
3 | Keysight | P-Series and EPM-P Power Meters for Bluetooth Testing – Technical Overview and Self-Guided Demonstration
Frequency Hopping
The ISM band used by Bluetooth®is available from 2.40 GHz to 2.4835 GHz in
most countries, although there are restrictions in some countries. In this band,
Bluetooth uses frequency-hopping spread spectrum (FHSS) techniques to
mitigate interference.
In countries without restrictions, the radio signals hop in pseudorandom
sequences around all 79 available channels with a channel spacing of 1 MHz.
Starting at the base channel of 2402 MHz, the frequency of the channels can be
expressed as below:
f = 2402 + n MHz
where n is the channel number with an integer value in the range from 0 to 78.
In countries with restrictions, a limited frequency hopping scheme with just 23
channels is used and accounted for in the Bluetooth specification. Both hopping
schemes have 1 MHz channel spacing. This allows a simple radio interface
design, whereby the baseband only has to specify a channel number and the
radio multiplies this up to the appropriate frequency offset.
In this FHSS scheme, there are 1,600 hops per second, which is a hop every
625 µs. Part of this hop timing is taken up by a guard time of 220 µs, allowing
the synthesizer time to settle. The frequency hopping implements time division
multiplexing graph is shown in Figure 1. During the first 625 µs slot, k, the
master device transmits while the slave receives. In the following slot, the
slave may transmit and the master listen.
Figure 1. Graph shows frequency hopping where master and slave interact on corresponding slots
The radio must be able to be retuned and stabilized to a new frequency within
tight time constraints. This is pushed further when establishing a connection;
the hop rate is can be shortened to every 312.5 µs. As the radio is constantly
hopping to different radio channels, this ensures that packets affected by
interference on one channel can be retransmitted on another channel. To
further enhance resilience, both automatic repeat request (ARQ) and forward
error correction (FEC) form part of the specification.
The Technology
Bluetooth operates within the
industrial, scientific, and medical
(ISM) band at 2.4 GHz. It is a short-
range wireless communication
standard defined as a cable replace-
ment for a personal area network
(PAN), an individual’s own personal
space.
A cable replacement standard has
been defined because cables limit
mobility. They are cumbersome to
carry around and are easily lost or
broken. It is frequently difficult to
diagnose failure in the connectors,
and they are often proprietary.
Bluetooth counteracts these
limitations by being light, portable,
robust, and not limited to one
manufacturer.
To serve effectively as a cable
replacement, Bluetooth keeps its
cost comparable to that of cable by
operating in the license-free 2.4 GHz
ISM band, while remaining backward
compatible whenever possible to
avoid upgrades. The relaxed radio
specification also enables single-chip
integrated circuit solutions.
f(k) f(k+1) f(k+2)
Master
Slave
625 μs
t
t