Digital Communications 2

3Digital Transmissions with Carrier Modulation: Problems 27 to 33

3.1. Problem 27 – Digital transmissions with carrier modulation

We consider the general system of transmission of digital information with modulation of a carrier represented in the block diagram of Figure 3.1.

The symbols b_n delivered by the binary source are emitted every T_b seconds. The baseband encoder of the transmission system generates two baseband signals I (t) and Q (t) as follows:

• – separation of the binary sequence {b_n} into two binary sequences {b_2n} and {b_2n+1};
• – transcoding of the sequences {b_2n} and {b_2n+1} into two sequences of symbols {a_n} and ∈ M₁ – ary and M₂– ary respectively;
• – pulse amplitude modulation using the basic pulse x (t):

The two baseband signals I (t) and Q (t) are expressed by:

and indicate that symbols {a_n} and are emitted every T seconds.

Figure 3.1. General block diagram of a digital transmission system with carrier modulation and demodulation (Transmitter-Channel-Receiver) ...