rejection of WLNA signal at input of colocated Wimax receive - RF
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Post subject: rejection of WLNA signal at input of colocated Wimax
Posted: Thu Nov 19, 2009 11:01 pm
Joined: Wed Aug 16, 2006
I am designing a transceiver where WLAN and WIMAX
radios are colocated (on the same PCB).
an usage scenariuo when the WLAN (2.45 GHz) systems
is transmitting and the WiMAX (2.51 GHz) is receiving.
due to finite isolation between the antennas (<15
dB) of WLAN and Wimax, the WLAN signal will either
saturate the WIMAX LNA or may produce some intermodulation
product by mixing with other signals.
standard the minimum rejection requirements of WLAN
signal at WIMAX receiver input is not specified.
I calculated that -40 dBm of any interferer
signal would be desired to keep all IMD products
at LNA input(IIP3=3) below the Noise Floor.
From this I found out the rejection required
for WLAN signal. For example it the receiver antenan
see a maximum of 5 dBm WLAN signal then the filter
before LNA should attenuate the signal to -40 dBm
so that any IMD generated by this signal is less
than Noise floor.
However it seems that such
filters are extremly difficult to realize.
I was wondering if this is the correct approach
of finding the filter rejection requirements.
I would be grateful if anyone can share
their views on this.
Post subject: Re: rejection of WLNA signal at input of colocated Wimax
Posted: Fri Nov 20, 2009 9:36 am
Joined: Fri Feb 17, 2006
Location: London UK
There are two separate issues here:
1) mutual interference that might be filtered
2) unacceptable high saturation of the non-linear
Regarding (1) I am not familiar with
the relative relationship between mutual interferers
in these particular systems, but in general with
digital modulation, it is possible to operate even
co-channel with interferers that are only -10dBc.
Thus typically one obtains a Bit Error rate of say
1 in 10^-8 for a carrier to noise ratio of 10dB.
This arises mainly because the modulation is quasi-synchronous,
and the interferer signal clock will be out of phase
coherence and thus appear as just noise.
(2) the resulting non-linearity products might be
capable of being out-filtered using an elliptic
function filter with the rapid roll-off side of
the BPF placed between the two carriers.
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