Telecommunications networks are rapidly shifting from circuit switched to packet switched technologies to meet the exploding demand for bandwidth. The transition from TDM to packet-based networks requires a change in the synchronization architecture as the TDM layer that inherently carried the sync signal is lost, and the sync signal is broken.
Asynchronous Ethernet networks do not provide physical circuits between network elements, and consequently, synchronization of base stations must be engineered into the packet backhaul using a packet timing technology such as IEEE 1588 (PTP) .
As network operators look to design the sync architecture for 4G/LTE networks, some considerations must be made for meeting must stricter synchronization requirements to support the latest mobile technologies and location based services including E911.
This will require network designers to implement a synchronization solution that can support both frequency and phase. And, they must define and architect a sync back-up in the event the primary sync signal is lost to ensure continuity of service.
IEEE 1588 (PTP)
IEEE 1588-2008 Precision Time Protocol (PTP) is a synchronization protocol that has gained traction as the technology of choice to deliver synchronization for packet-based networks because it delivers both the frequency and phase synchronization required for 4G / LTE networks. Figure 1 below shows a typical example of PTP synchronization for cellular networks.
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Mobile base stations that rely purely on frequency control, such as GSM and UMTS, have a requirement of 16 parts per billion (ppb) physical layer (G.823) clock on the E1/T1 backhaul connection (the transport interface) to lock their internal oscillators and generate the 50 ppb accuracy required to align the base stations with the mobile phones at the RF layer (the air interface).
Failure to meet the 50 ppb synchronization requirement will result in dropped calls. Figure 2 below shows the synchronization requirements for various cellular network types.
As the backhaul transitions to Ethernet, the TDM physical layer synchronization service chain is no longer available. The loss of physical layer sync has generated a requirement for base station designs to incorporate PTP slave clocks that will meet the 16 ppb requirement using packet technology. Such PTP slaves in the base stations rely on access to a carrier-grade PTP grandmaster clock deployed in the mobile switching center (MSC) or radio node controller (RNC).
With the network transition to 4G/LTE TDD (Time Division Duplex), more stringent phase synchronization is now required to support the tighter use of frequencies and emerging location based services (LBS) including E911 requirements. Figure 3 below shows what happens to service quality when the network is not synchronized to the required specifications.
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Figure 3: LTE Synchronization
Backing-Up the Sync Signal
Depending on the type or geographical location of the network, some networks have relied heavily on GPS technology to deliver synchronization. However, GPS synchronization is susceptible to jamming and spoofing, or simple signal fades where antennas are partially blocked, which disrupt sync in the network.
Regardless of the primary technology used to synchronize the packet-based network (PTP or GPS), rubidium holdover technology can perform a critical function within the specified requirements of the base stations to support 4G/LTE services (Figure 4, below ).
Clickon image to enlarge.
To ensure continuous network operations, it is recommended that service providers deploy rubidium atomic clocks in their base stations to ensure holdover for either a GPS or a PTP synched network.
Rubidium is the best technologies available in the market today to deliver holdover of up to 1.5 µs (required for LTE-TDD) over a 24 hour period.
Holdover Requirements and Technologies
Holdover (Figure 5 below ) is achieved by equipping cellular base transceiver stations (BTS) with oscillators or atomic clocks that temporarily holdover sync signals. Holdover periods can range from several hours to several days depending on the oscillator technology (crystal or rubidium), environmental factors (temperature and temperature variation), and the quality of the implementation (algorithms that account for and adapt to the effects of aging).
Holdover requirements vary depending on the type, complexity, and operator requirements. 4G/LTE (TDD) networks have more stringent timing requirements than 2G/3G networks and some applications such as location based services and E911 impose even more exacting sync requirements in order to accurately locate the handset by triangulating from base stations.
Even for 2G/3G environments where the accuracy requirements are not as stringent, rubidium provides a significant advantage as the much longer holdover period can save weekend or nighttime truck rolls.
It must be noted that different grades of oscillators deliver varying holdover performance, which of course will also vary in cost. The design implementation can also have significant impact; for example a software algorithm can compensate for accuracy changes due to the aging of the oscillator. The point is that under similar environmental circumstances and within the price/performance ranges targeted for base stations, rubidium provides holdover performance significantly better than crystal oscillators.
Other important factors favoring rubidium holdover technology are:
1. Rubidium atomic clocks have a much faster lock time, thereby reducing the impact of a power outage or other event that would require systems to cycle.
2. Innovation has yielded reduced size and lower power consumption making rubidium solutions easier to embed in equipment designs.
3. And most importantly, cost for rubidium clocks are on a steep decline: five years ago prices were double what they were two years ago, and technical innovation continues the downward trend today.
Sync and Holdover Technology for the Future
Carrier service availability has always relied on redundancy and backup solutions to meet the expectations of their customers. A reliable end-to-end synchronization solution for a packet-based network requires the use of a primary sync source (either IEEE-1588 PTP or GPS) and an embedded rubidium atomic clock at the base station.
In this solution, multiple technologies aid one another to extend the holdover time of rubidium and allow installation of base stations in locations that were not practical in the past. This approach has been tested for deployment and is ready for carriers to include in 4G/LTE build out plans.
With the evolution of mobile networks to 4G/LTE, the requirements for synchronization become more stringent with the advancement of tighter phase requirements. To meet the phase requirements of ±1.5 microseconds and ensure continuous network operations, rubidium atomic clocks are required to deliver network holdover as a back-up to either PTP or GPS based synchronization technologies.
Michelle Pampin , director of product and channel marketing at Symmetricom, is a graduate of Golden State University and McGill University.