Block 1 Part 3 Flashcards
1
Q
Noise and Interference
A
- noise from natural sources tends to be unavoidable
- interference from unwanted transmissions sometimes avoidable
2
Q
Electromagnetic compatibility (EMC)
A
- mitigates causes and effects of interference
- has standards to comply to
- emissions - amount of power device allowed to radiate at different frequencies (when not its intended function) limited
- immunity - device must function normally in presence of radio waves up to certain power at different frequencies
3
Q
Two types of fading commonly distinguished
A
- slow fading
- fast fading
- relates to how quickly signal changes as receiver or transmitter moves around
- fast fading changes happen only with small change in position
- both fading’s due to obstacles and reflections
4
Q
log-normal fading
A
- variation in power in slow fading can be modelled with this
- if variation in power expressed in decibels then it falls within familiar bell curve
- known as normal or Gaussian distribution
5
Q
Distance of fading
A
- fast fading cause large variations in received signal over short distances
- slow fading takes places over distances of metres
6
Q
Rayleigh fading
A
- when no line of sight in fast fading this gives good approximation
7
Q
Rician fading
A
- when there is line of sight in fast fading Rician distribution is used
8
Q
Doppler shift
A
- radio receivers in vehicles can suffer from problem due directly to their speed
- This is shifting of frequency when transmitter and receiver moving relative to each other
- when moving towards each other received signal higher in frequency than transmitted signal
- when moving apart it’s lower
9
Q
Beam steering or Beamforming
A
- technique that uses multiple transmitter antennas
-used when communicating with single receiver
purpose is to improve reception at target device - relative amplitudes and phases of signal from antennas are adjusted so when they arrive they add together constructively
10
Q
Multiple input multiple output (MIMO)
A
- using multiple antennas at receiver and transmitters
- increases paths available
11
Q
Frequency diversity
A
- provision for more than one way for signal to pass through channel in presence of fading
- spread spectrum and OFDM provide this type
- by spreading signal over range of frequencies, likely some signal will get through and reduce fading
12
Q
Spatial diversity
A
- MIMO provides this type of diversity
- multiple paths between transmitters and receivers
- When paths used in combination, effects of fading reduced and S/N ratio improved
13
Q
Spatial multiplexing
A
- MIMO can provide this too
- more than one stream of data can be sent at one time without taking additional spectrum
- increases overall data rate of the channel
14
Q
Multi-user MIMO(MU-MIMO)
A
- extends MIMO further to cover multiple users at different locations
15
Q
Signal-to-noise ratio
A
- signal power divided by noise power
- higher s/n ratio, less the signal is affected by noise, and higher data rate can be obtained
- can be expressed as decibels or simple ratio
16
Q
Sampling theorem
A
- puts lower limit on rate at which samples must be taken if signal is to accurately reconstructed
17
Q
Shannon limit
A
- theoretical upper bound to data rate that can be obtained with given bandwidth and s/n ratio
18
Q
Additive white Gaussian noise (AWGN)
A
- Shannon’s equation applies strictly to this noise
- white noise not concentrated at any frequency, equally spread over spectrum
- additive means different sources of noise added together
- Gaussian relates to statistical distribution of the noise, which follows a bell curve
19
Q
Bit error rate (BER)
A
- number of bits received in error divided by number of bits transmitted in total
20
Q
technical considerations when allocating spectrum
A
- Line-of-sight suitable for some applications but not others
- Using sky wave or surface wave limited to lower frequency bands
- Not possible to accommodate wide bandwidths in low frequency bands
- communications between ground and satellite requires window through atmosphere
- Transmitters and receivers for highest frequencies present engineering challenges
21
Q
International telecommunications union (ITU)
A
- radio spectrum management
- operates at world level
- developed global treaty Radio Regulations
- sets out general terms on how spectrum to be used
- updated at world radio conferences (WRCs) every few years
22
Q
Radio Regulations
A
- frequency allocations either global or apply to one, two or three regions
- Americas, Asia Pacific and Europe and Africa
23
Q
Ofcom
A
- responsible at local level for spectrum
24
Q
UK frequency Allocation Table (UK FAT)
A
- show permitted use of radio spectrum in UK
- lists use in all sections of spectrum, start at 8.3 kHz to 275 GHz
- primary and secondary allocations distinguished in UK FAT
25
Q
Radio regulations two classes of users
A
- Primary users, permitted to operate provided they don’t cause interference to existing primary users
- Secondary users, permitted as long as don’t cause interference to primary or secondary users
- must not cause interference to new primary users
26
Q
Channels
A
- frequency bands often organised into channels
- occupies range of frequencies that will accommodate bandwidth of radio signal
- usually consecutive and equally spaced
27
Q
Spectral mask or spectral envelope
A
- specified for channel and power density must not exceed it at any frequency
28
Q
Cognitive radio
A
- if radio device has sufficient info about other radio usage in area they could transmit on different frequencies
29
Q
How would radio device obtain info about environment
A
- detect what transmission already taking place
- maintain access to database of radio services
in area - device might need to know
- know about its location using GPRS
30
Q
White space
A
- refers to any unused spectrum
