Abstract:
A transceiver comprises a receiver, a transmitter, a signal transmission arrangement, a first signal transferring element, and a transformer having magnetically-connected first and second windings. The first signal transferring element is between the transmitter output and the signal transmission arrangement, which is arranged to transmit signals from the transmitter and to receive signals and provide them to the receiver. The first winding of the transformer is connected in parallel with the first signal transferring element, which has input and output impedances so that signals from the transmitter output reach the signal transmission arrangement, while signals from the signal transmission arrangement do not reach the transmitter output. As such, the first signal transferring element is arranged to transfer signals from the transmitter to the signal transmission arrangement such that the transmitter contribution to the signal in the first winding is suppressed.

Description:
TECHNICAL FIELD 
     The present invention generally relates to a transceiver, a method of operating the transceiver, and a computer program for implementing the method. The present invention also relates to a communication device capable of frequency division duplex communication comprising such a transceiver. 
     BACKGROUND 
     Transceivers comprise both a transmitter and a receiver, and are commonly used in a variety of communication apparatuses. Transceivers can be arranged to be operated in semi-duplex, i.e. the receiver and transmitter operates separated in time to prevent the transmitter signal from concealing the received signal. This approach is therefore commonly referred to as time division duplex (TDD). Transceivers can also be operated in full duplex, i.e. the receiver and transmitter operates simultaneously wherein some special arrangements are provided to prevent the transmitter from concealing the received signal. One approach to achieve this is to assign different frequencies for transmission and reception. This approach is therefore commonly referred to as frequency division duplex (FDD). 
     Often the receiver and the transmitter use the same antenna, or antenna system which may comprise several antennas, which implies that some kind of circuitry may be desired to enable proper interaction with the antenna. This circuitry should be made with certain care when operating the transceiver in full duplex since the transmitter signal, although using FDD, may interfere with the received signal, i.e. internal interference within the transceiver.  FIG. 1  illustrates an example of a communication apparatus  100  comprising a transceiver  102 , an antenna  104  connected to the transceiver  102 , and further circuitry  106  such as processing means, input and output circuitry, and memory means. The transceiver  102  comprises a transmitter  108 , a receiver  110 , and a duplexer  112  which is connected to the transmitter  102 , the receiver  110  and the antenna  104 . The duplexer  112  is arranged to direct radio frequency (RF) energy from the transmitter to the antenna, as indicated by arrow  114 , and from the antenna to the receiver, as indicated by arrow  116 , and can for example comprise a circulator. Duplexers are known in the art and for example described in U.S. Pat. No. 4,325,140. However, duplexers are not ideal and a leakage of transmitter signals from the transmitter to the receiver, as indicated by arrow  118 , is at least to some degree present. Further, duplexers are commonly costly, space consuming and challenging to be implemented on-chip. Therefore, efforts have been made in the art to achieve the similar effects with on-chip solutions. These are based on electrical balance by using a dummy load which is arranged to be equal to the antenna impedance.  FIG. 2  illustrates an example of such a structure  200 , which is also disclosed in WO 2009/080878 A1, comprising a transmitter  202 , a receiver  204 , and an antenna  206 . The transmitter  202  provides its output signal both to a branch towards the antenna  206 , the branch comprising a capacitor  208  and an inductor  210 , and to a branch towards a dummy load  212 , the branch comprising a capacitor  208 ′ and an inductor  210 ′. The dummy load  212  is arranged to mimic the impedance of the antenna  206 , and by the achieved symmetry, and, when using a differential input to the receiver  204  via a transformer  214 , the contribution at the receiver input from the transmitted signal can be suppressed. However, here it can be seen that transmission energy is lost in heat dissipation in the dummy load. 
     It is therefore a desire to provide an approach for transceivers where the above discussed drawbacks are reduced. 
     SUMMARY 
     An object of the invention is to at least alleviate the above stated problem. The present invention is based on the understanding that by matching a signal transferring element for connecting the transmitter to the signal transmission arrangement and a circuit path parallel thereto which includes the primary winding of a transformer to provide received signals to the receiver, the voltage of transmit signals across the primary winding will be zero or as close to zero as the quality of the implementation allows. Thus, the contribution from the transmitter at the input of the receiver will be suppressed accordingly. 
     According to a first aspect, there is provided a transceiver comprising a transmitter, a receiver and a signal transmission arrangement. The signal transmission arrangement is arranged to transmit signals provided from the transmitter, and arranged to receive signals and provide them to the receiver. The transceiver further comprises a first signal transferring element arranged between the transmitter output and the signal transmission arrangement, wherein the first signal transferring element is arranged to enable a signal from the output of the transmitter to reach the signal transmission arrangement through the first signal transferring element, and prevent a signal at reception frequency from the signal transmission arrangement to reach the output of the transmitter through the first signal transferring element. The transceiver further comprises a transformer comprising a first winding and a second winding being mutually magnetically connected, wherein the first winding is electrically connected in a circuit path which is connected in parallel to the first signal transferring element, and the second winding is connected to an input of the receiver. The first signal transferring element is arranged to transfer signals from the transmitter to the signal transmission arrangement such that a transmitter contribution in the first winding is suppressed. 
     The first signal transferring element may comprise an amplifier having a gain and phase shift that is matched to a gain and phase shift of the circuit path at the transmitter frequency such that a voltage contribution from the transmitter is equal at both sides of the first winding. Alternatively, the first signal transferring element may comprise a filter having an attenuation and phase shift that is matched to an attenuation and phase shift of the circuit path at transmitter frequency such that a voltage contribution from the transmitter is equal at both sides of the first winding. Further alternatively, the first signal transferring element may comprise an isolator having an attenuation and phase shift that is matched to an attenuation and phase shift of the circuit path at transmitter frequency such that a voltage contribution from the transmitter is equal at both sides of the first winding. The first signal transferring element may comprise an element having two or more of the abovementioned functions having an attenuation and phase shift that is matched to an attenuation and phase shift of the circuit path at transmitter frequency such that a voltage contribution from the transmitter is equal at both sides of the first winding. 
     The transceiver may further comprise a first controller arranged to control the gain or attenuation of the first signal transferring element such that the first signal transferring element and the circuit path have equal gain or attenuation impact on signals from the transmitter. 
     The first signal transferring element and the circuit path may have equal phase impact on signals from the transmitter. The circuit path may consist of the first winding and a second signal transferring element. The transceiver may further comprise a first controller arranged to control the phase shift of the second signal transferring element such that the first signal transferring element and the circuit path have equal phase impact on signals from the transmitter. The second signal transferring element may be connected between the first winding and the output of the transmitter. The second signal transferring element may be a galvanic connection. 
     The transceiver may further comprise a measurement circuit arranged to measure a signal significant for transmitter contribution in the first winding, wherein the first controller may comprise a feedback circuit arranged to perform the control based on the measured signal. The measurement circuit may be arranged to measure the significant signal as a voltage across the first winding. Alternatively, the measurement circuit may be arranged to measure the significant signal as transmitter leakage signal at the input of the receiver. 
     The first signal transferring element may be tuneable based on changes in impedance of the signal transmission arrangement. 
     The first signal transfer element may be any of a filter, amplifier, isolator and circulator. 
     According to a second aspect, there is provided a communication device, capable of frequency division duplex communication in a communication network, comprising a transceiver according to the first aspect. 
     Other objectives, features and advantages of the present invention will appear from the following detailed disclosure, from the attached dependent claims as well as from the drawings. Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the [element, device, component, means, step, etc]” are to be interpreted openly as referring to at least one instance of said element, device, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings. 
         FIG. 1  is a block diagram which schematically illustrates a conventional communication apparatus comprising a transceiver. 
         FIG. 2  is a schematic circuit diagram which illustrates a FDD transceiver arrangement without duplexer and based on electrical balance. 
         FIG. 3  is a block diagram which schematically illustrates a transceiver according to an embodiment. 
         FIG. 4  is a block diagram which schematically illustrates a transceiver according to an embodiment. 
         FIG. 5  is a flow chart which schematically illustrates a method according to embodiments. 
         FIG. 6  schematically illustrates a computer program and a processor. 
     
    
    
     DETAILED DESCRIPTION 
     The approach herein is based on the principle that a primary winding of a transformer arranged to couple e.g. an antenna signal of a transceiver to the receiver part is connected such that a transmitter signal across the primary winding is kept to unity gain, wherein the contribution from the transmitter to the receiver is zero. This can be made either by connecting the primary winding in parallel to an element of the transmitter path, which element is kept at unity gain, or by connecting the primary winding and a further element in series, and the series coupling is connected in parallel to the element of the transmitter path, wherein the difference from unity gain in the element of the transmitter path is compensated by gain of the further element. The same principle as mentioned for gain here is also applicable for attenuation. Due to the unity gain, the voltage at any moment emanating from the transmitter across the primary winding is always zero, and thus no contribution of the transmit signal is made to the signal being transformed and fed to the input of the receiver. The element in the transmit path can for example be a power amplifier, filter or isolator. A signal received by the antenna on the other hand provides its contribution to the primary winding and will be coupled to the receiver input. By the approach, there is no need for a duplexer or a dummy load, and the drawbacks of these components are avoided. 
       FIG. 3  is a block diagram which schematically illustrates a transceiver  300  according to an embodiment. The transceiver comprises a transmitter  302 , a receiver  304  and a signal transmission arrangement  306 , such as the depicted antenna arrangement, or a wired connection. The transmitter  302  is connected to the antenna arrangement  306  via a first signal transferring element  308 , which for example can be a power amplifier, filter or isolator. The first signal transferring element  308  should have properties such that signals from the antenna arrangement  306  are disabled, i.e. highly attenuated, from reaching the transmitter  302 , while signals from the transmitter  302  should be enabled to reach the antenna arrangement  306 . 
     The transceiver further comprises a transformer  310  where a primary winding is connected in parallel with the first signal transferring element  308 , and the secondary winding is connected to the input of the receiver  304 . In  FIG. 3 , a single-ended configuration of the receiver  304  is illustrated, but the secondary winding can be connected in a differential way to the receiver as well, e.g. as depicted in  FIG. 2 . 
     The gain, or attenuation, of the first signal transferring element  308  is kept to unity, either by design, or by control means, wherein the contribution of the transmit signal over the primary winding becomes zero. Here, “unity” should be construed in its technical context where gain or attenuation of the primary winding is considered as unity too. In practice, the first transferring element  308  and the circuit path from the transmitter to the antenna via the primary winding should have equal gain/attenuation such that voltage of a transmitter signal across the primary winding is zero or as close to zero as technically feasible by the implementation. 
     Of course, components are not ideal, but even with this consideration, the contribution is kept low and an advantageous isolation of the transmit signal from the receiver input is achieved. When considering control means, this will be further elucidated with reference to the embodiment illustrated in  FIG. 4 , but is also applicable to the embodiment of  FIG. 3  in sense of the options for measuring signals for a feedback structure of controlling, and in sense of controlling the first signal transferring element. 
       FIG. 4  is a block diagram which schematically illustrates a transceiver  400  according to an embodiment. The transceiver comprises a transmitter  402 , a receiver  404  and an antenna arrangement  406 . The transmitter  402  is connected to the antenna arrangement  406  via a first signal transferring element  408 , which for example can be a power amplifier, filter or isolator. The first signal transferring element  408  should have properties such that signals at reception frequency from the antenna arrangement  406  are disabled, i.e. highly attenuated, from reaching the transmitter  402 , while signals from the transmitter  402  should be enabled to reach the antenna arrangement  406 . The transceiver further comprises a transformer  410  where a primary winding is connected in series with a second signal transferring element  412  and a circuit path consisting of the series connection of the primary winding and the second signal transferring element  412  is connected in parallel with the first signal transferring element  408 , and the secondary winding is connected to the input of the receiver  404 . In  FIG. 4 , a single-ended configuration of the receiver  404  is illustrated, but the secondary winding can be connected in a differential way to the receiver as well, e.g. as depicted in  FIG. 2 . 
     The gain, or attenuation, across the primary winding is kept to unity by mutually matching the first and second signal transferring elements  408 ,  412 , either by design, or by optional control means  413 , wherein the contribution of the transmit signal over the primary winding becomes zero. Of course, components are not ideal, but even with this consideration, the contribution is kept low and an advantageous isolation of the transmit signal from the receiver input is achieved. The control means  413  can be an analog or digital control circuit which either controls the second signal transferring element  412  to match the first signal transferring element  408 , or controls both the signal transferring elements  408 ,  412 . The latter approach can have advantages for matching the transmit path to changes for example in antenna impedance and/or used frequency band. The control can be made by using a feedback structure where significant signals for the suppression feature, i.e. between the transmitter  402  and the receiver  404 , are measured. This can be made by a measurement circuit  415 . The measurement circuit  415  can for example measure transmitter contribution across the primary winding, as indicated as “Alternative  1 ” in  FIG. 4 . Another example is to measure the contribution at the receiver input, as indicated as “Alternative  2 ” in  FIG. 4 . The control can also be made based on knowledge about current operating conditions for the first signal transferring element  408 , and, for example, a look-up table can be used for setting the parameters for the second signal transferring element  412 , e.g. based on frequency band, transmit signal level, etc. 
     The transceiver according to the different embodiments and variants demonstrated above are particularly suitable for a communication device capable of frequency division duplex communication in a cellular communication network. The communication device can for example be a user device such as a cell-phone, a network adapter or card for a computer, or a device arranged for machine-to-machine communication. The communication device can be a wireless communication device such as a radio station capable of duplex communication or a cellular communication device, such as a mobile phone, cellular communication card, or Wide Area Network communication device, or a communication device for wired communication, such as a cable modem, a repeater device, or a wired network node. For the case of a wired solution, the antenna arrangement depicted for the transceivers in  FIGS. 3 and 4  is substituted with the wired connection. 
     A method for controlling the signal transferring element(s) of the transceiver is also suggested.  FIG. 5  is a flow chart illustrating a method according to embodiments. Although the illustration is illustrated as a number of steps, the nature of the method is different since the control procedure is preferably performed on real-time basis. The method comprises controlling  502  a first signal transferring element, optionally controlling  503  a second signal transferring element. Further optionally, signals are measured  501 , for example according to those alternatives illustrated in  FIG. 4  and discussed in connection therewith. 
     The methods according to the present invention is suitable for implementation with aid of processing means, such as computers and/or processors, especially for the case where the controller demonstrated above is a digital signal processor. Therefore, there is provided computer programs, comprising instructions arranged to cause the processing means, processor, or computer to perform the steps of any of the methods according to any of the embodiments described with reference to  FIG. 5 . The computer programs preferably comprises program code which is stored on a computer readable medium  600 , as illustrated in  FIG. 6 , which can be loaded and executed by a processing means, processor, or computer  602  to cause it to perform the methods, respectively, according to embodiments of the present invention, preferably as any of the embodiments described with reference to  FIG. 5 . The computer  602  and computer program product  600  can be arranged to execute the program code sequentially where actions of the any of the methods are performed stepwise. The processing means, processor, or computer  602  is preferably what normally is referred to as an embedded system. Thus, the depicted computer readable medium  600  and computer  602  in  FIG. 6  should be construed to be for illustrative purposes only to provide understanding of the principle, and not to be construed as any direct illustration of the elements. A particular advantage of the computer program is that the control approach can be applied in a flexible way when using a transceiver in different applications, and/or changing application of a transceiver. The flexible nature of the control in this disclosure makes this particularly advantageous. The new control approach is then applied as a software update. 
     The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.