MIMO

“MIMO or Multiple Input Multiple Output is a wireless
technology that helps in transmitting wireless signals over multiple antennas”.
Courtney Hamby, 2013.  “MIMO”
Multi Input Multi Output, refers to a system for transmitting data through
several transmitting antennas and receiving the same data through several
receiving antennas over the same radio frequency by exploiting numerous path of
the signal propagation established by the several antennas.

Fig 1. General form of MIMO.

Fig 1 shows a general idea of the MIMO technique, where Tx
is the transmitter station with several transmitting antennas, 1 to m and Rx is
the receiver stations with several receiving antennas, 1 to n h11 hnm are the
several paths of signal propagation.

In a normal network system, there are two antennas, the
sender and the receiving antenna, this type of setup gives rise to low signal
strength and low throughput.

MIMO can eliminate the effects of low signal and increase
throughput through diversity and spatial multiplexing techniques.

MIMO – Diversity and Spatial Multiplexing

Data transmitted through normal network channels are
susceptible to errors due signals defects such as interference and fading
e.t.c, in MIMO network system where different several transmitting and
receiving paths exist, data is sent across the different propagation paths.
There are two types of techniques used by MIMO system to achieve improve signal
strength and throughput. The techniques are diversity and spatial-multiplexing.
When the same data is sent through several paths in a MIMO system, this is
called spatial diversity or simply diversity. When bits of the same data are
sent through each path in a MIMO system, this is called spatial-multiplexing. These
two systems are listed below.

MIMO – implemented using diversity techniques – provides
diversity gain – Aimed at improving the reliability

MIMO – implemented using spatial-multiplexing techniques –
provides degrees of freedom or multiplexing gain – Aimed at improving the data
rate of the system

Diversity:

In diversity technique, same data is sent through different
paths to combat data error. When the same data is sent through different paths,
the amount of error on the data on different paths will be different. This type
of propagation guarantees that at least one copy of the data will reach its
destination with less error thereby increases the chance of receiving the data
accurately. Diversity improves the consistency of the system.

Consider a data (1 0 1 1 1) is transmitted through a channel
in a normal network system, due to effects, the data may get lost or corrupted.
The solutions for this problem is implementing the MIMO technique by increasing
the number of antennas.

The SISO antenna configuration will not provide any
diversity as there is no parallel link. Thus, the diversity is indicated as
(0).

Fig 2. Normal network system, zero diversity.

Using MIMO diversity technique, by increasing the number of
the transmitting and receiving antennas, if one of the links fails to deliver
the data, the chances of proper delivery of the data across the other link is
very high, this improvement in reliability translates into performance
improvement – measured as diversity gain. For a system with (N_T) transmitter
antennas and (N_R) receiver antennas, the maximum number of diversity paths is
(N_T imes N_R ).

Fig 3 MIMO networks with Diversity of 4.

By implementing the MIMO system, the data is transmitted
through the four links, h1 h2 h3 h4, the probability of having an error in the
data through all the four links is limited, this is called diversity.

The number of diversity is the multiplication of the
transmitting antennas and receiving antennas.

Diversity is (Tn multiply Rn) = 2*2=4.

Tn is the transmitting antenna.

Rn is the receiving antenna.

Spatial Multiplexing:

“In spatial multiplexing, each spatial channel carries
independent information, thereby increasing the data rate of the system”.
Mathuranathan, 2016. Each available channel in the network transmit independent
bits of the transmitted data and its assembled at the receiving antennas, this
is technique is the same as the orthogonal frequency division multiplexing
(OFDM), where different subchannels carry different part of the data. 

The multiplexing gain in MIMO is also referred to as degrees
of freedom with reference to signal. The number of degrees of freedom in
multiple antenna configurations is equal to (min(N_T, N_R)), where (N_T) is the
number of transmit antennas and (N_R) is the number of receive antennas. The
degrees of freedom in a MIMO configuration governs the overall capacity of the
system.

The difference between diversity and multiplexing.

Fig 4. Difference between diversity and spatial
multiplexing.

Diversity = Nt*Nr = 3.

Degree of freedom = min(Nt,Nr) = 3.

 

 

 

Space time coding.

“Space time coding is a technique that combines coding,
modulation and signal processing to achieve diversity”. Mehul R. Sameer D.,
2012.  Space–time code (STC) is a method
employed to improve the reliability of data transmission in wireless
communication systems using multiple transmit antennas.

Space time codes may be split into two main types:

Space–time trellis codes (STTCs) distribute a trellis code
over multiple antennas and multiple time-slots and provide both coding gain and
diversity gain. Vahid Tarokh and et al, 1998.

Space–time block codes (STBCs) act on a block of data at
once (similarly to block codes) and provide diversity gain but doesn’t provide
coding gain. S.M. Alamouti (October 1998).

Fig above showing space time coding.

From the figure above, the space time encoder encodes at the
transmitting antennas encode the data and send it through the several channels,
the encoded data is then decoded at the receiver antennas.

DIFFERENT TYPES OF MIMO

Multi-antenna types:

SISO – Single Input Single Output. 1 transmission antenna 1
receiving antenna. There is no diversity or complex process involved.

The advantage of SISO is its simple.

The disadvantage of the SISO channel is a limit throughput.
Interference and fading will impact the system.

 SIMO – Single Input
Multiple outputs – 1 Transmission antenna, NR Receiving antennas (NR>1).
Receiver diversity, this form helps the receiving antenna to receive multiple
signals from multiple sources. The advantage of SIMO is its simplicity, the
disadvantage is limited by size. Examples of mobile phones.

MISO – Multiple Input Single Output – NT transmission
antennas, 1 receiving antenna (NT>1): transmitter diversity, in this form
the data is transmitted from multiple transmitting antennas the receiving
antenna receives the data at the optimal signal. The advantage of MISO is that
the multiple processing is moved from the receiver to the transmitter which
makes it a good form for mobile phones.

MIMO – Multiple Input Multiple Output – NT transmission
antennas, NR receiving antennas (NT, NR > 1):  MIMO is both transmission diversity and
receiver diversity. It used to improve channel throughput and robustness.

Fig 6. Different types of MIMO.

SPATIAL DIVISION
MULTIPLE ACCESS.

Spatial Division Multiple Access is a communication mode
that optimizes, reuse and focuses the same radio frequency from the base
stations to different parallel narrow beam in a service area. Since the beams
are focused, this will increase the base station range. This characteristic of
SDMA allows base stations to have larger radio coverage with a lesser amount of
transmission energy, the focused narrow beam also allows better gain and
clarity. This is achieved by smart antennas that are highly directional allowing
same frequency to be used in several zones in the service area. The parallel
narrow beams from the smart satellite antennas ensure that interference will
not occur between zones using the same frequency.

In a traditional network setting, base stations transmit
signals in all directions within a cell or zone resulting in wastage of the
power of transmission when there are no mobile stations while in SDMA form,
signals are focused and transmitted in a narrow beam to each mobile station.

Fig 7. Spatial Division Multiplexing Access.

The figure above shows SDMA formation where different users
are accessing the same frequency beam from the base station.

Beamforming.

“Beamforming is a traffic-signalling system for cellular
base stations that identifies the most efficient data-delivery route to a user,
and it reduces interference for nearby users in the process”. Cruz, J, et al,
2005. Beamforming is bidirectional, the beamforming techniques likewise
concentrate the sensitivity of the receiver in the same direction, reducing
interference from other devices. Beamforming can be simply being defined as
focusing a WIFI signal in a specific direction with a smart router. The Router
sends a signal in the direction of the Client (mobile phones).

Benefits of beamforming are; extended WIFI coverage,
stronger signals, and reduction in interference.

To change the directionality of the network signals, a smart
router controls the signal path and amplitude of the signal at each by using
two beamforming techniques; the conventional beamforming and the adaptive
beamforming.

Conventional beamforming:

How beamforming.

Fig 9. Normal WIFI network and Beamforming technology.

In the figure above, the normal WIFI networks send out
signals in all direction thereby wasting bandwidths and power of transmission.
The second figure shows a beamforming where signals are transmitted in a
specific direction (the receiver) thereby reducing bandwidth and power wastage.

MASSIVE MIMO.

Massive MIMO simply means a normal MIMO system but with
hundreds or thousands more of antennas in the base station coordinating
together to provide greater through put and efficiency.

Sprint CTO John Saw simply explained Massive MIMO as having
128 ears to listen at once. He said, “You used to have one ear – 128 ears
in a sector where there are probably eight or 16 users at the same time.”

Fig Massive MIMO

 The figure above
shows a base station with hundreds or thousands of antennas spatially
multiplexing signals to many terminals.

The advantages of Massive MIMO

“The more antennas the transmitter/receiver is equipped
with, the more the possible signal paths and the better the performance in
terms of data rate and link reliability and will also limit interference”. Jon
M. by exploiting multipath through diversity and partial multiplexing, Massive
MIMO will increase through put and consistency of the system.

Massive MIMO and 5G.

Massive MIMO will be the foundation of the 5G network which
is due to roll out in 2020. With the concentration of many antennas, massive
MIMO will be handling the increase in the number of data usage in a zone. In
2016, Cisco estimates that by 2020 – when 5G is set to roll out – there will be
5.5 billion mobile users around the world, each consuming 20GB of data per
month.

Massive MIMO’s ability to serve multiple users – and
multiple devices – simultaneously within a condensed area while maintaining
fast data rates and consistent performance makes it the perfect technology to
address the needs of the forthcoming 5G era.