y 1,024 small orthogonal subcar- riers within the system bandwidth. Small subcarriers mean long symbols in time, SO the problem of intersymbol interference (ISI) is relieved. The big advantage of OFDM is that each subcarrier can be modulated differently, SO a robust BPSK (binary phase shift keying) can be used on frequencies where the channel is cur- rently bad due to fading, while, for example, QAM 64 can be used on more stable subcarriers with higher SINR values. Channel coding can also be adjusted in wide ranges to adapt to the subchannel conditions. This is called dynamic adaptive mod- ulation and coding (AMC) [26]. Channel adaptive behavior is an advanced topic. Together with adaptive power allocation (APA), it is far from comprehensive now. Orthogonal frequency division multiple access (OFDMA) means that within one OFDM symbol, several different UT receivers can be addressed. For example, 256 subcarriers are used for each of four UTs that receive the symbol. Dynamic subcarrier assignment (DSA) selects the best resources for each UT based on chan- nel state information (CSI), which is known due to channel quality indication (CQI). Because the distance and path loss may be very different among the UTs, it is likely that the PhyModes used for each UT are very different within the full OFDM symbol. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology TTI = 500 us (7 x 66.6 us + 6 x 4.687 us + 1 x 5.208 us) Signaling Scheduled overhead resources 1/6 for pilots 1 chunk = Cyclic 66.6 us 12x6=72 prefix subsyms Time slot (1 OFDMA symbol) Figure 8.7 The principal layout of the orthogonal resources within one LTE TTI frame [23]. This frame contains seven OFDM symbols in 500 us. The alternative is 14 symbols in 1 ms. Radio resources are the valuable goods of which nobody can get enough. Traffic and tariffs are typically proportional to the used or allocated resources of a user. The Shannon bound limits the number of bits that can be transmitted in a given resource block of f