This course will provide an introduction and history of cellular communication systems that have changed our lives during the recent four decades and will become an essential and inseparable part of human life. The principles of wireless communication theory are covered with emphasis on the essential concept delivery to non-major learners in the easiest way. Then, it will be covered how such principles are realized and how multimedia services can be delivered in practical LTE cellular systems by which learners are connected and enjoys together in their lives. After completing this course, learners will be able to understand 1. What a cellular system is and how it has been developed so far 2. The very basic principles how information can be delivered efficiently using radio 3. How such principles are realized in LTE systems. 4. How people can be connected and multimedia services can be delivered in LTE systems
4.3
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Del curso dictado por Yonsei University
Wireless Communications for Everybody
180 calificaciones
De la lección
Multiple Antenna Technologies
In this part of the course, we will learn how multiple antennas can be efficiently used in different strategies for communication system enhancement. First, we look in to the basics of antenna, and then learn about the three main gains that are achievable by multiple antennas: array, diversity, and spatial multiplexing. Lastly, we briefly look in to the concepts of single-user MIMO and multi-user MIMO.
Conoce a los instructores
Kwang-Soon KimProfessor
School of Electrical & Electronic Engineering
Seong-Lyun KimProfessor
School of Electrical & Electronic Engineering
Chan-Byoung ChaeProfessor
School of Integrated Techonology
Dae-Sik HongProfessor
School of Electrical & Electronic Engineering
Song-Kuk KimAssociate Professor
School of Integrated Technology
Welcome back to my lecture.
This is the third part.
I want to explain the diversity content.
The question now is, how can you improve a link reliability,
especially in the highest innovation?
Before, among the main three multiple-antenna gains,
we have covered the array gain.
And let's move on to the diversity gain.
The diversity gain is different from the array gain.
The diversity gain benefits from independent fading paths.
It could be a space, time, and/or frequency domains.
We reduce durability and eventually minimize error rates,
this gain can be achieved at both the transmitter and
the receiver side unlike an array system.
Arrays antenna system we cover in the previous part.
It is required to have uncorrelated channels for
diversity techniques since it relies on
statistically independent channel environment.
Thus, with the uncorrelated channel from now on.
In urban and indoor environments, there is no clear line
of sight between the transmitter and the receiver.
If [INAUDIBLE] the signal is refracted along multiple
passes before finally being received.
In addition to these macro level study obstacles,
we may also have more reflections and
doppler effect generated by the moving neighbors.
They see the change in frequency of a wave for
a moving relative to its source.
These macro and micro scatterers introduced three ships time the length,
attenuations and distorsion that can destruct heavily in
the fear with one another at a percher of the receiving antenna.
Here, we have plot that this cart the China fluctuations for really fading.
We could observe that there is a huge difference the maximum and
the minimum values of the amplitude.
It is approximately.
We also have similar percher in frequency and spacial domain.
The multipath angular spread is simply defined
by a ratio of the diameter of the here it's D and
the distance between the transmitter and the receiver L.
If there is no optical near the receiver,
angular spread becomes asymptotic laser.
Also, the spatial correlation is defined like this,
as simply the wider the agitation and entire distance.
The lower the space are correlation and which are default several times.
As the market pass angular spread increases,
this pressure correlation decreases which is what we desire
to achieve the diversity key to enjoy the independence of the channel.
Why is this independent passer important?
The antenna diversity is especially effective at mitigating
the multipath situation.
This is because the multiple antennas offer a receiver
several activations of the same signal.
Each antenna will experience a different one in a environment.
By simply summing up the two received signals here
the channel is not likely to fluctuate greater in amount.
It just looks like antenna.
Again, if one antenna is experiencing a deflate,
it is likely that another has sufficient signal,
reactively system can provide a link.
Well, this is [INAUDIBLE] in the receiving system, or receiver diversity,
diversity reception.
The analog has always proven valuable for
transmitting system and transmit diversity as well.
Transmit diversity is quite similar to the receiver diversity, except one thing.
No channel state information is available at the node.
Suppose we have three alphabets to transmit.
The single data string goes through the space-time and it repeats.
The guarantee its symbol suffers from two independent antennas.
You might think, it's not an efficient system because it adds some redundancy.
But a sense to days, we can improve the quality.
We might ask that or we'll write in wrong a technically,
we'd use to error rate then what is the difference between array gains,
a cheap buy the informing light solutions and special diversity?
The big difference is antenna structure.
Yes, as you correctly remember,
the perfectly correlated channel was chief rating.
And on correlated the diversity came.
So, the distance between adjacent antenna is different.
See half lambda, and oval, ten-lambda distances.
It gives quite good intuition for the array gain part,
the input signal is experiencing the same channel, and
the output signal is the same as the input signal.
But the received power increases.
Due to the increased power we have linear shift in
error probabilities for [INAUDIBLE] fading channel.
On the other side, the antenna diversity utilizes two independent channels.
And output signal becomes flat and it shows bit-error characteristics.
As the number of antenna increases, the slope of the pattern become increases.
From these curves, we could learn that the array gain
is quite useful for new SNR scenarios, like.
And the diversity gain is quite
powerful for high SNR.
So far, we have focused on the concept of the diversity system here.
Let me introduce some of the representative diversity techniques.
For simplicity, let's assume one Tx and
half and multiple Rxs antennas.
This kind of configuration has been used for over 50 years.
For this setup, we are not able to increase the data rate,
since we have a single mouth here at the transmitter but
we could achieve diversity gain.
The simplest way of enjoying the multiple antennas is antenna selection.
Here, as we played a lot before, two antennas or
separated with sufficient spacing.
That idea is quite simple,
we are quite familiar with the single antenna system and
we just select one antenna that has the highest antenna gain,
if you look at the channel variation of the first antenna,
we have red part and the green parts.
Red means bad channel, and green means good channel.
Thus, we select the second antenna for a certain period and
switch the first antenna, okay?
Switch to the second antenna and so on based on the channel condition.
In doing so, we have a new size assistant with quite good channel conditions.
However, as we all could guess the antenna selection is not the optimum solution.
We could do better if we use the channel coefficients at the receiver.
This is the optimal solution called the maximum ratio combining, MRC.
In this lecture, I'm not going to go to introduce all of the details,
but conceptually, this is the MRC.
The signal from each antenna is rotated and weighted according to the face and
strength of the channel such that the signals from all antennas are combined.
The maximum ratio between signal and noise terms.
More specifically, the received signal can be reach in.
Here, we have two unknown coefficients.
We next derived the SNRX pressure for the system and
optimize define two coefficients.
Then we have the a, b or
complex conjugates of the channel coefficients.
M or c is for simulcases, mesh memories are combing for simul cases.
It's not really straightforward how to achieve the diversity gain for
my two cases, multiple antennas at the transmitter side.
It's been an open question until Alamouti published his landmark paper in 1998.
Let me ask you a quick question,
wouldn't the Tx diversity be the same as the Rx diversity?
Do we just need to apply the same concept for Tx diversity?
And then the Tx diversity is the same as the Rx diversity,
if the signal from two different antennas are separated.
We could not rely only on the space domain.
We also need to exploit time and spacial domains to separate the signal.
Alamouti, he invented the simplest of all
the antenna Tx diversity in 1998.
It was designed for two a transing antenna system and
has the following putting matrix.
It's readily apparent that this is grade one code.
It takes two times slots to transmit two
symbols using the optimal decoding scheme here,
the beat that our rates of this Tx diversity is
equivalent to n Rx branch maximum ration combining.
That's, we already learned, deeper rights.
This is a picture of the perfect between the symbols after it's been processing.
There are two copies of its symbol transmitting.
Since they also want a this on a transmit signal.
We could easily decouple the signal by
multiplying a of the with H.
This means that the space diversity is designed such that
the vectors will project in pairs of columns taken from
the coding matrixes with original of this is simple,
linear, and optimal decoding at the receiver.
Here, we compare the bit error rate performances of MISO,
Alamouti, SIMO, MRC, and SISO systems.
It can be after that MRC and Alamouti have the same slope,
ensure a better performance or precise assistance.
Especially in the high senr reason.
All right, we have mentioned the third part.
We have learned the concept of space-diversity and
the difference between the diversity gain and array gain.
As a specific technique, we have also learned MRC for
Rx diversity and a Alamouti code for Tx diversity.
Please note that for space diversity, its most serious
disadvantage is that we must sacrifice some proportion of their data rates.
Indeed, the data rate is still one for Alamouti case like SISO/G systems.
In the next part, I will introduce a technique to enhance
the data rate by sending multiple messages.
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