Patent Publication Number: US-2010128825-A1

Title: Apparatus and method for efficient inter radio access technology operation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/278,878, filed on Apr. 6, 2006. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to communication systems and, more particularly, to inter radio access technology (RAT) operation in at least one communication system. 
     BACKGROUND 
     In the forthcoming evolution of cellular standards such as GSM and WCDMA, the use of additional modulation techniques such as Orthogonal Frequency Division Multiplexing (OFDM) can be expected. One proposal, referred to as Super 3G or S3G, can be seen as an evolution of the 3G WCDMA standard. Super 3G will likely use OFDM as a modulation technique, and will operate on a bandwidth that spans from 1.25 MHz to 20 MHz, with data rates up to 100 megabits per second (Mb/s) possible. Therefore, for smooth migration, within the existing radio spectrum, of the “old” cellular systems to the new high-capacity, high-data rate cellular systems, user equipment will need to be capable of operation on a flexible bandwidth. 
     The evolution of cellular standards will thus introduce new challenges in the design of terminals. Besides the need to support higher data rates, the variable bandwidth also puts new requirements on the receiver and transmitter. Furthermore, the mix of different access technologies within the same radio spectrum places new requirements on inter RAT operations, i.e., how the terminal handles handover (HO) and cell search among cells, including those that use different access technologies. 
     Conventionally, inter RAT operations involve adapting the receiver to the bandwidth and HO/cell search parameters of a certain cellular system, performing the cell search, measurements, etc., using the access technology of that cellular system, then adapting the receiver to a different access technology of another cellular system, and performing the cell search, measurements, etc., using that different access technology. During an ongoing communication session, such as a speech or data call, supported by a first access technology, if a handover to another cell that uses a second access technology becomes necessary, for reasons of capacity or coverage, the ongoing communication session is interrupted in order to permit cell search/measurements on the second access technology. In WCDMA systems, this is referred to as compressed mode, where an ongoing WCDMA communication session is interrupted so the terminal can measure on GSM. Interruptions in ongoing communication sessions reduces throughput and, with solutions such as WCDMA compressed mode, also reduce capacity. These reductions in throughput and capacity are disadvantageous from a system perspective. 
     Interruptions in ongoing communication sessions can be avoided if the terminal is provided with multiple receiver branches, including multiple antennas, multiple front end receivers, etc. This permits the ongoing communication session to continue on one receiver, adapted to a first access technology, while cell search/measurements are simultaneously performed using the other receiver, adapted to a second access technology. This two-receiver approach is expensive from the terminal perspective. 
     With Super 3G, the complexity of inter RAT operations is expected to be even greater as compared to inter RAT operations in WCDMA/GSM, because of the possibility of handovers to WCDMA/GSM on the same or adjacent carrier frequencies with different bandwidths. 
     It is therefore desirable to provide for an inter RAT operation that is more efficient than the conventional solutions. 
     SUMMARY 
     The present invention is an apparatus and method adapted to facilitate inter RAT operation in at least one communication system. The apparatus of the present invention includes an adjustable front-end receiver having an adjustable AF coupled to the front end receiver. Also included is an ADC coupled to the AF and a plurality of extraction units adapted to receive digital signals from the ADC. Coupled to the plurality of extraction units is a plurality of evaluation units and a plurality of detection units with a first control unit controlling a switching arrangement and fixing the frequency and bandwidths of at least one of the plurality of the extraction units based on signals received from at least one of the plurality of the evaluation units and detection units. The method of the present invention includes receiving frequency signaling throughout a first frequency bandwidth that includes a first carrier frequency and then converting the analog signal produced in the receiving step into a first digital signal that corresponds to the first frequency bandwidth. A further step is extracting from the first digital signal a secondary digital signal that is within a secondary frequency bandwidth which is equal to or within the first frequency bandwidth and which includes a secondary carrier frequency, the secondary frequency bandwidth and the secondary carrier frequency associated with communication in a first of a plurality of communication systems. While performing the step of extracting the secondary digital signal, a further step is extracting from the first digital signal at least one additional secondary digital signal that corresponds to a frequency bandwidth which is also equal to or within the first frequency bandwidth and which also includes a carrier frequency, the at least one additional secondary frequency bandwidth and its corresponding carrier frequency being associated with communication in a communication system similar to or different from that used the secondary digital signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  diagrammatically illustrates a terminal apparatus according to embodiments of the invention; 
         FIG. 1B  diagrammatically illustrates another aspect of the terminal apparatus of  FIG. 1A ; 
         FIG. 2  graphically illustrates an example of relative portions of the frequency spectrum that can be accessed by embodiments of the invention; 
         FIG. 3A  diagrammatically illustrates a terminal apparatus according to further embodiments of the invention; 
         FIG. 3B  diagrammatically illustrates a simplified version of the terminal apparatus of  FIG. 3A ; 
         FIG. 4  diagrammatically illustrates a terminal apparatus according to further embodiments of the invention; 
         FIG. 5A  is a flow chart illustrating a set of operations performed according to the invention; 
         FIG. 5B  is a flow chart illustrating a method of the present invention; and 
         FIG. 6  diagrammatically illustrates a terminal apparatus according to further embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention provide a terminal with a front end receiver operable throughout a bandwidth that is large enough to cover the entire bandwidth used by access technologies supported by the terminal. For example, in the Super 3G (S3G) case, the front end receiver would be operable across the entire 20 MHz bandwidth used by an OFDM signal, and could therefore receive a bandwidth that is larger than that of GSM (200 kHz) and WCDMA (5 MHz). In this manner, the present invention makes it possible, with only a single front end receiver, to perform HO measurements and cell searches with respect to one access technology, while simultaneously supporting, without interruption, an ongoing communication session on another access technology. It is also possible to perform HO measurements and cell searches on different access technologies, on different carrier frequencies, simultaneously, which can reduce the search time. 
       FIG. 1A  diagrammatically illustrates a terminal apparatus  1  according to embodiments of the invention. In the terminal of  FIG. 1A , a front end receiver (RX)  2 A is capable of receiving a signal with bandwidth BW 0  (e.g., 20 MHz) which can be tuned to a first carrier frequency f 0  and filtered via analog filter (AF)  2 B. Thus, for example, the frequencies in the interval f 0 +/− BW 0 /2 can be differentiated from unwanted signals by the front end receiver  2 A and AF  2 B. In some embodiments such as that described in  FIGS. 4 and 6 , the desired carrier frequency and bandwidth, e.g., f 0  and BW 0 , are adjustable, and can be specified at inputs  3 A and  3 B. 
     The terminal of  FIG. 1A  is capable of communicating with a communication system A (not explicitly shown) using a carrier frequency f A  and bandwidth BW A , both of which are included in the aforementioned operating bandwidth of the front end receiver  2 A and AF  2 B, as shown in  FIG. 2 . The example of  FIG. 2  is provided for general illustrative purposes only, and is not necessarily drawn to scale. In the front end receiver  2 A and AF  2 B, the input signal, at carrier frequency f 0  and having bandwidth BW 0 , is down-converted to an analog baseband signal, which is then converted from analog to digital format by an analog-to-digital converter (ADC)  4 . The resulting digital signal is fed to units adapted to extract signals, extraction units  5  and  6 . The use of two extraction units is illustrated in the Figures, however, the present invention encompasses as many extraction units and corresponding modules as there are signals to be processed. The extraction unit  5  uses the frequency f A  to down-convert the digital signal to a digital baseband signal, and also implements a low-pass filter of bandwidth BW A . The use of these types of digital processing operations to extract a desired frequency carrier and bandwidth are well known in the art. 
     The signal extracted by extraction unit  5  is routed by a switching arrangement  7  controlled by a control unit  10  to one of a plurality of AT 1 -ATm detection units  8  that is adapted to the access technology used by the communication system A. For example, if communication system A uses access technology AT 1  (e.g., GSM, WCDMA, OFDM-based system), then the AT 1  detection unit (adapted to access technology AT 1 ) is selected by switching arrangement  7 , and if communication system A uses access technology ATm, then the ATm detection unit (adapted to access technology ATm) is selected by switching arrangement  7 . In some embodiments, each of the illustrated detection units  8  is capable of performing signal detection according to techniques that are well known in the art.  FIG. 1B  diagrammatically illustrates in more detail the aforementioned AT 1 -ATm detection units  8  of  FIG. 1A .  FIG. 1B  shows a set of detection units for S3G and WCDMA signals. Note that the present invention encompasses any number of detection units optimized for any type of signal. 
     The signal detected by the selected detection unit  8  is provided to first control unit  10 , which forwards information contained in the detected signal for further processing at  11  as seen in  FIGS. 1A and 1B . Referring back to  FIG. 1A , the first control unit  10  also has a control output designated generally at  12 , which is used to provide control information for the extraction units  5  and  6  and path selection information to control the operation of switching arrangement  7 . Referring briefly to  FIG. 3B , the control output  12  is further detailed as control output  12 A, which is used to provide control information, such as the carrier frequency f A  and bandwidth BW A  for the extraction unit  5 , and control output  12 B, which is used to provide control information, such as carrier frequency f B  and bandwidth BW B , for the extraction unit  6 , and path selection information to control the operation of the switching arrangement  7 . Referring back to  FIGS. 1A and 1B , the first control unit  10  also uses neighbor list information (which is conventionally available in the signal produced by the selected detection unit) to learn which carrier frequencies, bandwidths and access technologies are utilized by neighboring cells. 
     Using this neighboring cell information, first control unit  10  can use the control output  12  to control extraction unit  5  and switching arrangement  7  to produce desired evaluations of neighboring cells. For example, the first control unit  10  can direct the extraction unit  5  to perform generally the same operations described above with respect to extraction unit  6 , but to use another carrier frequency f B  and another bandwidth BW B  (see also  FIG. 2 ). Referring to  FIGS. 1A and 1B , the first control unit  10  also directs switching arrangement  7  appropriately to route the output of extraction unit  5  to one of a plurality of AT 1 -ATn evaluation units  9  associated with the access technology (e.g., GSM, WCDMA, OFDM-based systems) used by the cell that is to be evaluated. There may be the same number, or a different number (more or less), of AT 1 -ATn evaluation units as there are AT 1 -ATm detection units. For example, if the cell uses access technology AT 1 , then the AT 1  evaluation unit is selected, and if the cell uses access technology ATn, then the ATn evaluation unit is selected.  FIG. 1B  diagrammatically illustrates in more detail the aforementioned AT 1 -ATn evaluation units  9  of  FIG. 1A . As seen therein, certain operations occur depending on the type of technology access. For a S3G signal, detector  8 A performs channel estimation and determines its signal to interference ratio (SIR) (techniques well known in the art), filters the signal through a matched filter (MF) and then decodes the signal. The same operations occur in block  8 B with respect to a WCDMA signal. Also, for S3G and WCDMA signals, evaluator  9  performs cell search and determines SIR in separate modules. Although  FIG. 1B  shows a set of evaluation units for S3G and WCDMA signals, the present invention encompasses any number of evaluation units optimized for any type of signal. 
     In some embodiments, each of the evaluation units  9  is capable of performing operations such as HO measurements and cell searches according to techniques that are well know in the art. In this manner, the terminal can, while simultaneously supporting an ongoing communication session with the aforementioned cell A (i.e., using extraction unit  5  as described above), perform operations such as HO measurements, cell searches, etc. with respect to communication systems in neighbor cells that use access technologies which differ from the access technology used by cell A. The ongoing communication session with cell A need not be interrupted. The evaluation results from the evaluation units are provided to the first control unit  10 , and can be processed therein, or forwarded at  11  for further processing. If a new cell becomes stronger than the current cell on f A , then a handover can be executed according to conventional practice. If the new cell uses an access technology that differs from the access technology of cell A, then this handover is an inter RAT handover. 
       FIG. 3A  diagrammatically illustrates a terminal that is capable of efficiently performing cell evaluations while the terminal is in standby mode, according to embodiments of the invention. In standby mode, the terminal is “on”, but not in use, so low power consumption is important. Typically, conventional terminals in standby mode will “wake up” periodically (e.g., once per second) and turn on its front end receiver to receive and read paging messages to determine if there are any messages for the terminal, and to perform cell measurement operations to determine whether there are any stronger cells on which to camp. 
     The terminal of  FIG. 3A  is similar to that of  FIG. 1A , but includes a digital data storage unit  31  coupled to the output of the ADC  4 . Each time the terminal “wakes up” to receive paging messages and perform cell measurements, the digital signaling output from the ADC  4  is stored in the storage unit  31 . The first control unit  10  can use its control output  12 A to effectuate the desired paging detection (e.g., detection on carrier frequency f A  and bandwidth BW A ) via extraction unit  5  and the appropriate detection unit, in generally the same fashion as any other communication session. When the front end receiver  2 A (including AF  2 B) is turned off at the end of the “wake up” period, the stored digital signal can be played back, and cell evaluation operations such as those described above can be performed for any desired combinations of carrier frequency, bandwidth and access technology within the bandwidth of the analog filter. Cell evaluations can therefore continue to be performed during standby mode, without requiring any additional use of front end receiver  2 A and AF  2 B, and thus without increasing the power consumed by front end receiver  2 A and AF  2 B. 
     When the paging message detection is completed, there is no communication session to support during the periods between “wake ups”. Accordingly, the first control unit  10  of  FIG. 3A  can, during those time periods, (1) control the switching arrangement  7  such that each of the extraction units  5  and  6  is coupled to a respective one of the evaluation units  9 , and (2) use both extraction units  5  and  6  to support cell evaluations. Cell evaluations can thus proceed simultaneously with one another, thereby reducing the time needed to perform the cell evaluations. In some embodiments, the first control unit  10  controls the cell evaluations based on neighbor list information obtained during the paging message detection for the cell being camped on. In some embodiments, the first control unit  10  controls the cell evaluations based on information about neighbor cells derived from historical information stored in the first control unit  10 .  FIG. 3B  diagrammatically provides a simplified version of the terminal apparatus of  FIG. 3A . As seen therein, the extraction units  5  and  6  are comprised of mixers  32  and  34  and corresponding low pass filters  33  having BW B  and  35  having BW A . These elements are adapted to permit tuning of f A  and f B  in the digital domain prior to the signals being fed via the switching arrangement to the detection units  8  and evaluation units  9 . 
     In some situations, very strong adjacent channel interferers can effectively prevent the use of the full bandwidth of the front end receiver  2 A. If there is a strong interferer close to the desired frequency or bandwidth, a considerable portion of that interferer may need to be filtered out upstream of the ADC  4  in order to avoid dynamics problems.  FIG. 4  diagrammatically illustrates a terminal that can minimize interference. The terminal of  FIG. 4  is similar to those of  FIGS. 1A ,  1 B,  3 A and  3 B, but includes the capability of adjusting AF  2 B relative to the full bandwidth BW 0  capability of front end receiver  2 A. For example, in some embodiments where the front end receiver  2 A has a 20 MHz bandwidth capability, AF  2 B is adjustable from 1.25 MHz to 20 MHz. 
     In  FIG. 4 , a second control unit  41  receives, from an output  43  of first control unit  10 , conventionally available information indicative of the signal quality of the ongoing communication session, e.g., the session on f A  described above. In some embodiments, this signal quality information is obtained from CQI (channel quality index) pilots in the system. The second control unit  41  continuously monitors the signal quality information, and provides to the front end receiver  2 A and AF  2 B control information  3 A and  3 B, respectively. The center frequency of the received signal is adjusted at front end receiver  2 A via control information  3 A and the analog selectivity filter of AF  2 B is adjusted in response to the control information  3 B, thereby adjusting the size of the bandwidth. In some embodiments, the carrier frequency f 0  and bandwidth of the receiver are initially set to f A  and BW A , respectively. Then, the receiver bandwidth is increased gradually, while continuing to monitor the signal quality information. This gradual increase continues until either the receiver bandwidth reaches its maximum available bandwidth, or the signal quality reaches a minimum acceptable quality threshold. In some embodiments, the threshold is a certain fraction of what the quality measure was when the bandwidth adjustment process began. 
     In some embodiments, adjustments of the bandwidth of front end receiver  2 A are performed periodically. In some embodiments, the adjustments are performed whenever the channel quality changes suddenly. In some embodiments, the adjustments are performed whenever the terminal executes a handover. 
       FIG. 5A  illustrates one of a plurality of possible bandwidth adjustment operations described above with respect to  FIG. 4 . Initially, at  51 , the receiver&#39;s bandwidth BW RX  is set equal to BW A . When an event that triggers adjustment occurs at  52 , the receiver bandwidth BW RX  is increased at  53 . The operations at  52  and  53  are repeated until either the receiver bandwidth BW RX  reaches its maximum available bandwidth, here BW 0 , at  54 , or the signal quality reaches a minimum acceptable quality threshold TH at  55 . 
     In some embodiments, the control information  3 A directs the front end receiver  2 A to adjust its carrier frequency f 0 , in order to permit use of the receiver&#39;s entire bandwidth capability, even in the presence of strong interference. Referring to  FIG. 2 , this carrier frequency adjustment has the effect of shifting the bandwidth BW 0  relative to carrier frequency f A . In some situations, the bandwidth BW 0  can be shifted enough to remove the problem interference with respect to carrier frequency f A . 
       FIG. 5B  is a flow chart illustrating a method of operating in a plurality of communication systems. As seen in step  501 , a frequency signaling is received throughout a first frequency bandwidth that includes a first carrier frequency. In step  502 , an analog signal produced in the receiving step is converted into a first digital signal that corresponds to the first frequency bandwidth. In step  503 , a secondary digital signal that is within a secondary frequency bandwidth is extracted from the first digital signal which is equal to or within the first frequency bandwidth and which includes a secondary carrier frequency. The secondary frequency bandwidth and the secondary carrier frequency are associated with communication in a first of a plurality of communication systems. In step  504 , at least one additional secondary digital signal that corresponds to a frequency bandwidth which is also equal to or within the first frequency bandwidth and which also includes a carrier frequency, is extracted from the first digital signal. The at least one additional secondary frequency bandwidth and its corresponding carrier frequency are associated with communication in a communication system similar to or different from that used the secondary digital signal. The aforementioned method can further include storing the first digital signal so that the steps of extracting the secondary digital signal and the at least one additional secondary digital signal from the stored first digital signal occur after completion of the receiving step. In addition, the receiving step can further include adjusting one of the first bandwidth and the first carrier frequency based on communication quality associated with one of the secondary carrier frequencies. For example, the receiving step can include adjusting the first bandwidth or the first carrier frequency based on communication quality associated with one of the secondary carrier frequencies. Further, the method illustrated in  FIG. 5B  can include performing the adjusting step at points in time that occur periodically or whenever a handover occurs. 
       FIG. 6  diagrammatically illustrates a terminal according to further embodiments of the invention. The terminal of  FIG. 6  is similar to those of  FIGS. 1A ,  1 B,  3 A,  3 B and  4 , but includes both the digital storage/playback capability described above with respect to  FIG. 3A , and the front end receiver adjustment capability described above with respect to  FIG. 4 . 
     Although several embodiments of the invention have been described above in detail, this does not limit the scope of the invention, which can be practiced in a variety of embodiments.