Best Practices to Accelerate 5G Base Station Deployment: Your RF Front-End Massive MIMO Primer

Introduction

Strategy Analytics predicts an explosive growth of emerging 5G networks. They forecasted the number of new base station sectors deployed to double between 2018 and 2024. This rapid 5G growth will result in equipment for nearly 9.4 million new and upgraded wireless base stations deployed by 2024.

Many of these 5G base stations will incorporate massive MIMO antennas. These new 5G network architectures incorporating massive MIMO antennas are pushing always-on connectivity to the outer edges of the cellular network. In this post, we cover everything you need to know about the fundamentals of the RF front-end in the massive MIMO base station.
 

 
Figure 1. Massive MIMO benefits.
 

Massive MIMO 5G and NR Standards

5G new radio (NR) specification’s first phase of 3GPP release 15 was published in June 2018. The specification focuses on mobile deployments using 5G NR non-standalone (NSA) and standalone (SA) standards. NSA is an evolutionary step for carriers that provides a pathway to SA (see Figure 2). NSA uses an LTE anchor band for control, with a 5G NR band to deliver faster data rates. NSA allows carriers to deliver 5G data speeds without requiring a new 5G core buildout. Because we are in the beginning stages of 5G NR design, most base station applications are NSA. But this will change as 5G evolves into SA type system deployments.

 
Figure 2. The Path to Standalone.
 

5G Frequency Bands for Massive MIMO Systems

A noted challenge for base station component suppliers and manufacturers is the number of stock-keeping-units (SKUs) required regionally. These fragmented band combinations at higher frequency ranges pressure suppliers and manufacturers to diversify product portfolios (see below figure). In addition, the increase in frequency and bandwidth requirements add another level of design difficulty for RF semiconductor technology providers. For example, power amplifiers’ (PA) gain and efficiency, which are interrelated, suffer with incumbent silicon LDMOS power technologies in the transmit path. Thus, system manufacturers are migrating away from silicon LDMOS to gallium nitride (GaN) with the capability for GaN to achieve up to 60% efficiency at average operating power levels over wide bandwidth, making it optimum for massive MIMO base station systems.
 

Exploring the RF Front-End of Massive MIMO Systems (a semiconductor versus manufacturer perspective)

So, what type of RF front-end (RFFE) components are needed for 5G massive MIMO base station systems? Highly linear, highly efficient, low-power consuming integrated front-end components. To break things down from a specification standpoint, manufacturers look to semiconductor suppliers to optimize the following parameters, to meet their system requirements.

• Key RF front-end specifications manufacturers require from semiconductor suppliers
• High Adjacent Channel Power Ratio (ACPR) also known as Adjacent Channel Leakage Ratio (ACLR)
• ACPR is the ratio of the transmitter power on the assigned channel to the leakage power in the adjacent radio channel. ACPR of a transmitter is mostly set by the performance of the PA. The higher the linearity of the PA, the better the ACPR because less distortion products are generated.
• High Power Added Efficiency (PAE)
• A metric for rating the efficiency of a power amplifier that takes into account the effect of the gain of the amplifier. It is best to choose amplifiers with high PAE because the benefits are less heat generated, higher reliability, lower weight as heat sinks are smaller or non-existent and overall higher performance is achieved.
• Low Noise Figure (NF)
• The noise figure of a Low Noise Amplifier (LNA), which is the first active stage in an Rx line-up, has a direct effect on receive sensitivity in the radio. Hence RF semiconductor vendors always try to achieve a lower NF as this is one of the most critical specifications in a radio design.
• Noise Figure is a dB measure of the ratio of signal to noise ratio at input of Rx (SNRi) to the signal to noise ratio at output of Rx (SNRo).
• Low-current consumption
• Low-power consuming devices are always a good choice for system applications. They reduce thermal heat, system operating expense and additional hardware like heat sinks. Given that massive MIMO has orders of magnitude more antennas in a single radio, the need for lower power consumption is paramount.
• High channel isolation
• Isolation refers to the ability to prevent a signal appearing at a node in the circuit where it is undesired. More isolation means less interference and clearer communications. Isolation is measured as the loss between two channel ports, either transmitter-to-transmitter or transmitter-to-receiver ports. The higher the isolation, the clearer the signal.
• With 5G massive MIMO architectures, channel isolation suddenly becomes a critical specification given the close proximity of multiple antenna chains in a single radio. Although TDD operation eases the need for Tx to Rx isolation, the Tx-Tx and Rx-Rx isolation is still required. With more of the small signal content integrated into a single chipset package and having multiple Rx front-end paths in this same package, isolation compliance is only achieved by innovative semiconductor circuit design and packaging techniques.

The semiconductor suppliers must optimize the above parameters so massive MIMO system manufacturers can easily meet their specifications. The following system specification correlates to the above RF front-end semiconductor parameters.
 

• Key manufacturer system specifications
• Optimized application Equivalent Isotropically Radiated Power (EIRP)
• The product of transmitter power and the antenna gain in a given direction relative to an isotropic antenna of a radio transmitter.
• For sub-6 GHz 5G systems, 16, 32 or 64 array elements are used, depending on the EIRP required for the application. Because of the large number of array elements and the output power required from each element, thermal dissipation becomes a critical challenge, driving designs toward technologies that provide the highest possible efficiency.
• Using a technology like GaN and GaAs helps reduce the required active elements in a massive MIMO array while meeting the base station EIRP system requirements.
• High receiver sensitivity
• Receive sensitivity is a measure of the ability of a receiver to detect weak signals and process them without error. The addition of noise is the biggest hindrance to achieving the sensitivity targets. Hence using components with excellent noise figure is a critical step in designing receiver systems.
• Another measure of sensitivity of a receiver is error vector magnitude (EVM) of the decoded received signal. The ability to minimize the error in EVM can only be achieved by using low noise figure and highly linear components in your line-up, hence minimizing the distortion of an already weak signal.
• Small form-factor
• Massive MIMO systems must also be lightweight and compact enough to mount in locations ranging from cell towers to streetlamp poles. It is also paramount to locate the front-end components as close as possible to the radiating antenna. This is driving front-end integration and high power efficient semiconductor technology and packaging.
• Low-power consumption
• To meet 5G high data requirements, we will need more infrastructure (i.e., macro and micro base stations, data centers, servers, and small cells). This means an increase in network power consumption and is driving a need for system efficiency and overall power savings. Ultimately, the carriers need more for less. Providing solutions that offer high output power, increased efficiency and low-power consumption is key.
• Additionally, massive MIMO systems with their 32 or 64 channels increase the probability of more heat sinks. However, by using technologies such as GaN to improve power added efficiencies, lowers the need for large heat sinks, thus minimizing weight and size.
• Passive cooling and highly reliable
• Lower power consumption has the added benefit of lower thermal heat, thus requiring smaller heat sinks and reducing size and weight. It is essential advanced antenna systems (AAS) are highly power efficient and robust to allow passive cooling of all outdoor tower top electronics. GaN allows manufacturers to use passive cooling in some applications, reducing the need for fans or air conditioning and allowing manufacturers to place the RF front-end at the antenna.

The 5G massive MIMO base station has arrived and carriers continue to ramp up deployments. The global demand for product with varying frequencies and power levels requires manufacturers to pull from a supply chain with a diversified product portfolio. Because massive MIMO systems require stringent parameter capability with higher frequency ranges and bandwidths, new technology capability is a must. As shown in the below tables, Qorvo has one of the largest 5G massive MIMO product portfolios on the market. We also create products using technologies best suited by each massive MIMO application. Qorvo not only has product stretching across all frequencies above 3.5 GHz, but these parts also provide best-in-class performance using GaN, GaAs and filter bulk acoustic wave (BAW) technologies.
 

5G massive MIMO sub-6 GHz and mmWave infrastructure designs are already being used. Technologies such as GaN, GaAs and BAW are helping carriers and base station OEMs achieve their goals for 5G massive MIMO – helping them stretch their coverage areas to the very edge of their networks. As consumers, we are only beginning to see the capabilities of what massive MIMO and 5G will bring.