become more powerful, with additional antennas, it is technically challenging to equip handsets with multiple antennas because space is required between them to avoid signal interference. See Massive MIMO below for information on antennas suitable for 5th generation (5G) networks' high-frequency bands. www.TechnicalPDF.com The LTE Orthogonal Frequency-Division Multiplexing Air Interface Orthogonal Frequency-Division Multiplexing (OFDM) is the air interface used in LTE mobile broadband protocols. It is not used in earlier protocols and is an important factor in increasing capacity in mobile networks. OFDM increases spectral efficiency by sending several multiplexed streams of data over separate, narrow bands of spectrum simultaneously in orthogonal streams. Orthogonal streams are those that are transmitted at right angles to one another. Figure 7-12 illustrates an example of orthogonal streams in OFDM. Signal A rives first Receiver www.TechnicalPDF.com Figure 7-12 Streams of bits sent by using the Orthogonal Frequency-Division Multiplexing (OFDM) protocol. 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. This narrowband, efficient use of spectrum is the main reason why LTE supports many more users in each cell site than the 3G technologies. Throughout this book, the term "carrier" has been used almost exclusively in relation to network service providers. However, as it applies to telecommunications, the term actually has two meanings. Thus, in addition to describing a network service provider, it can also refer to slices of spectrum as it does in Orthogonal Frequency- Division air interfaces. A Variety of LTE Flavors The evolution of LTE includes technical standards approved and developed by the 3GPP 3rd Generation Partnership Project, a worldwide technical standards group made