Cellular carriers are deploying a new technology known as License Assisted Access (LAA) utilizing unlicensed spectrum to offer increased data rates and an improved user experience. This article explores market drivers and RF solutions for LTE/LAA/Wi-Fi enabled devices.

Increased adoption of smartphones worldwide coupled with data-heavy applications like video streaming is creating an explosion in mobile data rate consumption (Figure 1).

2016 201820170

10

Exab

ytes

per

Mon

th

40

30

20

60

50

2019 2020 2021

46% CAGR

Figure 1. Expected mobile data growth (Source: Cisco VNI, 2017)

Integrated LAA/Wi-Fi RF Front-end SolutionsMeet New Requirements in 5G Smartphones

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To help deliver this data to mobile users, multiple technolo-gies are being deployed. The primary platforms being lever-aged are cellular (primarily LTE which is transitioning to 5G) and Wi-Fi (currently 802.11ac which is evolving to 802.11ax). However, in order to support increasing consumer demand for faster data and greater capacity, the industry must utilize additional spectrum and develop more spectrum-efficient technologies.

Today, there are two types of spectrum available – unlicensed spectrum used by 802.1X technologies (i.e. Wi-Fi, Bluetooth®, Zigbee®) and licensed spectrum used by cellular networks such as LTE and 5G. While unlicensed spectrum is free, carriers using the licensed spectrum pay substantial sums to acquire usage rights. Spectrum in licensed bands is in short supply and is also the reason behind current efforts to open more worldwide. On the other hand, there is substantial amount of unlicensed spectrum available in 2.4 and 5 GHz range with the possibility to extend up to 7 GHz (Figure 2).

Figure 2. Unlicensed spectrum 5 to 7 GHz

Channels20 MHz40 MHz80 MHz

160 MHzUNII Band

5150 5350 5490 5835 5935 6415 6535

820 MHz Unrestricted (60% more than 5 GHz)500 MHz Unrestricted Wi-Fi

1 2A 2B 2C 3 4 UNII - 5 6 7 8

Frequency (MHz)Cellular Bands

B46 (LAA)

V2X

B47

Current 5 GHz Wi-FiRestricted

New: Proposed UNII BandsNew: Possible Reduced PowerNot Available in Some Areas

6875 71257215

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Technical Article | Integrated LAA/Wi-Fi RF Front-end Solutions Meet New Requirements in 5G Smartphones

Figure 3. Sharing unlicensed spectrum between Wi-Fi and LTE using LAA

CellularInfrastructure

LAA Small CellPrimaryCarrier Download

and Uploadon Secondary

Carrier(s)

Unlicensed BandLicensed Band

1 GbpsUnlicensed(5 GHz)

Licensed

Anchor

CarrierAggrega�on

TM

Sharing 5 GHz Unlicensed Spectrum Between WLAN and LTE

R14-eLAAUplink and Downlink

Aggrega�on

R13-LAASupplemental

Downlink (SDL)

The substantial spectrum available in unlicensed 5 GHz (LTE Band 46) is very attractive to mobile carriers. As a result, industry efforts are underway to combine Wi-Fi and LTE to optimize licensed and unlicensed spectrum (Figure 3). LTE-LAA is a 3GPP standards-based technology that enables LTE transmission in unlicensed bands. LAA provides enhanced data capacity to mobile carriers without the additional cost for spectrum. In 3GPP Release 13, provisions for LTE-LAA sup-plementary downlink offer enhanced downlink-only capacity. LTE-LAA is capable of theoretical peak download speeds of up to 1 gigabit per second (Gbps) and deployment is currently underway at AT&T and T-Mobile, with ongoing trials at mul-tiple carriers such as China Mobile, SK Telecom, Verizon and others. Enhanced LAA (eLAA) (part of 3GPP Release 14) will offer both uplink and downlink capabilities.

At a high level, LAA technology is based on the use of carrier aggregation (CA) to tie licensed and unlicensed bands to-gether. An LTE carrier in cellular bands is the primary channel (anchor) and unlicensed carrier in 5 GHz band is the second-ary channel. LTE waveforms are used in this 5 GHz band to download large data payloads. The 3GPP standard calls for fair sharing of unlicensed spectrum – primarily listen before talk (LBT) without impacting Wi-Fi performance for nearby systems. There is joint collaboration on co-existence between the 3GPP standards group and the Wi-Fi Alliance® to ensure equitable use of shared spectrum between Wi-Fi and LTE.

System and Design ConsiderationsIn a smartphone, the Wi-Fi system-on-chip (SoC) and the cellular LAA SoC are operating at the same time to deliver extremely high download data rates (Figure 4). These SoCs will operate individually or together based on infrastructure availability as well as the network and smartphone functional-ity. Data streams are combined or used individually within the smartphone. Skyworks’ LAA/Wi-Fi front-end modules (FEMs) allow the flexibility to enable all operating modes to obtain the highest download data rates.

CellularInfrastructure

LAASmall Cell(5 + 2.4 GHz)

WiFi AccessPoint

CellularLAA SoC

LicensedAnchor

Unlicensed(5 GHz)WiFi Tx / Rx

WiFi SoC

LAA+WiFi5 GHz FEM

DATA!

CarrierAggrega�on

Figure 4. Smartphone system showing Wi-Fi and cellular-LAA SoCs

The LAA/Wi-Fi FEM combines the functionality of a full 5 GHz Wi-Fi transmit/receive module with the cellular 5 GHz-LAA receive function, allowing a single antenna

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Technical Article | Integrated LAA/Wi-Fi RF Front-end Solutions Meet New Requirements in 5G Smartphones

solution (Figure 5). The receive chain has been engineered to facilitate optimum performance for the simultaneous receive of both Wi-Fi signals and 5 GHz cellular LAA signals, which enables extremely high download data capacity. There are three elements that facilitate multiple operating states to accomplish this performance optimization.

LNA

PA

Cellular5 GHz SoCLAA

Rx

Tx

Rx

WiFi5 GHz SoC802.11ax

Spli�er

State Decision Control

Figure 5. Cellular/LAA and Wi-Fi Front-end modules

First, the receive chain contains a high-performance low-noise amplifier (LNA) with multiple low-noise gain-states as well as a high-linearity bypass-mode. This LNA allows the system to choose the correct amount of low-noise gain for low-level receive signals in addition to handling extremely high-level receive signals in the bypass-mode where noise figure is not a concern but where linearity now limits performance. This feature increases the input intermodulation intercept point (IIP) level from 6 dBm in the active state to 20 dBm in the bypass state.

Second, a splitter is incorporated–a key differentiator in allowing the simultaneous receive and routing of both Wi-Fi signals and 5 GHz cellular LAA signals. In many mobile archi-tectures, the Wi-Fi and cellular radios are often from different manufacturers and work independently. Skyworks’ solutions bridge this gap with the use of the splitter allowing both Wi-Fi and cellular receive signals to be amplified and routed to both receivers. Each receiver will then use baseband fre-quency-filtering to remove any unwanted signals. In a typical usage environment, the Wi-Fi and the 5 GHz cellular LAA transmitters may be in two different locations. This can cause these signals to be received by the user’s smartphone at two very different power levels (Figure 6). The splitter will route the two signals as well as adjust their amplitude to provide optimal receive levels to each transceiver. One challenge is the tradeoff between low-noise figure states for low-level signals and high-linearity states for high-level signals. The flexibil-ity of multiple states for the LNA in combination with the splitter provides flexibility to make these tradeoffs and priority decisions.

Rece

ive

Pow

er L

evel

Noise Floor

Frequency

WiFi Rx Signal

Cellular LAARx Signal

Figure 6. Example smartphone signal receiver power levels from separately-located transmit sources

The third section of the receive chain is the state-deci-sion-control circuit. This is a digital block that receives inputs from both the cellular SoC and the Wi-Fi SoC. This circuit synthesizes these inputs into many individual control signals for the internal functions of the FEM.

Another important function in Skyworks’ LAA/Wi-Fi FEMs is the built-in rejection for all other cellular bands below 3.8 GHz, particularly Band 40 through Band 43. This feature is important for maintaining noise figure and signal-to-noise ratios for the desired receive signals since the LAA feature will be using these lower cellular bands simultaneously with the 5 GHz receive. Unwanted signals injected into the receive chain can saturate the LNA or create intermodulation distortion, causing a severe degradation in system receiver sensitivity.

For next generation eLAA FEMs, architectures will add a 5 GHz LAA transmit function. The transmit section requires a PA path for cellular eLAA and Wi-Fi transmit signals. The PA system requirements for cellular and Wi-Fi standards are significantly different from each other and the same PA cannot be used for both. There are many differences in power control methods, modulation standards, key RF metrics like Error Vector Magnitude (EVM) and frequency mask-edge lev-els. Due to these challenges, eLAA FEMs will likely have two transmit ports and two internal optimized PAs with non-si-multaneous operation.

Skyworks’ Integrated LAA/Wi-Fi RF Front-end SolutionsSkyworks is sampling integrated LAA/Wi-Fi front-end mod-ules to leading OEMs. The SKY85774-11 (Figure 7) is a 5 GHz, dual connectivity (LAA/Wi-Fi) front-end module in a compact 2.2 x 3 mm footprint. It supports concurrent 5 GHz Wi-Fi and LAA receive chains. The device also incorporates 5 GHz Wi-Fi transmit chain, providing a very low EVM that supports next generation 802.11 requirements.

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Technical Article | Integrated LAA/Wi-Fi RF Front-end Solutions Meet New Requirements in 5G Smartphones

The SKY85817-11 (Figure 8) is a dual connectivity (LAA/Wi-Fi), dual band (2.4 and 5 GHz) front-end module in a compact 3 x 4 mm footprint. It incorporates 2.4 and 5 GHz Wi-Fi trans-mit chains, enabling very low EVM floor in order to support next generation 802.11 requirements. The module also pro-vides a high performance 5 GHz receive chain with low noise figure, high input IIP3 with low LNA supply current.

5G ANT

5G TX

Coupler

5G RX1

5G RX2

Figure 7. Skyworks’ SKY85774-11 5 GHz LAA/Wi-Fi FEM

2G ANT2G IO

BT_RF

5G Tx

5G ANT

RxOUT2

LAA_Rx

Coupler

Figure 8. Skyworks’ SKY85817-11 2.4 and 5 GHz LAA/Wi-Fi FEM

SummaryOne of the biggest challenges facing system engineers today is the increasing complexity in the RF front end and the new technologies required for supporting 5G. The need for back-ward compatibility with 4G networks, the ability to address multiple carrier aggregation combinations as well as the integration of Wi-Fi, Bluetooth®, Zigbee® and other wireless communication protocols are creating a perfect storm that system integrators must navigate. Meeting these design chal-lenges will require the ability to address signal transmission, conditioning, filtering, tuning, voltage regulation and battery charging.

Skyworks’ advanced wireless engines are highly integrat-ed, high performance transmit/receive front-end solutions designed specifically for spectrum in sub-6 GHz range and to support various network and communications protocols. Ultimately, they will enable faster time-to-market for next generation devices. For more information, please visit www.skyworksinc.com.

AuthorsDavid Whitefield Senior Technical Director Sanjiv Shah Director, Product Marketing

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