Subcarrier spacing - a neglected degree of freedom?

In this paper we assess the pains and potential gains a wireless communication system can leverage on, if we allow for multiplexing different subcarrier spacings within a single transmission time interval (TTI). CP-OFDM is nowadays most often the signal format of choice in wireless communication systems when serving multiple users. LTE/-A, WiFi and WiMAX have all adopted CP-OFDM due to its high degree of flexibility. Though, not all degrees of freedom have been exploited in those systems yet. While these systems are using tailored waveform parameters according to the respective setting - e.g. while WiFi is more tailored towards local area, 4G and WiMAX use settings more suited for wide area - 5G may go one step further by allowing for user specific settings within a single band. With allowing this the signal characteristics may be better matched to the respective channel characteristics such as high Doppler spreads and low coherence times. Our investigations have shown that with exploiting this degree of freedom a higher range of user velocities may be supported, especially with replacing CP-OFDM by UF-OFDM. UF-OFDM modifies the OFDM concept by subband-wise filtering, which mitigates inter-carrier interference (ICI) arising from different parallel subcarrier spacings. This makes it a natural fit for being combined with user-specific subcarrier spacings.