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