JPL's Wireless Communication Reference Website

Chapter: Data Networks
Section: Random access, CSMA

Hidden Terminal Problem

In Carrier Sense Multiple Access (CSMA), all terminals listen to the inbound (terminal to base) channel. No new packet transmission is initiated when the inbound channel is sensed busy. This requires that all mobile terminals can receive each others signals on the inbound frequency. However, in mobile radio nets with fading channels and path loss, a mobile terminal might not be able to sense a transmission by another (remote) terminal. This is called the hidden terminal problem.


Hidden terminal (blue) is busy transmitting. Other terminals (e.g. the green one) may not receive a signal strong enough to recognize this transmission.

Example

Assume that the base station antenna is place at a height of 100 meters, whereas mobile stations only have an effective antenna height of 2 meters. According to the 6 dB/oct height gain of the Plane Earth path loss model, mobile terminals that are near the base station observe a signal that is weaker, by a factor of



       100 m  2



     ( -------)  =   2500,



        2 m



i.e., by 34 dB. Moreover, remote terminals, at the opposite end of the cell as the transmitting terminal, face an additional 12 dB loss due to the two times larger propagation distance. Shadowing and fading also contribute to the hidden terminal problem.

Carrier Detection

The carrier sense decision should appropriately minimize These two requirements are conflicting, and the threshold selection must be a compromise.

A solution: Busy Tone

The hidden-terminal problem is avoided in Inhibit Sense Multiple Access (ISMA), where the base station transmits a busy signal on an outbound channel to inhibit all other mobile terminals from transmitting as soon as an inbound packet is being received.


JPL's Wireless Communication Reference Website 1993, 1996.