From its beginning, one of the challenges of mobile cellular communications systems has been delivering a very stable and accurate (as little as 16 parts per billion, ppb) long-term frequency reference at each base transceiver station (BTS) to maintain accurate radio transmit and receive frequencies needed for reliable inter-cell call handoff and interference control. An even bigger challenge comes with some existing and emerging cellular architectures that require a precision (as stable as ± 1 µs) time reference to support time division duplex (TDD) operational modes. By ‘time,’ a common (relative) sense of time across the network is meant, but not necessarily absolute time of day (such as 9:35 AM PST, 12 Dec 09). Be aware that some writers’ use of ‘time’ refers to relative time, while others mean absolute time of day.
Historically, syntonization means two or more oscillators having the same frequency while synchronization means two or more clocks indicating the same time. Syntonization is often confusingly referred to as synchronization. The syntonization reference was provided by the T1 (1.544 Mbps) or E1 (2.048 Mbps) backhaul lines that connect the Time Division Multiplexed (TDM) data to the wired network. The time reference required installing a Global Positioning System (GPS) downlink to each base station (see Figure 1).
Figure 1 Cellular network syntonization via TDM backhaul.
Exploding demand for data capacity for new cellular services like mobile Web browsing, music and video downloads, in combination with intense cost pressure to support mass service demand, have led cellular network equipment manufacturers and cellular service providers to look for alternate approaches to backhaul traffic. A promising approach is to use carrier-class Ethernet equipment to provide additional data capacity at approximately one-sixth the cost per bit, while preserving service quality. Unfortunately, using Ethernet to eliminate the expensive T1 or E1 connections breaks the frequency reference provided by the TDM link, forcing operators again to install a GPS downlink at each station (see Figure 2). Thus, the Ethernet cellular backhaul solution has a missing link in providing cellular backhaul references.
Figure 2 Ethernet backhaul with GPS reference.
Enter IEEE 1588, Precision Time Protocol
In 2002, the IEEE 1588-2002 Precision Time Protocol (PTP) standard was created to provide precision timing across Local Area Networks (LAN) for test and measurement and industrial control applications. Segments of these industries need to provide widely distributed sensors and actuators a common sense of time for coordinated measurement and control. For example, what is the distribution of stress in aircraft wings under dynamic vibration conditions? (Are the wings going to fall off?) And, how do we ensure that newsprint paper fed into a 100 meter long printing press at 30 meters per second also comes out at 30 meters per second? (Will it rip or end up in a pile on the floor?)
Interested in reading the complete article?
A complete view is available to registered MWJournal.com members.
Registration is FREE! Click here to register.
Already registered? Login >>
Already a member, but don't remember your username and/or password? Click here.
If you are a Microwave Journal monthly subscriber and would like to enable FREE Website access, please click here.
