Patent Application
20080159198
The present invention relates to processing of binary offset carrier (BOC) modulated signals (simply referred to as BOC signal hereinafter), more particularly, to a method and apparatus for processing BOC signals in acquisition and tracking modes of a satellite navigation receiver.
Nowadays, more than one Global Navigation Satellite System (GNSS) is available. A receiver supporting multi-specification LBS (location based service), wireless multimedia communication and broadcasting signals is becoming an expectation. Take multi-specification LBS as an example, such a receiver able to support multi-mode receiving for GNSS signals can enhance locating precision and access to more services. Among the GNSS systems, different signal frequency bands support different services. As more and more bands need to be supported, band overlapping occurs.
GPS is the U.S. navigation satellite system, which is a network of satellites continuously transmits high-frequency radio signals. The signals carry time and distance information that is receivable by a GPS receiver, so that a user can pinpoint the position thereof on the earth. Galileo, the emerging European satellite navigation system, offers higher signal power and more robust modulation that will enable users to receive weak signals even in difficult environments. When combined, Galileo and GPS will offer twice the number of satellite sources as currently available. This provides redundancy as well as greater availability for the user. The combination of GPS and Galileo basically has four bands, excluding SAR (Safe and Rescue) service. GPS and Galileo systems share some signal bands. That is, GPS and Galileo share some central frequencies and send signals on the same ones of carriers. For example, GPS L1 and Galileo E2-L1-E1 share the same band. To reduce inter-system and intra-system interference, specific modulation schemes are required. Binary offset carrier modulation (hereinafter simply referred to as “BOC”) is a widely used method.
The BOC modulation is done by multiplying a pseudo-random noise (PRN) spreading coded signal (simply referred to as PRN coded signal hereinafter) with a square wave subcarrier (SC). The SC has a frequency which is multiple of the code rate of the PRN spreading code.
The BOC signal has a symmetric split spectrum with two main lobes shifted from the center frequency by the frequency of the subcarrier. The characteristics of the BOC signal are dependent on the spreading code chip rate, the subcarrier frequency, and the subcarrier phasing within one PRN code chip. The common notation for a BOC-modulated signals in the GNSS field is represented as BOC(fc, fs), where fc is the code chip rate, and fs is the frequency of the subcarrier. Both fc and fs are usually represented as a multiple of the reference frequency 1.023 MHz. Therefore, the BOC signal can also be represented as BOC(n,m), where n is the multiple of 1.023 MHz; for the PRN code chip rate fc, and m is the multiple of 1.023 MHz for the subcarrier fs.
For satellite signal navigation, the BOC signal is preferably applied in tracking under white noises. Such scheme provides better inherent multipath mitigation compared to the spreading code alone. However, BOC scheme makes acquisition and tracking more difficult due to a multiple peak autocorrelation phenomenon. The presence of the subcarrier in the BOC signal introduces secondary peaks in a range of −1/+1 chip in BOC autocorrelation.
In addition, the width of the main lobe (main peak) of the BOC correlation result influences the performance of the receiver in acquisition and tracking. If the main lobe is narrow, it is good for tracking and position because a more accurate code phase can be tracked. However, a narrow main lobe makes it difficult to acquire the signal because the narrow correlation function leads to a finer code phase searching space, which needs longer acquisition time.
An objective of the present invention is to provide a BOC signal acquisition and tracking apparatus. The apparatus comprises a carrier unit generating a carrier; a code unit generating a plurality of subcarriers including at least a BOC subcarrier, a BOC-cos subcarrier, for example, as well as a PRN code, and outputting the BOC subcarrier as well as one of BOC-cos subcarrier and the PRN code; and a code delay estimator receiving a signal, removing a carrier component from said signal by using said carrier from the carrier unit, generating a BOC signal for the received signal by using the BOC subcarrier and generating a BOC-cos signal or a PRN coded signal for the signal by using one of the BOC-cos subcarrier and the PRN code, calculating an autocorrelation of the BOC signal and a cross-correlation of the BOC signal and one of the BOC-cos signal and the PRN coded signal, and combining said autocorrelation and said cross-correlation to generate a combined correlation. The apparatus has a controller controlling the carrier unit, the code unit and the code estimator. The controller controls the code unit to output the BOC-cos subcarrier or the PRN code. For example, in signal acquisition mode, the controller controls the code unit to output the BOC subcarrier and BOC-cos subcarrier; while in signal tracking mode, the controller controls the code unit to output the BOC subcarrier and the PRN code. Accordingly, the code estimator can properly generate a combined correlation based on built-in algorithms.
Another objective of the present invention is to provide a BOC signal acquisition and tracking method. The method comprising receiving a signal; generating a carrier; generating subcarriers including at least a BOC subcarrier, a BOC-cos subcarrier, for example, as well as a PRN code; outputting the BOC subcarrier and selecting to output one of the BOC subcarrier and said PRN code; removing a carrier component from the received signal by using said carrier; generating a BOC signal for the signal by using the BOC subcarrier; generating one of a BOC-cos signal and a PRN coded signal for the signal by using one of the BOC-cos subcarrier and the PRN code; calculating an autocorrelation of the BOC signal; calculating a cross-correlation of the BOC signal and one of the BOC-cos signal and the PRN code signal; and combining the autocorrelation and the cross-correlation to generate a combined correlation. For example, in signal acquisition mode, the BOC subcarrier and BOC-cos subcarrier are output; while in signal tracking mode, the BOC subcarrier and the PRN code are output. Accordingly, the combined correlation can be properly generated based on built-in algorithms.
The present invention will be further described in details in conjunction with the accompanying drawings.
As mentioned above, secondary peaks of a BOC signal due to a subcarrier is undesirable. A method to remove the secondary peaks is a combined correlation function. As shown in
R
combi
=R
2
BOC/BOC(τ)−α×R2BOC/PRN(τ) (1)
where τ is code delay in chips, and α is a variable coefficient. In
To rapidly acquire a signal, it is preferred that the correlation curve has a wide main lobe. A method to obtain a wide correlation function while reduce side peaks is combining BOC autocorrelation and cross-correlation of BOC and BOC-cos (QBOC).
R
combi
=R
2
BOC/BOC(τ)+β×R2BOC/QBOC(τ) (2)
where τ is code delay in chips, and β is a variable coefficient. In
It is noted that in either correlation combination, the coefficient (α, β) is variable as desired.
The BOC subcarrier is provided to the mixers 202 and 206, so that BOC signal is generated in I and Q channels. The selected output from the multiplexer 254 is fed to the mixers 204 and 208. When the selected output is the BOC-cos subcarrier, a BOC-cos (QBOC) signal is generated. When the selected output is the PRN code, a PRN coded signal is generated. The outputs of the mixers 202, 204, 206, 208, which are referred to as code mixers, are fed into integration and dump units 302, 304, 306, 308, respectively, to be integrated and dumped. Then the integrated results from the integration and dump units 302, 304, 306, 308 are fed to a combination unit 40. The combination unit 40 synthesizes the integration results by combining the integration results from units 302 and 304 to obtain combined correlation in I channel and combining the integration results from units 306 and 308 to obtain combined correlation in Q channel. The combination unit 40 combines the integration results based on the equations (1) or (2). In addition, the coefficient α or β is determined in the combination unit 40 in the present embodiment. However, the coefficient α or β can also be externally provided to the combination unit 40.
An output of the combination unit 40 is fed to a discriminator 50, which outputs a tracking error from the received correlation to feed back to the carrier unit 10 and code unit 20, so that these units can execute proper adjustments. The mixers 102, 104, 202, 204, 206, 208, integration and dump units 302, 304, 306, 308, combination unit 40 and discriminator 50 compose a code delay estimator 100.
The apparatus in accordance with the present invention further has a controller 60. The controller 60 controls the carrier NCO 12, the code NCO 22 and the multiplexer 254. For example, the controller 60 controls the multiplexer 254 to output the BOC-cos signal in signal acquisition mode while controls the multiplexer 254 to output the PRN code in signal tracking mode. The controller 60 can receive an external command and controls the respective units accordingly. In another embodiment, the coefficient α or β used in the combination unit 40 is determined by the controller 60.
Although the BOC-sine signal (BOC signal), the BOC-cos signal (QBOC signal) as well as PRN code are described in the embodiment, other signals combination, such as a BOC signal with a BOC harmonic signal and/or BOC-cos harmonic signal thereof, can be used. Here, the so called BOC harmonic signal indicates BOC of a multiple of fs, For example, a double frequency harmonic subcarrier of the BOC subcarrier is represented as BOC-sin(2fs), and a double frequency harmonic subcarrier of the BOC-cos subcarrier is represented as BOC-cos(2fs). The rest can be deduced accordingly.
In addition, in the above embodiment, the code unit 20 outputs PRN code, BOC subcarrier and BOC-cos subcarrier. However, more than the PRN code and the above two subcarriers can be generated and output by the code unit 20, such as harmonic of the BOC and BOC-cos subcarriers. The multiplexer 254 can outputs selected one or more among the PRN code and a plurality of subcarriers under the control of controller 60.
While the preferred embodiment of the present invention has been illustrated and described in details, various modifications and alterations can be made by persons skilled in this art. The embodiment of the present invention is therefore described in an illustrative but not in a restrictive sense. It is intended that the present invention should not be limited to the particular forms as illustrated, and that all modifications and alterations which maintain the spirit and realm of the present invention are within the scope as defined in the appended claims.
