we finally have only one component in the spectrum, the car- rier frequency. Bandwidth is one of the main restricting factors for transmission. The goal of the two preceding examples was to help us understand the connection between the data rate and the required bandwidth. By understanding this we can understand, for example, why efficient speech-coding schemes are required in cellular systems. By reducing the data rate we increase the Transmission network capacity in terms of maximum number of simultaneous calls via a radio-frequency band available for the system. 4.2 Radio Transmission In radio transmission we have to transfer the spectrum of the message into the radio-frequency band for transmission. For this we use continuous or car- rier wave (CW) modulation. 4.2.1 CW Modulation Methods The primary purpose of CW modulation in a communication system is to generate a modulated signal suited to the characteristics of a transmission channel. Modulation is needed in the transmission systems to transfer the message spectrum into high radio frequencies that propagate over radio channels. CW modulation is also used in voice-band modems where digital data modulate the carrier frequencies inside the voice frequency band. In CW modulation the message alters the amplitude, frequency, or phase of the high-frequency carrier (Figure 4.3). This alteration is detected in the demodulator of the receiver and the original message is reproduced. We saw in Section 3.3 that a cosine wave such as a carrier is defined by three characteristics: amplitude, frequency, and phase. In the CW modula- tion that we use in radio systems, we insert the message into the carrier wave by altering these three factors of the carrier wave according to the message to be transmitted. 4.2.2 The original carrier wave has a constant peak value (amplitude) and it has a much higher frequency than the modulating signal, the message. In AM the Transmitter Receiver Detected Message Transmission channel message Modulator Demodulator Modulated carrier wave Carr