Interference, Multipath, and Scintillation - Understanding GPS: Principles and Applications - page 277

When narrowband excision is not employed and NELP is used, the effect of
narrowband interference is obtained using (6.36) and (6.32), assuming small
early-late spacing,

− 1

() ()

fS f

2

Sf
T C
C

B C

N

C
C

1

n ι S ι S ι

σ ≅

S

+

ι

1 + + (6.40)

NELP D , → 0

T C
N

2 πβ

2

β 2

S

η

S

η

S 0 S S

0 ι

Figure 6.7 plots (6.39) and (6.40) for four different modulations, calculated
with B n of 0.1 Hz, ( C S / N 0 ) dB of 30 dB-Hz, ( C ι / C S ) dB of 40 dB, precorrelation band-
width of 24 MHz, correlation integration time of 20 ms, and very small early-spac-
ing. The results for NELP approach the lower bound for certain interference
frequencies. Interference very near band center degrades NELP code tracking accu-
racy less than interference further away from band center. The oscillatory behavior
of the NELP error for BPSK-R(1) and BOC(1,1) demonstrates that narrowband
interference away from band center can have the same effect on code tracking error
as interference nearer to band center, reflecting the frequency-squared weighting in
(6.34). The result for NELP BPSK-R(10) shows that the maximum error occurs

BPSK-R(1) NELP
BPSK-R(1) LB
BOC(1,1) NELP
BOC(1,1) LB

BPSK-R(10) NELP
BPSK-R(10) LB
BOC(10,5) NELP
BOC(10,5) LB

0.5

0.4

0.3

0.2

0.1

0

0 2 4 6 8 10 12

Interference Frequency (MHz)
Figure 6.7 NELP and lower bound (LB) code tracking error of different modulations in
narrowband interference, for different frequency interference, with 0 MHz corresponding to band
center, and 12 MHz corresponding to the edge of the band.