By Martin Nuss, Ph.D., Vice President, Technology and Strategy and Chief Technology Officer, Vitesse
LTE has been rapidly moving towards a packet based backhaul network, while still trying to cope with a dated and inadequate timing model based on TDM. Timing is necessary for both 3G and 4G deployments. Today, timing is typically delivered by either a SONET/SDH-based T1/E1 line or from GPS satellites. SONET/SDH is not only expensive, but also inadequate for next-generation wireless networks, since it can only deliver frequency synchronization, but not the time-of-day (ToD) synchronization that is mandatory for TD-LTE and LTE-Advanced (LTE-A) – the “true 4G.”
In many cases, operators use GPS satellites, rather than T1/E1, to provide both phase and frequency synchronization. However, GPS is prone to jamming and faces reception issues for small cell deployments inside a building or in urban corridors, due to the lack of direct GPS satellite visibility. With the advent of LTE-A, with support for multi-band operation, wide use of Multiple-Input/Multiple-Output (MIMO) antenna systems, and increased use of micro and picocells to expand capacity and coverage, the GPS issues become intractable for many operators. For example, many picocell and microcell base stations will be deployed in locations such as lampposts and at the sides of buildings. They are positioned in urban corridors where tall buildings likely will block access to multiple GPS satellites, which would preclude meeting the LTE-A timing specifications. The optimal solution is to deliver timing through the network, but this has challenges.
Synchronous Ethernet (SyncE) offers a direct replacement for TDM-derived timing as the backhaul network converts to all-Ethernet protocols. However SyncE, like T1/E1-based TDM timing, can support only frequency synchronization, not the newly required time-of-day synchronization. The good news is that the IEEE and the ITU have worked on standardizing a packet-based timing for both frequency and ToD delivery in telecom networks – IEEE 1588v2 Precision Timing Protocol, or PTP. IEEE 1588v2 or “1588v2” for short has now gained widespread acceptance as the de facto packet based timing protocol for mobile operators. Market analysts predict that by 2015, Synchronous Ethernet could be used in 30 percent of timing solutions, greater than T1/E1 or GPS deployments. However, in the same timeframe, 1588v2 is expected to grow to over 50 percent of all deployments.
IEEE1588v2: Better Timing Through Time Stamp Collection
Targeted at packet-based backhaul networks, IEEE 1588v2 carries time of day information (also known as timestamps) directly within the data packets. The packets carrying the timestamps flow along with the rest of the data traffic in the network from networking equipment that generates the timestamps (also known as primary reference clock) all the way to base station equipment where these timestamps are used to recover the original time using IEEE 1588v2. The difference between primary reference clock and recovered clock (i.e. the synchronization error) needs to be within the accuracy requirement shown in Figure 2. This synchronization error is cumulative across every node in the network in the path between the network node generating the master clock and the base station. Meanwhile, the challenge of migration from FD-LTE to TD-LTE/LTE-A, makes the synchronization error limits even more challenging.
Providing an IEEE 1558v2 implementation with timing errors below the network requirements is paramount to a successful deployment of this technology. The primary source of the IEEE 1588v2 synchronization error comes from the packet delay variations (PDV) that are inherent in any packet network. The key to meeting the accuracy requirements is an IEEE 1588v2 compliant solution that can compensate for the PDV in the most cost effective way. The IEEE 1588v2 standard specifies multiple clock types. Besides ordinary clocks at the beginning and the end of the timing chain, boundary clocks (BC) and transparent clocks (TC) are defined for network elements in between.
In general, a BC node is more complex and costly to implement than a TC node. A node that implements BC regenerates the timing based on the timestamps that it receives and a node that implements TC simply forwards the incoming timestamps after correcting for any error it may introduce. So a TC node requires only accurate time stamping and time stamp correction mechanism, while a BC node requires timestamping, time stamp correction, a reliable PDV filtering algorithm, and a IEEE 1588v2-aware timing complex that can be synchronized to the network.
The only way to meet LTE-A accuracy requirements for both BC and TC is with a well-architected time stamping architecture. As shown by silicon vendors, such time stamping and correction mechanism can be incorporated into port-level PHY silicon that can be universally used by the equipment vendors in the datapath without any other changes to the system. In fact, such a port-based PHY solution is completely sufficient to implement a highly accurate TC node, while any BC node also greatly benefits from port-level accurate time stamping. Under the bottom line, the most cost effective way to upgrading the network for LTE-A and future small-cell networks is to deploy distributed TC’s everywhere, augmented with BC’s only where necessary to segment timing domains. The use of TC’s can increase timing accuracy to the nanosecond range, as shown by Vitesse in a recent submission to the ITU-T standards committee. Silicon advances available today will insure that such solutions will carry only a nominal premium over non-IEEE 1588v2 aware systems designed for 3G networks today.
Careful TC planning can allow for picocell synchronization in an outdoor environment, and synchronization down to the femtocell in large indoor multi-floor installations. In the former case, TCs can even be carried over microwave and millimeter-wave links and still meeting TD-LTE and LTE-A specifications while eliminating the requirement for GPS signals or fiber links at the small cell. In an indoor environment, the access network itself can generate IEEE1588v2 timing, or a GPS antenna on the roof of the building can generate time packets for synchronization of services inside the building, using IEEE 1588v2 to distribute timing within the building.
Upgrading Via a Painless Path
IEEE 1588v2 promises a simple, low-cost option for packet network timing upgrades and an ideal alternative and backup to GPS for TD-LTE and LTE-A networks. In many instances, full IEEE 1588v2 awareness is only needed right at the base station or cell site where timing needs to be provided to the cell. Within the many hops in the network along the way, Transparent Clocks provide the most cost effective way to upgrade the network to LTE and LTE-Advanced simply by replacing port-level PHYs with new devices such as the ones from Vitesse, that can implement a nanosecond-accurate and highly stable time-of-day forwarding architecture, while fully compensating for transit time and PDV.
About the Author
Martin Nuss joined Vitesse in November 2007 as Vice President, Technology and Strategy and Chief Technical Officer. With more than 20 years of technical and management experience, Mr. Nuss was most recently, chief technology officer of Ciena's Optical Ethernet group where he led the successful integration of the products and technology acquired from Internet Photonics. Prior to Ciena's acquisition of the company, he was founder and CTO of Internet Photonics. He also served 15 years at Bell Labs in various technical and management roles including director of the Optical Data Networks Research Department, where he was responsible for research on Lucent's 10 Gigabit Multimode Fiber and innovations in the 10 GbE Metro Networking space. He 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.
