Pulse or digital communications – Pulse transmission via radiated baseband
Reexamination Certificate
1999-07-16
2002-07-16
Chin, Stephen (Department: 2734)
Pulse or digital communications
Pulse transmission via radiated baseband
C375S316000, C375S340000, C375S349000
Reexamination Certificate
active
06421389
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to radio receivers adapted to receive and process wideband impulse radio signals. More particularly, this invention pertains to devices and circuits for accurately converting in an impulse radio receiver a series of time-modulated radio pulses into a baseband signal.
There is a continuing need for the development of advanced wireless devices for communications of voice and data, for materials measurement, navigation, environmental sensing, radar, security and numerous other civilian and military applications of radio technology. Improvements are needed in the underlying technology to provide greater reliability, greater accuracy, lower power consumption, lower cost, reduced size, and efficient use of the limited available spectrum. Conventional narrow band AM, FM, CDMA, TDMA and similar wireless communications methods and systems have not fully met these needs.
However, there is an emerging technology called Impulse Radio (including Impulse Radar) (“IR”) that offers many potential advantages in addressing these needs. Impulse radio was first fully described in a series of patents including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990) and U.S. Pat. No. 5,303,108 (issued Nov. 8, 1994), all invented by Larry W. Fullerton and assigned to Time Domain Corporation. The disclosure of each of these patents is incorporated in this patent specification by reference.
Impulse radio systems are generally characterized by their transmission of short duration broad band pulses on a relatively low duty cycle. In some systems these pulses may approach a Gaussian monocycle, where the instantaneous pulse bandwidth is on the order of the center frequency. The short pulse, low duty cycle mechanism produces a processing gain that may be utilized for interference rejection and channelization. Because of the extremely wide instantaneous bandwidth of the pulse, the available processing gain far exceeds what is achieved using typical conventional spread spectrum methods. This enables the utilization of many more channels at higher dynamic ranges and higher data rates than are available in the typical conventional spread spectrum system.
Impulse radio systems have further advantages in the resistance to multipath effect. Because impulse radio signals are divided in time rather than in frequency, time related effects, such as multipath interference, can be separated, resulting in lower average power and higher reliability for a given power level.
Impulse radio techniques are also useful in radar systems. Impulse radar systems enjoy the combined advantages of very short pulses at relatively low frequencies. The short pulses result in high resolution and the low frequency gives relatively high material penetration. If a radar system used a pulse of equivalent bandwidth at a higher carrier frequency, the material penetration properties would usually be impaired. This combined advantage enables IR to be used for ground penetrating radar for inspection of bridges, roads, runways, utilities and the like, and security applications, and to “see” through walls radar for emergency management situations.
Existing IR receivers typically use mixer or sampling technology which is large in size, inefficient in power consumption and which is difficult to reproduce in a manufacturing environment. This results in a high cost to the user. Improvements are thus needed in converter technology to reduce size, weight, power consumption and cost and to improve the manufacturing yield and reliability of these systems.
Impulse radio systems are not limited to transmitting and receiving Gaussian monocycle pulses. However, some basic impulse radio transmitters attempt to emit short Gaussian monocycle pulses having a tightly controlled average pulse-to-pulse interval. A Gaussian monocycle is the first derivative of the Gaussian function. However, in a real world environment, a perfect Gaussian pulse is not achievable. In the frequency domain, this results in a slight reduction in the signal bandwidth. The signals transmitted by an IR transmitter, including Gaussian monocycles, signals having multiple cycles in a Gaussian envelope, and their real world variations, are sometimes called impulses.
The Gaussian monocycle waveform is naturally a wide bandwidth signal, with the center frequency and the bandwidth dependent on the width of the pulse. The bandwidth is approximately 160% of the center frequency. In practice, the center frequency of a monocycle pulse is approximately the reciprocal of its length, and its bandwidth is approximately equal to 1.6 times the center frequency. However, impulse radio systems can be implemented where the transmitted and/or received signals have waveforms other than an ideal Gaussian monocycle.
Most prior art wireless communications systems use some variation of amplitude modulation (AM) or frequency modulation (FM) to communicate voice or data with a radio carrier signal. However, impulse radio systems can communicate information using a novel technique known as pulse position modulation. Pulse position modulation is a form of time modulation in which the value of each instantaneous value or sample of a modulating signal (e.g., a voice or data signal) is caused to change or modulate the position in time of a pulse. In the frequency domain, pulse position modulation distributes the energy over more frequencies.
In some impulse radio communications, the time position (pulse-to-pulse interval) is preferably varied on a pulse-by-pulse basis by two separate components: an information component and a pseudo-random code component. Prior art spread spectrum radio systems make use of pseudo-random codes to spread a narrow band information signal over a relatively wide band of frequencies. A spread spectrum receiver then correlates these signals to retrieve the original information signal. Unlike conventional spread spectrum systems, impulse radio systems do not need the pseudo-random code for energy spreading. In some applications, impulse radio transmitters can use pulse widths of between 20 and 0.1 nanoseconds (ns) and pulse-to-pulse intervals of between 2 and 5000 ns. These narrow monocycle pulses have an inherently wide information bandwidth. (The information bandwidth, also referred to simply as the “bandwidth”, is the range of frequencies in which one or more characteristics of communications performance fall within specified limits.) Thus, in some impulse radio systems, the pseudo-random (PN) code component is used for different purposes: channelization; energy smoothing in the frequency domain; and interference resistance. Channelization is a procedure employed to divide a communications path into a number of channels. In a system that does not use a coding component, differentiating between separate transmitters would be difficult. PN codes create channels, if there is low correlation and/or interference among the codes being used. If there were a large number of impulse radio users within a confined area, there might be mutual interference. Further, while the use of the PN coding minimizes that interference, as the number of users rises the probability of an individual pulse from one user's sequence being received simultaneously with a pulse from another user's sequence increases. Fortunately, impulse radio systems can be designed so that they do not depend on receiving every pulse. In such systems, the impulse radio receiver can perform a correlating, synchronous receiving function (at the RF level) that uses a statistical sampling of many pulses to recover the transmitted information. Advanced impulse radio systems may utilize multiple pulses to transmit each data bit of information, and each pulse may be dithered in time to further smooth the spectrum to reduce interference and improve channelization. These systems may also include a sub-carrier for improved interference resistance and implementation advantages. In other embod
Jett Preston
Larson Lawrence E.
Pollack Bret A.
Rowe David A.
Chin Stephen
Ha Dac Vinh
Patterson Mark J.
Time Domain Corporation
Waddey & Patterson PC
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