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Understanding Sub-6 GHz Massive MIMO Infrastructure

The rapid adoption of 5G, the next generation of mobile connectivity, has created a lot of excitement and anticipation. Analysts predict that by the end of 2020, the number of commercial 5G networks will quadruple, the total number of 5G connections will increase from 5 million in 2019 to 2.8 billion in 2025, and the global market for 5G technology will reach 667.90 billion in 2026 Dollar. Unfortunately, achieving these ambitious coverage goals will not be easy and will require significant modifications to existing mobile network infrastructure, especially RF power applications.

Article by Roger Hall, Manager of High Performance Solutions at Qorvo

The rapid adoption of 5G, the next generation of mobile connectivity, has created a lot of excitement and anticipation. Analysts predict that by the end of 2020, the number of commercial 5G networks will quadruple, the total number of 5G connections will increase from 5 million in 2019 to 2.8 billion in 2025, and the global market for 5G technology will reach 667.90 billion in 2026 Dollar. Unfortunately, achieving these ambitious coverage goals will not be easy and will require significant modifications to existing mobile network infrastructure, especially RF power applications.

Understanding Sub-6 GHz Massive MIMO Infrastructure
(Source: Wikimedia Commons CC-BY-SA-4.0)

To meet RF front-end power needs, OEMs are turning to gallium nitride (GaN), a relatively new commercial semiconductor. Its power efficiency, power density, and wider-range frequency handling make it ideal for massive MIMO base stations.

Learn about MIMO

To realize the full potential of 5G’s high speed and ultra-low latency, customers need mobile operators to improve the performance of all network parameters. This means substantial investment in spectrum acquisition, network infrastructure and transmission technology. Regardless of how it’s done, rolling out 5G across the country will be prohibitively expensive for mobile network operators. Funding is also the biggest obstacle to 5G operations. Despite the high-frequency millimeter wave focus, operators are still implementing Massive MIMO technology in the sub-6GHz range to minimize costs and drive 5G.

MIMO, which stands for “Multiple Input Multiple Output,” is an antenna technology for wireless communications that uses multiple antennas to send and receive signals. MIMO replaces the classic single antenna used in traditional wireless communications, which sends multiple signals of the same data through different antennas. This allows for spatial multiplexing, where each channel conveys independent information to the receiver – MIMO has many advantages over traditional single antennas.

When an RF signal encounters an obstacle such as a building, the signal scatters and takes a different path to the target receiver. This multipath propagation can result in poor reception, dropped calls, and reduced rates. MIMO radios receive and combine multiple streams of the same data, so multipath propagation can be used to improve signal quality and strength. If the ambient scattering is rich enough, many independent sub-channels can be created in the same allocated bandwidth, resulting in quality and signal gain without the need for additional bandwidth or power. Instead of more base stations, network operators can focus on using larger antennas to meet demand.

MIMO antenna arrays can also focus signals in the direction of individual users through beamforming and beam steering. A single antenna can broadcast wireless signals in all directions, and through digital and analog methods, multiple antennas can also focus signals in specific directions to a receiver. This greatly improves spectral and power efficiency.

5G Massive MIMO

Past generations of wireless technology have taken advantage of MIMO’s advancements in antenna array technology to increase network speeds. 3G introduced single-user MIMO, which utilizes multiple simultaneous data streams to transmit data from a base station to a single user. 4G systems use multi-user MIMO to distribute different data streams to different users to achieve significant capacity and performance advantages. With the 5G new radio standard, MIMO has become “big enough”. 4G systems are usually equipped with four transmit and four receive antennas: a 4×4 antenna array. 5G Massive MIMO uses more transmit and receive antennas to increase transmission gain and spectral efficiency; some typical MIMO antenna sizes are 256×256.

Since Massive MIMO uses more antennas, the signal beam sent to the receiver is much narrower. It enables base stations to deliver RF energy to users more precisely and efficiently. The phase and gain of each antenna are individually controlled, and the mobile device does not need multiple receiver antennas since the channel information will be kept in the base station. The large number of base station antennas increases the signal-to-noise ratio in the cell, thereby increasing cell site capacity and throughput.

Just as importantly, 5G technology is built on 4G network infrastructure and can use dynamic spectrum sharing. This enables mobile network operators to increase network capacity, deliver high data rates and conserve spectrum while minimizing operator expenses.

The Promise of mmWave and the Reality of Sub-6 GHz

Millimeter wave technology (aka mmWave) and 5G are often mistakenly seen as synonymous. Millimeter wave is a band on the 24GHz to 100GHz radio frequency spectrum used by 5G networks, in parallel with the Sub-6GHz band. Millimeter waves were previously considered unsuitable for mobile communications because signals in this frequency band apparently have limited transmission distances and are easily blocked by buildings, leaves, rain, and people. However, these short wavelengths are able to transmit more data over short distances. It is clear that to achieve 5G’s 20Gb/s data rate target, the use of mmWave spectrum will eventually be necessary. While many mobile communications are excited about mmWave, not enough attention has been paid to the challenges of building base stations needed to roll out globally.

This becomes especially clear when looking at mmWave through base station construction. Millimeter-wave base stations have far less range than cell towers that transmit at lower frequencies. To achieve nationwide coverage, the researchers estimate that U.S. network operators will need to build 13 million base stations. Considering that today’s U.S. mobile network has roughly 300,000 base stations, the infrastructure is still a long way off. In addition, the high power requirements of mmWave further exacerbate capital expenditures, so it is not realistic to roll out mmWave to the United States in the next few years, except in stadiums and urban hotspots.

While OEMs work to reduce mmWave costs, the Sub-6GHz band will be an important frequency band for 5G network operators to rely on. The lower frequency signal penetrates further through obstacles such as buildings and covers a larger area around the tower before fading away, making it suitable for rural and urban areas. That means 5G at Sub-6 GHz can also do more with fewer base stations and use existing towers.

Massive MIMO Infrastructure Requirements

Even if 5G at Sub-6 GHz doesn’t offer the massive speed improvements that mmWave can provide, its massive MIMO antenna array can still enable more simultaneous connections, increase signal throughput, and make the difference between user coverage and capacity best balance. This is a more realistic path to implementation. The rollout of Sub-6GHz 5G will be faster than mmWave deployments, increasing the speed and consistency of mobile broadband. It offers improvements over current 4G systems while transitioning to a fully integrated 5G network. This is why many in the industry expect operators to bid on lower spectrum ranges where they can take advantage of dynamic spectrum sharing to offer 3G, 4G and 5G services in the same spectrum range. We are already seeing this approach in international 5G implementations. South Korea began rolling out 5G in lower frequencies two years ago, and China will overhaul its entire network infrastructure to achieve nationwide 5G coverage in the next few years.

This is not to say that 5G at Sub-6 GHz is easy to deploy; these new technologies come with significant system design challenges. To adopt Massive MIMO technology on 5G base stations, designers are tasked with developing highly complex systems containing hundreds of antenna elements. Many utilize active phased array antennas to provide beamforming and beam modulation capabilities to specific users. All of these extra antennas have better performance, but these large antenna arrays consume more power and require dedicated RF front-end (RFFE) chipsets and amplifiers.

Building an RF front-end to support these new Sub-6 GHz 5G applications will be a challenge. RFFE circuits are critical for power output, selectivity and power consumption of 4G systems. 5G modulation schemes bring additional demands, so wireless infrastructure power amplifiers (PAs) will need to be very efficient to achieve the necessary linearity. In addition, the large difference between the peak power requirements and the minimum power requirements also creates heat dissipation problems for the power amplifier and RF front end.

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