Benefits of RFSOI Technology for mmWave 5G Phased Array Antennas

Phased array antennas have proven their value for mmWave 5G radios. The improved beam reach and data throughput offered by a focused beam combines with the spatial multiplexing possibilities offered by directional beam steering to make array antennas essential at these frequencies. Driving these arrays of antennas with each antenna’s signal having a unique amplitude and phase setting requires significant circuit complexity. Distributing analog beamforming ICs, placing them in an array within the footprint of the array of antenna elements, has proven to be highly manufacturable, low in RF signal losses, and overall cost effective.

However, implementation of these beamforming ICs (BFICs) has suffered from some limitations due in part to the semiconductor technologies available – bulk CMOS, SiGe, GaAs, and GaN. RFSOI forms the basis for fully optimized BFICs and is enabling phased array antenna systems for 5G and other applications, such as flat panel electronically steerable SatCom antennas, which have higher performance and lower cost. In 5G infrastructure, these performance advantages extend further to allow for lower network deployment and OPEX costs.

Silicon Versus Silicon

First, let’s compare RFSOI with other Silicon processes which have been used thus far for BFICs – bulk CMOS and SiGe. RFSOI has several key fundamental advantages over these alternative Silicon technologies. The “SOI” in RFSOI stands for “Silicon on Insulator”. In SOI, the semiconducting Silicon substrate beneath the active layers is replaced with an insulating oxide. The high resistivity insulating substrate reduces parasitics in several key areas of the circuit and allows for higher isolation between transistors. Due to the high losses at mmWave frequencies, parasitics within active and passive elements are both critical.

Lower parasitics at the FET junction and FET perimeter allows for faster transistors. For example, GlobalFoundries 45RFSOI devices boast F_t and F_max of 305 and 380 GHz respectively. This is much higher than the mid 200 GHz achieved by comparable bulk CMOS. Beyond the high transistor speed, which is of course very helpful at mmWave bands of operation, reduced parasitics also allow for improved passive component quality factors and lower loss. BFICs require a complex routing network through each Tx and Rx chain of amplifiers, attenuators, and phase shifters. SOI allows for lower loss transmission lines, matching networks, splitting/combining, and filter structures. Improvements to these structures leads to significantly reduced signal loss within the RF chain. The resulting loss reduction then drives lower gain and RF output power requirements from the amplifiers in the chain, and thereby allows improved overall power efficiency.

RFSOI versus III-IV’s

Second, it’s important to compare RFSOI against traditional III-V compound semiconductors. Compound semiconductors like GaAs and GaN can have raw RF performance advantages over Silicon processes thanks to high carrier mobility and transistor breakdown voltages. However, they have several disadvantages that allow RFSOI to maintain the upper hand for these applications. These disadvantages include low levels of integration, higher costs, and very high RF power levels required to observe optimal efficiency.

Considering circuit integration: III-V compound semiconductors such as GaAs and GaN can in some, but not all cases, integrate amplifier functions (like PA’s and LNA’s) but fall short of complete RF front end integration. These integration limitations often end up forcing the use of a separate – Silicon based – companion IC for key functions. RFSOI allows for extremely high levels of integration with single chip integration of all the required functions in the RF/mmWave, analog, and digital domains.

• Tx and Rx chains with integrated Power Amplifiers and Low Noise Amplifiers
• Low loss RF switches
• Complex control circuits for bias, gain, and phase are easier to implement in Silicon
• Digital control, SPI and LVDS, on-chip Beam Table storage memory
• Digital to Analog DAC and Analog to Digital ADC conversion
• Analog functions such as Temperature and Power sensing

Compound semiconductor solutions for 5G phased array antennas also suffer from higher system costs. III-V GaAs and GaN chips are inherently more expensive to produce. Smaller wafer size of 75, 100, or 150 mm compared to RFSOI’s 300 mm, plus comparatively exotic and expensive substrate materials contribute to higher cost per square mm. Also, the higher RF power levels where GaAs and GaN perform best can force the use of additional distribution networks to send the signal energy to multiple antenna elements – increasing losses and PCB complexity and costs.

RFSOI’s Advantages Translate to Higher BFIC Performance

The advantages we’ve outlined show that RFSOI brings several advantages over both III-V’s and alternative Silicon technologies like bulk CMOS and SiGe, but how does that translate to advantages for mmWave BFICs? We already touched on how lower losses in passive structures can result in improved efficiency for the RF line up. However, there are several other aspects of BFIC operation which can be improved thanks to RFSOI. While it does require years of development, optimized building blocks like power amplifiers, low noise amplifiers, and beam forming gain and phase control circuits uniquely enabled by RFSOI can extract yet more efficiency and higher linear RF power levels per channel (more on why this is so important later).

Higher isolation = Higher linear RF power and more efficient power amplifiers

The improved isolation between the transistors offered by RFSOI enables “stacking” of transistor devices in power amplifiers and RF switches to allow for higher voltage operation. In power amplifiers this allows for higher supply voltages and higher output impedances which reduces matching loss and increases operational efficiency. Given the bulk of power dissipation in BFICs occurs in the power amplifier during Tx mode, efficiency improvements here mean substantial overall radio efficiency improvement. Higher voltage swings also result in higher linear RF output power capability, allowing each channel of the BFIC to deliver more RF power to the associated antenna element.

For 5G systems which operate in TDD, the BFIC must have several RF switches to go between Tx and Rx mode. In the TDD switches, the transistor isolation of RFSOI also allows for stacking of transistors. In switches, this allows for increased voltage swings across the switch legs, better switch isolation, as well as lower loss, all while maintaining low Ron*C_off figures of merit.

Better BFICs Mean Better Array Antennas

Moving up yet another level in the radio system, improved beam former ICs based on RFSOI enable improvements in flat panel electronically steerable phased array antennas and their application in mmWave 5G. Higher linear RF Tx power per channel from these BFICs means fewer antenna elements are required to achieve a desired EIRP (radiated power) target for the complete array. The antenna array can fully be optimized to balance gain, Rx sensitivity, and scanning performance. Fewer antenna elements also translate to smaller antenna dimensions and fewer BFICs required. Having both smaller boards and fewer components reduces system BOM cost.

The improved efficiency of these optimized BFICs also plays a part in array antenna cost. By reducing the total power dissipation, waste heat management requirements are reduced. Heat sinks can be reduced in size and weight, directly impacting the physical dimensions of the final product. Higher linear power and better efficiency from RFSOI BFICs allow for smaller, cheaper, and frankly nicer looking radios that are easier to hide on the side of a building or street furniture.

AiP – The “Grail” of mmWave Antenna Arrays

Lower power dissipation of optimized RFSOI based BFICS is also enabling unique array implementations. The promise of Antenna-in-Package (AiP) has long been recognized as it allows for antenna arrays to be cost effectively integrated directly into the package around the BFICs. AiP can allow for optimal antenna spacings at higher frequencies than standard PCB technology. It can also allow for lower loss connections between the BFICs and the antenna elements. Finally, AiP modules can be delivered with gain and phase calibration pre-programmed right to the antenna plane – reducing or eliminating the need for final product calibration (a major cost). Without the lower power dissipation offered by truly efficient BFICs, the AiP package concept falls apart as the circuits literally cook – shortening circuit life and threatening the mechanical integrity of the package laminations.

AiP reduces system costs in other ways beyond the reduced calibration test time. By putting the most sensitive routing within the module, lower tolerance PCB processes may be used. This reduces reliance on the comparatively small number of PCB suppliers who can produce traces with the tolerances required for high repeatability mmWave routing. The PCB to which the AiP is mounted can also now utilize fewer of the most expensive low-loss layers. Finally, by taking on the most difficult portions of the design, the antenna elements and the BFIC interface, AiP can radically reduce the engineering challenge, development time, and therefore cost of a new mmWave antenna system.

MixComm ECLIPSE3741: 39 GHz AiP with 16 element antenna array on left and four IC dual-polarization BFICs on right

Better Array Antennas Lead to Better Network Deployments

The first deployed mmWave 5G phased array antennas using bulk CMOS and other technologies like SiGe fell short in multiple areas. They were quite low in efficiency, which increased power consumption OPEX and created significant heat problems. Those beam former ICs also had very low linear output power per channel. This meant that extremely large arrays of ICs and Antennas had to be developed to reach the targeted radiated power. Naturally, this was not desirable from a cost, size, and aesthetics perspective. In fact, some of these arrays simply were not able to meet the desired EIRP levels within an array size that didn’t compromise other performance factors such as scan angle.

Better arrays enabled by RFSOI based BFICS lead to smaller and lighter antennas with small arrays and small heatsinks. These reduced dimensions improve deployments in several ways. Less wind and weight load means lower attachment costs. Smaller antennas are also more aesthetically pleasing, and can be easier to hide, making them more palatable to local regulatory bodies.

Better mmWave antenna array implementations also can reach the radiated power limits as specified by the FCC. This allows for maximum reach, which is of course critical given the higher atmospheric losses associated with mmWave. Higher radiated power and reach means fewer radios are required - which can add up quickly in terms of system deployment cost savings.

5G mmWave: Ready to Meet Its Full Potential

RFSOI is a foundational technology with numerous inherent advantages for mmWave. Optimized mmWave building blocks making full use of these advantages allows for higher performance beam forming ICs. Higher performance BFICs allow for smaller and cheaper phased array antennas, including the easiest and lowest cost format embodied in AiP implementations. By building layer by layer upon the powerful RFSOI foundation we’re able to see systems which enable mmWave 5G deployments to meet their true potential.

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