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IEEE 802.11 & Bluetooth Interference: Simulation and Coexistence

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IEEE 802.11 and Bluetooth, these two operating in the unlicensed 2.4Ghz frequency band are becoming more anymore popular in the mobile computing world. The number of devices equipped with IEEE 802.11 and Bluetooth is growing drastically. Result is the number of co-located devices , say within 10meters, grown to a limit, so that it may cause interference issues in the 2.4Ghz radio frequency spectrum. Bluetooth supports both voice(SCO) and data(ACL) packets. In this paper we investigate these interference issues and use a new Bluetooth voice packet named synchronous connection-oriented with repeated transmission (SCORT) to study the improvement in performance. For the sake of simulation results, we provide a comprehensive simulation results using MATLAB Simulink.
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IEEE 802.11 & Bluetooth Interference: Simulation
and Coexistence
Anil Mathew, Nithin Chandrababu, Khaled Elleithy, and Syed Rizvi
Department of Computer Science and Engineering, University of Bridgeport, Bridgeport, CT 06604
{amathew, nchandra, elleithy, srizvi}@bridgeport.edu
Abstract-IEEE 802.11 and Bluetooth, these two operating in
the unlicensed 2.4Ghz frequency band are becoming more and
more popular in the mobile computing world. The number of
devices equipped with IEEE 802.11 and Bluetooth is growing
drastically. Result is the number of co-located devices , say within
10meters, grown to a limit, so that it may cause interference issues
in the 2.4Ghz radio frequency spectrum. Bluetooth supports both
voice(SCO) and data(ACL) packets. In this paper we investigate
these interference issues and use a new Bluetooth voice packet
named synchronous connection-oriented with Repeated
Transmission (SCORT) to study the improvement in
performance. For the sake of simulation results, we provide a
comprehensive simulation results using MATLAB Simulink.
Index Terms—ACL, Bluetooth, SCO, SCORT
I. INTRODUCTION
The growth of wireless networks has transformed our daily life
into such a situation that we can't think of a life without
devices like computers, mobile phones like that. The wireless
networks interconnecting these devices are adding up more and
more nodes into it each minute. These devices communicate
with each other using many popular standards developed by
IEEE and such other groups.
The most popular among these communication standards are
IEEE 802.11 or Wi-Fi and the Bluetooth. Almost 75% of the
devices in the mobile computing world are equipped with
either one of these or both of them. These technologies use the
radio frequency for communication. The Bluetooth operates in
2.4GHz ISM band. Unfortunately IEEE 802.11 also operates in
the same 2.4GHz ISM band. There are different versions of
IEEE 802.11 like 802.11a, 802.11b, 802.11g, and 802.11n to
name a few. Some of them operate in a different frequency
range. However, in this paper we consider 802.11b which
operates in the 2.4GHz ISM band as shown in Fig. 1.
When IEEE 802.11b tries to send a packet through the
network, it will check whether the medium or the channel is
already occupied or is there any transmission already going on
through the channel. If it is not detecting any transmission, or
not sensing any RF energy in the channel, it will issue a CTS
or Clear To Send. That is the wireless network adapter will
now start transmitting the packet. Using the same technique,
while another co located IEEE 802.11b network tries to send
the packet, it will postpone the transmission.
This technique provides a good resolution for mutual
interference between co located IEEE 802.11 networks. But
when it comes to a co-located Bluetooth and IEEE 802.11
network they just don't communicate each other. So there is no
way they will identify each other. There is a definite chance of
collision when they use the same channel at a particular time.
A Bluetooth device may haphazardly begin transmitting
packets while an IEEE 802.11 device is sending a frame. This
may result in interference, which forces the IEEE 802.11
station to retransmit the frame when it realizes that the
destination station is not going to send back an
acknowledgment. This lack of coordination is the basis for
interference between Bluetooth and 802.11.
The objective of this paper is to build a simulation model
and study the impact of interference between IEEE 802.11b
and Bluetooth. We also study about a new Bluetooth voice
packet to reduce interference, which is proposed by IEEE
working group on co-existence.
Figure 1: 2.4 GHz ISM Spectrum
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the 7th Annual Conference on
Communication Networks and Services Research (CNSR2009). Restrictions apply.
This paper is arranged in different sections. In section II we
explain about Bluetooth specifications for voice and data
transmission. Section III presents the simulation model with a
brief discussion. In Section IV, we present “SCORT” the new
voice packet. Testing of the model and results are presented in
Section V. Finally, Section VI presents the conclusion.
II. BLUETOOTH SPECIFICATIONS
Bluetooth device can send both voice and data packet
through a radio channel with a data rate of 1Mbps. Bluetooth is
a short range Personal area network (PAN). Its operating range
is normally 10meters. Transmitting power of a Bluetooth Tx is
very low. It’s just 1mw. Bluetooth uses Gaussian Frequency
Shift Keying (GFSK) modulation technique. Bluetooth also
uses Frequency Hopping Spread Spectrum technique to reduce
interference from other devices operating in the same
frequency spectrum. Interference in Bluetooth system can be
recovered or sometimes avoided using various coexistence
techniques. Fig. 2 represents the utilization of time slot in
Bluetooth. In this paper we consider synchronous connection-
oriented with Repeated Transmission (SCORT) to reduce the
effect of interference in Bluetooth SCO voice links.
A time division multiplexing technique divides the channel
into slices of 625 µs slots as shown in Fig. 5. A new hop
frequency is used for each slot. Bluetooth supports both voice
and data transmission. Bluetooth voice transmission is called
Synchronous Connection Oriented (SCO) and data
transmission is called Asynchronous Connection Less (ACL).
Bluetooth SCO link is established between a master device and
a slave device in the Piconet as shown in Fig. 3. SCO link uses
reserved slots to communicate. Bluetooth master device use
these reserved slots to maintain the communication. Bluetooth
establishes an ACL link to transmit data. Unlike SCO, ACL
links can be established between one master device and up to
seven slave devices. ACL packets are transmitted in the free
slots after SCO transmission. An ACL packet can be occupy
up to one, three or five slots. All ACL packets other than
Broadcast from master are acknowledged.
A. Synchronous Connection Oriented (SCO) Link
Bluetooth voice transmission is done by SCO (Synchronous
Connection Oriented). The SCO link is a symmetric point to
point voice link for sending and receiving voice packets at
regular intervals of time. The SCO packets are transmitted in
only every sixth slot. This period of time is equal to 3.75ms.
The return path of transmission from the slave to master takes
place on the next slot. Bluetooth can support a maximum of up
to three voice calls at the same time. In Fig. 4, T1, T2, and T3
are the transmit slots for each SCO master link. Slots (R1, R2,
and R3) are the return path for the slaves.
A master device initializes and controls the SCO link. Up to
a maximum of three SCO links can be maintained by a master
device at the same time. When a master device sends a SCO a
packet in a slot, the slave device sends back in the following
slot. So it is symmetric. That is data rate is same in both
direction. The length of Bluetooth-SCO packet is always one
slot. There is no acknowledgement for SCO packets. SCO
packet transmission happens always in reserved slots at regular
time intervals, every two, four or six slots. There are different
types of SCO voice packets like HV1, HV2, and HV3. HV1
Figure 2: Bluetooth time Slot
Figure 4: Bluetooth SCO voice slot
Figure 3: Bluetooth SCO & ACL
Figure 5: Asynchronous Connection Less (ACL) link
carries 10 data bytes and is transmitted every 2 slots, HV2
carries 20 data bytes and is transmitted every 4 slots and HV3
carries 30 data bytes and is transmitted every 6 slots .The data
rate of HV1, HV2, HV3 packets are 64Kbps. HV1 and HV2
uses 1/3 and 2/3 rate Forward error correcting (FEC)
mechanisms respectively. There is no FEC in HV3.
B. Asynchronous Connection Less (ACL) Link
Bluetooth data transmission is called asynchronous
connection-less (ACL), which is different from SCO
transmission in many respects. In data transmission there is no
margin for error allowed.
If an error occurs, those packets must be transmitted again.
Different techniques can be used to implement it. In the case of
Bluetooth ACL transmission the system will wait for
acknowledgement from the receiver. It will send the packets
repeatedly till an acknowledgement is received. The receiver
will check the packet and verify the CRC to make sure the
packet is received correctly. In ACL Tx the through-put (in
bps) must be checked. The Bit Error Rate doesn’t matter much.
The through-put will go down if a packet has to be transmitted
again.
The receiver will set the ARQN bit in the header info. Then
it will send it to transmitter in the return path packet. That is
how receiver sends an ACK. By checking the ARQN,
transmitter senses if the transmission was successful. If the
value of ARQN is 1, it means a successful transmission, and if
ARQN is 0 it means a failed transmission. In the case of a one
way communication (master-to-slave) the slave sends back a
dummy packet in the next slot. NULL packet or dummy packet
does not have any payload. Fig. 3 shows the DM1 packet being
transmitted in the first slot, and the slave replying with a
NULL packet containing the ACK in the immediately
following slot. The master then transmits again in the next slot.
III. BLUETOOTH SIMULATION MODEL
Fig. 6 shows the simulation model of the network in
MATLAB Simulink. The above shown model simulates
Bluetooth Full duplex communication. We have to two similar
devices, each with a Transmitter and Receiver. One of them
should be set as master and the other as the slave. Other than
two Bluetooth devices, we also have an 802.11b packet
generating block as an interference source, error reading
meters and instrumentation.
A. Transmitter Design
The transmitter shown above performs data and voice input,
processing. Framing is also done. It also performs HEC, FEC.
Buffering and modulation is also done here. Frequency
hopping is the transmission technique used Fig. 7 shows the
state flow diagram of the data transmission. When the
“ACL_packets” is entered the transition to
“Transmit_blank_packet” will happen. The “Enable_Audio=0"
& "Get_blank_Packet=1" actions activates to disable audio and
Figure 6: Bluetooth Interference Simulation Model.
to generate a new data packet. When the next slot is about to
transmit, the transmitter will check the status of ARQN bit
returned from the receiving device.
If it’s in "Transmit_blank_Packet" ARQN is one, it stays in
the state and transmits another new packet. If ARQN is zero, it
shifts to the "Re_Transmit_Packet". This simulation model use
frame based processing. It can transmit samples having high
number of frames in each step of the simulation. This
technique enables quick simulation of digital systems. In this
particular model, a top sample rate of 100MHz is used.
Fig. 8 shows the state flow diagram of the data transmission.
When the “ACL_packets” is entered the transition to
“Transmit_blank_packet” will happen. The “Enable_Audio=0"
& "Get_blank_Packet=1" actions activates to disable audio and
to generate a new data packet. When the next slot is about to
transmit, the transmitter will check the status of ARQN bit
returned from the receiving device. If it’s in
"Transmit_blank_Packet" ARQN is one, it stays in the state
and transmits another new packet. If ARQN is zero, it shifts to
the "Re_Transmit_Packet". If the transmitter is in
“Re_Transmit_Packet", and ARQN is one, it shifts to
“Transmit_blank_Packet". Else it will not shift and will stay in
"Re_Transmit_Packet".
B. Receiver Design
The state flow diagram of receiver is shown in Fig. 9. It can
be seen in Fig.9 that the receiver waits a new packet all the
Figure 7: Bluetooth device having both Transmitter & Receiver.
Figure 8: Transmitter state flow diagram
time. When it senses the arrival of a packet it will register the
arrival. It will also make sure the decoder is enabled. The
above mentioned sequence of events is triggered because of the
detection of an arriving packet. The receiver has to make a
number of decisions to make sure whether the received packet
is correct or incorrect.
A DM1 packet will be checked for integrity. The receiver
performs a header error check (HEC).The address is also
verified. The receiver makes sure the packet is new and is not a
duplicate. It also checks the CRC.
Figure 9: Receiver state flow diagram
Figure 10: 802.11b Interference Source added to the channel
Figure 11: SCORT State Flow Diagram
If all these checks are correct then the packet will be
accepted. Else the packet will be rejected. This happens in the
case of a repeated packet arriving or in the case of its CRC
failing. This flow diagram is implemented in Stateflow
semantics as shown in Fig. 8. This image, captured during a
simulation, illustrates the animation provided with Stateflow,
which highlights the decision path (in bold) through the flow
chart.
C. Channel and Interferer Modeling
The 802.11b channel bandwidth is approximately 22MHz.
The Simulation model has a block which produces signals in
this bandwidth. This block can be configured to specify mean
packet rate, packet length, power, and frequency location in the
ISM band. This block is then connected to the channel where
the distance between the interference source and Bluetooth
system can be varied. Fig. 10 shows the addition of 802.11b
interference into the channel. We use this model in our
experimental verifications to determine the behavior of added
interference.
IV. COEXISTENCE SOLUTION - SCORT VOICE TRANSMISSION
The Coexistence task group working on co-existence has
suggested the use of a special voice packet to fight
interference. The synchronous connection-oriented with
Repeated Transmission (SCORT) packet achieves more robust
transmission by replacing bit-level redundancy with packet-
level redundancy. The state flow diagram of SCORT is
presented in Fig. 11. It works by repeating the transmission of
the same packet three times in one SCO interval. SCORT does
not have any error correction. SCORT is transmitted every
second time slot. As the same packet is being transmitted three
times in a row, only one voice link will be there, which is a full
duplex link. If interference destroys the transmission during
first slot, there are still three other slots, or opportunities to
communicate the packet, thus very much improving frame-
error rate (FER) in an interference scenario. It does not affect
the BER of the payload.
V. EXPERIMENTS AND RESULTS
Using the above model, we performed a series of tests to
evaluate the performance of a Bluetooth system under
interference. We used DM1 packet type to check the
performance of ACL transmission. Packet types HV1, HV2
and HV3 are used to evaluate SCO performance. Finally we
used SCORT packet type to compare its performance with
HV1, HV2 and HV3.
Fig. 12 represents the Bluetooth system throughput has been
evaluated by varying the distance between the device and the
interference source. It should be noted in Fig. 2 that a
consistent values of throughput is achieved with respect to a
constant increase in the distance between the Bluetooth
devices. From Fig. 12, we can see that the throughput of a
Bluetooth system is about 128kbps without 802.11b
interference source.
Figure 12: Bluetooth System throughput
ig. 13 shows the reduction in the throughput when 802.11b
Figure 13: Bluetooth Master and Slave device throughput in the presence of 802.11b
Figure 14: BER versus Eb/No
interfering source come closer to the Bluetooth system. Fig. 14
demonstrates the BER performance with respect to Eb/No. It
should be noted in Fig. 14 that the BER decreases linearly over
the values of Eb/No. However, the BER divergence in Fig. 14
is very rapid and acceptable for a maximum value of Eb/No.
For Fig. 15, we measured the difference in Frame Error Rate,
when using a SCORT voice packet, rather than the regular
HV1, HV2 and HV3 packet. From Fig. 15, we can see that
when using SCORT packets, there is a considerable reduction
in the Frame Error rate.
VI. CONCLUSION
Today Bluetooth and 802.11 network devices are part of our
daily life. This paper presented a model for the interference of
these two standards. Our analysis shows that situation gets
worse as more and more devices come into play. Such a
situation calls for the urgency of congestion free network.
Techniques such as SCORT are a big leap in the future for
such networks. By using SCORT packets we can minimize the
effect of interference. Hopefully in the future wireless industry
will mature in such a way that smooth data and voice
transmission will be achieved and finally a solution for Co-
existence without compromise can be realized.
REFERENCES
[1] N. Golmie, R. E. Van Dyck, and A. Soltanian, “ Interference of
Bluetooth and IEEE 802.11: Simulation Modeling and Performance”
National Institute of Standards and Technology.
[2] Matthew B. Shoemake, Ph.D., “Wi-Fi (IEEE 802.11b) and Bluetooth
Coexistence Issues and Solutions for the 2.4 GHz ISM Band” Texas
Instruments.
[3] Tsung-Chuan Huang and Shao-Hsien Chiang, “Coexistence Mechanisms
for Bluetooth SCO Link and IEEE 802.11 WLAN”.
[4] Mladen Russo, Dinko Begušić, Nikola Rožić, Maja Stella, “Speech
recognition over Bluetooth ACL and SCO Links: A Comparison”
[5] Steve Shellhammer, Symbol Technologies, “SCORT - An Alternative to
the Bluetooth SCO Link for Voice Operation in an Interference
Environment”
[6] Peter Dziwior.,“Specifications of t he Bluetooth System, Core v1.1,
www.bluetooth.org”
Figure 15: BER versus large values of Eb/No
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SCORT-An Alternative to the Bluetooth SCO Link for Voice Operation in an Interference Environment
  • Steve Shellhammer
  • Symbol Technologies
Steve Shellhammer, Symbol Technologies, "SCORT-An Alternative to the Bluetooth SCO Link for Voice Operation in an Interference Environment"
Specifications of the Bluetooth System, Core v1
  • Peter Dziwior