MIT Presents Mobile Receiver Chip to Block Interfering Signals
MIT’s receiver architecture offers effective harmonic cancellation all in the RF domain.
With the goal of making 5G New Radio (NR) communications more robust, MIT researchers have developed a mobile receiver chip that can effectively block some of the most troublesome interfering signals. As bands become more crowded and the number of connected devices increases, unwanted interference can quickly become a detrimental issue if not properly addressed.
The new receiver chip for mobile devices blocks unwanted radio frequency signals at the receiver’s input without lowering its performance or slowing the device. Image courtesy of MIT News
While they all fall under the same umbrella of interference, spurious signals can often be categorized into many different classes, such as co-channel or intermodulation interference. Some of these originate from external sources, while others can be traced right back to the mobile device itself, making robust receiver chips valuable even when not in a crowded stadium.
In order to give readers a general understanding of the operation of the receiver architecture, this article provides background information on harmonic rejection receivers and how the MIT architecture creates a beneficial tradeoff to eliminate a major source of interference.
Your LO Isn’t as Good as You Think It Is
In order to extract information from a wireless signal, it is generally beneficial to convert the high-frequency RF signal into a low-frequency baseband signal with the help of a mixer and a local oscillator (LO). The mixer effectively accomplishes voltage multiplication, while the LO provides a reference frequency to shift the RF signal down into the baseband.
A mixer can convert a high-frequency RF signal (ω1) into a low-frequency copy using an LO signal (ω2). In order to improve mixer noise and gain, hard-switching can be used, but it comes with the undesired effect of creating LO harmonics. Image used courtesy of IEEE Solid-State Circuits Magazine
In order to maximize gain and noise performance, the LO should be as strong as possible. While this does improve the gain of the mixer, it can also create undesired effects due to harmonics. For example, if the LO signal is driven so high that it effectively becomes a square wave, new harmonics are generated, which can impact the receiver's performance.
If a square LO is used, the odd harmonics in the LO signal create copies of the original signal in the baseband. While filters can remove these higher-order signals, if a signal is present at the harmonic frequencies, it will interfere with the signal of interest. Image used courtesy of IEEE Solid-State Circuits Magazine
These extraneous signals can be dealt with quite easily in an isolated system. However, if another signal is present near the LO harmonics, it will directly interfere with the received baseband signal, making detection more difficult.
Even with a high-performing mixer and LO, a strong out-of-band (OOB) signal can overload amplifiers in the receiver chain, again impacting the detection performance.
Put Your Best Component First
In response to these issues, MIT researchers developed a novel harmonic resilient receiver by taking notes on digital signal processing techniques and incorporating them into their own “mixer-first” receiver design.
Die micrograph of the MIT harmonic rejection receiver chip, highlighting the major components used in order to improve receiver performance in a cluttered environment. Image from All About Circuits’ attendance of ISSCC 2023
While I won’t pretend that the dynamic behavior of the receiver architecture is easy to understand, its overall operation can be understood as a weighted sampling of the input signal. By creatively sampling the input according to the LO frequency and phase, the output of the mixer will be boosted at the fundamental frequency of interest; at the odd harmonics, the unwanted signal can be canceled completely in the analog domain.
It's worth noting that this architecture by itself cannot cancel even-order harmonics. But, with the addition of a balun to create a differential signal, these signals are easily dealt with. In addition, the mixer-first receiver architecture prevents overloading the amplifiers in the receiver chain, negating a potential source of strong interference.
The mixer architecture can be used to negate odd-order harmonics from a receiver system, thanks to the sampling at critical points, which cancels the effects of odd harmonics. Image from All About Circuits’ attendance of ISSCC 2023
The overall receiver design has promising measurement results, sporting an overall RX gain of 35 dB and a low noise figure down to 3.5 dB, all the while maintaining strong harmonic rejection greater than 50 dB for the 3rd and 5th harmonics. The overall receiver power ranges from 32 to 54 mW, ensuring that its presence in a mobile system won’t create a hefty power requirement.
Robust Reception for 5G NR
While we've addressed harmonic interference, it is natural to wonder how well the receiver architecture performs against other forms of interference. One key problem that comes to mind is intermodulation distortion, where two closely spaced tones can create unwanted byproducts in the baseband signal. For a crowded spectrum or a device with multiple receivers and local oscillators, this can quickly become a problem.
Overall, the MIT receiver architecture appears to be a promising new template for designers to use when developing the next generation of wireless receivers. Especially in cases where harmonic interference is a major problem, the mixer-first harmonic rejection architecture offers clear advantages through purely analog signal cancellation and opens the door for increased spectrum usage without requiring highly selective receivers.