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# Bandwidth and frequency spectrum of QPSK signal

Posted on 2010-08-14
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I have this question for course work that i am stuck on. If anyone could point me to some examples of how to achieve this or just point me in the right direction or even let me know how i would go through it with different values, it would be much appreciated.

Assume that the QPSK signal has a carrier frequency is 1800 MHz and the bit rate is 270 kbit/s. Draw a diagram showing the frequency spectrum of the QPSK signal, labelling any important frequencies. Explaining your answer, calculate the bandwidth of the QPSK signal.

•      8-PSK
•      256-QAM
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Question by:Onity
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Expert Comment

ID: 33437626
The following link may be of help
-
Bit rate - Wikipedia, the free encyclopedia
The relationship between the gross bit rate and net bit rate is affected by the FEC ... Consequently the net bit rate is sometimes called digital bandwidth ...
Bit rates at various protocol ... -
en.wikipedia.org/wiki/Bit_rate -
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Expert Comment

ID: 33443500
we had to calculate stuff like this too during my study.
important to know ist that you understand the concept of how the data is transmitted (read for ex.: http://cnx.org/content/m10042/latest/). there are always 2 bits that are transmitted with one symbol (see: trheres 00, 01, 10 and 11) hence you can calculate how many symbols are transmitted and then calculate the needed bandwidth.
draw the frequency spectrum with the upper and lower band using

same with 8-psk - there are always 3 bits transmitted. and with 256-qam you have 8 bits. (however qam and psk is not tha same... look it up on wikipedia or ask your prof.)
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Accepted Solution

Kendor earned 2000 total points
ID: 33443534
to get back to this. 4-psk and 4-qam are the same modulation. 8-psk and 8-qam are not (but similar: 8-PSK uses constant levels while with 8-QAM the states lie on a square (Q,I). 8-PSK is simpler and more efficient to be realized which leaves th 8-QAM with no relevance to the practical use)

for your calculation again. I would say: you have 270kbit/s => 135ksymbols/sec
whats the needed bandwidth?

i don't know whether you know the "roll-off-factor" and whether and FEC (forward error correction) is used. then things change slightly.
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Author Comment

ID: 33466697
sorry for the late response it didn click in my head till last night.

as the baud rate is 135 the required bandwidth is 2 times that so 270kHz due to nyquists theory so the lower band is 1800*10^6 - 135*10^4 = 1798.65MHz and upper is 1800MHz + 135kHz. giving a bell shaped curve?

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