WCDMA Back-Off Power Calculation - RF Cafe Forums
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Post subject: WCDMA Back-Off Power Calculation Posted:
Mon Jul 07, 2008 8:01 am
Joined: Mon Jul 07,
2008 7:57 am
I want to know if there is any formula or calculation we can do
for back-off power in WCDMA systems.
Supposing I have the transistor
datasheet with P1dB, so how much power I must back off from that value
for one WCDMA carrier (considering WCDMA is a spread signal and P1dB
normally is measured with CW signal).
Furthermore if there is
any calculation involving WDCMA composite power and power per carrier.
I heard that Pcomp = Pcarrier + 20 Log (n), where n is the number of
users on that radio sector. Others say that this calculation follows
exponential rule and not logarithmic.
Please let me know
what you can do for help me.
Thank you in advance.
Post subject: Posted: Mon Jul 07, 2008 4:28
Joined: Mon Jun 27, 2005 2:02 pm
The most important
thing to know is the PAR (Peak to Average Ratio) of the signal, which
is affected by the modulation scheme, filter involved, clipping etc.
Once you know this value you can derive the average power in your system.
When you find the peak value (PEP), check if this value exceeds the
P1dB of the transistor. It should not exceed the P1dB.
the PA is working up to P1dB in order to allow reasonable efficiency
(Linearity and efficiency are opposite to each other). You have to see
if at P1dB, the ACPR level still meets the specification.
Post subject: Posted: Mon Jul 07, 2008
Joined: Mon Jul 07, 2008 7:57 am
Thank you for the reply.
You are absolutely right for the generic question. Let's try
to relate some specific numbers.
For UMTS (WCDMA) single carrier,
QPSK Modulation, Channel Bandwidth 3.84 MHz, the PAR is 7 to 10 dB depending
Question is : How much back-off in power is necessary
from the P1dB using a multicarrier system ?
I am not sure if
I should back-off 7 to 10 dB from the P1dB of transistor datasheet subtracting
for more carriers (n) 10 or 20 Log rule.
Post subject: Posted: Mon Jul 07, 2008 11:20 pm
Joined: Mon Jun 27, 2005 2:02 pm
From my experience you should back
off 7-10dB. Then, measure your ACPR and see if this is enough.
Post subject: Posted: Fri Jul 11, 2008
The 3GPP for WCDMA HSDP defines different
Tests Models. Each test model has a special configuration definition
that makes the Test Model proper to an specific measurement. For ex,
Test Model 1 is defined, so that the signal has the specific characteristics
to measure ACLR, Spurious Emission, IMD; Test Model 2 is for output
power dynamics, TM3 for Peak code demain power, TM4 for EVM...etc.
The most peaking model is the TM2 w/ 32 DPCH and the CCDF is as
10% --- 3.77dB
1% --- 7.27dB
0.1% --- 9.81dB
0.01% --- 10.71dB
0.001% --- 11.2dB
0.0001% --- 11.24dB