March 2004
doc.: IEEE 802.15-04/0106r01
Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: [WiQuest Proposal for Reduced PAR Channel Estimation Sequence for MB-OFDM]
Date Submitted: [ 13 March, 2004]
Source: [Matthew B. Shoemake and Sridhar Rajagopal] Company [WiQuest Communications, Inc.]
Address [8 Prestige Circle, Suite 100, Allen, Texas, 75013, USA]
Voice:[+1 214-547-1600], FAX: [+1 214-547-1606], E-Mail:[[email protected]]
Re: [N/A]
Abstract: [The channel estimation (CE) sequence of the current MultiBand OFDM proposal to Task Group 3a
has been found to have a very large peak to average ratio (PAR) of 8.86 dB. This high PAR increases the
probability of clipping the CE sequence on transmit and on receive thereby increasing the probability of packet
decode failure. An improved CE sequence is proposed herein that provides a 5.43 dB improvement in PAR for the
CE sequence. This improved CE sequence has been identified via an extensive computer search. The proposed
improvement does not increase computational complexity at the transmitter or the receiver.]
Purpose: [The authors would like the MultiBand OFDM proposal to be updated to include the channel
estimation sequence described herein. The authors believe that such action will increase the level of support for the
MultiBand OFDM proposal to Task Group 3a.]
Notice:
This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and
is not binding on the contributing individual(s) or organization(s). The material in this document is subject to
change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw
material contained herein.
Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and
may be made publicly available by P802.15.
Submission
Slide 1
Shoemake and Rajagopal, WiQuest
March 2004
doc.: IEEE 802.15-04/0106r01
Outline
• Overview of current packet preamble
• Desired properties in packet preamble
• Peak to Average Ratios for MB-OFDM and
IEEE 802.11a/g
• The problem of high PAR
• Improved Channel Estimation Sequence with
PAR reduction of 5.42 dB
• Conclusions and Proposal
Submission
Slide 2
Shoemake and Rajagopal, WiQuest
March 2004
doc.: IEEE 802.15-04/0106r01
Current PLCP Preamble (268r3)
• Symbols of Packet Sync Sequence
– Packet Detection
– AGC
– Frequency Estimation
• 3 Symbols of Frame Sync Sequence
– Boundary Detection/FFT Placement
• 6 Symbols of Channel Estimation Sequence
– Channel Estimation
– Frequency Estimation
0 ... 0 C0 C1 ... C127 0 0 0 0 0
0 ... 0 C0 C1 ... C127 0 0 0 0 0
PS0
PS1
PS20
Packet Sync Sequence
21 OFDM symbols
FS0
FS1
FS2
Frame Sync Sequence
3 OFDM symbols
CE0
CE1
CE5
Channel Est Sequence
6 OFDM symbols
9.375 s
Submission
Slide 3
Shoemake and Rajagopal, WiQuest
March 2004
doc.: IEEE 802.15-04/0106r01
Desired Properties of Preambles
• Packet Detection
– Good Correlation Properties
– Low Cross-Correlation (SOP)
• Frequency Estimation
– Multiple repeats
• Frame Sync
– Low correlation with packet sync
0 ... 0 C0 C1 ... C127 0 0 0 0 0
• Channel Estimation/AGC
0 ... 0 C0 C1 ... C127 0 0 0 0 0
PS0
PS1
PS20
Packet Sync Sequence
21 OFDM symbols
FS0
FS1
FS2
Frame Sync Sequence
3 OFDM symbols
CE0
CE1
– Low PAR
– Should have significantly lower clip
probability than payload
CE5
Channel Est Sequence
6 OFDM symbols
9.375 s
Submission
Slide 4
Shoemake and Rajagopal, WiQuest
March 2004
doc.: IEEE 802.15-04/0106r01
Why low PAR?
• Why is low PAR more important for the channel estimation sequence than it
is for the data portion of the packet?
• Channel estimation errors increase the probability of error on all data bits
• A clip in the payload affects the probability to decode a small number of bits
• The interleaver helps prevent payload clipping from causing errors
Data
FFT
CE
Sequence
Channel
Inversion
Deinterleaver
FEC
Decode
Channel
Estimator
Submission
Slide 5
Shoemake and Rajagopal, WiQuest
March 2004
doc.: IEEE 802.15-04/0106r01
Peak to Average Ratio
• PAR Calculation
– Possible for a given set of tones
– Difficult in general
• PAR is usually calculated numerically
• PAR is dependent on the sampling phase
Must oversample the signal to cover different
phases and find the true PAR
Submission
Slide 6
Shoemake and Rajagopal, WiQuest
March 2004
doc.: IEEE 802.15-04/0106r01
Legacy Channel Estimation Sequence (1 of
2)
• Legacy sequence PAR:
- 7.09 dB at NyQuist
- 8.86 dB oversampled
• Oversampling approaches:
- Use larger FFT
- Interpolate in the time
domain
• Results here use larger FFT
• Average energy: 122
Can see visually that signal must be
oversampled to obtain accurate PAR
Submission
Slide 7
Shoemake and Rajagopal, WiQuest
March 2004
doc.: IEEE 802.15-04/0106r01
Legacy Channel Estimation Sequence (2 of 2)
• The current sequence has a spike at
the end of the sequence
• The spike at the end of the sequence
causes variation in PAR measurements
under various interpolation methods
• The spike generates a transient that is
a function of design decisions and may
affect performance
• The spike at the end of the sequence
should and can be avoided
MBOA PAR
SRRC
7.31 dB
SRRC (repeat)
8.35 dB
SRRC (zero pad)
7.36 dB
IFFT (4x upsample)
8.85 dB
Sinc (zero pad)
7.29 dB
Submission
There is 1.49 dB difference in spike
between two reasonable but different
implementations.
Slide 8
Shoemake and Rajagopal, WiQuest
March 2004
doc.: IEEE 802.15-04/0106r01
PAR Properties for MB-OFDM and 802.11a/g
Oversample
Factor
CE
r2
CE
r3
LS
11a
1
2
4
1024
5.44
8.0
8.0
8.01
7.09
8.85
8.85
8.86
3.17
3.17
3.17
3.18
MB-OFDM CE r2: Channel Estimation Seq in rev 2 (112 tones)
MB-OFDM CE r3: Channel Estimation Seq in rev 3 (122 tones)
LS 802.11a/g: Long (CE) Sequence of 802.11a/g (48 tones)
Current CE PAR is too high thereby reducing probability of good
channel estimate and increasing packet error probability
Submission
Slide 9
Shoemake and Rajagopal, WiQuest
March 2004
doc.: IEEE 802.15-04/0106r01
PAR and Conjugation (1 of 2)
• Plot shows that the
constraint of
conjugate symmetry
tends to increase the
PAR on average by 1.7
- 2.0 dB
• Result of unnecessarily
forcing the time
domain signal to be
real valued
1.7 – 2.0 dB
Submission
Slide 10
Shoemake and Rajagopal, WiQuest
March 2004
doc.: IEEE 802.15-04/0106r01
PAR and Conjugation (2 of 2)
In addition to increasing the PAR
– Conjugation does not help channel estimation
– Conjugation does not reduce complexity or cost
• Mandatory rates of 110 and 200 Mbps do not use
conjugation
Conjugation should be removed from the
MB-OFDM channel estimation sequence
Submission
Slide 11
Shoemake and Rajagopal, WiQuest
March 2004
doc.: IEEE 802.15-04/0106r01
Improved Channel Estimation Sequence
Tones(-61:-1 1:61) = {-1 -1 -1 1 -1 1 1 1 -1 1 1 -1 1 1
-1 -1 1 -1 -1 -1 1 -1 1 1 1 -1 -1 -1 1 1 1 1 1 1
-1 1 1 1 -1 -1 -1 -1 1 -1 -1 1 1 1 1 1 -1 -1 -1 1 -1 1
-1 1 -1 -1 1 1 -1 1 1 1 1 -1 1 1 1 -1 1 -1 1 1 1 1
-1 1 1 -1 1 -1 -1 -1 -1 -1 1 -1 1 1 -1 -1 -1 -1 1 1 1 -1
-1 -1 1 -1 -1 -1 -1 1 -1 -1 1 1 1 -1 -1 -1 -1 1 -1 1 -1 -1 }
New sequence without conjugate symmetry has
PAR of 3.437 dB measured with oversampling
Submission
Slide 12
Shoemake and Rajagopal, WiQuest
March 2004
doc.: IEEE 802.15-04/0106r01
Reduced PAR Sequences
5.42 dB
Oversample
Factor
CE r3
WiQuest
Non-Conjugate
Symmetric
Improvement
1024
8.86 dB
3.44 dB
5.42 dB
Submission
Slide 13
Shoemake and Rajagopal, WiQuest
March 2004
doc.: IEEE 802.15-04/0106r01
Receive PAR
• The PAR at the receiver
depends on the multipath
channel
• To confirm that a PAR
reduction is maintained at
the receiver, cumulative
distribution functions were
generated for the PAR at
the receiver
~3 dB
~ 1 dB
~ 0.7 dB
• The reduced PAR CE
sequence was compared to
the CE sequence in the
current draft in AWGN,
CM1 and CM2
• A PAR reduction of 0.7
to 3.0 dB exists at the
receiver
Submission
Slide 14
Shoemake and Rajagopal, WiQuest
March 2004
doc.: IEEE 802.15-04/0106r01
Reuse of the Constellation Mapper
• Rotation of the CE sequence does not change the PAR
• The proposed non-conjugate symmetric sequence is
specified over {+1, -1}
• Rotating the proposed non-conjugate symmetric
sequence by 45 degrees would cause the sequence to be
over {+1+j, -1-j}/sqrt(2)
• If the sequence is rotated by 45 degrees, the current
QPSK mapper can be used without modification
• The authors are open to either approach
Submission
Slide 15
Shoemake and Rajagopal, WiQuest
March 2004
doc.: IEEE 802.15-04/0106r01
Conclusions and Proposal
• Current MB-OFDM channel estimation (CE) sequence has:
– A very high PAR of 7.36 to 8.86 dB
– An undesirable spike near the end of the sequence
– A higher probability of clipping on TX and RX
• Sequences exist that provide dramatic reductions in the PAR
– A 5.42 dB decrease
– Provides reduced DAC and ADC resolution
The authors propose to replace the channel estimation sequence
in the current MB-OFDM proposal with the non-conjugate
symmetric sequence on slide 11
Submission
Slide 16
Shoemake and Rajagopal, WiQuest