Space-Time Processing—Linux Style

Developing a new generation of wireless communications means you need FPGA development tools, a cluster for simulation and an embedded OS for prototype devices. Linux fits the need.
Zion

In late 2001, we obtained the old DEC Alphas (described as boat anchors) and decided to see what we could do with them. First, we modified the built-in bootloader to get Red Hat 7.1 running. We made two major changes. First, we chose one master machine and loaded up its SCSI interface with six hard disks and a DVD-ROM. Five disks became a level-5 RAID array (using raidtools) to store simulation or experimental data, and the remaining disk is for booting and recovery.

The second change was to install the MPI on all machines. Although installation was fairly easy, the need to derive all IP addresses through DHCP caused problems. In the end, we negotiated a fixed IP address for the master machine, now called zion. We run startup scripts on other machines that log their IP addresses onto an SMB mount using RPC (Listing 1).

Once MPI was working, we downloaded the latest Octave source and patched it with Octave-MPI. Since then, Octave-MPI seems to have been taken over by Transient Research (see the on-line Resources section). We set up our MPI system with a script that gathers the IP addresses stored by the script above, pings them and builds an rhosts file. Once dynamic generation of rhosts is complete, we simply run Octave-MPI over 4+1 machines with:

recon -v
lamboot
mpirun -v -c4 octave-mpi

With the system running, the Octave processing load can be shared across the cluster. We find that Alphas have significantly faster floating-point performance than do Pentiums, but using Ethernet to pass MPI messages slows the cluster down. We haven't benchmarked the system, but a simulation that takes a couple of hours to complete on a 2GHz Compaq PC runs about 10% faster on our first cluster of four Alphas (300MHz–500MHz).

We found GNU-Octave to be an excellent tool for numerical simulations. When executed with the -traditional option, also known as -braindead, it runs most MATLAB scripts. In some cases, Octave provides better features than MATLAB does, although MATLAB has a better plotting capability than the default gnuplot engine Octave uses.

Some engineers prefer to develop on Windows, so we provide a Web interface to MPI-Octave. It uses a JavaScript telnet client served from Apache on zion and some back-end scripting. Scripts were adapted from an on-line MUD game engine. For Windows users, the telnet client script automatically mounts their shared Windows drives on the zion filesystem, runs Octave over telnet and sets up plotting capabilities so that plots are written to a directory on the Web server in PNG format for displaying with a browser. Another option lets users save plots directly to their shared drives.

For debugging, a large debug buffer in the FPGA is accessible to the ARM. We memory-mapped the FPGA with 32-bit high-speed asynchronous access to the ARM, with access in userland or kernel space. Unsurprisingly, in kernel space we use a character driver module accessed as a file. This bursts up to 1,024 data words at high speed and handles all signaling, although the sustained speed is not so good. Userland access is accomplished through the neat method of mmapping to the /dev/mem interface, as long as you remember to create /dev/mem on your embedded filesystem first (Listing 2).

These tools allow us to upload known test vectors, saved from Octave on zion, to the debug buffer. Under ARM control, we route the debug buffer output to the input of a block under test, run the system for a few clock cycles and capture the output back in the debug buffer. Analysing the result in Octave tells us if the block works.

Figure 4. Visualising Channel Modes—The Complex Paths from Transmitter to Receiver

We found visualisation to be an important factor. After our first milestone, we invited some people to see the system. They saw boxes humming away with a couple of green LEDs to indicate everything was working. We noted a distinct lack of enthusiasm at what we believe was a world-first demonstration of the technology, so we realised something more was required. To this end, we chose to display channel models as they adapted in near real time. A channel is the complex path from a transmitter to a receiver, including reflections, multiple paths, dispersion and so on. The system we built sent training symbols over the air to sound the channel before sending data. Sounding gives us a picture of the channel, which we decided to display. We used the FPGA debug buffer on the receiver, with an ARM script running periodically to execute a program to extract the channel data from the buffer, format it and save in an Octave-compatible .mat file on zion. Octave was run non-interactively on zion to read the channel data periodically, analyse it and generate four plots as PNG image files, which a zion Web server PHP page displayed as a visualisation updated every four seconds (Figure 4).

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