The unlicensed 57-71-GHz wireless spectrum offers high throughput and scalable personal area connectivity. To implement high-speed energy-efficient signaling, we present a power back-off-efficient digital polar transmitter (TX) supporting multi-gigasymbol per second switching rates and higher order modulations up to orthogonal frequency-division multiplexing (OFDM) 64-QAM. Pulse shaping of the direct digitally modulated waveform outputs for spectral filtering is achieved in low power with a low oversampling ratio and analog filtering. Channel bonding and selection is supported with a frequency configurable local oscillator (LO) chain. Multiple-input multiple-output (MIMO) is implemented with low area overhead using a differential dual-polarization (DP) printed circuit board (PCB) antenna supporting simultaneous vertical/horizontal (V/H)-polarization data streams and high cross-polarization (X-pol) isolation. The 0.9-V 28-nm CMOS TX prototype with polarization MIMO achieves up to 42-Gb/s throughput at 4.3-pJ/b efficiency and 12 b/symbol of OFDM/single-carrier (SC) signaling.