anding of all the technologies would be required. Some technologies have been discussed in previous chapters. Here the focus will be on other technologies. 5.5 OFDM It has been stated in the previous section that wideband OFDM is the probable candidate for the radio access technique in 4G systems. The factors that make OFDM much better than its competitions will now be discussed in detail. The two main advantages that make OFDM preferred over other multiple access techniques are: OFDM can sustain a high level of frequency selective fading in mobile communications. This robustness is necessary for high speed data transmission. By combining OFDM with CDMA, the power of combating frequency selective fading increases and there is a high scalability in possible data transmission rates. 5.5.1 What Is OFDM? OFDM is a special form of multicarrier modulation (MCM) scheme, where a single data stream is transmitted over a number of lower data rate subcarriers. OFDM is actually a combination of both modulation and multiplexing. The question of multiplexing is applied to independent signals but these independent signals are themselves a subset of one main signal. In OFDM the signal itself is first split into independent channels, modulated by data and then re-multiplexed to create the OFDM carrier. This can be understood by making an analogy for using a shipment by truck. There are two options: one is to hire one large truck and the other a bunch of four smaller ones. Both methods carry exactly the same amount of data. However, in the case of an accident, only a quarter of the data on the OFDM trucking will suffer. Here the four smaller trucks when seen as signals are called the subcarriers in an OFDM system and they must be orthogonal for this idea to work. The independent subchannels can be multiplexed by using frequency division multiplexing (FDM), called multicarrier transmission, or can be based on code division multiplexing (CDM), called multicode transmission. Need for Orthogonal Signals The main concept in OFDM is the