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
- the missed detection rate.
It should be avoided that the terminal does not recognize ongoing transmissions.
This requires selection of a low detection threshold. If the received power exceeds this threshold, the terminal should delay its transmission
- the false alarm rate
It should be avoided that when the channel is idle, the terminal interprets noise or
weak signals from other remote co-channel cells as an ongoing transmission that should
not be interfered with.
This requires selection of a sufficiently high detection threshold. If the received power
is below this threshold, the terminal is free to transmit
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.
