Bluetooth, Wi-Fi and Coexistence Mastered in Handheld Devices

Bluetooth, Wi-Fi and Coexistence Mastered in Handheld Devices
WIRELESS ELECTRONICS
Bluetooth, Wi-Fi and Coexistence Mastered
in Handheld Devices
By Matthew B. Shoemake, Texas Instruments
Bluetooth and Wi-Fi coexistence has found its quintessential form factor, and it is small enough to fit in the palm of your hand.
For years companies and standards bodies have worked on making Wi-Fi (IEEE 802.11b) and Bluetooth (IEEE 802.15.1) coexist,
because there was a fear that these devices would generate catastrophic interference with one another. Although the concern was
justified, the reality was that very little agony occurred in solving this problem. The usage scenarios and proximities of devices
resulting in detectable performance degradation on either network were few and far in between.
As Bluetooth and Wi-Fi begin to take their
rightful places in the world, Bluetooth as a Wireless Personal Area Network (WPAN) technology
and Wi-Fi as a Wireless Local Area Networking
(WLAN) technology, the situation has started to
change. Wi-Fi has become the network of choice
for wireless Internet connectivity in offices,
homes and public hot spots. Bluetooth has
become the network of choice for cable replacement enabling wireless voice headsets, keyboards
and mice. These logical applications of Wi-Fi
and Bluetooth technologies are leading to the
natural convergence of the two into handheld
devices such as PDAs and smartphones.
With the migration of these two wireless networking technologies into handheld devices, the
coexistence of the two can no longer be achieved
by depending on distance or limited usage models. In fact, there are key applications that will
require both networks to operate simultaneously. Fortunately, solutions exist to enable simultaneous operation of these two networks when
embedded in the same device.
Keying in on the problem
Both Wi-Fi and Bluetooth operate in the unlicensed 2.4 GHz industrial, scientific and medical
(ISM) band. This band is 83.5 MHz wide, beginning at 2.4 GHz and ending at 2.4835 GHz.
Because Wi-Fi and Bluetooth approach spectrum use in different ways, they can cause considerable interference for one another.
Wi-Fi uses wideband stationary signals that
use direct sequence spread spectrum (DSSS),
single tone modulation, such as CCK (complementary code keying) and PBCC (packet binary
convolutional coding), and now OFDM (orthogonal frequency division multiplexing) as added
in the new IEEE 802.11g standard. Bluetooth
uses a frequency-hopping spread-spectrum
(FHSS) technique. While
Wi-Fi devices
occupy about
one quarter
of the 83.5
MHz available, Bluetooth devices
hop across
almost
the
entire band
with
an
instantaneous
bandwidth of
about 1 MHz.
Wi-Fi Block Diagram
Bluetooth Block Diagram
Distance
between WiFi and Bluetooth tends to
prevent them
from interfering, but in a
PDA this is
not possible.
When Bluetooth and WiFi are put into
the
same
device, the
tr ansmitted
signals from one network appear on the receiver
of the other making them interfere with the each
other. When one transmits while the other is
receiving, it is nearly impossible to decode the
incoming message, which can cause network
connections to drop if left unmitigated.
The problem is similar to a room full of people talking. There may be a lot of noise in the
room from all of the conversations, but as long a
people are spread out and not too loud, two people in reasonable proximity can hear one another and successfully carry on a conversation.
However, if one individual were to decide to talk
loudly right next to your ear, you would find it
very difficult to continue a conversation with
someone else.
Achieving coexistence
To alleviate the general coexistence problem,
various solutions have been considered including Adaptive Frequency Hopping (AFH), Transmit Power Control (TPC) and Time Division
Multiplexing (TDM). Using these mechanisms
in whole are in part enables Bluetooth and Wi-Fi
to enjoy simultaneous operation in handheld
devices.
Adaptive frequency hopping, while not on the
market today, allows Bluetooth and Wi-Fi networks to share frequency. When using AFH the
Bluetooth device no longer hops across the
whole band, but restricts its hop channels to
those frequencies not occupied by the Wi-Fi network. TDM techniques allow for both Bluetooth
and Wi-Fi to provide simultaneous operation.
And last but not least, each network can use TPC
to lessen the degree of interference it generates.
Returning the room full of people talking and
one rather loud individual near your ear, there is
an analogy for each of the coexistence techniques
mentioned above. If you were able to politely ask
the loud individual to speak at a higher frequency that you could not hear, you would not care
how loud he was talking. This is analogous to
AFH. Likewise, if you could reach an agreement
whereby the loud individual traded time speaking with you so you could still carry on another
conversation, is analogous to TDM. And if you
could ask the individual to lower his volume
when near your ear, you could also continue
your conversation without interruption, which is
analogous to TPC.
Reaching the goal
In handheld devices with Bluetooth and WiFi, coexistence can be achieved by using the
mechanisms described above. In practice, Bluetooth and Wi-Fi devices must be designed in
conjunction with each other to make sure they
have the required degree of interference rejection. In addition they must be able to communicate with one another to enable mechanisms
such as TDM and AFH.
Bluetooth and WLAN block diagrams are
shown in Figures 1 and 2, respectively. In coexistence enabled solutions, collaborative coexistence mechanisms are typically enabled by communication between the Wi-Fi Media Access
Controller (MAC) and the Bluetooth link manager. Texas Instruments has designed its Bluetooth and Wi-Fi solutions to allow the use of
coexistence mechanisms and enable simultaneous operation for the next generation of handheld communications devices. All this to ensure
that the first time you use your PDA to carry on
a VoIP phone call over your Wi-Fi connection
that is bridged to your Bluetooth wireless headset your call will maintain high quality due to
design for coexistence.
TEXAS INSTRUMENTS,
Tel+1-214-4807976 • Fax+1-214-4803386
enter 3001 at
www.epn-online.com
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