t the same and will incur additional conformance test time and cost. As always, the impact on user equipment cost and performance tends to get overlooked or ignored. Even at the system level it is arguable whether there is any efficiency gain. The argument is that if the uplink is lightly loaded in an FDD system then valuable bandwidth is being wasted. However, all that happens is that the noise floor at the e node B reduces. This improves the performance of all other FDD uplinks in the cell, reducing power drain for all users served by the cell site. So you might ask why TDD appears to work in WiFi. The answer is that WiFi is only spectrally efficient and energy efficient because it is low power, 10 milliwatts of transmit power from the user's device rather than 125 or 250 milliwatts in TDD LTE. The occupied channel bandwidth of WiFi at 2.4 GHz is 22 MHz with a channel spacing of 25 MHz within 80 MHz of operational bandwidth SO that's three channels within 80 MHz. This is not in itself efficient. The efficiency comes from the channel reuse that is in turn a function of the low Telecommunications Economies of Scale transmit power. There is some trunking gain achievable from a 20-MHz channel but much of this disappears if bidirectional differentiated quality of service needs to be supported. It is hard to avoid the conclusion that TDD at least for general use within licensed spectrum is one of those technology cul de sacs that the industry has managed to drive into with no reverse gear engaged. The combination of reasons outlined above explain why wide-area TDD WiMax networks never performed as well as expected as offered traffic loading increased over time. Anyway, back to FDD. As we have said, in an FDD band plan the bands are subdivided into transmit and receive duplex pairs. The duplex separation varies between 45 MHz (800/850/900 MHz bands) and 400 MHz (Band IV US AWS). The lower duplex in 2007 was always mobile transmit as the propagation conditions are more favourable. However, five years later the allocati