10GEPON Burst Receiver Ad-hoc

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10GEPON Burst Receiver Ad-hoc
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Goals

• The goals of this ad-hoc are to resolve the
  following issues
• TIA Gain
• Is different gain for 1G and 10G required?
• 10GEPON Burst Receiver architecture
• AC Coupled? or DC Coupled?
• Guard Time
• Is different Guard Time between Bursts of
  different data rate required?

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Outline

• Optical Receiver Basic
• TIA Parameters
• LIA - AC vs. DC Coupled
• Preamble Length

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Optical Receiver Architecture
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10GEPON TIA
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TIA Parameters

• The primary function of the TIA is to convert the
  small current, from the photodiode, into a
  voltage while adding as little noise as possible
  to the output signal

APD TIA optimized for 10G
Source 1 1.25Gbps 8B10B Coding
1.25Gbps LIA
Source 2 10.3125Gbps 64B66B Coding
10.3125Gbps LIA
?

• TIA is characterized by several parameters
• Transimpedance Gain
• Input Referred Noise
• Bandwidth

• One of the main problems of the TIA is the
  trade-off between Gain, Noise and Bandwidth

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TIA Bandwidth, Gain, and Noise

• All 3 parameters of the TIA are function of the
  RFB
• Bandwidth The Transimpedance gain is equal to
  the RFB, while the Bandwidth is determined by the
  RC time constant.
• Gain Transimpedance gain of the TIA is the
  ratio of the output voltage to the input current.
• Noise Noise contribution of the TIA is
  characterized by the input referred noise current
• The input referred noise current is related to
  the output noise voltage by the following
  equation

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TIA Gain Burst Parameter - Different for 1G and
10G?

• In order to optimize the performance of the TIA
  in both 1G and 10G we need to support Variable
  TIA Gain
• Gain can be varied between 1G and 10G bursts by
  changing the feedback resistor
• In order to analyze the impact on TIA performance
  in 1G, we need to calculate the SNR of the TIA

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TIA Input Referred Noise

• The ultimate limitation on the Optical Receiver
  sensitivity is the Noise
• The noise includes the Photodiodes Shot Noise
  and the noise added by the TIA
• The major noise sources are the Feedback Resistor
  and Voltage Amplifier

di²Rf
Rf
Rout
di²AMP
di²eq,in
gm
Cout
Cin
CDiode
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Preamble length function of maximum CID
(Consecutive Identical Digits)

• The data signal has a sequence of consecutive
  high and low bits in the middle of the sequence
• The DC level during the consecutive bits begins
  to droop
• Long sequence of consecutive bits can
  significantly change the DC level of the data and
  the optimum threshold voltage
• A poor low frequency cut-off vertically closes
  the eye diagram and can reduce the sensitivity of
  the system
• In order to achieve a lower low frequency
  cut-off, we need to extend the number of preamble
  bits
• For example, in GPON, the CID is 72 bits

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TIA AGC Loop Timing
Peak Detector output
Noise
idiode
TIA_Gain
Amplified Noise
TIA_Output

• TIA AGC initialization time parameter needs to be
  much longer than the CID bit time
• During 0 CID, the AGC loop should remain
  constant and not running to infinite gain
• In between Bursts, the AGC needs long preamble,
  greater than AGC_t to enable AGC to learn new
  peak value

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TIA AGC Response Delay
V
1.2
1.0
0.8
0.6
0.4
0.2
0
t
-120
-80
-40
0
40
80
120
tagc

• Practical AGC has delayed response to
  signal-level change

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10GEPON LIA
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The Problem AC or DC Coupled
APD TIA optimized for 10G
Source 1 1.25Gbps 8B10B Coding
1.25Gbps LIA
Source 2 10.3125Gbps 64B66B Coding
10.3125Gbps LIA
?
?
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10G LIA AC Coupled

• 10G LIA is simulated by the following Transfer
  Function
• The lower cut-off pole (f1) determine the CID
  length, while the higher cut-off pole (f2)
  determine the Bandwidth
• For the lower frequency, 3MHz was simulated
• For the higher frequency, 7GHz was simulated
• In order to maintains minimum DC droop from the
  baseline, we need at least factor 4 over the t

3dB
3MHz
7GHz
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10G LIA DC Coupled

• In DC-coupled the RC (t) is determined by
  internal capacitors
• One capacitor for fast acquisition during
  preamble - and the second for CID support
• During reception of the preamble, the threshold
  acquisition done by the high frequency cut-off
  pole, then switching to the low frequency
  cut-off pole to support CID

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Transfer Function
3dB
A forward gain. K feedback gain. WL low pass
pole frequency (in feedback loop). WH high pass
pole frequency (in forward path).
f1 3MHz
f2 80MHz

• f1 and f2 determine how much DC droop we are
  allowed from the baseline
• During preamble we use f1_high then after short
  time we switch to f1_low to support CID

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Preamble Length - A Formula

• Assuming that 4 time constants is needed

Where
Number of CID
Required Preamble length bits
Deviation of baseline permitted during CID
Just to the LIA
For 5 droop
-4 64
N
5000bits
ln(1 - 0.05)
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Open Questions

• AC or DC Coupled?
• In AC Coupled
• What should be the Maximum overhead?
• In DC coupled
• What should be the Minimum overhead?
• Different Guard time between different Bursts?

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