Abstract:
The invention relates to a RF front-end stage for user equipment that is designed for use in multiple communication bands and employs Frequency Division Duplex (FDD). The invention also relates to a FDD front-end module included in such a RF front-end stage. The object of the invention is to provide an RF front-end stage for user equipment that supports a plurality of operating bands and may be produced both at lower costs and with a reduced number of circuit element devices. This object is achieved with a FDD front-end module for a RF front-end stage of a FDD user equipment that supports at least two operating bands each comprising an uplink frequency sub-band and a downlink frequency sub-band wherein at least one of the uplink frequency sub-bands and the downlink frequency sub-bands of said at least two operating bands are not adjacent to each other.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a RF front-end stage for user equipment that is designed for use in multiple communication bands and employs Frequency Division Duplex (FDD). The invention also relates to a FDD front-end module included in such a RF front-end stage. The invention further relates to a method for transmitting signals between an user equipment and a base station by means of FDD. 
       BACKGROUND OF THE INVENTION 
       [0002]    Recently, frequency-division duplexing (FDD) has been increasingly used in communication applications such as ADSL and VDSL, IEEE 802.16 WiMax and most cellular systems, including the UMTS/WCDMA Frequency Division Duplexing mode. In FDD the transmitter and receiver operate at different carrier frequencies thereby allowing to send and to receive at the same time. In the case of symmetric traffic this may be benificial when compared to time-division duplexing (TDD) which tends to waste bandwidth during switch-over from transmitting to receiving, has greater inherent latency, and may require more complex circuitry. Another advantage of frequency-division duplexing is that it makes radio planning easier and more efficient since base stations do not interfere with each other, as they transmit and receive in different sub-bands. As such, FDD is used in applications that require a symmetric bandwidth in the uplink and downlink direction such as GSM, UMTS and 802.16, and may also find application in Long Reach Ethernet (LRE) and WiMAX. With the frequency channels being of the same width, this technology is especially useful in voice applications. 
         [0003]    In communications using FDD for transmissions between a user device and a base station a “frequency band” in the sense of a duplex frequency band does not mean a continuous range of frequencies but rather designates a pair of non-adjacent frequency sub-bands, one for each transmission direction, that are separated by a guard zone, such that more properly it will be referred to as an operating band below. 
         [0004]    A “mobile device” and an “user equipment” for the purpose of the present invention is defined as any device, used by an end user to connect to a base station for communication. This may be a cellular telephone, a personal digital assistant (PDA), a card in a laptop computer and the like. 
         [0005]    Nowadays, dual, tri, and quad band mobile devices are known and widely used in GSM systems, such devices supporting communication at two, three, and four different communication standards with different frequency bands, respectively, such as, e.g., GSM-900 and GSM-1800, which are used in Europe and other parts of the world. 
         [0006]    To support a specific number n of operating bands in a mobile device using FDD as communication technology, the RF front-end stage of the mobile device that connects the RF receiver/transmitter circuit to the device&#39;s antenna conventionally comprises an equal number n of separate front-end modules, one for each operating band, as is illustrated in  FIG. 1 . The n front-end modules ( 110 ,  120 , . . . ,  130 ) are connected in parallel between the antenna of the mobile device on one side, and both a RF transmitter circuit and a RF receiver circuit on the other side. The parallel connections on both sides are respectively provided by a SPnT (single pole n through) switch each, n again being the number of separate front-end modules comprised in the front-end stage and hence the number of operating bands the user equipment supports. To give an example, a quad band cell phone conventionally comprises four front-end modules, one for each operating band, and would require SP4T switches to connect the four front-end modules between the antenna at one side and both a RF transmitter circuit and a RF receiver circuit at the other side. The SPnT switches may be operated to automatically select an operating band for communication in response to the local availability of base stations within the reach of the mobile device. 
         [0007]    A known front-end module for a dedicated operating band substantially comprises an FDD-duplexer  112  connected between the SPnT switch on the antenna&#39;s side and both the transmit and the receive paths, and a power amplifier section  114  and a noise filter  116  in the transmit path, the power amplifier section being the most expensive circuit element both in terms of occupation of real estate on a PCB and in production costs. The duplexer allows the mobile device to send and to receive at the same time by operating the radio receiver and the radio transmitter at slightly different frequencies, i.e. the uplink (transmit) and downlink (receive) sub-bands are separated by a frequency offset. Thus, FDD implies a degree of complexity in that particular filters have to be used in the upstream and downstream direction. Conventionally, such as in all known GSM systems, the lower sub-band is allocated as an uplink sub-band, i.e. for transmitting to a base station, and the upper sub-band is allocated as a downlink sub-band, i.e. for receiving from a base station, as is shown in  FIG. 2 . It is known practice to implement an FDD duplexer by a pass-band/pass-band filter combination, each pass-band for a respective sub-band, the uplink and downlink sub-bands, respectively, as may be seen from the expanded partial view in  FIG. 1   a.    
         [0008]    A disadvantage of the prior RF front-end stages that are constructed as described above is that for each additional operating band that is to be supported by a user device an additional front-end module has to be integrated into the RF front-end stage adding both complexity and costs. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0009]    The object of the invention is to provide an RF front-end stage for user equipment that supports a plurality of operating bands and may be produced at lower costs and with a reduced number of circuit devices requiring smaller area on a circuit board. 
         [0010]    According to a first aspect, the invention provides an FDD front-end module for a RF front-end stage of a FDD user equipment that supports at least two operating bands each comprising an uplink frequency sub-band and a downlink frequency sub-band wherein at least one of the uplink frequency sub-bands and the downlink frequency sub-bands of said at least two operating bands are not adjacent to each other. The FDD front-end module according to a preferred embodiment of the invention includes a duplexer which comprises a filter combination of a pass-band filter and a notch filter. The pass-band filter may be designed to pass the at least two uplink frequency sub-bands and the at least two downlink frequency sub-bands, and the notch filter may be designed to pass the at least two uplink frequency sub-bands and to stop the at least two downlink frequency sub-bands. 
         [0011]    The invention advantageously uses one single front-end module, instead of at least two as in the prior art, for communicating in at least two operating bands. In this way, at least one power amplifier section is eliminated and smaller board area is required such that an FDD RF front-end stage for multiple operating bands for a mobile device may be produced at lower costs. 
         [0012]    According to another aspect, the invention provides a multi operating band RF front-end stage for user equipment employing frequency division duplex (FDD) for communicating both uplink and downlink frequency sub-bands of a plurality of operating bands, and connected between an antenna of the user equipment and an RF transmitter/receiver circuit. The RF front-end stage of the invention comprises a plurality of conventional front-end modules, each one including a frequency duplexer with a pass-band/pass-band filter characteristic for simultaneously communicating uplink and downlink frequency sub-bands of a single designated operating band. The RF front-end stage of the invention further includes at least one front-end module as described above which according to the invention comprises a frequency duplexer that is adapted to communicate a plurality of operating bands each comprising an uplink frequency sub-band and a downlink frequency sub-band wherein at least one of the uplink frequency sub-bands of said plurality of operating bands is not adjacent to the other uplink frequency sub-bands of said plurality of operating bands or at least one of the downlink frequency sub-bands of said plurality of operating bands is not adjacent to the other downlink frequency sub-bands of said plurality of operating bands. 
         [0013]    A further advantage of the invention is that a multi operating band RF front-end stage for user equipment that comprises a front-end module according to the invention may be implemented with switches having a smaller number of ports, i.e. reduced by at least one in comparison to the prior art, such switches being available more easily and at lower costs. Additionally, a shared FDD front-end module according to the invention allows the RF control interface to be simplified. 
         [0014]    In yet another aspect, the invention provides a method for simultaneously communicating uplink and downlink frequency sub-bands of a plurality of dedicated operating bands, wherein at least one of the uplink frequency sub-bands of said plurality of operating bands is not adjacent to the other uplink frequency sub-bands of said plurality of operating bands or at least one of the downlink frequency sub-bands of said plurality of operating bands is not adjacent to the other downlink frequency sub-bands of said plurality of operating bands. The method is characterized by communicating uplink frequency sub-bands and downlink frequency sub-bands of a plurality of operating bands over a single front-end module which includes a frequency duplexer that comprises a combination of a pass-band filter and a notch filter. 
     
    
     
         [0015]    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: 
           [0016]      FIG. 1  shows an RF front-end stage according to prior art; 
           [0017]      FIG. 1A  is an enlarged view of the duplexer of  FIG. 1 ; 
           [0018]      FIG. 2  is a schematic frequency diagram of a typical FDD operating band; 
           [0019]      FIG. 3  shows an RF front-end stage according to the invention; 
           [0020]      FIG. 3A  is an enlarged view of the duplexer of  FIG. 3 ; 
           [0021]      FIG. 4A  shows a frequency band plan with three FDD operating bands which can be supported by a prior RF front-end stage comprising three front-end modules; 
           [0022]      FIG. 4B  shows another frequency band plan with three FDD operating bands which can be supported by a prior RF front-end stage comprising two front-end modules; and 
           [0023]      FIG. 4C  shows yet another frequency band plan comprising three FDD operating bands which can be supported by a RF front-end stage of the invention comprising a single front-end module according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]      FIG. 4A  shows a frequency deployment plan with three FDD operating bands A, B and C, respectively. As stated before, an FDD operating band for the purpose of the invention is defined as a pair of frequency bands, an uplink frequency sub-band and a downlink frequency sub-band which are separated by a frequency offset. Each FDD operating band of  FIG. 4A  is arranged as described with reference to  FIG. 2 , i.e. each FDD operating band A, B and C comprises an uplink frequency sub-band for communication from a mobile device to a base station, and a downlink frequency sub-band for communication from a base station to a mobile device. Uplink and downlink frequencies for each band are separated by a guard zone. In each of the operating bands A, B and C the uplink carrier frequency is lower than the downlink carrier frequency, and operating bands A, B and C are well separated from each other, in other words, they are not interleaved with each other. Examples for such a frequency band deployment are, e.g. the GSM-450, GSM-900, and GSM-1800 operation bands. A mobile device that is to support these three operating bands will require a separate front-end module for each FDD operating band A, B and C, wherein each of these front-end modules conventionally comprise a duplexer made up of two pass-band filters, one for passing the uplink and the other one for passing the downlink. 
         [0025]    Another frequency deployment plan with three operating bands that are to be supported by a mobile device, is illustrated in  FIG. 4B . In this scheme two of the operating bands are interleaved such that the uplinks of operating bands D and E are adjacent to each other and the downlinks of operating bands D and E are adjacent to each other. This allows to simplify the front-end stage in that a single front-end module can be used to support both operating bands D and E. This front-end module will conventionally comprise a duplexer made up of a pair of pass-band filters, a first band-pass filter for passing the uplink frequency sub-bands of both operating bands D and E, and a second pass-band filter for passing the downlink frequency sub-bands of both operating bands D and E. A second front-end module will be required to support operating band F, which front-end module will again comprise a duplexer made up of a pair of pass-band filters, one for passing the uplink frequency sub-band of operating band F and one for passing the downlink frequency sub-band of operating band F. 
         [0026]    In each case described so far the uplink frequency sub-band is allocated to a lower frequency than the downlink frequency sub-band. However, the above allocation is not imperative. That means, deployments are conceivable where the uplink communication direction is allocated to a frequency sub-band at a higher carrier frequency than the frequency sub-band allocated for the downlink communication direction. Also, frequency deployments with a combination of the above discussed cases are conceivable, i.e. some of the operating bands that are to be supported by a front-end stage of a mobile device have its uplink communication direction allocated to a frequency sub-band at a higher carrier frequency than that of the frequency sub-band allocated for downlink communication, and some other operating bands that are to be supported by the same front-end stage have its uplink communication direction allocated to a frequency sub-band at a lower carrier frequency than that of the frequency sub-band allocated for the downlink communication direction. 
         [0027]    A non-limiting example for such a deployment is illustrated in  FIG. 4C . The diagram shows three operating bands G, H, and K. Operating bands G and H are interleaved and operating band K is not interleaved with G and H but adjoins operating band H. In particular, as may be seen from the figure, the uplink and downlink frequency sub-bands of operating bands G, H, and K are arranged such that the downlink frequency sub-bands of all three FDD operating bands are adjacent to each other but only two out of the three uplink frequency sub-bands are adjacent to each other, i.e. the uplink frequency sub-bands of FDD operating bands G and H are adjacent to each other and the uplink frequency sub-band of FDD operating band K is not adjacent to the other uplink frequency sub-bands of the three FDD operating bands that are to be supported by a front-end stage. There are two conventional ways to implement a front-end stage which is to support all three FDD operating bands G, H, and K of  FIG. 4C . A first way would be similar to the implementation described with reference to  FIG. 4A , i.e. to use a separate front-end module for each FDD operating band G, H, and K, wherein each of these front-end modules would comprise a duplexer made up of two pass-band filters, one for passing the uplink and the other one for passing the downlink which would be a rather expensive way to support FDD operating bands G, H, and K. 
         [0028]    A second and less expensive way would be to use an implementation as described with reference to  FIG. 4B , i.e. to use a first front-end module for FDD operating bands G, and H, and another front-end module for FDD operating band K, wherein the first front-end module would comprise a duplexer made up of two pass-band filters, one for passing the uplink sub-bands of both FDD operating bands G, and H which are adjacent to each other, and the other one for passing the downlink sub-bands of both FDD operating bands G, and H which are adjacent to each other. The second front-end module, likewise, would comprise a duplexer made up of two pass-band filters, one for passing the uplink sub-band of FDD operating band K, and one for passing the downlink sub-band of FDD operating band K. This would eliminate one front-end module and hence would reduce implementation cost in comparison to the front-end stage with three separate front-end modules. 
         [0029]    The invention, however, provides for an even more advantageous way to implement a front-end stage which supports communication in the three FDD operating bands of  FIG. 4C . A front-end stage according to the invention to support FDD operating bands G, H, and K such as illustrated in  FIG. 4C  will comprise a single front-end module such as illustrated in  FIG. 3  at  210 . 
         [0030]    Front-end module  210  is connected between the antenna of the user equipment at one side, and both a RF transmitter circuit and a RF receiver circuit at the other side. Front-end module  210  comprises an FDD-duplexer  212  connected between the antenna on the left hand side as shown in  FIG. 3  and both the transmit and the receive paths on the right hand side of  FIG. 3 . The transmit path of front-end module  210  further comprises a power amplifier section  214  and a noise filter  216 . Frequency duplexer  212  according to the invention is adapted to communicate all three operating bands G, H, and K of  FIG. 4C . All three downlink frequency sub-bands of each of operating bands G, H, and K are adjacent to each other but the uplink frequency sub-band of operating band K is not adjacent to the uplink frequency sub-bands of operating bands G and H. 
         [0031]    Frequency duplexer  212  of front-end module  210  of the invention comprises a combination of a pass-band filter and a notch filter which is detailed in the expanded oval of  FIG. 3A . The pass-band filter of frequency duplexer  212  is connected between the antenna and the receive path of front-end module  210  and is designed to pass the uplink frequency sub-bands of all three operating bands G, H, and K and the downlink frequency sub-bands of all three operating bands G, H, and K. The notch or stop band filter of frequency duplexer  212  is connected between the antenna and the transmit path of front-end module  210  and is designed to pass the uplink frequency sub-bands of all three operating bands G, H, and K and to stop the downlink frequency sub-bands of all three operating bands G, H, and K. In this manner, duplexer  212  allows the mobile device to send and to receive at the same time. 
         [0032]    A person skilled in the art will readily understand that there are many combinations of frequency allocations of several FDD operating bands where the principles of the invention can be applied benificially. For instance a frequency allocation similar to that of  FIG. 4C  is conceivable, but with the uplink and downlink directions swapped, i.e. with all three uplink frequency sub-bands adjacent to each other and one of the downlink frequency sub-bands not adjacent to the other two downlink frequency sub-bands. In this case the pass-band filter of frequency duplexer  212  will be connected between the antenna and the transmit path of front-end module  210  and the notch or stop band filter will be connected between the antenna and the receive path of front-end module  210 . 
         [0033]    A non-limiting example for a frequency band deployment for which the front-end module of the invention could be employed with benefit are the three operating bands in the 700 MHz spectrum which were auctioned in the U.S. and Canada in early 2008 for UMTS-FDD. A first of these operating bands has its uplink allocated to a lower frequency band (698-716 Mhz) than its downlink (728-746 Mhz). The second and third of these operating bands, however, have its uplinks allocated to a higher frequency band than its downlinks and are interleaved, i.e. have its uplinks adjacent to each other (777-787 Mhz and 788-798 MHz, respectively) and its downlinks adjacent to each other (746-756 Mhz and 758-768 MHz, respectively). It has to be noted that this is only to give an example for where the invention can be used and is not intended to limit the invention to any frequency deployment plan described herein. There are a variety of other frequency allocation plans conceivable for which the invention may be used as well. 
         [0034]    Besides front-end module  210 , the front-end stage of the invention may further comprise at least one conventional front-end module such as front-end module  220  shown in  FIG. 3  which is arranged as described for front-end modules  110 - 130  of  FIG. 1 , i.e. it includes a frequency duplexer with a filter combination of two pass-band filters for simultaneously communicating uplink and downlink frequency sub-bands of a single designated operating band. While the conventional front-end stage of  FIG. 1  requires SPnT switches (n being the number of operating bands supported) to connect the n conventional front-end modules between the antenna and both the receiver and the transmitter circuit, the front-end stage according to the invention at most requires SP(n−1)T switches that connect the (n−1) or even less front-end modules, depending on the number of operating bands the front-end module  210  according to the invention supports, between the antenna at one side and the receiver/transmitter circuits at the other side. To give an example, a front-end stage of a mobile device designed to support six operating bands and comprising one front-end module  210  according to the invention which supports three operating bands, and three conventional front-end modules  220 , will only require SP4T (Single Pole  4  Through) switches which are obviously less complex and available at lower costs. 
         [0035]    There have thus been described some embodiments of a front-end module for FDD user equipment and front-end stages comprising such front-end modules which is, however, solely intended to illustrate the principles of the invention. Various modifications will readily occur to those skilled in the art without departed from the scope and the spirit of the invention as set forth in the appended claims.