Communication - Topic 2 Flashcards
How to Choose a Baseband Digital Transmission Scheme?
– Energy efficiency, i.e., the Eb/N0 ratio required to achieve a specific error probability
– Bandwidth efficiency, i.e., the data rate per unit bandwidth
– Implementation cost and complexity
Digital transmission schemes - Schemes without memory
The mapping from a sequence of log2M bits to a symbol depends exclusively on the current sequence of log2M bits;
Digital transmission schemes - Schemes with memory
The mapping from a sequence of log2M bits to a symbol depends both on the current sequence of log2M bits as well as on past bits;
Linear vs Non-linear Schemes?
Linear schemes require that the principle of superposition apply in the mapping of the digital sequence into successive waveforms;
Nonlinear schemes do not require that such superposition applies.
Symbol rate Rs
Rs =1/Ts
Ts is the symbol interval, i.e. the duration of a symbol.
Bit Rate Rb
Tb =1 / Rb
Tb is the bit interval, i.e. the duration of a bit.
How are Symbol & Bit Rate / Duration related?
Average power associated with the digital signal
Average energy per symbol is:
Average Energy per bit:
Average energy associated with a unipolar signaling scheme
Average energy associated with a polar signaling scheme
Average energy per symbol and per bit- for a common m-ary scheme
What is an Eye Diagram?
A measurement over multiple symbol periods
‘Eye’ opening an indicator of signal quality
Average power density spectrum
The (average) power density spectrum corresponds to the Fourier transform of the (average) auto-correlation function:
AWGN Channel Model
The most simple channel model, it is simply noised addition.
Commonly used in deep-space communication.
Most of the course is going to be based on this model.
Band-limited channel model examples.
Twisted Cable.
Coaxial Cable.
Optical fibre (large bandwidth, but still limited)
Band-limited Channel Model Stages
First, the input signal is going to be modeled by an impulse response or by a frequency response.
This is where the bandwidth limitation comes from, if the channel doesn’t respond or attenuates certain frequencies there must be a fundamental bandwidth limitation.
This could distort the input signal.
In addition to that distortion by the channel itself, there is also noise that adds to the signal.
What is a channel?
To send information from A to B we need a channel.
Depending on the media, we have different channel models.
Channel that only adds noise.
Channel that on top of noise can distort signal because of bandwidth limitations
There is also a wireless channel.
For Wireless Channels why does Dispersion In Frequency occur?
It happens because of the Doppler Effect.
If you multiply two signals in the time domain, what will happen in the frequency domain?
Vice-versa?
Convolution in the frequency domain.
If there is convolution in the time domain the signals multiply in the frequency domain.
What does a binary transmission scheme mean?
This is when we send 1 bit per each signal interval.
What does a M-ary transmission scheme mean?
M-ary is when we send more than 1 bit per signal interval.
What does Xk refer to?
Refers to individual symbols one by one.
For example, in a binary polar scheme, X1 might refer to 1, X2 might be -1, X3 might be -1, etc.
Why does a Polar Binary Transmission scheme convery more energy per bit than in a Unipolar Binary Transmission?
Because polar has two amplitude levels, a positive and a negative amplitude level to encode 1 and 0.
On the other hand, unipolar has no amplitude level for 0, a null amplitude.
So unipolar uses have the energy per bit.
What does an envelope detector do?
An envelope detector measures the amplitude of a signal.
Ak = |xk|2
Noise is generally assumed to be…
Gaussian
Following a Guassian distribution.
What is the auto-correlation function?
Reveals how the correlation between any two values of the signal changes as their separation changes.
It is used to estimate periodicity in a time series.
What does p(t) represent?
It is assumed that p(t) is a (unit-energy) baseband pulse with duration Ts
Advantage and disadvantage of chosing Binary over M-ary
Binary has better power efficiency
However, it has worse bandwidth efficiency.
