This chapter considered the architectures of transmitters and receivers and in particular the implementation of modulators and demodulators. Since the beginning of radio there has been a tremendous increase in the sophistication of modulators and demodulators. The earliest AM demodulators required just a single diode and now the demodulators in software defined radio use hundreds of millions of transistors. Following early radios the discussion tracked the development of modulators and demodulators for cellular systems but the increasingly advanced techniques are used in other radio and radar systems. The 1G systems used FM modulation to transmit voice and simple FSK modulation to transmit limited digital data such as phone such as channel information. The 2G and 3G systems were confined to using just one or two types of modulation such four-state modulation. The level of complexity increased tremendously with 4G where now it it was essential to use software defined radio to support a very large number of different modulation formats including mostly various orders of QAM but also supporting legacy modulation formats. The 5G modulation and demodulation schemes are the same as those used in 4G with the 5G advances being in the move to millimeter-waves and beam steering. There is another layer of modulation used in 3G (i.e. wideband CDMA) which can layer multiple users in the same bandwidth. Strictly this is an access scheme and will be considered in Chapter 5. The 4G systems uses orthogonal frequency division modulation (OFDM) modulation. That is not considered here as it is a hybrid of an access scheme and a modulation scheme. It too will be considered in Chapter 5.