Multiplierless Filtered-OFDM Transmitter for Narrow-Band IoT Devices
In cyclic-prefix orthogonal frequency-division multiplexing (CP-OFDM) based radio access systems, the coexistence of different technologies without precise time-frequency synchronization is limited due to high out-of-band emissions. Therefore, spectrum enhancement techniques play a key role to relax the synchronization and power control requirements. This allows higher degree of opportunistic spectrum use with minimized interference. Filtering and time-domain windowing are two fundamentally different approaches considered for spectrum enhancement. Filtered OFDM (F-OFDM) provides better spectrum localization than the time-windowing based schemes (such as windowed overlap-add, WOLA), but at the cost of higher complexity. This paper presents low-complexity solutions for spectrally enhanced narrow-band OFDM transmitters based on the use of a look-up table (LUT) to store the waveform. Our focus is on F-OFDM, but the same ideas are directly applicable also for WOLA. The LUT approach requires only memory units and relatively low number of additions. We consider the LUT approach especially for the uplink scheme of the 3GPP narrowband Internet of things (NB-IoT) service, having a maximum bandwidth of 12 subcarriers. Moreover, we propose a lowcomplexity solution to deal with the CP-length variations within the transmission frame. We also propose a way to generate the NB-IoT uplink signal without additions, which could be realistic with 1 or 3 subcarrier allocations. A simplified addressing technique is formulated for the considered LUT arrangements and modulation schemes. Analytical expressions for the LUT contents are provided, along with expressions for the memory requirements and addition rates. Required memory wordlengths are evaluated through simulations, also considering the effects of a practical nonlinear power amplifier model. Comparisons with time-domain filtering, time-domain windowing, and fastconvolution- based filtering solutions are included as well.
