Bringing mm-wave beamforming to new applications
Until recently, defense and aerospace were the main markets using the 30 to 300 GHz, and specifically the millimeter-wave (mm-wave) bands, of the RF spectrum. With 5G rolling out, mainstream communications are now moving into mm-wave. The main reason for this RF expansion is demand for more data bandwidth.
But as the frequency increases, RF waves become increasingly directional. To achieve the highest data rates, the transmitter and receiver need to be aligned. A direct line of sight is optimal, but not essential. High signal-to-noise ratios can be achieved using RF energy reflected by hard surfaces in the pathway. This also creates multipaths, so directing the antenna at the source with the strongest signal helps achieve the performance needed for consistently high data rate communications.
Mechanical steering is often not required. Military applications use phased-array antennas for highly effective radar tracking systems. A transmitted beam is swept over a target area while receiving antennas attempt to pick up reflections. Active phased array antennas provide an efficient way to steer and direct the RF energy. Using phased arrays in communications systems achieves the level of radiated power and signal-to-noise ratios required.
Phased-array antenna fundamentals
A phased-array antenna employs multiple individual antenna elements. These antenna elements can be arranged in a regular pattern on a flat surface such as a printed circuit board or ceramic module. A beam is steered at the transmitter by altering the phase of the RF waveform sent to each antenna element. By adjusting each waveform’s phase, the composite antenna creates constructive interference in the desired direction combined with destructive interference for most of the unwanted directions.
The result, with enough elements at an appropriate element spacing for the wavelengths used, is a focused, narrow beam in the desired direction. Destructive interference for stray beams coming from other directions minimizes interference at the receiver.
Using multiple-input, multiple-output (MIMO) transmitters and receivers is fundamental to mm-wave band transmission in the 5G cellular communications standards. MIMO technology is also effective at lower operating frequencies, such as the 1GHz to 10GHz spectrum used for mainstream 5G and other protocols.
Beamforming is also instrumental to space and high-altitude communications, especially for systems that exploit low-earth orbits, where the angle of the beam will change significantly within a few minutes as each satellite passes overhead. Satellite and high-altitude communication will prove vital to bringing high-bandwidth coverage to areas where deploying terrestrial infrastructure is not feasible.
The automotive industry is using the mm-wave spectrum for both object tracking and high-speed communications, to help support autonomous driving. These joint radar-communication systems will be multifunctional. Vehicles will communicate with other vehicles, provide mapping data, and alerts of problems on the road.
High-speed control of phase shifts is essential
There are several key requirements for effective phased-array antenna signal processing. High-speed control is essential as the phase shifts need to be continuously assessed based on estimates of target position. Protocols in the standards let receivers send feedback on signal strength and quality, so the transmitter can adjust the relative phases to fine-tune the beam direction. Similarly, in radar systems, for an object being tracked actively, the receiver subsystems will provide directional information to the transmitter subsystem.
Sophisticated signal processing algorithms optimize the performance of the antenna, such as adaptive beamforming to mitigate interference or track moving targets. The Beamformer IC Evaluation Kit created by Avnet and Otava provides an example of that kind of control in action.
The kit incorporates an Avnet MicroZed system-on-module (SoM). The SoM is based on the AMD Xilinx Zynq 7010 system-on-chip (SoC). The SoC combines high-speed Arm processor cores with a large programmable logic array that can be combined to build sophisticated beamforming control algorithms. These algorithms track fast-moving targets for applications that require that level of performance.
A custom graphical user interface controls the board. This high level of abstraction lets engineers focus on manipulating the RF signals for best performance. Commands and results are shared through a USB port or Ethernet interface.
Control performance is only one part of the equation. Design choices in the RF path impact the performance of a phased array. One important choice is the operating bandwidth. High bandwidth increases the flexibility of the RF subsystem.
The Otava kit beamformer bandwidth is 24 GHz to 40 GHz. It covers four 5G FR2 bands (n257, n258, n260, n261), and is suitable for both Ka-band satellite communications and defence applications.
Bandwidth tradeoffs
A negative consequence of increased operating bandwidth is the higher risk of interference with high-power transmitters in adjacent bands. In receivers, the higher incoming RF energy reduces effective sensitivity. Reconfiguring the bandwidth reduces emissions and the effects of interference on a selected passband, by adding filters in the RF signal path.
Developers of phased-array transceivers can now employ wideband tunable filters to control the RF signal as it passes through the array hardware. The Otava OTFLx01 filters offer high linearity, power handling, and tuning range to make it easier to build highly flexible beamforming subsystems.
Otava's OTFL101, for example, is tunable to frequencies in the 2.5 to 7.5 GHz range with an instantaneous bandwidth of up to 1.5 GHz. The OTFL201 and OTFL301 extend the capabilities of tunable passbands to the mm-wave domain: 14 to 24 GHz and 24 to 40 GHz, respectively.
Tunable filters in mm-wave systems are now possible thanks to the capabilities of silicon-on-insulator (SOI) technology. SOI has inherent advantages over bulk silicon for RF applications, making it the technology of choice for many high-speed circuits and mm-wave and microwave applications. The buried oxide layer used by SOI helps isolate active devices from the substrate, which reduces parasitic capacitance.
The high-resistivity substrate also supports high-performance passives that exhibit lower insertion loss than those implemented on bulk silicon. The result is lower power dissipation due to less gain being required to compensate for losses.
Highly integrated ICs reduce bills of material and size
Another trade-off within phase-array subsystems relates to how the host processor or custom logic controls individual elements in real-time. Each element receives phase-shift commands. Tunable filters are programmed with the appropriate passband. Ideally, the phase-shifting elements will be mounted close to the antenna elements, which may limit the PCB routing area available for the command signals.
The phase-shifting elements are often mounted on the reverse of the antenna to provide very short signal paths. Otava's beamformer integrated circuit (IC) provides eight transmit and eight receive channels, each with an independent phase and 20dB of range-gain control. Real-time control is provided through a high-speed Serial Peripheral Interface (SPI) port that can be operated over low-voltage differential signaling (LVDS) or single-ended communication. A temperature sensor and RMS power detection are also integrated.
In the case of Otava’s filter ICs, programming is performed using a three-wire serial interface, reducing the number of signals that need to be routed. Each of the resonators in the filter design can be tuned with a five-bit coefficient. Full reconfiguration of the passbands takes just 1µs. The large number of degrees of freedom offered by the filters enables innovative approaches to tuning optimization.
To assist system architects in exploring these capabilities, Otava provides a companion behavioral model, capable of predicting filter passband and sideband skirts accurately, to support simulations. A more extensive model for The MathWorks Simulink environment enables bits-to-antenna system-level simulation to help drive RF signal-chain optimization and algorithm development.
Such techniques may prove vital to joint radar-communication algorithms that are highly sensitive to signal encoding. Those concepts can then be readily tested in the laboratory or the field with the help of the complete phased-array evaluation kit designed by Avnet and Otava, as well as evaluation boards for the individual beamformer and filter ICs.
Beamforming technology is becoming more accessible
Thanks to the integration and signal-processing advances made with SOI and other silicon-based technologies, beamforming for mm-wave bands is becoming increasingly practical and affordable. This is leading to an expansion in the range of applications that can take advantage of the higher bandwidths and lower congestion available with the mm-wave spectrum.
With its selection of devices for building phased-array antenna subsystems, Otava is now leading the market and the company's partnership with Avnet is making the technology more accessible through the evaluation kit and its software.
