hardware description and service troubleshooting in satellite communications

Slide Number 1Rev -, July 2001

Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada

6.8 Hardware Description and Service Troubleshooting



Hardware DescriptionPart 1

Vol 6: Digital Video

Sec 8: Hardware Description and Service Troubleshooting



Transmit Equipment for Satellite DistributionThe transmit equipment used for satellite distribution is typically comprised of:

Upconverter: Converts the modulated carrier from the modulator and up-converts its frequency from 70 MHz to one of the satellite frequency bands.

C-Band Transmit 5.9 to 6.4 GHz

Receive 3.7 to 4.2 GHz

Ku-Band Transmit 14.0 to 14.5 GHz

Receive 11.7 to 12.2 GHz

DBS Transmit17.3 to 17.8 GHz

Receive 12.2 to 12.7 GHzVol 6: Digital Video, Sec 8: Hardware Description and Service Troubleshooting

Part 1: Hardware Description

6.8.1.1: Transmit Equipment for Satellite Distribution



Transmit Equipment for Satellite DistributionPower Amplifier: Amplifies the signal from the output of the

upconverter. There are three types of amplifiers used in the transmission of digital video and/or audio signal:

SSPA (Solid State Power Amplifier)

TWTA (Travelling Wave Tube Amplifier)

KHPA (Klystron High Power Amplifier)

The next slides illustrate two different equipment configurations used to provide 1:1 redundancy at the transmit location.

Vol 6: Digital Video, Sec 8: Hardware Description and Service Troubleshooting








Figure 6.8.1.1a Typical RF Transmit (Uplink) Systems Used for Satellite Distribution 1:1 Redundant System (configuration #1)






Figure 6.8.1.1b Typical RF Transmit (Uplink) Systems Used for Satellite Distribution 1:1 Redundant System (configuration #2)



Transmit Equipment for Satellite DistributionWhen passing through the modulator, upconverter, power amplifier and the receive equipment (Low Noise Block Downconverter and receiver) the modulated signal is processed through multiple filters.

Every time the signal is passed through one of these filters a cumulative propagation delay is applied non-uniformly across the frequency span of the modulated carrier.

The propagation delay is referred to as the Group Delay. It is also referred to as Envelope Delay or Time Delay but in this presentation it will be referred to as Group Delay.

Group Delay can be very destructive to the distribution of digital signal over satellite.






Transmit Equipment for Satellite DistributionGeneral Instrument specifies for their DCII system (ATSC based system) that the upconverter, power amplifier, satellite transponder and LNB cumulative group delay shall not exceed:Frequency offset Max G.D. relative to center frequency forcarrier frequency 36 MHz transponder 54 MHz transponder

(MHz) (nsec) (nsec)

+ 8 9.0

+ 12 30.0 9.0

+ 14 70.0

+ 18 30.0

+ 21 70.0






Satellite Receive EquipmentTo receive a digital signal that is being distributed by satellite, we need:

• Receive antenna (size will depend on frequency band, BW of carrier, receive EIRP for the carrier) with or without receive feed.

• LNB or LNBF (low noise block downconverter, or low noise block downconverter with feed). Both the LNB and LNBF have similar function, they both take a signal from the satellite and convert it to the frequency band referred to as L-Band (in North America this band is from 950 to 1450 MHz).

• Satellite receiver or Integrated Receiver Decoder IRD

• Audio and video monitoring equipment.



6.8.1.2: Satellite Receive Equipment



Typical R F S ate llite R eceive (D ow nlink) S ystem s

IRDSate llite Receiver M onitor

Antenna

LNB orLNBF

Cable




Figure 6.8.1.2a Typical RF Satellite Receive (Downlink) Systems



Satellite Receive EquipmentThere are several considerations that must be investigated when designing a receive system:

• Antenna size - this will be determine in the link budget analysis.

• LNB or LNBF - will depend on the chosen antenna. Usually an LNBF is less expensive than LNB. The performance of an LNBF is not as good as that of an LNB.

• Noise figure of the LNB/LNBF - determined by the link budget analysis

• Phase noise of the LNB/LNBF

• Frequency stability of the LNB/LNBF






Satellite Receive EquipmentThe LNBs that were previously used for analog video transmission are usually not adequate for use with digital signal. You should inquire with the IRD manufacturer for their recommended low noise block downconverter.

One of the most common oversights when converting an analog receive station for reception of digital services is to use the wrong type of LNB.

The next slide is a comparison of an analog LNB versus an LNBF and two Phase Locked Loop (PLL) LNBs.






Satellite Receive Equipment

Analog LNBF PLL LNB

RF input freq. 11.7-12.2 GHz 11.7-12.2 GHz 11.7-12.2 GHzIF output freq. 950-1450 MHz 950-1450 MHz 950-1450 MHzNoise figure 0.6 to 1.0 dB 1.1 dB 0.8 dBGain 58-70 dB 50-62 dB 55-65 dBLO stability + 3.0 MHz + 2.0 MHz + 25 kHzPhase noise

@ 100 Hz -65 dBc/Hz@ 1 kHz -50 dBc/Hz -55 dBc/Hz -73 dBc/Hz@ 10 kHz -75 dBc/Hz -80 dBc/Hz -73 dBc/Hz@ 100 kHz -100 dBc/Hz -100 dBc/Hz -85 dBc/Hz@ 1 MHz -120 dBc/Hz -105 dBc/Hz






Service TroubleshootingPart 2





Service TroubleshootingWhen troubleshooting a digital video service, the possible causes of problems can be classified in the following manner:

• Customer video/audio signal

• Coding/compression/multiplex

• Modulator

• RF uplink • Group delay

• Transmit power to satellite



6.8.2: Service Troubleshooting

• Receive facility• Antenna

• LNB

• Receiver

• Video quality

• Audio quality



Service TroubleshootingCustomer Video and Audio Signals

One of the first things to check is the video, audio, or data signal being provided by the customer. The video signal should either meet the ITU-R BT.601-5 standard (D1) or an analog video/audio standard recommended by the manufacturer of the encoding system.

To facilitate troubleshooting, a means of decoding the video and audio signal within the D1 signal should be present at the transmit facility.

You cannot expect this digital receiver to provide good video/audio signal when there is a problem with the video and/or audio signal provided to the encoding system.






Service TroubleshootingCoding/Compression/MultiplexSome DVC systems provide an internal decoder that allows for monitoring of the video and audio signal after being processed through the coding/compression system.

The only method of verifying the state of the output of the multiplexer is to use an analyzer, sometimes called an MPEG analyzer (DVB and ATSC), DVB analyzer or ATSC analyzer. This analyzer takes the output of the multiplex and will identify:

• Loss of packets, loss of sync.• Information on the SI/PSI tables (MPEG/DVB system) such as

data rate, PID, content of PAT, CAT, PMT and SI tables.• Information on the video, audio and data PES, such as data rate,

PID.

Some analyzers will also decode the video and audio signal.Vol 6: Digital Video





Service TroubleshootingModulator

The only two things that you can check on a modulator are:

• The shape of its output spectrum (to make sure it matches the appropriate BW and slope)

• Back to back test with a demodulator. To do this, use a data test set to provide a data string at the input of the modulator (PRBS - pseudo random bit stream). The output of the decoder is then compared with the input to the modulator and a resulting bit error rate (BER) is calculated.

To make the PRBS BER test work you need a data test set that can operate at high data rate. With a DVB system you will need a data test set that can interface to an ASI system and operate at 270 Mbps.






Service TroubleshootingRF UplinkThe group delay characteristic of the transmit equipment is measured at the implementation of the service. Group delay should not vary too much over time but may change considerably if any of the equipment (i.e. upconverter or power amplifier) is replaced.

The power level of the transmit service should not vary over time. The Earth Station operator is responsible to ensure that the HPA is outputing a constant power level.

The level reaching the satellite or the level received at the received location may vary depending on climatic conditions. When operating above 10 GHz, the service can be significantly affected by rain fades at the uplink and downlink locations. In North America, some areas have had rain fades of more than 20 dB.






Service TroubleshootingReceive facility

When troubleshooting a receive facility you need to check the following

Antenna• Is the antenna properly peaked on the appropriate satellite?

• Is the antenna in good working condition?

• Is the antenna foundation solid?

• Is the wind making the antenna reflector move?

• Is there humidity or water in the antenna feed ?






Service TroubleshootingReceive Facility

LNB• Is an appropriate LNB used?

• Does the LNB have the appropriate noise figure, LO stability, or phase noise characteristic?

• Is there water or humidity in the feed part of the LNBF?

Receiver or IRD• Is the receiver providing the necessary power voltage to the LNB

or LNBF?

• What voltage level is measured on the cable before the LNB or LNBF?

• Does another receiver work with this antenna and LNB/LNBF?






Service TroubleshootingVideo and Audio QualityVideo and audio quality problems can be caused by:

• Using a very low video bit rate with a motion intensive program

• Low quality video or audio is provided at the input of the encoder

• The audio program is being clipped by the encoder because the average program level is higher than the clipping level of the audio encoder

• External interference is causing the receiver to lose lock or to lose packets of data

• Faulty hardware

Video and audio quality is usually based on human perception and is not quantifiable. Comments on video and audio quality measurement is covered in the next slides.






Video QualityPart 3





Video QualityVideo quality testing is a largely subjective process that is based on human perception. There are a few manufacturers that have developed processes and test equipment that can quantify the video quality of a signal. Here are two such pieces of test equipment:

Rhode & Schwarz DVQ (Digital Video Quality analyzer)

Tektronix PQA200 (Picture Quality Analyzer)

Even with such test equipment, subjective measurement is still required, and operators of a DVC system should be aware of the different artifacts and problems that may occur with digitally compressed video signals.



6.8.3: Video Quality



Video QualityThere are three types of artifact and four types of problems that the operator should be aware of:

Artifacts• Gibbs effect

• Blockiness

• Lossy compression

Problems• Aliasing

• Quantization noise

• Overload

• Degradation of the digital stream






Video QualitySince we are concerned about video quality, we will be using this picture has a reference. The effect of the problems and artifacts will be shown on the picture.

In some cases the effect was exaggerated to help in the explanation.

Original Picture




Figure 6.8.3a



The Gibbs EffectThe Gibbs effect is sometimes referred to as the mosquito effect.

As shown in the two pictures, the Gibbs effect is the haze that appears at the boundaries of objects in a picture after being processed through a MPEG or JPEG encoder.

It is usually seen around objects were a sudden transition is made between the object and the background.

Original Image

Image with Gibbs Effect


Part 3: Video Quality

6.8.3.1: The Gibbs Effect

Figure 6.8.3.1a



The Gibbs EffectThe Gibbs effect is most noticeable around text and high contrast objects such as a person and the background.

The effect is found in all DCT based compression system.

Gibbs effect on textFigure 6.8.3.1b



6.8.3.1: The Gibbs Effect



Blockiness When a DVC system uses a very high compression ratio, there is a chance that the frames will be subject to blockiness.

The changes within a pixel block are averaged amongst all the pixels in the block. This may cause a pixel block to adopt a common color. The original colour information within the pixel block is lost due to high compression rate.

Picture displaying blockiness

Figure 6.8.3.2a



6.8.3.2: Blockiness



BlockinessIn high compression systems, pixel blocks often select the same color. The effect is only noticed by a viewer when many adjacent pixel blocks adopt the same color.

The image will then seem to be made up of similar blocks. Within the block very little change in detail is provided. Picture displaying blockiness

Figure 6.8.3.2b



6.8.3.2: Blockiness



Lossy Compression MPEG compression is a lossy compression method. By its nature, a lossy compression method has to remove or disregard part of the original picture so that it can reduce the required amount of bits for a particular video signal.

In high compression-ratio systems there is a lot of information that needs to be removed. Some MPEG 2 services are transmitting video signals compressed to a data rate of 3 Mbps. When you consider that the uncompressed video signal requires a data rate of 270 Mbps you can easily visualize that a great deal of information was not sent. In this case, you hope that the compression system will be able to recreate a close representation of the original signal.



6.8.3.3: Lossy Compression



Lossy CompressionTo achieve this low compression rate a lot of information will be disregarded. In the picture, this could be perceived as loss of detail or resolution. In some extreme compression ratios, some small objects might be ignored or their detail could be such that they are no longer recognized by the viewer.



6.8.3.3: Lossy Compression



AliasingAliasing is an artifact that occurs during the sampling stage where a sample contains frequency components that are too high for the quantization level within that sampling range. This can happen when the sampling interval is too great and high frequency components are reproduced as low frequency components. Aliasing on a video signal appears has a jagged edge or stair-step effect.

Picture displaying aliasing

Figure 6.8.3.4a



6.8.3.4: Aliasing



AliasingThe aliasing effect can be avoided if the input video signal is passed through filters. These filters will remove the unwanted high frequency component in each of the samples.

The selection of filters is a tricky process. If filtering removes too much high-frequency component the image will appear to have softer edges as shown in this picture.

Picture pre-filtered to avoid aliasing

Figure 6.8.3.4b



6.8.3.4: Aliasing



Quantization NoiseWhen converting an analog signal to a digital format we first need to quantize the video signal to a finite number of levels. As the number of levels increase, the quantization noise diminishes. As the number of levels decrease, the quantization noise increases.

Within a quantization level the samples are averaged.

Original Picture

Figure 6.8.3.5a



6.8.3.5: Quantization Noise



Figure 6.8.3.5bPicture with

different number of color

resolution

Colour picture with 8 bits per Pixel Colour picture with 4 bits per Pixel

Greyscale picture with 8 bits per Pixel Greyscale picture with 1 bits per Pixel



6.8.3.5: Quantization Noise



Overload

Wraparound

OverloadOverload is another problem that can occur in the quantization process.

Samples are quantized and their value is set to one of the quantization levels. If the levels are not set properly, the samples could be represented by a value that is too high in amplitude. If this happens the image will appear to be bleached.

Figure 6.8.3.6aa

Figure 6.8.3.6ka



6.8.3.6: Overload



Overload

Wraparound

OverloadThe bleaching effect is shown in the top picture, where the ground, the antenna, the sky and part of the person’s face has been converted to a white level.

Another possible problem with overload occurs when low and high amplitude limits are set on the quantizer. Any sample whose level exceeds the limit is then converted to a set value.

Figure 6.8.3.6ab

Figure 6.8.3.6kb



6.8.3.6: Overload



Overload

Wraparound

OverloadIn this present example (bottom picture) the default value is the lowest value possible (black).

Care must be taken in setting level thresholds. Incoming signal should not exceed the level recommended by the manufacturer.

Figure 6.8.3.6ac

Figure 6.8.3.6kb



6.8.3.6: Overload



Degradation of the Digital StreamIn all of our previous explanation we have assumed that the data stream was received by the decoder without any loss of information (ie packets).

But what happens when the service is operating close to operating threshold and the forward error correction is not able to correct all errors in the bit stream?

MPEG image with multiple bit errors

Figure 6.8.3.7aa



6.8.3.7: Degradation of the Digital Stream



Degradation of the Digital StreamWhen packets are lost, the effect can be very noticeable, depending which packet was lost. In the video presentation, the loss of packets can be perceived as video blocks of errors, loss of frames (jerky motion) or popping in the audio signal.

Usually, blocks of errors are very noticeable as blocks of bad data. In the picture they appear as pink, purple and green blocks.

MPEG image with multiple bit errors

Figure 6.8.3.7ab



6.8.3.7: Degradation of the Digital Stream



Audio QualityPart 4





Audio QualityAudio quality problems are usually the results of:

• Aliasing

• Same as video, but results in a buzz or popping sound

• Overload

• Clipping of the audio signal, similar to clipping with analog audio systems

• Degradation of the digital stream

• The effect could be muting of audio, beeping or popping sounds

• There may also be problems with the synchronization of the video and audio frames



6.8.4: Audio Quality

hardware description and service troubleshooting in satellite communications

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