Cable TV Return-Path Transmission Characteristics
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Cable TV Return-Path Transmission Characteristics
The standard frequency plan for upstream transmission is shown in Table 1. The "T-channel" plan originated in about 1960 to identify the video channels that could be converted in a block to the FCC standard channel assignment T7 to T13. The current return-path or upstream transmission band is from 5–42 MHz in the United States (5–65 MHz in Europe). Recently, the same cable operators have proposed that the return-path band be located above the downstream channels in the 900 MHz to 1 GHz band. Although this upstream band is almost three times as large as the 5–42 MHz band, coaxial cable losses are almost 10 times larger than at the 5–42 MHz band and exhibit strong frequency dependence (see Table 2). On the other hand, the upstream noise in these RF frequencies is significantly less than in the 5–42 MHz band. To date, cable TV operators, because of the cost to upgrade their networks to 1 GHz, have mostly ignored this proposal.
Table 1 Standard Upstream Cable TV Frequency Plans in the U.S.
|
Channel Designation |
Visual Carrier Frequency (MHz) |
||
|
EIA |
STD |
HRC |
IRC |
|
T7* |
7.0000 |
|
|
|
T8* |
13.0000 |
|
|
|
T9* |
19.0000 |
|
|
|
T10* |
25.0000 |
|
|
|
T11* |
31.0000 |
|
|
|
T12* |
37.0000 |
|
|
|
T13* |
43.0000 |
|
|
|
2 |
55.2500 |
54.0027 |
55.2625 |
|
3 |
61.2500 |
60.0030 |
61.2625 |
|
4 |
67. 2500 |
66.0033 |
67.2625 |
|
5 |
77. 2500 |
N/A |
|
|
6 |
83. 2500 |
N/A |
|
|
5 |
N/A |
78.0039 |
79.2625 |
|
6 |
N/A |
84.0042 |
85.2625 |
Table 2 Maximum Loss for Drop Cable (dB/100 ft at 68F) with Four Different Cable Diameters as a Function of Frequency. To obtain loss in dB/100 m, multiply by 3.281.
|
Frequency (MHz) |
59 Series Foam |
6 Series Foam |
7 Series Foam |
11 Series Foam |
|
5 |
0.86 |
0.58 |
0.47 |
0.38 |
|
30 |
1.51 |
1.18 |
0.92 |
0.71 |
|
40 |
1.74 |
1.37 |
1.06 |
0.82 |
|
50 |
1.95 |
1.53 |
1.19 |
0.92 |
|
110 |
2.82 |
2.24 |
1.73 |
1.36 |
|
174 |
3.47 |
2.75 |
2.14 |
1.72 |
|
220 |
3.88 |
3.11 |
2.41 |
1.96 |
|
300 |
4.45 |
3.55 |
2.82 |
2.25 |
|
350 |
4.80 |
3.85 |
3.05 |
2.42 |
|
400 |
5.10 |
4.15 |
3.27 |
2.60 |
|
450 |
5.40 |
4.40 |
3.46 |
2.75 |
|
550 |
5.95 |
4.90 |
3.85 |
3.04 |
|
600 |
6.20 |
5.10 |
4.05 |
3.18 |
|
750 |
6.97 |
5.65 |
4.57 |
3.65 |
|
865 |
7.52 |
6.10 |
4.93 |
3.98 |
|
1000 |
8.12 |
6.55 |
5.32 |
4.35 |
Return-Path Noise Sources
The noise sources that impair the transmitted upstream signals in cable TV systems include ingress noise, common-path distortion, laser transmitter noise, and optical receiver noise. Ingress noise is the most important and dominant noise source in the return-path portion of the HFC network, and can be divided into three general types:
Narrowband shortwave signals, primarily from radio and radar stations, that are transmitted terrestrially and coupled to the return-path band at the subscriber's home or in the cable TV distribution plant.
Burst noise, which is generated by various manmade and naturally occurring sources, has time duration longer than the (symbol rate)-1.
Impulse noise, which is similar to burst noise, but has time duration shorter than (symbol rate)-1 such that the receiver impulse response is effectively being measured.
The propagation of narrowband interference signals depends on the atmospheric conditions as well as the 10.7-year solar cycle. Increased solar sunspot activity has been correlated with increases in the maximum usable frequency because of ionization in the upper layers of the atmosphere. There are various methods for reducing ingress in the return-path band at the subscriber's location, which are discussed later in this article. Burst and impulse noise are generated from various manmade sources such as electric motors and power-switching devices. Although these sources produce burst/impulse noise events in the 60 Hz to 2 MHz portion of the spectrum, their harmonics show up in the 5–42 MHz upstream frequency band. Naturally occurring burst/impulse noise events include lightning, atmospherics, and electrostatic discharge, which typically extend from 2 kHz up to 100 MHz. Figure 1 shows a typical return-path spectrum as measured at a cable head end with the average signal level in dBmV. Notice the presence of various ingress peaks, particularly below 10 MHz. As I will show later in this article, for robust data transmission using quadrature-phase-shift-keying (QPSK) modulation, a 16–20 dB margin above the noise level is needed, as indicated by the horizontal solid line in Figure 1.
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Figure 1 Typical return-path spectrum (5–30 MHz) as measured at the cable TV head end, with averaged signal levels (in dBmV). Notice the pronounced ingress peaks, particularly below 10 MHz. After Ref. [2] (" 1995 IEEE).
The time-varying ingress noise level also depends on the number of subscribers connected to the cable TV head end. Figure 2 shows the average ingress noise level as a function of the number of subscribers for T7, T8, T9, and T10 upstream channel frequencies (refer to Table 1), which is based on CableLabs field measurements.1 The worst-case ingress levels can exceed +10 dBmV within a 100 kHz bandwidth. Two distinct trends emerge from Figure 2. First, the ingress noise levels are generally higher in the low-frequency region of the upstream band. Second, ingress noise levels increase for cable TV distribution plants with larger numbers of subscribers per fiber node. The solid curves in Figure 2 can be approximated by A·log (N) – B, where N is the number of subscribers, A = 9, and B = 28, 33, 37, and 40 for T7 (top curve), T8, T9, and T10 channels, respectively.3
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Figure 2 Average ingress level (in dBmV) as a function of the number of subscribers for T7, T8, T9, and T10 upstream channel frequencies. After Ref. [3].
Another important noise source is common-path distortion, originating from various nonlinearities in the cable TV plant, such as oxidized connectors and bad amplifiers. The common-path distortion appears as discrete noise peaks in the return-path spectrum spaced by 6 MHz (8 MHz in PAL systems). This distortion can be contained in properly maintained cable TV plants. Other types of noise sources are the upstream laser transmitter noise and optical receiver noise.
The observed increase in the ingress noise levels as the number of homes passed is increased is due to the so-called noise-funneling effect. This effect is based on the assumption that the unwanted noise signals are located at the subscriber's location with time-dependent amplitude. Since the unwanted signals are uncorrelated (Gaussian noise, for example), the signals sum noncoherently. If the signals from each home passed were equal, the noise-funneling factor would be 10·log10 (N), where N is the number of homes passed. It turns out that according to the empirically derived expression based on Figure 2, the noise-funneling factor is slightly smaller than you obtain from a noncoherent noise power sum.
The time-varying narrowband interferers in the return-path cable systems limit channel availability for high-speed data transmission application. To quantify this parameter, automated spectral measurements of upstream ingress noise were taken every 1–5 minutes over an extended period of time (48–72 hours). The channel availability is the percentage of the time in which a channel of a specified bandwidth is available for transmission for a given modulation format such as QPSK or 16-QAM. For example, to transmit 256 Kbps, using QPSK modulation with FEC over a 192 kHz channel bandwidth, the required carrier-to-noise-plus-interference C/(N+I) must be at least 15.8 dB at BER of 10-7. Thus, if the C/N or CNR (in the presence of Gaussian noise only) is maintained at 21 dB, a single narrowband interferer as high as 10 dB below the equivalent unmodulated carrier (corresponding to C/I = 10 dB) can be tolerated.
In general, the upstream channel availability depends primarily on the number of homes connected to the fiber node, the channel bandwidth, and selected upstream frequency. Based on Figure 2, it's clear that by decreasing the number of homes passed in the fiber node, the noise-funneling effect is reduced. Also, selecting an upstream frequency, say above 15 MHz, is likely to improve the upstream channel availability based on Figure 1. Choosing a relatively wide upstream channel band, say 1 MHz or more, is also likely to reduce the channel availability. For example, Figure 3 demonstrates the availability of the 1 MHz channel in the 15–30 MHz frequency range of the upstream spectrum for a single fiber node with 1,500 homes passed.2 Notice the reduction in channel availability as the C/I is increased from 10 dB to 24 dB.
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Figure 3 Percentage of 1 MHz channel availability in the 15–30 MHz frequency range for a 1500 home node. After Ref. [2] (" 1995 IEEE).
Return-Path Noise Filtering
The appearance of ingress noise and other narrowband interference on the return-path cable network has introduced a new challenge for cable TV operators. Many companies have been working on developing methods to combat the effect of the ingress noise from reaching the cable TV head end. These methods include active or passive filtering techniques anywhere along the return-path channel from the subscriber to the fiber node.
One method consists of using a blocking filter, nominally between 15–40 MHz, between an in-home splitter and a coaxial termination unit (CTU) at the side of the home. The signals from the subscriber home terminals, such as CM and STB, can be transmitted from inside the home, but a low-pass filter prevents any signals originating from the home in the 15–42 MHz frequency band to enter the return-path cable network. Upstream-transmitted signals in the 15–42 MHz band can be added in the CTU after the blocking filter. Although this method reduces the amount of ingress noise coming from each home, it still allows the relatively high-ingress spectral region of the return path (5–15 MHz) to enter the cable network, and the method may be costly to implement. Another proposed method is to use a low-pass filter at the side of the house such that the filter is off except when the subscriber is transmitting data upstream.
Another method is using a bandpass filter with a switch at the side of the subscriber home. The upstream ingress noise from the subscriber home is filtered when the subscriber is not transmitting data. The disadvantage of this method is the large time delays from the switches when there are many subscribers connected to a fiber node ("H1200) who are trying to transmit data upstream.
In addition to the use of filters and similar approaches, upstream ingress and other impairments can largely be controlled by the use of quality materials, proper subscriber-drop installation practices, good network-maintenance programs, and aggressive signal-leakage monitoring and repair.4 This latter item has the benefit of reduced ingress where a leak exists and outside signals can also enter the network. To make all of these efforts successful requires effective training and quality control programs.