April 13, 2018
One of the earliest uses of 5G will be fixed wireless access (FWA), which promises to deliver gigabit internet speeds. FWA can be delivered to homes, apartments and businesses in a fraction of the time and cost of traditional cable/fiber installations. As with any technological advance, FWA brings new design hurdles and technology decisions. Let’s dig into five things to consider when designing FWA systems:
The first decision is whether to use mmWave or sub-6 GHz frequencies for FWA:
Efficient use of frequency range (sub-6 GHz or mmWave) is critical to scaling deployments. The choice for any situation will depend on balancing the goals of speed and coverage.
An FWA system will also need to employ active antenna systems (AAS) and massive MIMO (multiple input/multiple output) to deliver gigabit service.
Learn more about AAS and massive MIMO: How Carrier
Networks Will Enable 5G
A third element to consider is the type of beamforming to employ — all-digital or hybrid.
The most obvious choice in mmWave base station applications is to upgrade the current platform. You could explore extending all-digital beamforming massive MIMO platforms used for sub-6 GHz frequencies, but this isn’t a plug-and-play solution.
An all-digital approach faces these design constraints:
Remember: An array’s size is dependent on:
EIRP is the product of:
To achieve the target EIRP of 75 dBm and beamforming gain, an all-digital solution using today’s technology would need 16 transceivers. This would equal a total power consumption of 440 W. But for outdoor passive-cooled, tower-top electronics, it’s challenging to thermally manage more than 300 W from the RF subsystem. We need new technological solutions.
Efficient GaN Doherty PAs with digital predistortion (DPD) may provide the required margin, but these devices are still in development for mmWave applications. But it won’t be long before we see an all-digital beamforming solution. Several developments will make it a reality:
An alternative is hybrid beamforming, where the precoding and combining are done in both baseband and RF front-end module (FEM) areas. By reducing the total number of RF chains and analog-to-digital and digital-to-analog converters, hybrid beamforming achieves similar performance to digital beamforming while saving power and reducing complexity.
Another advantage of the hybrid approach is the ability to meet both a suburban fixed or limited scan range (<20º) and dense urban deployments with wide scan ranges in both azimuth (~120°) and elevation (~90°).
The bottom line: all-digital and hybrid approaches both have advantages and
disadvantages. We believe the hybrid approach is more appealing and doable
today, but new products on the horizon could make the all-digital approach
equally appealing in the future.
The technology you choose for the FWA front end depends on the EIRP, antenna gain and noise figure (NF) needs of the system. All are functions of beamforming gain, which is a function of the array size. Today, you can choose between a SiGe or GaN front end to achieve your desired system needs.
In the U.S., the Federal Communications Commission (FCC) has set high EIRP limits for 28 GHz and 39 GHz spectrum, as shown in the following table.
To achieve 75 dBm EIRP with a uniform rectangular array, the PA power output required per channel reduces as the number of elements increases (i.e., the beamforming gain increases). As shown in the below figure, as the array size gets very large (>512 active elements), the output power per element becomes small enough to use a SiGe PA, which could then be integrated into the core beamformer RFIC.
As you can see from the table below, a SiGe PA can achieve 65 dBm EIRP using 1024 active channels. However, by using GaN technology for the front end, the same EIRP can be achieved with 16x fewer channels.
A GaN FWA front end provides other benefits:
The takeaway: In wireless infrastructure applications, reliability is imperative because equipment must last for at least 10 years. For FWA, GaN is a better choice than SiGe for reliability, cost, lower power dissipation and array size.
The last consideration is selecting product solutions that are being used in real-world applications. Several RF companies are positioned to support the development of sub-6 GHz and cmWave/mmWave FWA infrastructure. Qorvo, for instance, is already supplying products for many Tier 1 and Tier 2 supplier field trials. Across the RF industry, examples of products for FWA include:
Additionally, in the 5G infrastructure space, several things are a must:
To support these trends, Qorvo has created integrated transmit and receive modules for cmWave/mmWave, as well as integrated GaN FEMs. These integrated modules include a PA, switch and LNA, and have high gain to drive the core beamformer RFICs. To meet the infrastructure passive-cooling specification, we use GaN-on-SiC to support the higher junction temperature.
For more information on Qorvo solutions for FWA, click on the images below or visit our 5G Infrastructure page, where you'll find product details and interactive block diagrams.
FWA implementation has begun, and full commercialization is approaching rapidly. Today, we believe hybrid beamforming is the best approach. Additionally, GaN, along with SiGe core beamforming, meets FCC EIRP targets of 75 dBm / 100 MHz base station targets. This approach also minimizes cost, complexity, size and power dissipation.
For more information on application-specific components, visit
Qorvo’s 5G Infrastructure
solutions online. For technical guidance and applications support, please
visit our Technical
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