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Reverse path design for DOCSIS 3.0 and beyond - Part 1: The long loop ALC

Sun, 11/30/2008 - 7:45pm
Lamar West, system engineering consultant for Cisco Systems Inc.

One of three related articles. The other two are:
Part 2: Optical Link Dynamic Range and Part 3: Link Budget and System Set-Up

The demands of DOCSIS 3.0 and 64-QAM (quadrature amplitude modulation) in the upstream make it necessary to revisit our network design approach. The networks must be approached from an overall perspective that considers cable modem termination system (CMTS), HFC network and subscriber devices (modems and set-top boxes) as part of an overall unified system design.

Those who have been in the business for several years know the importance of understanding and carefully optimizing the overall system (headend to set-top box) for the downstream delivery of services. All of the downstream characterization techniques – composite triple beat (CTB), composite second order (CSO), carrier-to-noise ratio (CNR), etc. – have been based on a thorough understanding of the requirements of the services to be carried. It is equally important to develop a system-level understanding of the upstream network in order to optimize the design for voice, video and data traffic.

One of the most misunderstood aspects of the DOCSIS network is the upstream long loop automatic level control (ALC). Conventional HFC plant designs have utilized automatic gain control (AGC)/ALC networks that operate on individual components (amplifiers, nodes, etc.). However, in the downstream there is no overall, network-wide AGC/ALC. In the DOCSIS network, there is an upstream ALC that operates over the entire network. This article will explain techniques that will allow the operator to understand this long loop ALC.

OVERALL NETWORK
Downstream balance and alignment is typically done one component at a time in a conventional system. The analog video modulators can be individually adjusted for depth of modulation, audio deviation and RF output power. QAM modulators can also be aligned for modulation order and RF output power. The headend-combining network can be set up for a given per-carrier output power. The downstream optical transmitter can be adjusted for a particular optical modulation index (OMI) by setting its input power and activating the modulator AGC, if present. The node can be padded and equalized for a given output level and tilt. The RF plant (amps and taps) can be designed and set up for downstream RF level and tilt. Set-top boxes are designed to operate over a sufficiently large input dynamic range to accommodate the various levels and tilts experienced at the end of a drop.

Set-up of the upstream direction in a DOCSIS network requires a different approach. AGC/ALC circuits in individual components in the downstream are intended to control the levels in that particular device only. However, the long loop ALC in the upstream creates interactions that affect the overall network from subscriber device through the CMTS. Let’s consider a simple example.

DOCSIS upstream block diagram
Figure 1: Simplified DOCSIS upstream block diagram.

In Figure 1, the source of upstream RF signal is the cable modem in the subscriber’s home. The RF/coaxial plant is represented by the three-directional couplers (taps). The node consists of RF input loss and a reverse optical transmitter. An optical receiver is located at the hub or headend. An optical attenuator is shown at the input to the optical receiver, and an RF attenuator (pad) is shown at the output of the optical receiver. In the hub or headend, there is a splitter/combiner network that routes RF signals from various optical receivers to the various CMTSs. This splitter/combiner network may also combine RF signals from multiple optical receivers before routing them to a CMTS. An attenuator (pad) is located at the CMTS upstream RF input. Finally, the CMTS receives and demodulates the upstream RF signals. Not shown in the block diagram is the downstream signal path from CMTS to the cable modem.

The long loop ALC is the method by which the CMTS sets the operating levels throughout the entire network. The CMTS is configured for a particular upstream RF carrier amplitude – called the commanded receive level – at the CMTS upstream RF input (typically 0 dBmV). The CMTS signals downstream to the cable modem in order to adjust the cable modem’s upstream transmit RF carrier amplitude. The upstream RF carrier amplitude is adjusted up or down until the desired amplitude at the CMTS upstream RF input is achieved. Since this RF carrier amplitude adjustment affects upstream RF carrier levels throughout the entire system, it is referred to as the long loop ALC. If designed properly from a system standpoint, this long loop ALC can maintain proper operating levels throughout the entire upstream plant.

However, if not designed properly, the long loop ALC can produce unexpected results. Consider a system that is in operation, as shown in Figure 2.

Long loop ALC
Figure 2: Long loop ALC example.

In this example, the CMTS is configured for an upstream input carrier level of 0 dBmV. The CMTS has adjusted the cable modem transmit level in order to achieve 0 dBmV at the CMTS input. In this example, the cable modem transmit power is 26 dBmV. The level at the node optical transmitter (after the input pad) is -10 dBmV due to the losses in the RF plant, input losses of the node and the reverse transmitter input pad selection.

Now imagine that the trouble center begins getting trouble calls from the subscribers served by this system. In this example, the likely cause is the low RF level (-10 dBmV) at the optical transmitter input. This low level is resulting in a poor CNR at the optical receiver output, and consequently at the CMTS input.

If the tech polls the CMTS, it will likely report the poor CNR. Let’s assume the value of the CNR as reported by the CMTS is 12 dB. This well-meaning but inexperienced tech might assume that the CMTS CNR is due to inadequate upstream RF signal level at the CMTS input. To fix this problem, he erroneously reduces the CMTS input pad by changing it from 12 dB to 6 dB. His expectation is that the 6-dB-higher RF level at the CMTS input will result in a new CNR of 18 dB. Unfortunately, his change will actually make things worse!

The new pad value will result in the RF input level at the CMTS upstream input increasing from 0 dBmV to 6 dBmV initially. However, the CMTS is set up for an upstream RF input level of 0 dBmV. The long loop ALC will try to correct this. The higher input level will cause the CMTS to send a downstream command to the cable modem indicating that the received RF input level is 6 dB too high. The cable modem will reduce its transmit level by 6 dB to a new level of 20 dBmV. The resulting new level at the node optical transmitter (after the input pad) will be -16 dBmV. This new lower level will reduce the CNR at the optical receiver output (and CMTS input) to 6 dB. The tech’s attempt to improve things has made the problem worse! The new levels are shown in Figure 3.

The “fix”
Figure 3: The “fix” makes things worse!

This is an unexpected consequence of the long loop ALC. Changing things in one part of the system may result in undesirable changes in another part of the system.

WHAT CONTROLS WHAT?
As it turns out, the long loop ALC makes things work in a way that is opposite of what you might expect. In a conventional downstream system, changing a pad will affect the RF level of the signals after that pad (in the direction of signal flow). However, this is not the case for the upstream with the long loop ALC in operation. In the upstream, changing a pad will affect the RF level of the signals before that pad (in the opposite direction of signal flow). A graphical representation of this is shown in Figure 4.

The long loop ALC is designed to maintain a fixed level at the CMTS upstream RF input. If the level at this point is wrong, the CMTS will communicate downstream and command the cable modem to increase or decrease its upstream RF transmit level to correct the error.

Consider what will happen if the value of the attenuator at the reverse optical transmitter is increased. This will initially reduce the RF levels through the optical link, the splitter/combiner network in the hub/headend, and the CMTS upstream input. The CMTS will react to this decreased level by telling the cable modem to increase its upstream RF transmit power. The power in the coaxial plant will increase until the original level at the CMTS input port is achieved. The net result of increasing the optical transmitter attenuator will not be a decrease in the RF levels further upstream, but rather an increase in RF levels at the cable modem output and in the RF plant!

What controls what
Figure 4: What controls what.

As shown in Figure 4, changes in the upstream plant will result in changes in the opposite direction from the signal flow. Consequently, this effect must be considered in the design of the loss-and- gain structure throughout the network. Improper attenuator values may cause the long loop ALC to force the upstream signals down into the noise, or raise them until severe clipping occurs.

COMMON MISCONCEPTIONS

1. The input attenuator in the upstream optical transmitter must be chosen properly to avoid severe clipping.

As it turns out, this attenuator does not affect the performance of the optical link at all. As shown previously, this attenuator actually affects the levels in the coaxial plant. The operating levels (OMI) in the optical link are actually set by the loss/gain structure between the optical receiver and the CMTS.

2. The only way to improve the carrier-to-ingress ratio is to roll a truck to every drop.

The easiest way to improve the carrier-to-ingress ratio is to make certain that all upstream transmitters in modems and set-top boxes are operating in the upper end of their transmit power ranges. This is accomplished by proper selection of the optical transmitter input pad, and by proper design and conditioning of the RF plant. The carrier-to-ingress ratio is set in the RF plant before signals get to the node. Forcing transmitters into the upper end of their power ranges maximizes carrier-to-ingress ratio at the point where most ingress gets into the plant (i.e., at the subscriber drop).

3. Changing the attenuators at the reverse optical receiver in the hub or headend will change the reverse carrier level at the CMTS input.

As described earlier, these attenuators (either the optical input attenuator or the RF output attenuator) affect the upstream RF levels in the coaxial plant, as well as the drive level (OMI) of the reverse optical transmitter. Improper selection of these attenuators and the losses between the reverse optical receiver and the CMTS might cause the laser in the reverse optical transmitter to clip severely, or might force upstream signals down into the optical link relative intensity noise (RIN) and optical receiver noise. The place to make sure that reverse optical link is optimized for best performance is between the reverse optical receiver and the CMTS.

CONCLUSION
The new challenges presented by DOCSIS 3.0 and upstream 64-QAM require a comprehensive approach to the overall system design. The long loop ALC results in interaction between all parts of the network. Therefore, all parts must be considered in the design and operation of the network.

In this and future articles, we will present a clear description of how the overall system operates. We will also suggest a straightforward way to design, implement and operate the network. The best strategy for the system operator is to partner with someone that has a clear understanding of all parts of the network, including CMTS, optics, coaxial RF plant and subscriber devices.

E-mail: lamar.west@sciatl.com

The two companion articles can be found in CED's Web Extra section: 
Part 2: Optical Link Dynamic Range
and Part 3: Link Budget and System Set-Up

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