ABSTRACT:
Last few years have seen rapid
changes in the wireless technologies. The stage is set for third generation (3G)
technology and R&D is aiming at fourth generation (4G) technology.
The future communication aims at
becoming all wireless and mobile supporting any communication, any where, any
time and for anybody with a single unique identification number of a person
world over for all communication.
It’s defined as personal
communication services (PCS) and may be supported by personal communication
network- a global wireless and mobile networks.
This
paper briefs out the consequences of multipath fading, the reason of poor
quality of services. This paper also put light on the concept of MIMO
technology and gives reason why it’s preferable over present day technology.
INTRODUCTION:
Comprehensive broadband, integrated
mobile communication will step into all mobile 4G service and communication.
The 4G will be the migration from the other generation of mobile services to
overcome the limitation of boundary and achieve the integration. The 4G of
mobile services aims to total wireless.
The 4G will be developed to provide
high speed transmission, next generation internet support, seamless integrated
services and coverage, utilization of higher frequency, lower system cost,
seamless personal mobility, mobile multimedia, sufficient spectrum use, quality
of service (QoS), reconfigurable network and end-to-end IP systems.
In conventional wireless
communication a single antenna is used at the source and another antenna is
used at destination. In many cases it gives rise to problem with multipath
fading, making difficult to meet promises aim by the 4G.
The solution to multipath fading can
be solved using MIMO technology. The following paper will outline the concept
of MIMO technology and why it’s superior to present the present day technology.
Multipath fading:
Wireless technologies are not free
from the limitations of the available frequency spectrum, fading and multipath
fading.
Multipath fading results when the
transmitted signal bounces off object like buildings, office cabinets and hills
creating multiple paths for signal to reach the receiver. The same transmitted signal
that follows the different path reaches the receiver at different time with
different phases. Added together, the several incidences of same signal with
different phases and amplitudes may cancel each other, causing signal loss or
drop of signal power.
The expectation from the future
wireless mobile networks are high data rate, higher network capacity, better
quality of service and lower probability of call drop. With increase data
rates, the problem of multipath fading becomes severe.
The consequences of multipath fading
may be delay spread, short time fading, long time fading and Doppler Effect.
In mobile environments as channel
condition changes with the motion of receiver, fading causes short term effect,
resulting fluctuation in the received power over time. The receiver may not
adapt to the changes .This may degrades quality of service. Short term fading
occurs over short time duration.
The long term fading results in the
decreased received power over long time/distance as time increases, the moving
receiver usually goes farther away.
The Doppler Effect occurs in the
moving mobiles. It results in the shift of the frequency randomly. Multipath
fading, in effect, either causes low received signal power or degraded quality
of services, both of which are highly unexpected in future all wireless and
mobile communication. The low received power increases the bit-error rate,
which in turn limits the data rate.
Concept of MIMO technology:
Wireless channels input and output
modulated signals. For the purpose of modulation, the two basic things are
considered are frequency and time. The frequency plan and time plan use ‘bits
per hertz’ and ‘bits per second’ as measures for data rate transportation.
A new dimension to upgrade the data
transportation rate is spatial dimension. This is the concept behind multiple input multiple output (MIMO) technology.
MIMO technology may be seen as an
upgrade of single input multiple
output
And multiple input single output
(MISO). All three technologies namely SIMO, MISO and MIMO uses multipaths
for increasing data rate, throughput and reliability. Multiple paths are used
by multiple transmit antenna and multiple receiver antenna.
Multiple antennas at one end either
at transmitter or at the receiver were in use long ago. The then use of
multiple antennas aimed at beam forming and spatial diversity, which are mainly
used to increase the signal to noise
ratio. The improved signal to noise
ratio decreases the bit-error rate.
The use of multiple antennas adds
the new dimension to digital communication technology which forms the basis of
3G and 4G. The natural dimension of digital technology is time. Added with
that, MIMO offers a new space time axis to digital technology. MIMO is
therefore termed as ‘space time wirelesses or ‘smart antenna’.
Digital MIMO is called volume to
volume wireless links as it offers parallel bit pipes between transmitter and
the receiver.
Why MIMO?:
MIMO technology promises higher data
rate, higher quality of service and better reliability by exploiting antenna
array at both the transmitter and the receiver. Signals at both sides (transmitter
and receiver) are mixed such that they either generate multiple parallel,
spatial bit pipes and /or add diversity to decrease the bit-error rate.
Diversity helps in selecting the
clearest signal out of many signals, resulting in lower bit-error rate. Multiple
bit pipes effectively increases the data rate (quantitative improvement),
whereas the reduced bit-error rate improve the quality of service, throughput
and reliability (qualitative improvement).
The fundamental gain in MIMO is
increased data rate. Why not use more
bandwidth or complex modulation scheme to
increase the data rate? The use of more bandwidth depends upon the
availability of spectrum and again the use may be difficult to meet the
spectral efficiency. All wireless devices use a particular part of radio
spectrum. Air traffic radar, for example, operates between 960 and 1215
megahertz and cellphone between 824 to 849 megahertz. As growing number of wireless
devices enter the consumer market, the spectrum becomes congested every year.
MIMO has potential to expand radio capacity and relieve the burden on existing
bandwidth.
By spreading the transmitted signal
over the multiple paths, the MIMO technology increases the chances of signal
reception at receiver. It also increases the range of operation.
Multipath fading causes the distortion
by scrambling the copy of the signals reaching the receiver via multiple paths
on bouncing of the objects. Then how does
the multipath signals work in MIMO?
Proper algorithms are used at both the transmitter and receiver to analyses the
signal received from different path and different antenna of array.
Proper spacing of antenna and signal
analysis via a matrix manipulation technology that cross-correlate the signals are
the requirement of MIMO technology.
Conclusion:
MIMO technology will prove basic
building block of total wireless communication. The problems of multipath
fading will fade away and uninterrupted service will be available to one and
all.
Future wireless technologies will
provide unique and single identification globally. In future this number will
be unique like our names.
With on going R&D in MIMO
technology is exploring way towards our fantasies.
If MIMO technology
stands on it promises that day is not far when the James Bond style video
conversation will be reality?
Reference:
- my oh MIMO, http:\\www.networkworld.com.
- www.ece.utexas.edu.
- www.engineer.ucla.edu.
- MIMO technology: future wireless, EFY,vol: 38 No.1, January 2006.
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