ys be better than PTP on its own. Cellular network synchronization requirements We now know all we need to know about how PTP and SyncE can be used to distribute time and frequency around a network. Now to look at how this applies to O-RAN and, in particu- lar, the synchronization of the fronthaul network between the Distributed Units (DUs) and associated Radio Units (RUs). To understand the role of synchronization in O-RAN, we need to take a step out and look at the requirements of the cellular networks that O-RUs are serving. The requirements for these networks are laid down by 3GPP in their Release 15, and beyond, standards. Specifically, the document of interest is 3GPP TS 38.104 (Base Station radio transmis- sion and reception). All of these requirements are defined in terms of the transmit signal mea- sured at the antenna port of the radio device, SO the O-RU output in this context. Frequency accuracy and stability TS 38.104 defines separate frequency accuracy limits for macrocells and pico/small cells: For a macrocell, the frequency must remain within +1 50 ppb (parts per billion) of its nominal value. For a pico or small cell, the frequency must remain within + 100 ppb of its nominal value. The relaxation of the accuracy requirements for smaller cells is possible because their sig- nal is much more local and, therefore, less likely to cause interference and is desirable since it could allow the use of lower cost components. Expecting the instantaneous frequency to fall within these limits at all times is both unre- alistic and unnecessary, so 3GPP further defines that the stability is observed over a 1 MS window. This means that if one was to continuously measure the frequency error over a period of time and then select any 1 ms period within that measurement time, the average frequency within that period has to be within 50 or 100 ppb of nominal. This definition allows short-term frequency errors outside of the limits, while ensuring that over longer peri- ods, the limits are met. Chapter 8 Synchronizati