WiFi, unlike mobile communication, has very limited mobility management, so is not regarded as mobile communication. It also operates at much lower power levels than mobile, giving a reduced range, and operates in unlicensed spectrum as opposed to the licensed spectrum allocated to mobile operators for their exclusive use. Anyone can use unlicensed spectrum (subject to certain restrictions), so WiFi has no guarantee of quality or reliability. Table 4.1 summarises some of the more significant versions of WiFi in chronological order.
Table 4.1 Some versions of 802.11
|Channel(s) /MHz||Modulation method||Highest modulation order||Highest code rate||MIMO streams||Max transmission rate per MIMO stream /Mb s–1|
|802.11n||2009||2.4/5||20/40||OFDM||64 QAM||5/6||up to 4 streams total|
(in 20 MHz)
|802.11ac||2014||5||20/40/ 80/160||OFDM||256 QAM||5/6||up to 8 streams||180 (in 40 MHz; highest coding rate in 20 MHz = 3/4)|
The ‘modulation method’ column shows that OFDM is standard in all versions apart from 802.11b. Not only has OFDM become standard, but the specification of OFDM has remained consistent. Subchannel width, is 312.5 kHz across all WiFi versions except 802.11ad.
The maximum transmission rates shown in Table 2.3 need to be read in conjunction with the number of channels used, as the 802.11n and ac standards allow channels to be combined. Combining two adjacent 20 MHz channels to make a 40 MHz channel yields slightly more than double the benefit, because the guard band between the channels can be used for data. Similarly, an 80 MHz channel gives slightly more than double the benefit of a 40 MHz channel.
802.11n and ac can use Multiple Input Multiple Output (MIMO), which takes advantage of the multiple routes radio waves can take in a reflective environment (such as indoors) to enable multiple streams of data to be transmitted on the same frequency. Using MIMO increases the transmission rate in proportion to the number of spatial streams used. Table 2.3 gives maximum transmission rates for a single spatial stream.
802.11ad is clearly exceptional, both in the band it occupies (60 GHz) and in its channel width (2160 MHz). The appeal of the 60 GHz band lies in the large amount of licence-free spectrum available – hence the prospect of channels that are about 100 times wider than the 20 MHz channel width of, for example, 802.11g. With such wide channels, transmission rates of several gigabits per second become possible; thus 802.11ad is also known as ‘WiGig’ or gigabit-rate WiFi.
A basic principle of the 802.11 standards is backwards compatibility, whereby later variants are compatible with earlier variants. For example, a device equipped for 802.11ac – which is a 5 GHz-only standard – will ‘fall back’ to earlier 5 GHz standards, namely 802.11a and n if there is a WiFi access point using one of those earlier standards. In those circumstances an 802.11ac device will perform no better than a device designed for the earlier standards.