Sensitivity Of Reciever

SENSITIVITY

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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.