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| 4 | | 27 | densely distributed throughout urban and suburban regions. There are 4G systems currently being upgraded from 2T2R and 8T8R MIMO to 32T32R and 64T64R mMIMO antennas in anticipation of leveraging mMIMO technology to aid in upscaling 4G services to meet with 5G expectations before the full-spectrum 5G (sub-1GHz, sub-6GHz, and millimeter-wave spectrum) can be deployed. These new 5G base stations and 5G-ready 4G upgrades require many more antenna elements, with a greater number of cellular transmitters. Careful design and selection of RF components are required to keep the size and weight of these new mMIMO antennas to a minimum. A common design decision to reduce the size and weight of mMIMO antennas is to replace existing 4G antennas with a combined 4G/5G mMIMO antenna with embedded RF hardware. This type of densification can significantly reduce costs, especially as it pertains to tower space and wind-loading charges. Still, it comes at the expense of requiring higher-power-density RF transmitters that must be extremely reliable to reduce the potential of increased maintenance and service failures that are a result of component failures. Although these concerns are significant for sub-6GHz 5G systems, they are even greater concerns for current prototype and future millimeter-wave 5G systems. Even for sub-6GHz 5G systems, the 3.5GHz to 5GHz 5G new radio (NR) cellular bands experience higher frequency-related RF losses than 4G cellular bands below 3GHz. These higher losses are also coupled with the need for greater amplifier efficiency to account for the more complex and higher peak-to-average-power-ratio (PAPR) modulation signals used in the latest communication technologies. There is a significant need for RF power amplifiers that can deliver multiple benefits. These benefits include high efficiency over several gigahertz of bandwidth, exhibit high reliability even while withstanding higher power densities, and be cost-effective and small enough to be assembled into compact mMIMO antennas with embedded hardware. 5G RF Front-End Specifications mMIMO 5G antenna systems have much of the similar performance considerations as 4G, with many added concerns and constraints and more stringent performance criteria. With mMIMO transmit and receive antennas placed in such proximity, there is a heightened consideration for performance factors, such as isolation and adjacent channel power ratio (ACPR)/ adjacent channel leakage ratio (ACLR). ACPR/ACLR is a measure of the leakage of power to the adjacent radio channel from a transmitter. The main contributor to ACPR/ACLR is the linearity of the transmitter's power amplifier. A more linear power amplifier will exhibit less distortion, which results in less distortion products appearing in adjacent channels. Power amplifier linearity and distortion—specifically, amplitude distortion and phase distortion—have other impacts on deeply modulated communication signals. Even aside from meeting the transmit mask required to meet FCC or other telecommunications regulations around the globe, excessive distortion can also lead to a power amplifier degrading its transmissions. Error-vector magnitude (EVM), the measure of the deviations of constellation points from the ideal, is due to power amplifier nonlinearity, phase noise, and amplifier noise. It is critical to use power amplifier technology that maintains a high standard of linearity and noise, even under high load and temperature. However, more linear power amplifiers don't necessarily deliver better isolation metrics—transmitter to transmitter, transmitter to receiver, or receiver to receiver. High isolation is critical for mMIMO systems to prevent unwanted signals from other spatial multiplexed signals from appearing in nearby MIMO antenna elements. Even though time-domain duplex (TDD), used with 5G technology, is less susceptible to transmitter-to-receiver isolation considerations, this still doesn't address transmitter-to-transmitter or receiver-to-receiver isolation concerns. To address isolation concerns, careful circuit and packaging design are necessary. This is only possible if large and high-power components, such as transmitter PAs, are compact and versatile enough to allow for creative configurations aimed at meeting stringent isolation requirements. Other power amplifier considerations include both low current consumption and high power-added efficiency (PAE). With mMIMO antenna systems requiring arrays of transmitters and receivers, the power consumption and efficiency of each element have become critical performance criteria. This effect is amplified as future 5G rollout plans include vast numbers of dense networks being placed throughout urban and suburban environments, from macro-cell towers to the sides/tops of buildings and telephone poles to street lamps and tunnel/subway structures. With so many more 5G base stations planned, there is a greater pressure to reduce overall power consumption, of which a transmitter's power amplifier is one of the highest-power-consuming components. Higher-PAE amplifiers lead to reduced overall energy consumption for the same output power but also have other beneficial effects.

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