S, RN, UT1, and UT2 for a small scenario are shown in Figure 8.9. In the downlink direction, either the BS sends to UTs and RNs or the RN uses this time slot. The uplink is simply used in parallel at the same time, but this is not necessarily required. For duplex and simplex (half-duplex) FDD UTs, the schedule is different. The right-hand side of Figure 8.10 shows that the send and receive phases must not happen simultaneously for simplex terminals. Without the need for a duplex filter, simplex allows for cheaper radio hardware. OFDMA offers some additional benefits for relaying. The resources can be subdivided in a finer granularity than what would be possible with OFDM only. Figure 8.9 shows that first-hop transmissions are all treated the same way. They just occupy the required resources for their traffic. There is no waste due to completely assigned but incompletely used frames. In the uplink, also, several UTs share the full bandwidth, each of them transmitting on a subset of subchannels. The BS or RN coordinates the orthogonal noninterfering use of these subchannels by the UTs. OFDMA subdivision can also take place in the downlink SO that BS and RNs send on distinct subchannels. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology BS Transmits & Receives RN Transmits & Receives Figure 8.9 Resource usage in uplink and downlink. Shades correspond to the ones used in the previous figure. The RN toggles its task phase between UT and BS modes. duplex duplex ((i)) 10 ms Switch ((9)) Rx + Tx duplex duplex Figure 8.10 Radio frame format for the LTE FDD operation. Frame at BS: BS sends to downlink and receives in uplink resources. Top and bottom frames: UTs send in uplink and receive in downlink resources. Frame 3: RN has transceivers that can send and receive in both UL and DL. © 2009 by Taylor & Francis Group, LLC Multihop Extensions to Cellular Networks 8.4.2 Scheduling In layer 2, the resource scheduler is one of the most important functional units. Its purpose is the