ried on separate frequency. A 2x2 antenna has two antennas and handsets communicating with it also have two antennas. This is analogous to multilane highways with two lanes in each direction. The antennas currently on the market are the 2x2 models; 4x4 models are expected to be available next. An 8x8 antenna is specified for LTE-Advanced, which is Release 10, the 100Mbps, true-4G air interface. It's technically challenging to equip hand- sets with multiple antennas because space is required between them to avoid signal interference. The LTE and WiMAX Orthogonal Frequency- Division Multiplexing Air Interface Orthogonal Frequency-Division Multiplexing (OFDM) is the air interface used in LTE and WiMAX mobile broadband protocols. It is not used in HSPA+. OFDM increases spectral efficiency by sending several multiplexed streams of data over separate, Mobile and Wi-Fi Networks Signal A arrives first Signal B arrives second Multiple streams of data at slightly different frequencies arrive at their destination at different times Receiver decodes signal A and then signal B Figure 7-4 Streams of bits sent by using the Orthogonal Frequency- Division Multiplexing (OFDM) protocol. narrow bands of spectrum simultaneously in orthogonal streams. Orthogonal streams are those that are transmitted at right angles to one another. Figure 7-4 illustrates an example of orthogonal streams in OFDM. In addition, guard bands between each stream of data are not required, which is an important factor underpinning the spectral efficiency of OFDM. Guard bands are unused channels of spectrum that provide a buffer between streams to protect data from interference. Guard bands carry no data. For information on guard bands, see the section "Unlicensed Spectrum for 'Super' Wi-Fi," earlier in this chapter. This narrowband, efficient use of spectrum is the main reason why LTE and WiMAX support many more users in each cell site than the 3G technologies WCDMA and CDMA2000. LTE supports 200 users per cell site, whereas CDMA2000 supports only 50. The Inc