Microwave Journal

High-Power Silicon Switches for Massive MIMO Front-Ends

August 14, 2018

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MIMO transceiver architectures are becoming widely used in the design of high-power RF wireless communication systems. As a step into the 5G era, massive MIMO (mMIMO) system covering the cellular bands are now being deployed in urban areas to meet the emerging demand for high data throughput and new services. The availability of highly integrated, single chip, baseband transceiver solutions, such as ADI’s new ADRV9008/9 family of products, made this achievement possible. Similar integration is needed at the RF front-end of these systems to reduce power consumption for thermal management, shrink size, reduce cost and accommodate more MIMO channels.

MIMO architectures allow the RF output power and power handling requirements for amplifiers and switches to be relaxed. However, as the number of parallel transceiver channels increases, the complexity of peripheral circuits and total power consumption scales accordingly. ADI’s new high-power switches, fabricated in silicon, are designed to simplify the RF front-end design, eliminating the need for peripheral circuits and reducing power consumption to a negligible level. ADI’s high-power silicon switches offer RF designers and system architects the flexibility to increase system complexity without the RF front-end becoming a bottleneck in the design.

Figure 1

Figure 1 Antenna switch.

Figure 2

Figure 2 LNA protection switch.

In time-division duplex (TDD) systems, a switch function is used at the antenna interface to isolate and protect the receiver input from the power of the transmitted signal. This switch can either be used directly at the antenna interface in relatively lower power systems (see Figure 1) or placed on the receive path for higher power applications, to ensure proper termination to the duplexer (see Figure 2). In both architectures, having a shunt arm on one of the switch outputs helps improve isolation.

Figure 3

Figure 3 PIN diode switch.

Figure 4

Figure 4 Silicon switch.

PIN diode switches have been the preferred solution, offering low insertion loss and high-power handling. However, they need high bias voltages, to reverse bias for isolation, and high current, to forward bias for low insertion loss, which are shortcomings in mMIMO systems. Figure 3 shows a typical application circuit for a PIN diode switch, including the peripheral circuitry, where three discrete PIN diodes are biased through bias-tees and controlled with a high voltage interface circuit.

Figure 5

Figure 5 PCB footprint comparison: PIN diode vs. silicon switch.

ADI’s new high-power silicon switches are better suited to mMIMO designs. They are biased with a single 5 V supply, draw less than 1 mA current and do not need external components or interface circuits. The FET circuit architecture (see Figure 4) operates on low supply voltages and draws low bias currents, making the power consumption negligible and aiding system thermal management. Additionally, the FET architecture yields better isolation, as more shunt arms are incorporated on the RF signal path. ADI’s silicon switches handle RF peak power up to 80 W, adequate margin to cover the peak-to-average power requirements for mMIMO systems.

Figure 5 shows a side-by-side comparison of the PCB artwork on a single-layer design for a PIN diode switch and ADI’s new silicon switch. The footprint of the silicon switch is more than 10× smaller than the PIN diode switch footprint.

Table 1 shows ADI’s family of high-power silicon switches, which are optimized for various power levels and package options. These devices reflect the intrinsic advantages of silicon technology: better ESD robustness and less part-to-part variation compared to alternative solutions.

Table 1

As mMIMO systems evolve, they will need even higher levels of integration. ADI’s new high-power silicon switch technology is well-suited for multi-chip module designs, enabling integration with LNAs to offer a complete single chip solution for TDD receiver front-ends. ADI plans to scale new designs to cover higher frequencies, leading to similar solutions for 5G mmWave systems. Other applications such as phased array radars will benefit from the advantages of the silicon switch architecture as ADI expands its product portfolio to X-Band and higher.

Analog Devices Inc.
Norwood, Mass.