Necessity of Cyclic Prefix in OFDM I have a question on CP-OFDM.
In recent years people are talking about filter bank approach where they are trying to filter out the out of band side lobes using filters. Besides they are presuming that this approach may provide some robustness and spectral efficiency since they can get rid of CP. Is this true and what aspects we can remove CP and make spectral efficiency equal one. 

Ref.
F. Schaich, T. Wild, Y. Chen , „Waveform contenders for 5G – suitability for short packet and low latency transmissions“, accepted for IEEE VTCs’14, Seoul, Korea, April 2014
In CP-OFDM, the transmitted signal is the output of the IDFT appended with a cyclic prefix. No pulse-shaping is used.

In so-called "new waveforms" proposed as candidate for 5G, pulse shaping is heavily used.

In FBMC, each subcarrier (i.e each complex exponential) is convoluted with a pulse with a certain shape (usually a bell shape with beginning and end at zero amplitude). As a consequence of convolution, the amplitude of each filtered subcarrier at the beginning and at the end of the multicarrier symbol goes to zero. 
Consequently, in case of multi-path, this zero-valued beginnings and ends avoid ISI, since the part of the previous symbol overlapping most with the current one is its end, which as we said is zero-valued (or almost) and the current symbol has little energy at its beginning (zero or almost).

Hope this helps.