Link budget
A link budget is an accounting of all of the gains and losses
from the transmitter, through the medium (free space, cable, waveguide,
fiber, etc.) to the receiver .
It accounts for the attenuation of
the transmitted signal due to propagation, as well as the antenna gains and other losses. Randomly varying channel gains such as fading
are taken into account by adding some margin depending on the
anticipated severity of its effects. The amount of margin required can
be reduced by the use of mitigating techniques such as antenna diversity or frequency hopping.
A simple link budget equation looks like this:
- Received Power (dB) = Transmitted Power (dB) + Gains (dB) − Losses (dB)
A link budget equation including all these effects, expressed logarithmically, might look like this:
where:
- = received power (dBm)
- = transmitter output power (dBm)
- = transmitter antenna gain (dBi)
- = transmitter losses (coax, connectors...) (dB)
- = path loss, usually free space loss (dB)
- = miscellaneous losses (fading margin, body loss, polarization mismatch, other losses...) (dB)
- = receiver antenna gain (dBi)
- = receiver losses (coax, connectors...) (dB)
The loss due to propagation between the transmitting and receiving
antennas, often called the path loss, can be written in dimensionless
form by normalizing the distance to the wavelength:
- (where distance and wavelength are in the same units)
When substituted into the link budget equation above, the result is the logarithmic form of the Friis transmission equation.
In some cases, it is convenient to consider the loss due to
distance and wavelength separately, but in that case, it is important to
keep track of which units are being used, as each choice involves a
differing constant offset. Some examples are provided below.
- (dB) = 32.45 dB + 20×log[frequency(MHz)] + 20×log[distance(km)] [1]
- (dB) = - 27.55 dB + 20×log[frequency(MHz)] + 20×log[distance(m)]
- (dB) = 36.6 dB + 20×log[frequency(MHz)] + 20×log[distance(miles)]
These alternative forms can be derived by substituting wavelength
with the ratio of propagation velocity (c, approximately 3×10^8 m/s)
divided by frequency, and by inserting the proper conversion factors
between km or miles and meters, and between MHz and (1/sec).
receiver sensitivity ودي اقل باور يستقبلها ال receiver علشان يقدر يرجع ال Signal لل baseband form تاني باقل bit error rate
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free space loss وده الفقد الناتج عن أن الاشاره مشيت مسافه معينه ودي تعتبر بتتغير علي حسب حاجتين مهمين وهم distance , frequency
FSL = 36.6 + 10 log frequency + 10 log distance
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antenna Gain وخد بالك ان الانتينا تعتبر passive Element يعني المفروض مش بتدي Gain ولكن لما نقول أن ال dish Antenna ليه Gain حوالي 18 dbi ده معناه أن عندها القدره علي توجيه الباور اللي داخلها كله في اتجاه معين اكبر من قدره ال istropic antenna بقيمه 18 . اذن ال Gain بيتترجم الي directivity
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feeder loss وده الفقد اللي بيضيفه الكابل اللي شايل اشاره الميكرويف من الكابينه تحت لحد ال dish فوق . وال loss ده بيتقاس بال DB و بيكون علي حسب نوع ال feeder وال cross section بتاعه وال distance طبعا
يعني مثلا 1 Db لكل 100 meter . ده مثال
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gas attenuation ودي بتتضاف في المعادله بي بال negative لانها loss وليست Gain ودي بتعبر عن قيمه امتصاص ال gases الموجوده في ال atmosphere لل Signal ودي بتتغير بتغير ال frequency
يعني مثلا بنحاول نتجنب دايما ال 24 - 25 GHz لان فيه Gas attenuation عالي
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rain attenuation ودي مهم جدا خصوصا في ال coastal area وطبعا ال width بتاعه ال rain drop بتفرق وكمان نوع ال polarization اللي بتبعت بيه هيفرق جدا . علشان كده لو في area فيها rain rate عالي بنستخدم معاها vertical علشان ال rain ليحصلها spread في ال horizontal plane وبتكون effective اوي فيه
