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Why should jitter be minimized in CPRI fronthaul ?
July 4, 2014 | By Steve Shin (firstname.lastname@example.org) and S.M. Shin (email@example.com)
In this post, we will talk about “jitter” introduced in
the fronthaul network of LTE C-RAN. As shown in the
figure on the right, jitter refers to the skew or
variation in time or phase of a digital signal exchanged
between communication systems. This jitter can
eventually cause errors during the data and clock
recovery process at RRH.
Then, why should we be worried about jitter in LTE C-
In LTE C-RAN (Centralized/Cloud RAN), BBU and RRH are placed tens of kms away from each other, and
connected through a fronthaul network that carries CPRI traffic. A remotely separated RRH should be
synchronized with a BBU in the clock frequency. In most cases, a BBU can generate a master reference clock
using its GPS receiver while a RRH, with no built-in or external GPS receiver, can't (Just for your reference, a
RRH unit is usually about a few thousand dollars, and a GPS unit is about a few hundred dollars. So, a RRH with
an extra GPS would result in extra costs). Thus, in LTE C-RAN, a RRH must obtain a reference clock by
recovering a timing clock from CPRI I/Q bit streams transmitted by BBU. Then it creates and distributes all sub-
system clocks to be used in the RRH system.
1/fo 1/(fo +Δf)
Master reference clock Recovered reference clock
I/Q bit stream
fo ≃ fo +Δf
▶Clock recovery through referencing to the CPRI I/Q bit stream received at RRH
▶Frequency synchronization between BBU and RRH?
Received signal (with Jitter)
Received signal (Ideal)
Netmanias Tech-Blog: Why should jitter be minimized in CPRI fronthaul ?
In a fronthaul network composed of Dark fiber only, jitter rarely occurs between BBU and RRH, because this
type of optic fiber seldom causes any jitter. On the contrary, in a fronthaul network containing active
equipment like “Active WDM” or “PON”, jitter can be generated and introduced into the fronthaul network
during signal processing such as mapping/muxing (i.e., mapping/demaping/multiplexing in OTN). CPRI I/Q bit
streams with such jitter can cause errors in the clock and data recovery process at RRH, consequently leading
to degraded system performance of RRH.
Degraded frequency accuracy of the reference clock recovered in RRH can affect the performance of all
relevant components that use the reference clock, subsequently. For example, an inaccurate reference clock
may cause errors in converting LTE digital signals (I/Q sample data) into LTE analog signals at DAC (Digital
Analog Converter), and also lead to inaccurate frequency of carrier signals used for radio transmission of LTE
analog signals. Eventually, jitter in the fronthaul network can cause significant impacts on the quality of LTE
radio signals transmitted through RRH antennas. Because of this risk, when building a fronthaul network for C-
RAN, thorough verification is required to ensure jitter introduced by active equipment is kept within the
Low deviation between BBU and RRH clocks: How low is low enough? This can be a tricky question. As seen
above, jitter introduced in a fronthaul network can degrade the accuracy of a recovered clock, which then
accordingly causes degradation of LTE signal quality at RRH. So, strict accuracy requirements for RRH clocks
are needed for performance management. For this, the CPRI standard specifies the maximum allowed impact
of a fronthaul jitter on the frequency accuracy of the clock recovered at RRH (in comparison with a master
reference clock of BBU).
The above graph depicts a frequency variation (fo+Δf) of a reference clock recovered at RRH, fluctuating
around the original frequency of the master reference clock (fo). It shows there is a deviation in the clock
frequency between BBU and RRH due to the clock frequency variation at RRH. R-18, one of the CPRI technical
requirements (CPRI Specification_v_6/2013-08-30), defines the clock frequency accuracy of RRH as
'±0.002ppm'. This requirement states that the maximum impact of jitter from the CPRI fronthaul on the
frequency accuracy of RRH should be less than '±0.002ppm', '±2ppb' or ‘two billionth of the reference clock
Other than jitter, there are many factors that may affect the clock frequency accuracy. The R-18 requirement
concerns only jitter, among all other factors, and defines the contribution of jitter to the total frequency
accuracy budget allowed at an LTE radio base station.
Note: ‘ppm’/’ppb’(parts per million/billion) unit is used to express a very small parameter/value using“one
millionth or one billionth” like 'percentage'. For example, assuming that the reference clock frequency of BBU
is 30.72MHz, 2ppb of clock frequency accuracy represents 0.06144Hz.
RRH clock frequency
For example, assuming that
the reference clock frequency
of BBU is 30.72MHz, 2ppb of
clock frequency accuracy