s, as described above. Instead of using 2 or 4 points, QAM defines a constellation of 16, 64 or more points, each with a particular phase and amplitude, and each representing a 4- or 6-bit (or chip) data symbol. 16-QAM and 64-QAM modulation techniques are used in the IEEE 802.11a and g specification, together with OFDM, to achieve data rates of 24 to 54 Mbps. Figure 4-25 shows the 16-QAM constellation - the 16 points on the IQ plane - used to achieve data rates of 24 Mbps and 36 Mbps. The points in the 16-QAM constellation can be alternatively numbered according to a Gray coding in which adjacent points differ only in the switching of one bit, as shown in Figure 4-26. Using this numbering reduces the chance of two-bit errors in the receiver - if a point is erroneously detected as a neighbouring point only one bit will be incorrect. This makes it easier to recover the bit error using error correction techniques. The next step, a 256-QAM modulation scheme, would further improve achievable data rate with no increase in the occupied bandwidth, but Radio Communication Basics Figure 4-25: 16-QAM Constellation Figure 4-26: Gray Coded 16-QAM Constellation Chapter Four generating and processing 256-QAM modulated signals is currently a significant challenge for hardware performance and cost. Dual Carrier Modulation Dual carrier modulation is a technique applied in multi-carrier systems such as OFDM to combat the loss of data due to the destructive interference, or fading, of individual carrier signals in a multi-path environment (see the Section "Multipath Fading, p. 114"). By modulating data onto two carrier frequencies rather than one, the transmission can be made more robust, although at the cost of using additional bandwidth. In multi-band OFDM (see the Section "Multiband UWB, p. 122") a 4-bit symbol is mapped onto two different 16-QAM constellations and the resulting symbols are transmitted on two OFDM carrier tones separated by at least 200 MHz. If reception of one of the tones is affected by fading, the data can be 