Welcome to the course on Fundamentals of MIMO
Wireless Communication. Today, we are in the
lecture number 3 were we will talk about overview
of MIMO communication systems.
In the previous lectures, we have seen till
know how wireless communication has emerged
from 1860s starting with the Maxwell equation.
Then we have seen how systems are grown from
the 1st generation to the 5th generation system.
One of the important things that we may have
noted is that between first to fourth generation
systems, one of the most important drivers
of growth has been high bit rate requirement
along with that a special in a 4th generation
systems there was requirement to support high
mobility whereas, when we looked at the 5th
generation communication systems that are
currently being designed we saw that there
is diverse set of requirements which does
not necessarily be in the direction of increase
of date rate.
Increase in data rate is definitely one of
the means, but there many other requirements
which have come up we have also seen some
of the important assumptions on digital communication
systems that we need to make while undergoing
this particular course. One of the first assumptions
we talked about was it requires ideal frequency
synchronization, second important assumption
we made is that it requires ideal timing synchronization
that means, there is no inter symbol interference.
The third important assumption we made is
that there should be perfect channel estimates
available at the receiver, so that there is
no error due to channel estimation, we can
concentrate mainly on the signal processing
algorithms and performance analysis of systems
based on ideal conditions.
We have also seen what is known as the radio
access network, where we have been discussing
about the last part which is containing of
the transmitter and the access point and we
have looked at wireless systems in this part.
We are also identified which is the core network.
In this course, we will be mainly concerned
with this access part of the network.
Moving ahead just to summarize, we know the
several OSI layers starting with the physical
layer then about that there is data link layer
or MAC layer, then there is a Network layer,
Transport layer, Session layer, Presentation
layer and Application layer. In the application
layer you are well aware of things like HTTP,
FTP, SMPT, and so on. In the presentation
layer it is mainly to do data formatting like
JPEG, etcetera. Then there is a session layer
which control session between communication
systems. Transport layer is usually there
to maintain quality of service between end
to end devices and network layer is there
to route a packet from the source to the destination
going through different nodes in the network.
The data link layer are also sometimes refer
to as the MAC layer is critical in controlling
the link parameters of or access to the medium
finally, there is the physical layer which
is the lowest layer and which interacts directly
with the medium.
In this particular course, we will be concerned
in the lower two layers or primarily will
be looking at the physical layer and also
some parts of the data link layer or the lower
MAC layer specially, we term the work in these
areas as cross layer designs most modern communication
systems are using advantage or knowledge of
the physical layer in the higher layers, so
that they can adapt their transmissions, as
to achieve reliable communication link as
well as to reduce the delay between the transmission
and the reception.
Now, we take a look at a typical blog diagram
of a digital communication receiver.
So, we have the source 
at the input, if it is an analog input usually
goes through a sampler 
followed by the Quantizer, followed by source
encoding 
examples which to compression of the source
what possible could be image compression algorithms.
This whole block is usually termed as source
signal processing are also some time referred
to as source encoding this is in some systems
followed by a scrambler; scrambler is usually
present to mix up the sequence of ones and
zeros, so that it avoids long sequence of
ones and zeros that helps in achieving synchronization
at the receiver.
In some systems you will find a cyclic redundancy
check being present which helps in checking
whether a packet is being received correctly
or not followed by the CRC check, there is
the forward error correction module and inter
leaver 
at the output. The output of the inter leaver
is fade to in digital communication systems
something known as the Symbol Mapper 
which the output of which is further put into
pulse shaping 
which may be further fade into a D/A converter
and then may or may not be presence of an
IF stage followed by the RF stage which includes
up conversion and spectral mask followed by
an antenna or a corresponding trans user depending
upon the medium.
This is the typical blog diagram of a transmitter
in case of MIMO communication systems. This
particular section is rather replicated to
transmit signal across multiple antennas which
we will see very shortly on the receiver front
the antenna receives the signal followed by
the RF section which includes a low noise
amplifier followed by a band pass filter,
followed by a down converter through a mixer
into either the IF stage or to the Base Band
stage depends upon the kind of mixer and the
particular specific communication system at
hand followed by sampler or A/D converter.
This will also vary depending upon the particular
communication system because they could be
match filter represented or utilize in the
digital domain or it could be in the analog
domain.
Then there would be the timing synchronization
which would include packet synch. The packet
synchronization is important in some systems
were burst mode communication is present from
the packet synch. They could be going into
frame synch packet; synch is sometimes is
also referred to as packet detection which
simply presents detects the presents of a
packet or non presence of a packet followed
by the packet detection. There is of frame
synchronization 
which identifies the exact start of a packet
which is followed by the carrier synchronization.
Sometimes, there is iteration between carrier
synch and frame synchronization. This carrier
synchronization after carrier synchronization
they would be the channel estimation equalization
block as we can shown, in the previous diagram,
but before that there would be the clock synchronization
followed by the clock synch there is will
make one module channel estimation 
and equalization. After we have channel estimation
equalization there could be an important module
known as combiner. Now, typically when we
talk about combiners, these combiners are
generally mentioned for communication systems
such as CDMA whereas, when we talk about MIMO
these combiners could be related to the combining
of signals arriving through multiple antenna
part or multiple antenna chains.
So, once we are through with the combiner,
then there would be the decision logic for
the base band processing or De Mapper. This
could be hard decision or this could be soft
decision from the De Mapper it goes into the
De inter leaver 
followed by decoder of the FEC followed by
cyclic redundancy check at the receiver we
checks whether redundancy added is received
in that. So, that it can identify the packet
received is correct or not followed by the
opposite part what is done at the transmitter
De scrambler.
Which is usually done using a seeded random
sequence and then passed on to; I will make
a large block for this source decoder which
would include the corresponding block of the
source encoding corresponding to source encoding
they would be source decoding ends from the
compression it is being to extract the information
table look up for container. They should be
a corresponding table look up followed by
d to a as may be necessary. So, this way we
can build up the entire block diagram of the
transmitter and the receiver.
We can clearly see that if he compare block
diagrammatic representation of the transmitter
and receiver the receiver has many more components
than the transmitter and some of the important
components which are additional at the receiver
are the synchronization modules, timing synchronization
modules, the frequency synchronization modules,
the clock synchronization modules, the channel
estimation equalization modules, the combiner
modules.
These are additional on top of what is present
at the transmitter and hence designing of
the receiver is very, very critical and what
we can see also from the figure that between
the transmitter and the receiver, there lies
the channel the design of the transmitter
is usually done in such a way, so that the
signals which have coming out from this section
are matched to the channel the receiver is
design in a such a way, that all the distortions
which the channel introduces is resolved.
So, that when the signal following through
all this process finally, comes at the destination.
It is with minimum errors compare to any other
situation if all these blocks were not present
there will be huge amount of errors because
of the channel as we move on with this back
ground that is already discussed.
We will now take an overview of MIMO communication
systems trying to see how typical MIMO communication
architecture are present, which will give
us an overview of what we are going to cover
in details in this particular course.
When we talk about MIMO what we mean is, we
have the transmitter which encompasses all
the blocks that we have already described
in the previous diagram. Instead of one antenna,
what we have describe in the previous figure
there are multiple transmitting antennas typically
in many references they will be refer d to
using N T. These signals are sent simultaneously
from these antennas and propagate through
the channel. So, instead of one transmit antenna
one receive antenna there would be multiple
transmit antennas and there would be multiple
received antennas at the receiver.
There will be multiple signals which is received
and processed instead of single antennas signal
partly what we have described in a typical
SISO link the received signal Y which is mentioned
over here can be represented as Y equals to
H times X, where X is the transmitter base
band signal and this what we have discussed
in earlier class that will not talk about
carriers. So, we talk about base band signal
plus noise; noise is due to receiver components
and H represents the channel.
Typically, you may have studied about convolution,
but this is the simplest form of relationship.
If it is the non convolving channel or a flat
fading channel for MIMO links on the contrary
what we have is we have X m going from one
to n t; that means, x 1, x 2, x 3 up to x
N T each getting multiplied by H m and then
at being received at let us say antenna Y
1. So, if I take Y 1 this is equal to some
over all the links that are present all the
transmitter signal multiplied by the corresponding
channel links and this will be for Y n equals
to 1 to N R. So, will be getting several such
signals one we get once we get several such
signals we have to process them at the receiver.
So, that our n goal is will be able to extract
the signals that have been sent from the transmitter.
If we look at the benefits from multiple antennas
the first important benefit that we gain from
multiple antenna transmission is known as
diversity gain. When we talk about diversity
gain it helps us over come small scale fading
details of which we are going to see very
soon and the next important benefit of spatial
diversity combining is path loss that means,
if there is arrays of antennas that are present
at the receiver. So, they will be used to
combine this strength of the signals. So,
that there is average increase in SNR, we
will disuse all this in details when we talk
about the diversity gain what we mean is.
Suppose, I have a signal which is coming in
this form and we have another signal which
comes in this way. So, by diversity combining
what we mean we will of course, see details
later is at any one instant of time we considered
this time since we have 1 and 2 copies of
signal either of these two signals will be
above certain threshold whereas, if we would
have taken only one they would be instances
when one of the signals is below threshold
or when other is below the threshold that
is the advantages of diversity combining whereas,
if we look at the array gain there what is
been done is the signal that is received from
one of the antennas and the signal which is
been received by the other antennas. They
are all combing in such a way that whereas,
each signal was arriving at different SNR
values after combining the combined SNR can
be at a much higher level and we get an increasing
average signal strength thereby improving
the performance of this communication systems.
If we take a look at the second important
gain which MIMO provided that is with increasing
in capacity that means, this is achieved through
something known as spatial multiplexing which
will again see very shortly. So, as the name
suggest there is multiplexing that is going
on. So, multiplexing means simultaneously
or mixing up several serial streams, so that
they can go together. So, there is parallel
stream transmission that means, at the same
time using multiple antennas it can send two
more than one user or even to one user it
can send two different data streams.
The next important method of using multiple
antennas is through beam forming as shown
in this particular figure that specific beams
are formed which are very, very directional
and users can be present or addressed in each
of the beams separately and this would also
help reduce interference that means, if one
user is radiating from this direction another
user is radiating from this direction because
of forming separate beams for this users the
interference from one user can easily be taken
care of by forming the spatial beams. This
also can be use for space division multiplexes
and other techniques.
If we look at the benefit that MIMO brings
is this is one diagrammatic representation
of one of the results which we will see of
course, later on the x-axis. So, shows SNR
in d B, the y-axis shows average capacity
in bits per second per hertz again will define
all these terms M equals to 1 means that is
one transmit antenna, one receive antenna
typically this is a SISO system what we are
usually used to as we go to M equals to 2
that means, we are going to this particular
curve that means, there are two antennas at
the transmitter two antennas at the receiver
we see that as SNR increases the spatial elections
increases in a such a fashion that compare
to SISO link here, the SNR increase is significant
as we move to the next one, which is M equal
to 3 or 3 cross 3 system then we get another
significant jump and then when we move to
4 cross 4 system or 4 transmitters and 4 receive
antennas we reach here.
So, apparently what we see there is approximately
linear increase in capacity or linear increase
in spectral efficiency as we increase the
number of antennas from 1 to 2 to 3 to and
4, this is one of the biggest gains that MIMO
brings to us and in this course, we are going
to study how we are going to achieve this
particular gains.
If we take a look at how MIMO is used in commercial
systems for example, LTE or others MIMO antenna
systems can be broken down into one of the
sub cases, SISO; single antenna single output,
SIMO; single input multiple output that means,
one transmit antenna multiple receive antenna
multiple receive antenna single output antennas
and finally, multiple input multiple output;
MIMO systems can provide us with diversity
gain and spatial multiplexing gain. As we
have discussed in spatial multiplexing we
have single user and multi user scenarios
as well as dedicated beam forming in single
user scenario we have open loop spatial multiplexing
closed loop spatial multiplexing one of the
objectives of this course is that we will
cover these parts as highlighted in this particular
slide.
In this diagram will try to briefly discus
what the overview about what is single user
MIMO. In single user MIMO, we have multiple
antennas at the transmitter multiple antennas
at the receiver and signals are all mixed
up when it comes to the receiver at the receiver
due to signal processing and due to prepossessing
at the transmitter it may be possible to create
parallel interference free channels thereby
separate streams which do not interfere with
each other may be possible to be transmitted
from the source to the destination and we
will be able to receive separate us interference
free data streams at the receiver. One of
the important objectives of this course is
to understand how this can be realized and
what capacity gains are achieved because of
such processing.
If we move further, if these receivers instead
of being connected together if they are now
separated whereas, at the transmitter the
antennas are co-located in that case what
we have is a multi user MIMO is these will
be multiply users and this will be the base
station, for example, in this mode the transmitter
would be able to send separate streams to
user 1 to user 2 and user 3 simultaneously
using similar, but improved signal processing
algorithms and when we move further taken
additional step at the transmitter if we have
these antennas now distributed, so that they
form parts of different antenna systems and
there are different receivers whereas, the
source they are somehow connected to each
other what we have is a multi point transmission
to multiple users which is one of the most
complicated part of MIMO systems.
Moving ahead further will take a brief look
of how, a typical communication system work.
Then will see what complex it is MIMO adds
to this system and understanding of MIMO would
help resolve designing this complexity complex
communication systems design better algorithms
at the transmitter and at the receiver a typical
communication system has experiences channel
which is describe by this particular figure.
Of course, will spend significant time try
to understand what we briefly see is that
across time and frequency domain there is
fluctuation of signal strength across this
whole space we have a typical cellular structure
were there might be base station across at
this central part of the nodes and there would
all been transmitting simultaneously.
One of the central nodes over here is the
base station which connects to multiple users,
this signals the users send back channel quality
indicator. At the base station there is cycle
of events that happen which include link adaptation
followed by packet scheduler in both transmit
and frequency domain there is transport block
formation there is QOS adjustment there is
beam forming or they could be spatial multiplexing
or diversity mode there is call admission
control. So, all these process keeps on happening
in one of the base stations and these keeps
on happening at all these base station simultaneously
with the advent of MIMO one, etcetera processing
comes in is selection amongst these different
modes and even when we go multi user MIMO
one has to select between multiple users.
If we look at when this gets added when the
MIMO techniques which briefly describes get
added toward typical large communication system,
what happen is all the processing that we
have discussed goes on at the base station
of one of the cells and there are other cells
which are nearby and a typical multi point
transmission, these base station 1 and base
station 2, they would typically communicate
to one central processing unit where by base
station 1, 2 and 3, for example, would jointly
sends signals to these users in such a way
that there is no interference from these which
apparently there is huge amount of interference
whereas, there is a constructive generation
of the signal, so that huge increase in spatial
efficiency is achieved outage is re dude especially
at the cell age which is otherwise not possible
without the use of multiple antennas and coordination
between these base stations.
As we move further, the other forms of coordination
could be that there are typical processing
going on in the base stations and these other
base stations, they do not do joint processing,
but there is communication amongst these base
stations, but they send independent signals,
but they doing such a way the selection of
MIMO mode which is also known as MIMO beam
forming is done in a coordinated fashions
so that they reduce interference specially
at the cell age thereby improving the quality
of service.
The other mode of coordination which is known
as coordinated scheduling and radio resource
allocation. Again these base stations do talk
to each other, but they generally do not do
joint signal processing, but through coordination
they exchange information about the channel
conditions and they together solve the problem
of allocating resources. To users in this
space who would be actually hearing to transmissions
from all these neighboring base stations they
solve it in such a way. So, that the interference
is reduced resource allocation is maximized
and overall there is gain in the performance
of the systems.
Just last few minutes for today’s lecture
were we talk about the prerequisites for this
subject. So, as we have been discussing till
know we would require you to revise digital
communications. We have briefly touched upon
some of the concepts we would require lot
of probability and statistics. So, your understanding
of probability and statistics is important
linear algebra it is required to be revised
the books which we will follow some of the
books would be wireless communications by
Rappaport principles of mobile communication
by Gordon Stuber.
These ones will specially refer in the beginning
part were will talk about the channel introduction
to space time wireless communication by Paulraj
et al are is one of the most important references
which we are going to follow through out when
we will talking about MIMO channels as well
as MIMO signal processing. So, this is one
of the important books to be followed, there
are other books like wireless communications
by Andrea Goldsmith elements of information
theory by Cover and Thomas which will also
be necessary.
Below there is list of a few important researches
in this domain who have been prolific authors.
So, beyond the course work I would encourage
you to go across and find even more authors
of in the domain of MIMO communications who
have been producing rich literature. So, that
this domain has been improving and we are
at a stage where we are able to take it as
a course. Other reference materials if you
do not limit yourselves to these materials
feel free to go into journals and conferences
which keep on getting published to provide
latest results.
So, we stop at this particular point in the
third lecture and will discuss some more things
in the coming lecture the fourth.
Thank you.
