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Continuously Adjusting, Base Station Independent Squelch Method for Control Channels in Communication System Controllers

IP.com Disclosure Number: IPCOM000022463D
Original Publication Date: 2004-Mar-16
Included in the Prior Art Database: 2004-Mar-16
Document File: 5 page(s) / 74K

Publishing Venue

Motorola

Related People

Sanjay Desai: AUTHOR [+4]

Abstract

Squelch feature allows the users of a radio receiver to set a noise threshold such that a received signal is further processed only if the noise levels are below the set threshold. Modern communication receivers typically employ squelch feature in a DSP. In this paper, we describe a method for implementing a robust squelch algorithm using a Digital Signal Processor (DSP).

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Continuously Adjusting, Base Station Independent Squelch Method for Control Channels in Communication System Controllers

         By: Sanjay Desai, Kevin Good, Jerry Capehart and Venkatesh Ramu

Abstract:

Squelch feature allows the users of a radio receiver to set a noise threshold such that a received signal is further processed only if the noise levels are below the set threshold. Modern communication receivers typically employ squelch feature in a DSP. In this paper, we describe a method for implementing a robust squelch algorithm using a Digital Signal Processor (DSP).

Introduction:

An FM (Frequency Modulation) receiver's discriminator possesses an interesting property. The high frequency noise of the discriminator output quiets very rapidly as the strength of the carrier signal at its input increases. This allows us to use the discriminator's output noise energy to make a determination about the presence or absence of a carrier signal at the receiver's input, thus making the output of a FM receiver's discriminator an excellent place to implement squelch.

Problem:

There are numerous problems however, in estimating the strength of the carrier present at the receiver’s input. The two most significant problems are as follows:

1) The noise quieting in the presence of a carrier varies for each different make/model of the receiver. Even within a specific make/model of receivers, the noise profiles vary significantly. To estimate carrier level present at the input, a typical algorithm measures noise energy at the discriminator output and refers to a look-up table of pre-calculated quieting values at various carrier levels. Then, based upon some linear or non-linear interpolation method the appropriate carrier level is determined. This approach makes the squelch measurement dependent upon the type of receiver being used. If a different type of receiver is to be used, it becomes necessary to refer to a table of pre-calculated noise quieting values at various carrier levels for that specific receiver. Another flaw with this approach stems from the fact that typically the table values are calculated using a small sample of receivers of a particular type. These values do not necessarily represent behaviors of all receivers of that particular type. Also, any changes in the hardware design can result in lengthy testing and tweaking of the pre-calculated quieting values at different carrier levels.

2) The noise measured at the receiver’s discriminator (demodulator) output may contain not just the true discriminator noise, but also noise added due to various other sources such as thermal noise, ambient noise in the environment due to other equipment etc. This results in an in-accurate squelch/un-squelch decision-making process. Due to its random nature, it is impossible to separate the discriminator noise from noise due to other sources.

Solution:

We describe a solution, which uses a statistical decision-making algorithm based upon the history of the noise ene...