- Different modulation methods corresponding to different transmitting abilities in air interface.
HSDPA: QPSK or 16QAM
- The UTRAN air interface uses QPSK modulation in the downlink, although HSDPA may also employ 16 Quadrature Amplitude Modulation (16QAM). 16QAM requires good radio conditions to work well. As seen, with 16QAM also the amplitude of the signal matters.
- in QPSK one symbol carries two data bits; in 16QAM each symbol includes four bits. Thus, a QPSK system with a chip rate of 3.84Mcps could theoretically transfer 2 × 3.84 = 7.68 Mbps, and a 16QAM system could transfer 4 × 3.84 Mbps = 15.36 Mbps. In 3GPP also the usage of 64QAM with HSDPA has been studied.