 and known to both the transmitter and receivers, which is the case in a Time Division Duplex implementation. The number of receivers (users) is arbitrary but limited by the number of transmit antennas. The number of served users thus becomes a resource allocation problem: The scheduler selects from all active users, at any chosen time, a subset of users (and their allocated resources such as OFDMA subcarriers and transmit power) to maximize performance. Performance can be defined as sum rate across users at a given power, or similar. Intuitively, it can be understood that selected (scheduled) users need to exhibit both high SNR and good separability in the special domain (i.e., orthogonality) to ensure multiplexing is successful. Popular algorithms are greedy, where the transmitter chooses the user with the highest channel capacity first and then finds the next user that provides orthogonality from the first user. Orthogonality can be measured as the coefficient of correlation:47 cos(coed((1,,11)) or by estimating the maximum sum rate from the remaining unselected users, which is pro- vided by the condition number of the channel matrix from all users combined48. where cond(A) The "next user selection" continues until all r users are selected. Chapter 7 Wireless scheduling and Quality of Service optimization techniques 249 Architectural framework for the base station wireless scheduler algorithm This chapter outlines a generic architecture for packet queueing and scheduling frameworks that allows different scheduling algorithms to be implemented independently of remainder MAC/Radio Link Control(RLC) processing stages. Given the complexity of the scheduling algorithm, practical implementations are broken down into simpler stages that are executed sequentially. This breakdown sequentially reduces the dataset (user database) under consideration for scheduling. Consider Fig. 7-7. This graph defines: Connected users are users that are associated with the base station. For example, mobile phones that have undergone RAC