GNSS Frontiers: BeiDou and Ubiquitous Location

BeiDou Signals, Future Receiver Design Highlighted at Stanford PNT Symposium

By James D. Litton and Tom Langenstein

James L. Litton

The Stanford Center for Position, Navigation and Time conducted its eighth symposium on PNT in October 2014. These symposia have always been a superb two (this year three) days of excellent presentations, ranging over the entire domain of PNT, including policy factors as well as technical ones.

This year the first day featured student speakers, either from Stanford or the students of former Stanford students who are now faculty at other universities. The conference is by invitation only; sponsors include Lockheed Martin, Boeing, and other companies involved with GNSS. This essay highlights two presentations that struck us as harbingers of change in the industry: Greg Turetzky’s paper on ubiquitous location, and Minquan Lu’s and Zheng Yao’s paper on new signal structures for BeiDou.

Brad Parkinson gave a keynote address mixing challenges and opportunities from the frontiers of policy formation. David Last did not fail to amuse with his lighthearted and satirical commentary on navigation and society at dinner. Many others gave noteworthy presentations, and all of the presentation slides can be found online.

Tom Langenstein

Both papers that we selected for this article have very broad scope with considerable strategic significance in GNSS design and applications. It seems a little impertinent, as well as superficial, to try to convey their essence in fewer than 2,000 words, but the material presented is available elsewhere, too.

New Signal Structures for BeiDou

Professors Mingquan Lu and Zheng Yao of Tsinghua University laid out in clear and detailed fashion the motivations for BeiDou’s choosing to introduce new signals for the Phase III global system, analyses of alternative modulations, and the results of bench testing in service to the desired properties (interoperability, acquisition and tracking thresholds, receiver complexity, in-band interference, and so on).

They emphasized one non-technical or operational motivation: independent proprietary designs for patent protection. No declaration of policy intention was made; however, the direction was clear, even though the authors are university professors and not government officials.

Some of this work has been published elsewhere in IEEE Transactions by the same authors and has a substantial history, reflecting the lessons learned from the predecessor system designs and very thorough analysis, simulation and bench testing. Space does not allow extensive citation, but the key drivers for the designs and the results are summarized below. The preferred modulations chosen or synthesized are quadrature multiplexed binary offset carrier (QMBOC) for B1C and asymmetric constant envelope-binary offset carrier (ACE-BOC).

The principal deficiencies cited of the earlier-proposed BeiDou Phase III signals (circa 2010-ICG) were given as:

• no independent intellectual property rights; thus, a big patent risk 
• signal performance needs to be improved
• more flexible receiving modes and more varied application scenarios should be considered.

The principal requirements for BeiDou Open Service signals were cited as:

• independent intellectual property rights
• better compatibility and interoperability with GPS and Galileo
• smooth transition from Phase II to Phase III
• improved performance

Separate requirements were stated for the B1C and B2 signals, as follows:

B1C: (QMBOC)

• compatibility with other signals of the same carrier frequency
• better interoperability with GPS L1 and Galileo E1 signals
• better ranging accuracy (than GPS C/A and BeiDou Phase II B1(I))
• receiving mode diversity for different receivers (low-end and high-end)
• independent Intellectual property rights

B2C: (ACE-BOC)

• multiplexed B2a and B2b into a constant envelope signal
• better interoperability with the GPS L5 and GALILEO E5 signals
• high ranging accuracy
• in-band interference-resistant ability (MAI, DME, TACAN, Near-far effect, etc.)
• joint optimization with B1C
• independent intellectual property rights

In the quoted case study tests, simulated ACE-BOC and AltBOC signals were generated at several fixed transmitting power levels and processed using software receivers. For each given transmit power level, the ACE-BOC was allotted three times power for the pilot channel over that of the data channel while the AltBOC allocated equal amount of power for both the pilot and the data channel, that is, 3:1 for ACE-BOC and 1:1 for AltBOC.

The resulting tracking performance of the ACE-BOC is more robust than that of the AltBOC.

Table 1, taken from the presentation, provides an overview of the signals.

Table 1. New signal structures proposed for BeiDou.

The compatibility properties of the new signals, if adopted, which seems quite likely, are desirable. The implicit intellectual property aspects of the development, both in motivation and in differential design of a signal structure which seems to be claimed as novel have a defensive basis, apparently, in earlier assertions of proprietary designs. It will be interesting to see whether similar international negotiations follow, or perhaps already have. The paper was well received and stimulated considerable hallway comment.

Ubiquitous Location

Turetzky’s paper laid out the phenomenal growth of location-based services and the implications of such growth for design requirements in GNSS-wireless at the user device level and at the silicon level. On growth (from various quoted sources):

• The compound annual growth rate of GNSS devices will continue, from its current 22 percent level to a robust 9 percent for the years 2016-2022; heading for seven billion installed units by 2022.
• The cumulative core revenue in the decade 2012-2022 will be 46 percent in LBS portable and wearable devices and 47+ percent in vehicles.
• There will be many billions of installations of indoor location technologies by 2018, in virtually every venue imaginable.

Some of the design implications of the requirements driving the growth in indoor location are:

• Always Located, or continuous location. For this case, the energy dissipated per day (16 hours) and signal availability (100 percent) are the featured specification and the secondary specification, respectively. These specifications, in turn, require hybrid constellations and minimal standby power consumption.
• The scaling down to very small (14 nanometer) dimensions enables much faster switching speeds, search rates and lower power dissipation in active modes and more complex algorithms, but at the expense of leakage current, which adversely affects standby power, an increasingly important factor.

Thus, for GNSS design, the challenges are to:

• Take advantage of benefits of smaller geometries to achieve higher clock speeds, more memory, lower active power and smaller size, while greatly reducing standby power from leakage;
• Incorporate new methodologies at chip and system design level; Integrate multiple radios on a single die to reduce cost and size without creating interference to a very sensitive GNSS radio;
• Integrate multiple radio sources into a single location solution;
• Bring together a disparate value chain;

Turetzky outlined a vision for his employer, Intel, to be a leader in all aspects of these revolutionary developments. The technology roadmaps embrace most modalities of positioning: GNSS, Bluetooth, WI-Fi, cellular, and SBAS, and cross most platforms, including wearables. We think that another, unemphasized challenge is in the increasing density of these units with the current specifications on out-of-band-emissions and the spectrum sharing and spectrum management factors in the ubiquity of the devices.

From Greg Turetzky’s Ubiquitous Location paper, presented at Stanford PNT Symposium.

Tune in to our free webinar Receiver Design for the Future, with Greg Turetzky of Stanford speaking on Ubiquitous Location, scheduled for Jan. 15 (1 p.m. EST/ 10 a.m. PST/ 6 p.m. GMT). Register today!

Both papers represented the dynamism of our industry and its diversity of technologies and practitioners and the service to that industry provided by the remarkably consistent excellence of this symposium.

James D. Litton heads the Litton Consulting Group and previously played key executive roles at NavCom Technology and Magnavox. 

Tom Langenstein is executive director of the Stanford Center for Position, Navigation, and Time, and deputy program manager of the Gravity Probe-B project.