This page highlights some of work we've been doing at UCLA on communication and radar systems. A good mixture of very low power applications like backscattering links, some spread spectrum data links, and a few new radar architectures.
Active Radiometry at Microwave to Millimeter-Wave
This is a neat project where we excite materials with an IR laser and image them using mm-wave radiometry. The laser only heats the surface of the material but the blackbody radiation the radiometer captures comes from the sub-surface so you can get a measure of the different thermal conduction leading to different time constant responses to pulses and CW signals.
100 GHz Sub-Thermal Spread Spectrum Data-links
In this project we explore the bandwidth of mm-wave communications not for high data-rate, but instead for spread spectrum links with extremely high spreading factors. In this demonstration we take a 57600 bit per second data stream and spread it out over several GHz of bandwidth leading to a crazy spreading factor of over 50dB. With this level of spreading you can set your transmit power so that the power density of your signal is below the thermal floor (KT) in the spread condition meaning the link is truly undetectable (as the SNR in the spread condition is a negative value). The most interesting part of the project is the technique we use for the despreading synchronization. Since the received signal is below the thermal floor there's no way to synchronize or recover packet or symbol level timing. Instead what we did was come up with a novel codeless DS/SS despreading technique where you use a delayed copy of the datastream to perform the despread with no synchronization needed.
100 GHz FMCW Radar with DiCAD-based Chirp Extensions
We've been exploring techniques to digitally enhance FMCW radars at UCLA. One of the neat tricks we came up with is to use the regular FMCW architecture where a PLL is phase-locked on a chirping reference signal, but adding digital trimming devices to the VCO and transmit/receiver chain. The control of the trimming devices runs in a sequence that is synchronized with the FMCW input chirp so certain portions of the FMCW chirp waveform correspond to certain digital settings on the digital trim elements. In this way the "lock-range" of the PLL and how far it can chirp over (related to range resolution) can be extended. One interesting property is the FMCW demodulation spreads the discontinuities produces by the switching trim elements across the entire chirp bandwidth resulting in only a tiny increase in effective noise floor. Of course nothing comes for free, but it's a neat way to trade a few dB of dynamic range across a large swath for substantially more range resolution.
6 GHz ASK/OOK Back-scatter Data-Links
Back-scatter wireless data links are a potential candidate for Internet-of-things (IoT) connectivity as their power consumption is extremely low for modest data-rates of 10s of Mb/s. Similar to RFID technology, these links work by reflecting or “back-scattering” a carrier from a base-station and modulating the reflection to carry data. This was our first and simplest demonstration which was a very straight forward OOK line of sight system capable of operating up to 330 Mb/s at several meters.
Back-scatter Data-Links with Pulse-Shaping and Pre-Distortion
As a follow up to the above work, we have developed a back-scatter link system, and have demonstrated both pulse-shaping, and pre-distortion in a reflective link. Pulse shaping the reflection allows the system to co-exist with other WiFi networks as the signal energy can be confined within the frequency boundaries of a defined network channel. Pre-distortion allows the link to compensate for distortion introduced by both the transmitter and receiver circuitry as well as frequency dependence of the antenna and indoor free-space channel. The demonstrated system supports QPSK & QAM signaling, over a distance of several meters and consumes only 2.6 mW of power at the IoT device.
Back-scatter Data-Links with Carrier Offset and DPD
The third generation of reflective link uses a combination of an offset-carrier and digital pre-distortion as well as the pulse-shaping techniques from the previous work to allow fully 802.11.b compatible signalling while avoiding all the issues related to a directly reflected carrier. The demonstrated backscattering system supports QPSK signaling up to 12.5MB/s, over a distance of several meters and consumes only 1.2 mW of power.
Back-scatter Data-Links with OFDM Modulation
The fourth generation of reflective link we've been developing at UCLA uses a full 32-sub-carrier OFDM modulation demonstrating compatibility with similar 802.11 a and g standards for IoT applications where very low power connectivity with 802.11 networks is desirable. The demonstrated back-scattering system supports signaling up to 12MB/s, over a distance of several meters and consumes only 1.77 mW of power.