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How to configure encoder for satellite TV signal transmission.

Encoder Configuration

Encoding  is an essential part for satellite communication. For that reason broadcast engineers need to know how to configure the encoder properly. There are a number of encoder brand in the market. Most of them have the almost same parameter to configure. So the engineers have to put some value of the parameter to the encoder to encode signal. It depends on the demand and criteria of the TV channel owner and the signal. Some of the most important and common parameter (with dummy value) are as follows.   



1) Program name: XYZ TV
2) Service ID: 101
3) Video Format: PAL/ NTSC/ SECAM
4) Video Resulation: 720*576, 1920*1080
5) Video Bit Rate Mod: CBR/ VBR
6) Video Bit Rate (kbps): 4000
7) Audio Bit Rate (kbps): 192
8) Brightness: 50
9) Contrast: 50
10) Saturation: 50
11) Hue: 50
12) Video Encode Mode: MPEG-2
13) Audio Encode Mode: MPEG-2

That parameters define the characteristic of the up-link / transmitting signal. After encoding the signal, it goes into the modulator. Then the signal is modulated for the long distance transmission. After that, the modulated signal is amplified by SSPA or HPA to increase the amplitude of the signals. Then finally it transmitted through the web guide to the satellite.  

What is the difference between symbol rate and bit rate?




A symbol can carry multiple bits (and usually does). So BPSK/BFSK has a symbol of 2 states and the bit rate is the same as the symbol. QPSK has 4 symbol states and each symbol can carry 2 bits. 8PSK has eight symbol states and each symbol can carry 3 bits. 16QAM has 16 symbol states and each symbol can carry 4 bits. I'm sure you get the drift. 

Terminology: EIRP, G/T, Input Back-Off, Output Back-Off

EIRP: Used to indicate the power transmitted from an antenna. EIRP=Power +Antenna Gain, Both EIRP and Power is expressed in dBW and Gain in dBi.

G/T: It is the figure of merit for an earth station and is expressed as dB/K (dB per K).
G/T= Antenna Gain-10*Log (System Noise Temperature) The higher the better-G/T can be raised by using a higher gain antenna or a lower temperature LNA. The earth station G/T determines the received carrier to noise ration. Increasing station G/T will increase the C/N of the received carrier.






Input Back-Off (IPBO): The level of a signal at the input of an amplifier relative to that level at the input that would result in the maximum possible output level. For example, if an input level of -20dBm causes max output and the actual input level is -25dBm, the IPBO is 5dB. Both IPBO and OPBO are commonly used to determine the operating levels in a satellite transponder TWTA.


Output Back-Off (OPBO) The level of a signal at the output of an amplifier relative to the maximum possible output level. For example, if a maximum output level is +40dBm and the measured output level is +34dBm, the OPBO is 6dB. 

Terminology: Bandwidth, LNA Noise Temperature, Saturation Flux Density and Polarization.

Bandwidth
Bandwidth is another fundamental antenna parameter. Bandwidth describes the range of frequencies over which the antenna can properly radiate or receive energy. Often, the desired bandwidth is one of the determining parameters used to decide upon an antenna. For instance, many antenna types have very narrow bandwidths and cannot be used for wideband operation.

Bandwidth is typically quoted in terms of VSWR (Voltage Standing Wave Ratio, and sounds very complicated. But it is simply a measure of how much power is reflected from an antenna. ) . For instance, an antenna may be described as operating at 100-400 MHz with a VSWR<1.5. This statement implies that the reflection coefficient is less than 0.2 across the quoted frequency range. Hence, of the power delivered to the antenna, only 4% of the power is reflected back to the transmitter. Alternatively, the return lossS11=20*log10(0.2)=-13.98 dB.

Note that the above does not imply that 96% of the power delivered to the antenna is transmitted in the form of EM radiation; losses must still be taken into account. 


LNA Noise Temperature: It is the measure of the amount of noise generated by the LNA. It is measured in Kelvin-the lower the better. Typical performance can be achieved for 30K to 70K. LNA noise performance is sometimes specified in terms of Noise Figure which is measured in dB.

NF=10*Log (1+T/290), NF in dB, T in Kelvins. 


Saturation Flux Density: Flux density is a measure of signal strength at a point in space and is measured in Watts/meter2 or dBW/meter2. It is usually applied to signals received at a satellite.


Polarisation: The polarisation of an RF wave in space is defined by the orientation of the electric vector (E) of the wave. The polarisation of an RF wave is used in satellite systems to separate two signals at the same frequency and allows frequency reuse in satellite systems. There are circular and linear polarizations and under each there are horizontal and vertical.




Frequency Band of Satellite communication

Due to lower frequencies, L-Band is easiest to implement for marine satellite stabilised systems. There is not much L-Band bandwidth available. The higher you go in frequency, the more bandwidth is available, but the equipment needs to be more sophisticated. 

L-Band (1-2 GHz)
Being a relatively low frequency, L-band is easier to process, requiring less sophisticated and less expensive RF equipment, and due to a wider beam width, the pointing accuracy of the antenna does not have to be as accurate as the higher bands.

L-Band is also used for low earth orbit satellites, military satellites, and terrestrial wireless connections like GSM mobile phones. It is also used as an intermediate frequency for satellite TV where the Ku or Ka band signals are down-converted to L-Band at the antenna LNB, to make it easier to transport from the antenna to the below deck, or indoor equipment. 


C-Band (4-8 GHz)
Satellite C-band usually transmits around 6 GHz and receives around 4 GHz. It uses large (2.4- 3.7 meter) antennas. These are the large white domes that you see on top of the cruise ships and commercial vessels.

C-band is typically used by large ships that traverse the oceans on a regular basis and require uninterrupted, dedicated, always on connectivity as they move from region to region.  The shipping lines usually lease segment of satellite bandwidth that is provided to the ships on a full time basis, providing connections to the Internet, the public telephone networks, and data back-hauls to their head office.

C-band is also used for terrestrial microwave links,  which can present a problem when vessels come into port and interfere with critical terrestrial links. This has resulted in serious restrictions within 300Km of the coast, requiring terminals to be turned off when coming close to land.


Ku-Band (12-18 GHz)
Ku-Band is most commonly used for satellite TV and is used for most VSAT systems on yachts and ships today. There is much more bandwidth available in Ku -Band and it is therefore less expensive that C or L-band. 

The main disadvantage of Ku-Band is rain fade. The wavelength of rain drops coincides with the wavelength of Ku-Band causing the signal to be attenuated during rain showers. This can be overcome by transmitting extra power but this of course comes with a cost as well. 

The pointing accuracy of the antennas need to be much tighter than L-Band Inmarsat terminals, due to narrower beam widths, and consequently the terminals need to be more precise and more expensive.


Ka-Band (26.5-40 GHz)
Ka-Band is an extremely high frequency requiring great pointing accuracy and sophisticated RF equipment. Like Ku-band it is susceptible to rain fade. It is commonly used for high definition satellite TV.  It is also used today for terrestrial VSAT services from companies like Hughes Networks.

Ka-Band bandwidth is plentiful and once implemented should be quite inexpensive compared to Ku-Band .  


Figure: Satellite Frequency Band of Operation