ofdm

\subsection{OFDMA}
OFDMA is a multiuser version of the Orthogonal Frequency Division Multiplexing digital modulation scheme. OFDMA systems are based on digital technology and one of the key concepts is the use of Discrete Fourier Transform (DFT) and its inverse operation (IDFT). With DFT and IDFT it is possible to move between time and frequency domain without losing any of the original information as long as the classical requirements for digital signal processing are full-filled. \cite{holma2011}

In OFDMA data is carried from baseband to user with sub-carriers. The spacing between sub-carriers is 15 kHz regardless of the total transmission bandwidth. Sub-carriers are orthogonal to each other meaning that in the peak of one individual sub-carrier in the frequency domain, other sub-carriers have zero value as shown in Figure X. In the transmitter, a serial input bitstream is converted into multiple parallel bitstreams, and each stream is mapped to a symbol stream using a modulation constellation. Symbol stream is then fed into IFFT block which gives a set of complex time-domain samples. Each sample corresponds to the input representing a certain sub-carrier. After the IFFT block, a cyclic prefix is added to each symbol to mitigate the Inter-Symbol-Interference (ISI) where adjacent symbols are interfering with each other due to the multipath propagation. The transmitted signal is the sum of a set of orthogonal sub-carriers. \cite{holma2011}

The cyclic prefix is added by copying the tail of the symbol and adding it to the beginning of the symbol, as shown in Figure X. Cyclic prefix extends the symbol time and the length of the cyclic prefix should be longer than the delay spread of the channel to remove ISI entirely. Cyclic prefix also makes the symbol periodic over the symbol duration which ensures orthogonality between sub-carriers and allows the use of DFT and IDFT in the receiver and transmitter respectively. \cite{holma2011}