Transmit Output Power
The first thing to remember when comparing 802.11ac and 802.11ad is that 11ad operates in 60 GHz resulting in a much shorter range than 11ac. The second thing to remember about any standard is that the theoretical maximum throughput will probably never be achieved in practice. Just because the 802.11ad standard talks about 7 Gbps that doesn’t mean you’re going to get 7 Mbps of actual TCP/IP throughput. There’s a reasonable possibility that you may see a 2 to 3 Gbps data transfer rate with 802.11ad by 2014. There’s a sense in which 802.11ac is “faster WiFi” and, while 11ad uses WiFi protocols and modulation, it’s more like a “faster Bluetooth”. We think of WiFi for data transfer and we think of Bluetooth for connecting headsets, keyboards and other shortrange implementations. 802.11n and even 802.11ad (500 Mbps per user) start to create limitations for largescreen, highdefinition, multiaudiochannel video delivery in the home. 802.11ad enters the scene as a “wireless HDMI” technology – short range, very high throughput.
While output power (EIRP) is limited to 36 dBm (4 Watts!) in the 5 GHz UNII bands it’s limited to 10 dBm (10 milliWatts) in the 60 GHz band. A typical commercial WiFi network may operate at between 25 mW and 100 mW which immediately differentiates 802.11ac (typical 25100 mW) from 802.11ad (typical 1 milliWatt). To put the difference in 11ac and 11ad power into perspective we’ll first review some basic decibel rules. 10 raised to the 0.301029 power equals 2. 0.301029 “Bels” = 3 decibels hence 3 dB is a factor of 2. The surface area of an expanding RF signal volume increases in size as the formula for the surface area of a sphere: 4 X pi X r^2. When the radius of a sphere doubles the surface area increases by a factor of 4 – this is the Inverse Square Law for RF. When the radius of a sphere decreases by half (a factor of ½) the surface area decreases by four (a factor of ¼). A receiving antenna presents a particular real (or calculated) surface area to the expanding spherical RF wavefront. As the surface area of the wavefront increases the energy density decreases as an inversesquare proportion. The reduction in signal strength due to distance is referred to as “geometric signal reduction.” As a side note, the calculated surface area of a dipole antenna is referred to as the “effective aperture” and it gets smaller as frequency gets higher. Effective aperture and geometric signal reduction are both considered when calculating link budget using conventional formulae.
You can put the decibel representation for ratios (2 = 3dB) into the Inverse Square Law ratio. If distance doubles then surface area increases by 4 which means that RF signal power density decreases by 4. The number 4 is simply 2 X 2 and, since 3 dB = 2 and since you add logarithms to multiply, a ratio of 4 = 6 dB. If you double the range from a transmitter the signal will decrease by 6 dB based on spherical geometric expansion of the signal volume. If you’re not 100% confident in your understanding of “6 dB = twice the range” then you should do some selfstudy and nail this concept. An 802.11ac may transmit with 30 mW (14.77 dBm) but 11ad will be at 1 mW (1 dBm) – “How many times is that different by 6 dB?”. 14.77 dBm – 6 dBm = 8.77 dBm – 6 dBm = 2.77 dBm = 1.89 mW; resulting in just over 2 reductions by 6 dB. Each reduction of 6 dB cuts the range in half. If an 802.11ac network offered a 150 foot range indoors then the 11ad network would be limited to 37 feet by transmit power alone. Very low levels of transmit power are only one limiting factor for 802.11ad; the physics of the 60 GHz frequency band are the other limiting factor.


