HK1228168B - Method and device for cell-search procedure in a cellular communication network - Google Patents

Description

Method and apparatus for a cell search process in a cellular communication network

Technical Field

The present invention relates to a cell search procedure in a cellular communication network.

Background Art

Further evolution of cellular communication systems, such as those sometimes referred to as fifth generation (5G) cellular communication systems, will typically require bit rate performance in the Gb/s range and a signal frequency bandwidth of approximately 100 MHz in the downlink. For comparison, the maximum signal bandwidth (for a single component carrier) in current 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) cellular communication systems is 20 MHz, i.e., one-fifth that of fifth generation (5G) cellular communication systems. In order to find such free bandwidth, the carrier frequency may need to be increased by a factor of 10-20 over the current (radio frequency) carrier frequencies (typically in the 1-3 GHz range) used in current second, third, and fourth generation (2G, 3G, or 4G) cellular communication systems.

Typically, low cost and low power consumption are desired for cellular communication devices. At the same time, it is also desired that cellular communication devices be able to operate in multiple radio access technologies (RATs). Devices with such multi-RAT functionality are referred to as multi-RAT devices hereinafter. For example, 4G devices typically also support operation in 2G and 3G communication systems. The reason for this is the gradual deployment of new RATs, whereby the use of a single new RAT is limited from the end user's perspective. Therefore, it is likely that in the near future, new devices that support 5G cellular systems will also need to support legacy systems, such as one or more of the 2G, 3G, and 4G systems.

The reference clock signal to the radio transceiver circuitry of a cellular communication device can be provided by a crystal oscillator. For example, the crystal oscillator can be designed to operate at 26 MHz and driven by a low-cost 32 kHz reference clock signal generator. To meet low-cost and low-power constraints, a certain degree of inaccuracy in the crystal oscillator must typically be accepted. The open-loop uncertainty (maximum deviation from the nominal value) of the crystal oscillator frequency can be on the order of 10-15 ppm. Consequently, once a cellular communication device is powered on, there is uncertainty in the device regarding the reference frequency, which needs to be addressed by the device during the initial cell search process as it searches for a cell to synchronize with.

In 2G systems such as the GSM (Global System for Mobile Communications) system (whose carrier frequency is slightly below 1 GHz), the frequency uncertainty when a cellular communication device is powered on can be around 10-15 kHz. The FCCH (Frequency Correction Channel) burst in GSM, which is a 67.7 kHz signal, is generally tolerant of frequency errors at that level, and typically no special measures need to be taken during the initial cell search due to crystal oscillator inaccuracies.

However, in 3G systems such as UMTS (Universal Mobile Telecommunications System) systems or 4G systems such as LTE (Long Term Evolution) systems, which typically operate at a carrier frequency of approximately 2-3 GHz, the frequency uncertainty when the cellular communication device is powered on can be approximately 20-45 kHz. At the same time, the PSCH/SSCH (Primary Synchronization Channel/Secondary Synchronization Channel) in UMTS systems and the PSS/SSS (Primary Synchronization Signal/Secondary Synchronization Signal) in LTE systems are typically robust to frequency errors of up to 3-4 kHz. For these types of systems, so-called frequency gridding can be used for initial cell search. The frequency gridding process is outlined below.

The actual carrier frequency of a (RF) carrier is hereinafter referred to as the nominal carrier frequency. In the case of zero frequency error in the cellular communication device, it appears to the cellular communication device as if the carrier is actually located (in terms of frequency) at this nominal carrier frequency. However, if a non-zero frequency error exists in the cellular communication device, it appears to the cellular communication device as if the carrier is located (in terms of frequency) at some other carrier frequency. When performing frequency gridding, the cellular communication device assumes a plurality of such other carrier frequencies. Thus, a set of assumed carrier frequencies is obtained around the nominal carrier frequency, which may also include the nominal carrier frequency. The cellular communication device then performs a search on the assumed carrier frequencies until the carrier is detected. Detecting the carrier may, for example, mean detecting a synchronization channel (such as the FCCH in GSM or the PSCH/SSCH in UMTS) or a synchronization signal (such as the PSS/SSS in LTE) modulated onto the carrier. Based on knowledge of the actual carrier frequency and the assumed carrier frequency at which the carrier was detected, the cellular communication device can then estimate the frequency error in the cellular communication device and take corrective measures to synchronize a reference frequency in the cellular communication device with a reference frequency of the cellular communication network.

In 3G and 4G systems, typically about 5-6 grid points are required to reliably detect PSCH/SSCH and PSS/SSS, respectively.

Summary of the Invention

The inventors have recognized that for upcoming 5G cellular communication systems or other systems expected to operate on carrier frequencies of approximately 10-30 GHz, the initial frequency error may be as high as 200-300 kHz at a 30 GHz carrier frequency. Furthermore, assuming that the sampling rate may be approximately 5 times that of LTE, the synchronization signal design for such a system may only be robust to a frequency error of approximately 5 times that of the LTE case, or 15-20 kHz. Therefore, using the frequency gridding approach outlined above, the search grid would have to be significantly increased compared to LTE in order to detect and register with cells in such a system. The inventors have therefore recognized that an alternative cell search approach is needed. Embodiments of the present invention are based on the inventors' insight that by first synchronizing to a cell of another RAT in a lower frequency region, the required search grid can be reduced, thereby reducing the uncertainty in the internal reference frequency of the cellular communication device.

According to a first aspect, a cell search method for a cellular communication device capable of communicating in a first frequency band via a first radio access technology (RAT) and in a second frequency band in a higher frequency region than the first frequency band via a second RAT is provided. The method includes performing a first cell search in the first frequency band to detect a first cell of the first RAT. Furthermore, the method includes synchronizing to the first cell without registering with the first cell if such a first cell is detected; determining a reference frequency error estimate between a local reference frequency of the cellular communication device and a reference frequency of the first cell; and thereafter performing a second cell search in the second frequency band to detect a second cell of the second RAT based on the reference frequency error estimate.

Performing the second cell search may include searching a frequency grid of the set of hypothesized carrier frequencies, wherein the frequency positions of the frequency grid are based on the reference frequency error estimate. The frequency positions of the frequency grid may also be based on relative frequency positions of the first frequency band and the second frequency band.

The method may further include performing a second cell search in the second frequency band to detect a second cell of the second RAT based on the default reference frequency error estimate if such a first cell in the first frequency band is not detected.

According to some embodiments, the first frequency band is located below 4 GHz and the second frequency band is located above 10 GHz.

The first RAT may be any one of a second generation (2G) cellular communication RAT, a third generation (3G) cellular communication RAT, and a fourth generation (4G) cellular communication RAT.

The second RAT may be, for example, a fifth generation (5G) cellular communication RAT.

According to a second aspect, a method is provided for the cellular communication device to connect to a cell of a second RAT. The method comprises performing the cell search method according to the first aspect and, if the second cell is detected, registering with the second cell.

According to a third aspect, a cellular communication device is provided, capable of communicating in a first frequency band via a first radio access technology (RAT) and in a second frequency band in a higher frequency region than the first frequency band via a second RAT. The cellular communication device includes a control unit. The control unit is adapted to perform a first cell search in the first frequency band to detect a first cell of the first RAT. Furthermore, if such a first cell is detected, the control unit is adapted to synchronize to the first cell without registering with the first cell; determine a reference frequency error estimate between a local reference frequency of the cellular communication device and a reference frequency of the first cell; and thereafter perform a second cell search in the second frequency band based on the reference frequency error estimate to detect a second cell of the second RAT.

To perform the second cell search, the control unit may be adapted to search a frequency grid of the set of hypothetical carrier frequencies, wherein the frequency positions of the frequency grid are based on the estimated reference frequency error. The frequency positions of the frequency grid may also be based on the relative frequency positions of the first frequency band and the second frequency band.

If no such first cell in the first frequency band is detected, the control unit may be adapted to perform a second cell search in the second frequency band to detect a second cell of the second RAT based on the default reference frequency error estimate.

According to some embodiments, the first frequency band is located below 4 GHz and the second frequency band is located above 10 GHz.

The first RAT may be any one of a second generation (2G) cellular communication RAT, a third generation (3G) cellular communication RAT, and a fourth generation (4G) cellular communication RAT.

The second RAT may be, for example, a fifth generation (5G) cellular communication RAT.

If said second cell is detected, the control unit may be adapted to register the cellular communication device with the second cell.

According to a fourth aspect, there is provided a computer program product comprising computer program code for performing the method according to any one of the first and second aspects when said computer program code is executed by a programmable control unit of a cellular communication device.

According to a fifth aspect, there is provided a computer readable medium having stored thereon a computer program product, the computer program product comprising computer program code for performing the method according to any one of the first and second aspects when said computer program code is executed by a programmable control unit of a cellular communication device.

Further embodiments are defined in the dependent claims. It should be emphasized that the term "comprising" when used in this specification is used to specify the presence of recited features, integers, steps or components, but does not exclude the presence or addition of one or more other features, integers, steps, components or combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of embodiments of the present invention will become apparent from the following detailed description with reference to the accompanying drawings, in which:

FIG1 illustrates a cellular communication environment;

FIG2 is a simplified block diagram of a cellular communication device according to an embodiment;

3-4 are flow charts of methods according to embodiments;

FIG5 shows a control unit according to an embodiment; and

FIG6 schematically illustrates a computer readable medium and a programmable control unit.

DETAILED DESCRIPTION

FIG1 illustrates an environment in which embodiments of the present invention may be employed. A cellular communication device 1 is within the coverage of a first cell 2 and a second cell 5. FIG1 illustrates the cellular communication device as a mobile phone. However, this is merely an example, and the cellular communication device may be any type of device capable of communicating over a cellular communication network, including a computer equipped with a cellular modem (such as a laptop or tablet computer) or a machine-type communication device equipped with a cellular modem (such as a sensor).

A first cell 2 is shown in FIG1 as being served by a first base station 3. A second cell 5 is shown in FIG1 as being served by a second base station 6. In the example of FIG1 , the first cell 2 is a cell of a first radio access technology (RAT) operating in a first frequency band 4. Furthermore, the second cell 5 is a cell of a second RAT operating in a second frequency band 7 that is in a higher frequency region than the first frequency band 4. This is illustrated in FIG1 , where the first frequency band 4 is located below frequency f1 and the second frequency band is located above frequency f2, where f2>f1. According to the examples used throughout this detailed description, frequency f1 can be, for example, 4 GHz, and frequency f2 can be, for example, 10 GHz. The first RAT can be, for example, any of a second generation (2G) cellular communication RAT, a third generation (3G) cellular communication RAT, and a fourth generation (4G) cellular communication RAT. Furthermore, the second RAT can be, for example, a fifth generation (5G) cellular communication RAT. Alternative network configurations may include those in which cells 2 and 5 cover overlapping areas and are served by the same base station.

FIG2 is a simplified block diagram of a cellular communication device 1 according to an embodiment of the present invention. In the embodiment shown in FIG1 , cellular communication device 1 includes a transceiver unit 10. Transceiver unit 10 may, for example, include a transmitter configured to transmit signals to a cellular communication network and a receiver configured to receive signals from the cellular communication network. The receiver may, for example, include one or more analog and/or digital filters, low-noise amplifiers, mixers, and/or other circuitry for receiving a radio frequency (RF) signal and converting it to a lower frequency signal, such as a baseband signal. Furthermore, the receiver may include one or more analog-to-digital converters (ADCs) for converting the lower frequency signal to the digital domain. The transmitter may, for example, include one or more digital-to-analog converters (DACs) for converting the digital baseband signal to be transmitted into an analog signal. Furthermore, the transmitter may include one or more analog and/or digital filters, mixers, power amplifiers, and/or other circuitry for up-converting the analog signal to an RF signal and amplifying the RF signal in a manner suitable for transmission. Such receivers and transmitters are well known in the art of cellular communications and are not further described herein.

In the embodiment shown in FIG1 , the cellular communication device 1 further includes a control unit 20. The control unit 20 may be, for example, a digital baseband circuit (such as a digital baseband processor) or a portion thereof. The control unit 20 is operatively connected to the transceiver 10 to control the operation of the transceiver 10. Furthermore, the cellular communication device 1 includes a reference frequency unit 30. The reference frequency unit 30 is arranged to provide a reference clock signal having a reference frequency to the cellular communication device 1, for example, to the transceiver 10 of the cellular communication device 1, and possibly also to the control unit 20 thereof. The reference frequency unit 30 may be, for example, a crystal oscillator or include a crystal oscillator.

The inventors have recognized that when a cellular communication device 1 is powered on, or is out of synchronization with respect to available RATs for some other reason (e.g., a prolonged period of inactivity or "sleep mode"), and is requested to search for a cell of a second RAT (e.g., second cell 5), frequency synchronization with the cell of the second RAT can actually be faster by first synchronizing with a cell of the first RAT (e.g., first cell 2) than by directly attempting a frequency gridding method to search for cells of the second RAT. If the cellular communication device 1 first synchronizes with a cell of the first RAT without registering with the cell of the first RAT, the uncertainty of the reference frequency in the cellular communication device is reduced. Using an LTE cell operating at 2.5 GHz as an example for the first cell 2, the following assumptions hold true. The frequency error in detecting the PSS/SSS may be as high as 1.5-2 kHz. Therefore, once the LTE cell PSS/SSS has been reliably detected, the residual frequency error can be expected to be less than 2 kHz. In addition, synchronization improvements using the common reference signal (CRS) (pilot symbols) can reduce the residual frequency error down to approximately 500 Hz, at the expense of slightly longer synchronization time compared to detecting only the PSS/SSS. Similar numbers are achieved if a WCDMA cell is used as the first cell; if synchronization is based on PSCH/SSCH detection, the residual frequency error is approximately 2 kHz, and if synchronization is based on CPICH detection, the residual frequency error is approximately 500 Hz. The residual frequency errors mentioned are errors in the carrier frequency of the first cell. When searching for cells of the second RAT, these residual frequency errors are then proportionally expanded to the ratio between the carrier frequency of the second RAT and the carrier frequency of the first RAT. For example, when the carrier frequency of the second RAT is 10 times that of the first RAT, it is expanded by a factor of 10. By first synchronizing to the first RAT, the number of hypothetical carrier frequencies used in the frequency gridded cell search of the second RAT can be reduced compared to directly attempting a frequency gridding approach to search for cells of the second RAT. Even if synchronization with the cells of the first RAT takes some time to perform, this must be taken into account (or included) in the total time spent performing the cell search in the second RAT, the total time can still be reduced compared to directly attempting a frequency gridding approach to search for cells of the second RAT.

Accordingly, according to some embodiments of the present invention, a cell search method for a cellular communication device 1 capable of communicating in a first frequency band 4 via a first RAT and in a second frequency band 7 via a second RAT is provided. The method may be applied, for example, when the cellular communication device 1 has just been powered on and is to perform a first cell search after power-up. It may also be applied in active mode when the cellular communication device 1 is operating in a discontinuous reception (DRX) mode with very long sleep times (e.g., sleep times of several minutes or hours), which is expected to be useful in some use cases in emerging 5G systems. In this case, the reference frequency unit may have drifted significantly, and thus a cell search similar to the initial cell search at power-up may be required. As indicated above, the method may also be applied when the cellular communication device 1 is out of synchronization with respect to the available RATs for any other reason and is requested to search for a cell of a second RAT (e.g., second cell 5).

The method may be performed, for example, by control unit 20 ( FIG. 2 ), which utilizes transceiver unit 10 ( FIG. 2 ) to receive signals from base stations (e.g., 3 and 6 in FIG. 1 ). According to an embodiment of the present invention, the method includes performing a first cell search in a first frequency band 4 to detect a first cell of a first RAT (e.g., cell 2). If such first cell 2 is detected, the method further includes synchronizing to the first cell without registering with the first cell and determining a reference frequency error estimate between a local reference frequency of cellular communication device 1 and a reference frequency of first cell 2. This reduces the uncertainty of the reference frequency in the cellular communication device. The method then includes performing a second cell search in a second frequency band 7 to detect a second cell of a second RAT (e.g., cell 5) based on the reference frequency error estimate. Due to the reduced uncertainty of the reference frequency in the cellular communication device achieved by synchronizing with the first cell, a relatively small search grid can be used when performing a cell search for the second cell, which speeds up the overall search time, even including the time spent synchronizing with the first cell. Avoiding registration with the first cell 2 before searching for the second cell 5 helps to shorten the overall search time compared to if the cellular communication device 1 were to first register with the first cell 2 before searching for the second cell 5. Parameters that affect the uncertainty of the reference frequency after synchronization with the first cell 2 may include the type of the first RAT (e.g., 2G, 3G, or 4G), which reference signals were used for synchronization (e.g., PSS/SSS, CRS, PSCH/SSCH, or CPICH as mentioned above), and receiver processing parameters used to synchronize to the first cell 2 (e.g., the amount of filtering or averaging of the reference signal).

The term "reference frequency error estimate," as used herein, refers to an entity that represents the limits or tolerances within which a frequency error lies, and these limits can be expressed, for example, in absolute terms such as ±X Hz or relative terms such as ±Z ppm. In some embodiments, such an entity may explicitly state the reference frequency error estimate (e.g., ±X Hz or ±Z ppm). In other embodiments, such an entity may take the form of an exponential, such as an integer, implicitly indicating the value of the reference frequency error estimate. For example, an exponent of "1" may imply "500 Hz," and an exponent of "2" may imply "2 kHz," and so on. The reference frequency error estimate can be determined, for example, based on the type of the first RAT, which reference signals of the first RAT have been used for synchronization (e.g., the PSS/SSS, CRS, PSCH/SSCH, or CPICH mentioned above), and/or receiver processing parameters used for synchronization to the first cell 2. The determination of the reference frequency error estimate can be performed, for example, by calculation within the control unit 20, or by looking up in a lookup table containing pre-calculated reference frequency error estimate values. Such pre-calculated values can be pre-calculated, for example, by simulation.

FIG3 is a flow chart illustrating an embodiment of a method, generally designated by reference numeral 90. The method begins in step 100. In step 110, a cell search is performed in a first frequency band 4 to detect a first cell 2 of a first RAT. In step 120, a check is performed to determine whether such a first cell 2 is detected. If such a first cell 2 is detected ("yes" branch), the cellular communication device 1 synchronizes to the first cell 2 in step 130 without registering with the first cell. In step 140, a reference frequency error estimate is determined between a local reference frequency of the cellular communication device 1 and a reference frequency of the first cell 2, e.g., based on the reference signal and receiver processing parameters for synchronization to the first cell 2 as outlined above. In step 150, a second cell search is performed in a second frequency band 7 to detect a second cell 5 of a second RAT based on the reference frequency error estimate, and the method proceeds to step 160, where the method 90 ends.

If the first cell 2 is not found in the first frequency band 4 during the first cell search, another type of cell search can be performed to search for a cell of the second RAT in the second frequency band 7. For example, when the reference frequency unit 30 is not synchronized with the reference frequency of any cellular network, a default reference frequency error estimate can be assumed based on a known tolerance of the reference frequency unit 30. The method may then include performing a second cell search in the second frequency band 7 to detect the second cell 5 of the second RAT based on the default reference frequency error estimate. This alternative of using the default reference frequency error estimate is illustrated in FIG. 3 with optional step 170 used in some embodiments. If the first cell is not found in step 110, operation of the method according to these embodiments follows the "No" branch from step 120 to step 170. In step 170, a second cell search for a cell of the second RAT is performed in the second frequency band 7. Operation then proceeds to step 160, where the method 90 ends. The frequency grid used in this case corresponds to the grid used when directly attempting a frequency gridding method to search for a cell of the second RAT (without first synchronizing with another cell in a lower frequency band). Due to the relatively wide (and growing) coverage of existing 2G, 3G and 4G networks, it is likely that failure to find any first cells 2 in the first cell search will be a relatively rare event. It should also be noted that since the cellular communication device does not register with the first cell 3 in step 130, but only synchronizes with it, the set of possible such first cells 2 is not limited to cells with which the cellular communication device has a valid subscription (for communicating through it), but can also include other cells (e.g., cells belonging to other operators).

As described above, a frequency gridding approach can be used to perform the second cell search. Thus, in the event that a first cell 2 is found during the first cell search, performing the second cell search (e.g., step 150 in the flowchart of FIG. 3 ) may include searching a frequency grid of a set of hypothetical carrier frequencies. The second cell search can be based on a reference frequency error estimate, in the sense that the frequency positions of the frequency grid (i.e., which frequencies are included in the set of hypothetical carrier frequencies) are based on a reference frequency error estimate. As also described above, in the event that no such first cell 2 is found during the first cell search, a second cell search (e.g., performed in step 170 of FIG. 3 ) can be performed in a similar manner based on a default reference frequency error estimate. Thus, in this case, performing the second cell search (e.g., step 170 in the flowchart of FIG. 3 ) may include searching a frequency grid of a set of hypothetical carrier frequencies, in the sense that the frequency positions of the frequency grid (i.e., which frequencies are included in the set of hypothetical carrier frequencies) are based on a default reference frequency error estimate. Qualitatively, the larger the reference frequency error estimate (the determined reference frequency error estimate used in step 150 or the default frequency error estimate used in step 170 ), the larger the frequency grid needs to be.

The reference frequency error can be expressed, for example, in absolute terms of ±X Hz of the nominal carrier frequency f _nom1 of the first cell of the first RAT. Let the corresponding reference frequency error of the nominal carrier frequency f _nom2 of the second cell be ±Y Hz. Since the relative error at these two nominal carrier frequencies should be the same, for example, ±Z ppm, we have

Therefore, if the reference frequency error estimate is determined in absolute terms at the location of first frequency band 4 (e.g., in step 140 of FIG. 3 ), it follows that the relative frequency locations of first frequency band 4 and second frequency band 7 may need to be taken into account when determining the frequency locations of the frequency grid used in the second cell search in step 150 ( FIG. 3 ). For example, let's say the first frequency band is located near 2 GHz, and after synchronization with the first cell, the uncertainty of the reference frequency at 2 GHz is ±500 Hz, i.e., the reference frequency error estimate determined for the carrier frequency at 2 GHz is ±500 Hz. Then, as a first example, if second frequency band 7 is located near 12 GHz, the corresponding uncertainty of the reference frequency in second frequency band 7 would be ±500·12/2 Hz = ±3 kHz. On the other hand, as a second example, if second frequency band 7 is located near 30 GHz, the corresponding uncertainty of the reference frequency in second frequency band 7 would be ±500·30/2 Hz = ±7.5 kHz. The second example will likely require a wider frequency grid with more hypothesized carrier frequencies than the first example used for the second cell search in step 150 (FIG. 3).

Therefore, in some embodiments, the frequency positions of the frequency grid used in the second cell search in step 150 ( FIG. 3 ) are also based on the relative frequency positions of the first frequency band 4 and the second frequency band 7 .

According to some embodiments, the cell search method described above can be used as part of a process for connecting to a cell of a second RAT. Thus, according to some embodiments of the present invention, a method for a cellular communication device 1 to connect to a cell of a second RAT is provided. The method includes executing the cell search method 90 described above. Furthermore, if the second cell 5 is detected during execution of the cell search method 90 (and referring to FIG. 3 , this can be in step 150 or in step 170), the method includes registering with the second cell 5.

FIG4 is a flow chart illustrating an embodiment of a method for connecting to a cell of a second RAT. The method begins in step 180 and then proceeds to perform a cell search according to method 90 described above. In step 185, a check is performed to determine whether the second cell 5 has been detected during the cell search. Referring again to FIG3 , this can be in step 150 or step 170 (in embodiments including step 170). If the second cell 5 has been detected (the "yes" branch from step 185), the cellular communication device 1 registers with the second cell in step 190 (according to a registration procedure defined by the standard of the second RAT), and the method ends in step 195. If no such second cell has been detected (the "no" branch from step 185), the method proceeds to step 195 without connecting to a cell of the second RAT (because no such cell has been detected), whereupon the method ends. In the latter case, as a fallback, the cellular communication device may, for example, attempt to connect to a cell of another RAT, such as the first RAT. In some embodiments, for example, if the cellular communication device 1 is capable of connecting to cells of multiple RATs simultaneously, the cellular communication device may register with a cell of the first RAT even if a cell of the second RAT is found during the cell search 90. This may be performed, for example, after or in parallel with the registration in step 190. As long as this is not performed before the search performed in steps 150 or 170 ( FIG. 3 ), such a registration will not adversely affect the overall search time for cells of the second RAT.

Embodiments of methods for operating a cellular communication device are described above. Some embodiments of the present invention, described further below, also relate to a cellular communication device 1 configured to perform any of the above-described methods. Thus, according to some embodiments of the present invention, a cellular communication device 1 is provided as shown in FIG. 2 , capable of communicating in a first frequency band (e.g., 4 in FIG. 1 ) via a first RAT and in a second frequency band (e.g., 7 in FIG. 1 ) in a higher frequency region than the first frequency band via a second RAT. According to these embodiments, a control unit 20 is adapted to perform a first cell search in the first frequency band 4 to detect a first cell 2 of the first RAT. If such a first cell 2 is detected, the control unit 20 is further adapted to synchronize to the first cell 2 without registering with the first cell; determine a reference frequency error estimate between a local reference frequency of the cellular communication device 1 and a reference frequency of the first cell 2; and thereafter, based on the reference frequency error estimate, perform a second cell search in the second frequency band 7 to detect a second cell 5 of the second RAT.

As described above in the context of embodiments of the method 90, for performing the second cell search, the control unit 20 may be adapted to search a frequency grid of a set of hypothesized carrier frequencies, wherein the frequency positions of the frequency grid are based on the estimated reference frequency error.

As also described above in the context of the embodiment of the method 90 , the frequency positions of the frequency grid may also be based on the relative frequency positions of the first frequency band 4 and the second frequency band 7 .

Furthermore, as also described above in the context of some embodiments of the method 90 comprising step 170, if no such first cell 2 in the first frequency band 4 is detected, the control unit 20 may be adapted to perform a second cell search in the second frequency band 7 to detect a second cell 5 of the second RAT based on the default reference frequency error estimate.

The control unit 20 may be adapted to register the cellular communication device 1 with the second cell 2 if the second cell 2 is detected, according to what was described above in the context of the method shown in FIG. 4 .

FIG5 is a simplified block diagram illustrating some embodiments of the control unit 20. As shown in FIG5 , these embodiments of the control unit 20 include a first RAT cell search unit 200 for performing a cell search in a first RAT, a first RAT synchronization unit 210 for synchronizing with a cell of the first RAT, an error estimate determination unit 220 for determining a reference frequency error estimate, and a second RAT cell search unit 230 for performing a cell search in the first RAT.

The first RAT cell search unit 200 is adapted to perform the first cell search in the first frequency band 4 in order to detect a first cell 2 of the first RAT.

If such a first cell 2 is detected, the first RAT synchronization unit 210 is adapted to synchronize to the first cell 2 without registering with the first cell.

The error estimate determination unit 220 is adapted to determine the reference frequency error estimate between the local reference frequency of the cellular communication device 1 and the reference frequency of the first cell 2 .

The second RAT cell search unit 230 is adapted to perform the second cell search (corresponding to step 150 in FIG. 3 ) in the second frequency band 7 to detect the second cell 5 of the second RAT based on the reference frequency error estimate.

To perform the second cell search, the second RAT cell search unit 230 may be adapted to search a frequency grid of the set of hypothetical carrier frequencies, wherein the frequency positions of the frequency grid are based on the estimated reference frequency error. In some embodiments, the frequency positions of the frequency grid may also be based on the relative frequency positions of the first frequency band 4 and the second frequency band 7.

In some embodiments, if such a first cell 2 in the first frequency band 4 is not detected, the second RAT cell search unit 230 may be adapted to perform a second cell search (corresponding to step 170 in FIG. 3 ) in the second frequency band 7 to detect a second cell 5 of the second RAT based on the default reference frequency error estimate.

As shown in Figure 5, the control unit 20 may in some embodiments also comprise a second RAT registration unit 240. The second RAT registration unit may be adapted to register the cellular communication device 1 with the second cell 2 if the second cell 2 is detected.

In some embodiments, the control unit 20 may be implemented as a dedicated application-specific hardware unit. Alternatively, the control unit 20 or parts thereof may be implemented by programmable and/or configurable hardware units, such as, but not limited to, one or more field programmable gate arrays FPGA, processors or microcontrollers. Therefore, the control unit 20 may be a programmable control unit. Therefore, embodiments of the present invention may be embedded in a computer program product that allows the implementation of the methods and functions described herein, for example, with reference to embodiments of the methods described in Figures 3 and 4. Therefore, according to an embodiment of the present invention, a computer program product is provided, which includes instructions arranged to cause the programmable control unit 20 to perform the steps of any embodiment of the method. The computer program product may include program code stored on a computer-readable medium 300, as shown in Figure 6, which can be loaded and executed by the programmable control unit 20 to cause it to perform the steps of any embodiment of the method. In some embodiments, the computer-readable medium is a non-transitory computer-readable medium.

The embodiments described herein enable relatively fast cell searches in RATs operating at relatively high carrier frequencies, e.g., about 10-30 GHz. An alternative solution for achieving relatively fast cell searches could be to use a reference frequency unit with higher intrinsic accuracy, such as a crystal oscillator, in the cellular communication device. However, this solution would likely be more costly, and therefore, embodiments of the present invention can provide lower costs compared to this alternative solution. Another alternative solution for achieving relatively fast cell searches could be to perform parallel cell searches, where cell searches are performed for several hypothetical carrier frequencies simultaneously. However, this solution would likely require more complex signal processing, increasing costs in terms of power consumption or required chip area (or both), and therefore, also compared to this solution, embodiments of the present invention can provide lower costs.

The present invention has been described above with reference to specific embodiments. However, other embodiments than those described above are possible within the scope of the invention. Within the scope of the invention, different method steps than those described above may be provided, where the method is performed by hardware or software. Different features and steps of the embodiments may be combined in other combinations than those described. The scope of the invention is limited solely by the appended patent claims.

Claims (17)

1. A cell search method (90) for a cellular communication device (1) capable of communicating in a first frequency band (4) via a first radio access technology (RAT) and in a second frequency band (7) at a higher frequency region than the first frequency band via a second RAT, the method comprising: Perform (110) a first cell search in the first frequency band (4) to detect the first cell (2) of the first RAT; and If such a first cell (2) is detected, then: Synchronize (130) to the first cell (2) without registering with the first cell (2); Determine (140) the reference frequency error estimate between the local reference frequency of the cellular communication device (1) and the reference frequency of the first cell (2); and then Based on the reference frequency error estimate, a second cell search (150) is performed in the second frequency band (7) to detect the second cell (5) of the second RAT. 2. The method (90) of claim 1, wherein performing (150) the second cell search comprises: Search for a set of frequency grids that assume carrier frequencies, wherein the frequency positions of the frequency grids are based on the reference frequency error estimate. 3. The method (90) of claim 2, wherein the frequency position of the frequency grid is also based on the relative frequency positions of the first frequency band (4) and the second frequency band (7). 4. The method (90) of claim 1, comprising: If no such first cell is detected in the first frequency band, then: Based on the default reference frequency error estimation, a second cell search (170) is performed in the second frequency band (7) to detect the second cell (5) of the second RAT. 5. The method (90) of any one of claims 1-4, wherein the first frequency band (4) is below 4 GHz and the second frequency band (7) is above 10 GHz. 6. The method (90) according to any one of claims 1-4, wherein the first RAT is any one of a second-generation 2G cellular communication RAT, a third-generation 3G cellular communication RAT, and a fourth-generation 4G cellular communication RAT. 7. The method (90) of any one of claims 1-4, wherein the second RAT is a fifth-generation 5G cellular communication RAT. 8. A method for a cellular communication device (1) to connect to a cell of a second radio access technology (RAT), the cellular communication device (1) being capable of communicating in a first frequency band (4) via a first RAT and in a second frequency band (7) in a higher frequency region than the first frequency band (4) via a second RAT, the method comprising: Perform the cell search method (90) as described in any of the preceding claims; and If the second cell is detected, then: Register with the second community (190). 9. A cellular communication device (1) capable of communicating in a first frequency band (4) via a first radio access technology (RAT) and in a second frequency band (7) at a higher frequency region than the first frequency band via a second RAT, comprising: Control unit (20), applicable to: Perform a first cell search in the first frequency band (4) to detect the first cell (2) of the first RAT; and If such a first cell (2) is detected, then: Synchronize to the first cell (2) without registering with the first cell; Determine the reference frequency error estimate between the local reference frequency of the cellular communication device (1) and the reference frequency of the first cell (2); and then Based on the reference frequency error estimation, a second cell search is performed in the second frequency band (7) to detect the second cell (5) of the second RAT. 10. The cellular communication apparatus (1) of claim 9, wherein the control unit (20) is adapted to: search a frequency grid of a set of assumed carrier frequencies for performing the second cell search, wherein the frequency position of the frequency grid is based on the reference frequency error estimate. 11. The cellular communication device (1) of claim 10, wherein the frequency position of the frequency grid is also based on the relative frequency positions of the first frequency band (4) and the second frequency band (7). 12. The cellular communication device (1) according to any one of claims 9-11, wherein the control unit is adapted to: if no such first cell (2) is detected in the first frequency band (4), perform a second cell search in the second frequency band (7) to detect a second cell (5) of the second RAT based on a default reference frequency error estimate. 13. The cellular communication device (1) according to any one of claims 9-11, wherein the first frequency band (4) is located below 4 GHz and the second frequency band (7) is located above 10 GHz. 14. The cellular communication device (1) according to any one of claims 9-11, wherein the first RAT is any one of a second-generation 2G cellular communication RAT, a third-generation 3G cellular communication RAT, and a fourth-generation 4G cellular communication RAT. 15. The cellular communication device (1) according to any one of claims 9-11, wherein the second RAT is a fifth-generation 5G cellular communication RAT. 16. The cellular communication device (1) according to any one of claims 9-11, wherein the control unit (20) is adapted to: register the cellular communication device (1) with the second cell (2) if the second cell (2) is detected. 17. A computer-readable medium having stored thereon computer program code, which, when executed by a programmable control unit (20) of a cellular communication device (1), performs the method as described in any one of claims 1-8.