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HF APRS - VARA vs MFSK vs OFDM Shootout!

 

Introduction

Having demonstrated that the VARA high-performance data modem was dramatically superior to classic AX.25 packet for APRS on HF, I am now attempting to compare the VARA-HF modem to both MFSK-16 and OFDM-500 modes generated by FLdigi.   [See previous results of the AX.25 vs VARA test here:  HF APRS Over VARA Modem ]

This test setup in the car will use VARA linked to one instance of UIview, FLdigi in MFSK-16 mode linked to a second instance of UIview, and a second copy of FLdigi in OFDM-500 mode linked to a third copy of UIview.   [Multiple copies of UIview can run on the same computer at the same time, as long as they are all installed in different folders.
    UIview's  KISS-over-serial outputs are bridged to the three modems (all of which only do KISS-over-TCPIP) with three stacks of the Eterlogic "VSPE" (Virtual Serial Ports Emulator) utility.  A separate VSPE "widget" splits the incoming data from the same GPS receiver on COM4,  so all copies of UIview can use it from multiple "COM5" ports at the same time.

Ever since Windows Vista, it has been possible for two or more programs to access sound devices simultaneously. In this setup, the VARA modem and both copies of FLdigi are accessing the same sound system at the same time.
    In the road test, all three modems will be linked to a Behringer UCA-202 USB "sound card" connected to one of my homebrew tone-activated sound card interfaces.  These interfaces DO NOT require a serial port, USB port or CAT control to key the radio transmit PTT line. The net result is that the same sound system & interface, same radio (Yaesu FT-891) and same mobile antenna (60 meter MFJ monoband "Ham Stick") will be used on all modes. Transmit power of the 100-watt FT-891 will be set to around 30 watts output to assure fully-linear low-distortion handling of the complex waveforms of OFDM and VARA.

This is a screen shot of the 1024x758 "XGA" screen of the Toughbook showing the three  instances of UIview and all three modems running.   Note the low CPU loading with all this stuff running at once on a 32-bit 1.5 GHz dual-core Intel "Centrino" CPU on the VARA CPU meter.  I made three copies of the same UIview map, each overlaid with the mode being used -- otherwise it would be impossible to tell what mode each UIview was linked to!

Testing the three modems side-by-side, one thing is already apparent.  For the identical APRS compressed position report and short comment string generated by UIview, MFSK takes about FIVE times longer to send than VARA. 
    It will be interesting to see if the much longer (slower) symbols of MFSK fare better in the real-life world of the lower HF frequencies plagued with multi-path and selective fading distortion, especially in the twilight periods of propagation.
    The test road trip for this project will be a round-trip drive from the East Lansing, Michigan area to Traverse City, Michigan and back, for an amateur radio swap meet on the 11th of February.  Traverse city is about 150 miles / 240 Km from East Lansing "as the crow flies". The driving (road) distance is about 180 miles / 290 Km. 
    The outbound trip will start around 3:00 AM local in order to arrive at the swap around 7:00 AM local to setup my seller's table. The outbound trip will start with full night-time propagation transitioning to twilight as I arrive.  The return trip will depart Traverse City probably around 1:00-2:00 PM, placing most of the drive in full daylight, when signals are weaker, but normally have far less multi-path distortion.
    From my previous tests, direct ground-wave propagation extends out from the home location about 25 miles / 40 Km. This should not be affected by changes in HF propagation.

Route of the planned trip. The route out at night, and the return trip in the daytime will be the same. Map image from Microsoft MapPoint 2011.

 

 

This is a shot of the VSPE stack linking UIview #1-to-VARA, UIview #2-to FLdigi in MFSK mode, and UIview #3 in OFDM mode at the same time. The VSPE  COM4-to-COM5  "splitter" creates multiple virtual "COM5" ports from a single physical COM4.  Data from the GPS receiver attached to COM4 is shared with multiple applications simultaneously connected to "COM5". This includes three copies of UIview, MapPoint, DeLorme Topo 10.0, and Visual GPS (a GPS status monitor) at the same time. (The splitter can service up to 8 applications on the same virtual COM port at once.)