OFDM - Muliple path mitigation capability

MULTIPATH MITIGATION CAPABILITY OF OFDM

OFDM Stands for Orthogonal Frequency Division Multiplexing . High spectrum efficiency of OFDM is already explained in earlier blogs ( Please refer to blog Wi-Max Technology).

Here I will explain about the multipath mitigation capability of OFDM.

In OFDM the frequency band available is multiplexed to multiple sub-bands.

And the data is transmitted in parallel over the multiple sub-bands for the longer period of time.

Now when the data is being transmitted serially  ( Non – OFDM ) and it is taking  multiple paths ( 2 paths in this example ) one is direct path and through reflection from any reflecting surface.

Lets say we have to transmit  1 0 1 0  and each bit will be transmitted for ta period of time now initially the transmitter will transmit bit 0 for ta period of time the bit 0 will be received in minimum time from direct path by the time 0 is received from shortest path the it will still be on the way on longer path after time ta collapses transmitter will start transmit bit 1 now 1 will be received by the receiver in shortest time from shortest path and it will also receive earlier bit 0 which was travelling through longer path so here the interference has occurred.

Now when the data is transmitted parallel ( OFDM ) The data which we were earlier transmitting serially will transmitted parallel and each bit will be transmitted for the longer period of time here the time period of each bit will become 4ta .

In this since each data stream is transmitted for a longer period of time so even if the time ta collapses same data will be travelling through the direct path and reflective path so interference will not occur.

Dual Polarized Antenna

DUAL POLARIZED ANTENNA

Antenna is the permanently attached part of wireless communication. Antenna is a transducing element that converts electrical energy to electromagnetic waves energy and vice versa . Antenna is used both for transmitting and receiving the electromagnetic waves.

What is dual polarized antenna?

Dual polarized antenna can be understood in one simple sentence , but here I will tell few thing things about polarization so that I can write the dual polarized antenna in one simple sentence.

Consider a 3D coordinate system X,Yand Z,

X axis is the direction perpendicular (Vertical) to the surface of the earth .

Y axis is the direction parallel (Horizontal) to the surface of the earth.

Z axis is the direction of wave propagation, perpendicular to both X and Y.

HORIZONTAL POLARIZATION :  The electromagnetic wave propagating in Z direction with electric field vector only in Y axis.

VERTICAL POLARIZATION :  The electromagnetic wave propagating in Z direction with electric field vector only in X axis.

Horizontal polarization and vertical polarization comes under the category of LINEAR POLARIZATION , Electric vector only in one direction

ELLIPTICAL POLARIZATION :  The electric vector of wave rotates as a function of time . Elliptically polarized wave is considered as the resultant of two linearly polarized wave(one horizontal polarized and another vertical polarized) of same frequency with different electric field intensity , And the wave seems to progress in z direction with elliptical rotation in X and Y axis.

CIRCULAR POLARIZATION :  Circular polarization is a special case of elliptical polarization where intensity of electric field component in X and Y axis are equal.

DUAL POLARIZED ANTENNA : Dual polarized antenna is combination of two antennas with one antenna at vertical polarization and another at horizontal polarization .
DUAL POLARIZED ANTENNA is used in MIMO technology.

EARTH CO-ORDINATE SYSTEM

CO-ORDINATES ON EARTH

Earth is in somewhat spherical in shape . North (N) , South (S), East (E) and West (W) are the four major direction , other directions lie in between these direction . North and South are the two poles , these are axis at which earth rotates . ( Please read the blog how to use campass to know about direction finding ).

Here I am telling you about the coordinate convention.

North and South are two poles .

First talk about the latitude . N and S are latitudes.

Consider the earth in two parts from equator.

Consider for N , at equator N=0⁰ , Now consider the horizontal parallel lines that moves towards the north pole, at exact north pole N=90^o.

Consider for S , at equator S=0⁰, Now consider the horizontal parallel lines that moves towards the south pole at eaxact south pole E=90⁰.

Conclusion is latitude varies from 0⁰  to 90⁰

Now discuss about the Longitude . E and W are the longitudes.

Now Consider earth in two parts from prime meridian and antemeridian.

E=W=0 at prime and E goes on increasing as move towards East direction and W goes on increasing as goes on moving towards the west direction and again E=W=180⁰ .

Conclusion drawn from the above statement is that longitude varies from 0⁰ to 180⁰ i.e E vaies from 0⁰ to 180⁰ and W varies from 0⁰  to 180 ^0.

latitude is N , S and Longitude can be E,W .

LATITUDE	LONGITUDE
N (00 to 900)	E (00 to 1800)
N (00 to 900)	W (00 to 1800)
S (00 to 900)	E (00 to 1800)
S (00 to 900)	W (00 to 1800)

Combined information from latitude and longitude will give the exact information in 3D. Lat – Long at particular location can be taken by the GPS ( See GPS in blogs for understanding GPS Working ).

^{We are what we repeatedly do. Excellence, therefore, is not an act but a habit.}

How GPS works ?

GPS stands for Global Positioning System , GPS is constellation of 29 satellites placed in 6 planes at inclination of 55 degrees from earth’s equator . Each satellite is equipped with the highly precise clock i.e atomic clock , Atomic clock are extremely necessary for synchronization of each satellite . The satellites are placed such that minimum of 4 satellites are visible at any time from any place of the surface of earth . The satellites are placed at 20180 km away from the earth surface and the time taken by one satellite to complete one revolution of earth is 11 hrs and 58 minutes.

GPS operates at the frequency of 1575.42 Mhz.

GPS was developed by US army in 1970s for defence purpose only and later on it was made commercial, Nowadays it a common device used for the purpose of navigation,site surveys,path finding and many other purposes.

GPS that we carry in our hand is a GPS receiver , That shows our location,our speed and direction in which we are moving  by receiving signals from minimum of 4 satellites.

How GPS works ?  

My motive has always been is to explain my understandings in simple words. For understanding the working of GPS you have to be little bit imaginative.

You can take Paper and pencil in your hand for better understanding.

Let us consider that a person with GPS is standing at position A on the surface of earth , GPS receives signal from first satellite , the signals are electromagnetic waves which travels at the speed of light i.e 3*10­⁸  meter per second , let the distance be ‘p’ (Distance is calculated by the time taken by the signal to travel from satellite to GPS )  but it will not be known in which direction and where the GPS is , so probable  position for GPS receiver according to the satellite will be any position at the surface of imaginary sphere with radius ‘p’ and satellite at its centre .

Draw a circle (In 3 D it will be sphere ) as Earth , consider a position of GPS on earth and consider a satellite  A (mark it as point A ) on Space , Now draw a Circle ( In 3 D it will be sphere ) with satellite A as centre and distance between GPS and satellite A as radius .

Now GPS  receives the signal from another satellite , Let the distance be ‘q’ the , the position of satellite according to second satellite will be any position at surface of imaginary sphere with radius ‘q’ and satellite ‘B’ at the centre of sphere .

But with the combined information from satellite A and Satellite B the probable position for GPS is the surface where sphere x and sphere y intersects , the two imaginary spheres at an exact imaginary circle , let us call the circle as X . Now we have reached a closer approximation.

In the diagram drawn earlier consider an another satellite , satellite B at space , Draw another circle with satellite B as centre and distance between satellite B and GPS as radius , the two circles drawn with satellites as circle will intersect at 2 points on paper but in 3D it will intersect to form an another imaginary circle . so now keep in mind the only the intersecting imaginary circle of two imaginary spheres.

Now GPS receives the signal from third satellite , Let the distance be ‘r’  , the probable position of GPS according to third satellite will be a surface of sphere with radius ‘r’ and  satellite C at the surface centre of sphere . Now the imaginary sphere with radius ‘r’ will intersect the imaginary X at two points one at the surface of earth and another at space , let the intersection point at surface of earh be ‘N’ and Intersection point at space be ‘M’ , since GPS receiver will be place at the surface of the earth so position ‘M’ will be ignored and the only possibility left will be position ‘N’.

In the diagram drawn earlier consider an another satellite , satellite C at space , Draw another circle with satellite C as centre and distance between satellite C and GPS as radius , The new circle drawn will intersect the imaginary circle discussed above at two points , Consider only the position on earth for GPS location.

Combined Information from three satellite will provide the position on earth in 3 dimension , 4^th satellite will give the more precise information about the AMSL (Above Mean Sea Level )

With combined information from 4 satellites the Lat , Long and AMSL at the point where GPS is placed is known .

Now GPS receiver can show you in which direction you are moving and can show you the speed at which you are moving.

GPS calculates the direction by computing the lat-long at every point  and speed by computing change of lat-long with time at every point.

As satellites are equipped with the atomic clocks the GPS receiver are widely used at Wimax BTS’s working on TDD for synchronization of uplink and downlink stream of each base station to avoid interference.

Akela he chala tha janib manzil , Magar log saath aatey gae aur karwan banta gaya ..

MIMO

MIMO

MIMO is multiple input multiple output, MIMO is a technology that is most commonlhy used these days in Wi-Fi and Wi-max technologies (refer to wimax technology in the blog) .

In MIMO multiple antenna are used at transmitter and receiver end. MIMO takes the advantage of multipath transmission.

MIMO can effectively enhance the data rate , range and reliability of the link by sending same data on multiple stream or by sending multiple data on multiple stream at same frequency .

Most of the wireless devices these days have the feature of adaptive switching between MIMO A and MIMO B  ( MIMO A and MIMO B will be discussed at the end of article).

In earlier SISO technology the signals arriving through different paths may cause distructive interference or multipath interference so the efforts was made to receive the signals arriving from direct LOS only and suppress the signals arriving from other paths , which is nothing but the wastage of power.

MIMO technology has effectively utilized the multipath signal reception for its advantage with minimum power wastage.

MIMO with OFDM has made the Wi-Fi and wimax technologies highly spectrum efficient these days.

MIMO 2*2  means 2 antennas at receiver and 2 antennas at transmitter , it increases the spectrum efficiency by two times.

Max channel capacity in MIMO can be defined by  

Channel capacity   = B log _{2 ­}(1+ M*N*SNR)

 Here

B is channel bandwidth.

M is number of antennas at transmitter

N is number of antennas at receiver.

SNR is signal to noise ratio in the link.

Channel capacity is maximum amount of data that can be carried by the cahannel.

MIMO A :  MIMOA comes in use when the overall SNR is low , It tries to maintain the link reliability and coverage by sending same data on multiple antennas.

MIMO B :  MIMO B comes in use when the overall signal strength is high , It increases the capacity of the link bt transmitting different data on multiple antennas.







                                              


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I am thankful to all my readers for viewing my blogs. I have started this journey to share  the knowledge I have gained from my professional experience and academic studies . End of journey is not yet decided .

Any suggestions and queries from readers will be highly appreciated.

Use Of Compass

USE OF COMPASS TO SEE THE DIRECTIONS

Compass is a very basic instrument to see the direction to align the directional antennas in the required directions .

How to Use Compass ?

All compasses comes with two needle always pointed in opposite directions. One of the needle is mostly red coloured which always points towards the north direction.

By default north is always taken as reference point  it is 0˚ , east is 90˚ south is 180˚ and west is 270˚.

If we say that we have to align the antenna at 70˚ we mean it 70˚ from north unless it is specified .

To see the proper azimuth , look at red needle of the compass , synchronize it with the 0˚ mark in the compass as red needle always points towards the  0˚ which is north , now look at the degrees marked in the compass , if for eg. It is 70˚ make a small straight line in that direction on ground , and look in the direction of this line , it is the direction 70˚ from north.

In point to point communication if the azimuth from 1^st location is  A˚ ( A<180) then azimuth from 2^nd location will be B ( B= A+ 180) . If the azimuth from 1^st location is C˚ (C>180) then azimuth from 2^nd location will be D (D=C-180) .

Eg. 

If Azimuth from 1^st location is 70˚  then the azimuth from the 2^nd location will be ( 180 + 70) = 250˚.

If Azimuth from 1^st location is 200˚ then the azimuth from the 2^nd location will be (200-180)= 20˚.




Compass must be kept away from magnet and heavy iron materials while doing observations.

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Refraction In Radio Communication

REFRACTION IN WIRELESS COMMUNICATION

Refraction is bending of signals as it passes from one medium to another . Refraction causes degradation in signal strength and sometimes loss of communication in Point to Point links.

Loss of communication in P2P link occurs in long distance links when there in change in atmospheric refractive index which causes signals to divert from original path and communication does not take place.

Refraction also occurs when some non reflective medium comes as obstruction the signals refract into the medium but in this case when the signals comes out after penetrating the medium the signal follows the path parallel to original but with signal power degraded.

Not to be confused with reflection , In reflection the signal after striking the surface of another medium return back to the same medium and in refraction the signal penetrates into another medium with diversion from original path of signals.

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Reflection in Radio Waves

REFLECTION IN RADIO WAVES

Reflection is the phenomenon in which the rays after striking the reflecting surface return back to the same medium. The law of reflection says that in reflection the angle at which wave is incident on surface equals the angle in which it is reflected.

Reflection can have advantage and disadvantage in radio communication.

In first case if the LOS is obstructed by the reflecting surface ,the EM waves will reflect and will not travel from transmitter to receiver and communication will not take place, or if there is any reflecting surface with in the 60% of  Fresnel zone the reflected rays will cause multipath interference.

In second case if the LOS is not clear , Face of both the antennas in P2P link can be faced towards the another reflecting surface which are not in the LOS path , There will be some loss in signal but the communication can take place. It has same application in mobile communication and TV transmitters and receivers

LOS , Near LOS and Non LOS

LOS , Near LOS and Non LOS

LOS  :   LOS means line of sight , line of sight in radio communication refers to the condition where minimum 60% of first Fresnel zone (refer to Fresnel zone in blog ) is clear, In modern radio communication systems operating above the 2.4 Ghz frequency requires clear LOS. 

Near LOS  :   Near Los means near line of sight , it refers to the condition in radio communication where a straight line from Site A to Site B is unobstructed but 60% of first Fresnel zone is not clear .

Wimax technology (refer Wimax Technology in blog) if developed to combat the interference occurred due to the near LOS reflection.

Non LOS  :   Non LOS means non line of sight , it refers to the condition where Fresnel zone is completely obstructed . this condition is avoided in modern microwave radio communications.

Non LOS communication can take place in low frequencies, nowadays very less in commercial use because of throughput requirement .

Sensitivity Of Reciever

SENSITIVITY

.

Sensitivity is the minimum receiving power required by the receiver to establish a reliable link having a minimum specified SNR. Signals below the sensitivity level will not establish the radio link and the noise above sensitivity level will cause interference with the usable signals and may cause fluctuation in usable signals.

If we say that the receiver sensitivity is -80dbm it means that the minimum signal strength required for link establishment is -80dbm , but at -80dbm fade margin will be zero so link will not be reliable for any fluctuation , for link to remain established the signal strength should not fall below -80dbm.

Fade Margin

FADE MARGIN

Fade margin is the amount by which the RSSI can be reduced without causing system performance to fall below a specified threshold value .

In easy words fade margin can be expressed as the RSSI above the sensitivity of the receiver.

e.g  

If the sensitivity of receiver is -80 dbm and the received signal is -65 dbm then the fade margin will be 15 dbm .

From above example we can say that greater the fade margin , more reliable the link.

RSSI ( Recieved Signal Strength Indicator )

RSSI

RSSI stands for Received Signal Strength Indicator .

RSSI is the power received by the receiver after all gains and losses from transmitter to receiver.

RSSI can be expressed in dbm , milliwatts or watts  but In modern P2P links the received power calculation is done in dbm and its value is in negative

RSSI (dbm) = 10 log ₁₀ ( P mw).

RSSI has a great significance in all radio technologies. It is recommended that the value of RSSI should not exceed to a specified upper level for safe operation and long life of the radio device and value of RSSI should not be lower than the specified lower limit value for reliable link establishment.

In modern radio link technologies the RSSI should not exceed -35 dbm and should not be less that -65 dbm.

P2P LOS Survey Procedure

PROCEDURE OF DOING P2P LOS SURVEY

LOS survey must for any radio link operating on or above the UHF band. Here I will discuss about the procedure of doing the LOS survey.

Assets Required for LOS surveys are

• GPS
• Compass
• Binocular 

 

PROCEDURE :

    1.   Go to 1^st location and note the Lat – Long and AMSL at  tower position and save it on GPS.

    2.   Go to 2^nd location and note the Lat- Long and AMSL at tower position and save it on GPS.

    3.   Make note that if the cable used is CAT5 , Max length supported will be 100 mtr .

    4.   Calculate the azimuth angle between the two locations and air distance from the taken Lat – Long of both the locations.

    5.   Now move from 2^nd location to 1^st location with route tracing using GPS.

    6.   Note the Lat- Long and AMSL of the higher obstacles such as natural terrains , buildings , flyover or any other obstacles coming in the line of sight .   
7. Note the height of obstacles.
8. Map the Lat-Long on google earth , observe the AMSL at every point on the LOS path.
9.  Now prepare the survey report and suggest the minimum height required for antenna mounting at both the locations with 60% of first Fresnel zone clear at every point on the LOS path. .

Fresnel Zone

FRESNEL ZONE

Fresnel zone is the primary factor while doing the LOS surveys for radio communication links.

After calculating the 1st Fresnel zone 60% of the radius is calculated which must be clear for reliable radio communication.

Fresnel Zone is calculated by the formula 

                                                 F­_n =  (( nλd₁d₂ / (d₁ + d₂ ))^1/2

Here n is the nth Fresnel zone

F­­_{n  ­} is the radius of nth Fresnel zone.

λ is the wavelength of frequency used.

d 1 and d2 are the distance from transmitter and receiver in the path LOS between transmitter and receiver.

For LOS communication only 60 % of 1^st Fresnel zone calculated  and rest are just ignored .

If there is any obstruction in the 60% of the first Fresnel zone the radio wave reflecting from the obstacle may arrive at out of phase and hence may cause the interference .

Free Space Path loss

FREE SPACE PATH LOSS

Free space path loss is the degradation of signal strength of electromagnetic wave due to LOS (Line Of Sight) path in  free space.

Free space path loss (FSPL) is strictly dependent only on distance and frequency.

FSPL = (4πd/λ)²

To clear the concept FSPL does not at all depend on the antenna gain , transmit power , receiver sensitivity or any other hardware imperfections.

Free space loss is calculated considering ideal atmospheric conditions with no obstacles in between to cause reflection, refraction or diffraction.

Wi-max technology

OFDM stands for Orthogonal Frequency Division Multiplexing .

OFDM technique is utilized in Wimax 802.16d and Wifi .

OFDM is a frequency division multiplexing technique with each career being orthogonal to each other .

In conventional FDM (frequency Division Multiplexing ) guard bands are required to prevent the interference so the spectrum is not fully utilized  ,and in OFDM as the signals are orthogonal with each other, no guard band is required and spectrum is fully utilized.

Other than efficient spectrum utilization, OFDM is used as a effective tool to overcome the multipath fading  , in this the sequential data is transferred  in parallel over a longer period of time ,so even if the data is received by the receiver directly and same data  with some reflection,the data over the certain period of time will be same and so data recieved will not be corrupted due to multipath reflection.

Conclusion : Advantage of OFDM is its spectrum efficiency and mutipath mitigation capability.