WiMAX supports very robust data throughput. The technology at theoretical maximums could support approximately 75 Mbps per channel (in a 20 MHz channel using 64QAM ¾ code rate). Real world performance will be considerably lower---perhaps maxing out around 45 Mbps/channel in some fixed broadband applications. Remember however, that service across this channel would be shared by multiple customers. Actual transmission capabilities on a per customer basis could vary widely depending on the carrier's chosen customer base, which is actually an inherent strength because it can be defined by QOS in a deliberate fashion to offer different bandwidth capabilities to customers with different needs (and different budgets). Mobile WiMAX capabilities on a per customer basis will be lower in practical terms, but much better than competing 3G technologies. WiMAX is often cited to possess a spectral efficiency of 5 bps/Hz, which is very good in comparison to other broadband wireless technologies, especially 3G.
The modulation scheme, whether quaternary phase shift keying (QPSK), quadrature amplitude modulation (16QAM, 64 QAM etc.) and their attendant code rate variations deliver varying bandwidth capabilities by channel size. Like most things wireless, the devil as they say is in the details. The good news is that pretty much all of the news is good in this regard relative to other broadband wireless and wireline competitors of WiMAX excepting LTE, which is still at least two years away from reaching the field. The OFDMA® technology actually supports multiple modulation schemes depending upon the users range from the cell with users at closer range receiving signal across more sub-channels at, for example, 64 QAM whereas a user at greater range would receive signal across fewer sub-channels (with higher gain or power per channel) using a lower bandwidth QPSK technique for example.
Many things affect transfer rate beyond simple radio capability---one major element being distance from the base station. The physics of radio cannot be avoided. Longer ranges result in lower bandwidth delivered. Also, the spectrum channel size (1.e. 20 MHz or other) that regulation defines as appropriate for different frequency bands will dictate bandwidth capabilities at least to some extent. Also, remember that the RF and physical environment play a strong role in throughput results. Essentially, the real world blunts theoretical performance.
The physics of frequency range plays a powerful role in bandwidth capability. The higher the frequency, the greater the bandwidth delivery potential and the shorter range potential. Lower frequencies enjoy much greater range capability, but trade that off with much lower bandwidth potential. Fortunately, even with disclaimers centered on real world impediments, WiMAX throughput is excellent. Perhaps no litmus test is as good as the results that carriers report and several carriers have shared that they are consistently achieving as much as 5 Mbps download speeds.