Active Channel Selection / Listen-Before-Talk - Siemens 1218RF User Manual

in time and to co-exist amicably with other fixed-frequency or adaptive frequency hopping systems, PurePath Wireless
uses an advanced adaptive frequency hopping (AFH) scheme that adapts to changing conditions within tens of
milliseconds.
The PurePath Wireless RF Communication System divides the 2.4 GHz ISM band into 18 RF channels with 4 MHz
bandwidth. The protocol master controls the adaptive frequency hopping scheme for the audio network, and
maintains a table with an entry for each RF channel and an associated quality-of-service (QoS) estimate for each. Each
time an RF channel is used the QoS estimate is updated based on what happens during the timeslot.
The frequency hopping algorithm separates the 18 RF channels into two sets:
- A set of 4 active channels
- A set of 14 trial channels
The active channel set contains the preferred RF channels that have proven that they provide sufficiently good quality-
of-service. The trial channel set contains the remaining RF channels that are only evaluated occasionally in order to be
able to maintain an accurate picture of their quality-of-service. If the QoS estimate of an RF channel in the active set
goes beyond a minimum threshold this channel is swapped out with the RF channel in the trial channel set that has
the best QoS estimate. Other factors play in when selecting a new RF channel to the active channel set, such as trying
to maintain a certain minimum distance in frequency between the different active channels.
The frequency hopping algorithm, employs the following macro-sequence, when no swaps between the active and
trial channel sets occur, it goes through a sequence of 70 frequency hops over the course of which every RF channel
has been used.
This 70-hop macro-sequence consists of 14 repetitions of 5-hop micro-sequence during which:
- Each of the four active RF channels is used once
- One of the trial RF channels is used once (cycling through all trial channels over the course of a macro-
sequence)
Four active channels are selected randomly at start-up seeded by a unique 32-bit device address, which is stored in
the hardware by Texas Instruments during chip production.

6 Active Channel Selection / Listen-Before-Talk

In addition to AFH, listen-before-talk (LBT) is used by the master at the start of every timeslot to determine the energy
in the current channel and these measurements are compared against a threshold and are used to detect a "bad"
channel. In addition, at the end of every timeslot one will detect if one or more slave packets are missing.
- Two failing LBTs in a row for a given channel will mark the channel as bad / occupied.
- Three failing LBTs during the last five timeslots at a given channel will mark the channel as bad / occupied.
- If three timeslots in a row for a given channel has missing slave packets, the channel is marked as bad.
If any of the active channels are marked as bad they are replaced by the best channel that is furthest away from the
remaining three good channels.
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