, H.; Kim, B.; Lee, Y. On the use of Linear Programming for Dynamic Subchannel and Bit Allocation in Multiuser OFDM; IEEE, 2001. Song, G.; Li, Y. Cross-layer optimization for OFDM wireless networks-part i: theoretical framework. IEEE Transactions on Wireless Communications 2005, 4 (2). Song, G.; Li, Y. Cross-layer optimization for OFDM wireless networks-part ii: algorithm development. IEEE Transactions on Wireless Communications 2005, 4 (2). Viswanath, P.; Tse, D. N.C. Sum capacity of the vector gaussian broadcast channel and uplink-downlink duality. IEEE Transactions on Information Theory Aug 2003, 49, 1912-1921. Wang, Y., Peng, F., Weidong, Z., Yuan, Y. MU-MIMO User Pairing Algorithm to Achieve Overhead- Throughput Tradeoff in LTE-A Systems. 2013 8th International Conference on Communications and Networking in China (CHINACOM), 2013. Yin, R., et al. Pricing-based interference coordination for D2D communications in cellular networks. IEEE Transactions on Wireless Communications 2015, 14 (3), 1519-1532. This page intentionally left blank Synchronization in open radio access networks In this chapter, we turn away from the primary data-plane functionality of open radio access networks (O-RAN) and look at all the aspects relating to synchronizing the nodes in an O-RAN network. We will start by revising the common concepts of frequency and time and discussing how networks can be synchronized and the tools made available for this. Then we look at the specific synchronization requirements for O-RAN networks and how these can be achieved in practice. Unfortunately, the synchronization aspect of a network is often a function that is not given the consideration it deserves in system planning-often with the result that it is left until the last minute. This is bad, since without proper synchronization, the performance of a cellular network can be significantly downgraded or, in extreme cases, compromised to the point where its transmissions could violate regulatory standards. Understanding frequency, time, syntonization, a