CATV service in FTTH
CATV service is one of the oldest services, but it is still offered by many telecommunications operators. Let’s meet!
Introduction
Video service can be CATV or IPTV. IPTV is short for interactive television. IPTV is a modern service available for the last ten years. It offers great possibilities compared to classic cable television – CATV, such as video demand, audio demand, chat up, etc. It can be distributed using multicast or unicast traffic. Users must have appropriate IPTV or hybrid boxes. But let’s get back to the main topic – CATV.
CATV is short for cable television, it started to develop more than 50 years ago. Even if it belongs to the old services, it is still represented in the offer of most telecommunications operators. Cable television can be analog – AM (Amplitude Modulation) or digital – DVB-C (the largest number of countries in the world use this standard).
AM CATV is less and less in use due to a number of disadvantages – small number of channels, large number of possible interferences, etc. The only reason why it still exists is the poverty of the citizens, who still use old TV sets.
DVB-C is short for Digital Video Broadcast – Cable. This is the most commonly used standard. The most important DVB-C features are the following:
- Input Interface – Single Transport Stream,
- FEC – Reed Solomon,
- Modulation – Single Carrier QAM,
- Modulation Schemes – 16- to 256-QAM,
- Interleaving – Bit-Interleaving, etc.
In the following figure, we can see the block scheme of a DVB-C transmission system.
IPTV and analog and digital CATV services can be distributed over the same access network. And that is not a rare case. IPTV and digital CATV are especially often used. In general, the future is in the IPTV service.
Optical transmitter and EDFA
TodistributeIPTV service via FTTH PON, an OLT is sufficient in headend or CO. However, if we want to distribute CATV via the FTTH PON system, we need an optical transmitter and an optical amplifier – EDFA in headend or CO. A special wavelength – 1550 nm is used for CATV service. In the next figure, we can see the whole system.
The complete CATV service is fed into the optical transmitter. There are two types of optical transmitters – with internal and external modulation. Optical transmitters with external modulation are used in PON systems.
The input signal is an RF signal, which is supplied via a coaxial cable (RG6 or RG59), and an F connector, in the frequency range of 85 to 1000 MHz. Typically, the required input RF level is around 75 dBuV.
At the output we have an optical signal, most often on one or two optical output connectors. The optical power is from 5 to 12 dBm. Only SC/APC are always used as optical connectors due to reflection (it should be as low as possible, that’s why APC is used, less than – 60 dB). SBS is one of the most important parameters for analog cable TV and usually ranges from 13 to 20 dBm.
There are a number of different optical amplifiers. However, considering the price and performance, the best solution is EDFA (Erbium Doped Fiber Amplifier). EDFA uses doped fiber with trivalent erbium ions(Er3+), which are excited by laser pumps (980 nm and 1480 nm). In this way, a large output of optical power is obtained. This amplifier works only with an optical signal, in the range of wavelengths around 1550 nm – L and C range.
The optical signal from the optical transmitter is fed into the EDFA. The most frequently required optical power ranges from -10 to +10 dBm. The most optimal optical power is around 0 dBm. EDFA can have one or two optical inputs. SC/APC or LC/APC optical connectors are always used. If there are two optical inputs, it is an optical switch. That optical switch has the task of automatically switching to another input if there is no optical signal at one input.
EDFA can have different amplification, and thus it affects the output optical power on all output optical connectors. Output optical connectors are always SC/APC or LC/APC. The maximum output optical power is about 22 – 23 dBm. The maximum number of output optical connectors is 128. EDFAs with 32 or 64 output optical connectors are most often used.
The connection between the EDFA and the OLT requires a WDM multiplexer/demultiplexer. This element can be integrated into EDFA (built-in WDM). Optical connectors for this connection can be SC/APC, LC/APC, SC/PC or LC/PC. In this case, on the output optical connectors of the EDFA, we receive unified services – voice, data (1490 nm and 1310 nm), and video (1550 nm). The next figure shows an example of EDFA.
ONT and user’s installation
In order to enable CATV service at the user’s premises, a suitable ONT device is required. The ONT must specify the RF port. Examples of Huawei ONT with RF port are HG8012H, HG8247H, HG8247, EG8247H5, EG8143A5, etc.
The output RF level is usually 75 to 80 dBuV. That is enough RF level for 2 to 3 TV sets. The user installation consists of RG6 coaxial cables, F connectors, and appropriate RF splitters. The RF output of the ONT is routed via a coaxial RG6 cable to the RF distributor and finally to the TV set or DVB-C box.
If you have any questions, please feel free to contact: [email protected].
