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- 26th March 2009, 13:41 #1

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## Explanatio of 16-QAM and QPSK statements

can anybody explaine the following please:

16 QAM with code rate of 1/2 coding to give 2 bits per symbol.

and

QPSK with code rate of 3/4 coding to give 1.5 bit per symbol.

also if you know the reference, please advise me

thanks in advance.

- 26th March 2009, 15:53 #2

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## qpsk coding rate

Hi,

16 QAM produces 4 bits/symbols with an (1/2) rate (bits/symbol) coding will produce

4*(1/2)=2 bits/symbols

QPSK with code rate of 3/4 coding to give 1.5 bit per symbol.

2 * (3/4) = 1.5bit/ symbol

whether the explantion is enough for you or are u expecting some other reply.

Happy Learning

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- 26th March 2009, 16:08 #3

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## 16qam

My Friend,

If i have 128 binary digits and need to use the QPSK modulation, then the number of output symbols will be 64, in each symbol there are 2 bits. Now if i used the QPSK with 3/4 coding, what is the outbut will be?

- 26th March 2009, 16:08

- 26th March 2009, 18:00 #4

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## qam 16 3/4

may i know the clarification also?

Thank you

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- 26th March 2009, 19:00 #5

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## 16 qam bitrate

At first what is the your bit rate(source bit rate=R_s)?????

if there are k=128 bits for (1/2) rate coder, there are 100% redundant bits (n-k)

i.e., 128 bits. ; n=128+128=256.

so code rate=r=k/n=128/256=1/2.

channel encoder bit rate is given by = R_c= (n/k)*R_s = (256/128)*R_s =2*R_s

where R_s is the bit rate generated by the source.

If R_s is the bit rate which is incoming bits to the encoder.

the o/p of the M-ary modulator in your case QPSK (M=4, in this case)

At the o/p of the M-ary modualtor: symbol rate R_sym

R_sym= R_c / log2 (M) = 2*R_s / log2 (4) = R_s in your case.

Happy learning

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- 26th March 2009, 19:28 #6

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## qam bit rate

My friend,

Thank you for your help, but i am asking about the number of output symbols in case of QPSK with code rate of 3/4 coding if the input is 128 bit.

please rectify me if i am asking something wrong. In OFDM i need to know what is the data that i have before i process it by the IFFT block.

Many Thanks

- 26th March 2009, 19:28

- 26th March 2009, 20:30 #7

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## 16 qam coding rate

If the i/p bit is transferred at the rate of 128 bits/ sec.

then in case of of QPSK (M=4) with code rate of 3/4 coding

Rc= (3/4)*128 = 96

NO. OF SYMBOLS TRANSFERRED AT THE O/P QPSK MODUALTOR IS : 96/log2 (4) = 96/2 = 48 symbols/sec

i think this will help you to further proceed in your learning.

Happy learning

Added after 5 minutes:

hVE YOU GOT CONVINCED OR ARE YOU EXPECTING ANOTHER BETTER REPLY.

WHETHER YOU CHECKED WITH MY ANSWER ???? I HAVE SHARED MY KNOWLEGE WITH YOU.

KINDLY CHECK WITH THE ANSWER AND DO REPLY ME.

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- 26th March 2009, 20:55 #8

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## qam code rate

I donate 50 points for you. Your discussion was very clear and good.

Many thanks.

by the way, there are about 10 new members from Malaysia joined our group all are students of PhD and M.s.c in communications. Most of them are Iraqi students. So that, Please rewrite your answer in the group as this very nice from you

- 27th March 2009, 09:07 #9

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## qpsk code rate

Hi Muntather,

"Please rewrite your answer in the group as this very nice from you "

i can't follow this line.

what do you mean by that.....

Happy learning

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- 27th March 2009, 12:56 #10

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## qam coding rate

just to put some force in the group. May be this will help some of the members. I do not mean other thing. My Friend,

My goal is to let every one learn.

I am sorry if you understand it in a wrong way.

- 2nd April 2009, 08:27 #11

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## 16 qam symbol rate

Originally Posted by**rramya**

2*R_s shows that after suppose convolutional encoding the data rate of the encoded symbols is greater than the data rate of the incoming bit stream.

I think the data rate remains same, only that now you have half of the user bits flowing in the same amount of time.

Secondly, the idea about the 1.5 bits/symbol ... I think this is related to information theory, we dont have a 0.5 bit practically. I am completely convinced by the math that you have provided but this shows that in one symbol after encoding there would be equivalent information to 1.5 bits. Even though the symbol would really be carrying 2 bits i.e. 2 bits/symbol

Do you agree? Y/N

- 2nd April 2009, 11:46 #12

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## qam 16 1/2

comm_eng wrote:

How can the (data) rate change? Doesn't the channel encoder just add redundency at the same data rate of the incoming stream.

2*R_s shows that after suppose convolutional encoding the data rate of the encoded symbols is greater than the data rate of the incoming bit stream.

I think the data rate remains same, only that now you have half of the user bits flowing in the same amount of time.

ANS: YOU R CORRECT:

i never talk abt data rate change!!!!

all i talk abt

the channel encoder rate ... because of adding redundant bits....

eg in case: for convolutional encoder rate of (1/2)

channel encoder bit rate is given by = R_c= (n/k)*R_s = (256/128)*R_s =2*R_s

where R_s is the bit rate generated by the source.

which means: we require double the bandwidth of the uncoded Systems.....and hence the capacity of the channel needs to be doubled for the . hence R_c=2*R_s

INTREPRETATION:

since BW is a constrainst,

if we cant double the bandwidth of the single user channel: automatically,

ONLY HALF OF THE USER BITS ARE TRANSMITTED in the given channel , utilizing the available BW.

Secondly, the idea about the 1.5 bits/symbol :

suppose if R_s = 3 bit/sec.... encoder rate = (3/4)

R_c=(4/3)R_s =4 bit/sec

Here , we have original bits = 3 bits

redundant bit = 1 bit

At the Modulator o/p , the symbol rate is R_Mod_sym = R_c/log2(M)

for QPSK M=4.

R_Mod_sym = R_c/log2(M) = ( 4 ) / log2(4) = 4/( 2) = 2 symbols /sec

now,

bits/symbol = 3 (bit/sec) / (2 ) (symbols /sec) = (3/2) bits / symbol...

= 1.5 bits/symbol

INTREPRETATION:

we have 3 bits as original bits there are 2 symbols (which has got 2 bits/symbol, in case of QPSK) we get at the o/p of the modulator ......

bits/symbol = 1.5 bits/symbol ..............

Happy Learning.

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- 2nd April 2009, 11:46

- 1st September 2009, 21:21 #13

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## 16 qpsk

Hi, Im looking for an answer with a problem that I have with my wireless connection.

The antena outside my house has very high signal but the modulation fluctuates too much. For example:

QAM64 3/4

QAM16 1/2

QAM16 3/4

QPSK 3/4

Data ranges for download and upload of 29+

It seems that this fluctuations are affecting my internet connection. I'm experiencing also fluctuations on connection timeouts (Sporadic connection timeout). This is affecting my downloads and other communication that requires stable connection.

Do anyone knows if its possible to fix a wireless antena with just one modulation?

Thanks for any help

QLands

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