Method and apparatus for converting RF signals to baseband
View Patent ↗The present invention relates to the conversion of signals from RF to baseband using transition functions, or edge functions. These functions typically transition from positive to negative, or from negative to positive, synchronously with the transition of the received pulse signal, effecting detection essentially by synchronously rectifying a signal cycle of the received pulse, producing a net signal at baseband that can be further processed to detect modulation according to the modulation format. It is further disclosed how to configure these systems for optimal reception with a filter optimized for a given detect signal function. Generalizations of the matched filter embodiment lead to further embodiments employing alternative detection functions. Also disclosed is a two-stage version which applies a decode signal to the rectified signal. This step can be performed by a single correlator or by a plurality of correlators in parallel. Coding methods are disclosed to employ two-stage systems for enhanced channelization, and interference rejection.
1. A method of converting a radio frequency signal to a baseband signal, said radio frequency signal comprising a plurality of impulses, comprising the steps of:
a. changing each of said plurality of impulses into rectified pulses by mixing with said radio frequency signal a detect signal, said detect signal comprising precisely timed detecting transitions, such that said detecting transitions occur substantially coincident with a zero-crossing of each of said impulses; and
b. obtaining said baseband signal by integrating one or more rectified pulses.
2. The system according to claim 1 , wherein said detecting transitions are substantially instantaneous.
3. The method according to claim 2 , further comprising the steps of:
a. obtaining a decoded signal by mixing with said rectified pulses a decode signal; and
b. obtaining said baseband signal by integrating said decoded signal.
4. The method according to claim 2 , wherein said detecting transitions are functions, M(f), such that
M
(
f
)
=
S
(
f
)
H
(
f
)
H
(
f
)
2
where S(f) is a received signal in the frequency domain, H(f) is a filter response in the frequency domain such that where f is frequency and t is time.
H
(
f
)
≠
{
ⅇ
-
2
π
j
f
t
S
(
f
)
,
1
.
5. The method according to claim 2 , wherein said detecting transitions are functions, E(f), such that
E
(
f
)
=
S
(
f
)
G
(
f
)
G
(
f
)
2
where S(f) is a frequency domain expression of a received signal, and G(f) is a frequency domain expression of a differentiated filter response where f is frequency.
6. The method according to claim 1 , wherein said detect signal comprises said detecting transitions and sloped non-detecting transitions.
7. The method according to claim 1 , wherein said detecting transitions are non-instantaneous.
8. The method according to claim 7 , wherein said detecting transitions are functions, M(f), such that
M
(
f
)
=
S
(
f
)
H
(
f
)
H
(
f
)
2
where S(f) is a received signal in the frequency domain, H(f) is a filter response in the frequency domain such that where f is frequency and t is time.
H
(
f
)
≠
{
ⅇ
-
2
π
j
ft
S
(
f
)
,
1
.
9. The method according to claim 7 , wherein said detecting transitions are functions, E(f), such that
E
(
f
)
=
S
(
f
)
G
(
f
)
G
(
f
)
2
where S(f) is a frequency domain expression of a received signal, and G(f) is a frequency domain expression of a differentiated filter response where f is frequency.
10. An apparatus for converting a radio frequency signal to a baseband signal, said radio frequency signal comprising a plurality of impulses, comprising:
a. an antenna for receiving the radio frequency signal;
b. a detect signal generator, an output of said detect signal generator being a detect signal comprised of a plurality of transitions substantially coincident with zero-crossing of each of said impulses, said detect signal generator comprising:
i. a code source;
ii. a timing generator coupled to said code source; and
iii. a transition signal generator responsive to said timing generator;
c. a first multiplier responsive to said antenna and to said detect signal generator, said first multiplier having a rectified output; and
d. a means for integrating said rectified output responsive to said first multiplier to produce the baseband signal.
11. The apparatus according to claim 10 , further comprising:
a. a decode signal generator;
b. a second multiplier responsive to said first multiplier and to said decode signal generator, said second multiplier having a decoded output; and
c. a means for integrating said decoded output responsive to said second multiplier.
12. The apparatus according to claim 10 , further comprising a filter responsive to said antenna coupled to said first multiplier, said filter having a filter response, H(f), such that
H
(
f
)
≠
{
ⅇ
-
2
π
j
ft
S
(
f
)
,
1
where S(f) is a received signal, f is frequency and t is time, and wherein said detect signal generator has an output, M(f), such that
M
(
f
)
=
S
(
f
)
H
(
f
)
H
(
f
)
2
.
13. The apparatus according to claim 12 , further comprising a differentiator responsive to said filter and an integrator responsive to said detect signal generator.
14. The apparatus according to claim 12 , further comprising an integrator responsive to said filter and a differentiator responsive to said detect signal generator.
15. The apparatus according to claim 10 , further comprising:
a. one or more second stage correlators coupled to said first multiplier and responsive to a second stage decode signal generator; and
b. means for integrating output of said one or more second stage correlators responsive to said second stage correlators.
16. The apparatus according to claim 15 , further comprising a buffer responsive to said first multiplier and having an output sent to said one or more second stage correlators.
17. The apparatus according to claim 15 , further comprising a high pass filter responsive to said first multiplier and having an output sent to said one or more second stage correlators.
18. The apparatus according to claim 15 , further comprising a low pass filter responsive to said first multiplier and having an output sent to said one or more second stage correlators.