Mastering Wireless Link Surveys

Building reliable, high-speed wireless links is the cornerstone of delivering exceptional service to customers. This involves meticulous planning and precise execution, starting with a thorough wireless link survey. This guide will delve into the critical aspects of conducting a wireless link survey, focusing on line of sight (LoS), Fresnel zones, distance calculation, Backhaul Design Indicator (BDI), antenna selection, and the mathematics behind signal strength and potential throughput. We will use MikroTik NetMetal 5 GHz radios as our example hardware to illustrate these concepts.

Understanding Line of Sight and Fresnel Zones

Line of Sight (LoS)

LoS is the direct visual path between the two endpoints of a wireless link. For a successful connection, it's essential to ensure that there are no physical obstructions between these points. Trees, buildings, and even geographical features can impede wireless signals, making LoS a critical first step in surveying for a wireless link.

Fresnel Zones

While LoS provides a visual path, Fresnel zones account for the electromagnetic propagation of the signal. These zones are elliptical areas around the LoS, where the signal spreads out and then reconverges. For optimal signal strength, at least 60% of the first Fresnel zone should be clear of obstructions, with 80% clearance being ideal.

Calculating Distance and BDI

The distance between the two points of your wireless link is a straightforward measurement, but its implications on the choice of equipment and configurations are profound. Backhaul Design Indicator (BDI) is a metric that takes into account the distance, desired throughput, and signal strength to help determine the feasibility and design of the link.

Antenna Selection

Choosing the right antenna is pivotal. Larger antennas typically offer more gain, which can be beneficial for longer distances but may be overkill for shorter links, leading to unnecessary expense and complexity. The antenna size should match the requirements dictated by the distance, desired throughput, and the specific characteristics of the area (e.g., urban vs. rural).

The Mathematics of Signal Strength and Throughput

To calculate signal strength and potential throughput, we consider several factors, including Received Signal Strength Indicator (RSSI) and the sensitivity of the receiving radio.

Example Calculation

Assume we have the following values for our MikroTik NetMetal 5 GHz radios:

• Transmit power: 30 dBm
• Antenna gain: 24 dBi (both transmitting and receiving antennas)
• Frequency: 5 GHz
• Distance: 10 km
• RSSI: -75 dBm
• Receiver sensitivity: -80 dBm for a 20 MHz channel at MCS7

Calculating Free Space Path Loss (FSPL)

FSPL is a fundamental formula to estimate signal loss over a distance in a free space environment:

Where:

• is the distance in meters,
• is the frequency in Hz,
• is the speed of light .

Substituting our values:

Determining Effective Signal Strength

To find the effective signal strength at the receiver, we consider the transmit power, antenna gains, and subtract the FSPL:

This effective signal strength is well above the receiver sensitivity of -80 dBm, indicating a strong and viable link.

Potential Throughput

The potential throughput can be estimated based on the modulation and coding scheme (MCS) index achievable at the calculated RSSI. Given our RSSI of -75 dBm, which is significantly better than the sensitivity threshold, we can operate at MCS7, allowing for a maximum theoretical throughput of up to 150 Mbps in a 20 MHz channel.

Conclusion

Conducting a wireless link survey for WISPs involves a comprehensive understanding of various technical aspects, including

LoS, Fresnel zones, distance, BDI, and antenna selection. By applying the mathematical principles behind signal strength and throughput calculations, WISPs can make informed decisions on the design and implementation of their wireless links. Utilizing MikroTik NetMetal 5 GHz radios as an example, we've demonstrated how to approach these calculations, ensuring that your wireless links are both efficient and reliable. Remember, each wireless link is unique, and factors such as environmental conditions, hardware specifications, and network requirements must all be considered to achieve optimal performance.