WCDMA uses QAM (Quadrature Amplitude Modulation) apart from QPSK and dual QPSK to achieve higher downlink throughput in HSDPA. QAM uses both phase and amplitude information when forming the signal constellation. The modulation level of QAM can range up to 64QAM, but due to cost, complexity 16QAM was considered reasonable for HSDPA applications. 16 QAM uses 4 bits/modulation and 64 QAM uses 6 bits/modulation


In theory modulcation can be changed based on link quality. i.e., if radio link is good 64 QAM can be used, otherwise 16 QAM. Not sure if anyone out there using it in UTRAN