May 2001
doc.: IEEE 802.11-01/257r1
Multipath comparison of
IEEE802.11g High Rate
Proposals
Sean Coffey, Anuj Batra, Srikanth Gummadi,
Chris Heegard, Matthew Shoemake
Texas Instruments
141 Stony Circle, Suite 130
Santa Rosa California 95401
(707) 521-3060,
[email protected]
Submission
Slide 1
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
Contents
 CCK-OFDM is not 802.11a
 Receiver structures
 Multipath performance comparisons
 Conclusions
Submission
Slide 2
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
What is wrong with CCK-OFDM that is
right with pure 11a OFDM?
Submission
Slide 3
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
Overhead and Data Payloads
PBCC
11a
CCK-OFDM
No acks, 500 byte packets
Submission
Slide 4
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
Relative throughputs
Submission
Slide 5
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
The CCK-OFDM Dilemma
Any receiver requires overhead for channel estimation,
tracking, etc:
– 802.11a is “pay as you go” - ultra-short preamble,
16 musecs
– 802.11b and PBCC-22 are “pay all up front” - 11b
“short” preamble, 96 musecs
– CCK-OFDM is “pay up front and again as you go”
- 11b “short” preamble, plus (non-standard) OFDM
preamble, 110 musecs
• “Double the pain”
Submission
Slide 6
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
Packet size & system performance
Compare performances – results are critically
dependent on packet size
– Short packets (e.g., MPEG-4 packets of 188
bytes) strongly favor “pay-as-you-go” approach 
802.11a/Hiperlan 2
– Long packets increasingly favor “pay-all-up-front”
approach”  PBCC-22 aimed at this application
– Short or long, it won’t work well if it’s CCK-OFDM
Submission
Slide 7
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
PBCC-22 features
 Excellent performance in full range of multipath
conditions - much better than CCK-OFDM at
comparable rates
– This is the central technical point in dispute
 Range advantage of PBCC 22 Mbps over CCKOFDM 24 Mbps in multipath conditions is 30-40%.
- These claims documented later; standard IEEE
models were used.
Submission
Slide 8
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
Multipath comparison of the proposals
First, for each proposal, assume same ground rules:
–
–
–
–
floating point implementation
full channel knowledge
standard IEEE multipath model
off-the-shelf algorithms
– assume each uses receiver structure presented by
proposers
Submission
Slide 9
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
PBCC-22 Receiver:
 treat multipath and code as forming a composite
state machine, or “super code”
 decode the “super trellis” using any standard reduced
state algorithm
Simulation results here assume whitened matched filter
plus M-algorithm; standard material, very well
understood:
– whitened matched filter - Forney, 1972.
– M-algorithm - Anderson, 1969.
Submission
Slide 10
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
M-algorithm decoder background:
 M-algorithm operates like regular trellis decoder, but
retains only best “M” paths at each depth
– No restrictions on choice of M
– straightforward way of trading performance versus
complexity
– natural receiver upgrade path
– Main results use M = 64
• we also present M = 8, M = 16, M = 32, M =
128.
Submission
Slide 11
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
M-algorithm decoder:
 Assume the “state” consists of input data bits at last 8
time units
– Compare last 4 time units to represent pure code
state
– Choice of 8 is arbitrary, other values possible
 Assume each “state” remembers the full impact of the
past on the future
– Curve shown in Doc. 01/140 assumes instead that
multipath is regenerated from last 8 time unit
inputs
Submission
Slide 12
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
Baseline comparisons, IEEE multipath
model, 100 ns
From Doc. 00/392r1
5 dB
Ideal channel knowledge, floating point implementations
Submission
Slide 13
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
Baseline comparisons, 100 ns, contd.
From Doc. 00/392r1
Submission
Slide 14
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
Implications of 5 dB advantage:
5 dB translates to a factor of 3.1
 For similar throughput and range, PBCC requires 3
times less received power than CCK-OFDM 24 Mbps
– translates to greater battery life
 For similar throughput and received power, PBCC
has 40% more range than CCK-OFDM 24 Mbps
– Assuming the “power of 3.3” model for path loss –
this is the standard model used in 802.15.2 (Doc.
802.15/138r0)
Submission
Slide 15
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
100ns: 40% PBCC range advantage
CCK-OFDM
PBCC 22 Mbps
24 Mbps
Double the coverage
Submission
Slide 16
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
Relative throughputs:
Submission
Slide 17
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
Actual PBCC receiver algorithms, 100 ns
2.9 dB
Ideal channel knowledge, floating point implementation for CCK-OFDM
Submission
Slide 18
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
Baseline comparisons, contd: 250 ns
4.5 dB
Ideal channel knowledge, floating point implementations
Submission
Slide 19
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
Baseline comparisons, 250 ns, contd.
Submission
Slide 20
Coffey et al, Texas Instruments
May 2001
doc.: IEEE 802.11-01/257r1
Conclusions
 PBCC-22 has a natural superiority over CCK-OFDM
in multipath.
• Established IEEE multipath model used.
 CCK-OFDM tries to juggle two incompatible things –
makes an underperforming system out of a merger of
two otherwise good components
 A better way of doing things – PBCC-22 + .11a!
Submission
Slide 21
Coffey et al, Texas Instruments