L8

Question 1

What is achieved by modulating signals? What are the three modulating mechanisms?

Answer 1

Higher bandwidth

Amplitude modulation
Frequency modulation
Phase modulation

Question 2

Explain shift keying with two examples.

Answer 2

PSK (Phase Shift Keying): the phase of a constant amplitude carrier is modified to represent different binary symbols
o Binary PSL: two phase values encode 1 and 0 logic values

FSK (Frequency Shift Keying): the phase of a constant amplitude carrier signal is modified to represent different binary symbols
o Binary FSK: the frequency changes between two values corresponding to binary 1 and 0

Question 3

Explain Spread Spectrum (SS) modulation techniques with examples.

Answer 3

• Spread Spectrum (SS) modulation techniques: utilise pseudo-noise sequences produced by appropriate circuits

o Pseudo-noise (PN) sequences: binary sequences that have an autocorrelation that eventually resembles the autocorrelation of random binary sequence

o Direct Sequence Spread Spectrum (DS-SS): spread the baseband data by directly multiplying the baseband data pulse with a PN sequence
 If an interference is detected, the interference is spread

o Frequency Hopped Spread Spectrum (FHSS): periodic change of transmission frequency

Question 4

What is channel coding?

Answer 4

Channel Coding: introduce redundant bits in the transmitted data to reduce errors through error detection codes and error correction codes

Question 5

Match the following categories of channel coding:
Block codes
Convolutional codes
Turbo codes

A. Detect specific number of errors and correct them.
B. Combines the capabilities of convolutional codes with channel estimation theory.
C. Map a continuous sequence of information bits into a continuous sequence of encoder output bits.

Answer 5

1 A
2 C
3 B

Question 6

Why are multiple access schemes important? What are two examples?

Answer 6

Multiple Access Schemes allow many users to share simultaneously a finite amount of radio spectrum

Frequency division duplexing (FDD): assign separate frequencies for transmitting and receiving

Time division duplexing (TDD): assign separate time slots for transmitting and receiving

Question 7

For the 4 channels: frequency, time, space, and code, explain the multiple access methods (excluding space).

Answer 7

Frequency division multiple access (FDMA): assigns multiple individual channels to individual users
* Ex: voice calls: each channel carries one phone circuit at a time

Time division multiple access (TDMA): splits the spectrum into time slots that are periodically repeate
* Ex: voice calls: users share a single frequency and each user uses a separate time slots
* Allows different bandwidths to be assigned to different users based on priority structures

Code division multiple access (CDMA): all users use the same frequency and may transmit at the same time
* Each user has a specific pseudorandom codeword which is almost orthogonal to all other code words
* Ex: voice calls: many users share the same frequency and the same time. Increasing the number of users increases the noise floor
o Near-far problem: when one user has higher power than the other, power control is enforced