Tham khảo tài liệu 'vehicular technologies increasing connectivity part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fast Power and Channel Adaptation for Mobile Users in OFDMA Multi-Cell Scenarios 141 our simulations the thresholds a 1 l L are assumed equally spaced between the minimum and the maximum values specified in modulation and coding profiles for IEEE S-OFDMA standard 1 and O-L . For the sake of generality instead of considering a number Cl of information bits provided per carrier that depends on the chosen modulation and coding we assume that an user whose SINR achieves the threshold a transmits with the theoretical Shannon efficiency ni bit s Hz log2 1 10ai 10 . 3 If none of the thresholds is exceeded sub-channel i is switched off for the k-th user. 3. Fast adaptive techniques In OFDMA systems smart allocation of radio resources is a crucial aspect for achieving excellent performance levels. In Fig. 2 we can observe a typical structure of an OFDMA time-frequency layer the set of sub-carriers and symbol times is divided into resource blocks RB which constitute the minimum amount of resources that can be assigned to an user connection. In fact each user is assigned a set of RBs generally but not necessarily contiguous Fig. 2 a . The sub-carriers of the same RB are interested by the same modulation coding profile and power. For the sake of simplicity but without loss of generality . the scope of this study we assume a resource division based on a one-dimensional approach where a user is assigned the same sets of sub-carriers for the entire allocation time Tupd Fig. 2 b . So a resource block is equivalent to a sub-channel and an user can share separate sub-channels in the same Tupd. The allocation techniques require knowledge of the function reported in Equation 1 with an updating time Tupd that should be shorter than the coherence time of the channel this constitutes the main limiting factor in mobile applications. When Tupd is comparable or greater than the coherence time of the channel the algorithm performance degrades rapidly as the channel
