Abstract:
A LTE compliant RF transceiver includes at least one transmit path and at least two receive paths. A switching arrangement connected between a transmit PLL synthesizer and at least one transmit path as well as between a receive PLL synthesizer and at least two receive paths allows the transmit PLL synthesizer to selectively be connected to the receive side of the transceiver as well as the receive PLL synthesizer to selectively be connected to the transmit side of the transceiver, thereby considerably increasing flexibility of the RF transceiver which enables both speed-up of handover procedures and power savings. A modem including the transceiver is also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Application No. 10152786.9 filed on Feb. 5, 2010 and European Patent Application No. 10157906.8 filed on Mar. 26, 2010, the entire disclosure of these applications being hereby incorporated herein by reference. 
       BACKGROUND ART 
       [0002]    The present invention relates to an LTE compliant RF transceiver. The invention also relates to a modem device comprising such a transceiver. 
         [0003]    RF transceivers are known to be a salient component of modem devices in wireless telecommunications devices to provide communication in both directions, i.e. the ability to send and receive at the same time. With the frequency-division duplexing (FDD) transmission technique, transmitter and receiver operate at different carrier frequencies, i.e. uplink (from terminal to base station) and downlink (from base station to terminal) sub-bands are separated by a frequency offset which enables a station to send and receive at the same time. In contrast, time-division duplexing (TDD) is the application of time-division multiplexing to separate transmit and receive signals. It uses the same frequency for uplink and downlink. 
         [0004]    3GPP LTE (3rd Generation Partnership Project Long Term Evolution) which is the upcoming standard of 4th generation radio access networks employs both FDD and TDD, and also employs Orthogonal Frequency Division Multiplexing (OFDM) as a downlink modulation scheme. Multiple transmit antennas at the base station side and the mandatory requirement of multiple receive antennas at the mobile terminal side, i.e. user equipment, permit simultaneous transmission of multiple data streams, or data layers, from one base station to one mobile terminal. This transmission method is known as Multiple-Input Multiple-Output (MIMO) OFDM. 
         [0005]    Many modern wireless communication systems use both Frequency-Division Duplexing (FDD) and Multiple Input Multiple Output (MIMO). Also, TDD/FDD dual mode devices are known. 
         [0006]    According to the LTE standard, the minimum requirements for antennas of a communications terminal is two receive antennas and one transmit antenna, i.e. a typical implementation of an LTE terminal provides a single transmit path and a pair of receive paths. Since in FDD transmit and receive operate at different frequencies, as stated above, two independent PLLs (Phase Locked Loops) are required in the RF transceiver of such a terminal. 
         [0007]      FIG. 1  shows a typical known RF transceiver as comprising one transmit (Tx) path and two receive (Rx) paths. Both the Tx side and the Rx side each have a dedicated PLL synthesizer associated therewith for setting the carrier frequency for the respective path. Each path comprises a mixer to receive said carrier frequency from the PLL and use it to convert a respective RF signal into a BB signal or vice versa, and a filter for adapting the signal to the desired bandwidth. In particular, as shown in  FIG. 1 , a Tx path comprises Tx filter  112  and Tx mixer  114 , a first Rx path comprises Rx filter  152  and Rx mixer  154 , and a second Rx path comprises Rx filter  162  and Rx mixer  164 . 
         [0008]    Tx mixer  114  receives a baseband signal (Tx BB in) which has been adjusted in bandwidth by Tx filter  112 , and receives a carrier frequency from dedicated Tx PLL  130  to convert the filtered baseband signal up to RF for output (Tx RF out) to a Tx antenna for transmission. 
         [0009]    Each Rx mixer receives an RF signal input, Rx RF in 1 and Rx RF in 2, from a separate Rx antenna, and a carrier frequency from a shared Rx PLL synthesizer  140  to downconvert the received RF signal to a baseband signal for supply, via a respective RF filter ( 152 ,  162 ) which adjusts the bandwidth of the signal, to a baseband unit for demodulation. 
         [0010]    PLL synthesizers such as Tx PLL  130  and Rx PLL  140  generally comprise a control loop which includes a voltage controlled oscillator (VCO), a reference clock generator, a phase detector charge pump and a loop filter. For more details about the internal structure of a typical PLL used in RF communication devices see e.g. U.S. Pat. No. 7,498,888 (WO2005062471 “Method and arrangement for interference compensation in a voltage-controlled frequency generator”). 
         [0011]    There is a continuing demand in mobile communications for increased data rates, speed-up of communication setup and/or communication handling, and power savings. 
         [0012]    A general object of the invention is to make the PLL configuration flexible, so that the RF transceiver can support more functionality. A further object of the invention is to provide an improved LTE compliant RF transceiver and an improved LTE compliant modem which allow to speed-up handover procedures in mobile communication. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    According to the invention there is provided an RF transceiver for an LTE compliant communication device that comprises a transmit side with a transmit PLL synthesizer, and a receive side with a receive PLL synthesizer. The transmit side further comprises at least one transmit path including a mixer unit and a filter unit, and the receive side comprises at least two receive paths, each path including a respective mixer unit and filter unit. The transmit PLL synthesizer is operative to provide a carrier frequency for the at least one transmit path, and the receive PLL synthesizer is operative to provide a carrier frequency for said at least two receive paths. The inventive RF transceiver is characterized in that the transmit PLL synthesizer is connected to the transmit side, in particular to the mixer unit, via a switching arrangement and the receive PLL synthesizer is connected to the receive side, in particular to at least one of the receive mixer units, via the same switching arrangement, wherein this switching arrangement is operable to selectively connect said transmit PLL synthesizer to said at least one receive mixer unit and to selectively connect said receive PLL synthesizer to said transmit mixer unit. 
         [0014]    According to a preferred embodiment, the transmit PLL synthesizer is connected to the transmit mixer unit via a first pole of a first two-pole switch, and the receive PLL synthesizer is connected to one of said receive mixer units via a second pole of a second two-pole switch, wherein the first poles of the first and second two-pole switches being interconnected and the second poles of the first and second two-pole switches being interconnected such that the first switch is operable to selectively connect the receive PLL synthesizer to said transmit mixer unit and the second switch is operable to selectively connect the transmit PLL synthesizer to said one of receive mixer units. Thus, by simply adding a few switches which imply only little implementation overhead the flexibility of RF transceivers is significantly enhanced. 
         [0015]    According to another embodiment a third switch can be connected between the Rx PLL synthesizer and the other one of the pair of receive paths in a way similar to that of the second switch to further enhance flexibility of the RF transceiver. 
         [0016]    According to a second aspect the invention provides a modem for an LTE compliant communication device, substantially comprising a baseband unit and an RF transceiver as described above. The baseband unit comprises a baseband transmitter, a baseband receiver, a baseband controller, said baseband controller providing control signals for the switching arrangement of the transceiver unit. 
         [0017]    According to a further preferred embodiment, the modem may further comprise a received signal strength indication measurement unit, in which case each of the first and second receive paths of the transceiver is connected to a first and second switch of said baseband unit, each switch being selectively connectable to the baseband receiver or to the baseband RS SI unit, and the switches of both the baseband unit the transceiver unit can be controlled by the baseband controller. 
         [0018]    In this way, the invention advantageously provides a flexible PLL configuration which allows the RF transceiver to support more functionality at very little implementation overhead by enabling use of the transceiver PLLs, and in particular use of the pair of Rx paths for other purposes than that to which they were basically intended, namely spatial diversity (MIMO) compliant to the LTE standard. 
     
    
     
       BRIEF DESCRIPTION OF DRAWING FIGURES 
         [0019]    Additional features and advantages of the present invention will be apparent from the following detailed description of specific embodiments which is given by way of example only and in which reference will be made to the accompanying drawings, wherein: 
           [0020]      FIG. 1  shows a schematic block diagram of a prior art RF transceiver of a wireless communication device; 
           [0021]      FIG. 2  shows a schematic block diagram of a first embodiment of an RF transceiver according to the invention in an ordinary LTE operation mode; 
           [0022]      FIG. 3  shows the RF transceiver of  FIG. 2  in a first alternative operation mode; 
           [0023]      FIG. 4  shows the RF transceiver of  FIG. 2  in a second alternative operation mode; 
           [0024]      FIG. 5  shows the RF transceiver of  FIG. 2  in a third alternative operation mode; 
           [0025]      FIG. 6  shows a schematic block diagram of another embodiment of an RF transceiver according to the invention including two transmit paths; and 
           [0026]      FIG. 7  shows a schematic block diagram of an exemplary embodiment of a modem according to the invention which comprises the RF transceiver of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0027]      FIG. 2  shows a schematic block diagram of an RF transceiver  200  according to the invention that can be used in an LTE communication device. RF transceiver  200  comprises an Rx side including two receive paths, Rx 1  and Rx 2 , and a Tx side including a single Tx path. Each of Rx and Tx paths comprise a mixer and a filter. Each of Rx mixers  154 ,  164  receives an RF signal from a separate Rx antenna and can receive a carrier frequency from a shared Rx PLL synthesizer  240  to downconvert the received RF signal into a baseband signal for supply to RF filter  252 ,  262 , respectively, which adjusts the bandwidth of the signal which is then supplied to a baseband unit for demodulation. A Tx mixer  214  receives a baseband signal which has been adjusted in bandwidth by a Tx filter  212 , and can receive a carrier frequency from an associated Tx PLL  230  to convert the filtered baseband signal up to RF for output to a Tx antenna for transmission. PLL synthesizers  230 ,  240  receive a common reference clock signal, Ref clk in, from a clock generator, which is either a separate device or integrated in the RF transceiver or BB unit. 
         [0028]    To the extend described so far, RF transceiver  200  is similar to RF transceiver  100  of  FIG. 1 . In contrast to RF transceiver  100 , Tx PLL  230  and Rx PLL  240  are not directly connected to the respective mixer. Rather, Tx PLL  230  is connected to Tx mixer  214  through a two-way switch  280  the second pole thereof being connected to Rx PLL  240 , and Rx PLL  240  is connected to Rx mixer  254  through a two-way switch  290  the second pole thereof being connected to Tx PLL  230 . 
         [0029]    With switches  280 ,  290  the flexibility for employing the transceiver PLLs is considerably enhanced in comparison to a known transceiver. 
         [0030]      FIG. 2  shows the ordinary operation mode of Rx transceiver  200 , which is the same as provided by hard-wired prior art transceiver illustrated in  FIG. 1 . In particular, Tx mixer switch  280  is switched to connect Tx PLL  230  to Tx mixer  214 , and Rx mixer switch  290  is switched to connect Rx PLL  240  to Rx mixer  254  such that both Rx mixers,  254  and  264 , use Rx PLL  240 , with Rx mixer  264  being fixedly connected to Rx PLL  240 . 
         [0031]    Switches  280 ,  290  enable RF transceiver  200  to be operated in alternative operation modes which, under certain preconditions, allow to speed-up communication handover and/or power savings. For example, if the terminal is connected to an FDD network and currently is receiving on one Rx path but not transmitting, the Tx PLL is free and can be switched to the second Rx path, allowing to simultaneously perform measurements on different frequencies while staying on the serving cell using the first Rx path. This may significantly increase the speed of handovers. In another example, the terminal can be connected to a TDD (Time-Domain Duplex) network which means that Rx and Tx operate on the same frequency. With the inventive arrangement the Rx PLL can be switched to both Rx and Tx paths such that the Tx PLL becomes free. The Tx PLL can then either be switched off to save power, or it can be used to perform measurements on different frequencies when the second Rx path is not required for normal operation. This is particularly useful for FDD/TDD dual-mode RF transceivers. 
         [0032]    Three different alternative operation modes of RF transceiver  200  will now be explained in detail with reference to  FIGS. 3 to 5 . 
         [0033]    In  FIG. 3 , the terminal does currently not transmit, so Tx side is off, as indicated at  212 ,  214 . The second receive path, Rx  2 , is used for reception. In this case, the first Rx mixer,  254 , can temporarily be switched to Tx PLL  230 , which then is configured to a Rx frequency different from the current receive frequency, allowing to perform measurements at that different frequency to look for an alternative carrier frequency exhibiting a better signal-to-noise ratio (SNR) for optionally switching over to another communication cell. 
         [0034]      FIGS. 4 and 5  illustrate two operation modes of the RF transceiver of the invention which can be employed in a case where the transceiver is part of a dual mode communication terminal which supports communication in both FDD and TDD (Time Division Duplex) networks. If the communication terminal is currently connected to a TDD network, which means that Rx and Tx operate on the same frequency, this common frequency may be generated by Rx PLL  240 , i.e. the Tx mixer switch  280  is connected to Rx PLL  240 , as shown in both  FIGS. 4 and 5 . 
         [0035]    Now, Tx PLL  230  is free which allows the following two options: 
         [0036]    According to a first option, the first Rx mixer  254  can be switched to Tx PLL  230 , as shown in  FIG. 4 . Tx PLL  230  will then be configured to tune to a different Rx frequency, which allows to perform RSSI measurements, as described above with reference to  FIG. 3 . This option allows to speed up RSSI measurements for mobile communication at very little implementation cost. 
         [0037]    According to a second option, Tx PLL  230  can be switched off to save power, as shown in  FIG. 5 . 
         [0038]    In another embodiment of the invention, not illustrated in the figures, a third switch can be connected between Rx PLL  240  and the second Rx mixer  264  in a similar manner the second switch is connected between Rx PLL  240  and the first Rx mixer  254 , to further increase flexibility. 
         [0039]    Returning to  FIG. 2 , the control path for controlling switches  280  and  290  is shown in dashed lines. In  FIGS. 3-5  these control paths have been omitted for sake of clarity. Control signals for switches  280  and  290  can be provided by a baseband controller of a baseband unit which together with RF transceiver  200  forms part of a modem in a wireless communication device. 
         [0040]      FIG. 7  shows a modem  700  as comprising RF transceiver  200  of  FIG. 2  and a digital baseband unit  750  which are coupled by analog-to-digital (ADC) and digital-to-analog (DAC) converters, respectively. 
         [0041]    An RF transceiver such as shown in  FIG. 2  can be implemented on one semiconductor chip, and a baseband (BB) unit such as baseband unit  750  of  FIG. 7  can be implemented on another semiconductor chip. DAC  710  and ADCs  722 ,  724  may reside on the RF or on the BB chip. It is even possible to integrate all components on a single chip. 
         [0042]    Switching of switches  280 ,  290  in RF transceiver  200  is initiated by baseband controller  756  in baseband unit  750  of the modem. 
         [0043]    Base band output interface generally is a serial interface; so merely one additional address per switch will be required to control the switches of the inventive RF transceiver. 
         [0044]    Besides baseband controller  756 , a baseband transmitter  752  and a baseband receiver  754 , the base band unit can comprise a baseband measurement unit such as unit  758  illustrated in  FIG. 7  for providing a received signal strength indication (RSSI) of a received signal at a frequency other than the current operation frequency of the communication device. 
         [0045]    Further the base band unit can comprise switches  760 ,  770  which selectively connect each of Rx BB inputs of baseband unit  750  to baseband receiver  754  or alternatively to baseband measurement unit  758 .  FIG. 7  shows switches  760 ,  770  in a position to support the ordinary LTE communication mode, i.e. reception through two receive paths and transmission through one transmit path. However, BB controller  756  is operable to control switches  760 ,  770  in a way to support a desired operation mode of RF transceiver  200  such as described above in conjunction with  FIGS. 3 to 5 . 
         [0046]    For certain modem applications, in particular those which are not battery powered, it may be advantageous to add another transmit path. Application examples for such embodiments are so called femtocells, i.e. tiny base stations, and so called residential units, i.e. stationary wireless terminals. The block diagram of an embodiment of an RF transceiver incorporating a second Tx path is given in  FIG. 6 . 
         [0047]    For a terminal, the primary application for the second Tx path is to increase the uplink range, which can be achieved in two ways: 
         [0048]    In a first operation mode, both Tx paths are used in parallel to increase output power. Also, transmit diversity or beamforming (as described in another patent application of the present applicant, EP 09 179 085.7) can further increase the range. 
         [0049]    In another operation mode, only a single Tx path is used at a time, but the best one is selected under base station control, in other words, an Tx antenna selection controlled by the base station is implemented to enhance communication performance. 
         [0050]    There have been disclosed an RF transceiver and a modem including a flexible PLL configuration which allows the RF transceiver to support more functionality at very little implementation overhead by enabling use of the transceiver PLLs for other purposes than ordinary LTE traffic communication. Other combinations of the embodiments and operation modes described above readily will suggest themselves to a person skilled in the art in view of the foregoing description.