Methods, apparatus, and systems for efficiently utilizing scattered narrow spectra without introducing interreference among adjacent frequency bands are described. In one example aspect, a wireless communication method includes determining a set of time-domain symbols by applying an inverse Fourier transform to data modulated on multiple subcarrier groups. Each subcarrier group comprises one or more subcarriers and adjacent subcarrier groups are separated by one or more unused subcarriers, and each symbol has a symbol length of T0. The method includes determining a set of modulated time-domain symbols each having a length of N×T1 by modulating the set of time-domain symbols using a waveform function, where N is a real number greater than 1. The method includes transmitting the set of modulated time-domain symbols.