New wireless standards promise multigigabit transmission rates. Delivering such rates requires some careful design of circuitry going into both small cells and receiving devices.
The wireless consumer market is looking for technologies capable of providing multi gigabits per second (Gbps) to satisfy the needs of low-latency high-definition applications such as high-definition video streaming and virtual reality or augmented reality (VR/AR) applications. These requirements have led to next-generation standards such as 5G and extended WiGig (802.11ay), which cover both user and infrastructure equipment.
Multiple Gbps communication speeds require wide bandwidth, which is available at high carrier frequencies in the millimeter wave (i.e., 30-300 GHz) range. For example, IEEE 802.11ay standard defines 6 channels of 2.16 GHz each from 57 GHz to 71 GHz. This gives the potential to go up to 35.4 Gbps coded data rate when four channels are bonded together.
Since transmission loss in air is inversely proportional to the square of the signal frequency, signals at mm-waves are highly attenuated. Therefore, multiple antenna paths are usually required to overcome signal losses. With a high-gain antenna array, the beam becomes too narrow, and electronic beam steering becomes important to establish a communication link. Two key components can be identified when using a large beam-steered antenna array: the transmit-receive (T/R) switch and the phase shifter.
Transmit-receive switch integration
The T/R switch is used to share the large-size antenna array between the transmit and receive modes without degrading the RF performance. The main two specifications of a T/R switch are insertion loss and isolation. As shown in Figure 1, there are three T/R switch types: the series switch, the series-shunt switch and the shunt switch. The series switch usually suffers from a high insertion loss due to Ron and poor isolation due to Coff. The parallel switch in the series-shunt topology improves isolation by grounding the signal but adds to the insertion loss. The shunt topology replaces the series switch with a λ/4 transmission line to provide a high impedance when the signal is grounded. The trade-off among the 3 topologies highly depends on the used technology and frequency of operation. For example, both the series switch and the 50Ω λ/4 TL (~ 600µm long) give an insertion loss of ~ 1.5dB in 28nm bulk-CMOS at 60 GHz.
Figure 2 Switch-less TRX front-end including PA and LNA
Phase shifting for electronic beam steering
Figure 4 Phase shifter schematic including the variable cascode array and its ideal settings per quarter.
A full 8-way transceiver front-end
Using the T/R switch of Figure 2 and the phase shifter of Figure 4, a 60 GHz phased array wireless module was developed including eight front-ends with antenna patches. Fabricated in TSMC 28nm CMOS technology, the chip (Figure 5) achieves a TX output P1dB of 10dBm and an RX noise figure (NF) of 6.8dB (per path) and consumes 231 mW in RX mode and 508 mW in TX mode. The phase shifter achieves an amplitude error within ±0.8dB and a maximum phase step of 6 degrees over the whole 360-degree phase difference without calibration. The amplitude error and phase step can be adjusted to ±0.35dB and 5 degrees, respectively, after calibration.
Figure 5 Block diagram and die micrograph of the 60 GHz phased array chip
The chip is flipped on a 12-layer Megtron 6 PCB for wireless testing using patch antennas placed in a circular shape for similar azimuth and elevation performance (Figure 6). The module uses the uncalibrated phase shifter settings and achieves less than 0.4dB peak-to-peak gain ripple over a 3dB scan angle of ±46 degrees. In this case, the gain and angle errors of a beam directed to a certain angle are a result of the average of the amplitude and phase errors of all phase shifters in the system. A robust active phase shifter enables steering the beam electronically without extensive calibration.
The 60 GHz in-out interface allows integrating the chip with another similar front-end chip (enabling range extension) or a baseband chip. This allows the chip to be useful for both mobile indoor applications such as 5G and WiGig as well as outdoor applications such as small-cell backhaul and fixed wireless access.
Wireless noise figure measurement and EINF
Figure 7 Wireless NF measurement setup