Very Often Radio signals reaching the receiver faces degradation due to Multipath Propagation.
Which reduces Link Capacity and Link Reliability.
So, how do we cope with these degradation ?
Hello Everyone, Welcome back to the Fundamentals of 4G.
Today we will discuss how link performance is enhanced using SPATIAL DIVERSITY.
We will also discuss how SPATIAL MULTIPLEXING is used in MIMO to enhance Link Efficiency.
And finally we will discuss some futuristic technologies such as ANTENNA BEAMFORMING which
will dynamically serve the User to improve the coverage and MASSIVE MIMO which has the
potential to increase the link capacity by several fold.
The principle of Diversity, is to provide the receiver with multiple versions of the
same signal which reduces the signal degradation considerably, and effectively improves the
link performance.
This can be done by,
TIME DIVERSITY - where same message is transmitted at different time slots.
FREQUENCY DIVERSITY - Where the same Message is transmitted at different frequencies, and
SPACE DIVERSITY - It uses separate antennas, which are located in different positions to
take advantage of the different radio paths, that exist in a typical terrestrial environment.
This concept is used in MIMO.
Up until the 1990's, Space diversity was used in systems that switched between two antennas,
or combined the signals to provide the best signals.
Such systems demanded high level of processing, but the systems processing were limited.
However with the improvement of processing power , it was possible to implement MIMO.
Between a transmitter and a receiver, the signal can take many paths due to obstacles
and object in the medium.
Traditionally, These multiple paths only introduced interference.
By introduction of MIMO, these additional paths can be used to provide additional robustness
to the radio link by SPATIAL DIVERSITY, or to increase the link data capacity by SPATIAL MULTIPLEXING.
So let us understand, how both of them work in MIMO.
Consider a system, where a data stream [10111] is transmitted through a channel with deep fades.
Due to fluctuation in the channel quality, the data stream may get lost or severely corrupted
that the receiver cannot recover it.
The solution to combat the rapid channel fluctuation is to, add independent fading channels by
increasing the number of "Transmitting Antennas" or "Receiving Antennas" or "Both".
So in brief, Spatial diversity techniques, where same information is sent or receive,
across independent channels to combat fading.
Here DIVERSITY GAIN is defined as, No of Tx antenna X No. of Rx Antenna
Which is equal to 1.
let’s increase the number of receiving antennas by one count.
The chances of proper delivery of the data across 2 link is very high.
Thus, Additional fading channel increases the reliability of the overall transmission.
Here DIVERSITY GAIN is 1 X 2 = 2
In this way, more diversity paths can be created by adding multiple antennas at transmitter side as well.
The following figure shows a 2X2 MIMO system with number of diversity paths equal to
2 X 2 = 4
Now, As per SHANNON’S CHANNEL CAPACITY Theorem, There is a limit on the capacity of a channel
for a given bandwidth.
The capacity is also limited by the signal to noise ratio of the received signal.
By using higher modulation scheme we can certainly increase channel capacity but for that we
have to have high signal to noise ratio.
This tradeoff can be difficult, expensive and some time we can not compromise.
Therefore it is necessary to look at other ways of improving the data throughput for
individual channels.
Using Spatial Multiplexing wireless communications can effectively utilize link capacity and
spectral efficiency.
In this technique Multiple antennas are used in transmitting and receiving end.
Each spatial channel carries independent information, thereby increasing the data rate of the system.
This can be compared to Orthogonal Frequency Division Multiplexing technique(OFDM), where, different
frequency sub channels carry different parts of the modulated data.
But in spatial multiplexing, If the scattering by the environment is rich enough, several
independent subchannels are created in the same allocated bandwidth.
Thus the multiplexing gain comes at no additional cost on bandwidth or power.
To recover the transmitted data-stream at the receiver it is necessary to perform a
considerable amount of signal processing.
First the MIMO system decoder must estimate the individual channel transfer characteristic (Hij)
to determine the channel transfer matrix.
Once all of this has been estimated, then the matrix [H] can be produced and the transmitted
data streams can be reconstructed by multiplying the received vector with the inverse of the transfer matrix.
This is how we can achieve Link Robustness using SPATIAL DIVERSITY, and Effective use
of Link Capacity using SPATIAL MULTIPLEXING.
Apart from this, there can be many configuration in MIMO and each one of them has specific
ADVANTAGE and DISADVANTAGE.
Such as Single Input Single Output, Single Input Multiple Output, Multiple Input Single
Output, Multiple Input Multiple Output, These can be balanced to provide the optimum solution
for any given application.
After discussing all the aspect of MIMO, there are few more aspect which needs to be looked upon.
Antenna Beamforming - With the development of more Adaptive systems
and greater levels of processing power, it is possible to utilise antenna beamforming
techniques with systems such as MIMO, In this technique smart antennas are used, Which can be of,
PHASED ARRAY SYSTEM - Phased array systems have a number of predefined patterns.
The required one is being switched according to the direction required.
ADAPTIVE ARRAY SYSTEM - This type of antenna uses what is termed as adaptive beamforming
and it has an infinite number of patterns and can be adjusted to the requirements in real time.
Now, Traditional MIMO systems have 2, or 4, or 8 antennas.
When number of antennas used in communication terminal are more than 10s, or 100s, Then
it is called MASSIVE MIMO system.
The concept of massive MIMO or large MIMO systems is becoming popular as it is able
to offer; Increased data rate,
Increased basic link signal to noise ratio, and
Channel hardening.
Key issue which is faced in Massive MIMO is antenna placement.
As a RULE OF THUMB, spacing of Lambda/2, (where Lambda is the wavelength of the signal)
is considered necessary to provide almost no correlation between the antennas.
And in order to achieve this, various approaches can be taken such as;
Use of higher frequencies, Use of volumetric spacing, and
Use of spatial modulation.
One of the key requirements for a large MIMO system is , Rich diversity of signal paths
between the transmitter and receiver.
This is normally present within a typical INDOOR and most URBAN ENVIRONMENTS.
Other environments where there are less paths, won’t be able to provide the same benefits
with a MIMO or large MIMO System.
So friends, in this brief video on MIMO we have seen how multiple path increases LINK
ROBUSTNESS in SPATIAL DIVERSITY, and LINK CAPACITY in SPATIAL MULTIPLEXING.
We have also seen, how in future, COVERAGE can be improved using ANTENNA BEAMFORMING,
and THROUGHPUT can be enhanced using MASSIVE MIMO.
In our next video on CARRIER AGGREGATION TECHNIQUE, we will discuss how Throughput is
Enhanced by combining multiple bands.
You can post your doubts in the comments section.
Don't forget to share it with your friends.
Happy learning.