u-blox have just introduced a new GNSS based timing product that achieves the nano-second phase synchronisation required to maximise 5G performance. Concurrently tracking multiple GNSS constellations in multiple frequency bands, it has demonstrated a tolerance of +/- 10ns with a standard deviation of 3ns over 24 hours. This is the kind of step change in innovation and technology that will be required to achieve the full potential of 5G.
Major changes introduced to enable 5G
Each generation of cellular technology takes a huge leap forward in performance. 5G has been designed to achieve up to 10 gigabit data rates, latency as low as 1 millisecond, and capacity for up to one million devices per square kilometre alongside high spectral efficiency. It also introduces new services, specifically very accurate position-finding together with a new cellsite architecture. All of these are strong drivers for increased timing precision.
4G networks removed the centralised BSC/RNC functions in 2G and 3G, distributing all of the radio control functions to the edge. 5G further disaggregates these into a centralised RF control unit (CU),
distributed units (DU) and remote RF radio heads (RRU). All three operate in a tightly co-ordinated and synchronised harmony with neighbouring cells to achieve peak performance. The tightest timing requirements are between closely-located cells with more relaxed tolerances between edge and core network.
|RRH: Remote Radio Head
DU: Distributed Unit
CU: Cerntralised Unit
Core: Core Network
Fronthaul 1: Typical throughput 25Gbps
Fronthaul/Backhaul Architecture for 5G
Several advanced RF techniques are exploited such as massive MIMO and beam forming. This allows each antenna array to discriminate and focus on a number of individual users, increasing the signal quality and thus throughput.
Low latency is achieved by adopting a much faster frame rate. Co-ordinated multi-point techniques combine simultaneous transmissions to and from each user device using several nearby cellsites to discern the best signal.
Higher throughput is achieved by aggregating multiple frequency bands with simultaneous transmissions from multiple cellsites. A likely technique will be to enhance capacity at cell edge by supplementing with lower-powered small cells.
A minimum bandwidth of 100MHz is required to make efficient use of 5G features, with contiguous blocks of up to 1GHz anticipated to achieve headline data rates.
Protection against disruption and outages is of increasing importance. Jamming of the primary GNSS frequency is not uncommon, while spoofing attacks are becoming more sophisticated.
Some governments and many network operators have mandated complementary timing sources that supplement GNSS, such as Synchronous Ethernet and IEEE 1588, which can extend holdover times during outage periods.
Within the GNSS receiver itself, diversifying the solution to operate across four GNSS constellations and multiple frequency bands significantly improves system resilience. Should any individual GNSS become unavailable, one or more of the remaining three systems and/or alternative frequency bands can be used.
Why is much higher timing accuracy required for 5G?
Recovering and processing signals received from multiple antennas and multiple sites is most effective where the delays and timing can be precisely compensated. This requires not just frequency alignment, but also phase-timing alignment.
Co-ordinated Multi-Point (CoMP) transmits and receives signals from multiple small cells simultaneously, creating more spatial dimensions. This allows simultaneous transmissions for multiple users in the same geographical area while minimising interference.
The effective use of co-ordinated multi-point drives a design target for phase synchronisation of not more than 90ns between base stations, compared with a minimum of 500ns for 4G.
The frame rate of 5G is four times faster than 4G, and the choice of cyclic prefix OFDM further shortens the required inter-site phase-timing tolerance from 1500ns for 4G to as low as 390ns.
CP-OFDM Frame Structure
High-precision network positioning services are the most stringent driver for phase-timing accuracy, targeting a synchronisation error between adjacent cells as low as 10ns.
The source of the phase timing must be very stable, reliable and have long-term accuracy for the highest performance. This is most important in busy, dense urban areas where signal paths constantly vary but capacity and throughput are in high demand.
How u-blox’s product meets those requirements
The latest ZED-F9T u-blox GNSS timing product takes a generational leap forward to match the pace of 5G innovation. Stress testing absolute timing over 24 hours achieved a tolerance better than +/-10ns with a standard deviation under 3ns. Additionally, ZED-F9T includes a new differential timing mode which delivers even higher timing accuracy locally
Rather than a single satellite constellation, it concurrently tracks all four international systems (America’s GPS, Europe’s Galileo, Russia’s GLONASS, and China’s Beidou), with a combined total of more than 100 satellites.
Furthermore, whereas previously only a single frequency has been used, now several satellite systems also broadcast on a second frequency band. This provides more than just a simple duplication of the signal. The variable delay travelling through the ionosphere is one of the major inaccuracies found in satellite navigation systems. A second frequency allows this to be more accurately determined and compensated for. There are already more than 70 satellites transmitting L2/L5 today, with more being deployed.
GNSS Satellite Band Plan
Implications for deployment
Universal deployment worldwide is assured from the huge number of satellites that can be tracked across four GNSS constellations. Resilience is further improved because chances of entire satellite-signal loss across multiple frequency bands are much reduced
The superior performance of the ZED-F9T obviates the need to use online correction services or satellite based augmentation. Nano-second level accuracy can be achieved from ionosphere correction and multi-band reception alone.
Security is enhanced to counter threats from jamming and spoofing, with built-in algorithms to detect and filter out rogue signals. Multi-band and multi-constellation reception provides more robust and resilient operation by design.
Further advanced security features include secure boot and secure API interfaces.
Overall, the ZED-F9T delivers a step change in timing performance, robustness and resilience that will allow 5G to achieve its full potential.