# WCDMA Back-Off Power Calculation - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

enricolia
Post subject: WCDMA Back-Off Power Calculation Posted: Mon Jul 07, 2008 8:01 am

Lieutenant

Joined: Mon Jul 07, 2008 7:57 am
Posts: 2
Location: Belgium
Hello All,

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.

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IR
Post subject: Posted: Mon Jul 07, 2008 4:28 pm

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello Enrico,

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.

Usually 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.

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enricolia
Post subject: Posted: Mon Jul 07, 2008 7:05 pm

Lieutenant

Joined: Mon Jul 07, 2008 7:57 am
Posts: 2
Location: Belgium
Hi IR,

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 on clippling.

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.

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IR
Post subject: Posted: Mon Jul 07, 2008 11:20 pm

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello Enrico,

From my experience you should back off 7-10dB. Then, measure your ACPR and see if this is enough.

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FSomma
Post subject: Posted: Fri Jul 11, 2008 8:50 am
Hello Enrico,

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 follow:
10% --- 3.77dB
1% --- 7.27dB
0.1% --- 9.81dB
0.01% --- 10.71dB
0.001% --- 11.2dB
0.0001% --- 11.24dB

Posted  11/12/2012

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