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An Opportunistic Frequency Channels Selection
Scheme for Interference Minimization
Syed Rizvi, Khaled Elleithy, and Mustafa Khan
Dept. of Computer Science and Engineering
University Of Bridgeport
Bridgeport, CT 06604, USA
elleithy@bridgeport.edu
Abstract—This paper presents a solution to resolve the
interference problems between the Wi-FiTM and
BluetoothTM wireless technologies. A new channel
selecting approach is being used to select the frequency
channel. The signal strength in a channel is assessed, and
that value is used to select the channels to send data
without interference. Thus we are trying to establish true
"Coexistence without Compromise" between
BluetoothTM and Wi-Fi TM.
Keywords—Bluetooth, Frequency hopping spectrum,
direct sequence spread spectrum, interference, Wi-Fi.
I. INTRODUCTION
owadays wireless access networks use many
different technologies. Standard 802.11b/g/n is the
most extended wireless technology to access Local
Area Networks (LAN), which is known as Wi-Fi
standard around the world. On the other hand,
Bluetooth standards are used frequently in Personal
Area Networks (PAN). PAN is low-cost, low-power,
secure and robust technology [2]. Both Wi-Fi and
Bluetooth are based on spread spectrum signal
structuring – a technique where a narrowband signal is
expanded to a wideband signal [1]. Both Wi-Fi (802.11)
and Bluetooth are located in unlicensed Industrial
Scientific and Medical frequency band, which is called
ISM. The frequency range of ISM band is 2.4 Ghz
(2.402 – 2.480 GHz).
In wireless PAN, Bluetooth is an industrial
specification. Connection and exchange of data
between devices such as mobile phones, laptops, PCs,
printers, digital cameras and video game consoles
became very much convenient by Bluetooth. It uses a
secure, globally unlicensed short-range radio frequency.
But Wi-Fi which was developed to be used for mobile
computing devices, such as laptops, LANs, is now
increasingly used for more services, including Internet
and VoIP phone access, gaming, and basic connectivity
of consumer electronics such as televisions and DVD
players, or digital cameras. Even more standards are in
development that will allow both Bluetooth and Wi-Fi
to be used by cars in highways in support of an
Intelligent Transportation System to increase safety,
gather statistics, and enable mobile commerce etc [3].
In wireless communication system, one or more
frequency bands (carrier frequencies) are used to
communicate. Both Bluetooth and Wi-Fi share the same
2.4 GHz band, which is under Federal Communications
Commission (FCC) regulations, extends from 2.4 to
2.4835 GHz. In this ISM band, a system can use one of
the two spread spectrum methods to transmit data.
FHSS (Frequency-hopping spread spectrum) and
(DSSS) Direct-sequence spread spectrum are the two
techniques used. FHSS enables a device to transmit
high energy in a relatively narrow band, but for a
limited time. On the other hand Direct-sequence spread
spectrum (DSSS) allows a device to occupy a wider
bandwidth with relatively low energy in a given
segment of the band, and it does not hop. Bluetooth
uses FHSS, which uses 1-MHz-wide channels and a
hop rate of 1600 hops/sec (625 microseconds in every
frequency channel). Bluetooth uses 79 different
channels in the United States. Wi-Fi opted for DSSS,
which uses 22 MHz of bandwidth (passband) to
transmit data with speeds of up to 11 Mb/sec. Wi-Fi
system uses any of 11 22-MHz-wide sub channels
across the allocated 83.5 MHz of the 2.4 GHz
frequency band. In the case of Wi-Fi, maximum three
networks can coexist without interfering with each
other. Regardless of the portion of the band in which
Wi-Fi operates, sharing with Bluetooth is inevitable.
Two wireless systems using the same frequency band
would have a high possibility to interfere with each
other.
N
II. PROBLEM IDENTIFICATION
Since both Bluetooth and Wifi devices operate at the
same 2.4GHz ISM band, the probability of interference
is very high. In case of Wi-Fi, the client or access point
will listen to the transmission medium to check whether
that channel is occupied or not. If the channel is
occupied, it indicates that data is transmitted at the
given point in time. In such an event, the Wi-Fi client
will hold off and will listen to a different channel. Once
it gets an unoccupied channel, the Wi-Fi client will start
transmitting the data using that particular channel.
Whenever interference occurs in a channel, Wi-Fi will
start retransmitting in the same channel. This technique
provides a fairly good method of sharing the radio
spectrum without interference.
But in case of Bluetooth, it does not have such
techniques like Wi-Fi. Therefore, it hops around the
entire 79 channels to transmit the data. The width of a
Bluetooth channel is 1MHz, but for Wi-Fi it is 22MHz,
(i.e., nearly one fourth of the entire radio spectrum of
83.5MHz). This implies that the Wi-Fi has a wider
bandwidth compared to narrow Bluetooth devices. This
makes the probability of narrow Bluetooth channels to
hop around the wider Wi-Fi channels high. Whenever
interference occurs, Bluetooth will hop away and will
start to hop again in a new channel.
A critical problem is that Bluetooth and 802.11b
neither understand each other nor follow the same rules
[7]. A Bluetooth radio may haphazardly begin
transmitting data while an 802.11 station is sending a
frame. This results in a collision, which forces the
802.11 station to retransmit the frame when it realizes
that the receiving station is not going to send back an
acknowledgement. This lack of coordination is the basis
for RF interference between Bluetooth and 802.11.
III. RELATED WORKS
Early Bluetooth devices interfered with 802.11b/g
Wi-Fi devices because both devices tried to use the
same channel for an extended period of time which
caused interference, lost data, and eventually a loss of
service for both devices (see Fig. 1). To enable
coexistence between Bluetooth and Wi-Fi, various
organizations like IEEE, Bluetooth Special Interest
Group etc are working on various techniques.
A. Adaptive Frequency Hopping (AFH)
AFH is one such solution that is widely used to nullify
interference between these two technologies [4]. In this
scheme, the normal Bluetooth frequency hopping
sequence is replaced with an adaptive frequency
scheme.
Presently a Bluetooth client must hop through the
entire 79 different channels (i.e., Wi-Fi is already
occupying the channels). The same concept is
illustrated in Fig.2, where Blue Square shows the
Bluetooth devices and the yellow square represents the
Wi-Fi.
This technique could add some degree of intelligence
into the process, so that a Bluetooth device would
analyze the available bandwidth and transmit the data to
those channels where interference has not encountered.
B. Transmission Power Control
Another technique involves adapting the transmit power
used by various devices in the ISM band. The reasoning
behind the notion of adaptive power control is based on
common sense. Transmitting data at a power level
above the minimum needed to meet a predetermined
level of acceptable data integrity unnecessarily causes
interference to other users in the band [5].
C. Adaptive selection of packet type
The type of Bluetooth packet being transmitted can also
affect coexistence performance. Bluetooth packets can
carry various payloads, depending on the number
Fig. 1. An Illustration of Bluetooth devices interference with
802.11b/g Wi-Fi standards [7]
Fig.2. Collisions resulting from random frequency hopping
adapting to the environment
of “slots” in the packet. Packets can occupy anywhere
from one to five time slots, according to the Bluetooth
specifications [6]. While carrying more than 10 times as
much data, a Bluetooth packet with five slots will
remain on a certain channel at a certain frequency five
times longer than a one-slot packet, increasing both the
vulnerability of this packet to interference, as well as
increasing the chance that the transmission will
interfere with others sharing the frequency.
Reducing the packet type to one slot, for instance,
would reduce the vulnerability of any one packet to
interference because the packet would have a shorter
duration. This would improve the chances that a
particular packet would be accurately received.
Research has shown that shorter Bluetooth packets
can improve data throughput in an environment with
interference. A throughput tradeoff arises from the
higher level of overhead that must be processed with
shorter packets, including additional address and packet
header processing, and the dead time between hops that
is needed for synthesizer and transmit/receive
switching. A point of diminishing returns is reached
where the overhead of processing a greater number of
smaller packets counterbalances the performance
improvements of the shorter packets.
IV. FREQUENCY CHANNELS SELECTION SCHEME
In this section, we present a new algorithm, which
modifies the original frequency hopping sequence
scheme. AFH is based on the convention that some
channels are good and some channels are bad for data
transmission. ‘Good’ and ‘bad’ are determined based on
whether the channel is already occupied or not. If a
channel is occupied, that channel is bad and if the
channel is unoccupied that channel is termed good.
In particular, our design modifies the Bluetooth
frequency-hopping scheme such that it can choose the
channels for transmitting the data intelligently. The goal
of our modified scheme is to provide a congestion free
scenario without modifying the Wi-Fi DSSS.
A typical Bluetooth network uses a Channel selector
to select the random frequency in which the data has to
be sent (see Fig. 3). For the intelligent channel
selection, our proposed scheme uses a special
parameter called RSSI, which stands for Received
Signal Strength Indicator. The IEEE 802.11 standard
defines a mechanism to measure RF energy. The RSSI
contains numeric value, an integer with an allowable
range of 0-255 (a 1-byte value). For example, when an
adapter wants to transmit a packet, it must be able to
detect whether or not the channel is clear (i.e., nobody
else is transmitting). If the RSSI value is zero, then the
chipset knows that the channel is clear (i.e., the “Clear
To Send”). Different vendors use different signal levels
for the Clear Channel Threshold, the Roaming
Threshold, and the RSSI value that represents these
thresholds differences from vendor-to-vendor because
different RSSI_Max values are implemented.
RSSI is an internal circuit which determines the
signal power in a frequency channel. The output value
of RSSI circuitry is used to determine the best possible
frequency channel to send the data without any
interference. The RSSI card will issue a CTS (Clear To
Send) signal to the network interface card (NIC).
Wireless NIC will select those channels whose RSSI
value is zero and begin transmitting the data between
Master and Slave devices.
Channel selection works as follows. Each Bluetooth
receiver will have a Frequency Status Table (FST),
where an RSSI value is associated to each frequency
channel, as shown in Table (Table 1) below.
Frequencies are classified “good” or “bad” depending
on whether their RSSI value is 0 or not. Each slave has
its own FST, which maintained locally. However, the
master has in addition to its
a copy of each slave’s FST. At regular time intervals
each slave updates its FST copy kept at the master
using a status update message that can be defined in the
Layer Management Protocol (LMP). Alternatively, the
master can derive information about each slave’s FST
by keeping track of the ACK bit sent in the slave’s
response packet. Fig. 4 shows the illustration of the
proposed scheme.
Table 1. Frequency Status Table
Status
Frequency Channel
RSSI
Good
Channel 17
0
Bad
Channel 26
2
good
channel 0
0
Good
channel 3
0
Bad
Channel 71
5
Good
Channel 78
0
Bad
channel 9
6
Bad
channel 6
12
Bad
Channel 46
58
If the status is good the network card is clear to send
(CTS) data in that channel. Once the card is clear to
send, a packet of information can be sent. On the other
hand, AFH requires a master to slave message
exchange in order to keep the piconet synchronized.
The following method has to implement at the
master device that postpones the transmission of a
packet until a slot associated with ”good” frequency
becomes available. The master device, which controls
all data transmissions in the piconet, uses information
about the state of the channel in order to avoid data
transmission to a slave experiencing bad frequency.
Furthermore, since a slave transmission always
follows a master transmission, using the same principle,
the master avoids receiving data on a ”bad” frequency,
by avoiding a transmission on a frequency preceding a
”bad” one in the hopping pattern. This simple
scheduling scheme needs only be implemented in the
master device and translates into the transmission rule.
This simple scheduling scheme is implemented as an
algorithm. From the Transceiver in the NIC, we will get
the RSSI value. That RSSI value is used as the input of
the algorithm. RSSI_val defines the RSSI value input
received from the NIC where n = 79 represent the
number of channels (see Fig. 5).
S: measured signal strength vector for n channel for
79 channels such as: S = (S0, S1, S2 ,..., S78. Each Si
where 0 <= I <=78, containing the RSSI value which is
calculated from RF power by the CC2420 transceiver
chip and set in all the vector element.
Algorithm: Opportunistic frequency selection algorithm
Start
Call RF_Power to compute RSSI value for each channel
While true do
Calc_Vec(Si); // where i= 0 to 79
For each RSSI_val є Si do
If RSSI_val == 0 then
CTS flag is set to true
Channel == Free
Exit
End If
End For
End While
End
When algorithm is implemented a loop will be
activated and a function Calc_vec(S) is called. The
Calc_vec(S) function will return RSSI value from NIC.
In the wireless Router (NIC), the NIC card will return
the RSSI values for each frequency channel. That RSSI
value is placed in the S vector.
Now the S vector is scanned for a Zero RSSI value.
Fig. 3. Modified Bluetooth Block Diagram using AFH
Wireless NIC
Master device
Slave 1
Channel
Selector
Slave 2
Fig. 4. Channel Selecting Flowchart
Stop
Start
Calc_vec(S)
While True
For each RSSI_val
in S
RSSI_val =0
CTS flag is activated
Yes
Yes
Yes
No
When a zero RSSI value is found a CTS flag will be
activated. The CTS flag will send a “Clear To Send
message” to the NIC. Then algorithm will exit and the
devices will start transmitting data using those
frequency channels with RSSI value of zero.
The master transmits in a slot after it verifies that
both the slave’s receiving frequency and its own
receiving frequencies are good”. Otherwise, the master
skips the current transmission slot and repeats the
procedure over again in the next transmission
opportunity.
V. CONCLUSION
In this paper, we presented an opportunistic
frequency channel scheme. We discussed how the
proposed scheme selects an available channel by
analyzing the signal strength and minimizing the
potential interference for data transmission. Several
illustrations were provided in the context of master-
slave scenario to show the practicality of our proposed
scheme. Finally, to support the implementation, we also
provided an algorithm for the opportunistic frequency
selection.
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Fig. 5. Collisions avoided using Adaptive Frequency
Hopping
