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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Reduced Duty Cycle Multiband OFDM Date Submitted: March 3, 2003 Source: Eric Ojard and Jeyhan Karaoguz Company: Broadcom Corporation Address: 190 Mathilda Place, Sunnyvale, CA 94086

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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

Submission Title:Reduced Duty Cycle Multiband OFDM

Date Submitted: March 3, 2003

Source: Eric Ojard and Jeyhan Karaoguz Company: Broadcom Corporation

Address: 190 Mathilda Place, Sunnyvale, CA 94086

Voice: 408 543 3320 E-Mail: eo@broadcom.com, jeyhan@broadcom.com

Re: [802.15.3a Call for proposal]

Abstract: Reduced duty cycle Multiband OFDM can dramatically improve performance in SOP environments, effectively solving the near-far problem.

Purpose: [TG3a-Broadcom-CFP-Presentation.]

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.

Eric Ojard, Broadcom Corporation

reduced duty cycle multiband ofdm

Reduced Duty-CycleMultiband OFDM

Eric Ojard

7-14-03

Eric Ojard, Broadcom Corporation

introduction
Introduction
  • Multiband OFDM has some well-known advantages for UWB:
    • Collects all multipath energy without high complexity
    • Decouples equalization & coding w/o transmitter channel knowledge
    • Facilitates suppression of narrowband interference (egress & ingress)
  • It’s less well-known that it has the potential to solve the near-far problem with Simultaneously Operating Piconets (SOPs)
  • This presentation shows how Reduced Duty-Cycle MB-OFDM can achieve excellent performance in SOP environments.

Eric Ojard, Broadcom Corporation

sop interference characteristics
SOP Interference Characteristics
  • White-Noise-Like Interference:
  • interference looks like gaussian white noise.
  • Matched filter (rake) coherently sums desired signal, while interference sums incoherently.
  • Spreading gain is realized.
  • Techniques:
  • different codesets for different piconets
  • long PN sequence
  • short PN sequence
  • time-hopping
  • baud/chip-rate offsets
  • Structured Interference:
  • Lower entropy rate than AWGN.
  • example: frequency-hopping, (a.k.a. time-frequency interleaving), with coding across frequencies. Interference level varies with time & frequency.
  • Well-designed receivers can exceed the nominal spreading gain.
  • Techniques:
  • use reliability metrics in viterbi decoder (smart receiver)
  • nonlinear limiter in receive path (dumb receiver)
  • Orthogonal Interference:
  • Examples:
  • - FDM (transmit power penalty)
  • Truly orthogonal codes (requires synchronization)
  • No theoretical limit to interference distance.

Correlated Interference:

interference looks similar to the desired signal.

Matched filter (rake) coherently sums desired signal... and undesired signal too.

Spreading gain is not realized.

In practice, multipath tends to decorrelate interference, but this isn’t guaranteed.

Fast Frequency Hopping

Slow Frequency Hopping

lowest capacity

highest capacity

longer channels

faster hopping

less structure

shorter channels

slower hopping

more structure

perfectly decorrelated

unstructured

perfectly correlated

perfectly orthogonal

Eric Ojard, Broadcom Corporation

fast hopping w multipath interference
Fast Hopping w/ Multipath & Interference

blue = piconet #1

red = piconet #2

Free Space:

one “collision” per cycle (if synchronized)

good immunity to near-far problem

single RF front-end captures all energy

CM1 channels:

interference is smeared over several pulses (not uniformly)

less immunity to near-far problem

single RF front-end can’t capture all energy

CM3 channels:

interference starts to like gaussian white noise

This behaves a lot like a wideband system, requiring a wideband front-end

Eric Ojard, Broadcom Corporation

slow hopping ofdm
Slow Hopping OFDM
  • Interference is highly structured even in the presence of multipath.
  • Unfortunately, for a 3-band system w/ one interferer, the receiver will typically see 2 collisions per 3 symbols.
    • requires a very low-rate code to work at high ISR
    • won’t achieve target rates at high ISR

collisiions

blue = piconet #1

red = piconet #2

Eric Ojard, Broadcom Corporation

reduced duty cycle sh ofdm
Reduced Duty-Cycle SH-OFDM

collisiions

  • Idea: Increase # information bits per symbol by eliminating conjugate symmetry, then reduce the symbol rate.
  • A 3-band system sees at most 1 collision per 3 symbols.
  • Advantages:
    • With well-designed codes, interleavers & receivers, we’ll show that we can reach target rates even at high ISR.
    • Bonus: reduced power consumption at receiver (turn off radio & analog front end during silence).
  • Disadvantages (compared to conjugate-symmetric QPSK):
    • 3 dB higher PAR
    • lost diversity on frequency-selective channels (not quantified yet... ~1 dB average loss?)
    • requires 2 DACs

blue = piconet #1

red = piconet #2

Eric Ojard, Broadcom Corporation

3 band simulations 1
3-Band Simulations (1)
  • Modulation Parameters
    • 3 bands (fc = 3.432 GHz, 3.960 GHz, 4.488 GHz)
    • 128 tones per band
    • 100 information tones per band
    • subcarrier spacing = 528 MHz / 128
    • 16-sample cyclic prefix (30.3 nsec)
    • ½ duty cycle
    • QPSK
    • rate 1/3 64-state convolutional code G = [117, 155, 127]8
    • coded bits interleaved across tones and bands (details not shown). For the 3-band case, each CC output stream is mapped to a different band.
    • Length 3 TFI pattern: signal [1 2 3], interference [1 3 2]
  • Derived Parameters
    • symbol length = (128 + 16) / (528 MHz) = 272.7 nsec
    • hop length = 272.7 nsec * 2 = 545.5 nsec
    • TFI cycle length = 3 * 2 * 272.7 nsec = 1.636 usec
    • Bit Rate = 100 * 2 * (1/3) / (545.5 nsec) = 122.2 Mbps

Eric Ojard, Broadcom Corporation

3 band simulations 2
3-Band Simulations (2)
  • Simulation Details
    • one interferer
    • 400 256-byte packets
    • signals over CM3 (cycled through all 100 channels in order)
    • interference over CM1 (one chosen randomly for each packet)
    • channels normalized to get desired ISR (no shadowing)
    • For interference simulations, we add AWGN 4 dB below the received signal PSD (~ 6 dB below sensitivity limit)
    • time-offsets between signal & interference randomly chosen for each packet
    • LPF: Transmitter & Receiver both use a 3rd order LPF
    • Other practical constraints (e.g. front-end dynamic range) not simulated
    • Channel Estimation & Timing Recovery not simulated (perfect channel estimate & timing assumed)
    • BER is averaged over 90 best channels

Eric Ojard, Broadcom Corporation

3 band simulation results 1
3-Band Simulation Results (1)

AWGN-like interference will always have a near-far problem

With a conventional receiver, structured interference looks worse than white noise (CC was designed for AWGN)

~16 dB

But with a simple smart receiver, performance is much better than white noise (limited only by out-of-band roll-off and front-end dynamic range)

Here, SNR is the ratio of the received signal PSD level to the AWGN PSD level.

Eric Ojard, Broadcom Corporation

3 band simulation results 2
3-Band Simulation Results (2)
  • Here, we hold the ISR constant and vary the noise.
  • When a high-level interferer is present, the coding scheme behaves exactly like a punctured code.
    • performance is predictable and easy to analyze
    • It’s important to choose a code that performs well for all possible puncturing patterns that collisions can cause
    • Example, for G=[133 145 175], G’=[145 175] is a catastrophic code
  • In free space, high-level interference introduces a penalty of only ~2.1 dB.

~2.1 dB

Note that this graph is for free space (hence better performance than CM3)

Eric Ojard, Broadcom Corporation

3 band comments
3-Band Comments
  • In the 2-SOP case, this simple scheme can theoretically eliminate the near-far problem.
  • The simulated system is limited by the small total bandwidth.
    • At 122 Mbps, only 2 SOPs can co-exist at very close range.
  • Increasing the number of close SOPs requires increasing the total bandwidth or decreasing the data rate.
    • but note that > 2 SOPs are supported at reduced ISR
  • Increasing the data rate in a 2-SOP environment would requires either...
    • higher SNR and a higher-rate code, or
    • increasing the total bandwidth

Eric Ojard, Broadcom Corporation

7 band simulation 1
7-Band Simulation (1)
  • Modulation Parameters
    • 7 bands (fc = 3.432, 3.960, 4.488, 5.016, 5.544, 6.072, 6.600 GHz)
    • 128 tones per band
    • 100 information tones per band
    • subcarrier spacing = 528 MHz / 128
    • 16-sample cyclic prefix (30.3 nsec)
    • ½ duty cycle
    • QPSK
    • rate 1/3 64-state convolutional code G = [117, 155, 127]8
    • coded bits interleaved across tones and bands (details not shown). For the 7-band case, we made no attempt to associate bands with CC output streams
    • Length-7 TFI patterns
  • Derived Parameters
    • symbol length = (128 + 16) / (528 MHz) = 272.7 nsec
    • hop length = 272.7 nsec * 2 = 545.5 nsec
    • TFI cycle length = 7 * 2 * 272.7 nsec = 3.818 usec
    • Bit Rate = 100 * 2 * (1/3) / (545.5 nsec) = 122.2 Mbps

Parameters that differ from 3-band tests are shown in RED

Eric Ojard, Broadcom Corporation

7 band simulation 2
7-Band Simulation (2)
  • Simulation Details
    • one, two, and three interferers
    • 400 256-byte packets
    • signals over CM3 (cycled through all 100 channels in order)
    • interference over CM1 (chosen randomly for each packet)
    • channels normalized to get desired ISR (no shadowing)
    • For interference simulations, we add AWGN 0 dB below the received signal PSD (~ 6 dB below sensitivity limit)
    • time-offsets between signal & interference randomly chosen for each interferer for each packet
    • LPF: Transmitter & Receiver both use a 3rd order elliptical filter (see appendix for response)
    • Other practical constraints (e.g. front-end dynamic range) not simulated
    • Channel Estimation & Timing Recovery not simulated (perfect channel estimate & timing assumed)
    • BER is averaged over 90 best channels

Parameters that differ from 3-band tests are shown in RED

Eric Ojard, Broadcom Corporation

7 band simulation
7-Band Simulation

AWGN-like interference will always have a near-far problem

1 interferer (2 SOPs including desired signal): performance is much better than w/ AWGN-type interference

2 interferers (3 SOPs): still works at high ISR

3 interferers (4 SOPs): still works at high ISR

Here, SNR is the ratio of the received signal PSD level to the AWGN PSD level.

Eric Ojard, Broadcom Corporation

7 band comments
7-Band Comments
  • 7-band solution appears to support 4 SOPs with much better performance than any current proposal.
  • 4 overlapping SOPs supported @ 122 Mbps, SNR = 4 dB, ISR ~= 16 dB
  • 2 overlapping SOPs should allow much higher rates (250 Mbps?)

Eric Ojard, Broadcom Corporation

general comments
General Comments
  • Well-designed hopping patterns, codes, and interleavers are required to achieve the theoretical benefits of structured interference.
  • The performance will be limited by filter roll-off and front-end dynamic range.
  • Interfering piconets don’t have to be “coordinated”, but they do have to be well-behaved. This only works if all piconets use a low duty-cycle when they sense other close piconets.

Eric Ojard, Broadcom Corporation

conclusions
Conclusions
  • Low Duty Cycle MB-OFDM shows promise for vastly improved performance with SOPs.
    • 3-band mode supports 2 overlapping SOPs @ 122 Mbps
    • 7-band mode supports 4 overlapping SOPs @ 122 Mbps
  • These results are a proof-of-concept. There may be potential to improve performance by optimizing the code, interleaver & modulation parameters.

Eric Ojard, Broadcom Corporation