Calibration of synthesizers in radio receivers

Abstract

A method of operating a radio receiver, the radio receiver being arranged to receive a plurality of data symbols transmitted at one of a set of predetermined frequencies. The method comprises receiving (204) a first set of data symbols at a first transmission frequency. The first set of data symbols comprises a message indicating a first subset of frequencies. A synthesizer is calibrated (208) for the one or more frequencies in the first subset of frequencies. A second set of data symbols is received (210) for transmission at at least one of the one or more frequencies from the first subset of frequencies. A determination is made based on the second set of data symbols whether a network quality metric exceeds a threshold (212). When the network quality metric exceeds the threshold, the synthesizer is calibrated (214) for one or more frequencies in a second subset of frequencies. The second subset comprises frequencies not in the first subset.

Description

Technical Field

[0001] The present invention relates to improving the efficiency with which calibration procedures can be performed by radio receivers, particularly (but not exclusively) in cellular based systems such as Long Term Evolution (LTE) systems suitable for Internet of Things applications. Background Art

[0002] The range and technical capabilities of cellular-based radio communication systems have expanded dramatically in recent years. Most recently, the 3rd Generation Partnership Project (3GPP) rd The Long Term Evolution (LTE) network, a fourth generation (or "4G") network standard specified by the 3GPP (3rd Generation Partnership Project), has gained popularity due to its relatively high uplink and downlink speeds and greater network capacity compared to previous 2G and 3G networks. More precisely, LTE is the access portion of the Evolved Packet System (EPS), a purely Internet Protocol (IP)-based communication technology that carries both real-time services (such as voice) and data services.

[0003] However, while "classic" LTE connections are becoming more common in the telecommunications industry, further development of communication standards is being carried out to facilitate the so-called "Internet of Things" (IoT) (IoT is a general name for the interconnection of physical devices (sometimes called "smart devices") to a network, providing physical objects that may not have been connected to any network in the past with the ability to communicate with other physical and / or virtual objects. Such smart devices include: vehicles; buildings; household appliances, lighting and heating (for example for home automation); and medical devices. These smart devices are typically real-world objects with embedded electronics, software, sensors, actuators and network connectivity, thereby allowing them to collect, share and act on data. These devices can communicate with user devices (for example, interfacing with a user's smartphone) and / or with other smart devices, thereby providing "machine-to-machine" (or "machine type") communication. However, the development of the LTE standard makes it more feasible to connect directly to cellular networks.

[0004] For this purpose, 3GPP specified two versions of LTE in Release 13 of the LTE standard. The first version is called "Narrowband IoT" (NB-IoT), sometimes referred to as "LTE Cat NB1", and the second version is called "Enhanced Machine Type Communications" (eMTC), sometimes referred to as "LTE Cat M1". It is foreseeable that in the near future, the number of devices using at least one of these standards for IoT will increase dramatically.

[0005] From a communications perspective, the LTE standard (including NB-IoT and eMTC) uses orthogonal frequency division multiple access (OFDMA) as the basis for allocating network resources. This allows the available bandwidth to be shared between user equipment (UE) accessing the network in a given cell provided by a base station, which is called "enhanced Node B", "eNodeB" or simply "eNB" in LTE. OFDMA is a multi-user variant of orthogonal frequency division multiplexing (OFDM), which is a multiplexing scheme in which the total bandwidth is divided into multiple non-overlapping subbands, each with its own subcarrier frequency. In OFDM, unlike other frequency division multiplexing (FDM) schemes, each of these subcarriers is orthogonal to each other, so that crosstalk between subbands is ideally eliminated and no inter-carrier guard bands are required.

[0006] To achieve this orthogonality, the spacing Δf between subcarriers is set so that Where T U is the "useful symbol duration" (receiver side window size), and k is a positive integer (and is usually set to 1). Therefore, with N subcarriers, the total bandwidth B can be expressed as B = NΔf. These subcarriers are then shared among multiple users, providing multiple access. This spacing Δf between subcarriers is usually called the subcarrier spacing. In LTE, the subcarrier spacing is usually 15kHz.

[0007] At the physical layer, in the downlink of an LTE connection, each data frame is 10ms long and consists of ten subframes, each of which has a duration of 1ms. Each subframe contains two equal-length slots, i.e., two 0.5ms slots. Each slot (and by extension, each subframe and each frame) will typically contain a certain number of "resource blocks" (where each subframe has twice as many resource blocks as a slot, and each frame has ten times as many resource blocks as a subframe). A resource block is 0.5ms long in the time domain and twelve subcarriers wide in the frequency domain. In general, there are seven OFDM symbols per slot, and therefore fourteen OFDM symbols per subframe. These resource blocks can be visualized as a grid of "resource elements", where each resource element is 1 / 14ms long and one subcarrier wide, so that each resource block has eighty-four resource elements (i.e., seven times twelve) and each subframe has one hundred and sixty-eight resource elements.

[0008] The exact number of resource blocks present in each time slot (and, in turn, each subframe and each frame) depends on the bandwidth configuration of the radio communication system. For example, in LTE eMTC Release 13, an LTE radio channel may have a bandwidth of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, or 20 MHz, where the channel is divided into narrowbands of 1.4 MHz width.

[0009] In LTE eMTC, communications undergo a 'frequency hopping' process whereby communications between the eNodeB and the User Equipment (UE) 'hop' between different narrowbands within the LTE channel, possibly changing narrowbands as frequently as every subframe, with the frequency hopping schedule being set by the network and communicated to the UE.

[0010] Typically, an LTE receiver (e.g., UE) uses a local oscillator (LO) to generate a reference signal that is used to receive and demodulate incoming radio signals. This reference signal must typically have the same frequency as the narrowband currently in use. The LO frequency can be synthesized using a voltage controlled oscillator (VCO), where a phase locked loop (PLL) is used to tune the VCO frequency to a reasonable multiple of a high-precision reference oscillator frequency.

[0011] Since VCOs typically oscillate at very high frequencies, VCO-based synthesizers may be sensitive to many parameters such as temperature, operating voltage, etc. In order to ensure correct operation and optimal performance, it is important to calibrate the VCO for operation at the specific frequency in use. One arrangement known in the art itself first calibrates the VCO frequency to the middle of its voltage tuning range so that the PLL and automatic frequency control (automatic frequency control, AFC) have the maximum frequency interval to control the VCO while keeping it within the tuning range. However, the applicant has learned that performing calibration in this way increases the total switching time of the synthesizer, that is, the time spent switching from one operating frequency to another operating frequency (which occurs, for example, when frequency hopping occurs). In addition, during calibration, the VCO is in an uncertain transient state. Therefore, calibrating the VCO in this way may be problematic for LTE eMTC communications, in which the UE may be expected to change the LO frequency during OFDM symbols typically used by conventional LTE for the physical downlink control channel (PDCCH) in continuous reception mode. Since calibration takes some time, there is no guarantee that the synthesizer frequency can be changed within a single OFDM symbol period.

[0012] Applicants are aware of a possible arrangement in which the VCO can be calibrated in advance for all possible frequencies that may be used, and the results of the calibration are stored in a memory and the relevant calibration information is retrieved when the frequency is changed. Texas Instruments ) uses a similar method to calibrate the VCO for frequency hopping in the 2.4 GHz communication protocol. However, the applicant has realized that although this method may be suitable for some wireless communication protocols, it may become too cumbersome when the number of possible frequencies is large.

[0013] This problem is exacerbated because any calibration of the VCO is only useful for a certain length of time. As outlined above, VCO calibration is sensitive to various factors including temperature and operating voltage, which may change during use. Thus, recalibrating the VCO for all possible frequencies takes more time and processing power than is desirable, especially for battery-powered devices. Summary of the invention

[0014] When viewed from a first aspect, the present invention provides a method of operating a radio receiver apparatus arranged to receive a plurality of data symbols transmitted at one or more frequencies from a set of predetermined frequencies, the method comprising:

[0015] receiving a first set of data symbols at a first transmission frequency, wherein the first set of data symbols comprises a message indicative of a first subset of frequencies, the first subset of frequencies comprising one or more frequencies selected from the set of predetermined frequencies;

[0016] calibrating a synthesizer for the one or more frequencies in the first subset of frequencies;

[0017] receiving a second set of data symbols from the first subset transmitted on at least one of the one or more frequencies;

[0018] determining whether a network quality metric exceeds a threshold based on the second set of data symbols;

[0019] When the network quality metric exceeds the threshold, the synthesizer is calibrated for one or more frequencies in a second subset of frequencies, wherein the second subset of frequencies includes frequencies in the set of predetermined frequencies that are not in the first subset of frequencies.

[0020] This first aspect of the invention extends to a radio receiver apparatus arranged to receive a plurality of data symbols transmitted at one or more frequencies from a set of predetermined frequencies, the radio receiver apparatus comprising a synthesizer, wherein the radio receiver apparatus is arranged to:

[0021] receiving a first set of data symbols at a first transmission frequency, wherein the first set of data symbols comprises a message indicative of a first subset of frequencies, the first subset of frequencies comprising one or more frequencies selected from the set of predetermined frequencies;

[0022] calibrating the synthesizer for the one or more frequencies in the first subset of frequencies;

[0023] receiving a second set of data symbols from the first subset transmitted on at least one of the one or more frequencies;

[0024] determining whether a network quality metric exceeds a threshold based on the second set of data symbols; and

[0025] When the network quality metric exceeds the threshold, the synthesizer is calibrated for one or more frequencies in a second subset of frequencies, wherein the second subset of frequencies includes frequencies in the set of predetermined frequencies that are not in the first subset of frequencies.

[0026] This first aspect of the invention extends to a radio communication system comprising a radio transmitter device and a radio receiver device, wherein:

[0027] The radio transmitter apparatus comprises a synthesizer and is arranged to transmit a plurality of data symbols at one or more frequencies from a set of predetermined frequencies;

[0028] Wherein, the radio receiver device is further arranged to:

[0029] receiving a first set of data symbols at a first transmission frequency, wherein the first set of data symbols comprises a message indicative of a first subset of frequencies, the first subset of frequencies comprising one or more frequencies selected from the set of predetermined frequencies;

[0030] calibrating the synthesizer for the one or more frequencies in the first subset of frequencies;

[0031] receiving a second set of data symbols from the first subset transmitted on at least one of the one or more frequencies;

[0032] determining whether a network quality metric exceeds a threshold based on the second set of data symbols; and

[0033] When the network quality metric exceeds the threshold, the synthesizer is calibrated for one or more frequencies in a second subset of frequencies, wherein the second subset of frequencies includes frequencies in the set of predetermined frequencies that are not in the first subset of frequencies.

[0034] Therefore, it will be appreciated by those skilled in the art that embodiments of the present invention provide a kind of operation radio receiver device (together with associated radio receiver device and the radio communication system comprising such radio receiver device) improved method, wherein the calibration for the synthesizer of different frequencies is split into multiple rounds.Still observe the advantage of performing calibration in advance (rather than performing calibration during any frequency hopping process when switching to new frequency), that is, receiver can change frequency quickly, and need not calibrate its synthesizer in a relatively short time window.However, compared with conventional methods of calibrating synthesizers before frequency hopping known per se in the art, the radio receiver device operated according to embodiments of the present invention can save time and / or power by calibrating for all possible transmission frequencies only when the network is suitable (that is, when the network quality metric is sufficient).

[0035] Those skilled in the art will appreciate that in some communication protocols, such as LTE-based communications, the reception of some initial data provides the receiver with information about the transmission channel, wherein this initial data is transmitted at only a few possible transmission frequencies used by the transmitter. Applicants have appreciated that a radio receiver device may advantageously be calibrated, at least initially, only for a subset of possible transmission frequencies in order to receive a set of data symbols by which the receiver determines whether the network is suitable. If the network is suitable (i.e., if the network quality metric exceeds a threshold), calibration may then be performed on the other frequencies in the set. However, if the network is not suitable (i.e., if the network quality metric does not exceed a threshold), the receiver may not perform any further calibration on the other frequencies, thereby saving power.

[0036] The applicant has learned that, despite several attempts, sometimes an LTE UE may not be able to decode the system information blocks (SIBs) it needs (SIB1 and SIB2) at all, in which case it may have to restart the cell search process. Otherwise, the UE may usually decode the SIBs. There are usually a large number of SIBs carrying various information. The UE usually decodes these in the appropriate order (SIB1 may provide the location in time and frequency of SIB2, which in turn may provide the time and frequency of SIB3, and so on). Sometimes, a SIB may provide the time and frequency of several subsequent SIBs. However, the applicant has learned that many of these SIBs may carry information that causes the UE to abandon a cell, for example, if the cell belongs to a non-preferred operator. In an alternative example, the use of the cell may simply be prohibited. If this happens, the UE may restart the cell search process.

[0037] If all SIB information is acceptable (e.g., if the cell belongs to a preferred operator and is not prohibited from use), one of the SIBs carries information about the parameters of neighboring cells. At appropriate times, the UE may perform periodic measurements on these neighboring cells to determine if any of the neighboring cells carry a stronger signal (or better SNR), in which case the UE will eventually switch to this neighboring cell and start the entire SIB process again. However, if none of the neighboring cells has a better signal strength than the current cell (or the neighboring cell is simply not listed), and the current cell does not meet the network quality metric, the UE may restart the cell search process.

[0038] The network quality metric may be any suitable metric known per se in the art for determining whether a network is suitable for connection. However, in some embodiments, the network quality metric comprises a signal-to-noise ratio. In some possibly overlapping embodiments, the network quality metric comprises a block error rate. In some further possibly overlapping embodiments, the network quality metric comprises a bit error rate. In one set of embodiments, these network quality metrics may be combined such that a plurality of network quality metrics are each compared to a corresponding threshold value in order to determine whether the network is suitable for connection.

[0039] Although the principles of the claimed invention may be applied to many different radio communications protocols that provide such information at a selected subset of transmission frequencies, in at least some preferred embodiments the radio receiver device (and where appropriate the radio transmitter device) is arranged to perform cellular radio communications, preferably LTE radio communications, and most preferably LTE eMTC and / or NB-IoT communications.

[0040] The applicant has appreciated that in LTE communications, such as in LTE eMTC communications, a receiver may first search for a primary synchronisation signal (PSS) and a secondary synchronisation signal (SSS) to find a cellular network, and then receive a physical broadcast control channel (PBCH) containing a master information block (MIB). Those skilled in the art will appreciate that the system bandwidth is generally not known to the receiver before the MIB is received. As outlined above, in LTE eMTC Release 13, an LTE radio channel may have a bandwidth of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz or 20 MHz, wherein the channel is divided into narrowbands of 1.4 MHz width. Therefore, the total number of frequencies at which the receiver needs to calibrate the synthesizer depends on the system bandwidth, i.e. the number of narrowbands used. An LTE system using a 10 MHz bandwidth provides eight narrowbands, while a system using a 20 MHz bandwidth provides sixteen narrowbands.

[0041] In addition, the number of narrowbands used to transmit other system information that the receiver may need typically depends on the system bandwidth in LTE eMTC, where this other system information can be LTE system information block 1 (SIB1-BR). For example, if the bandwidth is greater than 10 MHz, there may be four possible frequencies on which this other system information is located. In contrast, for lower bandwidths, there may be only one or two such frequencies on which other system information may be located. The radio receiver device can then determine whether the network is suitable for connection based on this other system information, and determine whether to calibrate for other narrowband frequencies or whether no further action is taken. In a system using a 20 MHz bandwidth and thus providing sixteen narrowbands, by only initially calibrating the synthesizer for four narrowbands, the quality of the network can be determined before performing calibration of the other twelve narrowband frequencies, thereby saving time and power compared to calibrating all sixteen narrowband frequencies at once.

[0042] In some embodiments, calibrating the synthesizer comprises storing calibration information associated with each frequency for which the synthesizer is calibrated in a memory, wherein the radio receiver device is arranged to retrieve said calibration information when switching to said associated frequency. As outlined above, the radio receiver device may switch frequencies intermittently, e.g. periodically (i.e. with a fixed period, e.g. each 1 ms corresponds to switching each subframe in LTE communications).

[0043] A person skilled in the art will appreciate that there are many synthesizers known per se in the art which can be readily used in a radio receiver apparatus. However, in some embodiments, the synthesizer comprises an oscillator circuit portion. In a preferred set of such embodiments, the oscillator circuit portion comprises a phase locked loop arrangement comprising a voltage controlled oscillator (VCO) arranged in a loop and a phase detector, wherein the phase detector is arranged to generate an error signal dependent on the difference between a frequency generated by the voltage controlled oscillator and a reference frequency, wherein the frequency generated by the voltage controlled oscillator depends on the error signal.

[0044] Those skilled in the art will appreciate that, as the name implies, the frequency generated by the VCO is controlled by a voltage applied to the input of the VCO (in this case, the error signal generated by the phase detector). In some embodiments, the oscillator circuit portion includes a filter, preferably a low pass filter, located between the phase detector and the voltage controlled oscillator and arranged to filter the error signal to generate a filtered error signal, wherein the frequency generated by the voltage controlled oscillator depends on the filtered error signal.

[0045] In some embodiments, the reference frequency may be generated by a crystal oscillator. Those skilled in the art will appreciate that a crystal oscillator may generate a relatively stable frequency, but the maximum frequency that may be generated by such a crystal oscillator is typically much less than the frequency required by the voltage controlled oscillator. In a set of possibly overlapping embodiments, the oscillator circuit portion includes a frequency divider located between the phase detector and the voltage controlled oscillator, the frequency divider being arranged to divide the frequency generated by the voltage controlled oscillator to generate a divided frequency, wherein the error signal depends on the difference between the divided frequency and the reference frequency. This may advantageously extend the range of frequencies that may be generated to multiples of the reference frequency.

[0046] In one set of such embodiments, the voltage controlled oscillator comprises a plurality of capacitors arranged to form a capacitor matrix, wherein a control signal applied to the capacitor matrix can selectively enable one or more capacitors such that the capacitance of the capacitor matrix varies, wherein the frequency generated by the voltage controlled oscillator depends on the capacitance of the capacitor matrix.

[0047] In some such embodiments, the step of calibrating the synthesizer comprises:

[0048] disconnecting the voltage controlled oscillator from the phase locked loop arrangement;

[0049] applying a fixed control voltage to an input terminal of the voltage controlled oscillator;

[0050] Changing the capacitance of the capacitor matrix;

[0051] measuring the frequency generated by the voltage controlled oscillator for a plurality of different capacitance values ​​of the capacitor matrix; and

[0052] The optimum capacitance value of the capacitor matrix is ​​determined by comparing the frequency generated by the voltage controlled oscillator with a calibration reference frequency. In some such embodiments, the calibration reference frequency is the same as the reference frequency used in normal operation (i.e., when the VCO is connected to the phase detector so that the PLL is complete).

[0053] Although calibration of the synthesizer may be relied upon over a period of time, those skilled in the art will appreciate that calibration may become less reliable as factors such as temperature and / or operating voltage vary during operation of the radio receiver device. Therefore, in some embodiments, the method includes calibrating the synthesizer for one or more frequencies in a set of predetermined frequencies prior to receiving another set of data symbols. Those skilled in the art will appreciate that while the another set of data symbols occurs later than the second set of data symbols (which in turn occurs later than the first set of data symbols), the another set of data symbols after the recalibration process may not necessarily follow immediately after the second set of data symbols.

[0054] The recalibration process may be performed intermittently throughout the operation of the radio receiver device.For example, in some embodiments, the method comprises: calibrating the synthesizer for one or more frequencies in a set of predetermined frequencies before receiving another set of data symbols if a predetermined period of time has passed since a previous calibration of the synthesizer.

[0055] However, in some potentially overlapping embodiments, the method includes comparing an operating temperature of the radio receiver device to a reference temperature, and when the difference between the operating temperature and the reference temperature exceeds a temperature difference threshold, calibrating the synthesizer for one or more of a set of predetermined frequencies before receiving another set of data symbols. In another set of potentially overlapping embodiments, the method includes comparing an operating voltage of the radio receiver device to a reference voltage, and when the difference between the operating voltage and the reference voltage exceeds a voltage difference threshold, calibrating the synthesizer for one or more of a set of predetermined frequencies before receiving another set of data symbols. Thus, in some embodiments, one or more operating parameters may be measured, and a determination may be made based on the measurements whether recalibration is required.

[0056] Although in some arrangements the radio receiver device may be arranged only to receive data, in at least some embodiments the radio receiver device is a radio transceiver device further arranged to transmit a plurality of data symbols transmitted at one or more frequencies from a set of predetermined frequencies. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Certain embodiments of the invention will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0058] Figure 1 is a block diagram of a typical LTE eMTC receiver;

[0059] Figure 2A and Figure 2B is Figure 1 A block diagram of a VCO-based synthesizer used in a receiver of FIG. 1 , showing normal operation and calibration of the synthesizer;

[0060] Figure 3 is a diagram of two typical consecutive LTE subframes;

[0061] Figure 4 is a diagram showing a prior art frequency hopping process;

[0062] Figure 5 is a flow chart illustrating a method of operating an LTE eMTC receiver according to an embodiment of the claimed invention; and

[0063] Figure 6 is arranged to perform according to an embodiment of the claimed invention Figure 5 A block diagram of an LTE eMTC receiver illustrating the method. DETAILED DESCRIPTION

[0064] Figure 1 is a block diagram of a typical LTE eMTC receiver 100 known per se in the art. The receiver 100 comprises: an antenna 102; a low-noise amplifier (LNA) 104; a frequency synthesizer 106; a mixer 108; and a demodulation module 110. Those skilled in the art will appreciate that this is a highly simplified overview of a real system that would normally contain many subsystems.

[0065] The signal received via antenna 102 passes through LNA 104 which amplifies the received signal before inputting it into mixer 108. Mixer 108 also takes as input the signal generated by frequency synthesizer 106 in order to mix the received signal down from the transmission frequency to a baseband frequency suitable for further processing by demodulation module 110.

[0066] In order to change between different radio channels, for example during a frequency hopping process, the frequency synthesizer 106 can be tuned to change the frequency of the signal it generates, which is input into the mixer 108 to match the frequency being used by the transmitter (e.g., eNodeB). The synthesizer 106 includes an oscillator circuit portion, which includes a voltage controlled oscillator (VCO) arranged in a phase locked loop (PLL) in a manner known per se in the art, and will be described in further detail below.

[0067] Figure 2A and Figure 2B is Figure 1 1 is a block diagram of a VCO-based synthesizer 106 used in an LTE eMTC receiver 100, showing normal operation and calibration of the synthesizer 106, respectively. The synthesizer 106 includes: a crystal oscillator 112; a phase detector 114; a low-pass filter 116; a VCO 118 including a capacitor matrix 120; a frequency divider 122; and an automatic frequency control (AFC) module 124. As shown in Figure 2B As can be seen in FIG. 1 , calibration unit 126 together with fixed control voltage source 128 can be used for calibration of synthesizer 106 as outlined below.

[0068] The crystal oscillator 112 provides a reference signal 130 which is input to a phase detector 114. The phase detector 114 provides an error signal 132 which is filtered by a low pass filter 116, thereby producing a filtered error signal 134. This filtered error signal 134 is input to a VCO 118 which produces a signal 136 at a frequency that depends on the filtered error signal 134 (and therefore on the error signal 132).

[0069] The signal 136 generated by the VCO 118 is divided by the frequency divider 122, wherein the factor by which the signal 136 is divided depends on the control signal 138 provided by the AFC module 124. The resulting divided signal 140 is fed back to the phase detector 114, which compares the frequency of the divided signal 140 with the reference signal 130 generated by the crystal oscillator 112 and changes the error signal 132, wherein the low pass filter 116 removes the high frequency components of the error signal 132 to reduce the effects of noise. Therefore, it should be understood that Figure 2A The arrangement shown in forms a PLL in which the frequency of the signal 136 generated by the VCO 118 iteratively converges to a multiple of the frequency of the reference signal 130 , where the multiple is the factor by which the divider 122 divides the frequency 136 generated by the VCO 118 .

[0070] In order to optimally control the VCO 118, the VCO 118 should generally first be calibrated to the middle of its voltage tuning range for a given frequency so that the control provided by the PLL and AFC module 124 has the maximum frequency spacing to control the frequency generated by the VCO 118. Figure 2B Describe the calibration process.

[0071] To calibrate the VCO 118, Figure 2B As shown, the VCO 118 is disconnected from the PLL, so that the input of the VCO 118 is disconnected from the low pass filter 116 and, therefore, from the phase detector 114. Instead, the VCO uses a fixed control voltage 142 generated by the control voltage source 128. The signal 136 generated by the VCO 118 is typically divided by the divider 122, but the divided signal 140 is input to the calibration unit 126 together with the reference signal 130 from the crystal oscillator 112.

[0072] The calibration unit 126 varies a control signal 144 supplied to the VCO 118, wherein the value of the control signal 144 changes the configuration of the capacitor matrix 120, which changes the capacitance of the capacitor matrix 120 presented to the VCO 118. The calibration unit 126 varies this control signal 144 within its range of possible values, comparing the divided signal 140 with the reference signal 130 in order to determine the best configuration of the capacitor matrix 120 for the current operating conditions, such that the desired frequency is given by the fixed control voltage.

[0073] Figure 3 is a diagram of two typical consecutive LTE subframes used in LTE communications.As explained previously, according to the LTE standard, each LTE data frame is 10 ms long and consists of ten subframes, each subframe being 1 ms in duration. Figure 3 Two consecutive subframes 2, 4 are shown, wherein each subframe 2, 4 contains two corresponding time slots 2a, 2b; 4a, 4b of equal length, i.e. each subframe has two time slots of 0.5 ms. Thus, the first time slot 2a of the first subframe 2 is at t 0 Starting at t 0 +0.5ms, and the second time slot 2b of the first subframe 2 ends at t 0 +0.5ms and starts at t 0 +1ms, and the first time slot 4a of the second subframe 4 ends at t 0 +1ms and starts at t 0 +1.5 ms, and the second time slot 4b of the second subframe 4 ends at t 0 +1.5ms and starts at t 0 Ends at +2ms.

[0074] Each time slot 2a, 2b, 4a, 4b contains a certain number of "resource blocks". A resource block is 0.5 ms long in the time domain and twelve subcarriers 6 wide in the frequency domain. In general, there are seven OFDM symbols 7 per time domain slot 2a, 2b; 4a, 4b, and therefore fourteen OFDM symbols 7 per subframe 2, 4. These resource blocks can be visualized as a grid of "resource elements", where each resource element is 1 / 14 ms long and one subcarrier wide, so that there are one hundred and sixty-eight resource elements per subframe (i.e., fourteen times twelve).

[0075] Each subframe 2, 4 is further split into a control channel portion 8 and a payload portion 10. The control channel portion 8 having a duration of the first two resource elements (i.e., the first 2 / 14 ms) is typically used by the UE to tune its receiver frequency synthesizer to the current narrowband frequency. Of course, it should be appreciated that in practice, the control channel portion 8 may have a duration of between one and four OFDM symbols.

[0076] The payload portion 10 contains the data itself, so that the plurality of resource elements in each frame 2, 4 contains only data symbols - that is, data that has been converted into OFDM symbols and transmitted over the air by the eNodeB to the UE. These data symbols are depicted as white blocks in the figure and are Figure 3 An exemplary resource element carrying data is labeled 12 in FIG.

[0077] A plurality of common reference symbols (CRS) are distributed throughout both the control channel portion 8 and the payload portion 10. These CRS are used to assist in the demodulation of the OFDM data symbols. In general, NB-IoT and eMTC are expected to operate under very low signal-to-noise ratio (SNR) conditions, and therefore it may be necessary to average or filter several of these CRS to obtain a suitable channel estimate, i.e., an estimate of channel characteristics such as attenuation, phase shift, and noise. These CRS are depicted as black blocks in the accompanying drawings and are Figure 3 The exemplary CRS resource element is labeled 14 in FIG.

[0078] These CRS 14 are used to demodulate the OFDM data symbols in a manner known per se in the art. In general, the averaging of the CRS 14 will be performed using a sliding window that is symmetric in both time and frequency, i.e., the CRS 14 from the past and the future relative to a given data symbol are used to demodulate the data symbol (e.g., the average of the CRS 14 received in the first five and the last five OFDM data symbols relative to the resource element containing the data to be demodulated).

[0079] Figure 4 is a diagram showing a prior art frequency hopping process. In this example, the transmission channel (i.e., narrowband) changes in each subframe. At a first time t 0 , LTE communications are performed over a first narrowband 20 (twelve subcarriers wide as described above). After 1 ms (i.e. the duration of a single subframe in LTE), once the first subframe 22 has been transmitted, the eNodeB changes its transmission frequency according to a predetermined pattern, and the UE 0 +1ms, its frequency synthesizer is retuned to the second narrowband 24. Of course, those skilled in the art will appreciate that this is only a single example, and in a practical system, the signal of interest may actually be multiple resource blocks wide. For example, the width of the incoming signal may be between one and six resource blocks, and these resource blocks do not necessarily have to be located adjacent to each other. For simplicity, the example shown here is only a single resource block wide, but it should be understood that the principles outlined herein apply equally to signals of any width in terms of resource blocks.

[0080] Similarly, 1 ms later, once the second subframe 26 has been transmitted, the eNodeB changes its transmission frequency again and the UE 0 +2 re-tunes its frequency synthesizer to the third narrowband 28 and then receives the third subframe 30.

[0081] When switching between these narrowbands, the LTE eMTC receiver 100 is expected to re-tune the synthesizer 106 during the OFDM symbol 38 typically used by conventional LTE systems for the physical downlink control channel (PDCCH). Since the VCO typically oscillates at a very high frequency, the VCO-based synthesizer 106 may be sensitive to many parameters such as temperature, operating voltage, etc. In order to ensure correct operation and optimal performance, it is important to calibrate the VCO for operation at the specific frequency in use (i.e., the frequency of the narrowband 20, 24, 28 in use).

[0082] In the conventional arrangement described above with reference to Figures 2a and 2b, calibration of the synthesizer 106 includes calibrating the VCO frequency to be in the middle of its voltage tuning range so that the PLL and AFC have maximum frequency separation to control the VCO while keeping it within the tuning range. However, performing calibration in this manner increases the overall switching time of the synthesizer 106, i.e., the time taken to switch from one operating frequency to another operating frequency (which occurs, for example, when frequency hopping occurs). In addition, during calibration, the VCO 118 is in an uncertain transient state.

[0083] As outlined above, calibrating the VCO in this manner may be problematic for LTE eMTC communications, where the UE is expected to change the LO frequency during OFDM symbols 38, which are typically used by conventional LTE for the physical downlink control channel (PDCCH). Since calibration takes some time, there is no guarantee that the synthesizer frequency can be changed during a single OFDM symbol period. It will be appreciated that the OFDM symbols 37 immediately preceding the marked OFDM symbols 38 may also be used for a change in LO frequency, however, the OFDM symbols 37 immediately preceding those marked symbols contain CRS that may be used for demodulation, and therefore the applicant has appreciated that the LO frequency may be changed during the OFDM symbol period. Figure 4 It is often advantageous to change the LO frequency during OFDM symbol 38 as marked in FIG. Those skilled in the art will appreciate that when the CRS in symbol 38 of subframe 26 is used for demodulation, it will be used in the demodulation of subframe 22 (i.e., a "continuation" of subframe 22 will be used on narrowband 20, which is not visible in the diagram but does exist because CRS is always transmitted by LTE on all narrowbands).

[0084] Conventional LTE receivers known per se in the art that require more than one OFDM symbol duration to change the LO frequency typically use both the marked OFDM symbol 38 and the OFDM symbol 37 immediately preceding the marked symbol.

[0085] Figure 5 is a flow chart illustrating a method of operating an LTE eMTC receiver 100 according to an embodiment of the claimed invention. Figure 5 The described process may be performed by any suitable processing arrangement, but in this exemplary embodiment, as shown in FIG. Figure 6 As shown, it is executed by a processor 300. As can be seen in this block diagram, the processor 300 is connected to the synthesizer 106 and is in communication with the calibration unit 126. The calibration information is stored in the memory 302, as outlined in further detail below.

[0086] Return to reference Figure 5 , the process starts at step 200, and after initialization, the receiver 100 starts scanning for a network at step 202 by scanning for PSS and SSS transmissions from an eNodeB across the frequency range. Once a network has been detected, the receiver 100 receives a PBCH transmission from the eNodeB at step 204. This PBCH transmission received at step 204 is a first set of OFDM data symbols, transmitted at a first transmission frequency (i.e., a channel frequency), and contains a master information block (MIB) in the payload that indicates the bandwidth being used by the network.

[0087] Once the eMTC receiver 100 determines the bandwidth in step 206 based on the MIB received via the PBCH transmission from the eNodeB, the receiver 100 can determine which set of frequencies the eNodeB can transmit the SIB1-BR message. An LTE radio channel can have a bandwidth of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz, where the channel is divided into narrowbands of 1.4 MHz width. For example, an LTE system using a 10 MHz bandwidth provides eight narrowbands, a system using a 15 MHz bandwidth provides twelve narrowbands, and a system using a 20 MHz bandwidth provides sixteen narrowbands.

[0088] Those skilled in the art will appreciate that the number of narrowbands used for transmission of SIB1-BR messages also depends on the bandwidth. For example, if the bandwidth is 15 MHz or 20 MHz, there are typically four possible frequencies at which SIB1-BR transmissions can occur. If the bandwidth is 3 MHz, 5 MHz, or 10 MHz, there are typically two possible frequencies for SIB1-BR transmissions. Conversely, if the bandwidth is 1.4 MHz, there is only a single frequency on which SIB1-BR transmissions can be located.

[0089] Once the receiver device has determined the bandwidth, at step 208, the receiver 100 uses the previously referenced Figure 2A and Figure 2B The described approach calibrates the synthesizer 106 for each of the possible frequencies on which the SIB1-BR message may be transmitted for the bandwidth used, and stores these frequencies in a memory. In step 210, the receiver 100 receives the SIB1-BR message at one of the frequencies for which the synthesizer 106 is calibrated. Then, in step 212, the receiver 100 determines whether the network is suitable for connection based on the received SIB1-BR message by comparing a signal quality metric with a threshold. This signal quality metric may be, for example, an SNR, a block error rate (BER), a bit error rate (BIR), or some other metric for measuring the quality of the network to determine whether it is sufficient for connection.

[0090] If the receiver 100 determines at step 212 that the network is not suitable, it returns to step 202 and searches for a new network. However, if at step 212 the network is deemed suitable, the receiver 100 at step 214 calibrates the synthesizer 106 for other narrowbands that were not previously calibrated.

[0091] Once the synthesizer 106 is calibrated for all narrowbands, with the calibration data stored in the memory 302, the receiver 100 receives incoming data at step 216. When receiving data, the receiver 100 may be required to change the receive frequency, for example, every subframe, which involves re-tuning the synthesizer 106. Figure 4 Rather than performing calibration during OFDM symbols 38 typically used by legacy LTE for the physical downlink control channel (PDCCH) as in the conventional procedure described, calibration data is stored for all narrowbands so that when frequency hopping, the synthesizer 106 can be easily tuned using the existing calibration data.

[0092] At step 220, the receiver 100 determines whether more data is to be received. If no further data is to be received, the process ends 222. However, if more data is to be received, then at step 224, the receiver 100 determines whether recalibration is required. This determination at step 224 may involve determining whether the operating temperature and / or operating voltage has changed significantly since the last calibration, or whether a predetermined period of time has passed.

[0093] If recalibration is not required, the receiver 100 returns to step 216 and continues to receive data using the current calibration information. However, if recalibration is required, recalibration is performed at step 226 for one or more frequencies before the receiver returns to step 216 to receive more data.

[0094] It will be appreciated that receiver 100 may determine that recalibration is required when receiving data, and if this is ultimately necessary, perform any necessary calibration prior to frequency hopping (ie, using conventional methods).

[0095] Those skilled in the art will appreciate that normal operating protocols may contain time gaps, for example between reception and transmission (or vice versa) or between subsequent receptions, which may be used for calibration.

[0096] Therefore, it will be appreciated by those skilled in the art that embodiments of the present invention provide an improved method of initializing a radio receiver device, wherein pre-calibration of a synthesizer is performed for a plurality of different receive frequencies, wherein a subset of frequencies are first calibrated so that the receiver can determine whether a network is suitable before calibrating other frequencies, thereby potentially saving time, processing resources, and / or power. It will be appreciated by those skilled in the art that the specific embodiments described herein are exemplary only, and that many variations within the scope of the present invention are contemplated.

Claims

1. A method of operating a radio receiver apparatus, the method being arranged to receive a plurality of data symbols transmitted at one or more frequencies from a set of predetermined frequencies, the method comprising: receiving a first set of data symbols at a first transmission frequency, wherein the first set of data symbols comprises a message indicative of a first subset of frequencies, the first subset of frequencies comprising one or more frequencies selected from the set of predetermined frequencies; calibrating a synthesizer for the one or more frequencies in the first subset of frequencies; receiving a second set of data symbols transmitted on at least one of the one or more frequencies from the first subset of frequencies; determining whether a network quality metric exceeds a threshold based on the second set of data symbols; When the network quality metric exceeds the threshold, the synthesizer is calibrated for one or more frequencies in a second subset of frequencies, wherein the second subset of frequencies includes frequencies in the set of predetermined frequencies that are not in the first subset of frequencies.

2. The method according to claim 1, wherein: The network quality metric includes a signal-to-noise ratio, a block error rate and / or a bit error rate. The method of claim 1 , comprising performing cellular radio communications.

4. The method of claim 1 , comprising storing in a memory calibration information associated with each frequency for which the synthesizer is calibrated; and When switching to the associated frequency, the calibration information is retrieved.

5. The method according to any one of claims 1 to 4, wherein: The synthesizer comprises an oscillator circuit portion, the oscillator circuit portion comprising a phase locked loop arrangement, the phase locked loop arrangement comprising a voltage controlled oscillator (VCO) and a phase detector arranged in a loop, wherein the method further comprises: An error signal is generated using the phase detector, the error signal being dependent on a difference between a frequency generated by the voltage controlled oscillator and a reference frequency, wherein the frequency generated by the voltage controlled oscillator is dependent on the error signal.

6. The method of claim 5, comprising filtering the error signal to produce a filtered error signal, wherein: The frequency generated by the voltage controlled oscillator depends on the filtered error signal.

7. The method of claim 5, comprising dividing the frequency generated by the voltage controlled oscillator to produce a divided frequency, wherein: The error signal depends on the difference between the divided frequency and the reference frequency.

8. The method according to claim 5, wherein: The voltage-controlled oscillator comprises a plurality of capacitors arranged to form a capacitor matrix, wherein the frequency generated by the voltage-controlled oscillator depends on the capacitance of the capacitor matrix, and the method comprises: applying a control signal to the capacitor matrix to selectively enable one or more of the capacitors so that the capacitance of the capacitor matrix changes.

9. The method according to claim 8, wherein: The steps of calibrating the synthesizer include: disconnecting the voltage controlled oscillator from the phase locked loop arrangement; applying a fixed control voltage to an input terminal of the voltage controlled oscillator; changing the capacitance of the capacitor matrix; measuring the frequency generated by the voltage controlled oscillator for a plurality of different capacitance values ​​of the capacitor matrix; and An optimum capacitance value for the capacitor matrix is ​​determined by comparing the frequency generated by the voltage controlled oscillator to a calibrated reference frequency.

10. A method according to any one of claims 1 to 4, comprising calibrating the synthesizer for one or more frequencies of the set of predetermined frequencies prior to receiving another set of data symbols.

11. A method according to claim 10, comprising comparing an operating temperature of the radio receiver device with a reference temperature, and when the difference between the operating temperature and the reference temperature exceeds a temperature difference threshold, calibrating the synthesizer for one or more frequencies in the set of predetermined frequencies before receiving the other set of data symbols.

12. The method of claim 10 , comprising comparing an operating voltage of the radio receiver device with a reference voltage, and when a difference between the operating voltage and the reference voltage exceeds a voltage difference threshold, calibrating the synthesizer for one or more frequencies in the set of predetermined frequencies before receiving the other set of data symbols.

13. A radio receiver apparatus arranged to receive a plurality of data symbols transmitted at one or more frequencies from a set of predetermined frequencies, the radio receiver apparatus comprising a synthesizer, wherein The radio receiver apparatus is arranged to: receiving a first set of data symbols at a first transmission frequency, wherein the first set of data symbols comprises a message indicative of a first subset of frequencies, the first subset of frequencies comprising one or more frequencies selected from the set of predetermined frequencies; calibrating the synthesizer for the one or more frequencies in the first subset of frequencies; receiving a second set of data symbols transmitted on at least one of the one or more frequencies from the first subset of frequencies; determining whether a network quality metric exceeds a threshold based on the second set of data symbols; and When the network quality metric exceeds the threshold, the synthesizer is calibrated for one or more frequencies in a second subset of frequencies, wherein the second subset of frequencies includes frequencies in the set of predetermined frequencies that are not in the first subset of frequencies.

14. A radio communication system comprising a radio transmitter device and a radio receiver device, wherein: The radio transmitter apparatus comprises a synthesizer and is arranged to transmit a plurality of data symbols at one or more frequencies from a set of predetermined frequencies; Wherein, the radio receiver device is further arranged to: receiving a first set of data symbols at a first transmission frequency, wherein the first set of data symbols comprises a message indicative of a first subset of frequencies, the first subset of frequencies comprising one or more frequencies selected from the set of predetermined frequencies; calibrating the synthesizer for the one or more frequencies in the first subset of frequencies; receiving a second set of data symbols transmitted on at least one of the one or more frequencies from the first subset of frequencies; determining whether a network quality metric exceeds a threshold based on the second set of data symbols; and When the network quality metric exceeds the threshold, the synthesizer is calibrated for one or more frequencies in a second subset of frequencies, wherein the second subset of frequencies includes frequencies in the set of predetermined frequencies that are not in the first subset of frequencies.

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