Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2001-04-05
2003-06-17
Bayard, Emmanuel (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06580750
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for receiving spread spectrum signals in which a receiving signal is received from a sending signal, which is encoded with a spread code, and is filtered. After this, in a first process part in a first RAKE finger, the filtered receiving signal, in an on-time correlation, is multiplied by a conjugated complex spread code, which is delayed by a base time delay in relation to the spread code of the sending signal, and the result is summed in a first adder. The result of the first summation is output as an information signal and the adder is reset. The filtered receiving signal is further processed in two alternative processes. In the first process, in a late correlation, the filtered receiving signal is multiplied by the conjugated complex spread code of the sending signal, which is delayed in relation to the spread code (c (t)) of the sending signal by a first base time delay and a positive additional time delay that increases the first base time delay, and is summed in a second adder. In an early correlation, the filtered receiving signal is multiplied by the conjugated complex spread code, which is delayed in relation to the spread code (c (t)) of the sending signal by the first base time delay and a negative additional time delay that decreases the first base time delay, and is then summed in a third adder. Then a correlation difference signal is determined as the difference between the results of the early and late correlations and the second and third adders are reset.
In the second process, the filtered receiving signal is multiplied by the difference between the conjugated complex spread code, which is delayed in relation to the spread code of the sending signal by the first base time delay and the positive additional time delay that increases the first base time delay, and the conjugated complex spread code, which is delayed in relation to the spread code of the sending signal by the first base time delay and the negative additional time delay that decreases the first base time delay, and the multiplied signal is summed in a fourth adder. This produces the correlation difference signal. After it is processed further, the fourth adder is reset. Then the real component of the difference signal, which is determined in the one or the other process, and the information signal is established as an error signal and used to control the magnitude of the base time delay.
Parallel to the above processing in a second RAKE finger, the same process steps are executed with a second base time delay which has a difference in relation to the first base time delay, and the information signals of the parallel executed processed parts are combined.
The invention also relates to a method which has the same sequence; but wherein, the receiving signal is not filtered but is instead multiplied by a filtered conjugated complex spread code, and the product signals are integrated, which achieves the correlation.
The process according to the invention uses RAKE receivers of the kind described in R. Price, P. E. Green, Jr., [In English:] “A Communication Technique for Multipath Channels,” Proc. IRE, Vol. 46, March 1958, pp. 555-570. The RAKE receiver is a receiver apparatus which is outstandingly suited for receiving spread spectrum signals and is used for this. The conventional RAKE receiver is comprised of a number of correlators which despread the spread spectrum signal with a different time offset and recover the narrowband signal.
Spread spectrum techniques of the type mentioned the beginning, as are described in R. L. Pickholtz, D. L. Schilling, L. B. Milstein, [In English:] “Theory of Spread Spectrum Communications—a Tutorial,” IEEE Transactions on Communications, Vol. COM-30, May 1982, pp. 855-884 and in M. J. Goiser Alois,
Handbuch der Spread
-
Spectrum
—Technik [Handbook of Spread Spectrum Engineering], Springer, 1998, have been used in the past exclusively for military applications for encoding and masking signals and for increasing jamming resistance. In this connection, a narrowband signal to be transmitted is multiplied through multiplication with a broadband pseudo-random spread sequence. The elements of the random sequence are referred to as chips. The resulting signal is likewise in broadband format. In other words, the signal to be sent, i.e. the sending signal, is encoded with a spread code.
In the receiver, this sending signal is received and then processed further as a receiving signal. After a time which is defined by the sending signal, the same broadband pseudo-random spread sequence is used in the receiver as is used for encoding the transmitted signal. This is possible because of the character of the pseudo-randomness, as a result of which the same pseudo-random sequence can be produced using the same technical means and assumptions. The sender and the receiver only need to know the means and assumptions for producing the pseudo-random sequence.
The original narrowband signal is then recovered in the receiver through multiplication by the conjugated complex spread.
In cellular mobile telephony, where a limited bandwidth must be made available to numerous subscribers, this process is likewise attractive. In this instance, different subscribers are simply associated with different pseudo-random spread sequences. For the receiver which uses the spread code of the desired subscriber to be detected, the signals of all the other subscribers behave like noise.
The information to be transmitted can be recovered in the receiver as long as the overall power of the interfering signals is compatible.
For a few years, spread spectrum has been used successfully in the American mobile telephone standard “IS-95.” Direct sequence spread spectrum has been proposed as the basic process for the mobile telephone standard of the third generation “IMT-2000” and it is probable that the mobile telephone standard of the third generation will be based on this process because it permits a simple and flexible allocation of the spectrum to different subscribers with different bandwidth requirements.
In mobile telephony, the transmitted signal of a base station usually does not travel directly to the receiver but instead arrives by a circuitous route through multiple reflections. The received signal is distinguished by an overlapping of these multiple reflections, which differ only in value, phase, and the transit time delay corresponding to the propagation path. Each signal component that has reached the receiver via reflections is in turn comprised of a series of separate signals with slight transit time differences so that the signal component that has reached the receiver via a particular path is subject to rapid fading.
Because of the favorable correlation properties of the spread spectrum signals, specific individual paths (signal components) of a multipath signal can be detected with a RAKE receiver through correlation with correspondingly delayed pseudo-random spread sequences. A combination of the correlation results permits a more reliable reconstruction of the information of the sending signal than when only a single correlation result is used.
Conventional processes use a time error estimator for each correlator of a RAKE receiver, which estimator, through a first correlation with an additional positive time offset and through a second correlation with a negative additional time offset, estimates the time delay in relation to the optimal time offset for the local random code generator, for which the actual correlator extracts the maximal signal strength from the multipath signal component. The correlator and time error estimator are often combined into one overarching unit which can also contain other estimators and which is referred to as a RAKE finger. The above-described process for time error estimation is therefore referred to as the early-late process. The estimated time delay is used by the RAKE finger itself or by an additional overarching unit for time tracking, for the so-called fine time
Baker & Botts LLP
Bayard Emmanuel
Systemonic AG
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