Carrier Detect and Busy Channel Lockout – A Collision Avoidance Mechanism

In a number of radio operating environments, whether data radios or voice radios, either system logistics or regulations may suggest a radio device listen for a signal on the intended frequency prior to transmission. This can serve a very useful purpose, as two radio transmissions occurring on the same frequency simultaneously within the same area will likely interfere with a receiver’s ability to clearly distinguish one transmission from another. This occurrence is conventionally called a collision, and thus collision avoidance is the goal.

One way to avoid two simultaneous transmissions on the same frequency interfering with a receiver is to physically space the two competing radio systems far apart. This is the primary reason why the Federal Communications Commission licenses radio frequencies to individual users. As the use of specific radio frequencies are licensed, their location of use is noted and the relicensing of the same frequency is coordinated to ensure that the competing systems are far enough apart to avoid interference with one another. This is also the reason why Raveon recommends the use of licensed frequencies in most applications. Still, upon occasion interfering signals will appear unexpectedly and a radio collision will occur. Certainly this occurs with regularity when unlicensed frequencies are available for anyone to use. Finally, even within a single radio system it is important to coordinate transmissions effectively and avoid collisions.

In systems operating within any particular area the most common technique utilized by transmitters in collision avoidance is to first listen on the air for potential interference before transmitting. This technique is called Carrier Detect or alternatively Carrier Sense. Raveon radios feature Carrier Detect capabilities where the radio modem listens on the intended transmit frequency. When the frequency appears busy either from a transmission within the system, from another system, or simply due to background noise, the radio can be prevented from transmitting, waiting a brief random duration before once again preparing to transmit the same message. This prevention of transmission feature is termed Busy Channel Lockout. It is important to note that the transmit and receive frequencies on the radio modem must be identical for this feature to work. Following is how Carrier Detect and Busy Channel Lockout features are used in concern on Raveon data radio modems for Collision Avoidance. The commands and parameters discussed can be found in the Raveon RV-M7 data radio modem Technical Manual.

First, a Carrier Detect Threshold is specified. This is the signal strength level the radio will listen for to determine another transmission may be taking place, and is measured in dBm. By factory default the threshold is -113, which correctly implies that Carrier Detect capability is always enabled. The larger the absolute value (the further from zero) the weaker the external signal strength is at the receiver, although it may still be detected. One immediate effect of signal detection is that the STAT LED on the radio (if available) will flicker green This is very roughly analogous to a squelch setting in a voice radio. The command mode command to set or read this value is ATCD. Typing ATCD will read back the current value, and typing ATCD “x” (where x is the dBm value) will reinitialize the parameter. For instance ATCD -105 will reset the parameter so that signal levels less than -105dBm (-106, -107 and beyond) are ignored. The command syntax calls for the minus sign to precede the integer value. The Carrier Detect Threshold may alternatively be set utilizing Raveon Radio Manager software.

radio_manager_carrier_detect

If the threshold is changed, the value should be carefully selected by comparing the background or interfering signal level to the received signal level of an intentional transmission. While Raveon radios are very good at distinguishing intentional signals above background noise and receiving data correctly, a minimum of a 20dBm difference should exist between intentional and background signals. A 30dBm difference is even better. For example, if your intentional transmission is received at -85dBm, you want to ensure that background signals stronger than -105dBm are heeded, and your system does not attempt to transmit. Thus you may set the Carrier Detect Threshold to -105dBm, a smaller value, or use the default of -113dBm.

You can take an instantaneous sample of the signal level heard by your receiver, by typing the command ATRQ. You can also determine the peak background signal found within a defined period by using the Bandscope feature of Radio Manager. Finally, you can determine the received signal strength of any specific transmitter in your network by the PING command and function. When a remote radio responds to a PING, the local radio will also display the RSSI (Relative Signal Strength Indicator) signal level of the response. The PING function is also available on the Interaction tab of Radio Manager.

Bear in mind that you want to be selective in determining your Carrier Detect threshold or trigger level. If you set your Carrier Detect threshold too close to the ambient noise level, any ambient noise variations may prevent the radio from transmitting. It is best to balance your Carrier Detect threshhold somewhere between your ambient noise level and weakest true signal level, trying to ensure your Carrier Detect threshold is at least 20dB stronger than ambient, if this is possible within your environment.

Once your Carrier Detect threshold is established (or if you choose to use the default), enabling Busy Channel Lockout is as simple as turning it on. It is important to note that the Raveon factory default has Busy Channel Lockout disabled. When this is the case, the radios still respond to background noise in excess of the threshold by blinking the STAT LED green. This STAT LED function is useful for a quick assessment of ambient interference, in an otherwise quiet system. To enable Busy Channel Lockout, either issue the command ATBC 1 (ATBC 0 turns BCL off), or check the Busy Channel Lockout box in the Basic Settings Tab of Radio Manager:

radio_mgr_busy_channel

When Busy Channel Lockout is enabled, the Raveon radio will first listen to the radio channel and if no signal is detected above the Carrier Detect Threshold, the radio will transmit. However, if an excessve carrier signal is detected the radio will not transmit, and will retry upon some random minimum delay. The delay period is randomized intentionally, and random backoff times are the de facto industry standard. The number of retries can be set by the ATRB command with 0-99 being the valid range of retries.

Use caution when setting your Carrier Detect Threshold and enabling Busy Channel Lockout, as a carrier wave interferer, PC with poor shielding, or some other source of RF can prevent the modem from transmitting.

Finally, while Carrier Detect with Busy Channel Lockout is an often used Collision Avoidance technique, it is not the most efficient use of bandwidth. Statistically, approximately 30% of overall bandwidth utilization is achieved with this technique. In systems where internal interference (where other in-system transmitters are the potential offenders), more sophisticated and deterministic mechanisms such as TDMA (Time Division Multiple Access) may be used. Statistically TDMA is projected to come close to 100% available bandwidth utilization, and for busy systems may be the technique of choice. A number of Raveon devices are available with TDMA. See this link or contact us for more details: http://ravtrack.com/GPStracking/tdma-time-slots/71/

Larry Top
lt@raveon.com

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