A8520E24A91 – User’s Manual
Page 9 of 34
Release Date 02/27/12
1.2.2. Adaptive Frequency Hopping
The purpose of using frequency hopping in a radio system is to provide diversity that allows
data throughput to be maintained even if the interfering radio systems or the physical
environment (e.g. multipath fading) renders some RF channels unusable. In the 2.4 GHz ISM
band, the sheer amount of radio systems and the severity and dynamic nature of indoor fading
phenomena in typical operating environments require the use of this kind of diversity if a
minimum data throughput is to be guaranteed (as audio streaming requires). Frequency
hopping systems can either implement a fixed sequence of channel hops or adapt its hopping
sequence dynamically to the changing environment it operates in. In order to maximize its own
chances of delivering audio data in time and to co-exist amicably with other fixed-frequency or
adaptive frequency hopping systems, PurePath™ Wireless uses an adaptive frequency hopping
(AFH) scheme that adapts to changing conditions within tens of milliseconds. PurePath
Wireless divides the 2.4 GHz band into 18 RF channels with 4 MHz bandwidth. A protocol
master that controls the adaptive frequency hopping scheme for the audio network, 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, when no swaps between the active and trial channel sets
occur, goes through a sequence of 70 hops over the course of which every RF channel has
been used.
This 70-hop macrosequence consists of 14 repetitions of a
o
5-hop microsequence during which
Each of the four active RF channels are used once
One of the trial RF channels is used once (cycling through all trial
channels over the course of a macrosequence)
illustrates this concept. This gives an average steady-state RF channel usage in an
audio network of:
Each of the four active channels are used 20% of the time
Each trial channel is used 1.43% of the time
