User's Manual

Vectron User manual
© Höft & Wessel AG
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Subject to amendment, errors excepted HW8660UserMan.doc 1.0 02.02.2001 Lin
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10.7 Interference avoidance strategies
10.7.1 Interference within the system
The medium access procedures provide sufficient mechanisms so that there is no relevant interference
within a system.
Only endpoints send out a signal that are synchronized to the basestation.
In a synchronized system only signals interfere that use the same bearer b(i,l,k). Due to the fact that the
basestation broadcasts a list of free channels and each endpoint performs RSSI measurements, the only
potential interference situation is a collision of two endpoints during connection setup. Such a collision is
detected at both endpoints which then re-attempt to establish the connection after a variable timeout. So
the probability is very low that the two endpoints collide again .
10.7.2 Interference between two systems of the same type
In case two systems of the described type are operated in the same location, they may interfere with each
other. There are two possible situations:
1. Both system use different generic hopping patterns. In this case connections belonging to different
systems may hit the same frequency at the same time. Due to the bursty transmission a collision oc-
curs only, if the connections use the same time slot and the same frequency channel. So the total
collision probability is (1/24)(1/48) < 0,09%. The system is tolerant to a certain amount of interfer-
ence. Packet loss is detected and corrected through re-transmission. Only if a significant amount of
packets is lost (say 10%) this will reduce performance.
2. Both system use the same generic hopping pattern. This case is further studied below
Each of two systems using the same generic hopping sequence recognizes (through RSSI measurement)
the bearer positions used by connections of the other system. So it avoids these bearer positions.
Since the two systems are mutually not synchronized, they drift against each other. The maximum rela-
tive drift is given by the oscillator accuracy of 10ppm. For drifting one slot position at least 2 seconds are
needed.
In this case slow sliding collisions occur. The system invokes bearer handover to solve sliding collisions.