An Introduction to LTE: LTE, LTE-Advanced, SAE, VoLTE and 4G Mobile Communications, 2nd Edition

Chapter 4Orthogonal Frequency Division Multiple Access

The technique used for radio transmission and reception in LTE is known as orthogonal frequency division multiple access (OFDMA). OFDMA carries out the same functions as any other multiple access technique, by allowing the base station to communicate with several different mobiles at the same time. However, it is also a powerful way to improve the system's spectral efficiency and to minimize the problems of fading and inter-symbol interference that we introduced in Chapter 3. In this chapter, we will describe the basic principles of OFDMA and show the benefits that arise when it is used in a mobile cellular network. We will also cover a modified radio transmission technique, known as single carrier frequency division multiple access (SC-FDMA), which is used for the LTE uplink.

OFDMA is also used by several other radio communication systems, such as wireless local area networks (IEEE 802.11 versions a, g and n) and WiMAX (IEEE 802.16), as well as in digital television and radio broadcasting. However, LTE is the first system to have made use of SC-FDMA.

4.1 Principles of OFDMA

4.1.1 Sub-carriers

In Chapter 3, we saw how a traditional communication system transmits data by modulating a carrier signal. LTE uses a modified version of this technique known as orthogonal frequency division multiplexing (OFDM). As shown in Figure 4.1, an OFDM transmitter takes a block of symbols from the outgoing information stream and transmits ...