Two Way Time Transfer

1
Two Way Time Transfer
Time of Flight (TOF)
Clock Offset (OS), 1 leads 2
Forward Rx _at_ TFR
Forward Tx_at_ TFT
Transciever 1
Transciever 2
Reverse Tx_at_TRT
Reverse Rx_at_TRR
TRRTRT OS TOF
TFRTFT OS TOF

• Can Measure TOF irrespective of clock offset!
• Can measure Frequency offset with multiple offset
  measurements!

TOF ½ TRR TRT TFR TFT
OS ½ TRR TRT TFR TFT
US Naval Observatory, Telstar Satellite, circa
1962 http//www.boulder.nist.gov/timefreq/time/two
way.htm
Multiband OFDM ranging proposal IEEE
802.15-04/050r0, http//multipbandofdm.org, JAN
2004
2
Signal Sequencing
OFDM sample clock at Node 1
OFDM sample clock at Node 2
OS
Clock offset(OS)
TX symbols during forward transmit
1-gt2
RX symbols during forward transmit
OS
OSTOF
Time of Flight (TOF)
RX symbols during reverse transmit
2-gt1
TX symbols during forward transmit
3TTOF
time
0
T
2T
4T
5T
6T
7T
3T
TOA12 OS TOF TOA21 (3TTOF (3TOS)) TOF
OS TOF 0.5(TOA12 TOA21) OS 0.5(TOA12 -
TOA21)
Assumption Time of Flight (TOF) does not exceed
the OFDM symbol period (T).
3
Sample System

• Sample channel had 5 taps.
• The LOS tap delay 17ns
• The delay spread was 213-14 199ns.
• 10ms of correlation time
• Fs128Msps, T 1µs
• 128 point FFT
• 64MHZ Total signal BW
• 6bit ADC
• PLL loop has 800kHz BW, -60dB in-band attenuation
• Single pole noise shaping filter

4
Channel and ToA Estimates
ToA 2-gt1
ToA 1-gt2

• Pilot Signal is a 128 sample PRN sequence.
• 10 OFDM symbols are averaged to reduce noise
  variance

5
Range Measurement performance
6
Muti Tone IFFT range measurement system block
diagram
Down convert the signal
LO
FFT
DEMUX
MEMORY
Sample
r(t)
. . .
BPF
ADC
. . .
. . .
/Pilot(?) (Remove Pilot seq.)
Time of arrival
IFFT
Determine Strongest tap
Range
. . .
. . .
. . .
. . .
K
Time of arrival in reverse dir
7
Multi tone IFFT approach highlights

• ?f carrier separation. (Fs 2NOC ?f), NOC
  number of carriers.
• Delay quantization is 1/Fs, Delay quantization
  error is 1/(2Fs)
• Range error due to delay quantization is
  150m/Fs/1Msps
• NOC is upper bounded by the Local oscillator
  phase noise and available signal bandwidth.
• NOC is lower bounded by the channel delay spread.
  Using few carriers cause and incomplete channel
  estimation.
• Can interpolate the spectrum and get more channel
  taps.
• Dont forget about DFTs that are not in 2i
  form.(eg. 384 pt DFT)
• Is also proposed in multiband OFDM proposal for
  802.15.3a, with some changes.

8
Advantages and Disadvantages of Multi tone IFFT
channel estimation method

• Much less computation required with comparison to
  an SVD.
• Since there is no random data, there is no real
  P/A power ratio problem at the power amplifier
• No need for UWB signals. 64-128MHz of BW is more
  than enough
• No timing synchronization needed

• Need for FFT.
• Two way transmission needed.
• ADC speed(power) requirement increases.
• Tones can not be very close due to Phase noise
  concerns
• Freq offset still lingering