This is an extremely misleading statement. The fact is that on xG's controlled and highly benign test link, WiFi could be adjusted to operate with the same low power, or even less. xG is implying that xMax represents a breakthrough in modulation and coding that requires vastly smaller amounts of transmitter power. And this is simply untrue.
Most WiFi units do not use automatic transmitter power control; they transmit a constant, relatively high power level (see footnote 1) to provide "link margin" to maximize range, penetrate walls and diffract around obstacles. This means that they often operate with much more power than required. 802.11 does automatically vary its data rate according to the received signal-to-noise ratio, with 802.11g ranging between 1 Mb/s and 54 Mb/s, and this can be seen as another form of automatic transmitter power control. But this only covers a 21 dB range (see footnote 2), and real-world links vary over a much larger range.
Although the lack of automatic transmitter power control is a definite 802.11 drawback, it certainly isn't the fault of its modulation and coding schemes.
xG's reports do not include all of the test parameters, particularly the antennas, but we can make some reasonable estimates that actually bias the calculation in favor of xMax.
WiFi equipment performance varies by manufacturer, but we can use a representative example: the Ruckus Wireless 802.11b/g WiFi transceivers. They specify a receive sensitivity of -96 dBm for the 6 Mb/s speed of 802.11g. This is the closest 802.11g speed that exceeds the specified 3.7 Mb/s speed of the xG xMax radio. Lower speeds are available in the 802.11b mode set, but since 802.11g uses convolutional error control coding while 802.11b does not, most of the 802.11b speeds require as much or more power as 802.11g's 6 Mb/s speed even though they're slower.
The path loss between two isotropic antennas is
path_loss(db) = 20log10(4 π d/λ), where100 feet is about 30 meters, and the wavelength of 915 MHz (the center of the 902-928 MHz band) is 32.8 cm. The path loss is therefore
d = distance
λ = wavelength in same units as distance
20log10(4 π 30 / .328)The required transmitter power over a 30 meter path between isotropic antennas to give a received power of -96 dBm is therefore
= 61.2 dB
Ptx(db) = -96 dBm + 61.2 dBThis is almost exactly the same as xG's much-touted figure of 300 nanowatts -- and for nearly twice the data rate (6 Mb/s vs 3.7 Mb/s). If we normalize the WiFi data rate to xMax's speed, the WiFi power drops to only 204 nanowatts.
= -34.8 dBm, or about 331 nanowatts.
Furthermore, isotropic antennas do not exist; they are just a benchmark for comparison. All real antennas have gain over isotropic, so while we don't know what antennas xG used for their test, their received signal strength was undoubtedly higher than we've assumed here for WiFi. So it's reasonable to conclude that 802.11g would require even less than 204 nW to carry data over the same link and at the same rate as xMax.
This is no surprise to those familiar with digital modulation and coding. The Shannon Channel Capacity Theorem places a firm upper limit on the error-free data rate that can be achieved over a noisy, bandwidth-limited radio channel (as all radio channels are). The current state of the art, Turbo coding, is only 0.5 dB away from the Shannon limit, that is, Turbo coding can already achieve 89% of the absolute limit. Even though 802.11g uses an older code (k=7 convolutional coding with Viterbi decoding) that's perhaps 5 dB from the Shannon limit, there simply isn't much room for additional improvement, and there is no chance for the kind of dramatic breakthroughs implied by xG's statement that xMax is 3,000,000 times more power efficient than WiFi. Such numbers can be achieved only in artificial comparisons to systems that are operated with extreme inefficiency.
Phil Karn, 7 June 2007; revised 29 June 2007