The way gain is implemented in a receiver usually requires a tradeoff between noise, distortion, and cost. The RTI noise contribution of any given component is the component's noise divided by the total signal gain between the input and the component. Thus, the earlier the gain occurs in the signal path, the fewer stages there are to contribute significant amounts of noise. Unfortunately, there are obstacles to taking large amounts of gain immediately. The first is distortion.
Cost is another limiting factor affecting where gain can occur in the signal chain.
Thus, to optimize the location of gain in the signal path, one must simultaneously trade off the constraints of noise, distortion, power dissipation, and cost. Selectivity indicates a receiver's ability to extract or select the desired signal in the presence of unwanted interferers, many of which may be stronger than the desired signals. For FDMA signals, selectivity is achieved through filtering with discrimination filters that block unwanted signals and pass the desired signal. Like gain, filtering is generally easier at lower frequencies. As noted above, filtering will be required early in the signal path to attenuate the strong interferers.