Patent Description:
Indoor location and navigation have gathered a great deal of attention recently. While the outdoor navigation is widely based on GNSS satellite services, there is currently no ubiquitous technology or service to facilitate indoor navigation. The existing indoor navigation solutions are based on e.g. wireless Local Area Network (wireless LAN), Bluetooth low energy (BLE), visible or infrared light etc. All of these solutions have more or less limited coverage and location accuracy.

An emerging technology solution for indoor location is based on radio direction finding systems. For that purpose, the radio direction finding systems may be divided into systems using Angle of Arrival (AoA) method and Angle of Departure (AoD) method. Both methods utilize an antenna array constructed of several distinct antenna elements, which are physically separated from each other. Having more antennas in the antenna array generally improves positioning accuracy and positioning system tolerance against impairments caused by radio signal propagation environment. In a simple direction finding solution one of the antenna elements of the antenna array is active at a time and the active antenna is changed over the time.

In the AoA method the antenna array is used for reception. The receiver antenna element is changed to find the direction of incoming radio wave-front from a transmitter and the position is defined from the observed phase differences collected from the receiver antenna elements. <FIG> schematically illustrates a simple example of the principle of AoA direction finding system <NUM>. The AoA radio direction finding system <NUM> comprises a transmitter unit <NUM> and a receiver unit <NUM>. The transmitter unit <NUM> comprises a transmitting antenna <NUM> and a RF transmitter <NUM>, e.g. BLE transmitter. The receiver unit <NUM> comprises a receiving antenna array, which comprises in this simple example four antenna elements 110a-110d. Furthermore, the receiver unit <NUM> comprises an antenna switch unit <NUM> to switch between the antenna elements 110a-110d, a RF receiver <NUM>, e.g. BLE receiver, and an AoA estimation unit <NUM>. The AoA estimation unit <NUM> of the receiver unit <NUM> estimates the angle of arrival of the signal by using the sampled receiver signal.

In the AoD method the antenna array is used for transmission. The transmitter signal is fed consecutively to each antenna element of the antenna array for a short time period to produce signal in receiving end. The transmitted signal is switched between the antenna elements according to a pre-determined antenna switching pattern. The antenna switching pattern defines the order in which the antennas in the antenna array are activated. The switching generates a directionally variable phase-modulated radiated signal, which may be used for defining estimating the relative direction between a distant receiver and the transmitter. The estimation is based on the received signal, and the receiver does not need to have a directional antenna. <FIG> schematically illustrates a simple example of the principle of AoD direction finding system <NUM>. The AoD radio direction finding system <NUM> comprises a transmitter unit <NUM> and a receiver unit <NUM>. The transmitter unit <NUM> comprises a transmitting antenna array, which comprises in this simple example four antenna elements 206a-206d. Furthermore, the transmitter unit <NUM> comprises an antenna switch unit <NUM> to switch the transmitted signal between the antenna elements 206a-206d and a RF transmitter <NUM>, e.g. BLE transmitter. The receiver unit <NUM> comprises a receiving antenna <NUM>, a RF receiver <NUM>, e.g. BLE receiver, and an AoD estimation unit <NUM>. The AoD estimation unit <NUM> of the receiver unit <NUM> estimates the angle of arrival of the signal by using the sampled receiver signal.

In the AoD method the switching of the transmitted signal from one antenna element to another antenna element may cause unwanted spreading of radiated RF spectrum, if the switching is done too rapidly. This spreading may cause interference to other radio receivers nearby the AoD transmitter and may potentially violate the regulatory requirements for spectral emissions, or cause non-compliance with relevant standards, e.g. Bluetooth.

In the AoA method the rapid switching from one antenna element to another antenna element may cause unwanted spurious responses from nearby radio transmitters.

A patent application <CIT> discloses a method of maintaining a communications link between a network node and a mobile node of a communications network and apparatus, particularly applicable to cellular networks, such as GSM or W-CDMA.

A patent application <CIT> discloses wireless communication systems using spatially diverse antennas for avoiding the effects of multipath interference, and more particularly approaches for selecting an active antenna in such systems.

A patent application <CIT> discloses cellular telephone systems and, more particularly, base stations using directional antenna arrays.

A document "Soft vs hard antenna commutation - RDF - Radio Direction Finding" discloses soft and hard antenna switching in radio direction finding. However, there is need to develop further solutions in order to reduce radiated spectral emissions of an AoD transmitter.

An objective of the invention is to present an AoD direction finding transmitter unit and an antenna soft switching method for an AoD direction finding transmitter unit. Another objective of the invention is that the AoD direction finding transmitter unit and the antenna soft switching method for an AoD direction finding transmitter unit reduces interference caused to radio receivers of other nearby systems.

The objectives of the invention are reached by an AoD direction finding transmitter unit, and an antenna soft switching method as defined by the respective independent claims. Further aspects of the invention are set forth in the dependent claims.

According to a first aspect, an Angle of Departure, AoD, direction finding transmitter unit comprising an antenna soft switching system is provided, wherein the antenna soft switching system is arranged between at least one radio frequency, RF, transmitter module and at least one antenna array, wherein the soft switching system comprises: a timing unit for obtaining at least a starting time of a switching event, a switching network arranged on an RF signal path between an RF port and a first antenna port and a second antenna port, and a generator unit for generating at least one waveform for controlling the switching network, wherein the generator unit is configured to reduce a level of unwanted emissions of a transmitted RF spectrum of the AoD direction finding transmitter unit by controlling the switching network so that the amplitude of the RF signal is switched smoothly from the first antenna port to the second antenna port.

The switching network may comprise at least two switching elements, wherein the at least two switching elements are resistive switching devices, fully reactive switching devices, partially reactive switching devices, or active switching devices.

The at least two switching devices may utilize junction gate field-effect transistors, JFETs; Metal-oxide-semiconductor field-effect transistors, MOSFETs; metal-semiconductor field-effect transistors, MESFETs; pseudomorphic high electron mobility transistors, PHEMTs; PIN diodes; or any other suitable RF switching devices.

The generator unit may comprise an analog ramp generator.

Alternatively, the generator unit may comprise a digital waveform generator, a digital to analog converter, and a reconstruction filter.

The at least two switching devices may be digitally controlled attenuators.

The generator unit may comprise a digital waveform generator and a waveform encoder configured to convert the waveform generated by the digital waveform generator into control signals of the digitally controlled attenuators.

Alternatively, the generator unit may comprise a counter and the waveform may be encoded into the at least two switching devices.

The switching network may be implemented as a part of an antenna switch configured to change active antenna port from the first antenna port to the second antenna port during the switching event, wherein the generator unit may be configured to control the switching network so that: the amplitude of the RF signal at the first antenna port is smoothly ramped down, and simultaneously the amplitude of the RF signal at the second antenna port is smoothly ramped up.

Alternatively, the switching network may be a separate switching network providing a centralized soft switching system, wherein the generator unit may be configured to control the switching network so that: the amplitude of the RF signal at the first antenna port is smoothly ramped down; and simultaneously the amplitude of the RF signal at a reference port is ramped up smoothly, wherein the antenna switch is configured to changing active antenna port while the RF signal is led to the reference port; and wherein the generator unit may further be configured to control the switching network so that the amplitude of the RF signal at the reference port is smoothly ramped down and simultaneously the amplitude of the RF signal at second antenna port is smoothly ramped up.

Alternatively, the switching network may be a separate switching network providing a centralized soft switching system and the first antenna port may belong to a first antenna group and the second antenna port may belong to a second antenna group, wherein the generator unit may be configured to control the switching network so that: the amplitude of the RF signal at the first antenna group is smoothly ramped down, and simultaneously the amplitude of the RF signal at the second antenna group is smoothly ramped up, wherein the second antenna port is pre-selected as the active antenna of the second antenna group.

According to a second aspect, an antenna soft switching method for an AoD direction finding transmitter unit comprising an antenna soft switching system is provided, wherein the antenna soft switching is performed by the antenna soft switching system arranged between at least one radio frequency, RF, transmitter module and at least one antenna array, the method comprises reducing a level of unwanted emissions of a transmitted RF spectrum of the AoD direction finding transmitter unit by controlling, by a waveform generated by a generator unit of the antenna soft switching system, a switching network of the antenna soft switching system so that the amplitude of an RF signal is switched smoothly from a first antenna port to a second antenna port.

The switching network may be implemented as a part of an antenna switch changing active antenna port from the first antenna port to the second antenna port during the switching event, wherein the controlling of the switching network may comprise: ramping down smoothly the amplitude of the RF signal at the first antenna port, and simultaneously ramping up smoothly the amplitude of the RF signal at the second antenna port.

Alternatively, the switching network may be a separate switching network providing a centralized soft switching system, wherein the controlling of the switching network may comprise: ramping down smoothly the amplitude RF signal at the first antenna port; simultaneously ramping up smoothly the amplitude of the RF signal at a reference port; changing, by a switching network of an antenna switch, active antenna port from the first antenna port to the second antenna port, while the RF signal is led to the reference port; ramping down smoothly the amplitude of the RF signal at the reference port; and simultaneously ramping up smoothly the amplitude of the RF signal at second antenna port.

Alternatively, the switching network may be a separate switching network providing a centralized soft switching system and the first antenna port may belong to a first antenna group and the second antenna port may belong to a second antenna group, wherein the method may comprise: ramping down smoothly the amplitude of the RF signal at the first antenna group, and simultaneously ramping up smoothly the amplitude of the RF signal at the second antenna group, wherein the second antenna port is pre-selected as the active antenna of the second antenna group.

<FIG> illustrates schematically an example an Angle of Departure (AoD) direction finding transmitter unit <NUM> according to the invention. The AoD direction finding transmitter unit <NUM> may comprise an antenna soft switching system <NUM>, <NUM>, <NUM> according to any of the embodiments of the invention as will be described in this application. The AoD transmitter unit <NUM> further comprises at least one radio frequency (RF) transmitter module <NUM> for generating at least one RF signal to be transmitted and at least one antenna array <NUM> comprising at least two antenna elements 332a-332n via which the at least one RF signal <NUM> may be transmitted by feeding the at least one RF signal <NUM> consecutively to each antenna element 332a-332n of the at least one antenna array <NUM>. The antenna soft switching system <NUM> may be implemented as a part of an antenna switch configured to perform switching between the antenna elements 332a-332n, i.e. the antenna switch comprises a switching network formed by a plurality of switching devices configured to select consecutively the antenna element 332a-332n of the at least one antenna array <NUM> via which the RF signal is transmitted. In other words, the antenna switch is configured to consecutively activate each antenna element 332a-332n of the at least one antenna array <NUM> at a time. The AoD transmitter unit <NUM> may transmit the RF signal via the active antenna element so that each antenna element is consecutively changed, i.e. switched, as the active antenna element. Each antenna element 332a-332n may be connected to respective at least antenna port 306a-306n to connect the antenna array <NUM> to the antenna soft switching system <NUM> and to the antenna switch. This means that the number or antenna elements 332a-332n of the at least one antenna array <NUM> defines the number of the antenna ports 306a-306n. For example, if the at least one antenna array <NUM> comprises <NUM> single-end antenna elements, the number of antenna ports 306a-306n is <NUM>. According to another example, if the at least one antenna array <NUM> comprises <NUM> differentially fed balanced antenna elements 332a-332n, the number of antenna ports 306a-306n is <NUM>. According to yet another example, if the at least one antenna array <NUM> comprises <NUM> differentially fed balanced antenna elements 332a-332n and <NUM> single-end antenna elements 332a-332n, the number of antenna ports 306a-306n is <NUM>. The antenna switch may comprise a conversion unit configured to provide single-end to differential conversion of the at least one RF signal for providing at least one positive phase RF signal RF+ and at least one negative phase RF signal RF-for each differentially fed balanced antenna elements 332a-332n of the at least one antenna array <NUM>.

In the AoD transmitter unit <NUM> the switching of the RF signal to be transmitted from one antenna element, i.e. a first antenna element 332a, to another antenna element, i.e. a second antenna element 332b, may cause unwanted spreading, i.e. unwanted emissions, of the transmitted, i.e. radiated, RF spectrum, if the switching is instantaneous. The unwanted emissions of the transmitted RF spectrum may be out-of-channel spectral emissions, i.e. emissions at frequencies being offset from the transmission frequency. This spreading may cause interference to radio receivers of other systems nearby the AoD transmitter unit <NUM>, may potentially violate the regulatory requirements for spectral emissions, and/or may cause non-compliance with relevant standards, e.g. Bluetooth. The AoD functionality in direction finding has been included in a Bluetooth SIG standard version <NUM>. The AoD functionality of the direction finding systems is enabled by adding so called Constant Tone Extension (CTE) period, i.e. CTE frame <NUM>, at the end of a transmitted Bluetooth low energy (BLE) advertisement packet frame. <FIG> illustrates an example structure of a CTE frame <NUM> as defined in BLE <NUM>. The CTE frame may comprise the following structure: a guard period <NUM>, a reference period <NUM>, and a plurality of alternating switch slots 430a-430n and sample slots 440a-440b. The guard period <NUM> is a first part of the CTE and no useful information is transmitted during the guard period <NUM>. The duration of the guard period <NUM> is <NUM> microseconds. The reference period <NUM> is for the phase reference for subsequent measurements. The duration of the reference period <NUM> is <NUM> micro-seconds. Each sample slot 440a-440n is the time reserved for phase measurement, i.e. the actual measurement, from the active antenna element. The duration of each sample slot 440a-440n is <NUM> microsecond or <NUM> microseconds. Each switch slot 430a-430n is the time reserved for changing, i.e. switching, the active antenna port. The duration of each switch slot 430a-430n is <NUM> microsecond or <NUM> microseconds. It means that the switching of the transmitted RF signal from the first antenna element 332a to the second antenna element 332b needs to be performed during <NUM> microsecond or <NUM> microseconds, which may be considered instantaneous switching and cause wideband spreading of the transmitted RF spectrum. <FIG> illustrates an example of transmitted RF spectrum, wherein the instantaneous switching causes spreading of the transmitted RF spectrum, which may be seen as high out-of-channel spectral emissions of the RF spectrum. In this example, the level of the out-of-channel spectral emissions may be approximately between -<NUM> dB and -<NUM> dB with small offset frequencies (less than ±<NUM> from the transmit frequency) and approximately between -<NUM> dB and -<NUM> dB with higher offset frequencies (between ±<NUM> - ± <NUM>).

The soft switching system <NUM> comprises a timing unit <NUM>, a generator unit <NUM>, and a switching network <NUM>. The switching network <NUM> is arranged on the RF signal path <NUM> between at least one RF port <NUM> and the antenna ports 306a-306n. The antenna soft switching system <NUM> may be connected to the RF transmitter module <NUM> via the at least one RF port <NUM> to provide at least one RF signal <NUM> from the RF transmitter module <NUM> via the antenna ports 306a-306n to each antenna element connected to the respective antenna port at a time. The timing unit <NUM> is configured to obtain at least a starting time of the switching event and optionally also a termination time of the switching event. The timing unit <NUM> may be configured to define at least the starting time of the switching event and optionally also a termination time of the switching event. Alternatively or in addition, the timing unit <NUM> may receive, e.g. as a control signal, the starting time of the switching event and/or the termination time of the switching event from an external control unit, e.g. a switch control unit of the antenna switch. Alternatively or in addition, the termination time of the switching event may be defined by a waveform generated by the generator unit <NUM>. The generator unit <NUM> is configured to generate at least one waveform for controlling the switching network <NUM>. The generator unit <NUM> is configured to control the switching network <NUM> so that the amplitude of the RF signal <NUM> is switched substantially smoothly, i.e. gradually, from the first antenna port 306a to the second antenna port 306b so that the level of unwanted emissions of the transmitted RF spectrum of the AoD transmitter unit <NUM> caused by the switching event are reduced. During the switching event the impedance seen by the at least one RF port <NUM> may be maintained constant. With the term "switching event" is meant throughout this application the switching the RF signal <NUM> from one antenna port to another. A sequence of switching events, i.e. switching sequence comprises a plurality of switching events. Above the invention is defined so that the switching takes place from the first antenna port 306a to the second antenna port 306a, however the inventive idea is directly applicable in switching between any two antenna ports 306a-306n. Moreover, because each antenna element 332a-332e of the at least one antenna array <NUM> is connected to at least antenna port 306a-306n, the switching from one antenna port to another antenna port causes also switching from one antenna element to another antenna element, i.e. switching from first antenna element 332a connected to the first antenna port 306a to the second antenna element 332b connected to the second antenna port 306b. The first antenna port 306a and the second antenna port 306b may be any two antenna ports 306a-306b. Respectively, the first antenna element 332a connected to the respective first antenna port 306a and the second antenna element 332b connected to the respective second antenna port 306b may be any two antenna elements 332a-332n of the at least one antenna array <NUM>.

<FIG> illustrates an example embodiment of the antenna soft switching system <NUM> according to the invention, wherein the switching network <NUM> is implemented as a part of the antenna switch configured to perform the switching of the RF signal from the first antenna port 306a to the second antenna port 306b, i.e. the switching network <NUM> is the switching network of the antenna switch configured to change active antenna port from the first antenna port 306a to the second antenna port 306b and correspondingly change active antenna element from the first antenna element 332a to the second antenna element 332b. In other words, the switching network <NUM> may be configured to activate the second antenna port 306b and the antenna element 332b connected to the second antenna port 306b and to inactivate, i.e. disconnect, the first antenna port 306a and the antenna element 332a connected to the first antenna port <NUM>. The generator unit <NUM> is configured to control the switching network <NUM> during the switching event so that the amplitude of the RF signal <NUM> at the first antenna port 306a is substantially smoothly ramped down, i.e. decreased, and simultaneously the amplitude of the RF signal <NUM> at the second antenna port 306b is substantially smoothly ramped up, i.e. increased. The switching network <NUM> may comprise at least two switching devices 602a, 602b. The at least two switching devices 602a, 602b may be resistive switching devices, fully reactive switching devices, partially reactive switching devices, or active switching devices. The at least two resistive switching devices 602a, 602b may utilize junction gate field-effect transistors (JFETs), Metal-oxide-semiconductor field-effect transistors (MOSFETs), metal-semiconductor field-effect transistors (MESFETs); pseudomorphic high electron mobility transistors (PHEMTs); PIN diodes, or any other suitable RF switches.

In the example antenna soft switching system <NUM> of <FIG> the generator unit <NUM> comprises an analog ramp generator and the two switching devices 602a, 602b are JFET, MOSFET, MESFET or PHEMT transistors. The transistors may be operated in a linear "triode" region, when used as a switching device 602a, 602b. In this region the channel of the switching device 602a, 602b acts as a variable resistor, which value is dependent on the gate voltage (Vgs) of the switching device 602a, 602b. By ramping up/down, i.e. increasing/decreasing, the gate voltage slowly during the switching event, i.e. during the switch slot 430a-430n, causes also RF attenuation of the switching device 602a, 602b to vary smoothly in proportion to the Rds/Vgs transfer function of the switching device 602a, 602b, wherein Rds is the resistance of the transistor. This smooth switching then reduces the out-of-channel spectral emissions of the transmitted RF spectrum in comparison to instantaneous on/off switching between the first antenna port 306a and the second antenna port 306b. In this example antenna soft switching system <NUM> the gate voltage of the switching device 602a ramps down when the gate voltage of the switching device 602b ramps up and vice versa. The waveforms of the gate voltages of the switching devices 602a, 602b are used as control signals of the switching devices 602a, 602b. <FIG> schematically illustrates an example of a time domain amplitude of the RF waveform of the second switching device 602b during the switching event, wherein the control signal of the second switching device 602b is a linear ramp waveform generated by the analog ramp generator to smoothly ramp up the amplitude of the RF signal <NUM> at the second antenna port 306b. The same linear ramp waveform may be used to control the first switching device 602a and the second switching device 602b by arranging an inverting component <NUM>, e.g. an inverting amplifier arrangement with a voltage gain arranged to <NUM>, between the generator unit <NUM> and the second switching device 602b to invert the linear ramp waveform generated by the analog generator unit <NUM>. As a result of the inversion of the waveform of the second switching device 602b is opposite to the waveform of the first switching device 602a in order to substantially smoothly ramp down the gate voltage of the switching device 602a when the gate voltage of the switching device 602b ramps up substantially smoothly. <FIG> illustrates an example of the resulting transmitted RF spectrum with the analog soft switching system <NUM> illustrated in <FIG>. From <FIG> may be seen that the level of the unwanted emissions of the transmitted RF spectrum are reduced in comparison to the instantaneous switching illustrated in <FIG>. In the example of <FIG> the level of the out-of-channel spectral emissions may be approximately between - <NUM> dB and -<NUM> dB with small offset frequencies (less than ±<NUM> from the transmit frequency) and approximately between -<NUM> dB and -<NUM> dB with higher offset frequencies (between ±<NUM> - ± <NUM>).

Different switching devices 602a, 602b require different control signals depending on the characteristics of the particular switching device. For example, if the switching device 602a, 602b is a PIN diode, the switch resistance may be controlled by DC or low-frequency current through the PIN diode. If the waveform is generated analogically by the generator unit <NUM>, the shape of the waveform may be practically limited to some simple waveforms, such as a linear ramp waveform. Also, the Rds/Vgs characteristics of the switching device 602a, 602b may have an impact on the resulting RF waveform of the switching device 602a, 602b.

More complicated waveforms may be generated digitally, which digital waveform may be converted into analog waveform by means of a digital to analog (D/A) converter. <FIG> illustrates another example embodiment of the antenna soft switching system <NUM> according to the invention, wherein the generator unit <NUM> comprises a digital waveform generator <NUM>, a D/A converter <NUM>, and a reconstruction filter <NUM>. The output of the D/A converter needs to be filtered to remove aliases and fed to the switching devices 602a, 602b, e.g. to the gates of transistor switching devices. Generating the waveform digitally requires some additional hardware, such as storage, e.g. a memory of the generator unit <NUM> or generating function for the waveform, the D/A converter <NUM>, and the reconstruction filter <NUM>, as well as the timing unit <NUM> needs to run at a higher clock rate than the actual switching frequency. Moreover, in digital soft switching the timing unit <NUM> may further be configured to time the events of the whole sequence of switching events comprising timings of e.g. reading the samples of the waveform from the memory of the generator unit <NUM>, updating the control signals of the switching devices 602a, 602b, etc. Otherwise, the operation of the soft switching system <NUM> of <FIG> is similar with the soft switching system <NUM> of <FIG>.

Alternatively, instead of using the resistive switching devices the at least two switching devices 602a, 602b may be digitally controlled attenuators. <FIG> illustrates another example embodiment of the antenna soft switching system <NUM> according to the invention, wherein the switching network <NUM> comprises at least two resistive switching devices 602a, 602b implemented as digitally controlled attenuators and the generator unit <NUM> comprises a digital waveform generator <NUM>. The switching device 602a, 602b implemented as the digitally controlled attenuator may be constructed from multiple resistive elements 902a-902n that are connected in series or parallel using controllable switching elements 904a-904n, such as JFET, MOSFET, MESFET or PHEMT transistors. The topology of the digitally controlled attenuator may be series, parallel, L-network, pi-network, T network, etc. <FIG> illustrates schematically an example of a switching device implemented as a digitally controlled attenuator. The controllable switching elements 904a-904n that are used for selecting the resistive elements 902a-902n may be controlled digitally according to a pre-determined timing pattern. The generator unit <NUM> may further comprises a waveform encoder <NUM> configured to convert the waveform generated by the digital waveform generator <NUM> into control signals of the individual controllable switching elements 904a-904n. Especially, when the number of controllable switching elements 904a-904n is higher than the word length of individual samples of the waveform, the use of waveform encoder <NUM> preferable in comparison to generating directly the control signals of the individual controllable switching elements or storing the control signals of the individual controllable switching elements in the memory of the generator unit <NUM>. The resulting RF waveform resembles a staircase, i.e. quantized waveform, with steps small enough to have reduced RF spectrum. In other words, the quantized waveform enables substantially smooth switching of the amplitude of the RF signal <NUM> from the first antenna port 306a to the second antenna port 306b. The word length means the resolution, i.e. dynamic accuracy, of the digital sample that has been taken, e.g. the word length may be <NUM>-bits to <NUM> bits. <FIG> schematically illustrates an example of a time domain amplitude of RF waveform of the switching device 602b during the switching event, wherein the control signal of the second switching device 602b is a quantized linear ramp waveform generated by the generator unit <NUM> comprising the digital generator <NUM> and the waveform encoder <NUM> to substantially smoothly ramp up the amplitude of the RF signal <NUM> at the second antenna port 306b. The target waveform is illustrated with dashed line and the quantized waveform is illustrated with solid (staircase) line. From <FIG> it may be seen that although the quantized waveform is formed by small steps, it approximates the target ramp waveform, i.e. the quantized waveform enables substantially smooth ramping up of the amplitude of the RF signal <NUM> at the second antenna port 306b. In this example, <NUM>-bit quantization is used, but also any other quantization may be used. <FIG> illustrates an example of the resulting transmitted RF spectrum with the digital soft switching system <NUM> illustrated in <FIG>. From <FIG> may be seen that the level of the unwanted emissions of the transmitted RF spectrum are reduced in comparison to the instantaneous switching illustrated in <FIG> and almost as good reduction of the level of the unwanted emissions of the transmitted RF spectrum is achieved as with the analog soft switching as illustrated in <FIG>. In the example of <FIG> the level of the out-of-channel spectral emissions may be approximately between -<NUM> dB and -<NUM> dB with small offset frequencies (less than ±<NUM> from the transmit frequency) and approximately -<NUM> dB with higher offset frequencies (between ±<NUM> - ± <NUM>). <FIG> illustrates an example of adjacent-channel leakage ratio (ACLR) at the transmitting frequency and at different offset frequencies with the digital soft switching system <NUM> illustrated in <FIG>. As comparison in <FIG> the ACLR with the instantaneous switching is illustrated. In this example, the improvement in the ACLR with the digital soft switching system <NUM> at the offset frequencies is <NUM> dB or even more when compared to instantaneous, i.e. hard, switching.

<FIG> illustrates another example embodiment of the antenna soft switching system <NUM> according to the invention, wherein the generator unit <NUM> comprises a counter <NUM> and the waveform is encoded into the at least two switching devices 602a, 602b, i.e. the values of the resistive elements 902a-902n of the switching devices 602a, 602b may be defined such that the resulting RF waveform approximates a desired waveform when the control signal is simply an ascending or descending count. This may simplify the generator unit <NUM>, because there is no need to store the waveform in the memory of the generator unit <NUM>. Otherwise the operation of the soft switching system <NUM> of <FIG> is similar with the soft switching system <NUM> of <FIG>.

There may be situations where having the resistive switching elements 602a, 602b at the antenna ports 306a, 306b is not practical due to size, cost or RF performance reasons. This may be the case especially with the digitally controlled attenuators, e.g. as in the example antenna soft switching systems <NUM> illustrated in <FIG> and <FIG>. In that case it may be preferable to use a single centralized antenna soft switching system <NUM> together with a switching network <NUM> of the antenna switch comprising the plurality of switching devices of the antenna switch configured to select consecutively the antenna element 332a-332n of the at least one antenna array <NUM>. In this case the antenna soft switching system <NUM> may steer the RF signal <NUM> into a dummy load <NUM> of a reference port <NUM> and the changing of the antenna element 332a, 332b takes place at a moment when there is no RF signal <NUM> present at any antenna port 306a, 306b.

<FIG> illustrates an example embodiment of the antenna soft switching system <NUM> according to the invention, wherein the switching network <NUM> is a separate switching network providing a centralized antenna soft switching system <NUM>. The switching network <NUM> is separate from the switching network <NUM> of the antenna switch, i.e. the plurality of switching devices of the antenna switch configured to select consecutively the antenna element 332a-332n of the at least one antenna array <NUM>. The generator unit <NUM> may be configured to control the switching network <NUM> so that the amplitude of the RF signal <NUM> at the first antenna port 306a is substantially smoothly ramped down and simultaneously the amplitude of the RF signal <NUM> at the reference port <NUM> is ramped up substantially smoothly. The switching network <NUM> of the antenna switch is configured to change the active antenna port and the active antenna element by connecting the RF signal path <NUM> from the first antenna port 306a to the second antenna port 306b while the RF signal <NUM> is led to the dummy load <NUM> of the reference port <NUM>. After that the generator unit <NUM> may be configured to control the switching network <NUM> so that the amplitude of the RF signal <NUM> at the reference port <NUM> is substantially smoothly ramped down and simultaneously the amplitude of the RF signal <NUM> at second antenna port 306b is substantially smoothly ramped up. In the example antenna soft switching system <NUM> of <FIG> the generator unit <NUM> comprises a digital waveform generator <NUM> and a waveform encoder <NUM>. <FIG> schematically illustrates examples time domain amplitudes of RF waveforms of the switching device 602a, switching device 602b, and the dummy load <NUM> during the switching event, wherein the control signals of the switching devices 602a, 602b are quantized linear ramp waveforms generated by the generator unit <NUM> comprising the digital generator <NUM> and the waveform encoder <NUM>.

One minor drawback of this embodiment may be that the RF signal <NUM> into the active antenna element, i.e. the first antenna element 332b, needs to be ramped down before the active antenna element is changed, and then ramped up again after the second antenna element 332b is activated. This means that the up/down ramps need to be twice as fast as in the embodiments, where the RF signal <NUM> is steered from one antenna element into another antenna element. For example, in the examples of <FIG> and <FIG> the duration of the switching event is <NUM> microseconds, during which the RF signal <NUM> is steered from one antenna element into another element. In the example of <FIG> the duration of the switching event is also <NUM> microseconds, but during that the RF signal <NUM> is steered from one antenna element into the dummy load and from the dummy load into another antenna element. Nevertheless, the reduction of the unwanted emissions of the transmitted RF spectrum may still be achieved, if there is enough time available for the complete switching event.

According to one example embodiment of the invention, if the transmitter module <NUM> is linear, or benignly non-linear, the common centralized soft switching system <NUM> may also be implemented in the analog or digital baseband of the transmitter module <NUM>. This enables that there is no need to use a dummy load <NUM> to absorb the RF signal, and the losses at high power level may be avoided.

<FIG> illustrates an example embodiment of the antenna soft switching system <NUM> according to the invention, wherein the switching network <NUM> is a separate switching network providing a centralized antenna soft switching system <NUM> and the antenna ports 306a-306n and the antenna elements 332a-332n are arranged to two groups, wherein the first antenna port 306a and the respective first antenna element 332a belong to the first antenna group <NUM> and the second antenna port 306b and the respective second antenna element 332b belong to the second antenna port group <NUM>. The switching network <NUM> is separate from the switching network 1010a of the first antenna group <NUM> and the switching network 1010b of the second antenna group <NUM> of the antenna switch, i.e. the plurality of switching devices of the antenna switch configured to select consecutively the antenna element 332a-332n of the at least one antenna array <NUM>. The active antenna element 332a-332n may be selected alternately from the first antenna group <NUM> and form the second antenna group <NUM>. The selection of the active antenna port and respective active antenna element of the antenna group may be performed when said antenna group is not active, i.e. the RF signal <NUM> is not led to said antenna group. For example, the second antenna port 306b may be pre-selected as the active antenna of the second antenna group <NUM> by the switching network 1010b of the second antenna group <NUM> when the RF signal is led to the first antenna group <NUM>. Alternatively, the first antenna port 306a may be pre-selected as the active antenna of the first antenna group <NUM> by the switching network 1010a of the first antenna group <NUM> when the RF signal <NUM> is led to the second antenna group <NUM>. To switch between the first antenna port 306a and the second antenna port 306b, the generator unit <NUM> may be configured to control the switching network <NUM> so that the amplitude of the RF signal <NUM> at the first antenna group <NUM> (to which the first antenna port 306a belongs) is substantially smoothly ramped down and simultaneously the amplitude of the RF signal <NUM> at the second antenna group <NUM> (to which the second antenna port 306b belongs) is substantially smoothly ramped up, wherein the second antenna port 306b is pre-selected as the active antenna of the second antenna group <NUM>. With this the above described drawback may be avoided. The two antenna groups <NUM>, <NUM> do not need to be exclusive, i.e. any physical antenna element may belong to both groups, if access from both RF paths is provided by the antenna switches 1010a, 1010b (dashed lines between the first antenna element 332a and the switching network 1010b of the second antenna group <NUM> and between the second antenna element 332b and the switching network 1010a of the first antenna group <NUM> in <FIG>). This enables flexible switching sequences, where the active antenna element may be selected arbitrarily at each switching event.

Above the different embodiments of the antenna soft switching system <NUM>, <NUM>, <NUM> are described by referring to JFET, MOSFET, MESFET or PHEMT transistors, PIN diodes, and/or digitally controlled attenuators as the at least two resistive switching devices 602a, 602b. Alternatively, fully or partially reactive, i.e. inductive or capacitive, switching devices 602a, 602b may be used in any of the embodiments of the invention described above. Alternatively, active switching elements 602a, 602b, e.g. amplifiers, may be used as the at least two resistive switching devices 602a, 602b in any of the embodiments of the invention described above. The transfer function, i.e. gain, of the active switching devices may be either continuously variable, i.e. analog control signals, or digitally programmable, i.e. digital control signals.

The connections, i.e. communicatively couplings, between any components, modules, and/or units according to the invention throughout this application (except between transmitter unit and receiver unit) may be based on any known wired communication technologies. The communication between the transmitter unit and receiver unit may be based on any known wireless communication technologies, e.g. Bluetooth low energy (BLE).

Above the invention is described relating to the antenna soft switching system of an AoD direction finding transmitter unit. Next an example of an antenna soft switching method according to the invention is described by referring to <FIG>. The method comprises controlling <NUM>, by a waveform generated by a generator unit <NUM>, the switching network <NUM> so that the amplitude of the RF signal is switched substantially smoothly, i.e. gradually, from the first antenna port to the second antenna port in order to reduce level of unwanted emissions of a radiated RF spectrum of the direction finding transmitter as described above. During the switching event the impedance seen by the at least one RF port <NUM> may be maintained constant. The controlling step <NUM> will be described more referring to <FIG> illustrating different embodiments of the method according to the invention.

According to an example embodiment of the method according to the invention, wherein the switching network <NUM> is implemented as a part of the antenna switch configured to perform the switching of the RF signal from the first antenna port 306a to the second antenna port 306b, i.e. the switching network <NUM> is the switching network of the antenna switch configured to change active antenna port from the first antenna port 306a to the second antenna port 306b and correspondingly change active antenna element from the first antenna element 332a to the second antenna element 332b (e.g. the example antenna soft switching systems <NUM> of <FIG>). In other words, the switching network <NUM> may be configured to switch the RF signal <NUM> from the first antenna port 306a to the second antenna port 306b in order to activate the second antenna port 306b and the antenna element 332b connected to the second antenna port 306b and to inactivate, i.e. disconnect, the first antenna port 306a and the antenna element 332a connected to the first antenna port <NUM>. The controlling step <NUM> may comprise: ramping down, i.e. decreasing, <NUM> substantially smoothly the amplitude of the RF signal <NUM> at the first antenna port 306a, and simultaneously ramping up, i.e. increasing, <NUM> substantially smoothly the amplitude of the RF signal <NUM> at the second antenna port 306a as discussed above. This example embodiment of the method according to the invention is illustrated in <FIG>.

According to an example embodiment of the method according to the invention, wherein the switching network <NUM> is a separate switching network providing a centralized soft switching system <NUM> (e.g. the example antenna soft switching system of <FIG>). The switching network <NUM> is separate from the switching network <NUM> of the antenna switch, i.e. the plurality of switching devices of the antenna switch configured to select consecutively the antenna element 332a-332n of the at least one antenna array <NUM>. The controlling step <NUM> may comprise: ramping <NUM> down substantially smoothly the amplitude RF signal <NUM> at the first antenna port 306a and simultaneously ramping <NUM> up substantially smoothly the amplitude of the RF signal <NUM> at a reference port <NUM>; changing, i.e. switching, <NUM> active antenna port and the active antenna element by connecting by a switching network of the antenna switch, the RF signal path from the first antenna port 306a to the second antenna port 306b, while the RF signal <NUM> is led to the dummy load <NUM> of the reference port <NUM>, ramping <NUM> down substantially smoothly the amplitude of the RF signal <NUM> at the reference port <NUM> and simultaneously ramping <NUM> up substantially smoothly the amplitude of the RF signal <NUM> at second antenna port 306b as discussed above. This example embodiment of the method according to the invention is illustrated in <FIG>.

Alternatively, according to an example embodiment of the method according to the invention, wherein the switching network <NUM> is a separate switching network providing a centralized antenna soft switching system <NUM> and the antenna ports 306a-306n and the antenna elements 332a-332n are arranged to two groups, wherein the first antenna port 306a and the respective first antenna element 332a belong to the first antenna group <NUM> and the second antenna port 306b and the respective second antenna element 332b belong to the second antenna port group <NUM> (e.g. the example antenna soft switching system of <FIG>). The controlling step <NUM> may comprise: ramping <NUM> down substantially smoothly the amplitude of the RF signal <NUM> at the first antenna group <NUM>, and simultaneously ramping <NUM> up substantially smoothly the amplitude of the RF signal <NUM> at the second antenna group <NUM>, wherein the second antenna port 306b is pre-selected as the active antenna of the second antenna group <NUM> as discussed above. This example embodiment of the method according to the invention is illustrated in <FIG>.

There may be also some other applications for the above described antenna soft switching system and method in addition to the AoD transmitter unit <NUM>. For example, the antenna soft switching system and method according to the invention may be used for smoothing of transmitter power on/off transients in time-domain duplex (TDD) radios with a single antenna. The antenna soft switching system <NUM> of <FIG> and the respective antenna soft switching method of <FIG> may be the most suitable for this application. The antenna soft switching system <NUM> may be used at any point of a linear transmitter chain, or between a power amplifier (PA) and the antenna in the case of a non-linear PA. Alternatively, the antenna soft switching system and method according to the invention may be used as a programmable matched attenuator. The resistive at least two switching devises 602a, 602b may be dimensioned such that the switching network <NUM> maintains a constant impedance over the entire attenuation range. The antenna soft switching system <NUM> of <FIG> and the respective antenna soft switching method of <FIG> may be the most suitable for this application. Alternatively, the antenna soft switching system and method according to the invention may be used for combining two or more antennas together into a single transmitter or receiver port, with adjustable power ratio between two antennas. This would be a lossy combiner in the case of resistive switching devices, but may be useful in some applications, such as beam-forming.

Claim 1:
An Angle of Departure, AoD, direction finding transmitter unit (<NUM>) comprising an antenna soft switching system (<NUM>, <NUM>, <NUM>), characterized in that the antenna soft switching system (<NUM>, <NUM>, <NUM>) is arranged between at least one radio frequency, RF, transmitter module (<NUM>) and at least one antenna array (<NUM>), the soft switching system (<NUM>, <NUM>, <NUM>) comprises:
a timing unit (<NUM>) for obtaining at least a starting time of a switching event,
a switching network (<NUM>) arranged on an RF signal path (<NUM>) between an RF port (<NUM>) and a first antenna port (306a) and a second antenna port (306b), and
a generator unit (<NUM>) for generating at least one waveform for controlling the switching network (<NUM>),
wherein the generator unit (<NUM>) is configured to reduce a level of unwanted emissions of a transmitted RF spectrum of the AoD direction finding transmitter unit (<NUM>) by controlling the switching network (<NUM>) so that the amplitude of the RF signal (<NUM>) is switched smoothly from the first antenna port (306a) to the second antenna port (306b).