Friday, September 28, 2018

Using True Time Delay to Increase Instantaneous Bandwidth in a Phased Array Antenna

The progression towards High Throughput Satellites is enabled by frequency reuse using larger and larger bandwidth signals.  Bandwidths exceeding 100 Mhz are commonplace, with 250 MHz and larger bandwidths evident. A phased-array antenna belies the concept of using phase control to form a beam (or inherently, steer the beam electronically.) 

Using an array of fixed phase shifting (the same phase shift at all frequencies) between elements creates a problem known as beam squint: the phase shift at the low end of the signal spectrum and the phase shift at the high end of the signal spectrum are different enough that the beam points differently from one extreme to the other.
Beam Squint: A signal at 9 GHz directed differently than a signal at 11 GHz
Beam Squint from a carrier at 6 Ghz to a carrier at 8 GHz

Typical aero beam steering accuracy is expected to be 0.2 deg or less. Contributions from latency, granularity, interference, accuracy of sensor data; consume the error budget.  Beam squint cannot be allowed to contribute significantly, and this restricts instantaneous signal bandwidth. Early phased array using fixed phase shifting appear to struggle in support of bandwidth broader than 100 Mhz.

The wavefront arrives at every element in a phased array of elements based on path distance. Path distance is related to the angle of incidence the distance between the elements. The phase difference is naturally scaled to the particular frequency.

The wavefront intercepts an array of elements based on the distance between them and the angle of incidence

True Time Delay applies variable phase shifting across the spectrum of the signal being steered between the elements.

One example of a something resembling a True Time Delay phased array is ThinKom VICTS technology, which uses an optical solution. The traveling wave front is manipulated in an analog manner, which naturally compensates the phasing to a large degree.

the wavefront passes to the right until reflected by the variable inclination surface

A fixed time delay line between elements can be used for a single steering solution. 

A set of time delay lines, each arranged progressively from short to long, and with enough to support necessary steering granularity, can selectively steer the beam.  The steering solution is based on setting True Time Delay, wherein the phase shift suitably matches signal spectrum.

A one dimensional array could have a columns of sub-array elements, with True Time Delay between each sub-array.  This could be used for azimuth steering a vertical oblong beampattern.

No Beam Squint using True Time Delay across 2 GHz carrier

A two-dimensional array would have a matrix between each elemental sub-array with true time delay.

It is possible to also provide multi-beam support. With phase shifters, another set of beam forming ICs is required for each beam, using the same elements.  True Time Delay is no different.  There are some optical methods that frankly, I just take it on faith that they work!

True Time Delay in Phased Arrays, Ruth Rotman, Moshe Tur, Lior Yaron
Proceedings of the IEEE | Vol. 104, No. 3, March 2016

A reader writes, "Time Delays are equally simple to imagine and hard to implement efficiently.  Digital Beam Forming depends on having enough processing power, with limited effectiveness. Most analog True Time Delay units suffer from high power consumption (e.g. 1-2W per channel)."

What gets RF engineers up in the morning is the challenge with optimizing the inherent tradeoffs of antenna design features.

In summary, a phased-array that applies fixed-phase-shift within each element is inherently limited in beamwidth because of beam squint.  Signal bandwidths more than about 100 MHz may not be useful. Implementing True Time Delay between elements using optical methods or by selecting amongst various path delays, provides variable phase shifting that matches signal spectrum, suitable for all future aero comm applications without degrading beam steering. True Time Delay implementations remain a challenge in processing or in power consumption.

Special thanks to Dr. Jörg Oppenländer, CTO at QEST, for his helpful tutorial on the floor at APEX 2018 and Bill Milroy, CTO at ThinKom, for his helpful commentary on the importance of optical beam forming.

Here are other references if you are still interested:

Stay tuned!

Peter Lemme

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Follow me on twitter: @Satcom_Guru
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Peter Lemme has been a leader in avionics engineering for 35 years. He offers independent consulting services largely focused on avionics and L, Ku, and Ka band satellite communications to aircraft. Peter chairs the SAE-ITC AEEC Ku/Ka-band satcom subcommittee, developing ARINC 791 and 792 characteristics and contributes to the Network Infrastructure and Interfaces (NIS) subcommittee developing Project Paper 848, standard for Media Independent Secure Offboard Network.

Peter was Boeing avionics supervisor for 767 and 747-400 data link recording, data link reporting, and satellite communications. He was an FAA designated engineering representative (DER) for ACARS, satellite communications, DFDAU, DFDR, ACMS and printers. Peter was lead engineer for Thrust Management System (757, 767, 747-400), also supervisor for satellite communications for 777, and was manager of terminal-area projects (GLS, MLS, enhanced vision).

An instrument-rated private pilot, single engine land and sea, Peter has enjoyed perspectives from both operating and designing airplanes. Hundreds of hours of flight test analysis and thousands of hours in simulators have given him an appreciation for the many aspects that drive aviation; whether tandem complexity, policy, human, or technical; and the difficulties and challenges to achieving success.

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