Introduction to FDM, OFDM, OFDMA, SOFDMA


SOFDMA (S-OFDMA) adds scalability to OFDMA. It scales the FFT size to the channel bandwidth while keeping the sub-carrier frequency spacing constant across different channel bandwidths. Smaller FFT size is given to lower bandwidth channels, while larger FFT size to wider channels. By making the sub-carrier frequency spacing constant, SOFDMA reduces system complexity of smaller channels and improves performance of wider channels.

As a reminder, IFFT (Inverse Fast Fourier Transform) is used in a WiMAX transmitter to create an OFDM waveform from modulated data streams, while FFT (Fast Fourier Transform) is used in a WiMAX receiver to demodulate the data streams. The FFT size equals the number of sub-carriers, e.g. in a OFDM/OFDMA system with 256 sub-carriers, the FFT size is 256.

Transmitter RF chain consists of encoder, modulator, IFFT, Filter, DAC, RF, and antenna; Receiver RF chain consists of antenna, RF, ADC, Filter, FFT, demodulator, decoder

Picture. Transmitter and Receiver RF chains in WiMAX
Basic building blocks of a Tx RF chain and a Rx RF chain.

SOFDMA is the OFDMA mode used in Mobile WiMAX. It supports channel bandwidths ranging from 1.25 MHz to 20 MHz. With bandwidth scalability, Mobile WiMAX technology can comply with various frequency regulations worldwide and flexibly address diverse operator or ISP requirements, that's whether for providing only basic Internet service or a broadband service bundle.

OFDM and OFDMA Symbol Structure

OFDM subcarrier structure

Picture. OFDM symbol structure in WiMAX

Both OFDM and OFDMA symbols are structured in similar way. Each symbol consists of:
- data sub-carriers (OFDM) or sub-channels (OFDMA) that carry data (information),
- pilot sub-carriers as reference frequencies and for various estimation purposes,
- DC sub-carrier as the center frequency, and
- guard sub-carriers or guard bands for keeping the space between OFDM/OFDMA signals.

OFDMA subcarriers (subchannels) structure

Picture. OFDMA symbol structure in WiMAX
sub-carriers of the same color represent a sub-channel.

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