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Why over-the air timing and synchronization testing is critical for 5G TDD networks

Time division duplexing (TDD) is the transmission technique of choice for 5G networks. It maximizes the spectrum efficiency achievable over a given frequency band and provides flexibility in spectrum allocation and management. These characteristics make TDD a critical technology for optimizing spectrum utilization and meeting the diverse requirements of 5G network deployments.

C-band spectrum frequencies are considered ideal for initial roll outs of 5G networks since c-band strikes the perfect balance between coverage and throughput. C-band operates in the mid-band frequencies (3.7 GHz – 4.2 GHz) and relies heavily on TDD technology for efficient uplink and downlink transmissions, so validating TDD transmissions is a critical step in getting 5G right.

To understand the importance of timing and synchronization to TDD transmissions (and consequently to 5G networks) and why testing timing and synchronization over the air is a test technique of choice, it helps to first understand how TDD works.

How does TDD work?

TDD is a transmission technique whereby uplink and downlink signals are transmitted over the same frequency using synchronized timed intervals. Its flexible design means that, with TDD, operators can manage how many time slots can be allocated to uplink versus downlink transmissions– making spectrum utilization more efficient.

Why is timing and synchronization important in TDD and 5G?

Since heavy, multi-directional traffic (i.e., signals) are constantly circulating along any given frequency, it’s important that signals are transmitted in an orderly, timely fashion—respecting the established signal-guard period. Signals sent too early or too late can result in overlay (or overlap) and ‘drift’ into other signals, which ultimately leads to interference.

If a cell site is out of sync–or radios are not synchronized–in a TDD channel, transmissions will drift outside the guard period and will cause intercell interference at nearby cell sites, regardless of who operates those cell sites. This impacts the performance of the wireless network for that entire area and ultimately negatively affects customer experience. This is why proper timing and synchronization of network traffic is critical to TDD, and ultimately to 5G.

Why over-the-air timing and synchronization testing?

Validating timing and synchronization over the air (OTA) is easier, more efficient and provides additional benefits, as we will discuss below.

Maximize efficiency and productivity

When technicians are at 5G cell sites, with access to transport links through cell site routers (CSR), they can validate timing and synchronization using Precision Time Protocol (PTP) by tapping into the transport interface feeding that site. This measures the network’s timing accuracy from the Grand Master to the base station.

However, since operators need to verify multiple PCIs (base stations) across a particular geographical area, this requires verifying equipment across multiple cell sites, not just at one location. If technicians attempt to perform that verification using a physical link, they must connect a fiber optic link to a cell site router. This requires that technicians have physical access to each cell-site router and that they travel from site to site to perform the verification across the desired area. With more and more small cells, c-band and mmWave radios in deployment it can be difficult (or too time consuming) to get physical access to so many transport links, or that access may be unavailable entirely.

In contrast, with OTA timing and synchronization, if technicians have cell-site GPS coordinates they can test multiple cell sites quickly without having to travel to each individual site under test. This speeds up the verification process and helps determine if all sites in a desired area are correctly synchronized.

See the true impact on user experience

Over-the air validation of timing and synchronization delivers a result that best reflects the impact of out-of-sync transmissions on user experience. Why? Since the equipment required to perform OTA timing and synchronization validation is portable and doesn’t require a physical connection to the network, measurements can be done close to the end customer. This means that results can truly capture the effect timing and synchronization has on user experience as they are obtained directly at the output of the device.

Quickly diagnose if a problem is related to timing and synchronization

Validating timing and synchronization accuracy through an over-the-air test should be the first step in determining if a timing problem is the cause of a performance issue on a network. By performing an over-the-air timing and synchronization test, a technician in the field can determine whether an entire network is synchronized. If the OTA test doesn’t uncover any issues, then timing and synchronization issues can be quickly ruled out as the source of a network problem. If the values indicate a problem, thorough protocol testing can be done across the fiber optic network to pinpoint the exact element causing the problem.

Flagging other unsynchronized networks in the vicinity

Testing timing and synchronization over the air is the only way for operators to quickly verify if other surrounding operator networks are properly synchronized. Why is that knowledge important? A network that is out of sync will not only cause interference within that one network, but that interference will spill over and affect surrounding networks. Therefore, it is in the best interest of all operators that traffic is correctly synchronized (by using an agreed-upon frame format) and that unsynchronized networks be quickly flagged and corrected.

Over-the-air timing and synchronization testing is available on the EXFO’s FTB 5GPro. Use this solution to help resolve C-band degradation issues quickly, easily, accurately. The solution features absolute time error results from multiple PCIs available at a glance.

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Timing and synchronization

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