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.


  1. make a video which will cover 4G LTE Network Architecture.
    your videos are very helpful..all the best , because of you we will keep learning.😎

  2. Kindly Make a video which will cover 4G LTE Network KPI's and Call flow .. So far I haven't seen similar sensory analogical videos , we are learning so many things because of your videos .. Thank you so much

  3. The roadmap for covering LTE is very very good and the explanation is great, but taking quite more time to upload videos

  4. Thanks for sharing, very useful information. Just regarding the part "Spatial Multiplexing", the transmitted antennas transmit the same signal and not different signals as showed in 4:36, in fact using in the example the MIMO used is 2X2 in spatial multiplexing, which mean the follow : 2 different codewords (2 different transport blocks) are mapped into 2 layers (2 data streams) and then there is a phase called precoding which consists of allocate weights for both layers (precoding matrix = PMI sent as feedback by the UE to the eNB using the UCI). After the precoding, the output signals are summed together and sent over the 2 available antennas of the transmitter => so ANT1 and ANT2 carry the same info 🙂

  5. Found very helpful videos… Easy to understand and best for Engineering Students…
    Thanks Guys…
    Keep it up….

  6. Due to fading is it guaranteed that the signal will increase the link capacity can't it degrade the signal ?
    I know that if the signal is out of phase then it can lead to destructive interference.
    Is there a way to counter it??

  7. Excellent tutorial. Thankyou
    Are you planning to make a video about different Beamforming techniques

  8. do you have any video regarding scheduling algorithm of the MAC ? round robin,?proportionnal fair? maximum C over I etc…

  9. Hello
    thanks a lot
    can you make videos about FDMC , GFDM , UF-OFDM
    these videos are helping me a loy with my studies in university

  10. I have a question spacial diversity and spacial Multiplexing are techniques that work at the same time or the is a especial condition to choose one of the techniques?

  11. Wow! beam forming up at 26-40Ghz with no pre-deployment studies on the biological effects. HB 2365 prevents any delay in tower deployments across cities in the USA. Israel ,and some cities in the USA have banned this technology (5G mmWave) without further studies. More should join in. We don't need more RF blanketing our work, home, schools, public spaces. All market driven for Telcom industry profits and share holders. Crime against wisdom!

  12. This is a very nice approach to learn many concepts from this only video. Sir, can you plz tell me the actual meaning of robustness and reliability in communication??

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