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Optimizing the Link Distance

distance between antenna and drone using embedded broadband router

One of the most frequent questions we get is “how far can your radios go?” This is not an easy question to answer, as there are many factors that go into this. We provide indicative range charts in each model’s datasheet. For more detailed link design, use of Link Budget Calculator is required. Here is a link to one of the many free online calculators.

This application note discusses methods to optimize the link performance of the Smart Radio. Our intention is not to go into detail on any of the topics, but to provide some useful insights which can help during the design of a RF link.

Our general recommendation is to design a link with about 10-15 dB fade margin to account for changes in the environmental conditions.  Use the highest gain antenna possible within the physical constraints of the system. The objective is to operate the link with the highest RSSI possible (Sweet spot is around -65 dB RSSI). This will result in the link operating at the highest modulation. provide the best performance and reliability.

Choosing the Operating Frequency

Operating frequency is one of the major factors in determining range. It requires careful trade-offs in the system design to achieve maximum range.

Lower frequencies have lower transmission losses, higher penetration and allow for longer-distance communication, which means that lower gain (i.e. smaller size) antennas may be utilized.  At lower frequencies, the Fresnel zones are larger. For drone operation, Fresnel zone obstruction is not a consideration since the communication link is pointing up. For this reason, many UAV manufacturers utilize the unlicensed ISM band 900 MHz Smart Radio.  

On the other hand, higher frequencies have higher transmission loss and hence reduced operating range. However, the antennas are smaller size and less bulky. Which means that a high-gain antenna will not be very bulky. For short range drones, it is advantageous to use higher frequencies. At higher frequencies, the Fresnel zones are smaller and signal penetration is lower.  So for AGV/UGV use cases that have direct Line of Sight, it is advantageous to use higher frequency to benefit from smaller Fresnel zones.

Hence, a careful balance of the required range, operating frequency, and antenna configuration must be made.

RF Power

Higher RF power makes the signals go farther. The Xtreme category of Smart Radios feature the maximum RF power (30 to 33 dBm, frequency dependent). The Pro category of Smart Radio feature 27 dBm of RF power. Hence the range will be about 75% to 50% that of the Xtreme models. 

Antenna Gain and Directivity

Antennas passive amplifiers of the RF signals. There are many variables that go into the proper selection of antennas. Visit our App Note – Antenna Selection and Overview.

We recommend to use the highest gain antenna that can be accommodated by your system design.

Fade Margin

Fade margin is the difference between the strength of the received signal at the antenna port and the minimum receive sensitivity for signal strength for reliable operation. The higher the fade margin, the more reliable the link will be. Smart Radio receive sensitivities are listed on each model’s datasheet. Maintaining a sufficient fade margin will create a buffer to account for natural degradation of the signal strength during operation. A fade margin of 10-15 dBm is a conservative value to target.

Our recommendation is to design a link with about 10-15 dB fade margin.

Channel Bandwidth  

 A wider channel bandwidth offers higher data throughput but lower Rx sensitivity. Wider bandwidths may interfere with other nodes operating in the area. So wider bandwidths are useful for high throughput, short range applications in low density environments.

Smaller bandwidth increases the SNR and hence, better Rx sensitivity. When the bandwidth is reduced from 20 MHz to 5 MHz, it provides 6 dB better Rx sensitivity. This translates into 2x longer range. However, the data throughput is reduced by 4x. Smaller channels give ability to choose from more channels to avoid interfering with other nodes in the area. So smaller bandwidth is useful for low throughput, long range applications operating in high density environments.

The Smart Radio channel bandwidth can be software configured between 3-40 MHz (10-40 MHz for Pro models). This helps accommodate a wide range of use cases and operating environments.

Overall, we believe wider channels offer better immunity to intermittent sources of interference as the data is transmitted at higher modulation rates and requires shorter airtime. In this case, the data packets have better probability of success in the presence of intermittent noise spikes.  

Our recommendation is to operate the radio at the widest channel bandwidth as allowed by link budget requirements and density of other nodes in the environment.

MIMO, and Antenna Diversity

Smart Radios employ 2×2 MIMO technology and require two antennas for MIMO operation. Having two antennas allows the radios to use either spatial-multiplexing to improve the data rate or spatial-diversity to improve the reliability of the link.

In spatial-multiplexing MIMO operation, two different streams of data are sent to the two antennas resulting in double the throughput.

In spatial-diversity, the same stream of data is sent redundantly over both antennas and combined at the receiver in an intelligent way to optimize the link quality and SNR. For mobile applications, where the orientation of systems constantly changes, it may be beneficial to mount one antenna horizontally and the other vertically to get the most diversity in the polarization.  

Doodle Labs recommends using two antennas. Smart Radios automatically switch between spatial-diversity to spatial-multiplexing modulations in order to optimize link quality. 

Fresnel Zone

An important factor to consider when establishing a link is potential obstruction of the Fresnel zone. The Fresnel Zone between a transmitter and a receiver is an elliptical zone within which RF propagation takes place. Therefore, objects within the Fresnel Zone can affect the link. If Freznel Zone clearance is not adhered to, multi-path interference will result, and destructive interference can result in significant degradation in the signal quality.

In the diagram below, the trees are considered to be obstructions because they are in the Fresnel Zone even though they do not obstruct the line of sight.

fresnel zone between antenna and drone using embedded broadband radio

When designing your link, Fresnel zone interference can be usually be overcome by having the antenna high enough off the ground, and in the case of drones, having the ground antenna pointing up towards the flyer.

Calculating the size of the Fresnel zone between two nodes can help to predict whether obstructions or discontinuities along the path will cause significant interference. Both distance and frequency affect the size of the Fresnel zone, which can be determined using a Fresnel Zone Calculator, like the one linked here. We’ve included a reference chart below that would give you an idea of the size of the Fresnel Zone based on distance and frequency.

There are many system design and use can related factors that go into optimizing the link distance. We provide indicative range charts in each model’s datasheet. For more detailed link design, use of Link Budget Calculator is required.

Our general recommendation is to design a link with about 10-15 dB fade margin to account for changes in the environmental conditions.  Use the highest gain antenna possible within the physical constraints of the system. The objective is to operate the link with the highest RSSI possible (Sweet spot is around -70 dB RSSI). This will result in the link operating at the highest modulation. provide the best performance and reliability.