IEEE 802.15 <subject>

1
Project IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs) Submission Title
Questions and Answers on the properties of the
MB-OFDM PHY Date Submitted 16 September,
2003 Source Charles Razzell Company Philips
Semiconductors Address 1109 McKay Drive, San
Jose, CA95131, California, USA Voice1
408-474-7243, FAX [phone],
E-Mailcharles.razzell_at_philips.com and
Anand Dabak Company Texas Instruments,
Inc. Address 12500 TI Blvd, Dallas, TX75243,
Texas, USA Voice1 408-474-7243, FAX 1
408-474-5343, E-Mailcharles.razzell_at_philips.com
Re IEEE P802.15-02/372r8 IEEE P802.15
Alternate PHY Call For Proposals dated 17
January, 2003 Abstract The following
contribution is offered in response to assertions
that have been made both formally and informally
by 15.3a members. These assertions address the
fundamental properties of the MB-OFDM signal
which has been down-selected for confirmation as
the alternate PHY for 802.15.3 wireless PANs.
Purpose This document reflects the contents
of 802.15-372r0 in a more presentation-friendly
format. 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.
2
Questions and Answers on the properties of the
MB-OFDM PHY

• Charles Razzell (Philips)
• Anand Dabak (TI)

3
Wont OFDM signals cause more interference than a
noise-like signal?

• The use of a scrambler for each sub-carrier
  ensures each 4MHz band has DSSS like spectral
  qualities
• Merely frequency shifting these signals will not
  alter their spectrum or statistics
• The IFFT can be seen as the summation of an
  efficient bank of frequency shifters
• Net result is very flat spectrum random and
  Gaussian amplitude distribution (CLT).

4
Isnt OFDM fundamentally the wrong choice in non
bandwidth limited systems?

• OFDM Orthogonal Frequency Division Multiplexing
• Choosing OFDM doesnt dictate high order
  modulation schemes.
• We chose QPSK with several additional forms of
  redundancy (FEC, repetition)
• Historical associations (11a, DSL) should not
  cloud our engineering judgment.

5
Doesnt OFDM require much higher SNRs than binary
DSSS modulations?

• QPSK modulation combined with soft decision K7
  convolutional FEC is very robust.
• For 110, 220 Mbps modulation/coding schemes
  MB-OFDM outperforms DS-CDMA
• The required operating Eb/No for the MB-OFDM
  scheme is approximately 4dB for 8 PER on
  1024Byte packets
• The MBOK modulation schemes proposed by the DSSS
  proponents are not that simple!

6
Doesnt the MB-OFDM system require extremely high
precision LO frequency matching?

• Tolerable frequency offset is proportional to
  sub-carrier spacing
• With QPSK, 5 of sub-carrier spacing is tolerable
  with low performance loss
• 5 of 4.125MHz is 206.25Hz, which is gt50ppm for
  Fc4GHz.
• This is much easier to accomplish than it was for
  802.11a with its 312.5kHz sub-carrier spacing.

7
Constellation Scatter Diagram with 206.25kHz LO
frequency offset
8
Dont OFDM systems require very high linearity
and I/Q matching for successful demodulation?

• Again in danger of guilt by association with
  802.11a
• We use 4-QAM at all data rates not 64-QAM.
• This implies we can tolerate higher levels of
  intermodulation and I/Q distortions
• The success of multiple, highly integrated
  802.11a/g receivers that dont enjoy this
  relaxation provides reason for confidence.

9
Doesnt the performance of OFDM systems suffer
due to non-coherent processing of bandwidth?

• Each of OFDM the sub-carriers potentially sees a
  different channel tap weight
• The phase derotator after the FFT block at the
  receiver applies a complex conjugate channel tap
  weight to each received FFT bin.
• The phase corrected and amplitude weighted
  symbols are then summed
• By additive combining of repeated symbols
• In the Viterbi decoder during Add-Compare-Select
• Both (1) and (2) can be seen as Maximal Ratio
  Combining of diversity components.

10
Illustration of Viterbi combining of Diversity
Symbols
11
The only way for OFDM systems to out perform
DS-CDMA is with a back channel to stuff more bits
on the tones with large SNR.

• With a strong code as used in MB-OFDM, MB-OFDM
  performs coherent processing
• Outperforms DS-CDMA
• No back channel is needed to achieve this.
• QPSK modulation is transmitted on all OFDM
  carriers of the MB-OFDM.
• Back channel based techniques further improve the
  performance of OFDM systems where bit stuffing
  can be done on a per channel basis.
• Back channel methods typically difficult for
  wireless channels due to bandwidth required
• MB-OFDM does not need to employ any back channel
  for its improved performance over DS-CDMA.

12
Given the coherent processing bandwith of MB-OFDM
is 528 MHz, how can you claim MB-OFDM has
comparable ranging capability to DS-CDMA?

• Each band of the MB-OFDM occupies 528 MHz, the
  overall bandwidth of MB-OFDM mode 1 is 3528 MHz
  1.584 GHz.
• Coherent processing techniques can be employed
  for combining ranging data from the different
  bands giving an overall coherent processing
  bandwidth of 1.584 GHz.
• MB-OFDMs coherent processing bandwidth is the
  same as DS-CDMA

13
Conclusions

• The MB-OFDM signal is a very close approximation
  to AWGN, both statistically and spectrally
• Use of OFDM for low bits/sec/Hz systems is both
  valid and advantageous with QPSK and multiple
  forms of redundancy
• The choice of QPSK with powerful convolutional
  coding gives MB-OFDM robust performance Eb/N04dB
• MB-OFDM does not require challenging frequency
  synchronization
• Linearity and I/Q matching requirement are
  relaxed for the MB-OFDM PHY due to the use of low
  order modulation (QPSK)
• MB-OFDM is well suited to (Maximal Ratio)
  coherent combining of multiple forms of
  redundancy
• MB-OFDM does offer accurate ranging capability
  using the time-domain preamble part of the bursts.