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Bluetooth radio is an integral part of a Bluetooth device as it
provides an electrical interface for transfer of packets on a modulated
carrier frequency using wireless bearer services (CDMA,
GSM, DECT). The radio operates in the 2.4 GHz ISM
(Industrial Scientific Medicine) band which requires a very small and
efficient antenna (smart antenna), a good RF front end (LNA, Up-converter,
down-converter) on chip, power controller, GFSK modulator and a
transmit/receive switch for it work as a transceiver. Below we discuss
radio architecture in reference to Bluetooth radio modem and controller
developed by SiliconWave.Com on
two separate chips using high performance silicon-on-insulator (SOI)
BICMOS process.
The radio modem performs the GFSK modulation and demodulation, symbol
and frame timing recovery. The modem also contains a fully integrated
radio transceiver and frequency hopping synthesizers on a single chip.
This radio essentially looks like below:
The controller implements the baseband protocol and functions. On the
receive side it performs error detection and de-scrambling. The link
controller hardware implements the basic, repetitive actions of paging,
inquiry, page and inquiry scans etc. It also provides a USB
and audio CODEC interface to the
host system. The controller is shown below:
As said before, the Bluetooth radio operates in the 2.4 GHz ISM band.
In the US and Europe, a band of 83.5 MHz is available; in this band, 79 RF
channels spaced 1 MHz apart are defined. Japan, Spain and France use only
23 RF channels spaced 1 MHz apart.
| Country |
Frequency Range |
RF Channels |
|
| Europe and USA |
2400 - 2483.5 MHz |
f = 2402 + k MHz |
k = 0,....,78 |
| Japan |
2471 - 2497 MHz |
f = 2473 + k MHz |
k = 0,....,22 |
| Spain |
2445 - 2475 MHz |
f = 2449 + k MHz |
k = 0,....,22 |
| France |
2446.5 - 2483.5 MHz |
f = 2454 + k MHz |
k = 0,....,22 |
The channel is represented by a pseudo-random hopping sequence hopping
the 79 or 23 RF channels. The hopping sequence is unique for the piconet
and is determined by the Bluetooth device address of the master; the phase
in the hopping sequence is determined by the Bluetooth clock of the
master. The channel is divided into time slots, each 625 microseconds in
length, where each slot corresponds to an RF hop frequency. The nominal
hop rate is 1600 hops/s. All Bluetooth units participating in the piconet
are time and hop-synchronized to the channel.
Transmitter uses GFSK (Gaussian Frequency Shift Keying) where a binary
one is represented by a positive frequency deviation and a binary zero by
a negative frequency deviation. The definition of Bluetooth modulated
signal is given below:
| Modulation |
GFSK |
| Modulation
index |
0.32 +/- 1% |
| BT |
0.5 +/- 1% |
| Bit Rate |
1Mbps +/- 1 ppm |
| Modulating
Data |
PRBS9 |
| Frequency
accuracy better than |
+/- 1 ppm |
The bluetooth devices are classified into three power classes depending
on the maximum output power of the transmitter. A power controller can be
used for limiting and optimization of the output power depending on the
power requirements of the device.
| Power class |
Maximum
Output Power |
Minimum
Output Power |
| 1 |
100 mW (20 dBm) |
1 mW (0 dBm) |
| 2 |
2.5 mW (4 dBm) |
0.25 mW (-6 dBm) |
| 3 |
1 mW (0 dBm) |
N/A |
The actual sensitivity level is defined as the input level for which a
raw bit error rate (BER) of 0.1 % is met. The requirement for a Bluetooth
receiver is an actual sensitivity level of -70 dBm or better.
Next step: Baseband
We now move from here to the Baseband level
where we will talk about the physical layer of the Bluetooth devices.
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