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 a set of processed data modulated on multiple subcarrier groups. Each subcarrier group comprises an even number of subcarriers and adjacent subcarrier groups are separated by one or more unused subcarriers. The set of processed data is determined by applying a first spreading code to data carried in subcarrier groups having 2×m subcarriers when m is a positive odd number and applying one or more spreading codes to data carried in subcarrier groups having 2×n subcarriers when n is a positive even number. The method also includes transmitting the set of time-domain symbols.
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