21581 - SCTE Broadband August2022 COMPLETE v1

62 Vol. 44 No. 3 - September 2022 Issue In TDD the same frequency is used for the transmission and reception of data. Adjacent cell sites must transmit or receive in the agreed-upon time windows. If a cell site transmits during a receive window, then interference is experienced. To avoid this, an absolute timing accuracy of about 1.5us is required from the core to the RU. The principle of CoMP is for a UE to communicate with multiple, geographically separated cell sites, at the same time, with the goal of enhancing system performance and quality of service. Timing is critical in being able to reconcile the signals received at the coordinating cell sites. CoMP requires a relative timing accuracy between these sites of around 260ns to 350ns. Emergency location services rely on observed time difference of arrival (OTDOA) technology where the UE measures the arrival time difference of the positioning reference signal (PRS) between a reference cell and several neighbouring cells. The more accurate the time measurement, the more accurate the location information that is derived. The timing accuracy required to meet the requirements of OTDOA based applications can be as low as 100ns. In all cases the need for stable, accurate and reliable timing is an essential requirement for the network to operate correctly. The use of a global network satellite system (GNSS) receiver to provide timing at each cell site would be expensive and not always practical when small cell locations do not have line of sight to a sufficient number of satellites, such as may be the case in a city centre (e.g., San Francisco or New York City). 5G networks will use IEEE1588/Precision Time Protocol (PTP) and SyncE to allow cell sites to be synchronised in both frequency and phase using the Ethernet network to distribute this information to each cell site and ensuring that all devices are in-sync with each other (see figure 8). technical Figure 8: 5G timing and synchronisation The specific timing limits for each network will depend on the network architecture and the type of applications being supported.