case study

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.

𝐹𝑆𝑃𝐿(𝑑𝐵) = 10 𝑙𝑜𝑔10 ( 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

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(a) Calculate the FSPL in dB for a distance of 5Km and 10 Km for two different frequencies 5.8GHz,

2.4GHz and construct a table as follows:

FSPL(dB)

Distance(Km) 2.4GHz 5.8GHz

5

10

(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: 5 km

Frequency: 5.8GHz

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 5Km 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