In V/UHF radios for airborne platforms Squelch is used to detect the presence of valid voice or tone signal over noise and interferer. An airborne radio receiver mentioned in annexure 1 with less complex processors will use Signal to Noise energy(S/N) method check as Squelch, due to low complexity of implementation. But in present airborne platforms interferences with strength -99 dBm are possible due to Electromagnetic Emissions (EMI) of other systems, leakage from other coexisting radios, telecommunication channels and intentional jammer signals. Secondly, due to gradual transition bandwidth of various filters used in the receiver mentioned in annexure 1, adjacent channel transmissions nearby to desired channel can act as interferer and degrades the adjacent channel rejection (ACR) performance of the airborne radio receivers. This high level of interference in the present airborne radios will lead to non- reliable squelch performance in the mentioned airborne V/UHF radios. For a reliable performance in this interference environment existing V/ UHF radios have minimum signal requirement is set above the average interference levels (more than -98dBm). This will in turn reduce the range of communication of airborne radios. Existing complex VAD (Voice Activity Detection) algorithms cannot be used in airborne receiver mentioned in annexure 1 as Squelch due to shortage of resources in digital signal processors (TMS 320 DSP or similar). Besides many of the VAD (Voice Activity Detection) algorithms fail in the aspect of differentiating tone from noise. The proposed Variance based squelch for V/UHF air borne radios is developed for detection of valid speech and tone signals in a noisy and dense interference environment using existing hardware resources. This method is based on the combination of two metrics of voice/single tone - variance of zero crossing rates and the RMS energy variation of the voice. The extraction of these properties will give a robust Squelch works under interference environment. This squelch is implemented in TMS320VC5416PGE Digital Signal Processor (DSP). The above squelch is implemented in DSP using a maximum of 300 MACs (Multiply and Accumulate Cycle). This squelch will detect voice/tone having S/N ratio greater than 6 dB, This robust squelch will perform well in wide band interferer (with strength -90dBm) and single carrier interferer (with strength -80dBm). A consistent adjacent channel rejection of 60dB at +-50kHz away from the tuned frequency and 80dB rejection at +-100kHz from tuned frequency can be achieved with the mentioned radio receiver hardware. Minimum energy requirement for satisfactory working of radios (in annexure 1) can.be confidently set.as -104dBm or low using this robust squelch.
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