The document summarizes MIMO-OFDM technology for high-speed wireless communication. It describes that MIMO uses multiple antennas at the transmitter and receiver to minimize errors and optimize data speed. It can increase channel capacity while obeying Shannon's law. OFDM divides data into small sub-signals transmitted through different frequencies using IFFT and FFT. Combining MIMO with OFDM provides higher throughput and link reliability. Industry standards like 802.11n, 802.16a, LTE/LTE Advanced have adopted MIMO-OFDM to achieve data rates up to 1Gbps.

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Technical paper
on
MIMO – OFDM :Technologyforhighspeedwirelesscommunica on
Sushil Kumar, ITS
Telecommunications Vol-61 Issue 1 , Nov -2014
The demand for increased channel capacity in wireless and mobile communication has been rapidly
increasing world wide due to multi fold increase in mobile telephony, High speed Internet users and
multimedia services coupled with a limited spectrum. In the present scenario, high data rate is required in
WLAN , Wi-Max and LTE/ LTE advanced-4G. In this paper MIMO antenna technique and OFDM has been
described. Industry standards, present applications and further research has also been taken up. MIMO
system with OFDM gives higher gain by using the direct and the reflected signals thus facilitating the
transmission at high data rate.
AboutAuthor
Sushil Kumar has done BE (E&C) in 1987
and M Tech (CST) in 1989 from IIT Roorkee.
He joined DOT as ADET in 1989 (87 batch)
and worked on various posts in DOT/BSNL.
He is presently working as DDG TEC New
Delhi.
1.0 MIMO (multiple input, multiple output) brief :-
I
n 1998 Bell Laboratories successfully demonstrated the
MIMO system under laboratory conditions. In the
following years Gigabit wireless Inc. and Stanford
University jointly held the first prototype demonstration of
MIMO. MIMO is an antenna technology for wireless
communications in which multiple antennas are used at
both the source (transmitter) and the destination
(receiver)[4]. The antennas at each end of the
communications circuit are combined to minimize errors
and optimize data speed. MIMO is one of several forms of
smart antenna technology, the others being MISO (multiple
input, single output) and SIMO (single input, multiple
output).
For example a 2*2 MIMO will have 2 antennas to transmit
signals (from base station ) and 2 antennas to receive
signals (mobile terminal).This is also called
1.1 Downlink MIMO.
General figure of a MIMO antenna system is as given in
figure 1:-
One of the core ideas behind MIMO wireless systems is the
use of multiple antennas located at different points.
Accordingly MIMO wireless systems can be viewed as a
logical extension to the smart antennas that have been used
for many years to improve wireless.
It is found that the signal can take many paths between a
transmitter and a receiver. Additionally by moving the
antennas even a small distance the paths used will change.
The variety of paths available occurs as a result of the
number of objects that appear to the side or even in the
direct path between the transmitter and receiver. Previously
these multiple paths only served to introduce interference.
By using MIMO, these additional paths can be used to
advantage. They can be used to provide additional
robustness to the radio link by improving the signal to noise
ratio, or by increasing the link data capacity. The two main
formats for MIMO are given below:
Spatial multiplexing : This form of MIMO is used to provide
additional data capacity by utilising the different paths to

2. carry additional traffic, i.e. increasing the data throughput
capability.
Spatial diversity: Spatial diversity used in this narrower
sense often refers to transmit and receive diversity. These
two methodologies are used to provide improvements in the
signal to noise ratio and they are characterized by improving
the reliability of the system with respect to the various forms
of fading.
As a result of use of multiple antennas, MIMO wireless
technology is able to considerably increase the capacity of a
given channel while still obeying Shannon's law. By
increasing the number of receive and transmit antennas it is
possible to linearly increase the throughput of the channel
with every pair of antennas added to the system (Figure -2).
This makes MIMO wireless technology one of the most
important wireless techniques to be employed in recent
years. As spectral bandwidth is becoming an ever more
valuable commodity for radio communications systems,
techniques are needed to use the available bandwidth more
efficiently. MIMO wireless technology is one of these
techniques. Figure -2 shows the comparison between
SIMO/ MISO and MIMO antenna techniques [2].
1.2 Uplink MIMO
Uplink MIMO schemes for LTE will differ from downlink
MIMO schemes to take into account terminal complexity
issues. For the uplink, MU-MIMO can be used. Multiple user
terminals may transmit simultaneously on the same
resource block. This is also referred to as spatial domain
multiple access (SDMA). The scheme requires only one
transmit antenna at UE side which is a big advantage. The
UEs sharing the same resource block have to apply
mutually orthogonal pilot patterns. To exploit the benefit of
two or more transmit antennas but still keep the UE cost low,
antenna subset selection can be used. In the beginning, this
technique will be used, e.g. a UE will have two transmit
antennas but only one transmit chain and amplifier.Aswitch
will then choose the antenna that provides the best channel
to transmit from user equipment to base terminal.
1.3 How MIMO works ? :- Traditional radio system either
do nothing to combat multipath interference, relying on the
primary signal to muscle out the interfering copies or employ
mitigation techniques. One technique uses a no. of
antennas to capture the strongest signal at each moment in
time. All techniques assume that the multipath signal is
harmful and strive it to limit the damage.
 On the contrary MIMO takes advantage of multipath
propagation (direct and reflected signals).
 MIMO uses multiple antennas to transmit multiple
parallel signals.
 In an urban environment, signals will bounce off trees,
high rise buildings and reach the receiver through
different path.
 Receiver end uses an algorithm / DSP to sort out the
multiple signals to produce one signal having originally
transmitted data.
 Multiple data streams are transmitted in a single channel
at the same time and at the receiver multiple radios
collect the multipath signal.
 MIMO OFDM uses IFFT in the transmitter and FFT in the
receiver.
 MIMO increase range, throughput and reliability .
2.0 OFDM :-
OFDM is a type of FDM modulation technique which is
used for large amount of data transmission. OFDM
works by dividing data into small sub signals and
transmitting them through different frequencies. It uses IFFT
in transmitter and FFTin receiver.
Efficient implementation of MIMO-OFDM system is based
on the Fast Fourier Transform (FFT / IFFT) algorithm and
MIMO encoding, such as Alamouti Space Time Block
coding (STBC), the Vertical Bell-Labs layered Space Time
Block code VBLASTSTBC, and Golden Space-Time Trellis
Code (Golden STTC)[3].
OFDM has been adopted for various transmission systems
such as Wireless Fidelity (WIFI), Worldwide Interoperability
for MicrowaveAccess (WIMAX), Digital Video Broadcasting
(DVB) and LongTerm Evolution (LTE).
The OFDM system assigns subgroups of subcarriers to
each user. With thousands of subcarriers, each user would
42Telecommunications Vol-61 Issue 1 , Nov -2014

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get a small percentage of the carriers. In a modern system
like the 4G LTE cellular system, each user could be
assigned from one to many subcarriers. In LTE, subcarrier
spacing is 15 kHz. Using a 10-MHz band, the total possible
number of subcarriers would be 666. In practice, a smaller
number like 512 would be used. If each subscriber is given
six subcarriers, you could put 85 users in the band. The
number of subcarriers assigned will depend on the user's
bandwidth and speed needs.
Combining OFDM with multiple input multiple output
(MIMO) technique increases spectral efficiency to attain
throughput of 1 Gbit/sec and beyond, and improves link
reliability.
3.0 Industry standards issued for various services:-
1. IEEE 802.11n for WLAN standards:- The IEEE
802.11n WLAN standards provides a series of
enhancement technique to both the physical layer and
MAC layers leading throughput of up to 100 Mbps.
The standards include Multiple input Multiple output
(MIMO) antenna technology and 40 MHz operation to
the physical layer.
2. IEEE 802.16a for WiMax standards :- WiMax is also
known as WMAN, used to provide broadband wireless
connectivity over a substantial geographical area such
as large metropolitan city. It has been designed to
evolve a set of air interfaces based on a common MAC
protocol but physical layer specifications having an air
interface support in 2-11 Ghz band having both
licensed and license exempt spectrum. Wi Max can
use radio bandwidth that can vary from 1.25 MHz to 28
MHz in steps of 1.75 MHz in 2GHz to 11 GHz band. It
also uses multicarrier OFDMA scheme with MIMO
antenna technique to achieve transmission data rate
as high as 155 Mbps. Wi Max equipment can operate in
different FDD or TDD configuration and operate in
different frequency bands of 5.8 GHz, 3.5Ghz and 2.5
Ghz [5].
3. LTE/ LTE Advanced :- Long Term Evolution (LTE) is a
4G wireless broadband technology developed by the
Third Generation Partnership Project (3GPP), an
industry trade group. 3GPP engineers named the
technology "Long Term Evolution" because it
represents the next step ( ) in a progression from4G
GSM UMTS 3G, a 2G standard, to , the technologies
based upon GSM. LTE provides significantly increased
peak data rates, with the potential for 100 Mbps
downstream and 50 Mbps/ 30 Mbps upstream,
reduced , scalable bandwidth capacity, andlatency
backwards compatibility with existing GSM and UMTS
technology. In LTE advanced - 4G, max down link
speed of 1 Gbps and beyond is expected in future.
The upper layers of LTE are based upon TCP/IP, which
will likely result in an all-IP network similar to the current
state of wired communications. LTE will support mixed
data, voice, video and messaging traffic. LTE uses
OFDM (Orthogonal Frequency Division Multiplexing)
and, in later releases, MIMO (Multiple Input Multiple
Output) antenna technology similar to that used in the
IEEE 802.11n wireless local area network (WLAN)
standard. The higher signal to noise ratio (SNR) at the
receiver enabled by MIMO, along with OFDMAand SC-
FDMA (Single channel orthogonal frequency division
multiple access in up link), provides improved
coverage and throughput, especially in dense urban
areas.
LTE 4G network will compete with WiMAX for both
enterprise and consumer broadband wireless
customers. Outside of the US telecommunications
market, GSM is the dominant mobile standard, with
more than 80% of the world's cellular phone users.As a
result, HSDPA and then LTE are the likely wireless
broadband technologies of choice for most users.
Nortel and other infrastructure vendors are focusing
significant research and development efforts on the
creation of LTE base stations to meet the expected
demand. When implemented, LTE has the potential to
bring pervasive computing to a global audience, with a
wire-like experience for mobile users everywhere. A
comparison beween 3G (WCDMA), HSPA, HSPA+,
LTE and LTE advanced is given on the next page.
Field results taken from” LTE-4G technology in today 's
spectrum” IEEE CVT Technical series, Ericssion, April
21,2009,[1] are as given below:-
 With 2*2 MIMOAntenna technology, peak data rate in
Down Link : 170 Mbps
Up Link : 56 Mbps (16 QAM)
 With 4*4 MIMOAntenna technology, peak data rate in
Down link : 325 Mbps
 RadioAccess
Down Link : OFDM
Up Link : SC- FDMA
4.0 Applications:-
As of today several companies have developed MIMO
OFDM solution based wireless routers, PDAs, Smart
Telecommunications Vol-61 Issue 1 , Nov -2014

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phones and other broadband devices. 802.11n based routers
can be used to create Digital home. Wi Max / LTE 4G can be
used for fixed, portable and mobile wireless broadband. Last
mile connectivity can be extended for various services such
as fleet tracking, VPN services, leased lines, Traffic control
system etc.
5.0 Future Outlook / research :-
As already discussed that efficient implementation of
MIMO-OFDM system is based on FFT algorithm and
MIMO encoding such as Alamouti space time block coding ,
vertical Bell labs layered space time block code. There is a lot
of scope for further research. It is proposed to have channel
coding using estimated channel parameters from a
transmitted pilot data at the receiver end. It may enhance the
performance and further make the channel more resistant to
noise.
Further studies can be made for constructing less complex
transceivers for multiple antenna technique.
A comparison between WCDMA, HSPA, HSPA+, LTE and
LTE advanced is as given below [6]:-
Item WCDMA
(UMTS)
HSPA HSPA+ LTE LTE Advanced
Max downlink speed
bps
384 k 14 M 28 M 100M 1G
Max uplink speed
bps
128 k 5.7 M 11 M 50 M 500 M
Latency
round trip time
approx
150 ms 100 ms 50ms (max) ~10 ms less than 5 ms
3GPP releases Rel 99/4 Rel 5 / 6 Rel 7 Rel 8 Rel 10
Approx years of
initial roll out
2003 / 4 2005 / 6
HSDPA
2007 / 8
HSUPA
2008 / 9 2009 / 10 2012/13
Access methodology CDMA CDMA CDMA OFDMA / SC-
FDMA
OFDMA / SC-
FDMA
References:-
1. LTE-4G technology in today 's spectrum” IEEE CVT
Technical series, Ericssion,April 21,2009.
2. Introduction to wireless MIMO- theory and application.
IEEE L1, Nov 15 2006. Dr. Jacob Sharony Director,
Network Technologies Division, Center of Excellence
in Wireless & IT, Stony Brook University
3. New Transmission scheme for MIMO OFDM systems.
A. Omri and R. Bouallegue, IJNGN vol 3, No. 1, march
2011.
4. Multiple antenna technique (MIMO) by Muhammad
Razin Ibn Azad. Helsinki Metropolia University of
applied science.
5. Wireless communications by T L Singal. Tata McGraw
Hill Education Pvt. Ltd. New Delhi, India.
6. 4 G LT E A d v a n c e d . h t t p : / / w w w . r a d i o -
electronics.com/info/cellulartelecomms/lte-long-term-
evolution/3gpp-4g-imt-lte-advanced-tutorial.php
Telecommunications Vol-61 Issue 1 , Nov -2014