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Low rate enabler
Low Rate Enabler

Authors:

Date: 2011-03-14

Rahul Tandra, Qualcomm Inc.


Abstract
Abstract

According to the latest FCC TVWS regulations [1], every Mode II device must have the ability to perform geo-location with a 50 m accuracy. This geo-location requirement is very difficult to meet in indoor environments. Existing indoor geo-location technologies are very expensive and hence may not be suitable for low-cost access points (APs).

In this presentation we propose a solution to circumvent the indoor geo-location problem. We propose the idea of a Low Rate Enabler which serves as a Mode II device and provides enablement to all mode I devices (APs and STAs) in a wide region (office buildings, malls or apartment complexes). The key idea is that a single device equipped with a standard GPS receiver, which can be located near a window or on a rooftop, can provide enablement for all other TVWS devices in wide area.

Rahul Tandra, Qualcomm Inc.


Comment id
Comment ID

Rahul Tandra, Qualcomm Inc.


Problem description
Problem description

  • FCC regulations [1]

    • All Mode II devices must have Geo-location with a 50 m accuracy

  • Meeting the geo-location requirement in indoor environments is extremely challenging

    • Building high sensitivity GPS is very expensive (massive correlators) [2]

    • Network assisted GPS requires the APs to have a cellular radio [3]

  • APs placed next to a window, or outdoors can meet geo-location requirement through existing GPS technology

    • Not a good location from a coverage standpoint, especially in enterprise scenarios

Rahul Tandra, Qualcomm Inc.


Proposed solution low rate enabler
Proposed solution: Low Rate Enabler

Low Rate

Enabler

  • Design a special Mode II device (can work as a Wi-Fi AP), called a Low Rate Enabler (LRE) that is placed near a window, or on the roof

    • Low Rate enabler has a GPS receiver

    • One or two low rate enablers can provide enablement for the entire building

      • Good for enterprise solutions, malls, apartment complexes or other large indoor facilities

  • Regular APs and STAs (mode I devices) may now be placed anywhere in the building

    • Do not require GPS receiver

    • Receive enablement and CVS through the low rate enabler

Mode I

AP

Mode I

STA

Rahul Tandra, Qualcomm Inc.


Low rate enabler requirements
Low rate enabler requirements

  • High-level design

    • Portable device

      • Statistically more channels available for portable devices than fixed devices

    • Low-complexity

  • PHY design

    • Have a low data rate PHY to provide wide coverage (e.g. building)

      • Initial enablement and periodic CVS for all mode I devices across a large area

  • MAC design

    • Medium access protocol for TVWS such that it can coexist with other networks

      • Should be able to share medium with other shorter range networks

Rahul Tandra, Qualcomm Inc.


Low rate phy design
Low-rate PHY design

  • Low Rate PHY requirements

    • Long communication range, to enable all devices in a wide area

    • Low-rate transmission to extend range

  • Two possible design choices to enable low rate modes

    • IEEE 802.11 DSSS based PHY

      • Original 1 Mb/s DSSS PHY scaled to 5 MHz gives 250 kb/s PHY

      • Possible to modify 1 Mb/s DSSS designs for 250 kb/s which would scale to 62.5 kb/s is useful to extend range further

    • OFDM based PHY

      • Lowest rate with current PHY design (clocking down by 4) for 5 MHz operation is 1.5 Mbps

      • Can lower data rates by repetition coding based ideas

Rahul Tandra, Qualcomm Inc.


Super long range slr phy
Super Long Range (SLR) PHY

  • Propose a 62.5 kb/s SLR PHY to provide sufficient coverage for the LRE

    • See MAPL calculations in Appendix B

  • Based on the 802.11 DSSS PHY Design

    • Step 1: Down-clock the 1 Mb/s 802.11b DSSS PHY by four

      • Effective data rate of 250 kb/s

    • Step 2: Every bit in the down-clocked packet is repeated by 4 to give a 6 dB gain

      • Provides data rate of 62.5 kb/s

    • Same 11-chip Barker sequence as in 802.11

    • Differential BPSK modulation

Rahul Tandra, Qualcomm Inc.


Slr phy frame format
SLR PHY: Frame format

  • SYNC sequence: Alternating 0,1 ‘s with a bit-level repetition of 4

    • Provide enough SLR symbol transitions such that the SLR detector can estimate the symbol boundary

  • SFD, SIGNAL, SERVICE, LENGTH, and CRC fields remain unchanged

    • Bit level repetition by 4 to give 6 dB link budget SNR gain

Rahul Tandra, Qualcomm Inc.


Low rate enabler operation
Low Rate Enabler Operation

  • Low rate enabler provides enablement on a single (base) channel

  • Low rate enabler visits the APs/STAs on their channels to provide CVS

  • Quieting on each channel during their scheduled CVS

  • Quieting on the base channel during enablement beacon (if it is a shared channel)

Rahul Tandra, Qualcomm Inc.


Mac design
MAC design

  • Advertizes enablement beacon

    • All mode I devices must listen to this beacon (in Ch A) to get enabled

      • Unicast message exchange between the TVWS enabler and each mode I device (FCC mandated enablement process)

    • Additional information elements can be exchanged during the initial enablement process

      • List of previously enabled channels (Ch A, Ch B, Ch C)

      • Period of the CVS message for each channel

  • CVS message

    • Enabler periodically hops across all the currently used channels (Ch A, Ch B, Ch C) to transmit CVS information for previously enabled APs and STAs

      • The CVS message can be sent through public action frames

  • Ensure power-save STAs receive CVS message

    • Mode I APs need to wake up power-save STAs to receive CVS message

  • Protect low rate enabler’s CVS/Enablement beacon from collisions

Rahul Tandra, Qualcomm Inc.


Protecting low rate enabler transmissions
Protecting low rate enabler transmissions

  • Low rate enabler may share the medium with other APs and STAs for

    • Enablement beacon (only in the case when the base channel is shared)

    • Transmitting periodic CVS message

  • Scheduling quiet-times:

    • Low rate enabler transmissions can be heard by all APs and STAs in a wide area (low rate packets)

    • Low rate enabler cannot hear the transmissions from APs and STAs which are far away (higher rate packets)

    • Low rate enabler cannot rely on CSMA to avoid collisions

  • APs must schedule quiet times to protect the TVWS transmissions from collisions

Rahul Tandra, Qualcomm Inc.


Scheduling quiet times
Scheduling Quiet Times

TVWS enabler

  • APs wake the STAs using the DTIM message described in the previous slide

  • APs know the approximate time to the next CVS or Enablement beacon

  • APs send a CTS to Self message with duration field set to cover the CVS/Enablement beacon

  • A CF End message can be used to reset the NAV field at the STAs if the duration setting in the CTS to self message was too long

Rahul Tandra, Qualcomm Inc.


Dtim messaging for power save stas
DTIM messaging for power save STAs

  • Low rate enabler sends CVS periodically

    • The period of the CVS is known to the Mode I APs

  • AP detects a CVS and sends DTIM to wake up all STAs

    • Chooses the TIM message nearest to the next CVS to send a DTIM

    • All the TIMs preceding this DTIM will contains the arrival time of this DTIM message

    • Broadcast bit in DTIM turned ON

  • STAs receive the next CVS signal from the enabler

  • STAs stay on till the next DTIM

Rahul Tandra, Qualcomm Inc.


Standards change
Standards change

  • MAC

    • Description of low-rate enabler

    • Some additional information elements

      • Operating channel of the low rate enabler

      • Operating channel of other Mode I APs

      • CVS schedule

  • PHY

    • Low-rate DSSS PHY

Rahul Tandra, Qualcomm Inc.


Concluding remarks
Concluding remarks

  • Low rate enabler solves the indoor GPS problem

    • Low rate enabler provides enablement and CVS for many APs and STAs

    • Other APs no longer need GPS. When there are multiple APs (e.g. enterprise scenario) which significantly reduces deployment complexity

  • Low rate enabler can reduce the search time for discovery since the LRE can tell new STAs which channels/bandwidths are available in the area

  • Minimal changes needed to the 802.11af MAC

    • Additional information elements

    • CTS to self message to schedule quiet-times for CVS transmissions

    • Reuse existing 802.11 DTIM messaging for power save nodes

  • Low rate PHY

    • Low complexity DSSS PHY based on simple modification of original 1 Mb/s DSSS PHY

Rahul Tandra, Qualcomm Inc.


References
References

  • Federal Communication Commission, Second Memorandum Opinion and Order In the Matter of Unlicensed Operation in the TV Broadcast Bands, Additional Spectrum for Unlicensed Devices Below 900 MHz and in the 3 GHz Band, Document 10-174, September 23, 2010

  • S. Schon and O. Bielenberg, On the capability of high sensitivity GPS for precise indoor positioning, Firth Workshop on Positioning, Navigation and Communication, March 2008

  • F. van Diggelen, Indoor GPS theory & implementation, IEEE Position Location and Navigation Symposium, August 2002

Rahul Tandra, Qualcomm Inc.


Annex - A

FCC Regulations

Qualcomm Incorporated



Definitions
Definitions

Rahul Tandra, Qualcomm Inc.


Definitions1
Definitions

Rahul Tandra, Qualcomm Inc.


Fcc regulations on enablement
FCC Regulations on Enablement

Qualcomm Incorporated


Fcc regulations on enablement1
FCC Regulations on Enablement

Qualcomm Incorporated


Fcc regulations on enablement2
FCC Regulations on Enablement

Qualcomm Incorporated


Fcc regulations on enablement3
FCC Regulations on Enablement

Qualcomm Incorporated


Fcc regulations on enablement4
FCC Regulations on Enablement

Qualcomm Incorporated


Annex - B

Pathloss calculations

Qualcomm Incorporated


Pathloss and range calculations
Pathloss and range calculations

  • Pathloss calculation

    • PL = PTX – SNRRX – N0

  • Numerical parameter & notation:

    • Noise figure = 8 dB

    • Bandwidth = 5 MHz

    • Transmit power , PTX= 16 dBm

    • Carrier frequency: 650 MHz

    • Dw --- separation between two walls

    • floor(d/Dw) --- average number of walls

    • n --- number of floors separating transmitter and receiver

    • Floor loss ~ 18 dB, wall loss ~ 5 dB

Rahul Tandra, Qualcomm Inc.


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