Next generation wireless communication systems face many challenges to increase the capacity and spectral efficiency of current solutions. The worldwide mobile data traffic increased 4000-fold over the last decade, and is forecast a 7-fold increase between 2016 and 2021. To cope with these stringent demands, prospective solutions are millimeter-wave (mmWave) technology and ultradense small cell networks, given that today most of the mobile traffic is offloaded from mobile networks, i.e., most of mobile users are connected to fixed networks. In addition, enabled by the fast development of electronics, digital signal processing has become essential to enhance the capacity and the performance of current communication systems. In this paper, by using the benefits of multiband modulation schemes and independent sideband (ISB) modulation, high-speed mmWave wireless transmissions in the D-band (110-170 GHz) are reported. D-band radio frequency carrier is generated by means of optical heterodyning, and ISB modulation is applied by means of the Hilbert transform and a double-nested Mach-Zehnder modulator. Total data rates up to 352 Gb/s and spectral efficiencies up to 7.7 bit/s/Hz are experimentally achieved in a single 2 x 2 multiuser multiple-input multiple-output system.