obi in rfog systems public

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OBI in RFoG systemsAn operator’s perspective

Vasile JercaSr. WAN PlannerRogers Communications PartnershipNovember 3rd, 2015

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Agenda

1. What is OBI?

2. Causes

3. OBI impact

4. Probability of OBI occurrence

5. 1310 nm or 1610 nm?

6. Historical and current solutions to minimize / eliminate OBI

7. Other solutions to eliminate OBI

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Agenda

1. What is OBI?

2. Causes

3. OBI impact


5. 1310 nm or 1610 nm?



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1. What is OBI?

OBI (Optical Beating Interference) is the phenomena that occurs when two (or more) optical signals with wavelengths separated by less than 15 GHz are fed into an optical receiver.

The effect at the RF output of the optical receiver is an elevated noise floor of up to 40 dB compared to the “normal noise floor”, dependent on the separation between the wavelengths.The “normal noise floor” is the composite RF noise floor with signals separated by more than 15 GHz.

The frequency domain 15 GHz corresponds to the following in the optical domain: 0.086 nm (86 pm) for 1310 nm band (O-Band) 0.130 nm (130 pm) for 1610 nm band (L-Band)

Optical RX RF output Noise Floor level

Normal

+10 dB

+20 dB

+30 dB

+40 dB

Optical signals separation

0 GHz 15 GHz15 GHz

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1. What is OBI?

Noise floor elevation is not linear with wavelengths separation

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1. What is OBI?

+13 to +17 dBmV

+3 ±1 dBm

ONUs are designed to include “Laser squelch” in order to minimize OBI risk

ONU laser squelch

A benefit of the laser squelch is the ingress-free low band of the return RF spectrum.Therefore, this RF band can be used for upstream channels.

There is no “funneling effect” like in HFC.

RF into ONU

Optical Power

This feature allows the laser to stay “warm” by having a very low optical signal (~-18 dBm) for RF input levels below a threshold, and fast turn on to the nominal optical output power once the RF drive is above the threshold.SCTE 174 2010 specifies the Turn-on and Turn-off parameters.

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Agenda

1. What is OBI?

2. Causes

3. OBI impact


5. 1310 nm or 1610 nm?



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2. Causes

±1σ±2σ±3σ

Q=68%

Q=95%

l Central

2.1. Manufacturing distributionTypically, manufacturers follow the “3 sigma” distribution. Approx. 68% of the ONU lasers have the wavelength within 1 sigma range; 95% of ONU lasers have the wavelength within 2 sigma range

SCTE Specification (174 2010) for ONU lasers range:1610±10 nm for operational range 0˚C to +50˚C1310±50 nm for operational range 0˚C to +50˚C

Given the wavelength drift of 1 nm/10˚C, SCTE specification becomes:1610±7.5 nm for operational range 0˚C to +50˚C1310±47.5 nm for operational range 0˚C to +50˚C

1610 nm

1602.5 1605.0 1607.5 1612.5 1615.0 1617.5

5.0 nm68% of 32=21 ONUs

0.24 nm apart

Min. l separation: 0.13 nm

1610 ± 7.5 nm

1310 nm

1262.5 1278.3 1294.1 1325.8 1341.6 1357.5

31.7 nm68% of 32=21 ONUs

1.51 nm apart

1310 ± 47.5 nm

Min. l separation: 0.086 nm

From the distribution graphs, it is observed that OBI risk is lower @1310 nm

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2. Causes

2.2 Wavelength drift caused by ONU unit temperature changesA laser wavelength drifts ~1 nm per every 10˚C change.Main contributors to ONU temperature variations:• Installation location

Outdoor ONUs are subjected to 100˚C (-40 to +60) temperature swings over a six month period. Indoor ONUs may be subjected to 30˚C - 40˚C variations if they are located in the path of forced

air.• Operational temperature increase (laser diode warms up during operation).

2.3 Faulty CPEs (ONUs, STBs, CMs, EMTAs)Any faulty CPE leading to ONU laser to continuously stay “ON” drastically increases the probability of OBI occurrence.

2.4 Number of potential ONUs transmitting simultaneouslyProbability of OBI occurrence increases with the number of potential ONUs having the laser “ON” simultaneously.OBI occurrence probability increase in an exponential relationship with the number of independently scheduled service platforms.If all three services (DTV, HSI and Voice) are controlled by a single upstream scheduler, there is no OBI occurrence because only one laser is on at one given time, unless there are faulty CPEs.Once there are more independently scheduled platforms, OBI occurrence becomes more probable, dependent on all above factors.

2.5 Laser chirpSCTE Specification (174 2010) for OMI is 35%. Therefore, the laser output, like any amplitude modulated signal, presents chirp.

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Agenda

1. What is OBI?

2. Causes

3. OBI impact


5. 1310 nm or 1610 nm?



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3. OBI impact

Dependent on OBI severity (wavelengths separation), higher modulation channels (e.g. 64QAM) are impacted first, followed by the lower modulation channels (e.g. 16QAM or QPSK) given the minimum SNR accepted by the platform modulation.The impact is a degraded (or loss of) service for the specific platform(s).• On the HSI platform, OBI will trigger the process of re-sending improperly received packets

by the CMTS. (TCP). This impacts the upstream traffic speeds.• On Voice platform, improperly received packets are not re-transmitted (UDP), therefore they

are lost. The OBI effect is currently not known, and difficult to predict. It may vary from “clicks” heard by customer’s party, to dropped calls, dependent on the duration and severity of the OBI.

• On the STB platform, the OBI effect is the same as for the HSI. The customer will need to push more than one time on the STB remote’s buttons to activate a service which requires upstream communication with the DTV platform.

We observed that operating HSI with a single upstream channel for a mixed population of DOCSIS 2.0 and 3.0 modems, and having three independently scheduled platforms (max. three ONUs transmitting simultaneously – no faulty CPEs), the RFoG system using 1310 nm return wavelength is fully operational without major (if any) impact on customer experience.

Once we increased the HSI upstream number of channels to three, with a mixed population of D2.0 and D3.0 modems, the number of potential ONUs transmitting simultaneously increased to 5 (the D2.0 modems share the return spectrum). The increase of simultaneous transmitting ONUs lead to severe packet loss percentages on HSI and dropped calls on Voice platform.Conclusion: OBI probability increases dramatically by adding just two more simultaneously transmitting ONUs to the initial three.

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3. OBI impact

Sample of OBI capture using a Spectrum Analyzer set on Max. hold

Although the entire noise floor is elevated (sometimes higher than the carriers), on the spectrum analyzer operator will see “spikes” due to SA sweeping the RF spectrum with a set RBW.

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3. OBI impact

OBI occurrence can be easily detected by measuring the lost packets in the Voice and Data platforms

HSI lost packets graph

Voice lost packets graph

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Agenda

1. What is OBI?

2. Causes

3. OBI impact


5. 1310 nm or 1610 nm?



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Given the number of variables (most important being: temperature, manufacturing distribution and probability to be activated simultaneously), it is very difficult (if not practically impossible) to calculate the probability of OBI occurrence.

However, considering HSI only platform, table below shows the probability of OBI occurrence for different instances.

Modems # Us channels OBI probabilityD2.0 + D3.0 1 0D2.0 + D3.0 >1 X (X>0)D3.0 Only 1 0D3.0 Only always bonded >1 0D3.0 Only not always bonded >1 Y (Y>0, but probably Y<X)

OBI probability in HSI only environment

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Agenda

1. What is OBI?

2. Causes

3. OBI impact


5. 1310 nm or 1610 nm?



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5. 1310 nm or 1610 nm?

• Initial RFoG systems (before the release of 174 2010 SCTE specification) used 1310 nm return ONUs.

• In order to allow PON overlay on the single fibre from LCP to the customer premise, and because the PON Specification is using 1310 nm return, 1610 nm return became the alternative for ONUs upstream.

Generic diagram of RFoG/PON overlay.

• As shown in section 2, due to the narrow optical spectrum in the 1610 nm band, the risk of OBI occurrence is higher than using 1310 nm band.

• However, using 1610 nm return assumes that operator overlays PON, and at least one of the independent scheduled platform (HSI, main contributor to OBI occurrence due to high utilization) is not present anymore in RFoG, and this reduces significantly the probability of OBI occurrence.

• Assuming that PON can deliver HSI and Voice services, the RFoG system has no OBI issues.

• Therefore, it is prudent to use 1610 nm return in RFoG systems only in conjunction with PON overlay.

HUB or RFoG extender

1550161014901310OLT

ONU

ONT

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Agenda

1. What is OBI?

2. Causes

3. OBI impact


5. 1310 nm or 1610 nm?



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• In 2009, SCTE Working Group 5 recommended the use of lasers from batches manufactured at different time periods in order to minimize the λc manufacturing Gaussian distribution. This recommendation is, in our opinion, hard (if not practically impossible) to implement. Adding into equation the temperature effect, the results are questionable.

• Use of cooled DFB lasers; given the narrow spacing, the lasers’ TEC (thermo-electric cooler) must be DWDM quality; beside the cost of the DWDM-like lasers, sparing and proper documentation make this option not practical.

• One manufacturer offers “OBI Free” ONUs, which operate in conjunction with a system analyzing the return noise and controlling the ONUs. This technology is patented and details on how it is accomplished are not disclosed. There is also a cost premium involved.

• Another manufacturer offers a system claimed to “Eliminate” OBI. This technology is patented and details on how it is accomplished are not disclosed. Involves field active deployment (power, enclosure).

• Given our experience in RFoG deployments (long haul), and the upstream capacity demand, our first solution is to use for HSI platform a scheduler capable to allow operation of both D2.0 and D3.0 modems by controlling a single modem to transmit in each time slot. Customers using D2.0 modems may experience a slower upstream throughput, but D2.0 modems are used only for lower HSI tiers.

• Because of the “clean” return RF spectrum in RFoG systems, we successfully activated three 64QAM/6.4 MHz HSI upstream channels, also using the 5 to 17 MHz band (dependent on the region). This gives RFoG systems a max. throughput of ~90 Mbps. Field tests showed that a 20 Mbps Us speed is achievable. Our lab tests and significant number of RFoG deployments showed the feasibility of the solution.

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Agenda

1. What is OBI?

2. Causes

3. OBI impact


5. 1310 nm or 1610 nm?



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Today’s high performance solid-state optical switches have switching time in the order of femto-seconds.(1 fs=10-15 seconds)By “sweeping” 32 ports corresponding to a standard group of 32 customers and allocating proper sampling time to each of the ports, the RX will receive only one optical signal at a time.This sampling is very similar to digitization of the RF return spectrum.Replacing the 32-way splitter with the switch increases the upstream optical budget by ~14 dB (assuming a 2 dB loss in the switch).

Given the (up to) 1GHz FWD spectrum, the sampling procedure above is not possible (Nyquist theorem requires sampling frequency twice the max. frequency).Consequently, FWD 1550 nm signal must be locally amplified and split 32-ways, and multiplexed on the single fibre serving the individual RFoG customers.

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Thank you!

obi in rfog systems public

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