Chapter 9: Upstream Cable Access Modules (UCAMs)
STANDARD Revision 1.0 C4® CMTS Release 8.3 User Guide
© 2016 ARRIS Enterprises LLC. All Rights Reserved. 312
time. Because of this if a single modulation profile is used, then this modulation profile must be able to handle the worst
case noise that is expected on the upstream channel and still achieve a reasonable level of performance.
Noise and SNR versus Modulation Symbol Rate
Unfortunately, different types of noise are typically seen on an upstream channel, and each type has a different effect on
the upstream channel. There is the Average White Gaussian Noise (AWGN) that is always present and is typically referred
to as the noise floor. There may be impulse and ingress noise, both of which may cause the noise floor to spike.
Note: Impulse noise is a spike in the time domain and ingress noise results in a spike in the frequency domain.
There are various techniques that can be used to reduce the effects of each of these types of noise. For example, having all
the cable modems transmit at a higher power level (assuming there is enough power headroom) gives a better Signal to
Noise Ratio (SNR) because the modem’s bursts are at a higher power level while the AWGN remains at about the same
level. Forward Error Correction (FEC), Ingress Cancellation Block (ICB), and interleaving can be used to correct ingress and
impulse noise.
Noise affects the SNR. The SNR is the primary indicator of what modulation rate can be used on the upstream channel. If
one assumes that an upstream channel has no ingress or impulse noise, then theoretically the following modulation rates
would work as long as the SNR of the upstream channel is higher than the stated threshold shown in the table below.
The table below is based on theory. In the real world the thresholds shown here would be too low. The amount of margin
that needs to be added is dependant upon the types of noise present in the plant and how that noise varies over time.
Even in a clean plant, we would recommend a margin of at least 4 dB. For a plant with a high noise level, the margin should
be increased. Additionally, the CMTS is effective in eliminating AWGN, less effective for impulse noise, and still less
effective for ingress noise. Therefore, the amount of SNR margin may also be varied depending upon the type of noise that
is present in the system.
Table 54. Minimum SNR Thresholds under Lab Conditions
If the Modulation Rate Is…
Then the SNR Threshold Must Be at Least:
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