Patent Application
20080310481
This patent document pertains generally to data communications, and more particularly, but not by way of limitation, to a system and method for transmitting and detecting wideband signals in the presence of noise.
Coherent detection is better than noncoherent detection for detecting extremely weak spread spectrum signals. That is especially so as signal to noise ratios fall below unity (0 dB). For communication systems employing high spreading factors, the coherency bandwidth of the system may, however, be too low to coherently detect the spread spectrum signal over the symbol period duration. The receiver must therefore employ noncoherent (i.e., energy accumulation) detection after the coherency bandwidth is reached and exceeded.
What is needed is a system and method for coherently detecting extremely weak wideband or spread spectrum signals.
In the drawings, which are not necessarily drawn to scale, like numerals describe substantially similar components throughout the several views. Like numerals having different letter suffixes represent different instances of substantially similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document.
In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.
As noted above, coherent detection is better than noncoherent detection for detecting extremely weak spread spectrum signals. That is especially so as signal to noise ratios fall below unity (0 dB). For communication systems employing high spreading factors, the coherency bandwidth of the system may, however, be too low to coherently detect the spread spectrum signal over the symbol period duration. The receiver must therefore employ noncoherent (i.e., energy accumulation) detection after the coherency bandwidth is reached and exceeded.
A system and method for coherently detecting extremely weak spread spectrum and other wideband signals is shown in
At receiver 120, the received signal 150 may be shifted in frequency due to the offset between the transmitter 102's local oscillator and the local oscillator in receiver 120, as well as any Doppler effects. The correlation of received signal 150 with the transmitted chirp is illustrated in
System 100 exploits this property of frequency offset tolerance in order to detect extremely weak signals at significant frequency offsets with respect to the receiver. This is accomplished at the transmitter by simply repeating the chirp sequence a number of times to construct a data un-modulated synchronization sequence.
Returning to
System 100 therefore achieves coherent detection of a signal beyond the system's coherency bandwidth through the use of chirp modulation for a data-unmodulated sync or preamble.
In one embodiment, such as is shown in
An embodiment such as is shown in
In one embodiment, as illustrated in
In summary, system 100 solves the problem of detecting a signal with an extremely low signal to noise ratio. It relaxes the coherency bandwidth requirements for a communication system, allowing low cost, lower stability oscillators to be used.
It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.
The Abstract is provided to comply with 37 C.F.R. §1.72(b), which requires that it allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment.
