Transcript EE 364 Communication Theory Spring 2000

Custom Coding,
Adaptive Rate Control,
and Distributed Detection
for Bluetooth
Matthew C. Valenti
Max Robert
Assistant Professor
Mobile & Portable Radio Research Group
Lane Dept. of Comp. Sci. & Elect. Eng. Virginia Tech
West Virginia University
Blacksburgh, VA
Morgantown, WV
[email protected]
This work was supported by
the Office of Naval Research under grant N00014-00-0655,
AOL, and the MPRG Affiliates Program
copyright 2002
Motivation & Goals

Motivation



Bluetooth enables low cost/power wireless connectivity.
However, range is restricted to ~10 m due to limited power,
inefficient modulation, and modest error control capabilities.
Goal of this study


Develop strategies for improving the performance of
Bluetooth in low SNR environments.
Benefits:
• Range extension.
• Operate in noisy industrial environments.
• Tolerate more interference.
© 2002

However, all proposed strategies comply with the standard.
• We are not suggesting changes to the standard.
2/16
Features of Bluetooth

Radio layer

Gaussian frequency shift keying (BT=0.5).
• Nonorthogonal: 0.28  h  0.35
• 1 Megabaud over 1 MHz occupied bandwidth.

Baseband layer

Transmissions are broken into 625 sec slots.
• A packet may be 1, 3, or 5 slots long.

Time-division duplexing (TDD).
• Master/slave take turns transmitting.

Packet-by-packet frequency hopping.
• 79 or 23 channels spaced 1 MHz apart.
• Piconet synchronized to master’s clock.
© 2002

ACL Packets for data.
• DHx (Data high rate): No FEC.
• DMx (Data medium rate): (15,10) Hamming FEC code.
• ARQ used by both DMx & DHx (assisted by CRC).
3/16
ACL Packet Structure
72 bits
Access Code
54 bits
Packet
Header
Payload
Header
8 or 16
bits

0-2744 bits
Payload
Payload Data
0-2712 bits
CRC
16
bits
Causes of frame error:

Failure to synchronize with access code.
• Sufficient for T>65 bits of the 72 to be correct.
© 2002

Failure to decode the packet header.
• Protected by triple redundancy code.

Failure to decode the payload.
4/16
Throughput over BSC Channel
800
Data Rate in kbps
700
600
Data bits per frame
DH5
K
R
bps
6
( DN )(62510 )
DH3
Slots per frame
500 DM5
Average number of
ARQ transmissions
400 DM3
300
200
100
DH1
DM1
0
10-5
10-4
10-3
e
10-2
10-1
5/16
Throughput in AWGN
800
DH5
Data Rate in kbps
700
Performance of
noncoherent & nonorthogonal FSK:
600
1 a 2 b 2 / 2
e ( ) Q1 (a, b)  e
I o (ab )
2
500
  sinc(2h)
400
a
300
b
200
1 
2


1
2

1  2
1  2


DH1
DM1
We assume
h=0.32
5
DM5
DM3
100
0
DH3
10 =E /N in dB 15
s
o
20
6/16
Throughput in Quasi-Static
Rayleigh Fading
Data Rate in kbps
800
Quasi-static Rayleigh fading:
•SNR constant for entire frame.
•Varies from frame to frame.
•SNR is exponentially distributed.
700
600
DH5
DH3
1
  
exp
u ( )

  
•Average throughput.
f ( ) 
500
400
DM5
DM3
Pr ()  E Pr e ( )
300
200
DH1
DM1
100
0
0
5
10
15
20
=Es/No in dB
25
30
7/16
Custom Error Control

The AUX1 packet



A seventh ACL packet type.
Occupies one slot.
CRC & ARQ are turned off.
• Operates as a “noisy bit pipe”.
• Whatever is received is passed up to application.


Can use AUX1 to transport a custom code

© 2002
29 bytes of payload data.
Implement FEC & ARQ on host computers.
• Sender: First CRC encode, then FEC encode.
 Any FEC code can be used: BCH, Reed Solomon, turbo.
 Some FEC codes can also perform error detection.
• Receiver: Decode FEC code, then CRC code.
 If errors, must manually ask for retransmission.

No modification of Bluetooth standard is needed.
8/16
Example: BCH Coding in AWGN
Data Rate in kbps
150
Notes:
Used 16 bit CRC plus (232,k) shortened BCH code
t is the error correction capability of the code
up to 2 dB gain by using custom coding
100
BCH
t=10
BCH
coding bound
1 t43
50
DM1
0
5
5.5
6
6.5
7
7.5 8
Es/No in dB
DM 3
8.5
9
9.5
10
9/16
Adaptive Rate Control


Optimal packet type depends on instantaneous SNR.
Can select the packet to match the current SNR.


Most of the benefit comes from selecting from among
a small set of packets.


© 2002

If custom coding is used, then can also pick the code
parameters (e.g. t).
Set {DH5, DM5, and DM1} gives most of the gain.
CQDDR is a protocol from CSR (David McCall) which
operates under same principle.
Problem is that the channel SNR must be known a
priori (predicted).

An alternative approach is to use hybrid-ARQ with
incremental redundancy (which is “blind”).
10/16
Adaptive Coding for
Quasi-Static Fading
800
Adaptive BCH:

Use AUX1 to transport custom code.

Adapt t to match instantaneous SNR
Fully Adaptive:

Choose from among 6 standard packets.

Can also choose a custom coded AUX1 packet.

Gain is 1.5 dB.

The set {DH5,DM5,DM1} yields almost same
performance (within 0.1 dB).
Data Rate in kbps
700
600
500
DH5
DM5
400
DM3
300
“Fully”
Adaptive
200
Adaptive BCH
BCH10
100
0
DM1
0
5
10
15
Es/No in dB
20
25
11/16
Antenna Diversity


Performance in fading can be improved by using
multiple (receive) antenna elements.
Best performance improvement is achieved using
maximal ratio combining.


Instead, we perform post-detection combining on a
packet level.



© 2002
However, this is too complex and requires coherent
detection.

Use CRC to determine if packet is correct or not.
If a packet is correct at any antenna, then it will be accepted
by the system.
Packet is only needs to be retransmitted if it is incorrect at all
antennas.
Note that this requires a separate receiver for each antenna.
12/16
800
Average Throughput (kbps)
700
M=6
600
M=2
M=1
Gain @500 kbps
3.2 dB for M=2
6 dB for M=6
500
400
300
Performance of Bluetooth
With M-antenna elements
Using packet-level combining
Of DH5 packets
In quasi-static Rayleigh fading
200
100
0
5
10
15
20
25
Average Es/No in dB
30
35
13/16
Distributed Detection

Packet-level combining required the
M antennas to be attached to M
transceivers.


AP #3
AP #2

AP
#4
MS
MS
location location
A
B
AP #5
AP
#1


No reason why they must be colocated.
The transceivers could be connected
through a backbone as in an
infrastructure-based WLAN.
Detection is distributed over space.
When the mobile is equidistant to the M
transceivers, performance is as if they are
connected to the same device.
However, the diversity advantage
diminishes if mobile not in center.
AP #6
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800
AP #3
Average Throughput (kbps)
700
AP #2
M=6
AP
#4
600
500
AP
#1
MS
MS
location location
A
B
AP #5
AP #6
400
Gain @500 kbps
0.4 dB for M=2
1 dB for M=6
Performance of Bluetooth
With M-antenna elements
Using packet-level combining
Of DH5 packets
In quasi-static Rayleigh fading
When mobile is at location B
300
200
100
0
M=1
5
10
15
20
25
Average Es/No in dB
30
35
15/16
Conclusion

Several strategies can be used to improve
performance of Bluetooth.


Each strategies complies with standard.
Custom coding:
• Use AUX1 to transport custom BCH code.

Adaptive rate control:
• Match the frame type to prevailing channel condition.

Antenna diversity:
• Use M antennas, but combine at packet level.
• Antennas don’t need to be co-located.
 Multiple Bluetooth devices can mimic antenna array.

Future work:

Channel tracking and prediction.
© 2002
• Hybrid ARQ with incremental redundancy.

Actual implementation of these strategies.
• Validation of channel models.

Application of similar concepts to 802.11
16/16