Types of OFDMA
OFDMA stands for Orthogonal Frequency Division Multiple Access. It is the most popular access technique adopted in next generation wireless technologies such as Mobile WiMAX, LTE, LTE Advanced etc. In OFDMA technique, number of frequency bins in a single IFFT is divided and allocated to different mobiles(users). In Mobile WiMAX resource allocation is based on number of slots. Slot definition varies based on zone types, but slot is composed of number of subcarriers and number of OFDMA symbols. One slot in PUSC is made of 1 subchannel and 2 OFDMA symbols. Here 1 subchannel consists of many subcarriers. For example in a 1024 point FFT, there are 30 subchannels and each subchannel is made of 24 data subcarriers. In LTE and LTE Advanced resource allocation is based on number of resource blocks. One Resource block consists of 12 subcarriers and 7 OFDMA symbols. In Mobile WiMAX terminal is referred as Mobile Subscriber Station and in LTE/LTE Advanced terminal is referred as UE.
There are two main types of OFDMA viz. distributed OFDMA and localized OFDMA depends on how the sub carriers are assigned to the terminals/mobile subscribers.
If the subcarriers are contiguously assigned to each terminal then it is called localized OFDMA. Number of subcarriers assigned to the terminal is determined based on data rate requirement as desired by the user. This is because one subcarrier carry one or more than one bits based on modulation technique. For BPSK it is one bit, for QPSK 2 bits,for 16QAM 4 bits and so on. Hence bits carried on total subcarriers in a symbol time gives us data rate.
If the distributed subcarriers are assigned to each terminal then it is called distributed OFDMA. This distributed subcarriers are obtained using permutation technique.
As mentioned and depicted in the figure above, resource allocation out of chunk of sucbcarriers of a IFFT size can be either continuous or permuted per user. Distributed OFDMA has advantage over localized OFDMA as it is more resistant to time varying fading channel. Localized OFDMA is simpler to implement compare to distributed one. Interleaving also does the same job as performed in a distributed OFDMA.