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In theory, OFDM [Orthogonal Frequency Division Multiplexing] is constructed on waveforms with right angles or perpendicular lines. An orthogonal shape contains only right angles. Planes or vectors that are mutually perpendicular. In systems, OFDM is a multi-carrier or multi sub-carrier modulation format. The data is split into many closely spaced sub-carriers. Here is a chart of the various modulation types, coding bits and transmission speeds in Wi-Fi 802.11a using OFDM. BPSK [Binary Phase Shift Keying], QPSK [Quadrature Phase Shift Keying], 16-QAM [Quadrature Amplitude Modulation] and 64-QAM are all covered elsewhere in TECHtionary. Subcarriers consist of a few or many bits. That is, the great the number of bits the higher the transmission speed. Subcarriers are parts or sub-channels within a channel. That is, the IEEE 802.11b standard defines 11 possible channels (frequencies bands) available for use numbered 1-11 as shown here. In each channel, there are 52 subcarriers - 48 for data transmission and 4 for path monitoring. Each subcarrier transmits 250,000 OFDM symbols. FFT [Fast Fourier Transforms] are used to create the waveform. A FT [Fourier Transform] separates a waveform or function into sinusoids waves of different frequency which adds to be the sum to the original waveform. FT separates the different frequency sinusoids and amplitudes.

FLASH [Fast Low-latency Access with Seamless Handoff] - OFDM is a proprietary system from Flarion uses fast hopping across all tones in a “pseudo-random predetermined pattern” (you wonder how can something be both random and predetermined at the same time) making it a spread spectrum technology. With fast hopping, a user that is assigned one tone does not transmit on the same tone every symbol but uses a hopping pattern to jump to a different tone every symbol duration. Different base stations use different hopping patterns and each uses the entire available spectrum.

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