There is an assumption that color of two neighboring pixels does not change and corresponds to transmitting phase φ. Before transmitting color signal its phase is changed every second pixel from φ to – φ. In decoder second pixel phase sign is returned from – φ to φ. As a result , phase deviations in channel are compensated, because phases half-sum does not depend on equal phase shifts (see slide 17 in FBS Presentation).
The first version of the FBS method (FBS-1) can be illustrated by transforming an OFDMA system with four one-carrier MPSK modulation signals to FBS-1 system with the same four signals on the same four carriers.
Let N = 4, each symbol of each signal is transmitted four times on four carriers. Sign of phase (+ or -) corresponds to one of Walsh functions.
Symbol of a single signal is transmitted four times (on four frequencies), but on the same frequencies we can transmit four signals.
The effective signal can be recovered from the sum of four signals, due to the orthogonality of Walsh functions. At that, Doppler shift and delay have no influence on FBS reception (see slides 19 and 24 in FBS Presentation).
Main idea of FBS-II (FBS): Instead of having two carriers in OFDMA with QPSK (4 bits), I signal (2 bits) and Q signal (2 bits) are used to transmit them on N carriers with sign variation corresponding to an individual Walsh function. Amplitude and phase changings are compensated in FBS-II method (see slides 25 and 26 in FBS Presentation).
In the FBS system the symbol, with the same duration as in OFDMA system, is transmitted in a wide frequency band.
Therefore FBS comprises the advantages of:
OFDMA system – weak influence of multi path propagation
CDMA system – narrow band interferences and selective fading insignificant influence.
FBS main benefit:
No need for any test, pilot signals, or equalizing process.
See detailed presentation.pdf in References Downloads
