I’m a sports car guy. It’s in my blood. Ever since I was old enough to drive, I’ve had sports cars. But now I have two growing sons, and my two-door is no longer feasible as a family car. So I decided to upgrade from a sports car to a roomier sports sedan.
After months of research, I settled on a great car with lots of power, fantastic handling, and great styling. I really love this car except for one thing—the remote keyless entry (RKE) system works from only 10 feet away. How can a car this nice have such a lackluster wireless remote?
When it comes to designing an optimal wireless system, many factors come into play, such as data rate, power consumption, and regulatory compliance. The one issue at the top of almost every wireless designer’s list is communication range. End users care about many features when buying the next gee-whiz wireless device, but if the range isn’t adequate, it’s just a fancy paperweight.
Output Power
What factors impact communication range, and what can engineers like us do to improve it?
One obvious factor is output power. Maximizing the output power can help drive longer-range communications. Like when yelling to your friends across a large, noisy room, increasing power can overcome a multitude of problems. However, output power has its limitations.
First, government regulations must be met, which can limit the amount of power you can legally use for wireless transmission. Increasing output power also consumes more current so battery life can become an issue. In addition, many radio devices require external power amplifiers to achieve high output powers, increasing the design complexity, size, and cost. While high output power is helpful for your system, it can pose problems for other systems trying to operate in the same region and frequency band.
Receive Sensitivity
The flip side of output power is receive sensitivity. Instead of transmitting at much higher power, we can employ a much more sensitive receiver. Improved receive sensitivity allows us to pick out very low power signals—to hear that faint whisper of a transmission. The combination of transmit output power and receive sensitivity is called link budget.
For example, a transmitter with 10 dBm of output power and a receiver with –110 dBm of receive sensitivity has a link budget of 120 dB. Increasing the transmit power to 20 dBm and the receive sensitivity to –120 dBm improves the link budget by 20 dB to 140 dB.
Increased link budget normally translates into increased communication range. Ideally, each 6-dB increase in link budget roughly doubles the communication range. If increasing output power is analogous to yelling in a crowded room, receive sensitivity is like having a hearing aid that allows us to hear whispers in a noisy room.
What about someone yelling (i.e., transmitting at a high output power) near someone with very sensitive hearing (i.e., excellent receive sensitivity)? This is where dynamic range and automatic gain control (AGC) come into play. Receive sensitivity is really helpful except when there is a noisy neighbor nearby.
In this case we either need to handle that high input signal without overloading (i.e., dynamic range) or use AGC to back down the gain to prevent overloading. Receivers with excellent dynamic range can keep their gain high in the presence of a loud neighbor while those with reduced dynamic range need to reduce their receive gain to prevent overloading.
Additional Options
A few other things can be done to extend the range. If you’re sitting next to a noisy person, you can move away to reduce the interference with your conversation. This is analogous to frequency hopping in wireless systems, where a communication link changes frequency to avoid a strong blocker nearby. In addition, you can filter out loud neighbors by running your system at a narrow bandwidth to remove the offensive signal.
This approach of reducing the bandwidth has the added benefit of improving the signal-to-noise ratio of your receiver. You can also improve the communication link with other techniques, such as modulation encoding, fading mitigation via antenna diversity, or using a licensed spectrum band. In all wireless systems, the antenna design can have one of the largest impacts on overall communication range.
While the list of challenges for achieving the required wireless link range can seem overwhelming, continued advances in wireless radio architectures are making life easier for system designers. Highly integrated transceivers that include high-power transmitters, low-noise receivers, adjustable channel bandwidths, and antenna diversity are now available to optimize wireless systems to meet unique design challenges.
Also, fast tuning times and integrated MCUs support frequency-hopping systems to provide blocker avoidance. Perhaps most exciting for developers are sophisticated automated antenna tuning capabilities that automatically measure and adjust antenna matching for optimal performance.
Advances in wireless radio devices can go a long way toward reducing the complexity and effort to achieve long-range communication links. However, system designers still must make critical tradeoffs to arrive at the optimal solution for their wireless systems.
I wish the developers of my automotive RKE system had optimized their design for a link range of greater than 10 feet. I’d even give up a few extra horsepower to have longer-range RKE.