DSSS modulation is a single-carrier modulation scheme, BPSK operates much in
the same way as DSSS which usually are much more robust then other forms of
modulation because they seem to run like a single carrier modulation

The neat thing about BPSK is its ability to take interference and still run
because it sends multiple bits of the same data and then aggregates it into
a single bit so it's not an all or nothing but more of a something.

As you start to walk up the modulation line you definitely need more C/I,
but you also start to loose the ability to use full power out of the radio.
As you turn the power up on the radio, the more distortion you get so you
can't achieve full modulation at full power where as you can run BPSK at
full power.  So dish size and quality becomes more important than radio
power when you are gunning for full modulation. 
Dustin Jurman
Rapid Systems Corporation
1211 N. Westshore Blvd
Tampa, FL 33607

-----Original Message-----
Behalf Of Tom DeReggi
Sent: Sunday, February 05, 2006 11:37 AM
To: WISPA General List
Subject: [WISPA] BPSK QAM16 DSSS interference

I have a problem where BPSK modulation has packet loss due to interference
greater than QAM16 at good RSSI levels.

I am using tons of DSSS all around the troubled OFDM link. (16 PtMP links
totalled from two cells, 2 miles apart). And my DSSS links most likely are
the culprits causing some self interference with the OFDM PtP link. So is
there anything about BPSK modulation that would make it more prone to
interferrence from DSSS radios apposed to QAM16?

RSSI at -65 when this occured most obviously.

QPSK had similar characteristic/loss as BPSK, when comparing to QAM16.

At -55 QAM modulations got worse/unusable, possibly because overloaded by
self noise.
At -75 QAM modulations got worse/unusable, possibly because to close to
noise floor (-80).
Testing at -65 was the sweet spot that QAM worked well, much better than
QOS loss was relatively consistent for BPSK/QPSK at -55,-65, rssi, with a
little more degregation at -75 for QPSK as getting close to noise.

Note: 10 mile link. Rssi adjustments accomplished by reducing radio TX power
on both ends, Antenna type remained constant.  For the purpose of this
discussion, I'm looking for theory pertaining to all radios, not a specific

Tom DeReggi
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband 

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