umber of public hotspots. Source: Ref. [8] With the exception of the 802.11b standard, all other 802.11 standards are based on OFDM for modulation. The carrier frequency bandwidth is typically 20 MHz, which fits in well within the ISM bands of interest. A typical device may use up to 3 subcarriers at any time, as illustrated in Figure 12.4. The subcarriers are used in a time division duplex (TDD) fashion, with two-way communications duplexed in the time domain [as there does not exist a central device such as a base station, a frequency division duplex (FDD) mode cannot work]. OFDM subcarrier bandwidth is 312.5 kHz, and therefore 64 subcarriers can fit in a 20 MHz carrier. Of these 64 subcarriers 52 are used: 48 for data and 4 for pilots. The remaining 12 subcarriers Personal Wireless Communications Systems Table 12.3 Features of IEEE 802.11 standards 802.11 Release Frequency Bandwidth Data rate Modulation Protocol (GHz) (MHz) (Mbps) June 1997 DSSS/OFDM Sep. 1999 Sep. 1999 June 2003 DSSS/OFDM Oct. 2009 2.4/5 20/40 7.2~150 Jan. 2014 20/40/80/160 87.6~866.7 Dec. 2012 Up to 6750 Single-carrier/ Frequency 20 MHz 2.4 GHz 2.484 GHz Figure 12.4 802.11 carrier allocation with the ISM band are left unused and act as guard bands. Data modulation may use any of BPSK, QPSK, 16- QAM, 64-QAM or even 256-QAM. Moreover, MIMO techniques have been specified for the 802.11n standard to further increase the possible transmission rate. Wider carrier bandwidth of 40, 80 and 160 MHz (104, 208 and 416 subcarriers) have been specified for the 802. 11n and 802.11a standards, with a corresponding 2, 4 and 8 times increase of transmission rate. IEEE 802.11 standard carrier allocation within the 2.4 GHz ISM band is illustrated in Figure 12.4. As noted above, the ISM band is shared with other ISM equipment, as well as other Wi-Fi devices. Multiple carrier allocation allows a Wi-Fi device to avoid interference from other devices by choosing a subset of carrier frequencies which are least occupied. The available transmission rate is calculated 