An interesting technical detail came to my attention today concerning Time Division Duplex (TDD) wireless systems such as WiMAX: Since uplink and downlink transmission is done in the same frequency band, uplink and downlink capacity can be adjusted based on demand. In theory this is an advantage over FDD (Frquency Division Duplex), used by most cellular 2G and 3G systems today. Here, uplink transmissions use a seperate frequency band which is just as large as the downlink frequency band (e.g. 5 MHz for UMTS). This means that FDD systems always have a 1:1 ratio between uplink and downlink. With TDD systems, this ratio can be changed, for example to 2:1, 3:1, etc. to give more capacity to the downlink. But there is one important thing to remember: The efficiency of uplink transmissions is much lower than in the downlink due to the lower transmission power and small antennas of the mobile device. Thus, even with a 1:1 ratio, uplink data rates are far lower than data rates in the downlink despite the using the same amount of bandwidth. I estimate that the maximum overall speed achieved in uplink direction is only 1/3 or 1/4 of the downlink. With rising uplink requirements of web 2.0 applications (picture, video uploads for example) I wonder if it will even make sense in practice to configure a 2:1 or 3:1 ratio in TDD systems as the uplink capacity would then be only a tenth of that of the downlink!? Opinions, anyone?