Antennas Everywhere

A simple rule when it comes to electronics is that while digital circuits scale, antennas do not. That may not sound like a serious problem until you consider that as more devices get connected—cars, watches, industrial equipment—and they add more features that require interaction with the outside world, they need more antennas to make it all work.

In the future, there literally will be hundreds of antennas inside of every home, and thousands inside many offices and industrial facilities. Those antennas will be connected via various wavelengths to other antennas, at the edge of the network, in the cloud, and even in outer space. What used to be a set of wires or conductors on a rooftop or out the back of an audio receiver has now evolved into a complex design challenge, and one that will grow more complex as antennas are designed into extremely confined spaces and called upon to do more than in the past.

“The challenge is that you can only obtain a certain level of performance from an antenna,” said Larry Williams, director of product management at Ansys. “So you put more antennas into a device, but then you have to limit how close they are together and how they are affected by a platform.”

But adding more antennas also increases the number of signals that need to be processed, in part because not all signals move at the same speed. A signal that is sent point-to-point with no interference will arrive intact at a predictable time, but engineering is never that easy. Signals are affected by everything from environmental conditions such as heat, electrical interference, noise from other signals, and even the physics of the wires in which they must travel. A signal bouncing off the sides of an antenna or a wire inside a device or a chip will arrive at a different time than one with a straight path.

“It’s like driving on a highway but all the cars are crashing,” said Williams. “You have to sort it out with signal processing. But to do that you need to understand each signal as it propagates from point A to point B. It’s not just line of sight. Signals are bouncing off things and arriving at different times. And even worse, when signals bounce around and can compete with the original signal. So you may have to slow down signals in the main path and combine those with others to create a more powerful signal.”

As more antennas are added and more signals are moving around, complexity increases proportionately. So does the amount of energy required to sort them out and turn them into something useful. Energy is required for analog to digital conversion, processing the signals, and even more energy is required to amplify the signals. The faster all of this occurs, the lower the latency. But in an area where reception is bad and signals need to be constantly amplified, it also can drain a battery more quickly than in an area with good reception.

“The power amplifier in a phone is well over 1 watt,” said Chris Rowen, a Cadence fellow. “This is why smaller and smaller cells are critical. It’s the only way to continue to make progress in data rates. You don’t broadcast with as much power and you make many more transactions in a given area because the cells are independent of one another. Bandwidths increase proportional to the reduction in cell area and power decreases. This is not directly about the antenna, but it is about what we do with the antenna.” (See related article on small cells here.)

MIMO and beamforming
Other things can be done to add efficiency and improve performance, as well. Signals can be combined with multiple input/multiple output (MIMO) technology, a way of interlacing signals inside the same channels.

This adds a couple of new challenges to the mix. First of all, it’s far more difficult to put multiple antennas into a small mobile device such as a smart watch or even a cellular phone because of space limitations. And second, with signals going out and back simultaneously they sometimes cross, depending on where the signals are being sent or received from, making it much more difficult to sort out the signals and extract clean data.

This has led to the next phase in technology evolution, which is beamforming. Rather than sending and receiving scattered signals, those signals can be confined to a single direction. It can even be used to direct signals from multiple devices.

“You have multiple subscribers communicating to a cell tower and you transmit to one but not to another,” said Williams. “So you’re beaming energy, but it’s multipath.”

Radar has used this directional approach for years. What’s changed is that next-generation technology uses multiple signals spaced out within the beam to optimize the flow of traffic.

Design changes
As with beamforming and MIMO technology, it’s not necessarily the antennas that change as much as the technology used to extract, organize and make sense of that data that needs to change. That also includes the overall SoC and larger system design, particularly with regard to mixed-signal technology.

“One novelty in analog design is that for the RF stage of the radio, the best approach for directional transmitting is to put a DSP in the middle of the analog for constructive/destructive purposes,” said Bernard Murphy, chief technology officer at Atrenta. “This is digital to analog with a small number of analog blocks. You can do digital EDA for this, with the normal synthesis, optimization, place and route, and put that inside of something else. The problem with analog is that it’s difficult to pin down until late in the design process, when you know what you need for shielding and size.”

He said that now this is handled with “smarts and iterations,” but with IoT designs there will be a premium on both cost and time to market.

A second factor involves the heat generated around antennas, particularly for large antennas such as those used in base-stations or for military equipment. As these large antennas are used to receive and send signals to an ever-larger number of devices, more processing and amplification will generate heat that affects the performance of these antennas.

“The key is to spread the heat as close as you can to the source,” said John Wilson, technical marketing engineer at Mentor Graphics. “But it’s not just about getting the heat out. The enclosure may be hotter than the components on the inside, so you don’t necessarily want to connect the inside and outside. What you do want, though, is more surface area for cooling. It’s a lot more thermal management than you might expect.”

A third factor involves the packaging on a device. One story that’s repeated among a number of vendors involves a wearable device where the antenna was designed into the device and then, for aesthetic reasons, a small piece of chrome was added to the wristband. The chrome blocked the antenna, forcing the signal to be routed around it, amplified and processed, severely reducing the battery life.