or band). OFDMA domain relaying. Resources for hop one and hop two are separated in frequency (subchannels). Frequency domain relaying is trivial. It simply means we need another center frequency for the second hop. Then the BS does not need to reserve extra (idle) frames. It treats RNs as UTs, and the second hop acts as if it were a standalone cell. The result in performance is the same as for a wireless feeder. The main drawback is the higher usage of spectrum (i.e., a reduced spectral efficiency of the system). With time domain relaying, for a normal single-hop transmission (upper termi- nal in Figure 8.8), the BS uses dedicated resources on the downlink channel, which it can allocate for itself, knowing the traffic demand. Transmissions toward the RN happen in the same frame, just as with any other terminal. The traffic is known as well because the downlink scheduler knows the number of packets in its queues. © 2009 by Taylor & Francis Group, LLC Multihop Extensions to Cellular Networks Figure 8.8 Simple multihop scenario with one local UT and one remote UT (RUT) fed by a relay node RN. Power control can be used to assign a different transmit power to both blocks; the station closer to the BS does not need the full power compared, for example, to a UT at the cell border. Because of the fast Fourier transform (FFT) operation for OFDM, the full DL bandwidth is in use by the RF back end of the BS. For the second-hop DL transmissions, the RF sender of the RN is active. Because RN and BS are hard to synchronize (especially for the required OFDM orthogo- nality), all the power of the other sender must be treated as side-band interference. Therefore, the transmission must happen in a separate frame to avoid interference and the BS must reserve the complete resources of this frame for the second hop. Thinking about guard band distances could be another approach; this is difficult, however, because of the huge variations of signal and interference power within the area covered by both senders. The frame schedules at B