MITS5003 Wireless Networks And Communication 2
Question:
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
- Available/permitted output power,
- Bandwidth,
- Receiver sensitivity,
- Antenna gains
- 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.
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: 
(a) 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:
|
|
FSPL(dB) |
|
|
Distance(Km) |
3.6GHz |
4.2GHz |
|
8 |
|
|
|
16 |
|
|
(b) 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.
(c) 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.
|
Time Availability (Percentage) |
Link/Fade Margin |
|
90 |
8 |
|
99 |
18 |
|
99.9 |
28 |
|
99.99 |
38 |
|
99.999 |
48 |
Answer:
Link budget for a LOS Wireless link
Q1
From this case study, it is identified that FSPL completely depends on the frequency of signal and distance among the transmitter and receiver [1]. In order to find FSPL the below equation can be used which includes distance in km and frequency in MHz:
From this question, it is found that there are two distances and frequencies for which the student needs to calculate FSPL in dB.
Distances= 8 km and 16 km, Frequencies= 3.6 GHz and 4.2 GHz
- D= 8 km and f= 3.6 GHz
- D= 8 km and f= 4.2 GHz
FSPL (dB) = 122.970 dB
- D= 16 km and f= 3.6 GHz
FSPL (dB)= 127.650 dB
- D= 16 km and f= 4.2 GHz
FSPL (dB)= 128.990 dB
2.
Part a: Received power
It is argued that received power is depended on conveyed power, gains and losses occurred in the system. The below table includes values of each element that help for finding link budget:
Now, total loss at 8 km distance is 122.97 dB which can be used for finding received power in the system.
From this question, it is found that both Tx and Rx antennas are the same that means the total gain is 2*24= 48.
So, received power in dB= -51.97 dB
Part b: Link margin
Or, link margin (dB)= 20.030 dB
3
The below table designates the relative among time obtainability and link margin which may help for finding the reliability of the networks:
It is argued that the value of link margin depends on the conventional power and headset sensitivity so, by managing received power and path loss. It is found that at 8 km distance total link margin is 20.03 dB that shows that the availability of data is around 99% which is better for performing link budget in wireless communication [2]. It is observed that in footings of availability or reliability the developed system is more appropriate and the adopted link margin is sufficient for a line of sight communication.
References
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