5G Wireless: Market Opportunities and Technical Challenges from Sub-6 GHz to Millimeter Wave

• For Massive MIMO systems, 4th Generation GaN technology and Multifunction Phased Array Radar (MPAR) architectures enable improved RF performance and assembly efficiency - David Ryan, Senior Business Development and Strategic Marketing Manager, MACOM

The move toward 5G mobile networks is accelerating, with explosive growth in wireless throughput and capacity. In the near term, we will see the deployment of Sub-6 GHz wireless infrastructure to bridge the bandwidth gap between existing 4G LTE networks and future millimeter wave (mmW) 5G implementations, which use frequencies much higher than 6 GHz.

Sub-6 GHz infrastructure will continue to utilize the large amount of available spectrum in the 2.5 to 2.7 GHz range, while adding frequencies in the 3.3 to 3.8 GHz range, and even reaching 4.4 to 5 GHz in some areas. China Mobile plans to conduct major pilot deployments in 2017 and 2018. Pre-5G sub-6 GHz infrastructure is expected to improve the spectrum efficiency of traditional mobile phone bands and expand capacity and coverage at data rates 10 times faster than existing 4G LTE within a comparable frequency bandwidth. Sub-6 Ghz 5G wireless infrastructure will be widely deployed using beamforming solutions, which can greatly expand network coverage and building penetration capabilities.

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Although the 3GPP Alliance's first set of 5G standards (Release 15) is not expected to be approved until June 2018, and 5G networks at mmW frequencies will not become commercially mainstream for several years, demonstration systems and pre-standards are being developed today, and some important milestones have been achieved. Earlier, Verizon and AT&T have announced tests/trials for deploying 5G mmW technology, mainly for fixed wireless applications, aiming to compete with traditional cable TV operators and provide each household with the bandwidth required to watch multiple 4K videos at the same time. 5G may also be used to provide massive capacity in densely populated environments, such as stadiums and subway shopping malls. As the technology develops, future uses will become more obvious.

However, 5G is more than just a faster network with higher frequencies. One of its key features is that 5G will enable operators to monetize their networks in new ways and evolve their business models through new features such as federated network slicing. With the ability to divide the physical network into several virtual mobile networks, operators can use the same hardware infrastructure used by consumer users to provide a wide range of quality of service (QoS) and security/encryption options to enterprise customers. In the long term, the federated network slicing feature can also enable a greater shared platform between operators, allowing them to coordinate the allocation of network resources between countries, thereby providing users with a seamless 5G roaming experience.

Massive MIMO also brings huge challenges

Sub-6 GHz and mmW 5G systems will rely on phased array technology to optimize signal links and data rates, which utilizes a large number of antenna elements configured in a 3D-MIMO (multiple-input, multiple-output) architecture. Traditional base stations can accommodate two to eight transmitters and receivers, while 3D-MIMO systems can be equipped with 64 transmit and receive (T/R) elements and can be expanded to 128 or 256 elements. These array antenna configurations increase the number of available T/R paths to maximize data rates and enable advanced beamforming capabilities that are critical to the 5G value proposition - however, the complexity and density of such systems pose many challenges to design and assembly.

3D-MIMO systems require compact front-end solutions given the reduced spacing between elements in tightly clustered antenna configurations, especially at higher frequencies. This in turn creates thermal challenges associated with generating significant RF power (up to 5W per element in some cases) and dissipating it in a small area.

Assembly of the final device is another major challenge. A 64-antenna array will house 64 power amplifiers, 64 switches, and 64 low noise amplifiers, among other devices. This large number of RF components and RF interfaces places the final yield at low risk. While some base station OEMs have production capabilities that can assemble thousands of components and handle PCB packaging in-house, other OEMs choose to purchase fully assembled modules as functional blocks in their radios to reduce complexity and yield risk. By utilizing higher-level components, component failures can be localized to each of the 64 subsystems, so the board can be reworked more easily than if a single failure compromised an assembly consisting of thousands of individual components.

Advantages of 4th Generation GaN

At the semiconductor level, the fourth generation of silicon-based gallium nitride (Gen4 GaN) has become a clear replacement for LDMOS to serve the next generation of base stations for 5G deployment, especially for frequencies of 3.5 GHz and above, where LDMOS has inherent technical limitations. The fourth generation of GaN technology has established its leading edge over LDMOS through 4G LTE infrastructure, with significant advantages in power density, space saving and energy efficiency, and also helps achieve a cost structure that is superior to LDMOS.

The raw power density of the fourth-generation GaN is 10% higher than that of the current LDMOS technology, and the power per unit area can be increased by 4 to 6 times, that is, the GaN die size is 1/6 to 1/4 of the LDMOS die size. The fourth-generation GaN has higher power density characteristics and can achieve smaller device packaging, making it very suitable for 3D-MIMO antenna systems.

In addition, Gen4 GaN offers more than 10 percent efficiency improvement over LDMOS. If properly exploited, this frequency efficiency difference can have a huge impact on commercial 5G applications at the system level, especially for advanced assemblies where multiple packaging layers require specialized solutions to high temperature issues (such as Gen4 GaN, which enables devices to operate at higher junction temperatures).

***It is important to note that the wide bandwidths that device designers can achieve with fourth-generation GaN technology are critical as operators transition to higher frequencies with wider frequency bands, which in turn provides the flexibility to achieve wider carrier aggregation bands. GaN-based PAs support wider bandwidths than LDMOS-based devices, thus reducing the number of components required to cover the main cellular bands within 5G base stations.

MPAR assembly efficiency

We know that the architecture and assembly of massive MIMO 5G systems have many similarities with the new generation of multi-function phased array radar (MPAR) systems used for military and civil air traffic control applications. Sub-6 GHz 3D-MIMO systems are particularly suitable for MPAR design and assembly strategies (assuming that both technologies cover the frequency band range of 2.6 to 3.5 GHz) and such systems share a 64-antenna architecture.

The first generation of MPAR systems utilizes Scaled Planar Array (SPAR™) chips in a planar configuration consisting of hundreds to thousands of T/R elements. Developed in collaboration between MACOM and MIT Lincoln Laboratory, SPAR chip technology enables a new, cost-sensitive approach to developing phased array radar systems by leveraging advanced RF assembly and high-volume commercial-grade packaging and manufacturing techniques.

The SPAR chip avoids the use of traditional slot array architecture and instead uses a planar chip array architecture in which antenna elements and RF beamformers are integrated into a single multi-layer RF board. In this way, the T/R module can be mounted to the PCB in SMT form using an industry-standard manufacturing process, which simplifies the system assembly process and minimizes yield risk. This phased array implementation shortens time to market and significantly reduces costs, which can drive MPAR technology to become a mainstream technology in commercial applications such as sub-6 GHz wireless applications.

5G systems using sub-6 GHz and mmW frequencies present unique design challenges from the semiconductor level to device packaging and final system assembly. Our continued innovation in GaN and phased array technologies such as MPAR will help realize the full potential of 5G, enabling base station OEMs to achieve the best balance of power output and energy efficiency in a compact form factor using modular subsystems that simplify design and manufacturing.