LTE Requires Synchronization And Standards Support
Mobile operators are embracing Long-Term Evolution (LTE) as their next-generation wireless infrastructure solution. Its growth is increasing demand for network robustness and reliability. In LTE deployments, network synchronization is key and solutions need to meet the rigorous timing and delivery requirements that ensure network quality and availability. Of course, synchronization technology and standards are necessary to support LTE deployments, and various synchronization options are available.
“Synchronization” refers to the technique applied to ensure the radios in the target LTE basestation are operating within the performance parameters defined by the appropriate 3rd Generation Partners Project (3GPP) standard. Synchronization is achieved by delivering a specifically formatted clock signal or signals to the basestation’s radio circuitry. These signals in turn are used to generate the modulation method’s RF air interface frequency/phase components.
The RF or air interface requirements of LTE are determined by the 3GPP, a collaboration between groups of telecommunications associations, known as the Organizational Partners. The 3GPP’s standardization encompasses radio, core network, and service architecture.
The Rise Of Data
Throughout the evolution of the mobile wireless network deployments of 2G and early 3G, the fundamental synchronization reference clock was delivered to basestations via the backhaul telecommunications network or GPS. In days past, when voice traffic consumed most of the basestation backhaul bandwidth, T1/E1 physical-layer (PHY) facilities delivered the base clock frequency for frequency division duplex (FDD) class radios. In time division duplex (TDD) class radios, GPS delivered both the frequency and phase signals. (For more, see “What’s The Difference Between FDD And TDD?” at http://electronicdesign.com/article/communications/whats-difference-fdd-tdd-74243.)
In the past few years, data traffic has surpassed voice as the preponderance of backhaul traffic. This dramatic change in the backhaul paradigm, much of it due to iPhone and Android smart phones providing access to Web and video services, has caused the backhaul delivery technology evolution to migrate to Ethernet. This backhaul migration has spawned new standardized synchronization techniques, such as synchronous Ethernet (SyncE) from the ITU, G.8262, and one from the IEEE called Precision Time Protocol 1588-2008 (PTP).
The SyncE standard is a method for carrying a primary reference traceable clock (PRC) via the Ethernet PHY much the way T1/E1 carried the traceable clock in the past. This clock provides a very accurate frequency with high stability and minimal wander so it easily can be used to synchronize radios in an LTE FDD basestation. SyncE cannot carry a phase component. When it’s used by itself, then, it’s appropriate for FDD-based radio synchronization.
PTP is a mechanism for transporting a value of time from a grandmaster clock in the form of a timestamp across packet networks. The syntax of the protocol is master-slave, and it has means for the slave clock to measure packet flight times on the uplink and downlink sides of the path. PTP also can deliver both a phase signal in the form of one pulse per second and frequency. It can provide time of day (ToD) as well, though this data isn’t used in basestation radio synchronization.
FDD and TDD refer to the radio technique used in that specific configuration. In general, an FDD radio system requires all radios participating in that cell, plus adjacent cells, to have a frequency alignment within some parts per billion (ppb) of the center frequency. Typically, a TDD radio system has a frequency, ppb, and phase microsecond alignment requirement in that cell and adjacent cells (see the table).
Potential Solutions
GPS receivers can satisfy these synchronization requirements. However, they might be impractical or too expensive since the GPS receivers used in these types of applications aren’t the same as those in cell phones or automobiles. These higher-performance devices require an expensive antenna and a high-quality holdover oscillator. When one includes antenna installation, they can cost several thousands of dollars.
Where possible, the most effective way to provide synchronization to these types of basestations is SyncE and/or PTP. In the case of the residential LTE basestation, also called a small cell, one can use PTP/NTP. NTP is Network Time Protocol, an Internet protocol designed to synchronize the clocks of computers and other devices to some available time reference. It works over packet-switched variable latency data networks. NTP may be a better alternative than other standards for some systems.
Timing and synchronization systems for LTE deployments that deliver packet-based timing protocols such as NTP, PTP, and SyncE are available from a number of sources. For example, the Symmetricom TimeProvider 5000 carrier-grade IEEE 1588-2008 grandmaster clock includes additional enhancements to match the stringent frequency and phase synchronization requirements of next-generation networks (see the figure). For residential and enterprise small cells, Symmetricom solutions support a combination of PTP/NTP clients and softGPS timing.