Thursday, April 14, 2016

Cheap channel sounding

In 2000, when I was a research engineer at Motorola, we bought a state-of-the-art channel sounder.  It  came with a transmitter that sent a wideband (80 MHz) spread spectrum signal in the 2.4 GHz band, and a receiver that sampled the signal and computed the complex-valued channel impulse response.  It was $150,000 USD from a small custom software-defined radio company called Sigtek.  And it was worth it; it allowed me to conduct measurement campaigns to determine what accuracy was possible from a TOA indoor localization system in that band and with that bandwidth.  This was valuable information at the time for my employer.

Today we put together a channel sounder with capabilities that significantly exceed that system for $600 USD, using off-the-shelf parts and the Atheros CSI Tool, developed by Yaxiong Xie and Mo Li at NTU Singapore.  Anh Luong and Shuyu Shi got the system up and running in our lab.  The Atheros CSI tool is a hacked driver that works for several Atheros WiFi cards that allow the channel state information (CSI) calculated on the card by the standard 802.11n receiver to be exported out of the card.  We used an Intel NUC, which is essentially puts low-end laptop components into a 4 x 4 x 1 inch box.  It has two PCI express slots, and we use one to plug in an Atheros AR9462 card (Fig. 1, left).  The NUC has two antennas on the inside of its case, but internal PCB antennas like these typically are poor for propagation research (because of a non-uniform and unknown radiation pattern), so we instead set it up to attach our own external antennas by snaking a 200mm uFL to SMA adapter cable from the Atheros card to the side of the NUC case (via two holes we drilled, on the right side of Fig. 1).

Fig. 1: Inside the NUC-based Splicer channel sounding system

For one of the projects we're going to be using it for, we wanted directional antennas.  The Atheros is a 2x2 MIMO transceiver, so we need two antennas.  Also the Atheros card is dual-band, capable of 2.4 and 5.8 GHz.  But directional antennas tend to be big and bulky, and too many antennas hanging off of this unit would make it look like Medusa.  So instead we attached a dual-band dual-polarization antenna, the HG2458-10DP from L-Com.  It is a box that contains two antennas, one vertically polarized and one horizontally polarized.  The Splicer tool measures the channel between each pair of antennas, so we can measure the H-pol channel, the V-pol channel, and measure propagation for signals changing polarization in the channel.

Fig. 2: Two transceiver systems

Plus it looks like a scaled model of a 1981 IBM PC.  Or a minecraft character.  I'm not sure.

Why is this $600 system better than the $150,000 Sigtek channel sounder from 2000?

  • It's dual band, so we can measure either at 5.8 or 2.4 GHz, instead of being only at 2.4 GHz.  In fact, it can measure up to 200 MHz in the 5.8 GHz band, which is a wider bandwidth than the Sigtek system was capable of.
  • It's MIMO: we can measure four channels simultaneously.  Actually, if we had used a 3-antenna Atheros card, we could have measured nine channels simultaneously.  The Sigtek used one transmit and one receive antenna.
  • It can make multiple measurements per second, significantly faster than the Sigtek system.
  • It is smaller and uses less power.  The Sigtek system had to be pushed around on a cart, and when it needed to be battery powered, we had to use 80-pound marine batteries to power it.
Fundamentally, this is just another example of technology scaling over time.  The reduced costs ensure that many more people are able to perform research and test new communications, localization, and other applications of radio channel sensing.  I hope that the increased focus will lead to new research discoveries, new products, and even further reductions in the costs of radio channel research.

1 comment:

  1. Neat, and this is a very helpful writeup too!

    I had a similar experience recently: for about $15 in (USB-based FM SDR) hardware and a free software I could see bandpass and baseband signals that once required several thousands of dollars worth of equipment.

    I believe these reductions in cost will dramatically change how we teach introductory communications courses (from a primarily math-oriented course, to a much more hands-on measurement-oriented approach)...