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Recently, multirate filter banks have been recognized as high performance signal processing tools that could significantly enhance the efficiency and flexibility of future wireless communications. Filter bank-based multicarrier (FBMC) modulation allows increasing the spectral efficiency of high data rate transmission, both through the lack of time domain guard periods and the reduced frequency domain guard bands. Further, the inherent frequency selectivity is regarded as a vital characteristic, which enables high quality radio scene analysis, necessary for successful deployment of dynamic spectrum access in the context of cognitive radio technology. When applied to multicarrier communications, the filter banks are operated in the so-called transmultiplexer configuration where typically equal-size synthesis and analysis filter banks are utilized at the transmitter and receiver, respectively. This paper presents a new approach to generate an FBMC signal waveform. We focus on contiguous narrowband subcarrier allocation, which is often encountered in uplink transmission. Instead of using a full-size M-subchannel synthesis filter bank, the proposed scheme relies on a cascade of a P-subchannel synthesis bank (P ≪ M), time domain interpolation, and a user-specific frequency shift. This method allows constructing a transmitter-receiver pair using synthesis and analysis filter banks of different sizes. The presented approach is shown to provide notable computational complexity savings over a wide range of practical user bandwidth allocations, when compared to the conventional implementation consisting of equally sized filter banks. Therefore, this novel scheme provides flexible and low-complexity synthesis of spectrally well-localized FBMC uplink waveforms, which show strong potential for future broadband mobile communications.