Diversity Flashcards
Explain the meaning of ‘deep fade’
A deep fade can be described as a strong destructive interference. It may case a failure in a communication system due to severe drop in channel SNR (a signal attenuation of more than 1/SNRawgn).
e.g. When a car is stopping at a red light and the FM radio goes into static.
The stopping point of the vehicle is where the signal experienced severe destructive interference.
Explain why diversity can be helpful in slow flat flading
What is diversity and why is it good for reducing fading.
Diversity is the idea of transmitting multiple copies of the same information on statistically independent paths or channels.
Therefore, the probability that all replicas of signals will fade simultaneously is reduced considerably.
Why should an interleaver be used and what is it’s affect on reducing errors?
Many comm. channels are not memoryless. That means most symbols aren’t independent on each other and most errors will come in error bursts.
A bitstream of data without an interleaver will look like something this:
x = (-1,-1,-1,+j,+j,+j,-1,-1,-1)
An error burst will look like something like this:
x = (-1,-1,-1,,,_,-1,-1,-1)
As you can see the symbols representing +j cannot be recovered.
Using an interleaver will look something like
x=(-1,+j,-j,-1,+j,-j,-1,+j,-1)
Here, we can have an error burst, but since we’ve decided to place spread the symbols in different places, we can still recover the signals. We only have to decode one symbol of each (i.e. in this case -1,+j,-j)
Motivate why Maximum Ratio Combiner maximises the SNR for a channel which introduces AWGN and fading.
MRC is the optimum combiner for independent additive white Gaussian noise channels.
MRC is implemented by:
1. signals from each channel are added together
2. gain of each channel is made proportional to the rms signal level and inversely proportional to the mean square noise level in that channel.
3. diff. proportionality constants are used for each channel.
Using the three combination methods you know, list the most optimum receiver to the least
MRC, EGC, SC
Which diversity combination method would you choose if tasked with the selection for a non-coherent communication system?
In a non-coherent detector, the phase can’t be sought out.
Therefore, selection combining should be used.
What’s the difference between a coherent and non-coherent communication system.
In a coherent detector, the receiver knows the signal that the transmitter is sending.
In an incoherent detector, we do not know this.
Name the different types of diversity and how they tackle the attenuation of fading
Time (interleaving)
Frequency (duplication of OFDM symbols)
Space (multiple receiver antennas, multi-path propagation)
Code (orthogonal codes)
Equation for transmission in baseband including diversity
Flashcards in GN
State and explain Diversity-Multiplexing-Tradeoff
The additional degrees of freedom through the generation of diversity can either be used to increase reliability or, alternatively, the bit rate can also be used
be increased. However, both at the same time are not possible.
