ace-based synchronization (timing and frequency information is decoded over the air from other base stations). Frequency synchronization requirements are defined in 3GPP as per the Table 2-4. Synchronization accuracy of <1.5 us to target a time alignment error across base stations of <3 us [as stipulated by 3GPP 38.133 (cell phase synchronization accuracy)] as shown in Fig. 2-36. Note that actually systems may require more tight timing requirements. Consider: massive MIMO and distributed Cooperative Multi-Processing (CoMP) where timing alignment will be (tens of) nanoseconds and O-RAN requirements which define timing jitter between DU and RU as discussed. Most 3GPP systems incorporate a board-level oscillator (e.g., running at 30.72, 38.4, or 122.88 MHz) as well as a pulse per second source to perform both time and frequency syn- chronization functions. The board-level oscillator can be fed from a other frequency synchro- nous source or tuned through a software servo algorithm that runs on the Layerscape host processor. Chapter 2 System components, requirements, and interfaces Table 2-4 Time and frequency accuracy requirements per 3GPP. Application Frequency accuracy (ppb) Phase/time accuracy LTE-FDD LTE-TDD < 1.5 us LTE-MBSFN < 1.5 us LTE-A < 1.5 us 5G/NR < 1.5 us 1.5us 1.5us FIGURE 2-36 Timing alignment between radios. PRACH Preamble sequences are used as a method to obtaining synchronization between a client and a base station, both in 4G/LTE and 5G/NR implementations. The physical channel that carries the preamble (in the uplink only) is the PRACH. The preamble is a pattern (one out of 64 options) that is either randomly selected by the client (contention-based random access)-and assumed to be likely unique given its selection from a pool of 64-or allocated by the base station to the client (contention-less random access). The preamble is sent by cli- ent to base station over a PRACH which is a time/frequency allocation (PRACH occasion), obviously aligned with the defined bandwidth partition to the targeted c