Essay on Time and Clocks

2084 Words Apr 8th, 2015 9 Pages
Making time, holding time with or without a GPS.
The most ubiquitous way to get accurate time and phase, every time, everywhere is to use a GPS disciplined oscillator (GPSDO). Over the last few years, some concerns have arisen due to the potential impact on critical infrastructure of malevolent attacks such as jamming or due to natural causes such as solar flares.
The GPS system has been designed to distribute time around the world. It gets time from a reference called a time scale. They are several time scales in the world. In the US, NIST and USNO are used for the UTC time scale and are referred to as UTC (NIST) and UTC (USNO). USNO also produces the GPS time scale. The time scales around the world are coordinated through an
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Many forms of two-way time transfer techniques are being deployed today over a dedicated network, over satellites (the Ku band is generally being used) and, to some extent, over non-dedicated networks. In the latter case, some careful engineering is required as time transfer over a non-dedicated network is very sensitive to network asymmetries. (see ITSF 2013 presentation on sources of asymmetries in a fiber network). The PTP protocol as defined by ITU uses the TAI timescale whereas NTP uses UTC. These are two mechanisms of time transfer over a non-dedicated network assuming that there are no asymmetries (or that asymmetries are externally compensated).
Why frequency, phase and time matter in cellular networks.
Frequency Division Duplex (FDD) has been the dominating Radio Access Network (RAN) technology since the inception of cellular networks. It requires paired frequency bands - one for transmission and another one for reception. Time Division Duplex (TDD) uses a single frequency sharing the channel between transmission and reception and spacing them apart by multiplexing the two signals in real time. Stable, known, and accurate frequencies are fundamental to enable user equipment (UE) to connect to base stations. Worse, Doppler shift due to UE in a train or in a car will render impossible any communication unless the UE can syntonize the receiving frequencies from nearby base stations. In both cases, FDD and TDD, without this

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