OFDM Baseband Receiver Design for Wireless Communications

2

Digital Modulation

Digital modulation is the process of representing binary information using segments of different sinusoidal waveforms. The parameters that can be adjusted in a sinusoidal wave are its amplitude, frequency and phase.

2.1 Single-Carrier Modulation

Single-carrier modulation techniques use only one sinusoidal wave at all times, while in the multi-carrier modulation techniques, several sinusoidal waves are transmitted simultaneously. Basic single-carrier modulation techniques modify only one of the three parameters–amplitude, frequency and phase–of the sinusoidal wave according to the binary information to be transmitted. These techniques are called amplitude shift keying (ASK), frequency shift keying (FSK) and phase shift keying (PSK), respectively, according to the parameter of the sinusoidal wave that is modified. The basic time unit in digital modulation techniques is a symbol, which is composed of a segment of the sinusoidal waveform. If there are only two possible different symbols in a digital modulation, then it is called a binary modulation. Figure 2.1 depicts sample waveforms of binary ASK, binary FSK and binary PSK.

Note that ASK and PSK are linear modulation, whose symbol waveforms are sinusoidal waveforms of the same frequency. A more compact and clear representation of all possible symbols in such modulation is drawn by the phasor representations of all the possible symbols on a phasor plane. This representation is called the signal constellation ...