User's Manual Part 2

Processing Algorithms (draft)
RVP8 User’s Manual
April 2003
5–7
5.1.4 Interference Filter
The interference filter is an optional processing step that can be applied to the raw (I,Q) samples
that emerge from the FIR filter chips. The intention of the filter is to remove strong but sporadic
interfering signals that are occasionally received from nearby man-made sources. The technique
relies on the statistics of such interference being noticeably different from that of weather.
For each range bin at which (I,Q) data are available, the interference filter algorithm uses the
received power (in deciBels) from the three most recent pulses:
P
n*2
, P
n*1
, and P
n
where:
P
n
+ 10log
10
ǒ
I
2
n
) Q
2
n
Ǔ
.
If the three pulse powers have the property that:
Ť
P
n*1
* P
n*2
Ť
t C
1
and
Ť
P
n
* P
n*1
Ť
u C
2
(Alg.1)
then (I
n
, Q
n
) is replaced by (I
n*1
, Q
n*1
) . Here C
1
and C
2
are constants that can be tuned by
the user to match the type of interference that is anticipated, and the error rates that can be
tolerated. For certain environments it may be the case that good results can be obtained with
C
1
+ C
2
; but the RVP8 does not force that restriction.
This 3-pulse algorithm is only intended to remove interference that arrives on isolated pulses,
and for which there are at least two clear pulses in between. Interference that tends to arrive in
bursts will not be rejected.
Two variations on the fundamental algorithm are also defined. The CFGINTF command
(Section 6.22) allows you to choose which of these algorithms to use, and to tune the two
threshold constants. You may also do this directly from the Mp setup menu (Section 3.3.2).
Ť
P
n*1
* P
n*2
Ť
t C
1
and P
n
* P
n*1
u C
2
(Alg.2)
Ť
P
n*1
* P
n*2
Ť
t C
1
and P
n
* LinAvg( P
n*1
, P
n*2
) u C
2
(Alg.3)
Where LinAvg() denotes the deciBel value of the linear average of the two deciBel powers. The
Alg.2 and Alg.3 algorithms also include the receiver noise level(s) as part of their decision
criteria. Whenever power levels are intercompared in the algorithms, any power that is less than
the noise level is first set equal to that noise level. This makes the filters much more robust and
properly tunable, so that interference is more successfully rejected on top of blank receiver
noise.
Optimum values for C
1
and C
2
will vary from site to site, but some guidance can be obtained
using numerical simulations. The results shown below were obtained when the algorithms were
applied to realistic weather time series having a spectrum width = 0.1 (Nyquist), SNR = +10dB,
and an intermittent additive interference signal that was 16dB stronger than the weather. The
interference arrived in isolated single pulses with a probability of 2%.
Performance of the three algorithms is summarized in the first three columns of Table 5–2, for
which C
1
and C
2
have the common value shown. The fourth column also uses Algorithm #3,
but with the value of C
1
raised by 2dB. The “Missed” rate is defined as the percentage of