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Return Path Field Training
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Return Path Field Return Path Field TrainingTraining
Presented By:Presented By:Fawad FaheemFawad Faheem
Return Path MaintenanceReturn Path Maintenance
OverviewOverview IntroductionIntroduction Modulation SchemesModulation Schemes Return Path NoiseReturn Path Noise DOCSIS SpecificationsDOCSIS Specifications
IntroductionIntroduction What is Return Path?What is Return Path? Why do we use Lower Spectrum?Why do we use Lower Spectrum? Different Standards Different Standards (DOCSIS / EURODOCSIS)(DOCSIS / EURODOCSIS)
m(t)
Sin
m(t)Sint
1)Modulation schemes being used in R/P?a) QPSK b) 16-QAM1)BPSK
One bit is transmitted in one symbol
Block Diagram For Bpsk(t) = m(t) Sin t
QPSKQPSK The input binary bit stream {dk}, dk = 0,1,2,..... The input binary bit stream {dk}, dk = 0,1,2,.....
arrives at the modulator input at a rate 1/T arrives at the modulator input at a rate 1/T bits/sec and is separated into two data streams dI bits/sec and is separated into two data streams dI (t) and dQ (t) containing odd and even bits (t) and dQ (t) containing odd and even bits respectively.respectively.
dI(t) = d0, d2, d4 ,...dI(t) = d0, d2, d4 ,...dQ(t) = d1, d3, d5 , ...dQ(t) = d1, d3, d5 , ...
90o
Digital bit stream
DI
DQ
Sin
Cos
(t) = DI Sin t + DQ Cos t
Block Diagram Qpsk (t) = DI Sin t + DQ Cos t
= A Cos (t + I)
Where A= √ (DI)2 + (DQ)2
= Tan-1 (DQ/DI)
Reason for using QPSK and it's specsReason for using QPSK and it's specs
Return Path Noise being the Return Path Noise being the biggest challenge to interactive biggest challenge to interactive
services in HFCservices in HFC
LEAKAGELEAKAGEAmount of RF signal that escapes out of the cable due to one Amount of RF signal that escapes out of the cable due to one
of the following reasons.of the following reasons.
1.1. Cable cracks.Cable cracks.2.2. Loose connectorsLoose connectors3.3. Damaged ConnectorsDamaged Connectors4.4. Corrosion of connectorsCorrosion of connectors
Types of Leakages:Types of Leakages: There are two types.There are two types. 1) Egress.1) Egress. 2) Ingress2) Ingress
Egress:Egress: Egress is RF signal leakage out of Egress is RF signal leakage out of
the Cable System the Cable System
Ingress:Ingress: Ingress refers to RF Noise that Ingress refers to RF Noise that
leaks into the cable system. RF leaks into the cable system. RF signal emitted from surroundings signal emitted from surroundings enters into the system to follow any enters into the system to follow any path that provides least resistancepath that provides least resistance
Possible causes of noise from distribution plant and their fixingPossible causes of noise from distribution plant and their fixing
Studies have revealed that almost 95% of the ingress comes from Studies have revealed that almost 95% of the ingress comes from
customer premises. Possible causes are:customer premises. Possible causes are:
CPD (Common path CPD (Common path distortion)distortion)
1) 1) Because this distortion arises at Because this distortion arises at points in the plant where both forward points in the plant where both forward and reverse signals are present ,it is and reverse signals are present ,it is called as common path distortion.called as common path distortion.
2)2) This type of noise is spurious in This type of noise is spurious in nature (whole noise floor raises) and nature (whole noise floor raises) and are composed of distortion products of are composed of distortion products of downstream signalsdownstream signals..
Second and Third Order BeatsSecond and Third Order Beats
Possible causesPossible causes
1.1. These usually include poorly installed connectors, corrosion, dissimilar These usually include poorly installed connectors, corrosion, dissimilar metals used in connectors, or kinks and cracks in rigid coax, weather metals used in connectors, or kinks and cracks in rigid coax, weather shielding, oxidation etc.shielding, oxidation etc.
DOCSIS SpecificationsDOCSIS Specifications Downstream
DOCSIS SpecificationsDOCSIS Specifications
Upstream
How to Improve C/N?How to Improve C/N?
Plant StatisticsPlant Statistics Plant leakage has been a Big Plant leakage has been a Big
Problem.Problem. Missing HPF’s should be looked afterMissing HPF’s should be looked after Weather protection of passives by Weather protection of passives by
using heat shrinksusing heat shrinks Documentation and improvement in Documentation and improvement in
troubleshooting techniquestroubleshooting techniques