what is OQAM (Offset QAM) and why it is required? Currently I am studying about a modulation technique FBMC (Filter Bank Multicarrier) which is a predecessor of OFDM. The difference with OFDM is that, FBMC does not have a CP (Cyclic Prefix) which is a better use of spectrum. On the other hand, it is susceptible to ISI (Inter Symbol Interference) due to the absence of GI (Guard Interval) when transmitted over wireless channel.1. FBMC uses prototype filters to shape the out of band emission to lower the interference.
There is a concept of FBMC-OQAM (sometimes referred as staggered multitone or SMT), where the input signal is first converted to QAM symbols and further processed into OQAM symbols[2].
Can anyone explain to me in very simple words / general context:
#. What is the purpose of OQAM (ref.2 has a figure) and how it is making the signal orthogonal?

1https://ieeexplore.ieee.org/document/5753092
[2]https://ieeexplore.ieee.org/abstract/document/8255374
Actually, in theory and error performance there is no difference. But in practice, OQPSK  decrases the maximum phase alteriation from 180 to 90 degrees which is better for RF power amplifiers.