1. BASIC PERSPECTIVE OF
MIMO-OFDM BASED SYSTEM
Rajat Dak (b.tech)
Department of Electronics and Communication Engineering
Pacific Institute Of Technology,
Udaipur, Rajasthan, India

2. Significance & Introduction of
MIMO-OFDM
 Ever-increasing demands of
multimedia services and the growth of
Internet related contents lead to
increasing interest to high speed
wireless communications
 Challenges:
- to improve spectral efficiency
- efficient bandwidth utilization
- high speed processing network
 Solution: MIMO-OFDM

3. Aspirations (Mathematical) of a
System Designer
Achieve
High data rate “Channel Capacity (C)”
Quality Minimize Probability of Error (Pe)
Minimize complexity/cost of
implementation of proposed
System
Minimize transmission power
required (translates into SNR)
Minimize Bandwidth (frequency
spectrum) Used
Real-life Issues

4. MIMO Concept
 Multiple i/p multiple o/p
antenna array
 Used to increase data rates,
improve capacity and BER (bit
error rate) of the system
Diagram of a MIMO wireless transmission
 Typically used with ofdm so as
system.
to suit best needs in next
generation comm. i.e. “4G”

5.  MIMO technology leverages multipath behavior by using
multiple, “smart” transmitters and receivers with an added
“spatial” dimension to dramatically increase performance and
range.
 MIMO allows multiple antennas to send and receive multiple
spatial streams at the same time.
A wireless adapter with 3 antennas may have a speed of 600mbps
while an adapter with 2 antennas has a speed of 300mbps.
MIMO makes antennas work smarter by enabling them to
combine data streams arriving from different paths and at different
times to effectively increase receiver signal-capturing power.
 If there are more antennas than spatial streams, the additional
antennas can add receiver diversity and increase range.

6. Principle of Diversity
The principle of diversity is to provide the receiver with multiple versions of
the same signal.
The probability that they will all be affected at the same time is considerably
reduced.
Diversity helps to stabilize a link and improves performance, reducing error
rate.
Different diversity modes are available and provide a number of advantages:
Time diversity: Using time diversity, a message may be transmitted at
different times, e.g. using different timeslots and channel coding.
Frequency diversity: This form of diversity uses different frequencies. It may
be in the form of using different channels, or technologies such as spread
spectrum / OFDM.
Space diversity: Space diversity used in the broadest sense of the definition is
used as the basis for MIMO. It uses antennas located in different positions to
take advantage of the different radio paths that exist in a typical terrestrial
environment.

7. MIMO Design Criterion
 MIMO Systems can provide two types of gain
Spatial Multiplexing Gain Diversity Gain
 Maximize transmission rate  Minimize Pe (conservative
(optimistic approach) approach)
 Use rich scattering/fading to  Go for Reliability / QoS etc
your advantage
 Combat fading

8. Spatial Diversity:
Spatial diversity used refers to
transmit and receive diversity.
The methodology is 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. Spatial Multiplexing:
This form of MIMO is used to
provide additional data capacity
by utilizing the different paths
to carry additional traffic, i.e.
increasing the data throughput
capability.

9. Orthogonal Frequency Division
Multiplexing (OFDM)
 As the data rate increases in a multipath environment, the
interference goes from flat fading to frequency selective
(last reflected component arrives after symbol period).
This results in heavy degradation
 Most popular solution to compensate for ISI: equalizers
 As we move to higher data rates (i.e.> 1 Mbps), equalizer
complexity grows to level of complexity where the
channel changes before you can compensate for it!
 Alternate solution: Multi-carrier Modulation (MCM)
where channel is broken up into subbands such that the
fading over each subchannel becomes flat thus
eliminating the problem of ISI
Multi-carrier Modulation
FDMA OFDM

10. Detailed block Diagram of OFDM
system

11. OFDM provides high-speed transmissions in a frequency selective fading environment.
 OFDM is modulation method known for its capability to mitigate multipath.
 In OFDM the high speed data stream is divided into Nc narrowband data streams, Nc
corresponding to the subcarriers or subchannels.
 As a result the symbol duration is N times longer than in a single carrier system with the same
symbol rate.
Symbol duration is made longer by adding a cyclic prefix to each symbol.
By the time cyclic prefix is longer than the channel delay spread OFDM offers inter-symbol
interference (ISI) free transmission.
 Another advantage of OFDM is that it dramatically reduces equalization complexity by
enabling equalization in the frequency domain.
 OFDM, implemented with IFFT at the transmitter and FFT at the receiver, converts the
wideband signal, affected by frequency selective fading, into N narrowband flat fading signals
thus the equalization can be performed in the frequency domain by a scalar division carrier-wise
with the subcarrier related channel coefficients.

12. OFDM Spectral Efficiency
• The spectral efficiency of an OFDM-
(PSK/ASK) system is same as compared to
using the (PSK/ASK) system alone
• Spec eff = log 2 M bits/s/Hz
• However, you have successfully converted
an ugly channel into a channel that you can
use Rs/ 3 symbols/s
Rs symbols/s
• easy to implement
• Used in IEEE 802.11A

13. MIMO-OFDM
 MIMO-OFDM decouples the frequency-selective MIMO channel into a set of parallel
MIMO channels with the input–output relation for the ith (i = 0, 2,…,L-1) tone,
yi = Hisi + ni i = 0, 2,…, L-1

14. The quality of a wireless link can be described by three basic parameters,
namely
Transmission Rate
Transmission Range
Transmission Reliability.
However, with the advent of MIMO assisted OFDM systems, the above-
mentioned three parameters may be simultaneously improved.
Field tests of broadband wireless MIMO OFDM communication systems
have shown that an increased capacity, coverage and reliability is achievable
with the aid of MIMO techniques.

15. Conclusion
 The results obtained from different tests indicate very good performance for
the MIMO-OFDM prototype.
 Concept is proving to be very robust in highly dispersive channels
 Cost reduction is being investigated through various methods
 Results show that at any given BER the adaptive SISO system will be
outperformed by the adaptive MIMO system
 Other future work areas involve the improvement of MIMO processing
complexity and practical implementation issues.

16. REFRENCES
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17. Thank You !!