t on the corresponding radio bearer is sufficient and can meet the end-to-end EPS bearer-level QoS guarantees. In summary, we can formulate the primary goal of RRM as to control the use of radio resources in the system such that the QoS requirements of the individual radio bearers are met and the overall used radio resources on the system level are minimized. That is, the ultimate goal of RRM is to satisfy the service requirements at the smallest possible cost for the system. 4.1.2 The Notion of Radio Resource in LTE The radio interface of LTE is based on the OFDM technology, in which the radio resource appears as one common shared channel, shared by all users in the cell. The scheduler, which is located in the eNode B, controls the assignment of time-fre- quency blocks to UEs within the cell in an orthogonal manner SO that no two UEs canthus intracell interference is avoided. One exception, though, is multiuser spa- tial multiplexing, also called multiuser MIMO (multiple input, multiple output), when multiple UEs with spatially separated channels the uplink of LTE. More details of MIMO support in LTE are discussed in Section 4.2.7. Such a scheduler function is needed for both the uplink (UL) and the downlink (DL) SO that it is compatible with frequency domain duplexing (FDD) and time domain duplexing (TDD) modes. Figure 4.2 shows the resource grid of the uplink and downlink shared channels. The smallest unit in the resource grid is the resource element (RE), which corre- sponds to one subcarrier during one symbol duration. These resource elements are organized into larger blocks in time and in frequency, where seven of such symbol durations constitute one slot of length 0.5 ms and 12 subcarriers during one slot © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology One resource element User A User B One resource block 12 Sub-carriers 7 Symbols (one slot = 0.5 ms) Figure 4.2 Uplink/downlink resource grid. form the so-called resource block (RB). Two consecutive time slots