4. Modems and Modulation

Question 1

What are the benefits of modulating a signal?

Answer 1

1. Lower the dimensional requirements of the antennas involved in transmitting the signals
2. To control which bands we carry signals on to allow multiple channels to carry the same signal
3. To select a more optimal frequency for propagation in a particular environment.
4. Increase the bandwidth of the channel

Question 2

Explain the roles of the source encoder, channel encoder and modulator in the transmitter of a communication link

Answer 2

Source encoder - converts input signal to digital bits and removes redundancies using a coding scheme (like MP4).

Channel encoder - applies the error detection and protection coding scheme

Modulator - converts the digital signal into a higher frequency analog signal

Question 3

What is the difference between a baseband and a passband signal?

Answer 3

baseband signals are non-zero within some frequency band.

Passband signals are non-zero outside of this band and 0 within the band, the opposite of a baseband signal

Question 4

What is meant by antipodal and orthogonal signalling?

Answer 4

Antipodal: the vectors used in the modulation scheme are polar opposites therefore 180 degrees phase difference in the polar graph

Orthogonal: Vectors cannot see each other as one is purely real and the other is purely imaginary i.e. 90 degree phase difference

Question 5

What is the baud rate?

Answer 5

It is the number of symbols transmitted per second. Each symbol carries bits of information

Question 6

how is the information bit rate determined from the baud rate?

Answer 6

baud rate . constellation size

Question 7

How is the constellation size determined?

Answer 7

It is log 2 the number of vectors in the modulation scheme. I.e. 16-QAM has a constellation size, k=4. log2(16)

Question 8

In an IQ constellation, what is meant by gray coded

Answer 8

When neighboring symbols only differ by one bit. i.e. 111 could be next 101 and 110 but not 001

Question 9

What are the three parameters considered when choosing a digital modulation scheme?

Answer 9

PAPR: Peak to average power ratio
Minimum phase rotation: Phase distance between neighboring symbols
distance between neighboring symbols