By Martin Nuss, Vitesse Semiconductor
Fourth-generation wireless services based on long-term evolution (LTE) have new timing and synchronization requirements that will drive new capabilities in the network elements underlying a call or data session. For certain types of LTE networks, there is a maximum time error limit between adjacent cellsites of no more than 500 nanoseconds.
To enable network operators to meet the time error requirement in a predictable fashion, the International Telecommunications Union is set to ratify the ITU-T G.8273.2 standard for stringent time error limits for network elements. By using equipment meeting this standard, network operator will be able to design networks that will predictably comply with the 500-nanosecond maximum time error between cellsites.
In this article, we look at the factors driving timing and synchronization requirements in LTE and LTE-Advanced networks and how the new G.8273.2 standard will help network operators in meeting those requirements.
Types of Synchronization
Telecom networks rely on two basic types of synchronization. These include:
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Frequency synchronization
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Time-of-day synchronization, which includes phase synchronization
Different types of LTE require different types of synchronization. Frequency division duplexed LTE (FDD-LTE), the technology that was used in some of the earliest LTE deployments and continues to be deployed today, uses paired spectrum. One spectrum band is used for upstream traffic and the other is used for downstream traffic. Frequency synchronization is important for this type of LTE, but time-of-day synchronization isn’t required.
Time-division duplexed LTE (TD-LTE) does not require paired spectrum, but instead separates upstream and downstream traffic by timeslot. This saves on spectrum licensing costs but also allows to more flexible allocate bandwidth flexibly between upstream and downstream direction, which could be valuable for video. Time-of-day synchronization is critical for this type of LTE. Recently TD-LTE deployments have become more commonplace than they were initially and the technology is expected to be widely deployed.
LTE-Advanced (LTE-A) is an upgrade to either TD-LTE or FDD-LTE that delivers greater bandwidth. It works by pooling multiple frequency bands, and by enabling multiple base stations to simultaneously send data to a handset. Accordingly adjacent base stations or small cells have to be aligned with one another – a requirement that drives the need for time-of-day synchronization. A few carriers, such as SK Telecom, Optus, and Unitel, have already made LTE-A deployments and those numbers are expected to grow quickly moving forward.
Traditionally wireless networks have relied on global positioning system (GPS) equipment installed at cell towers to provide synchronization. GPS can provide both frequency synchronization and time-of-day synchronization. But that approach will be impractical as networks rely more and more heavily on femtocells and picocells to increase both network coverage (for example indoors) and capacity. These devices may not be mounted high enough to have a line of sight to GPS satellites – and even if they could, GPS capability would make these devices too costly. There is also increasing concern about the susceptibility of GPS to jamming and spoofing, and countries outside of the US are reluctant to exclusively rely on the US-operated GPS satellite system for their timing needs.
IEEE 1588
A more cost-effective alternative to GPS is to deploy equipment meeting timing and synchronization standards created by the Institute of Electrical and Electronics Engineers (IEEE).
The IEEE 1588 standards define a synchronization protocol known as precision time protocol (PTP) that originally was created for the test and automation industry. IEEE 1588 uses sync packets that are time stamped by a master clock and which traverse the network until they get to an ordinary clock, which uses the time stamps to produce a physical clock signal.
The 2008 version of the 1588 standard, also known as 1588v2, defines how PTP can be used to support frequency and time-of-day synchronization. For frequency delivery this can be a unidirectional flow. For time-of-day synchronization, a two-way mechanism is required.
Equipment developers must look outside the 1588 standards for details of how synchronization should be implemented to meet the needs of specific industries. The ITU is responsible for creating those specifications for the telecom industry.
How the telecom industry should implement frequency synchronization is described in the ITU-T G.826x series of standards, which were ratified previously. The ITU-T G.8273.2 standard for time-of-day synchronization was developed later and is expected to be ratified next month (March 2014).
Included in ITU-T G.8273.2 are stringent requirements for time error. This is an important aspect of the standard because wireless networks can’t tolerate time error greater than 500 nanoseconds between adjacent cellsites.
ITU-T G.8273.2 specifies standards for two different classes of equipment. These include:
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Class A- maximum time error of 50 ns
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Class B- maximum time error of 20 ns
Both constant and dynamic time errors will contribute to the total time error of each network element, with both adding linearly after applying a 0.1Hz low-pass filter. Network operators that use equipment complying with the G.8273.2 standard for all of the elements underlying a network connection between two cell sites can simply add the maximum time error of all of the elements to determine if the connection will have an acceptable level of time error. Previously, network operators had no way of determining time error until after equipment was deployed in the network, and the operators need predictability in their network planning.
Conforming to the new standard will be especially important as network operators rely more heavily on heterogeneous networks, also known as HetNets, which rely on a mixture of fiber and microwave devices, including small cells and femtocells. Equipment underlying HetNets is likely to come from multiple vendors, complicating the process of devising a solution in the event that the path between adjacent cell sites has an unacceptable time error level.
What Network Operators Should Do Now
Some equipment manufacturers already have begun shipping equipment capable of supporting ITU-T G.8273.2, as G.8273.2-compliant components are already available. As network operators make equipment decisions for the HetNets they are just beginning to deploy, they should take care to look for G.8273.2-compliant products.
As for equipment already deployed in wireless networks, over 1 million base stations currently support 1588 for frequency synchronization and can be upgraded to support time-of-day synchronization with a software or firmware upgrade.
Some previously deployed switches and routers may support 1588, while others may not. While 1588 may be supported by most switches and routers deployed within the last few years, it is unlikely that they meet the new ITU profiles for Time and Phase delivery. IEEE1588 Boundary or Transparent Clocks with distributed time stamping directly at the PHY level will be required to meet these new profiles, and only few routers and switches have this capability today. Depending where in the network a switch or router is installed, network operators may be able to continue to use GPS to provide synchronization, gradually upgrading routers by using 1588-compliant line cards for all new line card installations and swapping out non-compliant line cards where appropriate.
Wireless network operators should check with small cell, femtocell and switch and router vendors about support for 1588v2 and G.8273.2 if they haven’t already.
About the Author
Martin Nuss joined Vitesse in November 2007 and is the vice president of technology and strategy and the chief technology officer at Vitesse Semiconductor. With more than 20 years of technical and management experience, Mr. Nuss is a Fellow of the Optical Society of America and a member of IEEE. Mr. Nuss holds a doctorate in applied physics from the Technical University in Munich, Germany. He can be reached at nuss@vitesse.com.
About Vitesse
Vitesse (Nasdaq: VTSS) designs a diverse portfolio of high-performance semiconductor solutions for Carrier and Enterprise networks worldwide. Vitesse products enable the fastest-growing network infrastructure markets including Mobile Access/IP Edge, Cloud Computing and SMB/SME Enterprise Networking. Visit www.vitesse.com or follow us on Twitter @VitesseSemi.
