5G Timing and Synchronization

What is Timing and Synchronization? 

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Essential Resources:
• Download Our In-Depth Handbook
• Read the Blog: Why Does Synchronization in 5G Matter?
• See the Infographic

Timing and synchronization are interrelated requirements for wireless network performance. Timing is based on the establishment of a precise, standardized time value that must be disseminated throughout the wireless network. The closely related concept of synchronization refers to the coordinated, precise cadence of network activities that can only be completed successfully with this common time reference. 

Timing and synchronization standards for mobile networks prevent messages from interfering with one another and enable smooth cell-to-cell transfers. The increased stringency of timing and synchronization requirements for 5G is being driven by exponentially faster speeds, lower latency, and increased densification. The ongoing migration to packet-based transport and time division duplex (TDD) technologies also demand precision and versatility from timing and synchronization test solutions.

The Importance of Timing and Synchronization

Timing derived from global network satellite system (GNSS) constellations, including the global positioning system (GPS) servicing the U.S., plays a pivotal role in wireless network operation. Without access to this consistent and reliable time standard, efficient spectrum utilization and the delivery of high-speed, high-bandwidth wireless services would not be possible.

• Synchronization Technology is a fundamental building block for all wireless communication networks. Duplexing, multiplexing, and packet-based strategies all rely heavily on timing and synchronization to coordinate data transmission, prevent interference, reduce error rates, and compensate for any frequency or phase shifts.  
  • Frequency Synchronization describes a state in which the frequencies (repeating intervals) of different system clocks are aligned, but the phase and time are not. 
  • Phase Synchronization is achieved when the clocks are aligned with respect to frequency as well as phase but lack a common time origin. 
  • Time Synchronization refers to a state where clocks are aligned in frequency and phase with a common time origin such as Coordinated Universal Time (UTC).  
    
  • 5G RAN Disaggregation has divided the traditional Baseband Unit (BBU) into a more flexibly configured centralized unit (CU) and distributed unit (DU). Tightly maintained absolute and relative timing between these elements is needed to meet demanding 5G use case requirements. Synchronization of the fronthaul network ensures the RAN operates in harmony, despite any physical distance between components.  

What is Time Division Duplex (TDD)?  

Duplex telecommunication is defined as two-way transmission over a communication channel. Time Division Duplex (TDD) accomplishes this by allocating different time slots for uplink and downlink signals over the same frequency. This ingenious method allows a full-duplex (simultaneous) communication channel to be emulated over a half-duplex (serial-binary) communication link.   

• Time Division Duplex Wireless is a fundamental technology for 5G deployments around the world. Spectral efficiency is bolstered by uplink (UL) and downlink (DL) signals operating over the same spectrum. The advantages of time division duplex are counter-balanced by the precise timing and synchronization required to prevent intra-cell or inter-cell interference. TDD requires both the frequency and phase to be synchronized. 
• TDD Slot Format for 5G breaks data content into a series of 10ms radio frames, each of which contains ten 1ms sub-frames. The 56 available frame and slot configurations contained in release 15 of 3GPP TS 38.213 accommodate a wide range of 5G use cases and traffic patterns. Some options include equal UL/DL times while others are more asymmetrical. The variation in 5G TDD time slot formats create further potential for cross-link interference. To prevent this, frame and slot structures must also be synchronized between adjacent networks. 
  
  Two networks with unsynchronized Slot format
• Frequency Division Duplex (FDD) is a full-duplex telecommunication method that predated TDD and requires two separate communication channels. Any discussion of 5G FDD vs TDD usually includes the larger portion of spectrum consumed by FDD technology. Guard banding is also required between FDD send and receive channels to minimize interference. Although FDD is more forgiving with respect to timing and synchronization requirements, compatibility with MIMO, beamforming, and the C-Band spectrum are additional factors favoring TDD vs FDD 5G.

What is the Precision Time Protocol?   

The Precision Time Protocol (PTP) defined by the IEEE 1588 standard establishes a method for precise clock synchronization to the sub-micron range for packet-based networks. This includes Ethernet based 5G mid-haul and fronthaul networks. PTP Version 2 (1588v2), released in 2008, improved the accuracy, precision, and robustness of the protocol. 

• PTP Infrastructure includes a grand master clock directly synced to a GPS satellite source that relays an absolute time based on UTC. This information is dispersed throughout the network using a combination of boundary clocks and slave clocks. Ensuring all radio units in the network are synchronized to a common time and phase reference enables the scheduler to minimize potential interference. 
• The O-RAN Alliance recommends that no more than two boundary clocks are traversed between a grand master clock and any endpoint. However, there is no established limitation on the overall distance traversed. 
• PTP Over Ethernet is replacing GPS as the primary timing source for the 5G fronthaul network. Although Ethernet is not intrinsically synchronous, timing and frequency information can be distributed over an Ethernet layer using PTP and Synchronous Ethernet (SyncE). This allows existing Ethernet cabling to be leveraged to synchronize clocks in a distributed system. 

5G Timing and Synchronization Requirements  

When 5G network nodes are not in sync, received signals cannot be demodulated properly. High BER, delay, and jitter that compromise customer experience can result. To address this, synchronization requirements have now been established by multiple standards bodies including the 3GPP and ITU-T 

• Synchronization definition and procedures vary, depending on the communication system. Carrier and timing accuracy requirements become more stringent for TDD vs FDD 5G. For each use case, the synchronization, type, requirements, and impact of non-compliance on performance also varies substantially. 
• Time Error (TE) is defined as the time difference between any two nodal clocks. Absolute time error between a grand master time reference and any node is limited to an exceptionally short 1.5μs for LTE/5G TDD. This includes 1.1μs of absolute time error to the access point and 0.4μs over the fronthaul link to the radio. 
• Relative Time Error is the time difference between the inputs into two radio units. Relative TE is an important metric for advanced 5G features like carrier aggregation and Massive MIMO. Coordinated multipoint (CoMP), which is used to coordinate signals to and from multiple cell sites, cannot tolerate more than 1.0μs of relative TE.

The Challenges of 5G Timing and Synchronization  

RAN disaggregation, TDD, and the adoption of MIMO, beamforming, and millimeter wave technology have enabled the full potential of 5G wireless to be realized. These innovations have also combined to drive timing and synchronization requirements for 5G to unprecedented levels. Real-time applications like autonomous vehicles and the IoT are changing the equation for the timing and synchronization aspects in packet networks.  

• Inter-Cell Interference can be an unwanted byproduct of 5G time division duplex wireless. A compatible frame structure between collocated networks with adjacent frequency assignments must be established. Carriers using TDD must also avoid simultaneous DL and UL transmission. DL signal can potentially leak into adjacent channels using TDD and FDD LTE guard banding is no longer in place to lessen the impact.  
• GPS Signal Quality acquired by the satellite antenna must be highly reliable to meet the requirements of 5G. GPS signal strength verification from multiple locations and full antenna validation can minimize potential for interference issues. 3G and 4G networks only require one satellite line of site for synchronization. The precise timing and synchronization requirements of 5G cells have made even minimal variation intolerable. A lock on 4 or more satellites positions can be used to minimize the impact of satellite location.
  
  GPS based synchronization

What Can We Test?  

Fortunately, ultra-demanding timing and synchronization requirements can be accurately and reliably verified, using the powerful suite of VIAVI test solutions. Problematic frame drops, interference, and handover issues can be prevented by adopting a proactive validation approach. 

• PTP Testing can be performed to verify all network clocks are properly aligned with the grand master and PTP frequency profile limits such as floor packet percentile are being met. Time and phase profile conformance to time error (TE) limits can also be established. The VIAVI T-BERD/MTS-5800 makes it easy to test timing error and connectivity by emulating PTP endpoints downstream from a grand master clock. 

• 5G NR Frame Format should be tested to establish adjacent networks are conforming to the agreed-upon slot and frame formats. Over-the-air testing, using the OneAdvisor 800 Wireless, can be used to validate TDD frame format for multiple operators. Intercell interference from time division duplex wireless can be prevented. 

  
• GPS Testing can also be performed with the T-BERD/MTS-5800 to establish the suitability of the GPS antenna location during installation and thereafter. The number of visible satellites, signal strengths, and diversity of satellite positions across sectors and site lines can be assessed over a single intuitive interface.

Timing and Synchronization Solutions from VIAVI  

VIAVI timing and synchronization solutions have the features needed to ensure conformance to rigorous PTP/1588v2 and ITU-T standards that keep 5G and LTE wireless networks running like clockwork. 

• OneAdvisor 800 Wireless allows cell site technicians to test fiber, RF, and CPRI/Ethernet from a single instrument. Real time spectrum analysis provides a detailed representation of TDD LTE and 5G carriers that can prevent sources of interference. MIMO verification and signal analysis can quickly detect impediments that disrupt coverage and service quality. 
• T-BERD/MTS-5800 supports 5G timing and synchronization testing throughout the entire network service lifecycle. In addition to GPS signal verification and time error testing to the PTP protocol, the T-BERD/MTS-5800 can also be used to verify SyncE performance and perform one way delay and PDV (packet delay) testing over the network.