Abstract:
A method of performing cell measurements in a telecommunications system. The method includes the steps of receiving a signal by a user equipment (UE) operating within the telecommunications system and served by a serving cell, storing the received signal in a buffer, and determining a first portion of the signal related to the serving cell and a second portion related to neighbor (NB) cells. The first portion of the signal is decoded by the Fast Fourier Transforms (FFT) and the signal strength is estimated by a measurement unit coupled to the FFT. Next, any approximately synchronized NB cell signals in the second portion of the signal are then decoded and a signal strength is estimated. It is then determined if sufficient time remains to process unsynchronized NB cell signals of the second portion of the signal. If it is determined that sufficient time remains, the second portion of the signal is played back and decoded using the FFT and a signal strength of any unsynchronized NB cell signals of the second portion of the received signal is estimated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/805,652, filed Jun. 23, 2006 and is hereby incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable  
       REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX  
       [0003]     Not applicable  
       BACKGROUND OF THE INVENTION  
       [0004]     This invention relates to communication systems. More particularly, and not by way of limitation, the invention is directed to a system and method of performing cell measurements in a telecommunications system.  
         [0005]     In the forthcoming evolution of the mobile cellular standards, such as Global System for Mobile Communications (GSM) and Wideband-Code Division Multiple Access (WCDMA), new modulation techniques such as Orthogonal Frequency Division Multiplexing (OFDM) are likely to occur. Furthermore, in order to have a smooth migration of the old cellular systems to the new high capacity high data rate system in the existing radio spectrum, the new system has to be able to operate on a flexible bandwidth (BW). A proposal for such a new flexible cellular system is 3G Long Term Evolution (3G LTE), which is an evolution from the 3G WCDMA standard. This system utilizes OFDM as a multiple access technique (called OFDMA) in the downlink and is able to operate on a bandwidth spanning from 1.25 MHz to 20 MHz. Furthermore, data rates up to 100 Mb/s will be possible in this high bandwidth system.  
         [0006]     The 3G LTE system allows for use in a “Reuse One” fashion (i.e., all cells share the same carrier frequency). Therefore, neighbor cell measurements needed for mobility (handover) purposes can be made in similar fashion as in WCDMA. Furthermore, the different BW possibilities in LTE introduces additional neighbor (NB) cell measurements that need to be considered. For example, in some scenarios there may be a “hot spot” cell with a specific bandwidth (e.g., 20 MHz), while neighboring cells may be using another bandwidth (e.g., 5 or 10 MHz). Similar scenarios may occur in the border between countries or other geographical or political boundaries.  
         [0007]     There are several scenarios where different NB cell configurations need to be handled in LTE. Intra-frequency neighbor cell measurements are performed by a user equipment (UE) when the current and target cell operates on the same carrier frequency. In this case, the UE is able to carry out such measurements without measurement gaps. Neighbor cell measurements performed by the UE are considered as inter-frequency measurements when the neighbor cell operates on a different carrier frequency in comparison to the current cell. In this situation, the UE is unable to carry out such measurements without measurement gaps.  
         [0008]     Depending on whether the UE needs transmission/reception gaps to perform the relevant measurements, measurements are classified as gap assisted or non-gap assisted. A non-gap assisted measurement is a measurement on a cell that does not require transmission/reception gaps to allow the measurement to be performed. A gap assisted measurement is a measurement on a cell that requires transmission/reception gaps to allow the measurement to be performed. Whether a measurement is non-gap assisted or gap assisted depends on the current operating frequency. The UE determines whether a particular cell measurement needs to be performed in a transmission/reception gap.  
         [0009]     In the situation where cells operate on the same carrier frequency, gaps are not needed to perform the measurements. If the cells&#39; carrier frequencies differ, gap assisted measurements are needed which are independent of the UE/cell bandwidth. These measurement gaps are provided and controlled by the network.  
         [0010]      FIG. 1A  is a simplified block diagram illustrating an intra-frequency measurement scenario in LTE. In  FIG. 1 , a UE  10  communicates with a current cell  12 . The current cell  12  and a target cell  14  have the same carrier frequency and bandwidth. This is the most common measurement scenario. In this scenario, measurement gaps are not required.  
         [0011]      FIG. 1B  is a simplified block diagram illustrating a second intra-frequency measurement scenario in LTE. The current cell  12  and the target cell  14  have the same carrier frequency. However, the bandwidth of the target cell is less than the bandwidth of the current cell.  
         [0012]      FIG. 1C  is a simplified block diagram illustrating a third intra-frequency measurement scenario in LTE. In this scenario, the current cell  12  and the target cell  14  have the same carrier frequency. However, the bandwidth of the target cell is greater than the bandwidth of the current cell.  
         [0013]      FIG. 2A  is a simplified block diagram illustrating a first inter-frequency measurement scenario in LTE. The current cell and the target cell have different carrier frequencies. In addition, the bandwidth of the target cell is less than the bandwidth of the current cell and the center part of the bandwidth of the target cell is within the bandwidth of the current cell. In this scenario, since it is an inter-frequency scenario, measurement gaps are utilized.  
         [0014]      FIG. 2B  is a simplified block diagram illustrating a second inter-frequency measurement scenario in LTE. The current cell and the target cell have different carrier frequencies. In addition, the bandwidth of the target cell is greater than the bandwidth of the current cell and the center part of the bandwidth of the target cell is within the bandwidth of the current cell.  
         [0015]      FIG. 2C  is a simplified block diagram illustrating a third inter-frequency measurement scenario in LTE. The center part of the bandwidth of the target cell is outside the bandwidth of the current cell. In this scenario, measurement gaps are required.  
         [0016]     The above figures show the different NB cell configurations that are encountered and require action in LTE. For handoff measurements, typically only a subfraction of the entire bandwidth is used for cell search (i.e., 1.25 MHz). Cell measurements are denoted in  FIGS. 1 and 2  as measurement bandwidth (Meas BW). In  FIG. 1A , this is the most common scenario corresponding to legacy intra-frequency measurements. In LTE, the scenarios of  FIGS. 1B and 1C  are also defined as intra-frequency measurements. Since the carrier frequency for the NB cells in  FIGS. 1B and 1C  is the same as the serving cell, the UE typically performs the measurements on these NB cells without interruption (i.e., no measurement gap) in the data reception. In  FIG. 2C , a pure (legacy) inter-frequency measurement scenario is shown (i.e., similar to the WCDMA case). For this scenario, a gap in the reception from the serving cell is needed to conduct the measurements, such that the radio can be retunded to the NB cell carrier frequency. For LTE, in  FIGS. 2A and 2B , the scenarios also are inter-frequency measurements and reception gaps are required. In these scenarios, the carrier frequency for the NB cells is not aligned with the carrier frequency for the serving cell. However, the reception bandwidth for the UE still covers the measurements portions in  FIGS. 2A and 2B , without changing the local oscillator frequency.  
         [0017]     A system and method of performing cell measurements for all the scenarios discussed above is needed. Currently, there are three different existing solutions for performing the measurements on all the scenarios. First, gaps may be created in the intra-frequency measurement scenarios. When the UE is close to the cell border, the UE requests an interruption in the reception in order to allocate the Fast Fourier Transforms (FFT) to the neighbor cell. Thus, a similar way is needed for inter-frequency handoff. The disadvantage with this solution is that a lower throughput is achieved due to the need for interrupting the data reception.  
         [0018]     In the second existing solution, synchronized base stations are utilized. To accomplish this solution, all cells have the same timing when using the FFT. All cells&#39; (both serving and neighbor) pilot signals are detected and the signal strength is estimated. The disadvantage of this solution is that the cells must be synchronized.  
         [0019]     In the third existing solution, two FFTs are utilized. One of the FFTs is used for serving cell detection and one of the FFT is used for neighbor cell measurements. Since two FFTs are used, an increased chip area cost in the UE is necessary.  
         [0020]     Therefore, a system and method for performing the cell measurements utilizing a single FFT in both the inter-frequency and intra-frequency scenarios is needed. The present invention provides such a system and method.  
       BRIEF SUMMARY OF THE INVENTION  
       [0021]     In one aspect, the present invention is directed to a method of performing cell measurements in a telecommunications system. The method includes the steps of receiving a signal by a user equipment (UE) operating within the telecommunications system and served by a serving cell, storing the received signal in a buffer, and determining a first portion of the signal related to the serving cell and a second portion related to neighbor (NB) cells. The first portion of the signal is decoded and the signal strength is estimated by a measurement unit coupled with a Fast Fourier Transforms (FFT). Next, any approximately synchronized NB cell signals in the second portion of the signal are then decoded and a signal strength is estimated. It is then determined if sufficient time remains to process unsynchronized NB cell signals of the second portion of the signal. If it is determined that sufficient time remains, the second portion of the signal is played back and decoded using the FFT and a signal strength of any unsynchronized NB cell signals of the second portion of the received signal is estimated.  
         [0022]     In another aspect, the present invention is a system for performing cell measurements in a telecommunications system. The system includes a receiver for receiving a signal by a UE operating within the telecommunications system and served by a serving cell, a buffer for storing the received signal, a FFT for processing a signal, and a measurement unit for decoding and estimating a signal strength. The signal includes a first portion relating to the serving cell and a second portion related to neighbor (NB) cells. The first portion is processed first by the FFT. Within the second portion of the received signal, it is determined if any NB cell signal is approximately synchronized with the serving cell. The measurement unit then estimates a signal strength of any synchronized NB cell signal of the second portion of the received signal. If sufficient time remains to process unsynchronized NB cell signals of the second portion, the second portion of the signal is played back and decoded using the FFT and a signal strength of any unsynchronized NB cell signals of the second portion of the received signal is estimated. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0023]     In the following, the features of the invention will be described in detail by showing preferred embodiments, with reference to the attached figures in which:  
         [0024]      FIGS. 1A-1C  illustrates intra-frequency measurement scenarios in LTE;  
         [0025]      FIGS. 2A-2C  illustrates inter-frequency measurement scenarios in LTE;  
         [0026]      FIG. 3  is a simplified block diagram of a telecommunications system in the preferred embodiment of the present invention;  
         [0027]      FIG. 4  is a block diagram of the components within the UE for conducting cell measurements in the preferred embodiment of the present invention;  
         [0028]      FIG. 5  illustrates a frequency shift on an FFT processed signal in the preferred embodiment of the present invention; and  
         [0029]      FIG. 6  is a block diagram illustrates the steps of conducting cell measurements according to the teachings of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     The present invention is a system and method of performing cell measurements in a telecommunications system.  FIG. 3  is a simplified block diagram of a telecommunications system  20  in the preferred embodiment of the present invention. The telecommunications system may be any type of system, however, in the preferred embodiment of the present invention, the telecommunications system is an 3G LTE system or an Worldwide Interoperability for Microwave Access (WiMax) system.  
         [0031]     A UE  22  is located within the telecommunications system  20  and is serviced by a serving cell (SC)  24 . The UE receives signals from one or more NB cells  26 . The present invention provides a method and system for NB cell measurements and cell searching for all the scenarios discussed in  FIGS. 1A, 1B ,  1 C,  2 A,  2 B. Inter-frequency measurements shown in  FIGS. 2A and 2B  can be treated in a similar way as intra-frequency measurements shown in  FIGS. 1A, 1B  and  1 C. Either based on a neighbor cell list or on detected cells, a receiver within the UE  22  measures NB cells  26 . The FFT (see  FIG. 4 ), through a measurement unit (Meas unit), conducts measurements (Meas BW) on the measurement portion of the signal corresponding to the respective NB cell  26  at time instances when the FFT is idle (i.e., when the FFT is not conducting processing of the serving cell). The present invention accomplishes this process by storing the received signal in a buffer within the UE and playing back the received signal to the FFT. If the NB cell carrier frequency is not the same as the SC  24  (e.g., due to some offset commanded by the network or due to Doppler), a frequency adjustment unit adjusts the frequency to match the FFT frequency bins prior to the FFT processing. Furthermore, in the preferred embodiment of the present invention, when measurements are conducted on the NB cells using gaps in the serving cell reception as ordered by the network (i.e., the scenarios of  FIGS. 2A and 2B ), the radio front end receiver of the UE is turned off, the data from the buffer is played back, optionally frequency adjusted and processed by the FFT. It should be understood that for “almost synced” cells, additional FFT processing is not required. For an “unsynced” NB cell, the playback is conducted corresponding to the NB cell, prior to FFT processing.  
         [0032]      FIG. 4  is a block diagram of the components within the UE  22  for conducting cell measurements in the preferred embodiment of the present invention. A receiver  30  receives a signal through an antenna  32 . The signal is down-converted and processed through a low-pass filter to an analog base band signal Fe Rx  34 , AD converted at an Analog-to-Digital Converter (ADC)  36 , and digitally filtered through a filter with bandwidth BW 0  at digital filter (DF) BW 0    38 . The BW 0  filtered signal includes the signal from the SC  24 , as well as signals from NB cells  26  having measurement information (meas BW) within the BW 0 . The signal is then fed to a buffer  40  and simultaneously fed to an FFT  42 . The FFT  42  generates f-domain samples that are then further processed. At a specified time instance, when the FFT is idle, a control unit (CU)  44  instructs the playback of the stored signal from the buffer to the FFT. The NB cell measurements are then made upon the signal by a measurement unit (meas unit)  43 . By letting the FFT processing time of one symbol be smaller than the OFDM symbol length, sufficient time is allotted for the meas unit to conduct measurements on neighboring cell signals. Typically different cells have different scrambling codes, hence the signal needs to be de-scrambled with the respective NB cell scrambling code before measurements are taken. The necessary information for NB cells  26  are either detected in the cell search unit (CS)  46  and/or received from the network (e.g., neighbor list, higher layer information). In the case where there is a frequency offset between the SC cell and the NB cell, either due to the NB cell having another carrier frequency or due to Doppler spread (estimated in a F err  unit  48 ), the signal is adjusted before the buffered signal is applied for processing by the FFT. The adjustment is done such that the measurement portion (Meas BW) portion of the FFT processed signal of the NB cells is on the same frequency grid as the SC  24  (see  FIG. 5 ). For “almost synced” cells, a new FFT is not required. For an “unsynced” NB cell, the playback is conducted corresponding to the NB cell, prior to FFT processing.  
         [0033]     The present invention utilizes the buffer  40  prior to processing the signal within the FFT  42 . The FFT processing of an OFDM symbol is preferably at least twice as fast as the OFDM symbol rate. In addition, the control unit  44  controls the FFT to first decode and estimate the signal strength for the SC  24  OFDM symbol, as well as the possible estimate of the signal strength for NB cells that is in synch with the SC  24 . The remaining time (until the next OFDM symbol) is utilized to playback information from the buffer to the FFT. The playback is based on timing on the NB cells  26  not in synch with the SC and intra-frequency measurements on these NB cells are made. Thus, an unsynced NB cell, the playback is conducted corresponding to the NB cell, prior to FFT processing. For an almost synced cell, a new FFT is not required.  
         [0034]      FIG. 5  illustrates a frequency shift on an FFT processed signal in the preferred embodiment of the present invention. The NB cell offset frequency (relative to the SC  24 ) is f c . The signal is shifted f d , where f d  corresponds to the difference needed for the NB cell frequency grid to become the same as the SC grid. In LTE, Δf is 15 kHz (or 7.5 kHz) and therefore, the maximum necessary f d  is 7.5 (3.75) kHz. However, there may be situations where f d =f c . F represents the frequency for a specified sub-carrier frequency. Thus, a frequency shift is added such that the sub-carrier frequencies for the NB cell are aligned with the SC subs-carrier frequencies.  
         [0035]     As in the scenarios described in  FIGS. 2A and 2B , a higher layer may inform the UE to conduct measurements during reception gaps, i.e., interruption in received data. In this case, the stored signal (prior to the gap) is played back, frequency adjusted and FFT processed. Furthermore, the NB cell measurements are conducted. Simultaneously, the front end RX and ADC are turned off, thereby saving power.  
         [0036]     For LTE, currently two different sub-carrier spacings, Δf=7.5 and 15 kHz are defined. Therefore, the UE needs to be able to do measurements on NB cells having different carrier spacings. In the 7.5 kHZ case, the symbols are twice as long and hence an FFT 2048 is need for 10 MHz (compared to 20 MHz in the Δf=15 kHz case). The present invention also covers this scenario. For example, in the case where the SC has a carrier spacing of 15 kHz and the NB cell has a 7.5 kHz sub-carrier spacing, when the FFT is idle, samples corresponding to one OFDM symbol from the NB cell (with 7.5 kHz sub-carrier spacing) are played back from the buffer  40  to the FFT  42 . The signal is then frequency adjusted to fit the FFT grid. The control unit takes the different sub-carrier spacing into account and computes the required frequency adjustment (i.e., Ω). The CU applies a frequency shift, e jΩt , to the signal form the buffer.  
         [0037]      FIG. 6  is a block diagram illustrates the steps of conducting cell measurements according to the teachings of the present invention. With reference to  FIGS. 1-6 , the method will now be explained. The method begins with step  100  where timing information on the SC  24  and the NB cell  26  is obtained. Specifically, the timing of the SC  24 , T sc , and the neighbor cells  26 , T NB , is provided during a cell search procedure. The method then moves to step  102  where an incoming signal is received. In step  104 , incoming RX samples are stored in the buffer  40 . Next, in step  106 , a portion of the signal corresponding to cell timing T sc  is processed by the FFT  42 . Next, in step  108 , the incoming samples are de-scrambled using a specific scrambling code of the SC  24 . The main task of the receiver  30  is then accomplished. Specifically, the detection of the data sent from the serving cell is completed. Furthermore, measurement on the received pilot signal strength from the SC  24  is performed. The method then moves to step  110  where it is determined if there are NB cells with a timing T NB  having an approximate equal value as T sc . Specifically, it is determined if any NB signals are synchronized with the SC. T NB  does not have to match exactly with T sc , rather the difference between the timing T NB  and the timing T sc  is within a length of a cyclic prefix of the OFDM symbol. In step  110 , if it is determined that there are NB cells with a timing T NB  having an approximate equal value as T sc , the NB signal is de-scrambled and the NB signal strength is measured (i.e., estimated) by the meas unit  43  in step  112 . The method then moves to step  114  where it is determined if there is sufficient time to complete additional measurements. If it is determined that there is sufficient time to conduct more measurements, the method moves from step  114  to step  110 . In step  112 , if there is not sufficient time to perform additional measurements, the method moves to step  102 .  
         [0038]     In step  110 , if it is determined that there are no NB signals synched with the SC, the method moves to step  116  where it is determined if there is sufficient time available to complete the processing of other NB cell signals and that there are other NB cells to measure and process. If it is determined that there is sufficient time, the method moves to step  118  where the data is obtained from the buffer. In addition, the data (including the T NB ) is provided to the FFT  42 . The FFT processed samples are de-scrambled to obtain the NB reference symbols and a measurement of the NB cells is accomplished by the meas unit  43 . Next, the method moves to step  120  where it is determined if there are any non-synched cells remaining. In step  120 , if it is determined that there is non-synched cells remaining, the method moves to step  116 . However, in step  120 , if it is determined that there are not any non-synched cells remaining, the method returns to step  102 .  
         [0039]     A list of the most relevant neighbor cells is preferably maintained in order to prioritize the measurements for the different cells. This list is preferably ordered such that stronger cells have higher priority since they are more likely to be selected as serving cells in the near future. Furthermore, the list preferably contains information about elapsed time since the last measurement on each cell in order to ensure measurements with reasonable regularity. Thus, steps  114  and  116  of  FIG. 6  include performing the actions based on the priority list. In addition, any metric for comparing the SC with NB cells may be utilized. However, typically either signal-to-noise ratio (SNR) or received signal strength is utilized as the metric for comparing the SC with NB cells.  
         [0040]     In the preferred embodiment of the present invention, a method and system are providing for conducting NB cell measurements and cell searching covering all the scenarios discussed in  FIGS. 1, 2A  and  2 B where all the NB cell measurements may be conducted without interruption in the reception of the data from SC, i.e., as intra-frequency measurements. Either based on a neighbor cell list or based on detected cells, the receiver measures specified NB cells. The signal corresponding to the respective NB cell is processed by the FFT. In addition, measurements are performed on the measurement portion of the signal, at time instances when the FFT is idle, i.e. when the FFT is not processing the signal of the SC. This can be achieved by storing the received signal in the buffer and playing back the received signal to the FFT. If the NB cell carrier frequency is not he same as the SC (e.g., due to some offset commanded by the network or due to Doppler) a frequency adjustment unit adjusts the frequency to match the FFT frequency bins prior to the FFT processing. Furthermore, in the preferred embodiment of the present invention, when measurements are done on NB cells using gaps in the SC reception, as ordered by the network (i.e.,  FIGS. 2A and 2B ), the radio front end receiver is turned off and the data from the buffer is played back, optionally frequency adjusted and processed by the FFT. Thus, mobile terminal power is conserved during the reception gap.  
         [0041]     Although preferred embodiments of the present invention have been illustrated in the accompanying drawings and described in the foregoing Detailed Description, it is understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the scope of the invention. The specification contemplates all modifications that fall within the scope of the invention defined by the following claims.