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OFDM PHY Proposal for IEEE 802.15.4g - Maximizing Performance and Spectral Efficiency

This submission proposes an OFDM PHY for the IEEE 802.15.4g amendment, offering unmatched performance in adverse multi-path conditions, maximizing spectral efficiency, and improving transmitter ACPR. The proposal includes parameters, symbol structure, and a TDD framing structure to support 100,000s of nodes.

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OFDM PHY Proposal for IEEE 802.15.4g - Maximizing Performance and Spectral Efficiency

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  1. Rishi Mohindra, MAXIM Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: OFDM PHY proposal for SUN Date Submitted: May 1, 2009 Source: Rishi Mohindra, MAXIM Integrated Products Contact: Rishi Mohindra, MAXIM Integrated Products Voice: +1 408 331 4123 , E-Mail: Rishi.Mohindra@maxim-ic.com Re: TG4g Call for proposals Abstract: PHY proposal towards TG4g Purpose: PHY proposal for the TG4g PHY amendment 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. Slide 1

  2. OFDM PHY Proposal for 802.15.4g IEEE 802 Interim Session Montreal, Canada May 2009

  3. Contents • Motivation for using OFDM • TDD Framing structure to support 100,000s of nodes • OFDM based PHY Proposal for IEEE 802.15.4g • OFDM parameters & Symbol structure • 2-Ray channel simulation • Transmit spectrum, ACPR

  4. Motivation for OFDM • Offer unmatched performance in adverse multi-path conditions • when channel coherence bandwidth >> subcarrier spacing, forward error correction and interleaving techniques completely recover the packets error-free. • E.g. when path delay is 10us in a 2-ray model, there is only 1 deep null in a 75kHz OFDM modulation bandwidth i.e. only 1 subcarrier is affected out of a total of say 15. • Maximize spectral efficiency and improve transmitter ACPR • Offer best compromise between battery life and Data Rate • Hardware complexity (including gate count and power) can be scaled with data rate and channel bandwidth: • without affecting the multi-path performance which only depends on subcarrier spacing and symbol duration that can be kept constant, independent of channel bandwidth • 16-FFT can be used in 75kHz channel, or 64-FFT in 300kHz channel. Both work equally well for >10us multi-path delays for coded data rates from 288kb/s to 1.152Mb/s for 64-QAM • Allow 100,000s of nodes to communicate to single Base Station in star network, and also allow mesh networking. No loss in SNR or noise floor increase ! No interference between nodes due to slotted structure. • Can fully re-use IEEE802.11a/g Phy technology for 64-FFT OFDM. 16-FFT OFDM is a greatly simplified sub-set.

  5. 16-FFT and 64-FFT OFDM cases • Use 5 kHz subcarrier spacing as example. • Both 16-FFT and 64-FFT will allow >10-20 us of multi-path maximum excess delays with no impact to throughput using Convolutional encoding, interleaving and FER. • 16-FFT allows 288 kb/s uncoded data rate in only 75 kHz modulation bandwidth (100 kHz channels) using 64 QAM. • 64-FFT provides 1.152 Mb/s in 300 kHz modulation bandwidth (400 kHz channels). • Modulation code set (BPSK to 64QAM and r=1/2 to r=3/4) can be adapted for individual nodes based on range and interference • Extremely degraded RF Transceiver phase noise can be implemented for BPSK and QPSK • 16-FFT enables –121 dBm Ideal-Receiver sensitivity in AWGN for BPSK (add noise figure and implementation loss). 64-FFT achieves –98 dBm sensitivity

  6. Basic TDD frame structure • Frame comprises of a short Down-link segment followed by long Up-link segment, followed by a short Slotted Contention segment. Local node to coordinator or Mesh communication segment can be allocated within the frame. • Each node can transmit 2 or more OFDM symbols per frame. • At 4000 OFDM symbols per second (for a 250 us OFDM symbols), up to nearly 2000 nodes can communicate per second using a 1-second TDD frame structure. • A super-frame of say 30 frames can support up to 120,000 nodes over 30 seconds. With 10kHz subcarrier spacing (125us OFDM symbols), up to 240,000 nodes can transmit in 30s. • Each frame, a node can send 288 uncoded data bits in a 64-QAM 64-FFT OFDM symbol in a 500 us transmit burst interval that includes one OFDM training symbol. More OFDM symbols can be allocated to a node.

  7. 16-FFT Down-link OFDM Subcarriers(also up-link OFDM subcarriers for 2nd or greater symbol number) DF = 5 kHz Pilot SC Null SC Data SC Frequency 75 kHz Over 15 sub-carriers (incl null SC)

  8. 64-FFT Down-link OFDM parameters(also up-link OFDM subcarriers for 2nd or greater symbol number) Re-use IEEE802.11a parameters, scaled for 5kHz subcarrier spacing: • Keep structure of Short and Long training sequences of Down-link as in IEEE802.11a/g. • For 16-FFT OFDM Down-link Short and Long training sequences, use corresponding subcarriers –7 to +7 of IEEE802.11a/g

  9. 16-FFT Up-link OFDM Subcarriers for 1st Symbol DF = 5 kHz All reference subcarriers Null SC Frequency 75 kHz Over 15 sub-carriers (incl null SC)

  10. 16-FFT Up-link OFDM Subcarriers for 2nd symbol and beyond DF = 5 kHz Pilot SC Null SC Data SC Frequency 75 kHz Over 15 sub-carriers (incl null SC)

  11. Down-link OFDM Symbol TCP = 50us TFFT = 200us . . . . . . Time TSIGNAL = 250us Up-link OFDM Symbol TFFT = 200us TCP2= 25us TGUARD = 25us TCP1= 50us Node # n-1 Node # n-1 Node # n Node # n . . . . . . Time TSIGNAL = 250us TSIGNAL = 250us

  12. For 3 or more Up-link OFDM Symbols for a node TFFT = 200us TCP2= 25us TCP = 50us TGUARD = 25us TCP1= 50us Node # n-1 Node # n Node # n Node # n . . . . . . Time TSIGNAL = 250us Trainning symbol for channel estimation Data symbols • 25us guard (blank) interval for timing error margin • TCP1 is used for Base Station receiver AGC

  13. Timing Parameters

  14. Timing and Synchronization • Each node is pre-allocated a 2 or larger OFDM symbol slot in every 1-sec frame, or in every 30-sec super frame, or once over a larger time interval. • Initial entry or out-of-turn access is done through a contention process. • At each node, a 32kHz crystal oscillator keeps running continuously, and its frequency error is calibrated regularly with respect to the base station frame timing. • A node that transmits once each frame, has to maintain a timing accuracy better than the 25us guard interval between symbols of different nodes, after considering the propagation delay between the node and the base station. A ranging mechanism can be used for improving the accuracy.

  15. Worst case 16-FFT Subcarrier EVM for 2-ray multi-path, 10us path delay, equal powers Only one subcarrier is destroyed in this channel

  16. 16-FFT OFDM transmit spectrum Red graph: without PA non-linearity Blue graph: PA with 6.5 dB backoff (saturation power to rms transmitted power ratio), Rapps model, rho=2 EVM = -26.5 dB with 64-QAM ACPR = -33 dBc in adjacent 100kHz receiver channel filter

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