25Jun12 Customer HFC Training v1.2

' 2007 Cisco Systems, Inc. All rights reserved.Presentation_ID.scr

1

{No notes associated with this slide}


2



3

Corning type SMF-28 optical fiber is the most popular fiber for HFC networks.

A single fiber consists of a glass core surrounded by a concentric glass cladding; the two glasses have different refractive indeces so that light is confined to the core by total internal reflection. The protective plastic jacket is color-coded so that individual fibers can be identified in multiple-fiber cable bundle.

Cables may contain as few as two fibers, or as many as 144.


4

Corning type SMF-28 optical fiber is the most popular fiber for HFC networks.

A single fiber consists of a glass core surrounded by a concentric glass cladding; the two glasses have different refractive indeces so that light is confined to the core by total internal reflection. The protective plastic jacket is color-coded so that individual fibers can be identified in multiple-fiber cable bundle.

Cables may contain as few as two fibers, or as many as 144.


5

Optical signal power in a fibre-optic cable is reduced by two primary mechanisms: absorption and scattering.

Hydroxyl ions (-OH) absorb light near 2730nm, with weaker absorptions at 1390, 950 and 720nm . The hydroxyl content must be kept below 1 part per million in order for the attenuation at 900nm to be less than 1 dB per km. Even if the water content of the fibre is minimised, residual absorption remains from the infrared absorption of the fundamental vibrations of the bonds that make up the glass, which occur at 730nm for boron-oxygen bonds, 800nm for phosphorous-oxygen bonds, 900nm for silicon-oxygen bonds, and 1100nm for germanium-oxygen bonds.

Scattering occurs when the light signal bounces off atoms or other particles within the fibre and is spread in all directions.

For glass fibres, the most significant form of scattering is Rayleigh scattering, which actually does not quite fit the accepted definition of scattering. With this process, atoms or other particles within the fibre fleetingly absorb the light signal and instantly re-emit it in another direction. In this way, Rayleigh scattering appears very much like absorption, but it absorbs and re-directs the light so quickly that it is considered scattering. Microscopic variation in material density also enhance Rayleigh scattering.

Both scattering and absorption are cumulative. Light is absorbed and scattered continuously, so the signal at the end of the fibre is almost never exactly the same signal as it was at the beginning.

The graph shows that attenuation minima occur around 1310nm and 1550nm, which are the dominant wavelengths used in HFC networks today. Typical high-grade singlemode fibre has a loss of about 0.2 dB per km at 1550nm. If this radiation fell in the visible part of the spectrum, and if ocean water were as transparent as modern fibre-optic cable, it would be possible to see clearly the bottom of the Marianas Trench from the deck of a ship.


6

These are the primary non-linearities that affect the quality of signals in an HFC optical link.

Items 3 (DRB) and 4 (SBS) result in an increase in noise in an optical transmission system, and item 1 (Chromatic dispersion) result in Composite Second-Order distortion. The other items (2, 5 and 6) give rise to cross-talk between signals in a multi-wavelength transmission link. These effects will be discussed in more detail in other presentations.

Lon Brillouin; French-American physicist.

Lord Rayleigh; British physicist.

Sir Chandrasekhar Venkata Raman; Indian physicist and mathematician.


The analogue and digital video signals, and the high-speed (Internet) and VoIP traffic are multiplexed and transported through the HFC network using Frequency Division Multiplexing (FDM). That is, each analogue or digital signal is assigned a unique channel frequency. This is exactly the same method that is used in terrestrial television and radio broadcasting. A set-top box must tune to a specific channel frequency in order to receive an analogue or digital signal.

Broadcast traffic consists of all the analogue and digital video signals that are available to all subscribers at all times. They can be compared to Multicast transmissions.

Narrowcast traffic consists of all the interactive signals. (Video on Demand, Internet traffic, and VoIP). These signals are targeted at specific customers, and therefore they can be compared to Unicast transmissions.

(NOTE: the diplex crossover is an unusable part of the spectrum created by the filter modules which separate the downstream and upstream signal paths in active devices such as optical nodes and amplifiers. The actual frequency limits of the downstream and upstream spectra differ from region to region: the figures shown here are typical of Latin American countries. Cisco and other HFC manufacturers have recently introduced a system which allows more bandwidth in the upstream signal path. The diplex crossover in this new system is 85 to 105 MHz).

State-of-the-art HFC networks have a downstream maximum frequency of 1,000 MHz. This provides capacity for 153 signal channels, each 6 MHz wide. In general, analogue video signals are restricted to frequencies below 550 MHz. This allows for 78 analog video channels. The reamining bandwidth can accommodate 75 QAM signals.

7


The Prisma II family of high-density, 1310nm transmitters are all Directly Modulated units. This is a simplified schematic diagram of a typical transmitter. Note that the RF signal, after amplificiation and pre-conditioning, is applied directly to the semiconductor laser, which is a DFB (Distributed FeedBack) device. DFB lasers incorporate a diffraction grating in place of one or both of the reflective faces of the semiconductor laser: this restricts the number of longitudinal modes, and results in an optical line-width of typically 0.1 nm.

8


An upstream optical transmitter, suitable for use in an Optical Node, is much simpler and more compact than a downstream transmitter in a Headend or Hub. Again, the laser is directly modulated, and is a DFB type. (Cisco no longer offers a Fabry-Perot transmitter. The F-P lasers were less expensive: they did not have diffration gratings and therefore the laser operated in multiple longitudinal modes, resulting in a large optical line-width of typically 1 4 nm. This in turn caused much more Rayleigh Backscatteri ng in the fiber, compared to DFB lasers, and limited the CNR performance of the optical link. F-P lasers are therefore not recommended for upstream transmission of large numbers of high-bandwidth and/or bonded DOCSIS signals, although small numbers of QPSK and 16QAM signals can be carried satisfactorily).

9


In order to generate extremely narrow optical line-widths, an Externally Modulated Transmitter (EMT) must be used. This is particularly true at wavelengths close to 1550nm, where the Chromatic Dispersion of the fiber will cause signal distortion if the line-width is large.

An EMT uses a DFB laser which generates a steady output at fixed intensity. This light is then amplitude-modulated by a second optically-active device; in most cases, it is a Mach-Zehnder modulator. The result is an optical output signal with a line-width of only 1 2 MHz (0.00 0008 to 0.000016 nm).

10


{no notes associated with this slide}

11


Optical transmitter data-sheets from Cisco contain recommendations for the RF input level with certain reference traffic loads. For the Americas, this reference load consists of 78 analog video signals between 54 and 550 MHz, and 75 digital (QAM) signals between 550 and 1,000 MHz.

However, many operators are reducing the number of analog signals, and increasing the digital content. Others may be using only a portion of the entire available spectrum. Should the RF input level be changed in these circumstances? The answer is Yes, if optimum CNR performance is desired.

Note that the recommended RF input level refers to the Peak Envelope Power of an analog video signal. For example, a 1310nm high density Prisma II transmitter has a recommended input level of 15.0 dBmV with the reference load shown here. The following slides will explan how this level can be adjusted to accommodate different traffic patterns.

12


In order to calculate the new RF input level, we must begin with the power of a modulated analog video signal in a 6 MHz channel. Using the 1310nm transmitter example, the recommended Peak Envelope Power (which is the power of an unmodulated carrier) is 15.0 dBmV. The average power in a 6 MHz channel, when the carrier is modulated with active video, will be approximately 3 dB less, or 12.0 dBmV.

13


Most vendors (including Cisco) and HFC network operators now recommend a difference in level of 6 dB between the Peak Envelope Power of an analog video signal, and the power (total, in a 6 MHz channel) of a digital signal.

It is still possible, however, to find HFC networks in which 64QAM signals are at a level of -10 dB, relative to analog Peak Envelope Power, and 256QAM signals are at -6 dB.

Again using the example of the 1310nm Prisma II transmitter, the recommended RF input level for the reference channel load is 15.0 dBmV (analog Peak Envelope Power), and therefore the digital (QAM) signals will have a level of 15 6 = 9 dBmV.

14


Many experiments have been conducted to show that this basic principle is valid.

The recommended RF input level, for the reference channel load, is designed to optimize the performance of the laser. That is to say, it achieves maximum CNR while ensuring that the laser operates in its linear region, and certainly does not produce clipping.

When the channel load is changed, the total RF input power will change. It must therefore be adjusted so that it is equal to the total power with the reference channel load.

15


The total power of the analog video signals and the digital signals must be calculated separately, then added on the basis of absolute power and converted back to dBmV. In this example, the total power of the combined analog and digital signals is 32.6 dBmV.

16



17


When the number of analog video signals is reduced to 30, and the number of digital (QAM) signals is increased to 123, the transmitter input level can be changed, so that the total power remains the same.

If the new peak envelope power of an analog signal is z, then the total power of the analog signals can be calculated, as shown, and also the total power of the digital signals. When these are added (on a power basis, of course), we find that the total composite power is z plus 16.6 dBmV.

As stated, this must be equal to the total composite reference power, which was 32.6 dBmV. Therefore z = 16 dBmV.

This means that the peak envelope power of an analog signal, with the new channel-load, will 16 dBmV; an increase of 1 dB.

18


This is a simplified formula for calculating the required change in the peak envelope power of an analog signal, when the numbers of analog and digitals are changed.

This formula also allows the user to modify the values of d and m, in the event that digital signals are not at -6 dB relative to peak analog envelope power, and there is disagreement regarding the amount of power reduction when an analog carrier is modulated. (Some system operators choose m = 6 dB).

AS stated earlier, numerous experiments have shown that this procedure is valid. When the number of analog signals is decreased, and the number of digital signals is increased, there will be a reduction in composite power at the transmitter input. The power can therefore be increased, resulting in an improved Carrier-to-Noise Ratio, without increasing the second- and third-order distortions in the overall optical link.

IMPORTANT NOTE: this procedure cannot be applied to the RF components in an HFC network. The tilt of the sig nals in a typical RF amplifier introduces a great deal of complexity and uncertainty.

19



20


The determination of the optimum RF input level for an upstream transmitter is not easy. In the downstream signal path, the problem is relatively simple: the traffic is continuous and predictable. But in the upstream signal path, the traffic consists (mostly) of DOCSIS Cable Modem transmissions, which are bursty, and signals from set-top boxes and (possibly) Element Management transponders, which are also sporadic.

In addition, it is not always possible to forecast the growth of traffic and, of course, an HFC system operator must be able to accommodate this growth without the need for repeated adjustments to Optical Nodes.

There is also the problem of ingress. This is the accumulation of noise and spurious signals that originate outside the HFC network, and which can result in serious degradation of traffic quality.

21


We must first address the problem of traffic volume.

HFC network operators have adopted one of the following strategies:

1. Maximize Carrier-to-Noise Ratio (CNR) by operating the upstream transmitter at close to the 100% OMI point. This strategy assumes that the total instantaneous traffic volume can be predicted.

2. Assume the worst-case scenario: the entire upstream bandwidth will be utilized at some point in the near future. It should not be necessary to visit the Optical Node and make any adjustments.

3. Do nothing. This strategy actually works. Because the upstream traffic is bursty, and because bandwidth demand has not been as great as expected in recent years, many HFC networks continue to provide reasonable upstream traffic quality, even when using Fabry-Perot transmitters. Of course, when subscribers monitor their data-rates, they may find that they are receiving sub-standard service.

22


A typical semiconductor laser will provide generate optical power in direct proportion to the input current over a limited operating range.

With a very low input current, the laser will (effectively) generate no light; when the input current reaches a certain threshold, however, the laser will produce coherent light. The intensity of the light will increase in direct proportion to the input current (a linear relationship), but eventually an additional increase in current produces a smaller increase in light output (a nonlinear relationship).

Ideally, the laser should always be operated within the linear region of this curve.

23


This slide illustrates the optimum performance of the transmitter. The input current to the laser is maximized but, at the peaks of the signal, the laser is not driven into a non-linear region.

At this point, the input signal is generating 100% Optical Modulation Index (OMI). Any further increase in input level will drive the laser into non-linearity.

24


When the OMI exceeds 100%, the laser is driven into a non-linear region of the curve, and the output signal is distorted. In particular, the laser output will be extinguished (driven below the threshold) at the peaks of the input signal.

The result is a complex mixture of distortions that appear as Noise.

25



26


This is a section of the data-sheet which describes upstream analog optical transmitters for Optical Nodes. Here we see that a single CW signal (sine-wave) will produce 100% OMI in the transmitter when it is at a level of 33 dBmV at the transmitter input.

(This is the high gain transmitter: the standard gain version requires a lower input level).

It is important to understand that this level is expressed in dBmV, and therefore it is a power level. It is the RMS value of the signal. For a sine-wave, the peak value of the signal, which is the key issue in laser clipping, will be 3 dB above the RMS value.

27


With simple mathematics, then, we can calculate the power of each DOCSIS signal so that the transmitter will be operated at the maximum possible input power (100% OMI; which maximizes the CNR).

But we must consider the signal peaks, which are the cause of laser clipping. The ratio of the peak value to the rms value (also called the Peak Factor) of a noise-like DOCSIS signal is not the same as the peak factor for a sine-wave. The sine-wave has a constant and well-defined peak factor of 3dB. What is the ratio for a noise-like signal from a Cable Modem?

And what allowance should be made for the Ingress noise, which also adds to the power input to the transmitter?

28


The mathematics is complicated.

In simple terms:

We cannot predict the peak factor of a noise-like signal at any instant. We can only say that, if we select a certain value, there is a probability that this value will be exceeded at any instant.

If we select a value of 9.5 dB, we can show (mathematically), that the probability that this value will be exceeded at any instant is less than 0.01%.

If we want to achieve a lower probability, we must choose a higher peak factor. But remember that DOCSIS transmissions are error-protected, and increasing the peak ratio implies a lower level for the DOCSIS signals.

NOTE: there is no industry standard for this calculation.

So, the difference in peak factor between a sine-wave and a QAM signal is 6.5 dB.

29


This method ensures that the laser in the transmitter is operated at its optimum performance: close to (but not exceeding) 100%, so that the CNR is maximized, without causing laser clipping.

The problem, of course, is that we do not know whether the traffic will increase in the future, and we cannot account for the effects of ingress noise (RF interference).

30


The Noise Power Ratio (NPR) test is a measurement of the quality of a complete upstream optical link. (Transmitter, fiber, passive losses and receiver).

Because, in many cases, we cannot predict the total volume of upstream traffic, and we certainly cannot predict the volume of RF ingress, the NPR test simulates a full loading of the upstream bandwidth by using white noise, covering the entire upstream bandwidth.

The quality of the upstream signal path is determined by examining a narrow slot in this bandwidth, using band-stop and band-pass filters, and measuring the depth of this slot as the noise power is increased.

31


The Noise Power Ratio (NPR) test is designed to fully load a device or system with a broad spectrum of white (gaussian) noise, and to determine the degree of Intermodulation Distortion created by this noise signal, as its level is increased.

32


At the output of the device or system, a spectrum analyzer is used to measure the depth of the notch. The level of the noise will have risen, due to the creation of Intermodulation products.

The NPR is measured for a range of input noise levels.

33


The graph in this slide shows the variation in NPR versus the input level to the device or system under test. (NPR is expressed as the ratio between the carrier and the combined thermal and intermodulation noise).

At low input levels, the intrinsic (thermal) noise of the device or system is dominant, and an increase in the white noise input level leads to an improvement in NPR (1 dB for 1 dB). This is the linear portion of the NPR curve.

A point is reached, however, when Intermodulation noise begins to appear in the notch, and the curve is no longer linear. Increases in input level produce increasing deviations from the straight line. This is the transition portion of the NPR curve.

Finally, the Intermodulation noise is completely dominant, and an increase in the white noise input level causes a decrease in the NPR, in the nonlinear portion of the curve. The slope of the curve in this region will indicate the dominant order of the Intermodulation products.

34


This is the NPR curve for a typical upstream optical transmitter. It is a 1310nm DFB high gain transmitter.

The input level on the horizontal axis of the NPR curve is usually expressed in dBmV per Hertz. (The power density)

The value of the input level at the optimum loading point can be determined.

To determine the optimum level of any given signal, its bandwidth must be known, and then the required level can be obtained by taking the optimum input level from the NPR curve, and normalising to the bandwidth of the desired signal.

From the previous example: a 64QAM signal with a bandwidth of 6.4 MHz must be carried.

The optimum input level to the transmitter, as shown by the NPR curve, is -57 dBmV/Hz.

The level of the 64QAM signal should therefore be -57 + 10.log (6 400 000) = 11.1 dBmV.

Then an additional back-off for plant thermal effects and aging may be applied. A typical back-off is 3dB, but many operators are now eliminating this additional safety margin, reasoning that the figures obtained from the NPR curve already represent a worst-case transmitter loading scenario.

35


Although a peak factor of 9.5 dB is reasonable for a single QAM signal, the peak factor for white noise in the entire Upstream spectrum (5 to 42 MHz) may be much larger. In fact, in Cable Telecommunications Testing Guidelines (ANSI/SCTE 96 2008), the peak factor of the noise is expected to be at least 13dB.

36



37


38



This HFC architecture applies only to Upgrade projects, in which the existing network has a Trunk and Feeder architecture with a large number of amplifiers in each cascade. (Perhaps five or six). In these architectures, the Trunk had lower RF signal levels than the Feeder, and it will be advisable to maintain these differences in the Upgrade.

The reason for this difference in level is that the Trunk must maintain high quality (low distortion) as the signal passes through a relatively large number of amplfiiers. The Feeder, on the other hand, must maintain a higher level, in order to provide signal to as many subscribers as possible.

Note that this architecture employs the concept of unity gain: the output levels of all the amplifiers in the Trunk are identical. This greatly simplifies record-keeping and maintenance.

Upstream RF signal levels always obey the unity gain principle, as shown.

In an architecture which draws a distinction between trunk and feeder, the trunk will have lower RF signal levels than the feeder. This is done to reduce distortion accumulation in the trunk, which may consist of a cascade of four, five or six amplifiers, while maintaining a high level for the feeder, which has to deliver signal to as many potential subscribers as possible.

This architecture is a direct descendant of the old Trunk and Feeder (or Tree and Branch) designs that characterised the all-coaxial systems before the development of low-cost and reliable optics

39


The star architecture comprises an array of Tap Amplifiers, which receive signal from the Node via Express cable runs. There are no subscriber taps in the express cable, and Tap Amplifiers are not cascaded.

The same principle described in the previous slide applies here: amplifiers which feed subscriber taps are operated with a high output level, while the express amplifiers operate at a lower level, to minimise distortion.

Despite the different operating levels in the downstream signal path, the upstream RF signals continue to obey the unity gain principle.

This architecture is suitable in a new-build or re-build scenario, with low to moderate population density. A cascade of one or two Express amplifiers may be needed in order to ensure that the Serving Area is not too small.

40


In new-build or re-build scenarios, where the population density is high, it may be possible to reach a large number of subscribers by eliminating amplifiers entirely: this is the Node-plus-zero architecture. (But NOTE: it is generally assumed that a high-density area will contain a high proportion of Multiple-Dwelling Units (apartment blocks), and it is extremely likely that these building will contain small, indoor amplifiers, referred to as MDU amplifiers. (These could be amplifiers from the Compact product family, for instance).

Lacking any amplifier cascade, these systems can operate with high RF signal levels. Here, the downstream levels are the maximum recommended levels for the GS7000 node and the GainMaker nodes.

41


This is the schematic diagram of a GainMaker dual-output amplifier.

(NOTE: an internal jumper or directional coupler can be installed at the location labeled Signal Director to create a third o utput)

The downstream RF output levels are adjusted by changing the values of the input pad and equalizer, indicated by green arrows. The values of the interstage pad and equalizer (indicated by solid red arrows) are selected at the factory, and should not be changed. The output pads (indicated by dashed red arrows) are also selected at the factory, but may be changed if different output levels are required at the Main an d Auxilliary outputs: this is not a frequent practice among most Operators.

42


In the amplifier data-sheets, the gain of the amplifier, and its internal tilt, is specified.

These figures refer to the performance of amplifier when the input pad value is 1dB and the input equalizer value is 0dB.

This is because a 0dB equalizer is simply a jumper with no loss, but all other values of equalizer have 1dB loss at 1 GHz. Therefore, in all real situations, there will be a minimum loss of 1dB at 1GHz: the gain specification of the amplifier (as stated in the data-sheet) takes this loss into account.

43


We always begin with the required RF signal levels at the output of the amplifier: these levels determine the reach of the amplifier (the length of coaxial cable that can be installed between this amplifier and the next), and the number of subscribers that can receive signals from the amplifier (ahigher level allows more subscribers to be connected). HOWEVER, higher output levels also cause more distortion.

The gain of the amplifier, in the data-sheet, is always specified at the highest frequency (1 GHz, in this example). But the internal tilt of the amplifier means that the gain will be less at the lowest frequency (54 MHz in this example). Therefore, if the gain of the amplifier at 1 GHz is 43dB, and the internal tilt is 14.5dB, the gain at 54 MHz will be (43 14.5) = 28.5dB.

Now it is possible to calculate the required input levels.

44


Assume that each amplifier in this network has the same RF output levels as the previous amplifier. This is standard practice, and is called the unity gain principle.

In this example, the coaxial cable has 37dB loss at 1 GHz, and 8dB loss at 54 MHz. Therefore the actual input levels to the next amplifier are as shown. Of course, they do not match the required input levels.

NOTE: this diagram also shows the benefit of having a large positive tilt at the output of an amplifier: if we did not, then the signal level at 1 GHz would be very low, resulting in poor CNR.

45


Now we know the required amplifier input levels, and the actual input levels, we can easily calculate the values of the input pad and equalizer that will convert the actual levels to the required levels.

In this example, the required values are:

Input pad: 6dB

Input equalizer: 14.5dB tilt.

46


In summary, it is necessary to match the actual input levels to the requiredinput levels.

Therefore, we can ask the question: What pad and equal izer should be added to the coaxial cable so that the overall loss and tilt meet the input requirements of the amplifier?

So it is easier to visualize the situation if we imagine that the input pad and equalizer are part of the coaxial cable, and are external to the amplifier!

47


Signal levels in the upstream path of a DOCSIS modem/CMTS link arecontrolled by a long-range AGC system to optimize BER (bit-error-rate) performance and signal acquisition. This means that the level at which each CM (cable modem) transmits is governed by the level received at the CMTS in the headend or hub.

If an attenuator (B) is placed at the input to the CMTS, then all the modems transmitting to that port will be instructed to increase their output levels.

If an attenuator (A) is placed at the output of a Node optical receiver, the only those modems in that node serving area will increase their output level.

If attenuation is added to the upstream path in one feeder, then only the modems attached to the taps in that feeder will increase their output level.

48


This table shows that, as the number of upstream bonded channels and modulation complexity increase, the maximum output level required for a DOCSIS 3.0-compliant cable modem decreases.

(NOTE: the levels required for DOCSIS 2.0 modems are the same as those for TWO bonded channels in a DOCSIS 3.0 modem).

This means that an HFC network intended for high-speed DOCSIS 3.0 service must be designed to accommodate the lowest maximum cable modem output levels.

49


The maximum required transmit level for a DOCSIS 3.0 compliant cable modem is 51 dBmV, if it is transmitting four bonded channels and using the most complex modulation schemes for highest throughput. However, it is not good practice to design the HFC network with this maximum level in mind: any inaccuracies in design and construction, and changes in attenuation due to temperature fluctuations, may require the modem to exceed this level, and this can lead to failure of the modem to register when it is first installed. Therefore, a backoff is recommended. In this example, the maximum design level is assumed to be 47 dBmV.

In calculating the total attenuation of the Drop system, it is necessary to consider the worst case scenario, as represented here. In many homes, there will be an RF splitter to allow connection of several TVs, Set-TopBoxes, and the Cable Modem. Also, the maximum drop-cable length should be assumed to be 50m (150 ft.). Thus, the RF signal level arriving at a tap port will be about 38 dBmV.

50


This slide shows an example of a portion of the feeder plant, designed for a downstream upper frequency limit of 1000 MHz using 0.625 inch coaxial cable. If no constraints are employed during the upstream system design, and if standard (symmetrical) taps are installed, it is possible to find that the modem transmission levels will cover a significant range, and the lower levels may result in poor carrier-to-ingress ratios. It will also be seen that the highest levels are above the design level for DOCSIS 3.0 cable modems that was calculated in the previous slide.

This diagram shows the reverse signal level required, at the tap port, to provide a signal of 13 dBmV at the input to the amplifier. The highest level is 42.8 dBmV, and the lowest is 23.5 dBmV. There is therefore a range, or window of reverse levels, of 19.3 dB.

51


Here a 14 dB upstream pad is placed in the in-line equaliser/conditioner to force the modems at the end of he Feeder to transmit at a higher level. The result is a reduction in the window from 19.3 dB (see earlier slide) to 9.5 dB.

However, some of the levels are still too high for DOCSIS 3.0 cable modems.

(Typically,a maximum upstream window of 10 dB is allowed by many system operators). This corrective measure has therefore allowed the designer to meet most requirements. But further improvements are possible.

The 14 dB pad is effectively providing a partial correction for the difference in cable loss at upstream and downstream frequencies.

52


By adding reverse window taps and an upstream pad in the LEQ/RC, the upstream window can be further reduced, to 4.3 dB.

The reverse window taps are asymmetric: they have higher loss at higher frequencies than low. This allows the cable modems to transmit at a lower level. They are installed at the beginning of the feeder only. This solves the problem that was evident in the two previous slides.

53


The answer to this question is Yes, but at this time there is no precisemethod of calculating the increase in level.

The reason is that, at high output levels, amplifiers produce distortions which cannot be computed using the standard methods which apply to CSO (Composite Second Order) and CTB (Composite Triple Beat) products. This will be explained in the following slides.

54


55

Here is a review the structure of the traffic in the coaxial cable network.

Unless instructed otherwise by a customer, Systems Engineers will assume a full-spectrum traffic load for the purpose of analyzing system performance. In most Western Hemisphere countries, this is understood to mean 78 analog video channels between 50 and 550 MHz, and about 75 digital (QAM) signals between 550 and 1000 MHz. In the majority of HFC networks, the power of a digital channel is set at 6 dB below the power of an unmodulated analog video carrier at the same frequency, as shown in this diagram.


56

To simplify the discussion, we will remove the tilt in this and the following diagrams, and focus on the Second- and Third-Order distortion effects.

For analog video signals, most of the power of the modulated signal is concentrated close to the video carrier, and consists of the synchronizing pulses and the lower-frequency luminance components.


57

Because most of the analog video signal power is concentrated close to the carrier, intermodulation between analog signals results in distortion products that also have a fairly narrow bandwidth, and that can be easily seen on a spectrum analyzer and distinguished from noise.

Composite Triple-Beat distortion products lie at frequencies that are at, or close to, the analog video carrier frequencies.

Composite Second-Order distortion products lie at


58

As shown in a previous slide, CSO components are produced by the second harmonics of the analog video carriers, and by intermodulation between any two carriers, f1 and f2, resulting in unwanted signals at f1 + f2 and f1 - f2.

An example of the production of CSO components at frequencies 1.25 MHz above and below an analog video carrier frequency is shown in this slide.

NOTE: many CSO components of the form f1 + f2 will fall above the highest frequency of the network (1 GHz) and therefore will not directly interfere with traffic.


59

High RF signal levels are desirable, because this allows more subscribers to be powered by each RF amplifier. This in turn results in fewer amplifiers, and reduced equipment and maintenance costs to the customer.

However, as RF levels increase, the distortion products also increase. Note that, for every 1 dB increase in signal level, CTB distortion products increase by 2 dB.


60



61

The power of a digital (QAM) signal is spread evenly across the 6 MHz channel, unlike an analog video signal. This means that the products of intermodulation between digital signals do not produce discernible beats: rather, the distortion products are spread across a band of frequencies, and cannot be distinguished from the familiar thermal noise. Therefore the combined effect of second- and third-order distortion produced by digital signals is referred to as Composite Intermodulation noise (CIN).

With low and moderate RF signal levels, the CIN is also at a low level, and does not add appreciably to the thermal noise.


62

As signal level is increased, the CIN increases, and adds to the thermal noise, causing an increase in the total (Composite) noise. The ratio of the signal level to the composite noise level is referred to as the Carrier-to-Composite Noise (CCN) ratio.


63

We are not aware of any simple and reliable mathematical procedure that can predict the amount of CIN that will produced by a particular traffic load at a particular RF level. However, two important observations have been made during testing of a large number of amplifiers and nodes:

1. The amount of CIN produced by a particular traffic load at a particular RF level (and tilt) varies significantly from one amplifier or node to another.

2. After CIN begins to show itself as an increase in the composite noise, any further increase in RF signal level will produce a disproportionate increase in CIN.

The result of these two effects is that, when amplifiers or nodes are operated at high RF levels, there is a danger that a small increase in signal level could cause a dramatic increase in CIN, and a correspondingly large reduction in CCN.


64

This graph summarizes the effects that were shown in the previous slides.

In general, as amplifier output level is increased, the traditional Carrier-to-Noise Ratio will also increase.

However, the Composite Intermodulation Noise (CIN) also increases, so that the composite Carrier-to Noise ratio (CCN) decreases.

The CCN ratio can decrease very rapidly, as amplifier output levels are increased. It is an avalanche effect.


65

This is an Excel spreadsheet which is used to calculate end-of-line noise and distortion performance, based on the characteristics, quantities, and operating levels of the Node and the RF amplifiers.

In this example, a 1310nm optical link connects the Headend to a GainMaker Node, and this node is followed by two GainMaker RF amplifiers. The node and the first amplifier have RF output levels of 52 dBmV (at 1 GHz), and the final amplifier has an RF output level of 56 dBmV (at 1 GHz).

Note that this spreadsheet can calculate the traditional Carrier-to-Thermal-Noise ratio (CTN), and can also estimate the CIN and the CCN by using look-up tables. The total network CCN is approximately 49 dB. This is acceptable to most system operators.


66

The spreadsheet can show the variation in CCN (Carrier-to-Composite Noise) ratio as the RF signal level is changed. The zero point in the center of this graph corresponds with the signal levels shown in the previous slide. At this point, the CCN is approximately 49 dB. However, if the RF signal level increases by 2 dB, for any reason, the CCN will degrade to 48 dB. This also is acceptable to most operators.


67

If the RF output levels of all amplifiers in the cascade are increased by 2 dB, the CCN will be approximately 48 dB. This is acceptable to most customers. But if the levels increase by 2 dB for any reason, the CCN will be reduced to 40 dB. This is a 8 dB reduction in CCN for a 2 dB increase in RF signal level.


68



69

Returning to the original question: What increase in RF output level can be tolerated when all analog video signals are replaced by digital (QAM) signals?

Because the QAM signals are at a level of -6dB, relative to analog peak envelope power, an all digital traffic load results in a lower overall RF power in the amplifier. Therefore it is reasonable to suppose that the output levels can be increased.

Limited testing of GainMaker amplifiers has shown that the output level can be increased by approximately 2dB.

NOTE: exhaustive testing of all Cisco amplifiers will begin this year (2012).


70


25Jun12 Customer HFC Training v1.2

Documents

Transcript of 25Jun12 Customer HFC Training v1.2