HK1228168A1 - Method and device for cell-search procedure in a cellular communication network - Google Patents

Description

Cell search and connection procedures in a cellular communication device

Technical Field

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

Background

Further evolution of cellular communication systems, such as what is sometimes referred to as a fifth generation (5G) cellular communication system, will typically require bit rate performance on the Gb/s scale and a signal frequency bandwidth of about 100 MHz in the downlink. For comparison, the maximum signal bandwidth (for a single component carrier) in current 3GPP (third generation partnership project) LTE (long term evolution) cellular communication systems is 20 MHz, i.e. one fifth of that of fifth generation (5G) cellular communication systems. To find such spare bandwidth, the carrier frequency may need to be increased by a factor of 10-20 over current (radio frequency, RF) carrier frequencies (typically in the 1-3 GHz range) used in current second, third and fourth generation (2G, 3G or 4G) cellular communication systems.

Generally, low cost and low power consumption are desired for cellular communication devices. At the same time, it is also desirable for a cellular communication device to be capable of operating in multiple Radio Access Technologies (RATs). Devices having such multi-RAT functionality are referred to hereinafter as multi-RAT devices. For example, 4G devices also typically 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 point of view. Thus, it is likely that in the near future new devices supporting 5G cellular systems will also need to support legacy systems, such as one or more of 2G, 3G and 4G systems.

The reference clock signal to the radio transceiver circuitry of the cellular communication device can be provided by a crystal oscillator. The crystal oscillator can be designed to operate at 26 MHz, for example, and is driven by a low cost 32 kHz reference clock signal generator. To meet the low cost and low power constraints, some degree of inaccuracy of the crystal oscillator must typically be accepted. The open loop uncertainty (maximum deviation from nominal) of the crystal oscillator frequency may be about 10-15 ppm. Thus, once the cellular communication device is powered on, there is uncertainty in the device with respect to the reference frequency, which needs to be handled by the device during an initial cell search procedure when the device searches for a cell with which to synchronize.

In 2G systems, such as the GSM (global system for mobile communications) system, whose carrier frequency is slightly below 1 GHz, the frequency uncertainty at power-up of the cellular communication device can be about 10-15 kHz. FCCH (frequency correction channel) bursts in GSM, which are 67.7 kHz signals, are typically tolerant of frequency errors at that level, and typically do not require special measures to be taken during 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 carrier frequencies of about 2-3 GHz, the frequency uncertainty at power-up of the cellular communication device can be about 20-45 kHz. Meanwhile, PSCH/SSCH (primary/secondary synchronization channel) in UMTS systems and PSS/SSS (primary/secondary synchronization signal) in LTE systems are generally robust to frequency errors up to 3-4 kHz. For these types of systems, so-called frequency meshing (frequency meshing) can be used for initial cell search. The frequency gridding process is outlined below.

The actual carrier frequency of the (RF) carrier is hereinafter referred to as the nominal carrier frequency. With zero frequency error in the cellular communication device, it appears to the cellular communication device that the carrier is actually at (in terms of frequency) this nominal carrier frequency. However, if there is a non-zero frequency error in the cellular communication device, it appears to the cellular communication device as if the carrier is located (in frequency) at some other carrier frequency. In performing frequency meshing, the cellular communication device assumes a plurality of such other carrier frequencies. Thereby, a set of hypothetical 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 hypothesized carrier frequency 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 the knowledge of the actual carrier frequency and the assumed carrier frequency on which the carrier is detected, the cellular communication device can then estimate the frequency error in the cellular communication device and take corrective measures in order to synchronize the reference frequency in the cellular communication device with the reference frequency of the cellular communication network.

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

Disclosure of Invention

The inventors have recognized that for upcoming 5G cellular communication systems, or other systems intended to operate at carrier frequencies of about 10-30 GHz, the initial frequency error may be as high as 200-300 kHz at a 30 GHz carrier frequency. Furthermore, given that the sampling rate may be about 5 times that of LTE, the synchronization signal design for such a system may be robust only for frequency errors of about 5 times or 15-20 kHz as is the case with LTE. Thus, using the frequency meshing method as outlined above, the search grid will have to be substantially increased compared to LTE in order to detect and register with cells in such a system. The inventors have therefore recognized that alternative cell search methods are needed. Embodiments of the present invention are based on the following insights of the inventors: by synchronizing first to a cell of another RAT in a lower frequency region, the required search grid can be reduced, thereby reducing the uncertainty of the internal reference frequency of the cellular communication device.

According to a first aspect, a cell search method is provided for a cellular communication device capable of communicating via a first Radio Access Technology (RAT) in a first frequency band and via a second RAT in a second frequency band in a higher frequency region than the first frequency band. The method includes performing a first cell search in a first frequency band to detect a first cell of a first RAT. Further, the method includes synchronizing to the first cell without registering with the first cell if such first cell is detected; a reference frequency error estimate between a local reference frequency of the cellular communication device and a reference frequency of the first cell is determined, and then a second cell search is performed 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 a set of hypothetical carrier frequencies, wherein a frequency location of the frequency grid is based on a reference frequency error estimate. The frequency location of the frequency grid may also be based on the relative frequency locations 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 no such first cell in the first frequency band is 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 of a second generation (2G) cellular communication RAT, a third generation (3G) cellular communication RAT, and a 4 th 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 for the cellular communication device to connect to a cell of a second RAT is provided. The method comprises performing the cell search method according to the first aspect and registering with the second cell if the second cell is detected.

According to a third aspect, a cellular communication device is provided that is capable of communicating via a first Radio Access Technology (RAT) in a first frequency band and via a second RAT in a second frequency band that is in a higher frequency region than the first frequency band. The cellular communication device comprises a control unit. The control unit is adapted to perform a first cell search in the first frequency band in order 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; a reference frequency error estimate between a local reference frequency of the cellular communication device and a reference frequency of the first cell is determined, and then a second cell search is performed in the second frequency band to detect a second cell of the second RAT based on the reference frequency error estimate.

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 a frequency position of the frequency grid is based on the estimated reference frequency error. The frequency location of the frequency grid may also be based on the relative frequency locations 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 of a second generation (2G) cellular communication RAT, a third generation (3G) cellular communication RAT, and a 4 th generation (4G) cellular communication RAT.

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

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

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

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

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:

FIG. 1 illustrates a cellular communication environment;

fig. 2 is a simplified block diagram of a cellular communication device according to an embodiment;

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

FIG. 5 shows a control unit according to an embodiment; and

fig. 6 schematically shows a computer readable medium and a programmable control unit.

Detailed Description

FIG. 1 illustrates an environment in which embodiments of the present invention may be employed. The cellular communication device 1 is in the coverage of a first cell 2 and a second cell 5. The cellular communication device is shown in fig. 1 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 cellular modem equipped computer (such as a laptop computer or tablet computer) or a cellular modem equipped machine type communication device (such as a sensor or the like).

The first cell 2 is shown in fig. 1 as being served by a first base station 3. The second cell 5 is shown in fig. 1 as being served by a second base station 6. In the example of fig. 1, 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 in a higher frequency region than the first frequency band 4. This is shown in fig. 1, where the first frequency band 4 is located below the frequency f1 and the second frequency band is located above the frequency f2, where f2> f 1. According to examples used throughout this detailed description, the frequency f1 can be, for example, 4 GHz, and the frequency f2 can be, for example, 10 GHz. The first RAT may be, for example, any of a second generation (2G) cellular communication RAT, a third generation (3G) cellular communication RAT, and a 4 th generation (4G) cellular communication RAT. Further, the second RAT may 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 from the same base station.

Fig. 2 is a simplified block diagram of a cellular communication device 1 according to an embodiment of the present invention. In the embodiment shown in fig. 1, the cellular communication device 1 comprises a transceiver unit 10. The transceiver unit 10 may for example comprise a transmitter arranged to transmit signals to a cellular communication network and a receiver arranged to receive signals from the cellular communication network. The receiver may include, for example, 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. Further, the receiver may include one or more analog-to-digital converters (ADCs) for converting the lower frequency signals into the digital domain. The transmitter may, for example, include one or more digital-to-analog converters (DACs) for converting digital baseband signals to be transmitted into analog signals. Further, the transmitter may include one or more analog and/or digital filters, mixers, power amplifiers and/or other circuitry for upconverting 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 fig. 1, the cellular communication device 1 further comprises a control unit 20. The control unit 20 may be, or be part of, for example, digital baseband circuitry, such as a digital baseband processor. A control unit 20 is operatively connected to the transceiver 10 for controlling the operation of the transceiver 10. In addition, the cellular communication device 1 comprises 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 or comprise a crystal oscillator, for example.

The inventors have realised that when the cellular communication device 1 is started up, or is not synchronised with respect to the available RATs for some other reason (e.g. longer inactivity time or "sleep mode"), and is requested to search for a cell of the second RAT (e.g. the second cell 5), frequency synchronisation with the cell of the second RAT can actually be faster by first synchronising with the cell of the first RAT (e.g. the first cell 2) than by attempting the frequency meshing method directly to search for the cell of the second RAT. The uncertainty of the reference frequency in the cellular communication device is reduced if the cellular communication device 1 first synchronizes with the cell of the first RAT without registering with the cell of the first RAT. Taking an LTE cell operating at 2.5 GHz as an example of the first cell 2, the following assumption is valid. The detection of PSS/SSS may be up to a frequency error of 1.5-2 kHz. Thus, once the LTE cell PSS/SSS has been reliably detected, the residual frequency error can be expected to be less than 2 kHz. Furthermore, synchronization improvements using Common Reference Signals (CRS) (pilot symbols) can reduce the residual frequency error down to about 500Hz at the cost of slightly longer synchronization time than detecting only PSS/SSS. A similar number is achieved if a WCDMA cell is used as the first cell; the residual frequency error is about 2 kHz if the synchronization is based on PSCH/SSCH detection and about 500Hz if the synchronization is based on CPICH detection. The mentioned residual frequency error is the error in the carrier frequency of the first cell. These residual frequency errors are then scaled to the ratio between the carrier frequency of the second RAT and the carrier frequency of the first RAT when searching for cells of the second RAT. For example, when the carrier frequency of the second RAT is 10 times the carrier frequency of the first RAT, it extends by a factor of 10. By first synchronizing to the first RAT, the number of hypothetical carrier frequencies used in frequency meshed cell search in the second RAT can be reduced compared to directly attempting a frequency meshed approach to search for cells of the second RAT. Even if the synchronization with the cell of the first RAT takes some time to perform, which must be taken into account (or included) in the total time taken for performing the cell search in the second RAT, the total time can still be reduced compared to directly attempting the frequency meshing method to search for the cell of the second RAT.

Accordingly, according to some embodiments of the present invention, a cell search method for a cellular communication device 1 is provided, the cellular communication device 1 being capable of communicating via a first RAT in a first frequency band 4 and via a second RAT in a second frequency band 7. The method may be applied, for example, when the cellular communication device 1 has just started up and a first cell search is to be performed after start-up. It may also be applied in active mode when the cellular communication device 1 is operating in Discontinuous Reception (DRX) mode with very long sleep times, e.g. sleep times of minutes or hours, which is expected to be useful for some use cases in emerging 5G systems. Then, the reference frequency unit may have drifted too much and thus may require a cell search similar to the initial cell search at start-up. As indicated above, the method may also be applied when the cellular communication device 1 is not synchronized with respect to the available RATs for any other reason and is requested to search for a cell of the second RAT (e.g. the second cell 5).

The method may be performed, for example, by control unit 20 (fig. 2), control unit 20 utilizing 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 invention, a method includes performing a first cell search in a first frequency band 4 to detect a first cell (e.g., cell 2) of a first RAT. If such a first cell 2 is detected, the method further comprises synchronizing to the first cell without registering with the first cell and determining a reference frequency error estimate between the local reference frequency of the cellular communication device 1 and the reference frequency of the first cell 2. Thereby, the uncertainty of the reference frequency in the cellular communication device is reduced. Thereafter, the method includes performing a second cell search in the second frequency band 7 to detect a second cell (e.g., cell 5) of the second RAT based on the reference frequency error estimate. Due to the reduced uncertainty of the reference frequency in the cellular communication device achieved by synchronization with the first cell, a relatively small search grid can be applied when performing a cell search for the second cell, which speeds up the total search time even including the time taken to synchronize with the first cell. Avoiding registration with the first cell 2 prior to searching for the second cell 5 helps to shorten the overall search time compared to if the cellular communication device 1 would register with the first cell 2 prior to 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 have been used for synchronization (e.g., PSS/SSS, CRS, PSCH/SSCH, or CPICH as mentioned above), and receiver processing parameters (e.g., amount of filtering or averaging of reference signals) used for synchronization to the first cell 2.

The term "reference frequency error estimate" as used in this specification refers to an entity that represents a limit or tolerance 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 exponent, such as an integer, that implicitly indicates the value of the reference frequency error estimate. For example, an index of "1" may imply "500 Hz," and an index of "2" may imply "2 kHz," etc. The determination of the reference frequency error estimate can be based on, for example, the type of the first RAT, which reference signals of the first RAT that have been used for synchronization (e.g., PSS/SSS, CRS, PSCH/SSCH, or CPICH as mentioned above), and/or receiver processing parameters used for synchronization to the first cell 2. The determination of the reference frequency error estimate can for example be performed by means of a calculation within the control unit 20 or can be looked up in a look-up table with pre-calculated reference frequency error estimate values. Such pre-calculated values can be pre-calculated, for example, by means of simulations.

Fig. 3 is a flow chart illustrating an embodiment of a method, indicated by reference numeral 90. The operation of the method starts in step 100. In step 110, a cell search is performed in the first frequency band 4 in order to detect the first cell 2 of the first RAT. In step 120 it is checked whether such a first cell 2 is detected. If such a first cell 2 is detected (the "yes" branch), the cellular communication device 1 synchronizes to the first cell 2 without registering with the first cell in step 130. In step 140, a reference frequency error estimate between the local reference frequency of the cellular communication device 1 and the reference frequency of the first cell 2 is determined, e.g. based on the reference signal as outlined above and receiver processing parameters for synchronization to the first cell 2. In step 150, a second cell search is performed in the second frequency band 7 to detect the second cell 5 of the second RAT based on the reference frequency error estimate, and proceeding to step 160, in step 160 the method 90 ends.

If the first cell 2 is not found in the first frequency band 4 in 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 may be assumed based on the 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 a 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 shown in fig. 3 with an optional step 170 used in some embodiments. If the first cell is not found in step 110, the 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 cells of a second RAT is performed in a second frequency band 7. Operation then proceeds to step 160, where the method 90 ends at step 160. The frequency grid used in this case corresponds to the grid used when directly attempting the frequency gridding method to search for a cell of the second RAT (without first synchronizing with another cell in the lower frequency band). Due to the relatively wide (and growing) coverage of existing 2G, 3G and 4G networks, it would be a relatively rare event that it is possible to fail to find any first cell 2 in the first cell search. It should also be noted that since the cellular communication device is not registered with the first cell 3 in step 130, but is only synchronized therewith, the possible set of such first cells 2 is not limited to the cells with which the cellular communication device has a valid subscription (in order to communicate through) but can also comprise other cells (e.g. cells belonging to other operators).

As shown above, the second cell search may be performed using a frequency gridding method. Thus, for the case where the 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 that assumes a set of carrier frequencies. The second cell search can be based on a reference frequency error estimate in the sense that the frequency locations of the frequency grid (i.e. which frequencies are comprised in the set of hypothetical carrier frequencies) are based on a reference frequency error estimate. As also indicated above, for the case where 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 based on the default reference frequency error estimate in a similar manner. Thus, in this case, performing the second cell search (e.g., step 170 in the flowchart of fig. 3) may comprise searching a frequency grid of a set of hypothetical carrier frequencies, wherein the second cell search can be based on a default reference frequency error estimate in the sense that the frequency locations of the frequency grid (i.e., which frequencies are included in the set of hypothetical carrier frequencies) are based on the default reference frequency error estimate. Qualitatively, the larger the reference frequency error estimate (either 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, for example, at a nominal carrier frequency, such as in a first cell of a first RATIs represented by the absolute term of ± XHz. Let the nominal carrier frequency in the second cellThe corresponding reference frequency error of (d) is ± Y Hz. Since the relative error at these two nominal carrier frequencies should be the same, e.g., + -Z ppm, it follows that

。

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

Thus, in some embodiments, the frequency location of the frequency grid used in the second cell search in step 150 (fig. 3) is also based on the relative frequency locations of the first frequency band 4 and the second frequency band 7.

According to some embodiments, the above cell search method can be used as part of a procedure for a cell connected to a second RAT. Thus, according to some embodiments of the invention, there is provided a method for a cellular communication device 1 to connect to a cell of a second RAT. The method includes performing the cell search method 90 described above. Furthermore, if said second cell 5 is detected during the execution of the cell search method 90 (and with reference to fig. 3, this can be in step 150 or in step 170), the method comprises registering with the second cell 5.

Fig. 4 is a flow diagram illustrating an embodiment of a method of connecting to a cell of a second RAT. The operation of the method starts in step 180 and then proceeds to perform a cell search according to the method 90 described above. In step 185 it is checked whether the second cell has been detected during the cell search and, referring again to fig. 3, this can be in step 150 or step 170 (in embodiments comprising step 170). If said 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 is 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 is detected), and the method ends at step 195. 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 with cells of multiple RATs simultaneously, the cellular communication device may register with a cell of a first RAT even if a cell of a second RAT is found during the cell search 90. This may be done, for example, after registration in step 190 or in parallel therewith. As long as this is not performed before the search performed in step 150 or step 170 (fig. 3), such registration will not adversely affect the total search time for the cell of the second RAT.

Embodiments of a method for operating a cellular communication device are described above. Some embodiments of the 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, there is provided a cellular communication device 1 as shown in fig. 2, the cellular communication device 1 being capable of communicating via a first RAT in a first frequency band (e.g. 4 in fig. 1) and via a second RAT in a second frequency band (e.g. 7 in fig. 1) being in a higher frequency region than the first frequency band. According to these embodiments, the control unit 20 is adapted to perform a first cell search in the first frequency band 4 for detecting the 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; a reference frequency error estimate between the local reference frequency of the cellular communication device 1 and the reference frequency of the first cell 2 is determined and then a second cell search is performed in the second frequency band 7 to detect the second cell 5 of the second RAT based on the reference frequency error estimate.

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

As also described above in the context of an embodiment of the method 90, the frequency location of the frequency grid may also be based on the relative frequency locations 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 based on the default reference frequency error estimate to detect the second cell 5 of the second RAT.

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

Fig. 5 is a simplified block diagram illustrating some embodiments of the control unit 20. As shown in fig. 5, these embodiments of the control unit 20 comprise a first RAT cell search unit 20 for performing a cell search in the 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 for detecting the 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 said reference frequency error estimate between said local reference frequency of the cellular communication device 1 and said reference frequency of the first cell 2.

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

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 a frequency location of the frequency grid is based on the estimated reference frequency error. In some embodiments, the frequency location of the frequency grid may also be based on the relative frequency locations of the first frequency band 4 and the second frequency band 7.

In some embodiments, if no such first cell 2 in the first frequency band 4 is detected, the second RAT cell search unit 230 may be adapted to perform a second cell search in the second frequency band 7 (corresponding to step 170 in fig. 3) based on a default reference frequency error estimate to detect a second cell 5 of the second RAT.

As shown in fig. 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 said 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, FPGAs, processors or microcontrollers. Thus, the control unit 20 may be a programmable control unit. Thus, embodiments of the invention may be embedded in a computer program product, which allows implementation of the methods and functions described herein, e.g. the embodiments of the methods described with reference to fig. 3 and 4. Thus, according to an embodiment of the invention, a computer program product is provided comprising instructions arranged to cause the programmable control unit 20 to perform the steps of any embodiment of the method. The computer program product may comprise program code stored on a computer readable medium 300, as shown in fig. 6, which can be loaded and executed by said programmable control unit 20 to cause it to perform the steps of any embodiment of said method. In some embodiments, the computer-readable medium is a non-transitory computer-readable medium.

Embodiments described herein enable relatively fast cell search in a RAT operating at a relatively high carrier frequency, e.g., about 10-30 GHz. An alternative solution for enabling a relatively fast cell search 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 than this alternative solution. Another alternative solution for enabling a relatively fast cell search could be to perform a parallel cell search, wherein the cell search is performed for several assumed carrier frequencies simultaneously. However, this solution would likely require more complex signal processing, increasing the cost in terms of power consumption or required chip area (or both), and therefore embodiments of the invention can provide lower costs than this solution as well.

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

Claims (18)

1. A cell search method (90) for a cellular communication device (1), the cellular communication device (1) being capable of communicating via a first radio access technology, RAT, in a first frequency band (4) and via a second RAT in a second frequency band (7) in a higher frequency region than the first frequency band, the method comprising:

performing (110) a first cell search in the first frequency band (4) for detecting a first cell (2) of the first RAT; and

if such a first cell (2) is detected:

synchronizing (130) to the first cell (2) without registering with the first cell (2);

determining (140) 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

Performing (150) a second cell search in the second frequency band (7) to detect a second cell (5) of the second RAT based on the reference frequency error estimate.

2. The method (90) of claim 1, wherein performing (150) the second cell search comprises:

searching a frequency grid of a set of hypothetical carrier frequencies, wherein a frequency location of the frequency grid is estimated based on the reference frequency error.

3. The method (90) of claim 2, wherein the frequency location of the frequency grid is also based on the relative frequency locations 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 in the first frequency band is detected:

performing (170) 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.

5. The method (90) according to any of the preceding claims, wherein the first frequency band (4) is located below 4 GHz and the second frequency band (7) is located above 10 GHz.

6. The method (90) according to any of the preceding claims, wherein the first RAT is any of a second generation 2G cellular communication RAT, a third generation 3G cellular communication RAT and a 4 th generation 4G cellular communication RAT.

7. The method (90) according to any of the preceding claims, 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 via a first RAT in a first frequency band (4) and via a second RAT in a second frequency band (7) in a higher frequency region than the first frequency band (4), the method comprising:

performing the cell search method (90) of any one of the preceding claims; and

if the second cell is detected, then:

registering (190) with the second cell.

9. A cellular communication device (1) capable of communicating via a first radio access technology, RAT, in a first frequency band (4) and via a second RAT in a second frequency band (7) in a higher frequency region than the first frequency band, comprising:

a control unit (20) adapted to:

performing a first cell search in the first frequency band (4) for detecting a first cell (2) of the first RAT; and

if such a first cell (2) is detected:

synchronise to the first cell (2) without registering with the first cell;

determining 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

Performing a second cell search in the second frequency band (7) to detect a second cell (5) of the second RAT based on the reference frequency error estimate.

10. The cellular communication device (1) according to claim 10, wherein for performing the second cell search the control unit (20) is adapted to search a frequency grid of a set of hypothetical carrier frequencies, wherein a frequency position of the frequency grid is based on the estimated reference frequency error.

11. The cellular communication device (1) according to claim 10, wherein the frequency location of the frequency grid is also based on the relative frequency locations of the first frequency band (4) and the second frequency band (7).

12. The cellular communication device (1) according to any of claims 9-12, wherein if no such first cell (2) in the first frequency band (4) is detected, the control unit is adapted to perform a second cell search in the second frequency band (7) based on a default reference frequency error estimate to detect a second cell (5) of the second RAT.

13. The cellular communication device (1) according to any of claims 9-12, wherein the first frequency band (4) is located below 4 GHz and the second frequency band (7) is located above 10 GHz.

14. A cellular communication device (1) according to any of claims 9-13, wherein the first RAT is any of a second generation 2G cellular communication RAT, a third generation 3G cellular communication RAT and a 4 th generation 4G cellular communication RAT.

15. A cellular communication device (1) according to any of claims 9-14, wherein the second RAT is a fifth generation 5G cellular communication RAT.

16. A cellular communication device (1) according to any of claims 9-15, 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 program product comprising computer program code to perform the method of any of claims 1-8 when the computer program code is executed by a programmable control unit (20) of a cellular communication device (1).

18. A computer readable medium (300) having stored thereon a computer program product comprising computer program code for performing the method of any of claims 1-8 when the computer program code is executed by a programmable control unit (20) of a cellular communication device.