KNOWLEDGE OF THE VHF RADIO BAND in Java

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KNOWLEDGE OF THE VHF RADIO BAND
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Ducting
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Tropospheric Refraction Fresnel Zones
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Knife Edge Diffraction
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FIGURE 7-8
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Spatial distribution of energy in VHF.
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Propagation modes include skywave (radio line of sight (LoS), direct and multipath (e.g., air ground), some ducting beyond LoS in the lower end of the band below 100 MHz, refracting or diffracting beyond LoS) and ground wave (short ranges, <10 km) with multipath delay spread of 1 10 s. 7.5.2.1 Delay Spread Knowledge Chunk The delay spread of 1 10 microseconds supports instantaneous modulation bandwidths of hundreds of kilohertz in simple receiver architectures (e.g., single-channel push-to-talk with AM conversion or FM discriminator receivers; or FSK mark/space lters for data signals). Using That Knowledge: The AACR should know how to measure delay and Doppler spread from known broadcast stations with controlled time and pulsed emissions and by collaborating with peers. It should know by introspection (not by being told) whether a given waveform has an adaptive equalizer and how to compute the number of half-channel symbol period taps needed to compensate observed delay spread. 7.5.3 Spatial Distribution of VHF Energy
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Typical spatial distributions of energy are illustrated in Figure 7-8. 7.5.3.1 VHF Fresnel Zones Knowledge Chunk Upper VHF includes Fresnel zones, knife edge diffraction, ducting, and tropospheric refraction like LVHF. VHF has less lling of low lying and
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RADIO KNOWLEDGE
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shadowed regions because the shorter wavelengths set up spatially smaller interference patterns. These patterns have smaller angles between successive constructive and destructive interference zones. This ne structure supplies less total power to the shadowed regions. Using That Knowledge: The AACR should know how far away from a knife-edge obstruction the user should be to avoid shadowing. It should map constructive interference zones, to guide user mobility, and should share that knowledge with CWNs for HDR when in a Fresnel zone of high Eb /No. 7.5.3.2 Tropical Ducting Knowledge Chunk Wavelengths from 1 to 3 meters typical of this band are readily trapped in thermal inversions in the atmosphere in subtropical climates, leading to beyond LoS ducting at the day night boundary. Using That Knowledge: The AACR should know how location in the tropics implies occasional BLoS propagation. It should search for reception of BLoS known broadcast stations to calibrate the ducting. It should enhance QoI by adapting HF ALE to VHF ducting. 7.5.4 Available VHF Communications Modes
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VHF communications modes include AM (LSB, USB, VSB), FM (voice, fax), narrowband data (FSK, PSK, 75 bps to 9.6 kbps typical), multichannel radio relay (4 60 channels), and spread spectrum formats that include frequency hop (slow hop (<100 Hps); medium (100 1000 Hps), fast (>1000 Hps); wide hop (>10 MHz) and wideband BPSK with hop hybrids possible. 7.5.4.1 VHF Modes Knowledge Chunk AM, FM, data modes, and FH spread spectrum such as the US/NATO HAVE QUICK I & II slow FH air interface are common in VHF. Wide hop separations are more practical in these bands than in HF because about 300 MHz less prior allocations could be available for VHF FH. Using That Knowledge: The AACR should know about AM, FM, and FH and it should be able to observe the spectrum available for FH and explain this to a nonexpert user. It should be able to propose and validate FH style waveforms appropriate to current link conditions, QoI needs, and resource availability. 7.5.4.2 AM Voice Robustness Knowledge Chunk The AM air interface waveform is particularly appropriate for emergency communications with aircraft. AM waveforms are intelligible at negative
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KNOWLEDGE OF THE VHF RADIO BAND
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SNR, just as speech is intelligible in low SNR. This feature extends the range and robustness of analog AM voice. FM voice, also a popular military mode, provides greater clarity of voice communications at channel SNR greater than 9 dB. Below this SNR, the FM discriminator will not lock to the carrier, yielding only noise. Thus, AM voice is intelligible at total power levels well below those that render FM unintelligible. Consequently, AM voice is ideal for emergency reporting radio channels. Analog voice does not fully leverage modern signal processing technology. Recent research suggests extending analog modes via wavelet-based DSP [176]. Using That Knowledge: The AACR should know about AM and FM and should be able to explain to a nonexpert user why general aviation persists in using AM analog voice when so many other more modern channel coding methods are available. An iCR should know about wavelet signal processing and should be able to synthesize a wavelet-based air interface. 7.5.4.3 Channel Packing Knowledge Chunk Improvements in components have reduced channel bandwidths from 100 kHz or more in the early days of radio to typically 25 30 kHz today, with 81/3 and 61/4 kHz modes recommended by APCO. Congestion of air traf c control radio bands in Europe constrains AM/FM to 81/3 kHz. This packs three SCPC subscribers into the 25 kHz formerly occupied by one. Using That Knowledge: The AACR should know why analog voice was designed with 25 or 30 kHz channel spacing. An aircraft-aware AACR should know the new rules for denser spectrum packing. AACR should be able to set parameterized waveforms to achieve the denser spacing. It should cooperate with its peers to check air interfaces for spectrum mask conformance.
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