Question about OpenBTS BM 2009 RF chains

Hi,

I read the following article describing the RF chain of OpenBTS will be
used
in BM’09.
http://sourceforge.net/apps/trac/openbts/wiki/OpenBTS/BM2009RF
In the “RX/TX isolation” part, it says that “The USRP outputs noise in
the
receive band about 80 dB below the transmit signal level, or around -60
dBm.”. Does the USRP outputs noise in the receive band mean the out-band
leakage of transmitted signal? And why it is around 20dBm - 80dB =
-60dBm,
but not 36dBm - 80dB = -44dBm. In my opinion, the leakage signal is also
amplified by PA.

Bests,
Hanwen Cao

Hanwen -

The -60 dBm refers to the signal level at the output of the USRP, not
the final ouput of the BTS.

The leakage signal is amplified by the PA, but in the full BTS design
you reference it is subject to an additional 70 dB attenuation in the
duplexer. So the actual output level at the antenna is 30 dBm - 80
dB - 70 dB = -114 dBm. This is still high enough to limit uplink
receiver performance and will be reduced further in future revisions
of the hardware.

– David

On Sep 8, 2009, at 6:22 AM, hanwen wrote:

opinion, the leakage signal is also amplified by PA.

Bests,
Hanwen Cao


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David A. Burgess
Kestrel Signal Processing, Inc.

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