Page 14 - EETimes Europe June 2021

14 EE|Times EUROPE

BEYOND 5G, TOWARD 6G
Testing Times for 5G Network Synchronization

By John Walko

ne of the most important functions
of a radio network system is syn-
chronization. With the advent of
O5G, both synchronization and the
related issue of precise timing have become
critical considerations, mainly because the
latest generation of cellular relies on time-
division duplexing (TDD) radio technology.
Synchronization refers to the time-
accuracy requirements that must be enforced
among different radio components within
a network to ensure and maintain reliable
transmission. The key issue is that if the radio
clock of a communications network loses
synchronization accuracy — or the radios are
not synchronized — in a TDD channel, fram-
ing will drift outside of the guard period and
interfere with adjacent cell sites.
One solution to avoid this is to equip all
base stations with a common phase clock
reference. Timing and sync requirements across a 5G NR network (Source: Viavi Solutions)
Different timing synchronization tech-
niques are available to ensure that all the both transmitter and receiver use the same 3GPP Ultra-Reliable Low-Latency Communi-
radio units in a network are synchronized and frequency but transmit and receive traffic cations (URLCC) protocol. Typical examples
allow the scheduler at the base stations to at different times, using synchronized time include IoT devices, machines on a factory
ensure that interference is minimized. intervals. TDD allows for improved spectral floor, connected cars, and robotics.
“The thing is, with wireless communica- efficiencies but brings synchronization and On the transport side, for 4G LTE, the
tions, the receiver has no prior knowledge of timing challenges. Because downlink and industry currently relies on the Common
the channel or propagation delay associated uplink share the same spectrum, tough timing Public Radio Interface (CPRI), a synchronous
with the transmitted signal,” Kashif Hussain, restrictions are necessarily imposed on a TDD fronthaul interface that enforces stringent
wireless solutions director at network testing system to avoid interference. delay requirements, which are fine for central-
and monitoring specialist at Viavi Solutions To mitigate such interference, all base ization but create challenges when it comes to
(Scottsdale, stations in a network need to be synchronized bandwidth and node flexibility. This may not
Arizona), told EE with a common phase clock reference signal, necessarily be practical for all such use cases.
Times Europe. noted Hussain. According to the guidelines The interface provides a dedicated transport
“And typical of the ITU-T standards, both 5G NR TDD and protocol that was specifically designed to trans-
receivers use low- LTE-DD networks need to be phase- port radio waveforms between the radio unit
cost oscillators to synchronized to limit the end-to-end time and the digital unit. In operation, CPRI frames
keep the cost of error to within 1.5 µs — comprising no more expand with increased radio channel band-
devices manage- than 1.1 µs up to the access point and 0.4 µs width and the number of antenna elements.
able. Inherently, over the fronthaul to the radio. “The bottom line is CPRI is not very effi-
such oscillators cient in terms of statistical multiplexing and
will have a degree TESTING TIMES cannot scale to the demands of 5G, in partic-
Viavi Solutions’ of rift; thus, the Another complication in 5G — where TDD is the ular for massive MIMO and larger bandwidth
Kashif Hussain need for timing only option for the C-band — is that frame and requirements,” said Hussain.
synchronization.” slot synchronization also needs to be applied to He added that the required bandwidth and
Hussain noted that “previous generations avoid inter-network interference. And, with the antennas for a typical 5G scenario would push
of cellular also mandated frequency synchro- increasing interest in open radio access network the CPRI bandwidth above 100 Gbits/s. That’s
nization, mainly to prevent interference when (Open-RAN) architectures, timing and syn- one of the reasons that using Ethernet for
cells overlap. But with 5G, we are dealing chronization will be even more crucial because fronthaul and midhaul is so practical, he said.
with extremely stringent requirements, with additional delays from open interface network To improve bandwidth efficiency, 5G NR
TDD streams in both the uplink and downlink nodes will need to be considered for seamless typically uses the new eCPRI protocol, which
sectors. And to complicate matters, here you 5G services, Hussain cautioned. is claimed to reduce bandwidth requirements
are also dealing with MIMO.” Of course, 5G is not simply about deliver- tenfold. Equally importantly, unlike its pre-
ing voice and data via enhanced broadband; decessor, eCPRI is not designed specifically
TDD CHALLENGES the whole point is that it will be needed to to carry synchronization information; hence,
TDD offers a full-duplex channel over a connect and support other demanding and the need for new synchronization protocols,
half-duplex communications link. Thus, emerging use cases and services based on the such as Precision Time Protocol (PTP), and

JUNE 2021 | www.eetimes.eu