Browse Prior Art Database

Non-Coherent Collision Detection

IP.com Disclosure Number: IPCOM000059616D
Original Publication Date: 1986-Jan-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 3 page(s) / 51K

Publishing Venue

IBM

Related People

Fallwell, BN: AUTHOR [+3]

Abstract

This article describes a circuit arrangement for non-coherent frequency shift keying (FSK) collision detection. The use of coherent and non-coherent detection of radio frequency (RF) signals is well known in the industry. There are a number of advantages to coherent detection that limits the use of non-coherent detection today. One of those advantages is its tolerance to noise. Just as lightning has little effect on a frequency modulated (FM) radio, noise has little effect on coherently detected RF signals. Once a receiver has locked onto the carrier, noise (or a distant transmitter) has very little effect. This is known as the capture effect and is beneficial in recovering data from a network.

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Non-Coherent Collision Detection

This article describes a circuit arrangement for non-coherent frequency shift keying (FSK) collision detection. The use of coherent and non-coherent detection of radio frequency (RF) signals is well known in the industry. There are a number of advantages to coherent detection that limits the use of non-coherent detection today. One of those advantages is its tolerance to noise. Just as lightning has little effect on a frequency modulated (FM) radio, noise has little effect on coherently detected RF signals. Once a receiver has locked onto the carrier, noise (or a distant transmitter) has very little effect. This is known as the capture effect and is beneficial in recovering data from a network. Non-coherent detection modems do not exhibit this effect since they detect both the mark carrier and the space carrier simultaneously. In the carrier sense multiple access/collision detect (CSMA/CD) protocol that is used on the PC network, each modem continuously monitors traffic on the data channel. Anytime there is a pause on the channel, any modem can begin transmitting. If only one modem begins transmitting during a pause, that modem gains control of the channel and transmits its message without interruptions. If two or more modems begin transmitting during the same pause, their signals collide and the garbled data is detected by their receivers (collision detection). All transmitting modems must detect the collision (the garbled data). Coherent detection and the CSMA/CD protocol present a conflict in requirements. Coherent detection of the data improves noise rejection at the same time it reduces the likelihood of detecting a collision. Since the receiver can lock onto the stronger of two transmitters, it might not detect the weaker one. The circuit disclosed herein is illustrated in block diagram in Fig. 1, which shows the use of coherent detection of the data recovery and use of a separate non-coherent detection for the collision detection circuitry. This yields the desired noise rejection for the data recovery while guaranteeing collision detection over a wide dynamic range of input signals. Block 1 is the bandpass filter, mixer and amplifier that converts the input channel into a lower intermediate frequency (IF) for further processing. Block 2 is the quadrature detector. The...