requency (bandwidth) and time (transmission duration), given a certain SINR situation. In OFDMA, the granularity of these resources is typically a chunk (resource block)-that is, a group of subcarriers in frequency (f) direction and a number of OFDM symbols in time (t) direction. Figure 8.7 shows the frame format proposed for 3GPP-LTE [8]. In a real system, the pilot symbols are distrib- uted over the resources. For simplicity, they are shown concentrated in symbol 0 in the figure. Resources shown in gray are (just arithmetically) reserved for signaling purposes (i.e., for announcing the outcome of the resource allocation decision). A chunk of size-12 subcarriers times six symbols can carry up to 360 b in PhyMode 64 QAM-5/6. Six symbols is the equivalent net size of a resource block. Smaller resources with finer granularity could fit smaller traffic better, but the overhead for segmentation, signaling, and control increases and leads to inefficiency. An obvious additional resource dimension is space. Resources used at one loca- tion (cell) can be reused at another location (see reuse distance 3 in Figure 8.2). Also, within one cell, there can be a spatial reuse by using beam forming, multipath transmission, and spatial multiplexing by the use of distributed antenna elements. Spatial multiplexing is beneficial in multihop situations where relays belonging to one cell are separated enough from each other and can use the same f+ t resources © 2009 by Taylor & Francis Group, LLC Multihop Extensions to Cellular Networks in parallel [12]. The remaining orthogonality dimension is the code. CDMA (code division multiple access) systems excessively utilize this dimension. However, together with OFDMA, there is currently no useful case of combination. 8.2.6 Cross-Layer Issues The support of sending PDUs with different Phy. Modes requires an interaction between layer 2 and layer 1 (e.g., an "interface for adaptive modulation and cod- ing"). Adaptive power allocation per subchannel also requires a cross-layer interface. In th