Embodiments of the invention relate generally to the field of processing spread spectrum signals and more specifically to methods and apparatuses for effective signal acquisition and receiver tracking for spread spectrum signals.
Spread spectrum (SS) systems employ various techniques to spread energy generated at a given frequency or frequency band over a much wider band of frequencies. These techniques may be employed for many reasons including providing increased resistance to natural or intentional interference. In telecommunication applications, SS systems may employ direct-sequence (DSSS), frequency hopping, or a hybrid of these techniques, among others. SS communications systems use a sequential noise-like signal structure to spread the typically narrowband information signal over a relatively wideband range of radio frequencies. The receiver correlates the received signals to retrieve the original information (e.g., telecommunication signal). Such systems decrease potential interference to other receivers while achieving an acceptable degree of privacy. Moreover, such systems are ideal candidates for ranging and target detection. For instance, global navigation satellite systems (GNSS) such as Global Positioning System (GPS) and the European Geostationary Navigation Overlay System (EGNOS), utilize the SS signal received from multiple satellites to accurately estimate the position of the user. Additionally, SS systems are highly advantageous for applications that require robustness towards thermal noise, interference and multipath. For example, code division multiple access (CDMA) systems and ultra wideband (UWB) systems utilize the SS signal characteristics for thermal noise and interference immunity and to provide multiple access.
In a SS system, the transmitted SS signal reaches the receiver with an unknown timing and frequency offset. For example, even after down-conversion, the received SS signal is not pure baseband as there is still some residual frequency offset due to receiver motion, transmitter motion, oscillator inaccuracies, or a combination thereof. Additionally, the SS signal incurs an unknown time delay prior reaching the SS receiver owing to the transmitter/receiver separation. In conventional SS receiver systems, the time delay and frequency offset are determined prior to any further processing. That is, a two-dimensional search in time and frequency is performed to provide the initial estimates of code/frequency offset. The acquisition and tracking unit accomplishes the task of coarse and fine timing and frequency estimation in a SS receiver. The timing offset is determined by correlating the received SS signal with pluralities of locally generated signals having varying start timing (e.g., code offset) and finding the maximum of the output, while the frequency offset is determined by demodulating the received SS signal with pluralities of locally generated intermediate carrier signals to determine the maximum of the output. When the estimates are within the pull-in region, the SS receiver initiates the tracking unit that accurately tracks these parameters in a continuous fashion.
As discussed above, SS transmitters spread the transmit power over a relatively large signal bandwidth and consequently the received signal power is often below the thermal noise floor. Hence, the acquisition and tracking of SS signals, especially in low-transmit power situations, is a difficult task. The acquisition and tracking performance of SS receivers is restricted by such factors as signal attenuation (e.g., indoors), interferences emanating from similar SS receivers and other co-existing narrow and wideband systems, and also intentional interference (e.g., jamming). These restrictions to effective acquisition and tracking can be reduced by increasing the coherent observation period. However, the coherent observation period is also severely limited by factors such as transmitter and receiver oscillator stability, time varying propagation characteristics, transmitter and/or receiver dynamics, and data modulation. Furthermore, increasing the coherent integration period reduces the frequency search bin size, which significantly increases the search space and therefore, search complexity. Moreover, other issues like interference from similar SS receivers operating in the same spectrum can also be a detriment to effective acquisition and tracking.
Such disadvantages of conventional, standard SS receivers, which may cause operational failure in degraded signal environments, may be overcome using high sensitivity (HS) SS receivers equipped with significant signal processing capabilities. Such HSSS receivers are able to acquire and track much weaker SS signals. For example, the HS-GPS receivers may either utilize short coherent integration followed by a large number of noncoherent accumulations or increase coherent integration using the information obtained through dedicated backbone networks. Highly parallel architectures of searching code/frequency offset using massive number of correlators may also be utilized to reduce the mean acquisition time.
Such schemes retain the disadvantage of requiring some type of two-dimensional search in the time/frequency domain.
The invention may be best understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings:
FIGS. 12 (a) through 12 (d) illustrate correlation combining techniques in accordance with alternative embodiments of the invention;
In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail in order not to obscure the understanding of this description.
Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
Moreover, inventive aspects lie in less than all features of a single disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention.
System Overview
As shown in
Multiplier 105 multiplies the incoming complex samples by a complex residual frequency carrier received from the oscillator 109. The output of the multiplier 105 is supplied to the correlator 106. The correlator 106 correlates the complex samples with a locally generated replica of the PRN code obtained from the PRN code generator 108. The output of the correlator 106 is coherently integrated in the integrator 107. The output of the integrator 107 is input to a micro controller 110. The micro controller 110 generates the required information for code/frequency acquisition or tracking including both carrier and code phase information.
The SS receiver operates in two modes namely the acquisition and tracking modes. The ATU 101 initially operates in the acquisition mode where it performs a serial or a parallel search by trying different combinations of residual frequency and code phase until the output of the integrator 107 exceeds a certain predefined threshold level, indicating that a match has been obtained for the particular SS transmitter. For multiple SS transmitters the search is typically performed in a parallel fashion (e.g., GNSS). Generally, during acquisition mode, the PRN code phase is allowed to vary for each residual frequency and is exhausted for other residual frequency offsets. For every combination of PRN code phase and frequency offset the output of integrator 107 is tested in the micro controller 110. Once the threshold is exceeded, the micro controller 110 sets the flag for tracking mode.
In the tracking mode, the ATU 101 operates to continuously update the code phase and residual frequency. Code phase tracking is generally assisted in a well-known manner using early and late PRN code generators respectively, and may also use punctual code generator. The micro controller 110 reduces the phase delay if the received complex samples correlate better with early code and vice versa. Carrier tracking can be accomplished through frequency or phase tracking. The micro controller 110 typically increases the phase or frequency by examining the phase rotation at the output of integrator 107. Additionally, the unit also aids in demodulation of data encoded in the SS transmitter using the punctual code. For longer observation time, the micro controller 110 processes the output from integrator 107 coherently using external aiding information. Alternatively, the micro controller 110 processes the output from integrator 107 noncoherently.
For one embodiment of the invention, the signal processing unit 200 includes one or more complex differential detectors (CDDs) and one or more pre-filtering blocks that effect the conditioning of the complex pseudo baseband samples. For one such embodiment, an initial pre-filter is matched to the spectrum of the incoming signal in order to suppress noise by averaging. That is, since the signal is periodic whereas the noise is aperiodic, the initial pre-filter will enhance the signal (e.g., relative to the noise).
The signal from each CDD is processed in a parallel fashion over the entire bank. The pre-filter bank 203 enhances the signal in a similar fashion as the initial pre-filter 201.
The collective output of the pre-filter bank 203 is fed to the modified correlator bank 204. The modified correlator bank 204 is comprised of individual modified correlators, which obtain the primary PRN code from PRN code generator 108 and perform delay-and-multiply operations similar to the operation performed in the CDD bank 202. The modified correlator bank 204 provides signal correlation to determine timing offset. The collective output of the modified correlator bank 204 is supplied to the integrator bank 205. The integrator bank 205 consists of individual integrator units each of which functions similarly to that of the integrator unit 107 discussed above.
As shown in
Signal Processing Method
For one embodiment of the invention, the pre-filtering operation reduces the bandwidth on the final low pass signal after despreading by a factor of (LTP)−1 Hz. However, the pre-filter has a periodic response (e.g., a comb response) of TP−1 Hz with the bandwidth of (LTP)−1 Hz. For one such embodiment of the invention, the frequency search is incremented in steps that are smaller than (LTP)−1 Hz within ±TP−1 Hz to properly despread the received SS signal.
Referring again to
The individual differential detection delay (i.e. Tm) can either be an integer or fractional delay of the chip duration TC (i.e. Tm=mTC) and could take values larger than the code repetitive period NC. For one embodiment of the invention, the resulting samples are repetitive PRN code with a constant phase offset (except for the data boundaries). That is, the time varying phase caused by the residual frequency offset and data modulation is transformed into a phasor. The phasor or the phase offset at the output of individual differential detectors embodies the time-varying phase over the delay Tm.
Therefore, while the frequency information is lost in individual differential detector outputs, the frequency information is still present across the differential detection outputs. But, the residual frequency carrier is now being sampled at integer or fractional multiples of Tm as opposed to TS, which is the sampling duration.
At operation 515 the residual frequency is estimated by processing the outputs of across each CDD of the CDD bank. That is, when the PRN code is stripped off, the resulting CDD outputs carry only the frequency information.
For one embodiment of the invention, the individual differential detection delay Tm, or integer multiples of it, is set to the code repetitive period NC (i.e. Tm=NC), and the PRN code is stripped off in a differentially coherent fashion. Thus, the subsequent outputs carry only the frequency information, which can be processed to estimate frequency offset that is independent of code estimation. Note that, the transmitted PRN code in the received SS signal is eventually transformed after the differential detection output.
At operation 520 a secondary pre-filtering operation is performed to effect additional delay and sum operations by inputting the individual outputs of the CDD bank to a pre-filter bank.
The ultimate order of the individual pre-filter units in the pre-filter bank may be limited by the code Doppler and second order transmitter/receiver constraints. The individual pre-filter units in the pre-filter bank assume a structure similar to that of the pre-filter of the initial pre-filter operation. The individual gain obtained from the secondary pre-filtering operation in the individual pre-filter units is given by G2=10log10(W), where W is the number of delay and sum or the number of recursive summation operations. For one embodiment of the invention, the individual filter delays may be an integer multiple of code repetitive period NCTC (i.e. TW=wNCTC).
At operation 525 the collective outputs from the pre-filter bank are supplied to the modified correlation bank where a delay and multiply transformation operation is effected on the original PRN code (i.e., the PRN code from the PRN code generator). The transformed PRN code is then supplied to the complex multiplier.
For one embodiment of the invention, as shown in
At operation 530 the collective correlator outputs from the modified correlation bank are supplied to the integrator bank. The individual integrator units in the integrator bank are similar and perform the same function as that of the integrator 107 discussed above in reference to
At operation 535, the collective integrator outputs are then fed to the correlation combiner where they are combined. The combined integrator outputs are then input to the microcontroller as discussed above.
Exemplary Embodiments
As discussed above, the combining of the collective integrator outputs, to suppress noise and other interferences, may be effected in a variety of ways in accordance with various alternative embodiments of the invention. For example, in accordance with various embodiments of the invention, the combining of the integrator outputs may be effected through coherent correlation combining, differential correlation combining, non-coherent correlation combining, and combinations thereof, among other combination techniques
FIGS. 12 (a) through 12 (d) illustrate correlation combining techniques in accordance with alternative embodiments of the invention. For one embodiment of the invention, the individual inputs could be multiplied with a complex residual carrier (as set by the micro controller) and the corresponding outputs could be combined in a coherent fashion.
General Matters
Embodiments of the invention include systems and methods to address various disadvantages in SS receiver systems. Various embodiments of the invention may be combined in a single system to address such disadvantages. One embodiment of the invention provides a SS receiver system having initial and secondary pre-filtering blocks together with a bank of one or more CDDs together with corresponding correlators and correlation combiners.
Alternative embodiments of the invention may effect the combining of the integrator outputs through coherent correlation combining, differential correlation combining, non-coherent correlation combining, and combinations thereof, among other combination techniques
While discussed generally in the context of systems employing particular SS techniques (e.g., DSSS systems), embodiments of the invention are equally applicable to systems employing other SS techniques including, but not limited to, frequency-hopping SS (FHSS), PN spreading, time scrambling, chirp, UWB, and combinations of these techniques.
Embodiments of the invention have been described as including various operations. Many of the processes are described in their most basic form, but operations can be added to or deleted from any of the processes without departing from the scope of the invention.
The operations of the invention may be performed by hardware components or may be embodied in machine-executable instructions, which may be used to cause a general-purpose or special-purpose processor or logic circuits programmed with the instructions to perform the operations. Alternatively, the steps may be performed by a combination of hardware and software. The invention may be provided as a computer program product that may include a machine-readable medium having stored thereon instructions, which may be used to program a computer (or other electronic devices) to perform a process according to the invention. The machine-readable medium may include, but is not limited to, floppy diskettes, optical disks, CD-ROMs, and magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, magnet or optical cards, flash memory, or other type of media/machine-readable medium suitable for storing electronic instructions. Moreover, the invention may also be downloaded as a computer program product, wherein the program may be transferred from a remote computer to a requesting computer by way of data signals embodied in a carrier wave or other propagation medium via a communication cell (e.g., a modem or network connection). All operations may be performed at the same central site or, alternatively, one or more operations may be performed elsewhere.
As discussed above, embodiments of the invention may employ DSPs or devices having digital processing capabilities.
Referring to
Main memory 1304 may be, e.g., a random access memory (RAM) or some other dynamic storage device, for storing information or instructions (program code), which are used by CPU 1302 or signal processor 1303. Main memory 1304 may store temporary variables or other intermediate information during execution of instructions by CPU 1302 or signal processor 1303. Static memory 1306, may be, e.g., a read only memory (ROM) and/or other static storage devices, for storing information or instructions, which may also be used by CPU 1302 or signal processor 1303. Mass storage device 1307 may be, e.g., a hard or floppy disk drive or optical disk drive, for storing information or instructions for processing system 1300.
While the invention has been described in terms of several embodiments, those skilled in the art will recognize that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.
This application is a non-provisional application claiming priority to U.S. Provisional Application Ser. No. 60/716,530, filed on Sep. 13, 2005, entitled “Differential Signal Processing Schemes for Enhanced GPS Acquisition,” which is herein incorporated by reference in its entirety.
Number | Date | Country | |
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60716530 | Sep 2005 | US |