ARSAdvances in Radio ScienceARSAdv. Radio Sci.1684-9973Copernicus GmbHGöttingen, Germany10.5194/ars-13-181-2015Coexistence issues for a 2.4 GHz wireless audio streaming in presence of bluetooth paging and WLANPfeifferF.pfeiffer@perisens.deRashwanM.BieblE.NapholzB.perisens GmbH, Munich, GermanyFachgebiet Höchstfrequenztechnik, Technische
Universität München, Munich, GermanyDaimler AG, Sindelfingen, Germanyformerly at: Daimler AG, Sindelfingen,
GermanyF. Pfeiffer (pfeiffer@perisens.de)3November201513518118814December20145March201510March2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://ars.copernicus.org/articles/13/181/2015/ars-13-181-2015.htmlThe full text article is available as a PDF file from https://ars.copernicus.org/articles/13/181/2015/ars-13-181-2015.pdf
Nowadays, customers expect to integrate their mobile electronic devices
(smartphones and laptops) in a vehicle to form a wireless network. Typically,
IEEE 802.11 is used to provide a high-speed wireless local area network
(WLAN) and Bluetooth is used for cable replacement applications in a wireless
personal area network (PAN). In addition, Daimler uses KLEER as third
wireless technology in the unlicensed (UL) 2.4 GHz-ISM-band to transmit
full CD-quality digital audio. As Bluetooth, IEEE 802.11 and KLEER are
operating in the same frequency band, it has to be ensured that all three
technologies can be used simultaneously without interference. In this paper,
we focus on the impact of Bluetooth and IEEE 802.11 as interferer in
presence of a KLEER audio transmission.
Introduction
This paper addresses the impact of an IEEE 802.11b/g and a Bluetooth
system on a KLEER audio transmission. The considered wireless architecture
includes a WiFi (IEEE 802.11b/g) combo module and three independent KLEER
sources which allow three independent audio streams (see Fig. ).
Considered wireless architecture.
Bluetooth, IEEE 802.11b/g and KLEER channels in the 2.4 GHz-ISM-band.
KLEER from SMSC is a short-range radio interface in the 2.4 GHz frequency
band that is designed for lossless 44.1 kHz-sampled 16 bit stereo audio
transmission in full CD quality see. A lossless audio compression
is used to reduce the required net data rate of 1.4112 Mb s-1 to
approximately 1 Mb s-1 on average. But the short term compression ratio
depends on characteristics of the audio signal. Therefore the full data rate
of 1.4112 Mb s-1 has to be supported to achieve lossless full CD quality
audio streaming. KLEER is using 16 equally spaced RF channels from
2.403 to 2.478 GHz (having a frequency spacing of 5 MHz) and
each channel occupies an RF bandwidth of 3 MHz. For independent audio
streams, each stream requires its own RF channel. Thereby KLEER is offering a
gross data rate of 2.37 Mb s-1. The resulting excess net data rate is required
for retransmission of corrupted packets in case of interference. An audio
buffer ensures a continuous audio stream in case of packet loss. KLEER uses a
configurable buffer size of up to 100 ms. The buffer size has to be high
enough to assure a sufficient number of hops to find a channel without
interference. As additional coexistence mechanism for interference
mitigation, KLEER uses a dynamic frequency diversity to change the current
channel if it is experiencing bad channel conditions. A channel change has to
be initiated if the retransmission bandwidth is not able to cope with the
packet loss rate over a defined period of time. At the audio sink side (at
the wireless headphones), Kleer uses antenna diversity by switching two
orthogonal polarized antennas which provides additional interference
mitigation in small scale fading for in-vehicular communication. In Table
all important details on KLEER are shown and compared to
Bluetooth and IEEE 802.11b/g. Figure shows the channel
distributions of Bluetooth, IEEE 802.11 and KLEER in the ISM-band.
In the first section, the impact of an IEEE 802.11b/g interferer is
discussed and measurement results are presented. The next section analyzes
the impact of a Bluetooth interferer in connected and connecting state using
an analytical packet error model. Typical Bluetooth states are classified
according to their impact on a KLEER transmission. It turns out that
Bluetooth page state is very critical regarding interference. The following
section gives details about the packet timing and the hopping sequence during
page state. Furthermore, the characteristic behavior of a page hopping
sequence is discussed. In the last section, a statistical analysis is done to
carry out a relevance analysis concerning the impact of Bluetooth page state
on KLEER.
Impact of IEEE 802.11b/g on KLEER
In the network architecture considered in this paper, IEEE 802.11 is using
channel 6 with a center frequency of 2.437 GHz. An IEEE 802.11b/g
channel occupies a 20 dB-bandwidth of approximately 16/17 MHz.
According to the channel map, shown in Fig. , four KLEER
channels overlap completely and one partly with a single IEEE 802.11 b/g
channel. This assumes an IEEE 802.11 bandwidth of 22 MHz according to a
more conservative bandwidth definition which is often used in literature. We
used KLEER evaluation boards to analyze the impact of an IEEE 802.11 b/g
interferer on a KLEER audio streaming. This allows disabling KLEER's dynamic
channel switching. By applying an IEEE 802.11 b/g signal on a KLEER
transmission we could evaluate the signal to interferer ratio (SIR) at which
audio dropouts occurs. For a SIR region of 1 to -20 dB, audio
dropouts occurred in up to four KLEER channels (out of 16) in presence of
IEEE 802.11b/g traffic. The exact number of disturbed channels (channels
where audio dropouts occur) depends on the interferer signal (either IEEE
802.11b or g) and the overlapping spectral power density of the
interfering signal in a KLEER channel. Regarding the SIR values, it is
important to note that IEEE 802.11b/g is transmitting with a maximum
power of 20 dBm compared to KLEER with 1.5 dBm. Therefore a SIR of
-20 dB represents almost equal path loss conditions between KLEER victim
receiver and KLEER transmitter and IEEE 802b/g transmitter, respectively.
Under more unfavorable path loss conditions even more than four KLEER
channels can be disturbed. For SIR values between -20 and -30 dB,
IEEE 802.11b disturbs five KLEER channels. On the opposite side, KLEER is
not affected at all by IEEE 802.11b/g for SIR values higher than 1 dB.
Considering the difference in transmitting power between IEEE 802.11 and
KLEER, interference is likely in an in-vehicular environment. Therefore KLEER
has to avoid IEEE 802.11b/g signals. But if only a limited number of the
total 16 channels are affected by IEEE 802.11 signals, KLEER's dynamic
channel switching (DSC) is an effective method to avoid the disturbed
channels. Our measurements showed that DSC enables KLEER to dynamically
change the affected channels during an IEEE 802.11 disturbance with an SIR
of -30 dB without audio quality degradation.
Impact of Bluetooth on KLEER
KLEER needs a minimum SIR of about 13 dB to prevent audio dropouts in
presence of a constantly transmitting Bluetooth GFSK-signal. The SIR
difference compared to IEEE 802.11 signals can be explained by the lower
spectral bandwidth of Bluetooth. In contrast to IEEE 802.11, Bluetooth
communication standard uses Frequency Hopping Spread Spectrum (FHSS) which
combines TDMA (Time Division Multiple Access) and FDMA (Frequency Time
Division Multiple Access). The TDMA divides the channel in 625 µs slots
resulting in 1600 slots s-1. For the connecting state (inquiry or paging)
also half slots of 312.5 µs are used. The FDMA is dividing the ISM band
in 79 channels of 1 MHz width starting at 2.402 and ending at
2.480 GHz. Each packet is transmitted in a different channel than the
previous packet following a pseudo-random hopping sequence. In Bluetooth
specification version 1.2, Bluetooth Special Interest Group (SIG)
introduced Adaptive Frequency-hopping (AFH) as additional coexistence
mechanism for connected devices . When AFH is in use,
channels which are classified as “bad” are removed from the hopping sequence.
The Bluetooth core specifies a minimum number of 20 RF channels. In
presence of an interfering FHSS communication system – like Bluetooth – with
uniformly distributed channels over the whole band, a channel switch is
unnecessary and even bares risks: a channel change always implies a data
overhead and reduces the net data rate. Moreover, the Bluetooth channel
classification is influenced negatively by a constantly switching system. In
best case, Bluetooth will classify the channels inside the current KLEER
channel as “bad” and stop using these channels. Therefore, the interference
is only temporary during the Bluetooth AFH-adaptation time. Nevertheless,
KLEER's retransmission bandwidth has to be able to compensate the packet loss
during an occurring Bluetooth interference to provide overall coexistence. To
calculate the resulting packet loss, the packet distribution in time and
frequency has to be known. As Bluetooth supports multiple applications in
different profiles, the paper focuses on typical in-vehicle scenarios:
paging/inquiry,
A2DP audio streaming (ACL connection type),
hands-free telephony (SCO connection type),
and connected state without any data transmission.
The measurements were conducted with evaluation boards using an Ellisys
Bluetooth sniffer. In Table the average packet distribution in
time and frequency is shown.
Average packet distribution in time and frequency for different Bluetooth states;
data derived from an exemplary measurement using Ellisys Bluetooth sniffer.
Packet distribution time Packet distribution in frequency domainStateAverage Slot rate (Master to Slave)Average Slot rate (Slave to Master)Average Slot rate (both directions/all packet types)Page/Inquiry1 slot/ID-packet (68 µs)–1 slot/ID-packet (68 µs)Divided in A/B trains with a simultaneous use of 16 channelsA2DP audio streamingapprox. 16 slots/2-DH3-packet (1.4 ms mainly 2-DH3 packets are used)approx. 8.4 slots/NULL-packet (126 µs)approx. 5.5 slots/packet20–79 channels with AFHHands-free12 slots/2-EV3-packet (392 µs mainly 2-EV3 packets are used) + ca. 40 slots/NULL-packet (NULL 126 µs)12 slots/packet (mainly 2-EV3 packets [392 µs])+ ca. 70 slots/packet (NULL packets [126 µs]) approx. 4.8 Slots/packet20–79 channels with AFHConnected state without data transmission40 slots/POLL-packet (126 µs)40 slots/packet (NULL-packets [126 µs])20 Slots/packet20–79 channels with AFH
Comparing the slot rates for both directions as worst-case interference
scenario, it shows that paging/inquiry achieves by far the highest slot rate,
however, with very short packets of only 68 µs length. In order to make
a statement of the interference impact, it is necessary to calculate the
resulting packet error rate. According to , an analytical
approach is used to model the occurring interference. Figure
shows the timing of the desired packets with respect to the interfering
packets seen at the victim's receiver. The model assumes that the desired and
interfering packets are sent periodically in a fixed time period of
Tsig and Tint. This simplifies the calculation,
but is only valid in case of paging/inquiry and SCO connection. But
nevertheless, it also gives a reasonable estimation for the other cases. The
packet length is denoted with tsig and tint .
Collisions at the victim's receiver.
The variable Δt defines the time offset between a desired and an
interfering packet. Assuming that Δt is uniformly distributed, the
average number that an interfering packet hits a desired packet in time can
be calculated, as follows:
hi=tsig+tint,iTint,i.
Considering different interfering packets i=1,2,3,..,M the total number is
a sum of the individual numbers.
h=∑i=1Mtsig+tint,iTint,i
Assuming that every collision in time and frequency causes a packet error,
the packet error rate is:
PER=h⋅nintntotalifh≤ntotalnint1ifh>ntotalnint,
where ntotal is the total number and nint the number of
channels overlapping with the desired signal. The channel distribution is
assumed to be uniformly distributed. The above mentioned formula assumes that
the collision in time and frequency are independent. Even if not all
assumptions of the packet error model are completely valid, it gives a
sufficient well estimation of the interference. Using the derived model and
assuming a KLEER packet length of 1.3 ms an average packet error rate can
be calculated. For A2DP, Hands-free and no data transmission two different
interfering scenarios shall be regarded: in both scenarios, three BT channels
are overlapping with a KLEER channel. But one scenario assumes BT to use all
79 channels and the other only 59 channels. The latter represents a
situation where AFH avoids 20 channels due to additional IEEE 802.11
interference. In both cases, only co-channel interference with three BT
channels inside one KLEER channel is considered. For Bluetooth paging/inquiry
16 channels are used simultaneously in one interval of 10 ms. A
characteristic of the paging/inquiry sequence is that the adjacent channels
always remain unused. More details about the characteristics of Bluetooth
page state are given in the next section. Considering co-channel interference
using KLEER's channel bandwidth of 3 MHz three cases have to be
distinguished: no interference occurs and one or two channels out 16 are
disturbed. The resulting probabilities that a BT packet hits a KLEER packet
are shown in Table .
Average probability that a Bluetooth packet hits a 1.3 ms long
KLEER packet in frequency and time for different Bluetooth states.
StateBT channels inside KLEERAverage probability that a BT packet hits a KLEER packet in time and frequency and total # of channelsPacket from MasterPacket from SlavePacket either from Master or SlavePage0 of 160%0%0%Inquiry1 of 1613.7%0%13.7%2 of 1627.4%0%27.4%A2DP3 of 79 (only BT)1.0%1.0%1.0%audio3 of 57 (BT & IEEE 802.11)1.4%1.4%2.8%Hands-free3 of 79 (only BT)1.1%1.0%2.1%streaming3 of 57 (BT & IEEE 802.11)1.4%1.3%2.7%Connected3 of 79 (only BT)0.2%0.2%0.4%state w/o data3 of 57 (BT & IEEE 802.11)0.3%0.3%0.6%
Constant channel switch of a single KLEER transmission in presence of
BT paging and IEEE 802.11b/g – measurement done with signal generator and cable setup.
The table clearly shows that the critical states are page/inquiry with a
possible packet error rate of 13.7 and 27.4 % in average. For
Bluetooth connected states, the probabilities lie between 0.2 and
2.8 % which can be compensated by retransmitting the lost packets. The
reader could notice that the interference due to paging/inquiry is not
relevant as it is a very rare event. This is true for inquiry but not for
page state: sometimes it is desirable that a device is able to connect at
anytime. For example, in a vehicle the head unit enters periodically the page state
every 20–30 s if no device is connected. In such a case, interference
from Bluetooth paging has definitely to be excluded. As KLEER's
retransmission bandwidth is not able to cope with the packet error rates for
noise-equivalent audio signals which cannot be sufficiently compressed. Thus,
KLEER's DSC has to be capable to find a free channel fast enough without
audio degradation. To analyze the coexistence capabilities of KLEER in
presence of BT paging/inquiry we generated a BT page state sequence using a
vector signal generator. The sequence was generated with a periodic interval
of 1.28 s long A and B trains. As BT paging always leaves a region of
16 MHz free, we added a noise signal to fill this gap. This bad case
interference scenario was applied to a KLEER transmission. The power ratios
are chosen in such a manner that a collision in frequency and time leads very
likely to a packet loss. To manage this bad case scenario KLEER has to be
able to constantly switch in a free channel. The Fig. shows the
measured power density of the described scenario against frequency and time
in a waterfall plot.
As shown in Fig. , KLEER initiates a channel switch if the
Bluetooth page sequence changes from A to B train and vice versa. After every
train change, KLEER is able to find an undisturbed channel without any audio
degradation. Under these difficult conditions, KLEER's channel switching
algorithm is working well.
Statistical interference analysis of page (inquiry) sub state
As shown in the last section Bluetooth page (inquiry) sub state is a very
serious interferer. For a Bluetooth connection setup the master sends two
68 µs long ID packets each second slot. The ID packets are spaced by
312.5 ms. Each packet is sent on a different frequency according to a
short pseudo-random hopping sequence with a total period length of 216
slots (= 40.96s). The hopping sequence is determined by the ULAP of the
Bluetooth device which is paged. In case of inquiry, the ULAP is usually
derived from the general Access Code (GIAC). A total page (inquiry) hopping
sequence consists of 32 dedicated wake-up frequencies. These 32
frequencies are divided into two partial sequences of 16 frequencies. The
page (inquiry) hopping sequence does not use a hop adaptation. During an
interval of 10 ms the master transmits sequentially ID packets at 16
frequencies. The partial sequences – the so called A and B trains – are
shifted by half of a page sequence period (=40.96s/2). In modes
R1 and R2, the trains are repeated 128 and 256 times (1.28 and
2.56 s) to assure that the slave is able to detect at least one message.
After one repetition, it is switched to the other train. Figure
shows the used channels of an exemplary page hopping sequence.
Exemplary Hopping sequence in page state for ULAP = 0x8B6949.
The figure illustrates the characteristics of a page (inquiry) hopping
sequence:
The sequence always leaves a region of 16 channels free.
There is a gap of at least one frequency between two used channels.
The page/inquiry hopping sequence repeats every 40.96 s.
Every 1.28 s one (out of 16) frequencies changes.
Between 20.48 s (=16×1.28 s – phase difference between
A and B train) two different sets of 16 channels are used.
As described before, every page hopping sequence uses 16 channels per train
and has a free region of 16 frequencies. The spectral location of the free
region depends on the Bluetooth device address and is uniformly distributed
(for random device addresses). Furthermore, we know that an IEEE
802.11b/g signal is able to disturb up to 5 KLEER channels – assuming
only co-channel interference. Assuming a bad case scenario, an IEEE
802.11b/g signal lies into the free region of the page hopping sequence.
In an absolute worst-case scenario, the 16 frequencies of A or B train are
disturbing the remaining 11 KLEER channels outside IEEE 802.11. In this
case, all 16 KLEER channels are disturbed either with the IEEE
802.11b/g signal or the paging signal. KLEER would constantly initiate
channel switches without the chance to find a free channel – at least during
a single page train of 1.28 or 2.56 s. It is important to know the
probability of occurrence for this worst case scenario to evaluate the
relevancy. Based on 10 000 calculated page hopping sequences with random
ULAP addresses a statistical analysis was performed. For every ULAP address
32 sequences of 16 frequencies were calculated to cover all possibilities
over time. In total 320 000 page hopping sequences were calculated and
statistically evaluated. Figure shows the probability of
occurrence for the number of interfered channels. In the upper figure the
probability is given for the maximum number (considering the 32 possible
sequences over time for one ULAP address). The lower figure shows average
number of interfered channels.
Number of disturbed channel by a Bluetooth page state with probability
of concurrency calculated with 10 000 different Bluetooth device addresses.
As result it can be stated that the above mentioned worst-case situation is
possible: the statistical probability that IEEE 802.11b/g and Bluetooth
page state overlap with all 16 KLEER channels is about 0.02 %. In other
words, two out of 10 000 randomly generated ULAP addresses are manifesting
this behavior. In such a case, the interference on all channels occurs during
1.28 of 40.96 s. It should be emphasized that this worst-case
situation only occurs if several factors come together:
Decrease amount of transmit data (e.g. using adaptive lossy compression in case of interference)
A channel switch becomes unnecessary with a sufficient retransmission bandwidthIncrease KLEER's number of channelsDecreasing KLEER's channel spacing of 5 MHzA larger number of channels increases the chance of having non-disturbed channelsImprove SIR at the victim's receiver
Increase KLEER's Tx power
Decrease Bluetooth Tx power (in page state)
Increasing the power of the desired signal or reducing the power of the interferer signal improves the SIR at the victim's receiver
Bluetooth is enabled at the vehicle's head unit (HU) without any connected device.
IEEE 802.11b/g packet load is high and the SIR is low enough to disturb five KLEER channels.
The critical sequence is appearing while paging is active.
A packet collision between a Bluetooth ID packet and a KLEER packet leads very likely to a KLEER packet loss.
From a customer view, a page state interference is incomprehensible as
Bluetooth is switched off at the mobile device and only IEEE 802.11 as single
system is used simultaneously with KLEER. In such a situation the customer
cannot understand why he is experiencing audio dropouts. If there is more
than one KLEER connection active, of course more than one free channel is
needed. At maximum three independent streams (on three channels) are
possible. The probability that three or more channels are disturbed during a
page state is already 12.4 %. Thus is it certainly necessary to take
steps to minimize the interference in such a critical state. Table gives an overview of possible measures for interference
reduction: beside of the pure technical aspects, it was important for Daimler to avoid
time-consuming costly firmware and hardware changes. Considering the cost and
interference aspects, the following two measures were taken: the audio level
was lowered digitally on the transmitter's side to improve the audio
compression rate. A smaller amount of transmit data increases the
retransmission bandwidth. By adjusting the channel switch algorithm to the
new retransmission bandwidth, a channel change become more unlikely during
page state even if one Bluetooth channel lies inside the KLEER channel. It
must be taken into account that an audio level reduction implies an increase
of the audio quantization noise. Additionally to the reduction of the audio
level, the Bluetooth transmit power was reduced during page/inquiry state to
avoid packet loss in case of packet collision. In an in-vehicle situation the
distances to the mobile device are very short (typically below 3 m) which
usually allows a certain power reduction. First antenna based measurements on
random positions inside a vehicle cabin showed that a power reduction of up
to 20 dB does not affect the connect ability of Bluetooth devices.
Conclusions
In this paper we evaluated the coexistence of KLEER, a proprietary wireless
standard in the 2.4 GHz-ISM-band used for audio transmission in full CD
quality, in presence of Bluetooth and IEEE 802.11b/g. KLEER provides a
coexistence mechanisms with packet retransmission and dynamic channel
switching (DCS). The DCS is able to cope with static (or slowly switching)
interferers like IEEE 802.11b/g without audio degradation. Unavoidable
packet loss of a frequency hopping spread spectrum (FHSS) Bluetooth system in
connected state can be compensated by retransmitting lost packets. The
missing packets are compensated using a buffer management. The investigation
showed that a very serious interference scenario consist of a Bluetooth
system in page state in combination with an IEEE 802.11b/g link. This
case is particularly critical when the Bluetooth master device enters
periodically the page state to allow a connection of slave devices at any
time. In an absolute worst-case scenario, all 16 KLEER channels can be
disturbed either by Bluetooth ID packets or IEEE 802.11b/g packets. Two
out of 10 000 ULAP addresses are showing this worst-case behavior. Measures
are presented to improve the coexistence in this situation. Most of the
publications concerning interference are dealing with systems operating in
connected state. But this study shows that it is also important to include
unconnected states into a full coexistence investigation. Under certain
conditions Bluetooth connecting state (paging/inquiry) can be more critical
as connected states. The packet rate of Bluetooth paging is approximately
five times higher compared to typical connected states as Hands-Free
telephony or A2DP streaming. Furthermore, the packets are sent over the whole
band (only a region of 16 MHz is free) without channel adaptation (AFH)
for interference mitigation. Another important aspect is that a user is not
able to understand a Quality-of-service (QoS) degradation during page state
interference as it occurs when Bluetooth is switched off at his mobile
device. The interference issues of Bluetooth page state should also be viewed
from the interferer side. For interference mitigation, methods are needed to
avoid interference from other devices sharing the same band – but as second
coexistence objective the interference on other devices should be minimized
as well. During a single page sequence of 1.28 s (2.56 s) Bluetooth
master device sends 2048 (4096) identical 68 bit-packets in a very
high rate containing a high amount of redundant information. On the slave's
side, this is favorable as the scan time can be minimized and thus energy
saved. But for other systems, this could cause serious interference as shown
in this paper. An approach to minimize the interference without changing the
connecting procedure, is to adaptively reduce the transmit power during the
connection state. It has however be assured that the devices are close enough
to each other that a reduction in transmit power will not affect the ability
to connect devices.Edited by: F. Gronwald Reviewed by: M. Koch and one anonymous referee
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