source allocation. This stage is known as "frequency-domain scheduling." MIMO user pairing and remainder user selection. Power allocation. These stages are discussed in more detail in the following sections. Static and regular scheduling This scheduling stage establishes a logical grid of the air interface resources and preallocates those resources that are known in advance, such as broadcast channels, regular allocations from static resource schedulers (3GPP: semipersistent scheduling), etc. Static and regular scheduling is exe- cuted before dynamic scheduling as shown in Fig. 7-8, given that it has higher priority by design. Downlink Resource Static Channels Allocation Regular Channels Dynamic Algorithmic Scheduling Scheduler Dynamic HARQ re-tx Scheduler Scheduling buffers UE dbase measurements, stats Dynamic Algorithmic Scheduling Scheduler FIGURE 7-8 Breaking down the scheduler algorithm. Chapter 7 Wireless scheduling and Quality of Service optimization techniques 251 Active user selection Per the definition, active users are simply defined as connected users that can technically be scheduled to and have (sufficient) data available for transmission, either in the transmit direction by a data queue transitioning from empty to nonempty, or on the receive direction by a buffer status indication. Additional limitations on eligibility for scheduling can include DRX considerations or available HARQ resources. Algorithmically, there is no complexity in active user selection as such, but it is important to note that efficient implementation of this step in software can greatly reduce computational resources spent in the next phases through elimination of unnec- essary compute resources (do not even consider scheduling a user that has no data to transmit). Primary user selection (time-domain scheduling) The target of a PF scheduler algorithm is to balance system spectral efficiency (and system- wide aggregate throughput) while ensuring individual user satisfaction through providing a minimum throughput to each user. T