Patent Description:
To avoid interfering with current existing navigation signals, new signals have been designed with their frequencies offset from the carrier frequency. These signals have been designed to reduce the bit error rate for communication signals and increase accuracy and robustness for navigation signals.

These multi-carrier signals offer increased tracking performance and robustness for navigation signals and a reduction in bit error rate due to the diversity of frequencies over a wide bandwidth.

The main drawback of these signals is the acquisition process. Before starting the tracking of any signal and/or demodulating any data, the signal needs to be time synchronized and frequency Doppler synchronized. This process is rendered particularly difficult due to the large bandwidth that the signals cover.

A combination of coherent and non-coherent signal acquisition is already known to limit the complexity of wide band signals and can be applied to the acquisition of multi-carrier signals.

A method of receiving multiple signals is disclosed, for example, in <CIT> which generally regards a cell search in a wireless communication network.

The signal goes through a front end which converts the signal from an analogue signal to a digital signal. The wide-band signal is divided into P sub bands, each sub-band comprising one or more sub-carriers and each sub-carrier accommodating a respective band of the N narrow-bands. For each sub-band, the one or more sub-carrier cross correlation functions for that sub-band creating a sub-band cross-correlation is coherently summed. The sub-band cross-correlation functions of all of the P sub-bands to produce a cross-correlation function for the wide-band signal is non-coherently summed.

This layout can however lead to major performance losses in the acquisition process, leading to greatly deteriorated signal output and a higher receiver complexity.

There is therefore a need for improvement in the field of acquiring of multiple signals, in particular in which the losses during the acquisition process are greatly reduced, in which the power consumption is reduced and in which there is a simplification of the method of acquisition.

Prior art can be found in <NPL>", in <NPL>", in <NPL>", and in <NPL>.

Specific embodiments of the invention are outlined in the dependent claims.

According to a first aspect, a method for the acquisition of symmetric signals is described. The method comprises receiving one or more signal pairs symmetric about a carrier frequency. The method further comprises isolating the two signals of each of the one or more signal pairs from each other. The method further comprises modulating the two signals of each of the one or more signal pairs so that the two signals are modulated to have a same frequency. The method further comprises combining the two modulated same frequency signals of each of the one or more signal pairs to generate one uniform signal. The method further comprises implementing a Code Doppler search on the uniform signal to adjust the uniform signal. The method further comprises correlation of the adjusted uniform signal with a local replica of the expected signal. The method further comprises outputting the adjusted uniform signal.

The term modulated same frequency signals may be understood as the modulated signals having a same frequency.

The receiving of the one or more signal pairs symmetric about a carrier frequency may allow for an increase in the power and strength of the incoming signal. This may mean that the device that uses this method of acquiring symmetric signals may receive stronger, clearer signals than a conventional method of acquiring signals. A better acquisition of symmetric signal pairs may lead to a better synchronization of the symmetric signal pairs and thus, a lower bit error rate The modulation of the two signals of each of the one or more signal pairs so that the two signals are modulated to be at a same frequency may allow for a reduction in processing power. The reduction in processing power may be due to the following steps requiring the processing of one signal at a single frequency as opposed to two or more signals at different frequencies as seen in orthogonal frequency-division multiplexing.

The modulation may also reduce the complexity of the method of acquiring symmetric signals. This may be due to the signal frequency offset changes over time, as seen in orthogonal frequency-division multiplexing within <NUM> or <NUM> environments for example, not being needed to be known prior to the receipt of the signals. As a priori knowledge of the signal frequency offset sequence may not be needed, it may mean that the signal frequency offset is able to change over time without effecting the processing of the symmetric signal pairs.

The combining of the two modulated same frequency signals of each of the one or more signal pairs to generate one uniform signal may result in an increase in the power and strength of the signal being processed. This may result in a lower error rate in the Code Doppler search.

It also may reduce the power consumption of the device that uses this method. This may be due to the number of code chip hypotheses being limited to the chip rate of a single modulation as opposed to the full bandwidth of the received signal.

The implementation of a Code Doppler search on the uniform signal to adjust the uniform signal may allow for the device that uses this method to locate the correct signal pairs. The search may ensure that the correct information is output to the device.

The correlation of the adjusted uniform signal with a local replica of the expected signal may allow the device using this method to determine if the received signal pair is the correct signal pair.

In some examples, there is a plurality of signal pairs. This may allow for a great increase in the power and strength of the received signal. This in turn may lead to a lower bit error rate due to a better acquisition of signal pairs and thus, a more accurate receipt of information by the device.

In some examples, the plurality of received signal pairs are symmetric about a plurality of carrier frequencies. This may lead to a lower bit error rate due to a better acquisition of signal pairs and thus, a better synchronization of signals.

In some examples, the plurality of received signal pairs are symmetric about the same carrier frequency. This may allow for an increase in the power and strength of the incoming signal. This may mean that the device that uses this method of acquiring symmetric signals may receive stronger, clearer signals than a conventional method of acquiring signals. This in turn may lead to a reduction in the bit rate error of the signal which is output by this method. It also may allow for an increase in the amount of data and information to be transmitted to the device that uses this method as at least two full strength signals may be used. It may also allow for a reduction in the complexity of the processing of the signal pairs as only a single frequency may be needed to be processed.

In some examples, the two signals in each of the one or more signal pairs are modulated to have the same frequency, wherein the same frequency is the carrier frequency. This may lead to a reduction in the complexity of the device that uses method. This may be due to the device using this method not needed any a priori knowledge of the signal frequency offset varying over time and thus, no need for a frequency offset search.

It may also allow for a reduction in power consumption of the device that uses this method. This may be due to the code chip hypothesis being limited to the chip rate of a single modulation as opposed to the full bandwidth of the transmitted signal.

In some examples, the two signals in each of the one or more signal pairs are isolated from each other by a high pass filter and a low pass filter. This may reduce the complexity of isolating signals as only two relatively simple components are needed. It also may mean that the signal frequency offset is not needed to be known ahead of time. This may be due to the same frequency being known ahead of time and so the filters only need to filter frequencies outside the same frequency and the signals across the entire bandwidth may not matter.

Alternatively, the two signals in each of the one or more signal pairs may be isolated from each other by a down converter and an up converter.

In some examples, the Code Doppler search is implemented via an active search and/or a passive search and/or a transform search. These search methods based on a single code chip hypothesis may result in a reduction in power consumption in the processing of the uniform signal when compared to known methods of acquisition of symmetric signals. The reduction in power consumption may be due to the search being conducted in only two dimensions as opposed to three dimensions as seen in conventional methods for acquisition.

In some examples, a 3D cross-correlation function product is derived from the adjusted uniform signal and used to correlate the adjusted uniform signal with a local replica of the expected signal. This may allow the device that uses this method to recognize if the signal being received is the correct signal. The device may then recognize that the data being received is the correct data via, for example, a decision statistic and use it in the application that the device is designed for.

In some examples, the one or more 3D cross-correlation function products derived from the adjusted uniform signal are added coherently or non-coherently. This may allow the device that uses the method to receive one or more signal pairs that have equal or unequal phase offsets. This may mean that the signal frequency offset is not needed to be known prior to the receipt of the one or more signal pairs. This may increase the versatility of the device and may allow the device to acquire any number of signal pairs.

According to a second aspect, an apparatus for the acquisition of symmetric signals is described. The apparatus comprises a receiving unit and a processing unit. The receiving unit is configured to receive one or more signal pairs symmetric about a carrier frequency. The processing unit is configured to isolates the two signals of each of the one or more signal pairs from each other. The processing unit is further configured to modulate the two signals of each of the one or more signal pairs so that the two signals are modulated to have a same frequency. The processing unit is further configured to combine the two modulated same frequency signals to generate one uniform signal. The processing unit is further configured to implement a Code Doppler search on the uniform signal to adjust the uniform signal. The processing unit is further configured to correlate the adjusted uniform signal with a local replica of the expected combined reference signal. The processing unit is further configured to output the adjusted uniform signal.

For example, the adjusted uniform signal may be output to a device the apparatus is coupled to. In order to physically receive the one or more signal pairs, the apparatus may comprise an antenna. The antenna may be part of the receiving unit. In other words, the receiving unit may include the antenna.

In some examples, the processing unit may be configured to isolate the two signals of each of the one or more signal pairs by a high pass filter and a low pass filter. In these examples, the processing unit may comprise a high pass filter and a low pass filter. Alternatively or additionally, the processing unit may be configured to isolate the two signals of each of the one or more signal pairs by a down converter and an up converter. In these examples, the processing unit may comprise a down converter and an up converter.

In some examples, the modulation of the two signals of each of the one or more signal pairs results in the two signals to be modulated to have the same frequency, wherein the same frequency is the carrier frequency.

In some examples, the Code Doppler search is an active search and/or a passive search and/or a transform search.

In some examples, a 3D cross-correlation function product is derived from the adjusted uniform signal and used to correlate the adjusted uniform signal with a local replica of the expected signal
The advantages of these parts, steps and processes have been discussed above.

In some examples, the timing of each signal in a signal pair is the same. In other words, the first signal in a signal pair has a signal frequency offset symmetrical to the second signal in a signal pair as the two signals of the signal pair arrive at the device or receiving unit at the same time. This may allow for the difficulty of the receipt and processing of the signals to be greatly reduced as there is no need for extra components to adjust the signal frequency offset of each symmetric signal pair. This in turn may lead to a reduction in power consumption of the device that uses this method or of the unit for receiving and processing signals symmetric about the carrier frequency.

If the two signals of each signal pair arrive at the device or receiving unit at the same time, there may be no need for a priori knowledge of the signal frequency offset sequence versus time of each signal and therefore, any number of symmetric signal pairs about the same carrier frequency can be synchronized at the same time. This in turn may allow the signal frequency offset sequence to change over time and as no a priori knowledge of the signal frequency offset sequence of each signal may be needed.

In some examples, the signals that are isolated from each other are base band signals. The received signals may be changed to baseband signals via an orthogonal frequency-division multiplexing receiver. This receiver may comprise one or more of a band pass filter, an amplifier, an automatic gain control, and an analog to digital converter. This may allow for the power needed to process the signal pair to be reduced and simplify the process of acquisition and/or the processing unit. For example, the apparatus may comprise the orthogonal frequency-division multiplexing receiver. The orthogonal frequency-division multiplexing receiver may be part of the receiving unit. In other words, the receiving unit may include the orthogonal frequency-division multiplexing receiver.

It is clear to a person skilled in the art that the statements set forth herein may be implemented under use of hardware circuits, software means, or a combination thereof. The software means can be related to programmed microprocessors or a general computer, an ASIC (Application Specific Integrated Circuit) and/or DSPs (Digital Signal Processors). For example, the processing unit may be implemented at least partially as a computer, a logical circuit, an FPGA (Field Programmable Gate Array), a processor (for example, a microprocessor, microcontroller (µC) or an array processor)/a core/a CPU (Central Processing Unit), an FPU (Floating Point Unit), NPU (Numeric Processing Unit), an ALU (Arithmetic Logical Unit), a Coprocessor (further microprocessor for supporting a main processor (CPU)), a GPGPU (General Purpose Computation on Graphics Processing Unit), a multi-core processor (for parallel computing, such as simultaneously performing arithmetic operations on multiple main processor(s) and/or graphical processor(s)) or a DSP.

It is further clear to the person skilled in the art that even if the herein-described details will be described in terms of a method, these details may also be implemented or realized in a suitable device, a computer processor or a memory connected to a processor, wherein the memory can be provided with one or more programs that perform the method, when executed by the processor. Therefore, methods like swapping and paging can be deployed.

Even if some of the aspects described above have been described in reference to the apparatus, these aspects may also apply to the method and vice versa.

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures, wherein like reference numerals refer to like parts, and in which:.

<FIG> shows a frequency-time graph of two symmetric signal pairs transmitted to a receiver according to an embodiment as described herein.

In the embodiment of <FIG>, there are two signal pairs <NUM>, <NUM> illustrated however it will be appreciated by the skilled person that this method <NUM> for acquiring symmetric signals <NUM>, and the unit <NUM> used to receive and process these signal pairs <NUM>, <NUM>, could be used for any number of signal pairs <NUM>, <NUM>.

The graph depicts a first signal pair <NUM> represented by a white circle <NUM> and a black circle <NUM>, and a second signal pair <NUM> represented by a light grey circle <NUM> and a dark gray circle <NUM>. As can be seen, each pair <NUM>, <NUM> is symmetric about the carrier frequency <NUM> fc. That is to say, a first signal <NUM>, <NUM> of a signal pair <NUM>, <NUM> will have a frequency of -f<NUM> and a second signal <NUM>, <NUM> of a signal pair <NUM>, <NUM> will have the frequency f<NUM>. Each signal may have any frequency as long as the other signal of the signal pair <NUM>, <NUM> is symmetric to it about the carrier frequency <NUM>.

In this embodiment, the first signal <NUM>, <NUM> and the second signal <NUM>, <NUM> of each signal pair <NUM>, <NUM> are received at the same time <NUM> ti. The signals of the signal pair <NUM>, <NUM> arrive at the device <NUM> or unit <NUM> at same time <NUM> so that they can be processed at the same time and thus combined. If the signals <NUM>, <NUM> do not arrive at the same time, a method of rectifying the phase of each signal <NUM>, <NUM> may be needed. This may greatly increase the power consumption and cost of the device <NUM> which uses the method <NUM> and/or the unit <NUM> that is used to receive and process the symmetrical signals <NUM>. The first <NUM>, <NUM> and second <NUM>, <NUM> signals of each signal pair <NUM>, <NUM> may arrive at any time as long as they arrive same time.

In this embodiment, each signal at each epoch <NUM> ti is modulated by a chip <NUM> that, for example, may be overlaid with a data symbol or represents a pure pilot signal. In some examples, this may be a chip multi-processor. Alternatively, it may be any type of chip, processor or electronic device which has the ability to modulate each signal.

<FIG> shows a block diagram of a receiver receiving at least one signal pair, e.g. the signal pairs of <FIG>, from a transmitter and processed by a down conversion with appropriate filtering and an up conversion with appropriate filtering, and a transform Code Doppler search according to some example implementations as described herein.

In the embodiment of <FIG>, the signal pair <NUM> is received <NUM> by an antenna <NUM> however they may be received by any method that allows signals to be received. The code rate <NUM> of the two symmetric signals <NUM>, <NUM> are considered to be the same in this embodiment however, they can use different chipping rates. The only constraint is that each signal is symmetric with another signal in each time epoch <NUM> ti.

In this embodiment, the signal pair <NUM> is converted to a base band signal <NUM> in order to ease the processing of the signal pair <NUM>. This is done via a digital front-end <NUM>.

The digital front end <NUM> is not shown in detail but may comprise one or more band pass filters, and amplifier, an automatic gain control and an analog to digital converter. The signal pair <NUM> is merged together at the antenna <NUM> to create one wide frequency band signal <NUM>. This signal <NUM> may be passed through a band pass filter in order to eliminate the fringe frequencies and narrow the signal <NUM>. This signal <NUM> is then passed through a low noise amplifier to boost the signal <NUM> and then passed through a second band pass filter to further eliminate fringe frequencies and interference. The signal <NUM> then passes through an automatic gain control to increase the dynamic range and control the quantization levels. This signal <NUM> is then passed through an analog to digital converter which converts the signal <NUM> to a base band signal <NUM> which can then be processed.

After being processed by the digital front end <NUM> and being converted to a base band signal <NUM>, the generic expression of the modulated symmetric signal pair <NUM> can be expressed as: <MAT> <MAT>.

Where the symbols have the following definitions:.

Where a first signal <NUM> of the signal pair S<NUM>,Rec has a positive frequency +Δfm with respect to the carrier frequency and the second signal <NUM> of the signal pair S<NUM>,Rec has a negative frequency -Δfm with respect to the carrier frequency <NUM>.

This isolation <NUM> of the two signals after the conversion to a base band signal <NUM> is achieved via the isolation of the first <NUM> and second <NUM> signals from each other.

In this embodiment, this isolation <NUM> and modulation <NUM> of the two signals <NUM>, <NUM> is achieved via an up converter <NUM> and a down converter <NUM>. Alternatively, this isolation may be achieved by a high band pass filter <NUM> and a low band pass filter <NUM> or any other method of separating the signals <NUM>, <NUM> of one or more signal pairs <NUM>, <NUM>.

The up converter <NUM> comprises in this embodiment, a digital up converter and a high band pass filter. The digital up converter converts a digital baseband signal <NUM> into a higher frequency signal in an intermediate frequency position. The intermediate frequency signal is then passed through a high band pass filter only the signals with the frequency below the carrier frequency <NUM> fc remain. As this signal comprises frequencies below the carrier frequency <NUM>, this signal can be expressed as SN <NUM>.

The down converter <NUM> comprises in this embodiment, a digital down converter and a low band pass filter. The digital down converter works in a similar way to the digital up converter by converting a digital baseband signal <NUM> into a lower frequency signal and intermediate frequency position. This intermediate frequency signal is then passed through a low band pass filter so only the signals with a frequency of the carrier frequency <NUM> fc remain. As this signal comprises frequencies above the carrier frequency <NUM>, this signal can be expressed as SP <NUM>.

In this embodiment, there are only two signals <NUM>, <NUM> symmetric to each other. The two signals can be generalized as negative <NUM> and positive <NUM> depending on Δfm. The negative signal <NUM> can be expressed as: <MAT>.

And the positive signal <NUM> can be expressed as: <MAT>.

As a result, when these two signals <NUM>, <NUM> are combined, it can be expressed as: <MAT> <MAT>.

The acquisition will be applied to the product signal SNP <NUM>. The two signals of the signal pair <NUM>, <NUM> are now modulated to be at the same frequency <NUM>, the carrier frequency <NUM> in this embodiment, and have been combined <NUM> to generate a uniform signal <NUM> that is double the power of the individual signal <NUM>, <NUM> received by the unit <NUM> or device <NUM> which uses the method <NUM>.

As shown in equation <NUM>, Δfm and -Δfm have canceled each other out. Consequently, only the classical two dimensional search remains. This two dimensional search is the Code Doppler search <NUM>.

The Code Doppler search <NUM> is implemented <NUM> in order to determine if the received signal <NUM> is the signal <NUM> intended for the device <NUM> that uses this method <NUM> or the device <NUM> that the unit <NUM> is coupled to. Firstly, the uniform signal <NUM> is rotated by the estimated amount of carrier phase. This accounts for the observed Doppler frequency, increasing the reliability of the search <NUM>. A local Pseudo Random Noise (PRN) code is generated based on the current estimate of the code delay and the Doppler frequency. This local code is correlated <NUM> with the PRN code of the incoming uniform signal <NUM> and then it is determined if the incoming uniform signal <NUM> is indeed the signal that the device <NUM> is searching for. The code delay and the carrier phase of the incoming signal <NUM> are continuously updated by a series of tracking loops to ensure accuracy of the acquisition method <NUM>.

In this embodiment, the Code Doppler search <NUM> is a transform search <NUM>. The transform search <NUM> uses the properties of the Fast Fourier Transform (FFT) to correct and correlate the uniform signal <NUM> with the locally generated PRN sequence.

The Code Doppler search <NUM> may also be a passive search <NUM> or an active search <NUM>. In the passive search <NUM>, the uniform signal <NUM> is passed through a delay line whose tap values equal the samples of the PRN sequence to be correlated. In this search <NUM> however, the PRN sequence and the uniform signal <NUM> is correlated before being filter matched with the waveform. In this case, the correlation process <NUM> runs quicker with the rate being the sampling rate. In this search method <NUM>, the sampling rate can be expressed as fs = <NUM>/Ts where Ts is the sample step.

The active search method <NUM> is also known as a "correlator receiver" where the generator which generates the local PRN code is slewed for each new Code Doppler hypothesis. The correlation <NUM> then takes place between the incoming signal <NUM> and the modified PRN sequence which is modulated with the pulse waveform and the Doppler rotation. The correlation <NUM> runs at a rate proportional to the integration time equivalent to the minimum data period.

If the two signals <NUM>, <NUM> have no data and are pilot signals, there is no restriction on the coherent integration time for the correlation process <NUM>. If the data rates of the two signals <NUM>, <NUM> of each signal pair <NUM> are equal over time and the data transmitted on the two signals <NUM>, <NUM> of each signal pair <NUM> are equal over time, there is also no restriction on the coherent integration time for the correlation process <NUM>.

If the data rates of the two signals <NUM>, <NUM> of each signal pair <NUM> are equal over time and the data transmitted on the two signals <NUM>,<NUM> of each signal pair <NUM> are potentially different over time, the maximum coherent integration time for the correlation process <NUM> can be expressed as: <MAT> where data_rate is the data rate of S<NUM>,Rec and S<NUM>,Rec.

Alternatively, two replica hypotheses SNP <NUM> can be tested at each Tint to extend the coherent integration time. An extended coherent integration time may improve the sensitivity to the acquisition in harsh environment.

If the two data rates are not equal over time, the coherent acquisition may only be done during the integration time equivalent to the minimum data period: <MAT>.

Where data_rate_1 is the data rate of S<NUM>,Rec and data_rate_2 is the data rate of S<NUM>,Rec.

Alternatively, for each data rate modulated in a symmetric signal pair <NUM>, two replica hypotheses SNP <NUM> can be tested at each Tint to extend the coherent integration time. An extended coherent integration time may allow for the acquisition of be more accurate.

When the device <NUM> using the acquisition method <NUM> is trying to acquire the signal <NUM>, <NUM>, the device <NUM> looks for all possible combinations of code delay and Doppler frequencies by generating a local set of possible replica signals <NUM> and correlating each of them with the incoming signal <NUM>. The result of the correlation output is a measure of the accuracy of the carrier phase and code delay estimates.

The reference signal replica <NUM> for the acquisition of the signal pair <NUM>, <NUM> uses a method of combining the pulse shape and the PRN codes of the two signals <NUM>, <NUM> of the signal pair <NUM>. For the pulse shape, the reference signal uses p<NUM>(t)p<NUM>(t) and for the PRN code, the reference signal uses the PRN of S<NUM>,Rec multiplied by the PRN of S<NUM>,Rec.

The normalized 3D plot <NUM> of the correlation process <NUM> and thus, the test acquisition criteria can be expressed as a function of code delay misalignment error and frequency Doppler misalignment error: <MAT> Where the symbols have the following definitions:.

If the errors are low, it may then signify that the acquired signal pair <NUM> is close to the locally generated expected signal <NUM> and such, may be the signal pair the unit <NUM> or device <NUM> is looking for.

This 3D plot <NUM> is then assessed using a decision statistic <NUM>. This decision statistic <NUM> can be expressed as: <MAT>.

Where M is the number of non-coherent integrations, Y is the output of the 3D correlation function <NUM> of the incoming uniform signal <NUM> with the local replica <NUM> and k refers to the kth coherent integration interval.

This decision statistic z <NUM> is then output and compared to a detection threshold <NUM> predetermined by the device <NUM> or by the unit <NUM> coupled to the device <NUM> to assess whether the signal is present or if the signal is the correct signal. This detection threshold <NUM> is defined according to a target probability of false alarm. The adjusted uniform signal <NUM> is output <NUM> to the device <NUM> only if the decision statistic <NUM> is greater than the detection threshold <NUM>.

The correlations are integrated over time before further processing. There are two main types of integration that can be considered and these are discussed further below.

<FIG> shows a block diagram of a receiver receiving two signal pairs from a transmitter where the phase offset of the two pairs are unequal and where the signal pairs are processed by a down conversion with appropriate filtering and an up conversion with appropriate filtering, and a transform Code Doppler search according to some example implementations as described herein.

In the embodiment of <FIG>, there are two signal pairs <NUM>, <NUM>. However, it will be appreciated by the skilled person that this method can be used for any number of signal pairs symmetric about a carrier frequency.

In this embodiment, the phase offset of the first signal pair <NUM> is different to the phase offset of the second signal pair <NUM>, i.e. <MAT>.

Where ϕ is the phase offset and the subscript indicates which signal the phase offset belongs to.

As the phase offset of the two signal pairs <NUM>, <NUM> are unequal, the 3D CCF <NUM> of the two products of the two signal pairs <NUM>, <NUM>3DCCFp1p2 and 3DCCFp3p4 must be added non-coherently <NUM>. A non-coherent integration <NUM> consists of adding the results of the correlations <NUM> together before entering the decision process. This can be seen in <FIG> as the decision statistic <NUM> summation occurs before the combination of the two signal pairs <NUM>, <NUM>.

<FIG> shows a block diagram of a receiver receiving two signal pairs, e.g. the signal pairs of <FIG>, from a transmitter where the phase offset of the two pairs are equal and where the two signal pairs are processed by a down conversion with appropriate filtering and an up conversion with appropriate filtering, and a transform Code Doppler search according to some example implementations as described herein.

In the embodiment of <FIG>, there are two signal pairs <NUM>, <NUM>. However, it will be appreciated by the skilled person that this method can be used for any number of signal pairs symmetric about the carrier frequency.

In this embodiment, the phase offset of the two signal pairs <NUM>, <NUM> are equal i.e. <MAT>.

As the phase offset of the two signal pairs <NUM>, <NUM> are equal, the 3D CCF <NUM> of the two products of the two signal pairs <NUM>, <NUM>3DCCFp1p2 and 3DCCFp3p4 can be added coherently <NUM>. A coherent integration <NUM> consists of using longer integration times, such as multiples of the code length, before the correlation output. This can be seen in <FIG> as the decision statistic <NUM> summation occurs after the combination of the two signal pairs <NUM>, <NUM>.

A coherent integration <NUM> may be preferable to a non-coherent integration <NUM> as there are fewer squaring losses in a coherent integration <NUM> however, coherent integration <NUM> may be limited by the bit boundaries that carry the data of the incoming signal pair.

Referring back to <FIG>, the shape of the signal <NUM> after the digital front end <NUM> is symmetric in nature and comprises two vertical drops in power symmetric about the carrier frequency <NUM>. This is due to the symmetric nature of the two signals <NUM>, <NUM> in the signal pair <NUM> and the band pass filters in the digital front end <NUM> which eliminate the fringe frequencies. In these examples, the signal <NUM> after the digital front end <NUM> is limited to ±<NUM> with respect to the carrier frequency <NUM>.

The base band signal <NUM> then passes through an Rx filter which is the up converter <NUM> with the appropriate band pass filter and the down converter <NUM> with the appropriate band pass filter. In these examples, the signals <NUM>, <NUM> after the up <NUM> and down <NUM> converters are ±<NUM> with respect to the carrier frequency <NUM> and the two signals are almost identical to each other. These two signals are then combined <NUM> and processed by the Code Doppler search <NUM>. It will be appreciated by the skilled person that the bandwidth of the signal <NUM> and the bandwidth of the two modulated signals <NUM>, <NUM> may be different to those mentioned in this disclosure.

<FIG> shows a block diagram of a real world embodiment of a signal pair being received by the receiver where the signal pair is processed by a down conversion with appropriate filtering and an up conversion with appropriate filtering, and a transform Code Doppler search according to some example implementations as described herein.

In the embodiment of <FIG>, the signal is a Galileo E5 AltBOC signal transmitted over a <NUM> f<NUM> Tx bandwidth.

It can be seen in <FIG> that the shape of the signal <NUM> after the digital front end <NUM> is symmetric in nature and comprise two vertical drops in power symmetric about the carrier frequency <NUM>. This is due to the symmetric nature of the two signals <NUM>, <NUM> in the signal pair <NUM> and the band pass filters in the digital front end <NUM> which eliminate the fringe frequencies.

The base band signal <NUM> then passes through an Rx filter which is the up converter <NUM> with the appropriate band pass filter and the down converter <NUM> with the appropriate band pass filter. It can be seen, for example, in the branch comprising the SN(t) signal <NUM> that the signal that was received from the Tx filter was shifted upward by 15fo. That is, the frequency was increased and that the digital band pass filtering restricts the signal to 25f<NUM> bandwidth i.e. ±<NUM> f<NUM> with respect to the carrier frequency <NUM>. It can be seen that this signal is identical to the signal output <NUM> of the other branch. These two signals are then combined <NUM> and processed by the Code Doppler search <NUM>.

<FIG> shows a block diagram flow chart of the method of acquisition according to some example implementations as described herein.

The method of acquisition <NUM> of one or more pairs of signal pairs <NUM>, <NUM> symmetric about a carrier frequency <NUM> begins with receiving <NUM> the one or more signal pairs <NUM>, <NUM> via an antenna <NUM> or any other method of receiving a signal pair <NUM>, <NUM>. Each of the signals <NUM>, <NUM> in the one or more signal pairs <NUM>, <NUM> are then isolated <NUM> from each other and each one modulated <NUM> to be at a same frequency. These signals <NUM>, <NUM> are then combined <NUM> to generate one uniform signal <NUM>. A Code Doppler search <NUM> is then implemented <NUM> on this uniform signal <NUM> and correlated <NUM> with a local replica of the expected signal <NUM> and the result of the correlation <NUM> is then output <NUM> and compared to a detection threshold <NUM> which confirms if the received signal pair <NUM>, <NUM> is the correct signal pair.

Claim 1:
A method (<NUM>) for the acquisition of symmetric signals (<NUM>) performed by an apparatus, the method comprising:
receiving (<NUM>) one or more signal pairs (<NUM>, <NUM>) symmetric about one or more carrier frequencies (<NUM>),
isolating (<NUM>) the two signals (<NUM>, <NUM>) of each of the one or more signal pairs (<NUM>, <NUM>) from each other,
modulating (<NUM>) the two signals (<NUM>, <NUM>) of each of the one or more signal pairs (<NUM>, <NUM>) so that the two signals are modulated to have a same frequency,
combining (<NUM>) the two modulated same frequency signals to generate one uniform signal (<NUM>), that is double the power of the two signals,
implementing (<NUM>) a Code Doppler search (<NUM>) on the uniform signal (<NUM>) to adjust the uniform signal (<NUM>),
correlation (<NUM>) of the adjusted uniform signal (<NUM>) with a reference signal (<NUM>), and
outputting (<NUM>) the result of the correlation (<NUM>).