Case Scenario Problem: Designing a High speed Wireless Data Link - Line of Sight Link Budget Analysis
Several major factors that can impact the performance of a radio system are
a) Available/permitted output power,
b) Bandwidth,
c) Receiver sensitivity,
d) Antenna gains
e) Environmental conditions
In this case study, the students will be required to calculate the link budget for a LOS Wireless link. Read through the following to understand some of the aspects and elements that need to be considered when calculating a Link Budget.
Received Power:
If the estimated/calculated received power is sufficiently large (relative to receiver sensitivity), the link budget is sufficient for sending data.
Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) - Losses (dB)
Gains(dB) = Transmit Antenna gain+ Receiver Antenna Gain
Receiver sensitivity is the lowest power level at which the receiver can detect an RF signal and demodulate data.
Link Margin/Fade Margin:
The amount by which the received power exceeds receiver sensitivity is called the link margin/fade margin.
In wireless systems, multipath propagation results in multiple copies of a signal to arrive at different signal phases at the receiver. If these signals add up destructively, the resulting signal power can be lower by a factor of 100 or 1000 (20 or 30 dB). The signal level relative to the noise declines making signal detection at the receiver more difficult. It is therefore highly recommended to keep a link margin of 30 dB when designing a wireless system.
Link Margin = Received Power - Receiver sensitivity
Losses:
In a line-of-sight wireless system, losses are mainly due to free-space path loss (FSPL). Other losses are due to antenna cabling and connectors. Generally, 0.25dB loss per connector and 0.25dB loss for every 3-ft of antenna cable should be included in the link budget calculations. However, for the sake of simplicity, you can neglect these losses for your calculations.
FSPL(dB) = 10log10(4πdf/c)2
In dB's the path loss when the distance (d is in km) and the frequency (f is in MHz) can be simplified as follows:
FPSL(dB) = 20log10(d) + 20log10(f) + 32.45
Question 1: Calculate the FSPL in dB for a distance of 8Km and 16 Km for two different frequencies 3.6GHz, 4.2GHz and construct a table as follows:
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FSPL (dB)
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Distance (Km)
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3.6 GHz
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4.2 GHz
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8
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|
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16
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|
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Question 2: For the information given below, calculate the link budget analysis for the wireless system, which will involve calculating the following:
- Received Power
- Link Margin
Distance: 8 km
Frequency: 4.2 GHz
Link Type: Point-to-Point, Line-of-Sight
Tx power: +23dBm
Antenna gain is 24dBi
Assume negligible loss for cabling and connectors
Receiver Sensitivity: -72dBm
Assume that we are using the same antennas at the transmitter and the receiver side, the link budget calculations will be the same as the link budget in both directions is expected to be symmetrical. In cases where 2 types of antenna systems with different transmit power are used, we must perform two link budget analysis, one in each direction.
Question 3: For the calculated Link Margin at 8Km what can you say about the availability/reliability of the link based on the following Table which shows the relationship between the available link margin and link availability as a percentage of time.
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Time Availability (Percentage)
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Link/Fade Margin
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90
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8
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99
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18
|
|
99.9
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28
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99.99
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38
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99.999
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48
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