As 5G introduces new concepts & new network architectures that will have a deep impact on core, transport and radio network design, how will synchronisation be affected? More than half of the Communication Services Providers surveyed at Mobile World Congress this year said they were in the early stages of rolling out 5G, so timing & synchronisation options should be at the centre of architecture discussions now! In the same survey respondents said the most important requirement in switch/router hardware was "quality 5G timing & synchronisation".
5G's RAN functional decomposition brings some of the philosophies of NFV & SDN to 5G and its use of CPRI/eCPRI to fronthaul data brings its own synchronisation challenges, with IEEE & ITU liaisons looking at timing aspects of this part of the network. Time Alignment Errors proposed in the new 3GPP specifications for 5G are in some cases many orders of magnitude smaller than current requirements, with 65ns being the tightest requirement. (It is worth noting however that some of the most stringent tolerances are intra-node only, not transport/network wide requirements.) Many operators are still in the process of moving from frequency-only sync to providing time & phase sync distribution for existing 4G/LTE-A services.
Timing and synchronisation quality can be described by two factors:accuracy and stability.Accuracy is the measure of how closely the clock compares to a reference or target value; this can be a global standard for time such as UTC or a desired frequency such as 10MHz; stability is the measure of the variance of the clock when observed over a period of time.
In order to quantify the quality of timing and synchronisation, many different timing metrics exist.These are specifically designed for the timing technologies they are measuring and the characteristics of the signal that are of interest.Measurements must be made against another clock of known and dependable quality during relevant network or environmental conditions and over a period of time long enough to fully characterise the measured clock.
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