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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [ Bridging the gap between SC and OFDM as a basis for the 802.15.3c PHY ] Date Submitted: [14 November, 2006] Source: [Slobodan Nedic, Nedics Associates]

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slide1

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

Submission Title: [Bridging the gap between SC and OFDM as a basis for the 802.15.3c PHY]

Date Submitted: [14 November, 2006]

Source: [Slobodan Nedic, Nedics Associates]

Address [336 Sonoma Aisle, Irvine, CA 92618]

Voice: [949-748-7020], Cell: [732-421-2045], E-Mail:[nedics@aol.com]

Re: []

Abstract: [This document highlights a signaling/accessing format with potential to provide good basis for PHY implementation, with high flexibility in terms of trade-offs between power and spectral efficiency, and incremental future enhancements. It essentially conciliates the SC and OFDM signaling formats by filling the gap that currently exists between them. The basic element that enables orthogonal frequency division multiplexing of spectrally shaped (sub-)channels is staggering between in-phase and quadrature components. This feature is inherent to MSK modulation, and can be extended to more spectrally efficient O(ffset)QAM-MC, or OFDM/OQAM flavors. A rough estimation of achievable data rates is provided.]

Purpose: [For information only.]

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.

Slobodan Nedic, Nedics Associates

introduction
Introduction
  • When considering PHY for the 802.15.3c, it is often stated that power efficiency is primary importance, but very soon spectral efficiency will also be needed, no matter how wide bandwidths
  • The first 60Gz range WPAN standard should not impede enhancements due to backward compatibility issues; should also serve as basis for some form of 60GHz 10Gbps WLAN ?!
  • This contribution brings to the attention of the standardization group a signaling format that can meet the short-term and long-term objectives with best performance/complexity trade-off, by bridging the gap between SC and OFDM

Slobodan Nedic, Nedics Associates

pros and cons for single carrier and ofdm
SC

High complexity for good spectral efficiency

Moderate PAPR in such case

Long equalizer and slow rate (adaptation, and tracking)

Directly exploits multipath diversity through DFE

Insensitivity to frequency offset and phase jitter

Can be concatenated with CC for better performance

OFDM

Needs large number of subchannels for reasonable spectral efficiency, due to CP

Reduced power efficiency

Simple equalization, but high loss due to imperfect channel estimation

Reliance on coding overhead and interleaving

High sensitivity to frequency offset and phase jitter

Needs SDM/STC to be effective ?!

Pros and Cons for Single-Carrier and OFDM

Conciliatory solution: To use a relatively small number of sub-channels for best trade-off between complexity and performance

Slobodan Nedic, Nedics Associates

slide4

Visualization of broad-band signaling/accessing formats and the “technology gap”

Legend: OFDM – Orthogonal Frequency Division Multiplex; SC – Single Carrier, and CDMA – Code Division Multiple Access, SF – spreading factor in CDMA, (SF=1 for SC); TDMA – Time Division Multiple Access; F-H – Frequency Hopping; UWB – Ultra Wide-Band; PPM – Pulse Position Modulation and FPM – Frequency Position Modulation, as two forms of M-ary orthogonal signaling; OQAM – Offset, or Staggered Quadrature Amplitude Modulation; MC-CDMA – Combination of OFDM and CDMA

Slobodan Nedic, Nedics Associates

slide5

The key – alternating T/2 staggering of I and Q sub-channel components:

numerous systems dating back from early sixties (Chang, Saltzberg, Hirosaki …), known under different names (OFDM/OQAM, WDMT, TLO, IOTA …), essentially “differing” only in the type of the (filter-bank) referent base-band impulse response.

- I component

- Q component

f

1/T

Sub-channel k

1/T

t

0

T

T/2

T – QAM signaling interval (AM at T/2 instants)

Slobodan Nedic, Nedics Associates

slide6

Three channel system – development Stage 1 with using the power efficient MSK (shown approximately as 100% roll-off Nyquist spectral shaping, and as regular MSK in time)

≈22 dB

Ch3

Ch2

Ch1

B = 3/T’

1/T’

1/T’

f

In-phase signaling element

t

Quadrature signaling element, staggered

t

T’/2

Slobodan Nedic, Nedics Associates

slide7

Three channel system – development Stage 1 with orthogonal stacking of MSK (shown approximately as 100% roll-off Nyquist spectral shaping, and as regular MSK in time)

≈22 dB

Ch1+

Ch2-

Ch3

Ch2

Ch1

B = 3/T’

1/T’

1/T’

f

Quadrature signaling element

t

T’/2

In-phase signaling element, staggered

t

Slobodan Nedic, Nedics Associates

slide8

‘Aggregate’ data rates (Gbps) per single, and four overlapped channels

Slobodan Nedic, Nedics Associates

slide9

1/T

Three channel system – development Stage 1 with using the power efficient MSK (shown approximately as 100% roll-off Nyquist spectral shaping, as modified MSK in time)

>30 dB

Ch3

Ch2

Ch1

B = 2/T

1/T

f

In-phase signaling element

t

Quadrature signaling element, staggered

t

T/2

Slobodan Nedic, Nedics Associates

slide10

1/T

Three channel system – development Stage 1 with orthogonal stacking of “MSK” (shown approximately as 100% roll-off Nyquist spectral shaping, as modified MSK in time)

Ch±

>30 dB

Ch3

Ch2

Ch1

B = 2/T

1/T

f

Quadrature signaling element

t

T/2

In-phase signaling element, staggered

t

Slobodan Nedic, Nedics Associates

slide11

‘Aggregate’ data rates (Gbps) per single, and three overlapped channels

Slobodan Nedic, Nedics Associates

slide12

Increasing spectral efficiency and multipath resilience by OFDM with gradual increase of number of MSK bins to keep an acceptable power efficiency and to retain low complexity

Ch3

Ch2

Ch1

f

B = 8/To

1/To

1/To

To – QAM (extended) signaling interval in each of (K=7) sub-channels of each channel

▪ Basic spectral efficiency per channel increased from 0.5 to 7/8, (K-1)/K

▪ For simultaneous use of two adjacent channels, even more – 0.5 to 15/16

▪ Coping with multipath propagation in N-LOS by simple per-bin equalizers

▪ Only logarithmic increase in complexity for 10Gbps per channel with 16-QAM

(effectively 3 bits/symb with ¾ coding rate …)

▪ Number of sub-carriers to select as trade-off between reduced equalization/ implementation complexity and the increase in PAPR (reach & rate reduction)

Slobodan Nedic, Nedics Associates

slide13

Three channels with frequency dispersed sub-channels – with Nyquist roll-off of 50%;

(Partial overlapping of sub-channels belonging to different user channels …)

Ch3

Ch2

Ch1

Ch3

Ch2

Ch1

• • • • • • • • • • • • • • • •

f

B = 8/To

1/To

1/To

To – QAM (extended) signaling interval in each of sub-channels of each channel

▪ Basic spectral efficiency per channel increased from 7/8 to 23/24

▪ Reduced roll-off factor still just moderately increasing implementation complexity

▪ Very efficient (accelerated) LMS equalizer adaptation possible

▪ Moderate spectral overlapping – reduced inter-channel crosstalk partially based on inherent co-channel attenuation

▪ Increased frequency diversity by convolutional coding along frequency …

▪ Relatively small number of sub-channels for resilience against phase jitter/fr. offset

Slobodan Nedic, Nedics Associates

slide14

System-level and networking aspects

  • Some equalization will quite likely be needed
    • Available for staggered QAM, both in SC and MC
      • Even w/ directional antennas, MSK may need more than T/2 fractional equalizer (same complexity as in T-spaced QAM)
      • With omni-directional antennas, Nyquist shaping is preferable
  • With synchronized TDD MAC, fully overlapped sub-channels of different users may be possible
  • Equalization overhead low for longer packets
  • At early development stages, multiple channels for the same user through transceiver replication
    • In future enhancements to be used for SDM-diversity
  • Relatively small number of sub-channels may allow for relatively simple PAPR reduction

Slobodan Nedic, Nedics Associates

slide15

Conclusions

  • Starting with the conventional (“bookish”) MSK, in its spectrally maximally efficient QPSK form, and one plausible basic partition of the available bandwidths s, incremental modifications have been suggested to meet the current and future PARs
  • The basis is the use of OFDM of sub-channels with staggered QAM, enabling relatively small number of sub-carriers for bridging the gap between SC and traditional OFDM for an optimal trade-off between performance and complexity
  • This signaling format has been around for quite some time, and virtually all aspects of its implementation have been well researched and elaborated in numerous publications

Slobodan Nedic, Nedics Associates

slide16

References

[1] 15-06-0306-01-003c-msk-system-multi-gbs-wireless-communications-60ghz

[2] 15-06-0325-00-003c-chennelization-requirements-802-15-3c

[3] 15-05-0299-00-003c-preliminary-channel-proposal

[4] 15-06-0387-02-003c-phy-layer-modulation-802-15-3c-system-level-issues

[5] R.W. Chang, “Synthesis of band-limited orthogonal signals for multicarrier data transmission,” The BSTJ, Vol. 45, pp. 1775-1796, December 1966.

[6] B.R. Saltzberg, “Performance of an efficient parallel data transmission system,” IEEE Tr. On Communications Technology, December 1996.

[7] B. Hirosaki, “An orthogonally multiplexed QAM system using discrete Fourier transform,” IEEE Tr. On Communications, July 1981.

[8] R. Lee and G. Stette, “Time-limited orthogonal multi-carrier modulation schemes,” IEEE Tr. On Communications, February/March/April 1995.

[9] A. Valin and N. Holte, “Optimal finite duration pulses for OFDM,” IEEE Tr. On Communications, January 1996.

[10] B. Hirosaki, “An Analysis of Automatic Equalizers for Orthogonally Multiplexed QAM Systems”, IEEE Trans. on Communications, Jan 1980.

[11] P.A. Bello and K. Pahlavan, “Adaptive equalization for SQPSK and SQPR over frequency selective microwave channels,” IEEE Tr. On Comm., May1984

[12] S. Nedic, “An unified approach to equalization and echo cancellation in OQAM-based multi-carrier data transmission”, Globecom’97.

[13] H. Bölcskei, “Blind estimation of symbol timing and carrier frequency

offset in pulse shaping OFDM systems,” in Proc. Int. Conference on Acoustics,

Speech, and Signal Process., vol. 5, Phoenix, AZ, 1999, pp. 2749–2752.

[14] P. Ciblat and E. Serpedin, “A Fine Blind Frequency Offset Estimator for OFDM/OQAM Systems,” IEEE Tr. On Signal Processing, January 2004.

Slobodan Nedic, Nedics Associates