he radio interface. A high-level overview of 5G scheduler functionality is depicted in Figure 10.49. The Medium Access Control (MAC) scheduler is the controlling entity for multi-user radio resource allocations, which is subject to several constraints but also has many options for efficiently serving the different devices. The enlarged number of options for the 5G scheduler offers performance improvements, but also presents the nontrivial problem of how to best utilize those degrees of freedom in an efficient manner. The 5G scheduler works by dynamically allocating radio transmission resources on a per-user basis for downlink and uplink transmissions. The scheduler aims at fulfilling the QoS service targets for all the bearers of the served users. The scheduler must support multi-cell connectivity mode, where devices are configured to be simultaneously served by multiple cells. There may also be other multi-cell coordination constraints including inter-cell interference coordination 10.8 Packet Scheduling Algorithms Flexible radio Different New scheduling Service-specific EnhancedMIMO frame numerologies formats Agile 5G scheduler Service Multi-cell Multi-cell awareness coordination connectivity Figure 10.49 Agile 5G packet scheduler. between neighboring cells where certain radio resources are dynamically muted, and hence not available for dynamic scheduling of users. 5G comes with a new flexible structure, consisting of 10 ms radio frames and 1 ms sub- frames. The subframes are constructed of slots that consist of 14 Orthogonal Frequency Domain Multiplexing (OFDM) symbols in the default setting. To support operation in different frequency bands, the physical layer numerology is configurable. The subcar- rier spacing can scale from the base value by a factor 2 to the power of N from 15 to 120 kHz, or even larger values. The slot duration is 1 ms for 15 kHz, while it equals 0.5 ms for 30 kHz. Furthermore, mini-slots of two, four, or seven OFDM symbols are defined as well, with two-symbol mini-slots typically consid