-We previously saw the impact of channel coding on the bit error rate. Let us now have a look at the impact of channel coding rate on the bit error rate. For this purpose, we will consider the following four schemes. We drew four curves that correspond, from right to left, to a scheme without coding. Then, the second with the circles, corresponds to the bit error rate of a system for which we used a channel coding with a 7/8 rate. The third curve corresponds to a system for which we used a channel coding with a 3/4 rate. Finally, a system with a rate of 1/2. Let us first have a look at the rate 1/2. For a bit error rate set to 10 to the power of -4, Eb/N0 equals 8.4, for the scheme without coding. When we add the rate 1/2 channel coding, the corresponding Eb/N0 equals 3.4. So the coding gain equals 5 dB. In the second case, with a rate 3/4 channel coding, for the channel with coding, we still have Eb/N0 = 8.4. With coding, Eb/N0 equals 4.4. In this case, the coding gain now only equals 4 dB. Finally, in the third case, for the 7/8 efficiency, Eb/N0 for the curve without coding still equals 8.4. Finally, the Eb/N0 corresponding to the curve with a rate 7/8 equals 5.8. So the coding gain, here, equals 2.6 dB. This is less than the previous one, already less than the first one. So we may wonder why we use different rates, because if we target a 10 to the power of -4 bit error rate, one can think that we just need to use a rate 1/2 channel coding. Indeed, but do not forget that the occupied band is inversely proportional to the coding efficiency. So it is in our interest to use the highest possible efficiency. That is to say as close to one as possible. One may also wonder why we use different coding schemes within the same system. To answer this question, next week you will see in the "Link budget" sequence, that when the sky is clear, Eb/N0 is strong. Thus to get a 10 to the power of -4 bit error rate we just need to use the rate 7/8 coding. Whereas when it is raining, Eb/N0 is weak. In order to get a 10 to the power of -4 bit error rate, we are forced to choose a rate 1/2 channel coding.