Electronic Design

Build A Phased Array On A Wafer To Boost Antenna Performance

The need for significant improvements in radar continues to be an ongoing concern for the Department of Defense (DoD). One of the most impressive developments addressing this need is a high-resolution RF beam-forming system using phased-array antennas.

While phased arrays are far from new, a new wafer-scale antenna demonstrates important benefits that can support low-power-density arrays for a variety of radar applications. In the military arena, uses include:

  • ground-based midcourse defense radars
  • sea-based radars
  • future block ballistic-missile defense system radars
  • space-based radar constellations with missions that include persistent near real-time tracking of Moving Targets Indications
  • high-resolution terrain information
  • synthetic aperture radar

High-resolution RF beam-forming technology also has terrific potential in commercial wireless applications, such as ad hoc communication, point-to-point and point-to-multipoint wireless connectivity, radiometry, passive-medical imaging, mobility aids for the visually impaired, surveillance, and collision-avoidance systems.

Designers can take advantage of a compact, reliable, efficient, low-cost semiconductor and ceramic material solution for V-band (50- to 75-GHz) radars that support affordable, full field-of-view (FOV) operation while decreasing the hardware, logistics, and operating costs of current systems.

This solution is realized in a single wafer-scale antenna module (WSAM) that holds all of the antenna components and functions (such as RF beam forming).1 The WSAM concept, which provides an agnostic solution for phased-array antennas, significantly enhances the signal-to-noise ratio (SNR) of the radar systems.

At X-band (8 to 12 GHz), the concept involves a 64-element array containing the antenna elements, as well as all transmitter (Tx) and receiver (Rx) circuits on a standard 8-in. silicon wafer (Fig. 1a). At the higher V-band frequencies, with much smaller antenna elements, the wafer can handle up to 1024 elements and related circuitry (Fig. 1b).

CHALLENGES OF WAFER-SCALE INTEGRATION
Wafer-scale integration isn't without its challenges. For one, the design must maintain a uniform phase and amplitude from the central feed point to all antenna elements. Yet at 60 GHz, line attenuation can be severe, varying from less than 10 dB for 1 mm to about 200 dB for 150 mm.2 Therefore, the wafer uses a balanced Htree and distributed amplification along the signal path to help compensate for line attenuation (Fig. 2).

The width of the transmission line, also called a coplanar waveguide (CPW), is another issue. A wider CPW means more parasitic coupling due to the capacitance and inductance of the transmission line and its surrounding environment. But the narrower the CPW, the higher the resistance caused by the skin effect and surrounding parasitics.

We chose a 4-µm line because parasitic attenuation is more dominant than the skin effect. Though that narrow a line will create a higher resistance, a wider line's larger capacitance and inductance would deteriorate the signal significantly.

DISTRIBUTED AMPLIFIER DESIGN
The distributed amplification scheme employs an innovative approach that's based on load-balancing amplification (LBA) between the matching circuits and the driver amplifier pair (Fig. 3). By changing the ratio of TL1/TL2 (the combined length of TL1 and TL2 equal a multiple of a quarter wavelength), a nominal stable gain of around 10 to 15 dB per LBA stage (including the transmission-line attenuation) can be obtained. The power consumption for the LBA unit is less than 40 mW with a 1.5-V supply for the standard silicon-based process.

Figure 4 illustrates a proposed ultra-wideband (UWB) beam-forming Tx/Rx unit. During transmission, the RF signal is routed and phase-shifted, amplified, and coupled to the transmit antenna. The advantage of such a beam-forming function is that it provides the enhanced range and coverage that's required. It does so by improving the SNR of the transmit channel for a point-to-point or point-to-multipoint broadcast or for a fine-resolution radar function.

Similarly, during the receive operation, the RF signal is routed and amplified, phase-shifted, combined, and delivered to an external digital-signal-processing unit. The phase-shifting function, if repeated for multiple input antennas with fixed increments and properly weighted and combined, will result in spatial beam forming that can be applied for target search and tracking applications.

The RF unit consists of a distributed low-noise amplifier (DLNA), a CMOSbased low-power controller, analog switches to select the Rx or Tx mode, and delay-line routing or a phase-shifter unit (Fig. 4, again). The power-amplifier (PA) function is replaced with the output of a low-power DLNA, so ample signal energy is transmitted without resorting to a high-power PA.

The controller function addresses power-management based on a peak-detection mechanism, DLNA gain control for optimum and constant SNR performance, and a phase shifter (delay-line array) that's based on at least a 2-bit digitally-controlled shifter. The RF unit's communication with an external electronic unit (a baseband processor and media-accesscontroller functions) is based on a series of received pulse blocks that contain information to address.

This includes initialization, element selection in case of array-antenna-deployment (row and/or column selection) capability for addressing a tile array,3 selecting the amount of the phase shift/delay, an optional field to address LNA gain control, Tx or Rx switch selection, and power management to address sleep mode of operation.

The DLNA and integrated amplifier section can be a multistage bipolar-or CMOS-based design using less than 100 mW to provide a 10-dB gain in active mode. The phase shifter provides at least 2 bits of controllable phase shifting with a maximum variation of 10° rms. This translates to an accuracy of 0.6 ps rms at 4.15-ps resolution operating at 60 GHz. The RF switches, DLNA, phase shifter, and controller are all included in a die that fits underneath the antenna plate as an element within the whole wafer. Proper shielding and isolation are provided to eliminate electromagnetic coupling to active devices.

For data communications, stringent limitations are imposed on the unit's design. To attain a 70% eye opening (the capture window time slot) for signal detection, the maximum tolerable total jitter should be less than 1.2 ps. As a result, the total jitter limitation—including rise time and fall time on precise location of the beam, the SNR of the RF data path, and the active circuitry—should be less than 2 ps at 60 GHz.

This requirement clearly is beyond the capability of current technology for low-power operation. As a reference, the total jitter for OC-192 (10 Gbits/s) and OC-768 (40 Gbits/s) is less than 2 ps.4 This kind of limitation on a Tx/Rx millimeter-wave signal at V-band makes the design of a UWB impulse radio very attractive. A pulse-position modulation (PPM) scheme appears to be the most attractive solution.5

ANTENNA ARRAY GAIN AND BEAM WIDTH
The isolated antenna element provides a bandwidth of 7% for a 2:1 voltage standing wave ratio (VSWR) at a gain of better than 7 dB. The element is excited through a metal rod connected to a via that's filled with a deposited metal layer. That layer is connected to the output of an analog RF switch powered by a PA, or an equivalent DLNA after proper phase shifting.

Similarly, the element can deliver received radiation to the input of the RF switch feeding the DLNA and then perform phase shifting.

Figure 5 shows the simulation results for an array of antenna dipole loops built on a silicon substrate. This simulation was based on a honeycomb substrate with relative permittivity of 2.2. At 60 GHz, 32 elements per row provide a beam width of less than 2°, yielding a 1-m resolution at a 150-m far field. A 2° beam with a side lobe of less than 5 dB has been obtained for a 32-by-2 tile array (two rows of 32 antenna loops). The 32-by-2 antenna array's gain is about 21 dBi. A matrix of 32 by 32 (1024 elements) provides an antenna gain of better than 37 dBi.

With proper phase and amplitude management, the array's beam can be adjusted to address a variety of adaptive functions. These can include high-resolution scanning of far and near targets, dynamically controlled distant tracking, and power management to reduce RF interference on mass-market guidance systems, such as automotive collision-avoidance radars.

SCALABILITY AND INTEGRATION
As noted, packing more antenna elements on a single wafer enhances the antenna gain and provides a finer beam width, which is essential for high-resolution sensor or radar scanning applications, as well as very-high-data-rate wireless links.

Figure 6 shows a prototype of such an array. The 1024 antenna elements are integrated in a multiple-input, multiple-output (MIMO) configuration beneath each sub-array of the antennas. Due to the spatial power-combining property of waves, the beam-forming function provides enhanced range and coverage, which is needed to improve the SNR of the transmit or receive channel over long distances and for fine resolution.

One of the key limitations in designing a WSAM is dissipating the heat from the wafer. A practical limitation for heat dissipation without a need for an external cooling system is about 1 W/cm2 for a silicon substrate. That translates to about 375 W/wafer (for an 8-in. substrate). Obviously, the power dissipation is directly related to the number of antenna elements per wafer. With various packing densities that are the function of the frequency of operation, 64 elements and antennas can be placed at X-band and 4096 elements at V-band.

The key contributors to the integrated solution are the power consumed by the PA (DLNA) and by the controller to address global control of the WSAM. It should be emphasized that a WSAM integrated solution includes building the antenna array and the active electronics on the same substrate.

The key point is that for a 4096-element array (at 60 GHz), the weight of the WSAM module is at least 2000 times less than that of a discrete-component version—a result of extensive reduction of package and substrate material. Notably, the volume of the WSAM module is 4000 times less than the discrete-component-based implementation.

ACKNOWLEDGMENT
The author acknowledges significant support of the WSAM project by various DoD agencies. The development of RF and control-signal distribution across the WSAM has been sponsored by the Defense Advance Research Projects Agency (DARPA). The development of the electronic-scanning capability of the WSAM has been supported by the Air Force Research Laboratory.

References

  1. F. Mohamadi, "A proposed completelyelectronically controlled beam-forming technology for coverage enhancement," IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs), March 2005, Atlanta, Ga.
  2. B. Cleveland, et al., "Exploiting CMOS reverse interconnect scaling in multi-gigahertz amplifier and oscillator design," IEEE Journal of Solid-State Circuits, Vol. 36, No. 10, Oct. 2001, p. 1480-1487
  3. F. Mohamadi, "Si integration with millimeter-wave phased array antenna," RF Design, Feb. 2004, p. 40-48
  4. IEEE Standards 802.3ae, "Media Access Control (MAC) Parameters, Physical Layer, and Management Parameters for 10 Gb/s Operation," http://grouper.ieee.org/groups/802/3/a e/public/
  5. M.Z. Win and R.A. Scholtz, " Ultrawide bandwidth time-hopping spreadspectrum impulse radio for wireless multiple-access communications," IEEE Trans. Comm. vol. 48, p. 679-689, April 2000
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