Modulation scheme

[Mobileace]

Anyone into the transmission of DVB-T? Anyone know which is the best modulation scheme to use for transmission of digital channel? Is it QPSK or 16QAM?

[drmpeg]

The best scheme is a tradeoff between bitrate and signal to noise ratio required by the
receiver. Here's some numbers from this paper:

http://www.itu.int/ITU-D/tech/digital-broadcasting/kiev/Presentations/brugger/Kiew_rev1_R_Brugger.pdf


mode QPSK-2/3 16QAM-2/3 64QAM-2/3
s/n ratio 11 dB 17dB 23dB
bitrate 6.6 Mbps 13.3 Mbps 19.9 Mbp


So it depends on your application. Bitrate is determined by how many channels of SDTV
or HDTV you want to have in your multiplex.

Signal to noise ratio translates to how much transmit power (or how many network
nodes) you need to cover your intended audience. It takes 16 times the transmit
power to have the same coverage with 64QAM versus QPSK.

Ron

[Mobileace]

ohhh.i get what you mean... Thanks thanks.. I got another question for you, What is the bitrate of the whole DVB channel? How can we find out? Is there any formula to work it out? Who will determine the bitrate? Is is depending on the modulation scheme or coding of the video or wat??

Sorry for my little knowledge!It is really an blessing to found experts like you...keke...

[drmpeg]

The bitrate of the channel is a function of the bandwidth of the channel (usually 6, 7 or
8 MHz), the modulation format (QPSK, 16QAM or 64QAM), the Viterbi convolutional coding
rate (1/2, 2/3, 3/4, 5/6 or 7/8) and the guard interval. Here's a table for 6 MHz channels
from this website:

http://web-star.com/hdtv/DVB-T20Mexico.html

Modulation Code rate Guard interval

1/4 1/8 1/16 1/32
½ 3,732 4,147 4,391 4,524
2/3 4,976 5,529 5,855 6,032
QPSK ¾ 5,599 6,221 6,587 6,786
5/6 6,221 6,912 7,318 7,540
7/8 6,532 7,257 7,684 7,917

½ 7,465 8,294 8,782 9,048
2/3 9,953 11,059 11,709 12,064
16-QAM ¾ 11,197 12,441 13,173 13,572
5/6 12,441 13,824 14,637 15,080
7/8 13,063 14,515 15,369 15,834

½ 11,197 12,441 13,173 13,572
2/3 14,929 16,588 17,564 18,096
64-QAM ¾ 16,796 18,662 19,760 20,358
5/6 18,662 20,735 21,955 22,620
7/8 19,595 21,772 23,053 23,751

The actual equation is in DVB-T standard: EN 300 744. I used to have a copy, but I
don't know what I did with it. I'll see if I can get another.

The bitrates scale linearly with channel bandwidth. For instance, 31.67 Mbps is possible
with 64QAM , 7/8 code rate and 1/32 guard interval in an 8 MHz channel.

The guard interval doesn't affect S/N ratio, but longer guard intervals allow for longer
duration multipath echoes (at the expense of channel bitrate).

Ron

[drmpeg]

You can download DVB-T specification EN 300 744 from:

http://www.com-tech.it/downloads/en_300744.pdf

It contains bitrate tables for 6, 7 and 8 MHz channels. The equation for useful bitrate is:

useful bitrate = (clock rate) * (6048/8192) * (code rate) * (modulation bits/symbol) * (188/204) * (8192 / (8192 + (8192 * guard interval)))

The clock rate is 64/7 MHz for 8 MHz channels, 8 MHz for 7 MHz channels and 48/7 MHz
for 6 MHz channels.

6048/8192 is the number of useful carriers.

code rate is the Viterbi code rate, 1/2, 2/3, 3/4, 5/6 or 7/8

Modulation bits/symbol is 2 for QPSK, 4 for 16QAM and 6 for 64QAM.

188/204 is the Reed-Solomon block code for 188 byte MPEG-2 Transport Stream packets

For example, 64QAM, 7/8 code rate, 1/32 guard interval in a 7 MHz channel

8000000 * (6048/8192) * (7/8) * 6 * (188/204) * (8192 / 8448) = 27.71 Mbps

Ron

[Mobileace]

Thanks alot for the quick and prompt reply with the relevant answers i want, now i am getting clearer on these modulation schemes. More question to come.....hehe...

Which bandwidth to choose? 6,7,8 MHz?Why choose them?
When to choose 6MHz , 7MHz or 8MHz?
Why is it that when i choose 8MHz, it changed to 7.6MHz?

Althought it did mention in books, but i still dun get it.

[drmpeg]

The 6, 7 and 8 MHz values are used to match the analog television channel spacing
that DVB-T is replacing. 6 MHz is the channel spacing in the US, with 7 MHz used
in Australia and 8 MHz used in Europe.

If you are going to be transmitting DVB-T at a power level that covers more than your
backyard or RF laboratory, then it's almost certain you will need a license from the
government of the country you will be transmitting in. Usually that license will
dictate what bandwidth you are allowed to use.

In the US, there are many unlicensed frequency bands that are currently being used
for wireless telephones, wireless networks, microwave ovens, etc. The power levels
allowed are very low and you are not offered any protection from interference from
other transmitters. Currently, the 2.4 GHz band is becoming heavily used in the US
and devices are beginning to interfere with each other.

Another option for transmitting DVB-T is amateur radio. As it turns out, I'm a licensed
amateur radio opererator here in the US (my callsign is W6RZ). I'd have to check the
exact rules, but I believe I could transmit DVB-T on any amateur frequency band above
420 MHz with a maximum output power of 1500 Watts. I'd also have to check the
rules for maximum bandwidth, but certainly 6 MHz would be allowed since regular
analog NTSC television is allowed as a transmission mode. The only catch is that
one-way transmissions are not allowed. As an amateur, you can't "broadcast". You
must communicate with another station. Also, you can't make any money from your
amateur radio activities.

Whatever you do, don't fire up a DVB-T transmitter of more than 1 watt into a real
antenna without any license at all. You will be sure to be visited by men in lumpy
black suits driving unmarked black cars.;) Fines in the US for unlicensed operation
start at $10,000.

Ron