Weeks 5-8

Question 1

Whats the difference between analogue and digital signals?

Answer 1

Analogue: Continuous in time and amplitude

Digital: Discrete in time and amplitude

Question 2

What are the two conditions for error-free reconstruction of of a signal from its sample.

Answer 2

1. Signal must be bandwidth limited (B<∞)
2. sampling frequency must be sufficiently high (fs≥fnyquist) (fnyquist=2B)(no overlaps on graph)

Question 3

Whats is the Shannon Nyquist sampling theorem?

Answer 3

A signal band limited to 𝐵 Hz can be reconstructed exactly from its samples as long as the sampling frequency is greater than twice the signal bandwidth, 𝑓𝑠 ≥ 2𝐵. 2𝐵 is referred as the Nyquist rate.

Question 4

What is pulse amplitude modulation (PAM)

Answer 4

varies the amplitude of a constant- width, constant-position pulse according to the amplitude of the sample of the analogue signal

Question 5

What is pulse position modulation (PPM)

Answer 5

varies the position of a constant- width pulse within a prescribed time slot is varied according to the
amplitude of the sample of the analogue signal

Question 6

What is pulse width modulation (PWM)

Answer 6

varies the width of a constant amplitude pulse is varied
proportional to the amplitude of the analogue signal at the time the signal is sampled

Question 7

What is Quantisation?

Answer 7

Maps samples of a continuous amplitude waveform
to a finite set of amplitudes

Quantisation noise is introduced by the difference between the real signal value and its approximated value

Question 8

What is line-coding and what re the two main types?

Answer 8

Converts the bit stream produced by a source encoder to electrical pulses, for the purpose of transmission over channel

1. On-Off: 1 is transmitted as a positive pulse and 0 by no pulse.
2. Polar: is transmitted by a positive pulse and 0 by a negative pulse.

Question 9

What is pulse code modulation (PCM)?

Answer 9

Transmits a message using a sequence of coded pulses
representing the sampled, quantised and encoded signal

At the transmitter:
Low-pass filtering is used prior to sampling to prevent aliasing (anti-aliasing filtering).
 The message signal is sampled at a rate of at least twice its bandwidth (the Nyquist
rate).
 Quantisation assigns the sample values to one of L discrete levels.
 Encoding represents each discrete level in binary form, where each level is encoded
with the same number of N=log2L binary digits (bits).
 The digital output of the encoder are converted into electrical pulses by line coding

Question 10

Whats important about the transmission bandwidth of PCM?

Answer 10

To transmit a message signal with bandwidth of 𝐵 Hz, the required data rate is at least 𝟐𝒏𝑩 bits/s.

A unit channel bandwidth (1 Hz) can transmit a
maximum of two pieces of information per second, which is 2bits/

Question 11

Whats the difference between binary amplitude shift keying (BASK), binary phase shift keying (BPSK) and binary frequency shift keying (BFSK)

Answer 11

BASK: Similar to On-Off shift keying, carrier signal is a high frequency sinusoid, Modulating signal is a binary digital signal the one and zero
states are switched with bit duration period 𝑇

BPSK: Similar to Polar shift keying

BFSK: transmits zero by a pulse of frequency 𝑓଴, while one by a pulse of frequency 𝑓. BFSK can be viewed as two ASK signals with different carrier frequencies

Question 12

Whats is differential phase shift keying (DPSK)?

Answer 12

Transmits data encoded as the phase change between consecutive symbols;

Question 13

Whats the differences between ASK, PSK AND FSK

Answer 13

ASK:
– Simple; related to AM.
– Low bandwidth.
– Susceptible to interference

PSK:
– Constant envelope; related to PM.
– More complex.
– Robust against interference

FSK:
– Constant envelope; related to FM.
– Simple to generate and detect.
– Insensitive to amplitude fluctuations on the
channel.
– Larger bandwidth.

Question 14

What are three ways that you can increase the signal to noise ratio of a signal?

Answer 14

1. Increase transmitter power
2. Using high gain antennas
3. Cooling Electronics

(bit error rate is lower for high SNR or larger channel bandwidth)

Question 15

Whats the definition for channel capacity?

Answer 15

maximum amount of information that can be reliably transmitted over a communication channel per second

Question 16

What is multiplexing and where do multiplexors and demultiplexers do?

Answer 16

Multiplexing: Simultaneous transmission of multiple signals

Multiplexor: Two or more simultaneous signals on a single communication channel.

Demultiplexer: Recovers the original signals

Question 17

Whats are the two different types of multiplexor?

Answer 17

Frequency Division: Each signal is allocated a frequency range

Time division: Each signal is allocated a periodic time slot

Question 18

What is frequency division multiplexing (FDM) and what are the steps to do it?

Answer 18

FDM = several signals share the band of the channel

1. Each signal is modulated by a different carrier frequency (these are known as subcarriers and may use a different modulation method)
2. Composite signal is obtained by adding all the modulated signal
3. (optional) The composite signal may be considered as a baseband signal to further modulate a radio-frequency carrier for the purpose of transmission.

• NOTE: Bandwidth of the medium must be greater than the total bandwidth of the three signals.

Question 19

What is Time division multiplexing (TDM)

Answer 19

• Type of digital multiplexing
• Multiple low-bit-rate signals can be multiplexed to form one high-bit-rate signal.

– The medium is time-shared by various incoming signals.

– In TDM, bit rate of the medium should be greater than the total bit rate of alltransmitted signals

Question 20

What is the spread spectrum and what are the problems with it?

Answer 20

• Aims to minimise the amount of bandwidth consumed by the modulated signal during transmission.
• Narrow digital communication so easy target for detection from unintended users
• More susceptible to jamming

Question 21

How does a Radar work?

Answer 21

Produces pulses of EM-waves – the pulse is transmitted and then the radar switches to receive mode

Question 22

Explain the three different frequency stages that a radar uses?

Answer 22

1. Radio frequency for transmission/reception.
2. An intermediate frequency (often 30-60 MHz)
   is used because it provides a common frequency stage and makes the signal handling easier for wide-band signals, e.g., better filters and amplifiers are available.
3. The signal is lowered to the baseband
   frequenciesthe low-frequency baseband
   signal is dependent on the pulse properties
   and the target.

Question 23

Whats the definition for modulation and demodulation in terms of radar?

Answer 23

• Modulation (i.e. amplitude modulation) is the process of taking the pulse waveform (baseband) and mixing it with a local oscillator to raise its frequency to RF
• Demodulation is the reverse process, also achieved by multiplying the received signal by the local RF oscillator which (in radar) is normally donein two stages:
  – RF → IF
  – IF → baseband/video

Question 24

How does a matched filter work?

Answer 24

Matched filter is a special Low-pass filter (LPF) that correlates the received signal with the transmitted pulse shape to maximise the chances of detecting echoes in noise

Question 25

What is reciprocal spreading?

Answer 25

In general, if a signal/pulse has a duration T, the baseband bandwidth of the signal will be approximately fbbw=1/2T . This inverse relationship is referred to as
reciprocal spreading.

Question 26

What is pulse compression?

Answer 26

Generates a longer pulse that happens to have a large
bandwidth – this gives the effect of a short pulse but requires smaller peak power levels.

Question 27

What is pulse integration?

Answer 27

Integrate return of several pulses to obtain a larger signal. The integration must be coherent and will not be perfect is used to improve performance further.

Question 28

Explain how parabolic antennas work?

Answer 28

The feed antenna generates the signal, which is at or near the focal point of the parabolic dish, and the parabolic dish forms this into a radar beam with far superior directionality, although it is still limited by diffraction effects and gives a beam width that is limited by the wavelength and the size of the dish.
– Moving the dish allows the radar to sweep out an area of interest, although great care has to be taken
to ensure that the ‘side-lobes’ of the antenna are not generating spurious results…

Question 29

What is the radar pattern of an antenna and define reciprocity and radar beam width?

Answer 29

Radar Pattern: strength of the electro-magnetic waves from the antenna as a function of the angle to the antenna boresight

Reciprocity: Radiation pattern in transmit mode is the same as the sensitivity in receive mode

Radar Beamwidth: point at which the radiation pattern (antenna gain) drops to 3 dB below the maximum

Question 30

What is polarisation?

Answer 30

• Radar transmits one polarisation and receives both, the polarised wave that is transmitted is
  reflected differently from different surfaces.

– The ratio of the received power in each polarisation carries some information about the surface that it
was reflected/scattered off – which can be useful to identify objects.

Question 31

Why is radar cross section important?

Answer 31

• If there is a target at a distance R, some of the radar power falling on the target will be
  reflected back toward the transmitter.

– The amount of power re-radiated toward the transmitter will depend on the size and the
nature of the target (its cross section)

Question 32

What two things does the level of receive noise on a radar depend on?

Answer 32

1. Temperature of electronics (higher temp = higher noise)
2. Bandwidth (higher B= Higher noise)

Question 33

Whats the definition for range accuracy and range resolution

Answer 33

Range Accuracy: the ability to identify the peak of the returned pulse – the range accuracy is the error in the (absolute) range measurement that the radar produces.

Range Resolution: refers to the ability to resolve the returns from two different objects separated by a given distance.

Question 34

What Is a doppler radar and why are they useful?

Answer 34

• Uses long pulses with narrow band signals
• They can find doppler shift associated with the received signal (dependant on radial velocity)

Question 35

What is a secondary surveillance radar, How does it work and what are its advantages/disadvantages?

Answer 35

• A radar signal from a ground antenna (normally 1030 MHz) is received by the aircraft antenna at one frequency and then rebroadcast at a slightly different frequency (1090 MHz)
• An aircraft system (transponder) receives the radar signal and interprets the pulse sequence as a request for information. It then rebroadcasts the required information (usually an aircraft identifier and altitude) in all directions – the directionality of the ground
  radar allows the aircraft bearing to be determined and the time delay (including the delay due to the transponder processing) gives the range.
• Advantages: Signal power can be reduced as because signals go one way instead of two ways.

Disadvantages: Requires a transponder

Question 36

Where can errors propagate from a secondary surveillance radar?

Answer 36

1. Differences in delays in the transponder give range errors.
2. If an aircraft is in range of two ground interrogators then confusion can arise when the transponder
   replies to one ground station but the reply is received by the other – this is called ‘fruiting’ (FalseReplies Unsynchronised to Interrogator Transmissions).
3. If two aircraft are too close together, then they may produce overlapping replies – called ‘garbling’