RF TESTING OF MODEMS EMBEDDED IN RADIOS

Transcription

1 RF TESTING OF MODEMS EMBEDDED IN RADIOS J. W. Nieto Senior Scientist Harris Corporation RF Communications Division HFIA 2002, #1

2 Presentation Overview Overview Motivation Baseband Testing RF Testing RF Test Setups Recommendations Summary HFIA 2002, #2

3 Motivation Most HF waveform standards define a set of minimum performance requirements which must be met in order for a modem to be compliant with the standard These performance requirements have been determined using a baseband HF channel simulator STANAG bps adds baseband radio filters (digital filter taps provided in STANAG) If minimum performance numbers have no margin, demodulation process may meet performance specs when tested at baseband but not when embedded in radios HFIA 2002, #3

4 Motivation Differences between baseband testing and RF testing Transmit Side Transmit radio filter Transmit Gain Control (TGC) Automatic Level Control (ALC) Receive Side Receive radio filter Automatic Gain Control (AGC) To complicate things further, radio manufacturers will likely implement all of the above differently HFIA 2002, #4

5 Motivation Example: US MIL-STD B Appendix C bps waveform utilizing radios with poor AGC characteristics (e.g. too fast) bps is 64-QAM constellation with no coding/interleaving Waveform will run with errors even in a noise free environment BER will never be less than 1e-5 at any SNR! Will fail certification! At baseband, same demodulation process likely exceeded performance requirement of standard HFIA 2002, #5

6 Motivation Why did modem fail at RF? AGC causes sudden changes in amplitude of waveform. Since amplitude contains information for M-QAM constellations, errors have been introduced TGC and ALC can drive signal too high into Power amplifier, distorting signal and possibly modulating signal. This effectively limits the maximum SNR measured at the demodulator and may cause bit errors TX and RX radio filters spread impulse response of system. Demodulator process has to use more taps for channel estimate which allows more noise into system. Typical radio filter losses between 1-3 db HFIA 2002, #6

7 Baseband Testing Goal of testing Repeatable and comparable long-term BER modem performance measurement HF Channel Simulator Watterson Model Mechanism for simulating fading characteristics very important for achieving repeatable and comparable results No radio filters or known (specified) digital radio filters No TGC, ALC, or AGC Signal to Noise Ratio (SNR) is average long-term SNR of waveform in 3 KHz For 110B Appendix C and F Waveforms, SNR calibration may require several minutes Test should ignore first few minutes of results in order to avoid Acquisition related bit errors (specially in fading channels) HFIA 2002, #7

8 RF Testing RF setup extremely important Improper setup can limit SNR of system with no noise (RX SNR must be checked!) If possible, TGC and ALC should be turned off at transmit end Test should be performed at low power with enough attenuation between radios Slowest AGC setting should be selected for RX radio If AGC is turned off for RX radio, RF level into radio must be carefully selected HFIA 2002, #8

9 RF Test Setup 1 RF MODEM RADIO 1 X Baseband BER TESTER Synthesizer HF_SIM RADIO 2 RF X HFIA 2002, #9

10 RF Test Setup 2 MODEM HF_SIM Baseband BER TESTER Radio 1 Radio 2 RF HFIA 2002, #10

11 RF Test Setup RF Test Setup 1 (Traditional) Using low-cost mixers, maximum SNR achievable close to 30 db but very sensitive to input/output signal levels at both mixers Reliable BER measurements up to 20 db unless SNR in channel simulator is selected based on combined channel simulator and setup SNR (For example, if max SNR = 30 db and the channel simulator is set to 30 db SNR, effective SNR is 27 db, 3 db off from desired point) RF Test Setup 2 (courtesy of Fort Huachuca) Maximum SNR achievable > 40 db Radio 1 should have TGC and ALC disabled (or preferably, an exciter with no PA is used) Reliable BER measurements > 30 db HFIA 2002, #11

12 Recommendations Overall testing strategy needs to be defined Standards should: Have performance specifications for baseband testing and for RF testing Provide more details on how testing should be done for baseband testing and RF testing Specify characteristics of HF channel simulators Provide details on how HF channel simulators characteristics can be verified Provide details on how calibration for proper SNR should be done HFIA 2002, #12

13 Summary Modems which are embedded in radios suffer more performance losses than modems tested at baseband HF waveform standards currently don t address this issue which can result in embedded modems not meeting performance requirements with same demodulation process that meets performance at baseband HF waveform standards should provide more detail as to how testing should be done HF waveform standards should provide more detail about channel simulator calibration, generation of fading characteristics, noise, filtering, etc HFIA 2002, #13