-This sequence is dedicated to the modulation and demodulation operations. We will first remind you the communication chain that you saw this with Nathalie during week 3. This communication chain is made up of three elements, a transmitter, a communication channel and a receiver. In this sequence, we will not address the channel encoding and decoding operations that you will see with Tarik in the next sequence. We will address the modulation and demodulation operations. We will remind you a few features of the transmission channel. The modulation operation consists in transforming the binary signal made of 0s and 1s into a continuous analog signal, suited to the communication channel. The communication channel is the physical environment in which the signal will travel. In the case of satellite communication, it is free space. This channel will modify the transmitted signal. In the case of satellite communications, these modifications are due to thermal noise, that you will see in week 5 during the "Link budget" sequence, and to distortions generated by power amplifiers that you will also see during week 5. Demodulation consists in estimating the transmitted bits from the received signal, noisy and distorted, at the output of the transmission channel. Let me remind you the two key parameters seen in week 3, the bit rate to transmit and the bit error rate, or BER, to achieve. We will define two criteria to select a modulation. The first criterion is the bandwidth occupied by the signal for the bit rate to transmit. We will try to minimize this bandwidth since spectral resources are rare and expensive. The second criterion is the received power required for a given bit rate. We will also try to minimize this power in order to minimize the consumption and cost of the receiver and transmitter. Satellite communications are carrier frequency communications. This means that the central frequency of the signal is not null. Let us remind you a few bands used in satellite communication systems as seen in week 3, the L, C, Ku and Ka bands.