Telecommunications – Transmitter – Plural modulation
Reexamination Certificate
1999-08-18
2003-11-11
Nguyen, Lee (Department: 2682)
Telecommunications
Transmitter
Plural modulation
C455S061000
Reexamination Certificate
active
06647250
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
This is a continuation of pending application Ser. No. 09/176,415, filed Oct. 21, 1999
The present invention relates generally to electromagnetic communications, and more particularly, to a method and system for ensuring reception of a communications signal.
II. Description of the Related Art
Communication links utilize electromagnetic signals (EM), in the form of electromagnetic waves, to carry analog or digital electronic information from a first location to a second location. In doing so, a baseband signal, containing the information to be transmitted, is impressed on an oscillating signal to produce a modulated signal at the first location. The modulated signal is sent over the communications link to the second location. At the second location, the modulated signal is typically down-converted to a lower frequency, where the baseband signal can be recovered.
All EM signals can be sufficiently described in both the time domain and the frequency domain.
FIG. 1A
depicts a baseband signal
102
in the time domain that starts at time to and ends at a time t
1
. The baseband signal
102
can represent any number of real world occurrences. For example, baseband signal
102
could be the voltage output of a microphone for a given acoustical input.
FIG. 1B
illustrates spectrum
104
, which is the frequency domain representation of baseband signal
102
. Spectrum
104
depicts the relative amplitude of the sinusoidal components that when summed together with the correct relative phase will construct baseband signal
102
in the time domain. In other words, the spectrum
104
represents the relative amplitude and phase of the sine waves that constitute baseband signal
102
in the time domain.
Theoretically, a time-limited baseband signal (like baseband signal
102
) has an infinite number of sinusoidal frequency components. That is, the “tail” of spectrum
104
will continue to infinity. However, the amplitude of the sinusoidal components in spectrum
104
decrease with increasing frequency. At some point, the higher frequency components can be ignored and filtered out. The highest frequency remaining defines the “frequency bandwidth” (B) of the spectrum
104
. For example, if spectrum
104
corresponded to a human voice signal, the bandwidth (B) would be approximately 3.5 KHz. In other words, those sine waves beyond 3.5 KHz can be filtered out without noticeably affecting the quality of the reconstructed voice signal.
The signal with the simplest frequency domain representation is that of a single sine wave (or tone) at a given frequency f
0
. Sine wave
106
having a frequency f
0
, and its spectrum
108
are shown in
FIGS. 1C
, and
1
D, respectively. Sinusoidal signals are one type of periodic signals (or repeating signals) that may also be referred to as “oscillating signals”.
Amplitude modulation, a common modulation scheme, will be explored below to illustrate the effects of modulation.
FIGS. 1E and 1F
illustrate modulated (mod) signal
110
and its corresponding modulated spectrum
112
. Modulated signal
110
is the result of amplitude modulating sine wave
106
with baseband signal
102
. In the time domain, the amplitude of modulated signal
110
tracks the amplitude of the baseband signal
102
, but maintains the frequency of sine wave
106
. As such, sine wave
106
is often called the “carrier signal” for baseband signal
102
, and its frequency is often called the “carrier frequency.” In this application, information signals that are used to modulate a carrier signal may be referred to as “modulating baseband signals”.
In the frequency domain, amplitude modulation causes spectrum
104
to be “up-converted” from “baseband” to the carrier frequency f
0
, and mirror imaged about the carrier frequency f
0
, resulting in modulated spectrum
112
(FIG.
1
F). An effect of the mirror image is that it doubles the bandwidth of modulated spectrum
112
to
2
B, when compared to that of modulated spectrum
104
.
Modulated spectrum
112
(in
FIG. 1F
) is depicted as having substantially the same shape as that of modulated spectrum
104
(when the mirror image is considered). This is the case in this example for AM modulation, but in other specific types of modulations this may or may not be so as is known by those skilled in the art(s).
Modulated spectrum
112
is the frequency domain representation of what is sent over a wireless communications link during transmission from a first location to a second location when AM modulation is used. At the second location, the modulated spectrum
112
is down-converted back to “baseband” where the baseband signal
102
is reconstructed from the baseband spectrum
104
. But in order to do so, the modulated spectrum
112
must arrive at the second location substantially unchanged.
During transmission over the wireless link, modulated spectrum
112
is susceptible to interference. This can occur because the receiver at the second location must be designed to accept and process signals in the range of (f
0
−B) to (f
0
+B). The receiver antenna accepts all signals within the stated frequency band regardless of their origin. As seen in
FIG. 1G
, if a second transmitter is transmitting a jamming signal
114
within the band of (f
0
−B) to (f
0
+B), the receiver will process the jamming signal
114
along with the intended modulated spectrum
112
. (In this application a jamming signal is any unwanted signal regardless of origin that coexists in a band occupied by an intended modulated spectrum. The jamming signal need not be intended to jam.) If the power of jamming signal
114
is sufficiently large, then modulated spectrum
112
will be corrupted during receiver processing, and the intended information signal
102
will not be properly recovered.
Jamming margin defines the susceptibility that a modulated spectrum has to a jamming signal. Jamming margin is a measurement of the maximum jamming signal amplitude that a receiver can tolerate and still be able to reconstruct the intended baseband signal. For example, if a receiver can recover info signal
102
from spectrum
112
with a maximum jamming signal
114
that is 10 dB below the modulated spectrum
112
, then the jamming margin is said to be −10 dBc (or dB from the carrier).
Jamming margin is heavily dependent on the type of modulation used. For example, amplitude modulation can have a typical jamming margin of approximately −6 dBc. Frequency modulation (FM) can have a jamming margin of approximately −3 dBc, and thus more resistant to jamming signals than AM because more powerful jamming signals can be tolerated.
The Federal Communications Commission (FCC) has set aside the band from 902 MHZ to 928 MHZ as an open frequency band for consumer products. This allows anyone to transmit signals within the 902-928 MHZ band for consumer applications without obtaining an operating licence, as long as the transmitted signal power is below a specified limit. Exemplary consumer applications would be wireless computer devices, cordless telephones, RF control devices (e.g. garage door openers), etc. As such, there is a potentially unlimited number of transmitters in this band that are transmitting unwanted jamming signals.
The 900-928 MHZ frequency band is only a single example of where jamming is a significant problem. Jamming problems are not limited to this band and can be a potential problem at any frequency.
What is needed is an improved method and system for ensuring the reception of a modulated signal in an environment with potentially multiple jamming signals.
What is also needed is a method and system for generating a modulated signal that is resistant to interference during transmission over a communications link.
What is further needed is a method and system for generating a modulated signal that has a higher inherent jamming margin than standard modulation schemes (e.g. AM, FM, PM, etc.), without substantially increasing system complexity and cost.
SUMMARY OF THE INVENTION
The present invent
Bultman Michael J.
Cook Robert W.
Looke Richard C.
Moses, Jr. Charley D.
Sorrells David F.
Nguyen Lee
ParkerVision, Inc.
Sterne Kessler Goldstein & Fox P.L.L.C.
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