Patent Publication Number: US-2022224385-A1

Title: An antenna soft switching solution of an aod direction finding transmitter

Description:
TECHNICAL FIELD 
     The invention concerns in general the technical field of direction finding systems. Especially the invention concerns soft switching of a direction finding transmitter. 
     BACKGROUND 
     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. 1  schematically illustrates a simple example of the principle of AoA direction finding system  100 . The AoA radio direction finding system  100  comprises a transmitter unit  102  and a receiver unit  104 . The transmitter unit  102  comprises a transmitting antenna  106  and a RF transmitter  108 , e.g. BLE transmitter. The receiver unit  104  comprises a receiving antenna array, which comprises in this simple example four antenna elements  110   a - 110   d . Furthermore, the receiver unit  104  comprises an antenna switch unit  112  to switch between the antenna elements  110   a - 110   d , a RF receiver  114 , e.g. BLE receiver, and an AoA estimation unit  116 . The AoA estimation unit  116  of the receiver unit  104  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. 2  schematically illustrates a simple example of the principle of AoD direction finding system  200 . The AoD radio direction finding system  200  comprises a transmitter unit  202  and a receiver unit  204 . The transmitter unit  202  comprises a transmitting antenna array, which comprises in this simple example four antenna elements  206   a - 206   d . Furthermore, the transmitter unit  204  comprises an antenna switch unit  212  to switch the transmitted signal between the antenna elements  206   a - 206   d  and a RF transmitter  208 , e.g. BLE transmitter. The receiver unit  204  comprises a receiving antenna  210 , a RF receiver  214 , e.g. BLE receiver, and an AoD estimation unit  216 . The AoD estimation unit  216  of the receiver unit  204  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. 
     Typically, soft switching has been used in commercial direction finding receivers, e.g. AoA receivers. 
     However, there is need to develop further solutions in order to reduce radiated spectral emissions of an AoD transmitter. 
     SUMMARY 
     The following presents a simplified summary in order to provide basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention. 
     An objective of the invention is to present an antenna soft switching system, 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 antenna soft switching system, 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 antenna soft switching system, an AoD direction finding transmitter unit, and an antenna soft switching method as defined by the respective independent claims. 
     According to a first aspect, an antenna soft switching system of an Angle of Departure, AoD, direction finding transmitter unit is provided, 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 a radio frequency, 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 control the switching network so that the amplitude of the RF signal is switched substantially smoothly from the first antenna port to the second antenna port so that level of unwanted emissions of a transmitted RF spectrum of the AoD direction finding transmitter unit are reduced. 
     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 substantially smoothly ramped down, and simultaneously the amplitude of the RF signal at the second antenna port is substantially 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 substantially smoothly ramped down; and simultaneously the amplitude of the RF signal at a reference port is ramped up substantially 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 substantially smoothly ramped down and simultaneously the amplitude of the RF signal at second antenna port is substantially 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 substantially smoothly ramped down, and simultaneously the amplitude of the RF signal at the second antenna group is substantially 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 Angle of Departure, AoD, direction finding transmitter unit is provided, wherein the AoD direction finding transmitter unit comprises the antenna soft switching system described above. 
     According to a third aspect, an antenna soft switching method for an AoD direction finding transmitter unit is provided, wherein the method comprises controlling, by a waveform generated by a generator unit, a switching network so that the amplitude of a radio frequency, RF, signal is switched substantially smoothly from a first antenna port to a second antenna port so that level of unwanted emissions of a transmitted RF spectrum of the AoD direction finding transmitter unit are reduced. 
     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 substantially smoothly the amplitude of the RF signal at the first antenna port, and simultaneously ramping up substantially 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 substantially smoothly the amplitude RF signal at the first antenna port; simultaneously ramping up substantially 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 substantially smoothly the amplitude of the RF signal at the reference port; and simultaneously ramping up substantially 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 substantially smoothly the amplitude of the RF signal at the first antenna group, and simultaneously ramping up substantially 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. 
     Various exemplifying and non-limiting embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying and non-limiting embodiments when read in connection with the accompanying drawings. 
     The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of unrecited features. 
     The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of “a” or “an”, i.e. a singular form, throughout this document does not exclude a plurality. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       The embodiments of the invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings. 
         FIG. 1  illustrates schematically a simple example of the principle of Angle of Arrival (AoA) direction finding system. 
         FIG. 2  illustrates schematically a simple example of the principle of Angle of Departure (AoD) direction finding system. 
         FIG. 3  illustrates schematically an example an Angle of Departure (AoD) direction finding transmitter unit according to the invention. 
         FIG. 4  illustrates schematically an example structure of a CTE frame. 
         FIG. 5  illustrates schematically an example of transmitted RF spectrum with instantaneous switching. 
         FIGS. 6A-6D  illustrate schematically examples of an antenna soft switching system according to the invention. 
         FIG. 7A  illustrates schematically an example of a time domain amplitude of RF waveform of a switching device with a linear ramp waveform. 
         FIG. 7B  illustrates schematically an example of a time domain amplitude of RF waveform of a switching with a quantized linear ramp waveform. 
         FIG. 8A  illustrates an example of a resulting transmitted RF spectrum with an analog soft switching system according to the invention. 
         FIG. 8B  illustrates an example of a resulting transmitted RF spectrum with a digital soft switching system according to the invention. 
         FIG. 8C  illustrates an example of adjacent-channel leakage ratio (ACLR) with instantaneous switching and digital soft switching according to the invention. 
         FIG. 9  illustrates schematically an example of a switching device implemented as a digitally controlled attenuator. 
         FIG. 10  illustrates another example of an antenna soft switching system according to the invention. 
         FIG. 11  illustrates schematically examples of RF waveforms of switching devices and a dummy load with a quantized linear ramp waveform. 
         FIG. 12  illustrates another example of an antenna soft switching system according to the invention. 
         FIGS. 13-16  illustrate examples of a method according to the invention. 
     
    
    
     DESCRIPTION OF THE EXEMPLIFYING EMBODIMENTS 
       FIG. 3  illustrates schematically an example an Angle of Departure (AoD) direction finding transmitter unit  300  according to the invention. The AoD direction finding transmitter unit  300  may comprise an antenna soft switching system  310 ,  1000 ,  1200  according to any of the embodiments of the invention as will be described in this application. The AoD transmitter unit  300  further comprises at least one radio frequency (RF) transmitter module  320  for generating at least one RF signal to be transmitted and at least one antenna array  330  comprising at least two antenna elements  332   a - 332   n  via which the at least one RF signal  305  may be transmitted by feeding the at least one RF signal  305  consecutively to each antenna element  332   a - 332   n  of the at least one antenna array  330 . The antenna soft switching system  310  may be implemented as a part of an antenna switch configured to perform switching between the antenna elements  332   a - 332   n , i.e. the antenna switch comprises a switching network formed by a plurality of switching devices configured to select consecutively the antenna element  332   a - 332   n  of the at least one antenna array  330  via which the RF signal is transmitted. In other words, the antenna switch is configured to consecutively activate each antenna element  332   a - 332   n  of the at least one antenna array  330  at a time. The AoD transmitter unit  300  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  332   a - 332   n  may be connected to respective at least antenna port  306   a - 306   n  to connect the antenna array  330  to the antenna soft switching system  310  and to the antenna switch. This means that the number or antenna elements  332   a - 332   n  of the at least one antenna array  330  defines the number of the antenna ports  306   a - 306   n . For example, if the at least one antenna array  330  comprises 10 single-end antenna elements, the number of antenna ports  306   a - 306   n  is 10. According to another example, if the at least one antenna array  300  comprises 10 differentially fed balanced antenna elements  332   a - 332   n , the number of antenna ports  306   a - 306   n  is 20. According to yet another example, if the at least one antenna array  330  comprises 10 differentially fed balanced antenna elements  332   a - 332   n  and 10 single-end antenna elements  332   a - 332   n , the number of antenna ports  306   a - 306   n  is 30. 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  332   a - 332   n  of the at least one antenna array  330 . 
     In the AoD transmitter unit  300  the switching of the RF signal to be transmitted from one antenna element, i.e. a first antenna element  332   a , to another antenna element, i.e. a second antenna element  332   b , 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  300 , 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 5.1. The AoD functionality of the direction finding systems is enabled by adding so called Constant Tone Extension (CTE) period, i.e. CTE frame  400 , at the end of a transmitted Bluetooth low energy (BLE) advertisement packet frame.  FIG. 4  illustrates an example structure of a CTE frame  400  as defined in BLE  5 . 1 . The CTE frame may comprise the following structure: a guard period  410 , a reference period  420 , and a plurality of alternating switch slots  430   a - 430   n  and sample slots  440   a - 440   b . The guard period  410  is a first part of the CTE and no useful information is transmitted during the guard period  410 . The duration of the guard period  410  is 4 microseconds. The reference period  420  is for the phase reference for subsequent measurements. The duration of the reference period  420  is 8 micro-seconds. Each sample slot  440   a - 440   n  is the time reserved for phase measurement, i.e. the actual measurement, from the active antenna element. The duration of each sample slot  440   a - 440   n  is 1 microsecond or 2 microseconds. Each switch slot  430   a - 430   n  is the time reserved for changing, i.e. switching, the active antenna port. The duration of each switch slot  430   a - 430   n  is 1 microsecond or 2 microseconds. It means that the switching of the transmitted RF signal from the first antenna element  332   a  to the second antenna element  332   b  needs to be performed during 1 microsecond or 2 microseconds, which may be considered instantaneous switching and cause wideband spreading of the transmitted RF spectrum.  FIG. 5  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 −30 dB and −15 dB with small offset frequencies (less than ±2.5 MHz from the transmit frequency) and approximately between −45 dB and −30 dB with higher offset frequencies (between ±2.5 MHz-±10 MHz). 
     The soft switching system  310  comprises a timing unit  301 , a generator unit  302 , and a switching network  303 . The switching network  303  is arranged on the RF signal path  305  between at least one RF port  304  and the antenna ports  306   a - 306   n . The antenna soft switching system  310  may be connected to the RF transmitter module  320  via the at least one RF port  304  to provide at least one RF signal  305  from the RF transmitter module  320  via the antenna ports  306   a - 306   n  to each antenna element connected to the respective antenna port at a time. The timing unit  301  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  301  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  301  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  302 . The generator unit  302  is configured to generate at least one waveform for controlling the switching network  303 . The generator unit  302  is configured to control the switching network  303  so that the amplitude of the RF signal  305  is switched substantially smoothly, i.e. gradually, from the first antenna port  306   a  to the second antenna port  306   b  so that the level of unwanted emissions of the transmitted RF spectrum of the AoD transmitter unit  300  caused by the switching event are reduced. During the switching event the impedance seen by the at least one RF port  304  may be maintained constant. With the term “switching event” is meant throughout this application the switching the RF signal  305  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  306   a  to the second antenna port  306   a , however the inventive idea is directly applicable in switching between any two antenna ports  306   a - 306   n . Moreover, because each antenna element  332   a - 332   e  of the at least one antenna array  330  is connected to at least antenna port  306   a - 306   n , 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  332   a  connected to the first antenna port  306   a  to the second antenna element  332   b  connected to the second antenna port  306   b . The first antenna port  306   a  and the second antenna port  306   b  may be any two antenna ports  306   a - 306   b . Respectively, the first antenna element  332   a  connected to the respective first antenna port  306   a  and the second antenna element  332   b  connected to the respective second antenna port  306   b  may be any two antenna elements  332   a - 332   n  of the at least one antenna array  330 . 
       FIG. 6A  illustrates an example embodiment of the antenna soft switching system  310  according to the invention, wherein the switching network  303  is implemented as a part of the antenna switch configured to perform the switching of the RF signal from the first antenna port  306   a  to the second antenna port  306   b , i.e. the switching network  303  is the switching network of the antenna switch configured to change active antenna port from the first antenna port  306   a  to the second antenna port  306   b  and correspondingly change active antenna element from the first antenna element  332   a  to the second antenna element  332   b . In other words, the switching network  303  may be configured to activate the second antenna port  306   b  and the antenna element  332   b  connected to the second antenna port  306   b  and to inactivate, i.e. disconnect, the first antenna port  306   a  and the antenna element  332   a  connected to the first antenna port  306 . The generator unit  302  is configured to control the switching network  303  during the switching event so that the amplitude of the RF signal  305  at the first antenna port  306   a  is substantially smoothly ramped down, i.e. decreased, and simultaneously the amplitude of the RF signal  305  at the second antenna port  306   b  is substantially smoothly ramped up, i.e. increased. The switching network  303  may comprise at least two switching devices  602   a ,  602   b . The at least two switching devices  602   a ,  602   b  may be resistive switching devices, fully reactive switching devices, partially reactive switching devices, or active switching devices. The at least two resistive switching devices  602   a ,  602   b  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  310  of  FIG. 6A  the generator unit  302  comprises an analog ramp generator and the two switching devices  602   a ,  602   b  are JFET, MOSFET, MESFET or PHEMT transistors. The transistors may be operated in a linear “triode” region, when used as a switching device  602   a ,  602   b . In this region the channel of the switching device  602   a ,  602   b  acts as a variable resistor, which value is dependent on the gate voltage (Vgs) of the switching device  602   a ,  602   b . By ramping up/down, i.e. increasing/decreasing, the gate voltage slowly during the switching event, i.e. during the switch slot  430   a - 430   n , causes also RF attenuation of the switching device  602   a ,  602   b  to vary smoothly in proportion to the Rds/Vgs transfer function of the switching device  602   a ,  602   b , 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  306   a  and the second antenna port  306   b . In this example antenna soft switching system  310  the gate voltage of the switching device  602   a  ramps down when the gate voltage of the switching device  602   b  ramps up and vice versa. The waveforms of the gate voltages of the switching devices  602   a ,  602   b  are used as control signals of the switching devices  602   a ,  602   b .  FIG. 7A  schematically illustrates an example of a time domain amplitude of the RF waveform of the second switching device  602   b  during the switching event, wherein the control signal of the second switching device  602   b  is a linear ramp waveform generated by the analog ramp generator to smoothly ramp up the amplitude of the RF signal  305  at the second antenna port  306   b . The same linear ramp waveform may be used to control the first switching device  602   a  and the second switching device  602   b  by arranging an inverting component  604 , e.g. an inverting amplifier arrangement with a voltage gain arranged to 1, between the generator unit  302  and the second switching device  602   b  to invert the linear ramp waveform generated by the analog generator unit  302 . As a result of the inversion of the waveform of the second switching device  602   b  is opposite to the waveform of the first switching device  602   a  in order to substantially smoothly ramp down the gate voltage of the switching device  602   a  when the gate voltage of the switching device  602   b  ramps up substantially smoothly.  FIG. 8A  illustrates an example of the resulting transmitted RF spectrum with the analog soft switching system  310  illustrated in  FIG. 6A . From  FIG. 8A  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. 5 . In the example of  FIG. 8A  the level of the out-of-channel spectral emissions may be approximately between −40 dB and −18 dB with small offset frequencies (less than ±2.5 MHz from the transmit frequency) and approximately between −60 dB and −40 dB with higher offset frequencies (between ±2.5 MHz-±10 MHz). 
     Different switching devices  602   a ,  602   b  require different control signals depending on the characteristics of the particular switching device. For example, if the switching device  602   a ,  602   b  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  302 , 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  602   a ,  602   b  may have an impact on the resulting RF waveform of the switching device  602   a ,  602   b.    
     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. 6B  illustrates another example embodiment of the antenna soft switching system  310  according to the invention, wherein the generator unit  302  comprises a digital waveform generator  606 , a D/A converter  608 , and a reconstruction filter  610 . The output of the D/A converter needs to be filtered to remove aliases and fed to the switching devices  602   a ,  602   b , 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  302  or generating function for the waveform, the D/A converter  608 , and the reconstruction filter  610 , as well as the timing unit  301  needs to run at a higher clock rate than the actual switching frequency. Moreover, in digital soft switching the timing unit  301  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  302 , updating the control signals of the switching devices  602   a ,  602   b , etc. Otherwise, the operation of the soft switching system  310  of  FIG. 6B  is similar with the soft switching system  600  of  FIG. 6A . 
     Alternatively, instead of using the resistive switching devices the at least two switching devices  602   a ,  602   b  may be digitally controlled attenuators.  FIG. 6C  illustrates another example embodiment of the antenna soft switching system  310  according to the invention, wherein the switching network  303  comprises at least two resistive switching devices  602   a ,  602   b  implemented as digitally controlled attenuators and the generator unit  302  comprises a digital waveform generator  606 . The switching device  602   a ,  602   b  implemented as the digitally controlled attenuator may be constructed from multiple resistive elements  902   a - 902   n  that are connected in series or parallel using controllable switching elements  904   a - 904   n , 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. 9  illustrates schematically an example of a switching device implemented as a digitally controlled attenuator. The controllable switching elements  904   a - 904   n  that are used for selecting the resistive elements  902   a - 902   n  may be controlled digitally according to a pre-determined timing pattern. The generator unit  302  may further comprises a waveform encoder  612  configured to convert the waveform generated by the digital waveform generator  606  into control signals of the individual controllable switching elements  904   a - 904   n . Especially, when the number of controllable switching elements  904   a - 904   n  is higher than the word length of individual samples of the waveform, the use of waveform encoder  612  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  302 . 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  305  from the first antenna port  306   a  to the second antenna port  306   b . 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 4-bits to 24 bits.  FIG. 7B  schematically illustrates an example of a time domain amplitude of RF waveform of the switching device  602   b  during the switching event, wherein the control signal of the second switching device  602   b  is a quantized linear ramp waveform generated by the generator unit  302  comprising the digital generator  606  and the waveform encoder  612  to substantially smoothly ramp up the amplitude of the RF signal  305  at the second antenna port  306   b . The target waveform is illustrated with dashed line and the quantized waveform is illustrated with solid (staircase) line. From  FIG. 7B  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  305  at the second antenna port  306   b . In this example, 4-bit quantization is used, but also any other quantization may be used.  FIG. 8B  illustrates an example of the resulting transmitted RF spectrum with the digital soft switching system  310  illustrated in  FIG. 6C . From  FIG. 8B  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. 5  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. 8A . In the example of  FIG. 8B  the level of the out-of-channel spectral emissions may be approximately between −45 dB and −15 dB with small offset frequencies (less than ±2.5 MHz from the transmit frequency) and approximately −60 dB with higher offset frequencies (between ±2.5 MHz-±10 MHz).  FIG. 8C  illustrates an example of adjacent-channel leakage ratio (ACLR) at the transmitting frequency and at different offset frequencies with the digital soft switching system  310  illustrated in  FIG. 6C . As comparison in  FIG. 8C  the ACLR with the instantaneous switching is illustrated. In this example, the improvement in the ACLR with the digital soft switching system  310  at the offset frequencies is 20 dB or even more when compared to instantaneous, i.e. hard, switching. 
       FIG. 6D  illustrates another example embodiment of the antenna soft switching system  310  according to the invention, wherein the generator unit  302  comprises a counter  614  and the waveform is encoded into the at least two switching devices  602   a ,  602   b , i.e. the values of the resistive elements  902   a - 902   n  of the switching devices  602   a ,  602   b  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  302 , because there is no need to store the waveform in the memory of the generator unit  302 . Otherwise the operation of the soft switching system  310  of  FIG. 6D  is similar with the soft switching system  310  of  FIG. 6C . 
     There may be situations where having the resistive switching elements  602   a ,  602   b  at the antenna ports  306   a ,  306   b  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  310  illustrated in  FIGS. 6C and 6D . In that case it may be preferable to use a single centralized antenna soft switching system  1000  together with a switching network  1010  of the antenna switch comprising the plurality of switching devices of the antenna switch configured to select consecutively the antenna element  332   a - 332   n  of the at least one antenna array  330 . In this case the antenna soft switching system  1000  may steer the RF signal  305  into a dummy load  1020  of a reference port  1030  and the changing of the antenna element  332   a ,  332   b  takes place at a moment when there is no RF signal  305  present at any antenna port  306   a ,  306   b.    
       FIG. 10  illustrates an example embodiment of the antenna soft switching system  1000  according to the invention, wherein the switching network  303  is a separate switching network providing a centralized antenna soft switching system  1000 . The switching network  303  is separate from the switching network  1010  of the antenna switch, i.e. the plurality of switching devices of the antenna switch configured to select consecutively the antenna element  332   a - 332   n  of the at least one antenna array  330 . The generator unit  302  may be configured to control the switching network  303  so that the amplitude of the RF signal  305  at the first antenna port  306   a  is substantially smoothly ramped down and simultaneously the amplitude of the RF signal  305  at the reference port  1030  is ramped up substantially smoothly. The switching network  1010  of the antenna switch is configured to change the active antenna port and the active antenna element by connecting the RF signal path  305  from the first antenna port  306   a  to the second antenna port  306   b  while the RF signal  305  is led to the dummy load  1020  of the reference port  1030 . After that the generator unit  302  may be configured to control the switching network  303  so that the amplitude of the RF signal  305  at the reference port  1030  is substantially smoothly ramped down and simultaneously the amplitude of the RF signal  305  at second antenna port  306   b  is substantially smoothly ramped up. In the example antenna soft switching system  1000  of  FIG. 10  the generator unit  302  comprises a digital waveform generator  606  and a waveform encoder  612 .  FIG. 11  schematically illustrates examples time domain amplitudes of RF waveforms of the switching device  602   a , switching device  602   b , and the dummy load  1020  during the switching event, wherein the control signals of the switching devices  602   a ,  602   b  are quantized linear ramp waveforms generated by the generator unit  302  comprising the digital generator  606  and the waveform encoder  612 . 
     One minor drawback of this embodiment may be that the RF signal  305  into the active antenna element, i.e. the first antenna element  332   b , needs to be ramped down before the active antenna element is changed, and then ramped up again after the second antenna element  332   b  is activated. This means that the up/down ramps need to be twice as fast as in the embodiments, where the RF signal  305  is steered from one antenna element into another antenna element. For example, in the examples of  FIGS. 7A and 7B  the duration of the switching event is 2 microseconds, during which the RF signal  305  is steered from one antenna element into another element. In the example of  FIG. 11  the duration of the switching event is also 2 microseconds, but during that the RF signal  305  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  320  is linear, or benignly non-linear, the common centralized soft switching system  1000  may also be implemented in the analog or digital baseband of the transmitter module  320 . This enables that there is no need to use a dummy load  1020  to absorb the RF signal, and the losses at high power level may be avoided. 
       FIG. 12  illustrates an example embodiment of the antenna soft switching system  1200  according to the invention, wherein the switching network  303  is a separate switching network providing a centralized antenna soft switching system  1200  and the antenna ports  306   a - 306   n  and the antenna elements  332   a - 332   n  are arranged to two groups, wherein the first antenna port  306   a  and the respective first antenna element  332   a  belong to the first antenna group  1210  and the second antenna port  306   b  and the respective second antenna element  332   b  belong to the second antenna port group  1220 . The switching network  303  is separate from the switching network  1010   a  of the first antenna group  1210  and the switching network  1010   b  of the second antenna group  1220  of the antenna switch, i.e. the plurality of switching devices of the antenna switch configured to select consecutively the antenna element  332   a - 332   n  of the at least one antenna array  330 . The active antenna element  332   a - 332   n  may be selected alternately from the first antenna group  1210  and form the second antenna group  1220 . 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  305  is not led to said antenna group. For example, the second antenna port  306   b  may be preselected as the active antenna of the second antenna group  1220  by the switching network  1010   b  of the second antenna group  1220  when the RF signal is led to the first antenna group  1210 . Alternatively, the first antenna port  306   a  may be pre-selected as the active antenna of the first antenna group  1210  by the switching network  1010   a  of the first antenna group  1220  when the RF signal  305  is led to the second antenna group  1220 . To switch between the first antenna port  306   a  and the second antenna port  306   b , the generator unit  302  may be configured to control the switching network  303  so that the amplitude of the RF signal  305  at the first antenna group  1210  (to which the first antenna port  306   a  belongs) is substantially smoothly ramped down and simultaneously the amplitude of the RF signal  305  at the second antenna group  1220  (to which the second antenna port  306   b  belongs) is substantially smoothly ramped up, wherein the second antenna port  306   b  is pre-selected as the active antenna of the second antenna group  1220 . With this the above described drawback may be avoided. The two antenna groups  1210 ,  1220  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  1010   a ,  1010   b  (dashed lines between the first antenna element  332   a  and the switching network  1010   b  of the second antenna group  1220  and between the second antenna element  332   b  and the switching network  1010   a  of the first antenna group  1210  in  FIG. 12 ). 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  310 ,  1000 ,  1200  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  602   a ,  602   b . Alternatively, fully or partially reactive, i.e. inductive or capacitive, switching devices  602   a ,  602   b  may be used in any of the embodiments of the invention described above. Alternatively, active switching elements  602   a ,  602   b , e.g. amplifiers, may be used as the at least two resistive switching devices  602   a ,  602   b  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. 13 . The method comprises controlling  1310 , by a waveform generated by a generator unit  302 , the switching network  303  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  304  may be maintained constant. The controlling step  1310  will be described more referring to  FIGS. 14-16  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  303  is implemented as a part of the antenna switch configured to perform the switching of the RF signal from the first antenna port  306   a  to the second antenna port  306   b , i.e. the switching network  303  is the switching network of the antenna switch configured to change active antenna port from the first antenna port  306   a  to the second antenna port  306   b  and correspondingly change active antenna element from the first antenna element  332   a  to the second antenna element  332   b  (e.g. the example antenna soft switching systems  310  of  FIGS. 6A-6D ). In other words, the switching network  303  may be configured to switch the RF signal  305  from the first antenna port  306   a  to the second antenna port  306   b  in order to activate the second antenna port  306   b  and the antenna element  332   b  connected to the second antenna port  306   b  and to inactivate, i.e. disconnect, the first antenna port  306   a  and the antenna element  332   a  connected to the first antenna port  306 . The controlling step  1310  may comprise: ramping down, i.e. decreasing,  1410  substantially smoothly the amplitude of the RF signal  305  at the first antenna port  306   a , and simultaneously ramping up, i.e. increasing,  1420  substantially smoothly the amplitude of the RF signal  305  at the second antenna port  306   a  as discussed above. This example embodiment of the method according to the invention is illustrated in  FIG. 14 . 
     According to an example embodiment of the method according to the invention, wherein the switching network  303  is a separate switching network providing a centralized soft switching system  1000  (e.g. the example antenna soft switching system of  FIG. 10 ). The switching network  303  is separate from the switching network  1010  of the antenna switch, i.e. the plurality of switching devices of the antenna switch configured to select consecutively the antenna element  332   a - 332   n  of the at least one antenna array  330 . The controlling step  1310  may comprise: ramping  1510  down substantially smoothly the amplitude RF signal  305  at the first antenna port  306   a  and simultaneously ramping  1520  up substantially smoothly the amplitude of the RF signal  305  at a reference port  1030 ; changing, i.e. switching,  1530  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  306   a  to the second antenna port  306   b , while the RF signal  305  is led to the dummy load  1020  of the reference port  1030 , ramping  1540  down substantially smoothly the amplitude of the RF signal  305  at the reference port  1030  and simultaneously ramping  1550  up substantially smoothly the amplitude of the RF signal  305  at second antenna port  306   b  as discussed above. This example embodiment of the method according to the invention is illustrated in  FIG. 15 . 
     Alternatively, according to an example embodiment of the method according to the invention, wherein the switching network  303  is a separate switching network providing a centralized antenna soft switching system  1200  and the antenna ports  306   a - 306   n  and the antenna elements  332   a - 332   n  are arranged to two groups, wherein the first antenna port  306   a  and the respective first antenna element  332   a  belong to the first antenna group  1210  and the second antenna port  306   b  and the respective second antenna element  332   b  belong to the second antenna port group  1220  (e.g. the example antenna soft switching system of  FIG. 12 ). The controlling step  1310  may comprise: ramping  1610  down substantially smoothly the amplitude of the RF signal  305  at the first antenna group  1210 , and simultaneously ramping  1620  up substantially smoothly the amplitude of the RF signal  305  at the second antenna group  1220 , wherein the second antenna port  306   b  is pre-selected as the active antenna of the second antenna group  1220  as discussed above. This example embodiment of the method according to the invention is illustrated in  FIG. 16 . 
     There may be also some other applications for the above described antenna soft switching system and method in addition to the AoD transmitter unit  300 . 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  1000  of  FIG. 10  and the respective antenna soft switching method of  FIG. 15  may be the most suitable for this application. The antenna soft switching system  1000  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 nonlinear 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  602   a ,  602   b  may be dimensioned such that the switching network  303  maintains a constant impedance over the entire attenuation range. The antenna soft switching system  1000  of  FIG. 10  and the respective antenna soft switching method of  FIG. 15  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. 
     The present invention comprises an antenna soft switching system  310 ,  1000 ,  1200 , of an AoD direction finding transmitter unit, an antenna soft switching method and an AoD direction finding transmitter unit comprising the antenna soft switching system  310 ,  1000 ,  1200 . All these aspects of the invention comprise the same sub-features, sub-parts and sub-functionalities which are comprised in the dependent system claims. 
     The specific examples provided in the description given above should not be construed as limiting the applicability and/or the interpretation of the appended claims. Lists and groups of examples provided in the description given above are not exhaustive unless otherwise explicitly stated.