mpared with an isotropic antenna. The received power and overall radio link loss when antenna gains are considered becomes (Figure 4.12) (4.6) In decibel format received power levels and link loss become (4.7) Note that both antennas have equal impact on the received power level and use of a directional receiving antenna at, for example, the base station site of a cellular network reduces link loss and required transmission power of the mobile station. Introduction to Telecommunications Network Engineering Figure 4.12 Attenuation of the radio wave. In this section we have reviewed radio transmission at different fre- quencies and modulation methods that are used to transfer a message to the radio-frequency band for transmission. We have also examined the propaga- tion loss of radio waves. Many other things must considered in radio system engineering but they are beyond the scope of our brief introduction to radio transmission. In the following section we look at the general characteristics of trans- mission channels and how the maximum transmission data rate depends on the bandwidth and noise of the channel. Maximum Data Rate of a Transmission Channel A fundamental limit exists for the data rate through any transmission chan- nels, as we will see later in this section. The main restricting factors are the bandwidth and the noise of the channel. 4.3.1 Symbol Rate (Baud Rate) and Bandwidth Communication requires a sufficient transmission bandwidth to accommo- date the signal spectrum; otherwise, severe distortion will result. For exam- ple, a bandwidth of several megahertz is needed for an analog television video signal, whereas the much slower variations of a telephone speech signal fit into a 4-kHz frequency band. Every communication channel has a finite bandwidth. The higher the data rate to be transmitted, the shorter the digital pulses that can be used, as we saw in Section 4.1. The shorter the pulses used for transmission, the wider the bandwidth required, as we saw in Figure 4.2. When a signal changes rapidl