally, since B4 TX is active during UL CA operation, the in-band isolation between B4 TX and B4 RX is also a concern. Consequently, as for inter-band UL CA, we have to deal with intermodulation issue for (LB-LB) or (MB-MB) combination, and harmonic product issue for (LB- MB) combination. 29

  30. Among the three UL CA scenarios: Intra-band contiguous Intra-band non-contiguous Inter-band The scenario, Intra-band contiguous, is the most challenging. As for Intra-band non-contiguous case, as long as the duplex spacing and band gap as well as ACLR are low enough, the RX band noise would be mitigated. As for Inter-band case, as mentioned earlier, each UL CC has 3 dB less than a non‐CA signal. The intermodulation product can be mitigated by high linearity of ASMs and high antenna-to-antenna isolation. As for harmonic product issues, which can be mitigated by high trace-to-trace isolation, high port-to-port isolation, and high antenna-to-antenna isolation. Nevertheless, high PAPR due to intra-band contiguous case is inevitable, which means this issue can NOT be mitigated whatever method you adopt. 30

  31. Reference [1] ABCs of Carrier Aggregation [2] Exploring 5G RF Technology [3] Throughput for TDD and FDD 4 G LTE Systems [4] Carrier Aggregation in LTE-A (LTE Advanced) [5] A novel fuzzy scheduler for cell-edge users in LTE-advanced networks using Voronoi algorithm [6] FRONT-END MODULE FOR CARRIER AGGREGATION MODE, US patent 31

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