Differential quadrature phase shift keying. DQPSK is a digital modulation technique commonly used with cellular systems. Motorola's CyberSurfr cable modem uses DQPSK to carry data upstream from the subscriber's computer to the Internet on a narrower frequency band than standard QPSK. Narrower bands allow more upstream channels, so the CyberSurfr has additional noise-free channels to choose from when it's installed.