 evolution to multi-carrier transmission will have a negative impact on wider bandwidth can be designed SO that, for legacy ter- the transmitter power-amplifier efficiency, implying in- minals not capable of multi-carrier reception, each creased transmitter power consumption and increased downlink 'subcarrier' will appear as an original, more power-amplifier cost. Alternatively, the average transmit narrowband carrier, while, for a multi-carrier-capable power must be reduced, implying a reduced range for terminal, the network can make use of the full multi- a given data rate. For this reason, similar to the use of carrier bandwidth to provide higher data rates. higher-order modulation, multi-carrier transmission is The next chapter will discuss, in more detail, more suitable for the downlink (base-station trans- a different approach to multi-carrier transmission, based mission), compared to the uplink (mobile-terminal on so-called OFDM technique. Chapter 4.4 OFDM transmission Erik Dahlman, Stefan Parkvall, Johan Skold and Per Beming In this chapter, a more detailed overview of OFDM or Orthogonal Frequency Division Multiplexing will be Subcarrier (sincertian)2 (f/Af) given. OFDM has been adopted as the downlink spectrum transmission scheme for the 3GPP Long-Term Evolu- tion (LTE) and is also used for several other radio Pulse shape technologies, e.g. WiMAX and the DVB broadcast technologies. Tu=1/Af -4Af-3Af-2Af- Time domain Frequency domain 4.4.1 Basic principles of OFDM Figure 4.4-1 Per-subcarrier pulse shape and spectrum for basic Transmission by means of OFDM can be seen as a kind OFDM transmission. of multi-carrier transmission. The basic characteristics of OFDM transmission, which distinguish it from Figure 4.4-2). The subcarrier spacing is thus equal to a straightforward multi-carrier extension of a more nar- the per-subcarrier modulation rate 1/Tu. rowband transmission scheme as outlined in Figure 4.3-5 An illustrative description of a basic OFDM modulator is in the previous chapter, are: provided in Figur