Abstract:
Techniques for fast cell search, selection and reselection for wireless communication systems such as OFDM or OFDMA communication systems. In various implementations of the described techniques, downlink subframes from base stations to mobile stations are designed to include information in form of preambles or post-ambles in one or more downlink subframes to facilitate cell search at receiving mobile stations at high speeds. The described preambles and post-ambles may also be used to improve the accuracy of signal synchronization in time and frequency.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of provisional U.S. application Ser. No. 60/601,790, entitled “PREAMBLE SEQUENCES FOR FAST CELL SEARCH” and filed Aug. 16, 2004, which is incorporated herein by reference in its entirety as part of the specification of this application. 
     
    
     BACKGROUND  
       [0002]     This application relates to cell search and signal synchronization in wireless communication systems including systems based on orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA).  
         [0003]     Wireless communication systems use a network of base stations to communicate with wireless devices registered for services in the systems. Each base station emits radio signal that carry data such as voice data and other data content to wireless devices. Such a signal from a base station can include overhead load other than data for various communication management functions, including information to allow a wireless device to identify a cell sector of a base station, to synchronize signaling in time and frequency. Each wireless device processes such information in the overhead load of reach received signal prior to processing of the data.  
         [0004]     OFDM and OFDMA based communication systems are based on the orthogonality of frequencies of multiple subcarriers and can be implemented to achieve a number of technical advantages for wideband wireless communications, such as resistance to multipath fading and interference. However, many OFDM and OFDMA based wireless communication systems can be sensitive to frequency offsets and phase noise. In an OFDM or OFDMA based wireless communication system, the wireless service to a geographic area is provided by dividing the area into a plurality of cells, which can be further divided into two or more cell sectors. The base stations, which conceptually locate at the center of respective cells of their coverage, transmit information to a mobile subscriber station (MSS) via downlink (DL) radio signals sent out from the base stations. A mobile subscriber station is also known as a mobile station (MS) or the wireless station. The mobile stations transmit information to their serving base stations via uplink (UL) radio signals.  
         [0005]     The downlink radio signals from the base stations to mobile stations may include voice or data traffic signals or both. In addition, the base stations generally need to transmit preamble signals in their downlink radio signals to identify to the mobile stations, the corresponding cells and corresponding segments in the cells from which the downlink radio signals are transmitted. Such a preamble signal from a base station allows a mobile station to synchronize its receiver in both time and frequency with the observed downlink signal and to acquire the identity, such as IDcell and Segment, of the base station that transmits the downlink signal.  
         [0006]     IEEE 802.16 OFDMA has been developed to provide wireless communications based on an orthogonal frequency division multiple access (OFDMA) modulation technique. In the DL preambles currently defined in IEEE 802.16 OFDMA, the MSSs store predefined and handcrafted pseudo-noise (PN) like sequences for identifying IDcell numbers and segment numbers of the adjacent cells. In operation, an MSS captures the preamble symbols in received downlink signals and correlates the preamble in each received downlink signal with the stored pseudo-noise (PN) like sequences to determine IDcell and Segment of a specific sector for that received downlink signal. These preamble sequences are handcrafted in advance and are processed by the MSS one at a time. There are more than 100 such sequences (e.g., 114) in some implementations of the current IEEE 802.16 OFDMA. Performing the cross-correlation with such a large number of preamble sequences can be time consuming, and thus can adversely affect the quality of service to a mobile station, especially when the mobile station is rapidly moving.  
         [0007]      FIG. 1  shows an example of the signal subframe format in the time domain for an OFDMA downlink signal in an OFDMA system. This subframe structure is defined in IEEE P802.16-REVd/D5-2004 standard and includes a number of sequential OFDM symbols  110 ,  120 ,  130 , and  140 . Each OFDM symbol has a cyclic prefix (CP) ( 112  or  142 ) and a fast Fourier transform (FFT) symbol ( 114  or  144 ) which is the inverse FFT (IFFT) of the payload sequence in frequency. The CP  112 ,  142  is a copy of the last portion  116 ,  146  of the FFT symbol  114 ,  144  that the CP  112 ,  142  is prefixed to. The CP  112 ,  114  is used to combat adverse multipath effects in a DL signal received at a mobile station.  
         [0008]     In the illustrated example, the first OFDM symbol  110  in the downlink subframe contains a payload of the preamble in frequency. Each of the subsequent OFDM symbols  120 ,  130 , and  140  contains a payload of data in frequency. The BS transmits the cell-specific preamble symbol  110  in each downlink subframe in order for the MSS receiver to synchronize with the received downlink signals in both time and frequency, and to perform cell search, cell selection, and cell reselection.  
         [0009]      FIG. 2  illustrates an exemplary detection procedure in an MSS under IEEE P802.16-REVd/D5-2004. In this preamble detection scheme, the MSS receiver first performs time synchronization in step  202  by performing a CP correlation procedure. Once the time is synchronized, the CP  112  is removed and the FFT operation is performed in step  204  on the remaining time sequence that has a length of the FFT size in order to restore the payload sequence in frequency. In step  206 , the output sequence of the FFT operation is correlated with each of all preset cell-specific preamble sequences in frequency, one sequence at a time. For example, in a system with 114 cell-specific preamble sequences, the correlation procedure is performed 114 times. Next in step  208 , the MSS receiver determines whether the largest correlation output from step  206  is greater than a predetermined detection threshold. If the largest correlation output is greater than the threshold, the cell-specific preamble sequence corresponding to the largest correction output is identified and the associated BS is selected as the current serving BS. Next, the MSS receiver further processes the rest of the downlink subframe signal such as extracting the data in the data symbols. If the largest correlation output is not greater than the detection threshold, the MSS receiver moves on to the next received OFDM symbol and repeat the steps from step  202  to step  208  to search for a correlation output greater than the correlation threshold.  
         [0010]     IEEE P802.16-REVd/D5-2004 has 114 unique preamble sequences to represent up to 114 combinations of BS cell sites and antenna segments. Table 1 below shows a portion of the 114 preamble sequences in frequency, with the associated cell identities (i.e. IDcell and Segment). Each preamble sequence is a handcrafted pseudo-noise (PN) sequence that has relatively good autocorrelation characteristics in the frequency domain and low peak to average power ratio (PAPR) in the time domain. However, it is time- and power-consuming to perform the correlation process for all 114 candidate preamble sequences. Also, because the CP is only a fraction of the FFT size, the CP based symbol timing detection method may not be sufficiently accurate and may introduce inter-symbol interference (ISI), thus degrading the performance of the cell search process that follows. Furthermore, the initial frequency estimation based on CP correlation can be coarse and thus may not be reliable. These technical limitations of the preamble design under IEEE P802.16-REVd/D5-2004 can lead to a long cell search time, which may not be acceptable in some communication applications, such as wireless mobile communication services.  
                                             TABLE 1                       Preamble                   Index   IDcell   Segment   Preamble Sequences (in Hexadecimal format)                                0   0   0   A6F294537B285E1844677D133E4D53CCB1F182DE00489E53E                   6B6E77065C7EE7D0ADBEAF       1   1   0   668321CBBE7F462E6C2A07E8BBDA2C7F7946D5F69E35AC8AC                   F7D64AB4A33C467001F3B2       2   2   0   1C75D30B2DF72CEC9117A0BD8EAF8E0502461FC07456AC906                   ADE03E9B5AB5E1D3F98C6E       .   .   .   .       .   .   .   .       .   .   .   .       31   31   0   4841AFC277B86A0E067AF319422F501C87ACBFBDD66BFEA36                   44F879AE98BA8C5D605123       32   0   1   F35EA87318E459138A2CE69169AD5FD9F30B62DA04ED21320                   A9F59893F0D176752152FD       33   1   1   A0C5F35C5971CD3DC55D7D2B9FD27AA17A198583F580EB080                   0744EE5B6B3648DEA95840       34   2   1   A6D3D33AD9B56862DBF076E3ACE6A3150510CCC8BE77DE4E6                   E10EB5FE163765647D07DF       .   .   .   .       .   .   .   .       .   .   .   .       95   31   2   E7FDDCEED8D31B2C0752D976DE92BEA241A713CF818C274AA                   1C2E3862C7EB7023AF35D4       96   0   0   0D26B8D5452948E30FD29D36E8404C8456657A6CBEEFCD91B                   B14F91E291F2C1C8F4119F       97   1   1   2CC0EEF167D75102669A814D11A51E569D07A7433A9762A29                   2D7E2A4FE35B9130FD67DE       98   2   2   FB5CBD0CB6FA80C8B3560B4C443BA4900BF2729B160098C2F                   783A7752B8BA235010A1DA       99   3   0   87BF4954022D30549DF7348477EACB97AC3565B838460CC62                   F242883313B15C31370335       100   4   1   076BF72542FB9FEBDEC3C316BC28CF0607BCEC5399EC92289                   05375D3D15F929B586D7E6       .   .   .   .       .   .   .   .       .   .   .   .       110   14   2   503F196BBF93C238BFD5E735E5AE52E0DAE64F5E2F4C3B92E                   553F51303C4A64C4403BF3       111   15   0   CA346FCCF511822F524C043D2003F3B12DCFEA276B91BA98E                   B3984BBCFDF75C2A2E1B27       112   16   1   5FD4A6894566678C95B9D5A59DDE5366799045FEB03A2BAA7                   4094140E9068C61C2E972C       113   17   2   E68AE62D2EE51B14F9D33ED737253CDB55A6E277254050830                   F2FE409E5EE284534FC3E5                  
 
       SUMMARY  
       [0011]     This application describes, among others, techniques for fast cell search, cell selection, and cell reselection wireless communication systems such as OFDM or OFDMA communication systems. In various implementations described in this application, downlink subframes from base stations to mobile stations are designed to include information in form of preambles or post-ambles in one or more downlink subframes to facilitate cell search at receiving mobile stations at high speeds. The described preambles and post-ambles may also be used to improve the accuracy of signal synchronization in time and frequency.  
         [0012]     In one described implementation, for example, a cell identification sequence is used as a leading symbol of and as at least part of a preamble of every downlink subframe in time of a downlink signal from a base station to a mobile station. The cell identification sequence is one of a set of cell identification sequences and is designated to identify a base station and a cell sector of the base station. At least one data symbol is included, at a position subsequent to the cell identification sequence in time, in every downlink subframe of the downlink signal. In every other L frames of the downlink signal, a symbol is included at a position subsequent to the cell identification sequence in time and has a plurality of copies of a sequence placed sequentially in time. The parameter L is an integer greater than 0 and the sequence is common to at least a portion of base stations with which the mobile station is capable of communicating.  
         [0013]     In another example, a group identification sequence is used as part of a downlink subframe from a base station to a mobile station to identify a group of base stations. Different group identification sequences are respectively assigned to different groups of base stations and different base stations in one group have a common group identification sequence. In the downlink subframe, a cell identification sequence is included to identify a specific base station and a cell sector of the base station in the group identified by the one group identification sequence in the downlink subframe. Various examples with specific subframe designs are described.  
         [0014]     Processing techniques at mobile stations are described for cell search based on the various downlink subframes described in this application. For example, one method for operating a mobile station for wireless communications in this application includes the following. The mobile station is controlled to detect a downlink signal received from a base station. the downlink signal comprises a cell identification sequence as a leading symbol of and as at least part of a preamble of every downlink subframe in time, at least one data symbol, at a position subsequent to the cell identification sequence in time, in every downlink subframe of the downlink signal, and, in every other L frames of the downlink signal, at a position subsequent to the cell identification sequence in time, a symbol comprising a plurality of copies of a sequence placed sequentially in time. The parameter L is an integer greater than 0 and the sequence is common to at least a portion of base stations with which the mobile station is capable of communicating. the cell identification sequence is one of a set of cell identification sequences and is designated to identify a base station and a cell sector of the base station. In addition, the mobile station is controlled to perform synchronization in time and frequency based on the sequence and to perform a correlation between the cell identification sequencer and the set of cell identification sequences to identify the cell sector and the base station.  
         [0015]     These and other implementations, variations and modifications are described in greater detail in the attached drawings, the detailed description and the claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  shows an example of the time domain OFDMA downlink subframe structure.  
         [0017]      FIG. 2  illustrates a typical preamble detection procedure.  
         [0018]      FIG. 3A  shows an example of downlink subframe structure with a common preamble added before the legacy preamble.  
         [0019]      FIG. 3B  illustrates the drawback of having different locations for the legacy cell-specific preamble when the common preamble is not transmitted in every subframe.  
         [0020]      FIG. 4A  shows an exemplary downlink subframe structure with a common post-amble.  
         [0021]      FIG. 4B  illustrates that the legacy cell-specific preamble remains as the first OFDM symbol in every subframe when the common post-amble is not transmitted in every subframe.  
         [0022]      FIG. 4C  shows an exemplary downlink subframe structure with a common preamble added after the legacy preamble.  
         [0023]      FIG. 4D  shows an example of a cell structure that divides 19 adjacent base stations into different groups with group identification numbers.  
         [0024]      FIG. 5A  illustrates an exemplary OFDM symbol structure of a common preamble or post-amble that contains two common preamble sequences in one OFDM symbol.  
         [0025]      FIG. 5B  illustrates the interlacing assignment pattern of preamble sequence in frequency in order to obtain a repetition pattern in time after the IFFT operation.  
         [0026]      FIG. 6A  shows an exemplary OFDM symbol structure of a common short preamble or post-amble symbol.  
         [0027]      FIG. 6B  shows an exemplary OFDM symbol structure of a common long preamble or post-amble symbol.  
         [0028]      FIG. 6C  shows an exemplary transmission of downlink subframes wherein the common short and long preamble symbols are inserted before the legacy cell-specific preamble in different subframes.  
         [0029]      FIG. 6D  shows an exemplary transmission of downlink subframes wherein the common short and long preamble symbols are inserted after the legacy cell-specific preamble in different subframes.  
         [0030]      FIG. 6E  shows an exemplary transmission of downlink subframes wherein common short and long post-amble symbols are inserted in different subframes.  
         [0031]      FIG. 6F  shows an exemplary downlink subframes wherein the common short and long preamble symbols are inserted before the legacy Cell-specific preamble in the same subframes.  
         [0032]      FIG. 6G  shows an exemplary downlink subframes wherein the common short and long preamble symbols are inserted after the legacy Cell-specific preamble in the same subframes.  
         [0033]      FIG. 6H  shows an exemplary downlink subframes wherein the common short and long post-amble symbols are inserted in the same subframes.  
         [0034]      FIG. 7A  shows an example of the short preamble bits and subcarriers assignment in the case of 2 antennas configuration.  
         [0035]      FIG. 7B  shows an example of the long preamble bits and subcarriers assignment in the case of 2 antennas configuration.  
         [0036]      FIG. 7C  shows an example of the long preamble bits and subcarriers assignment in the case of 3 antennas configuration.  
     
    
     DETAILED DESCRIPTION  
       [0037]     The example illustrated in  FIGS. 1 and 2  shows that various OFDM and OFDMA systems under in IEEE 802.16 perform the correlation between a received cell identification sequence in a downlink signal and all possible cell identification sequences of the systems in processing each downlink subframe. For a 3-tier cellular system with 114 available preamble PN sequences for nineteen adjacent base stations, the mobile station computes  114  correlations when processing each downlink subframe during the cell search.  
         [0038]     Existing preamble PN sequences for OFDM and OFDMA systems are referred to as “legacy preamble sequences” in part to indicate distinctions with new preambles described in this application. The amount of processing for the cell search at the mobile station can be significantly reduced by using one or more preambles that are common to some or all of the base stations in downlink subframes. Similarly, common post-ambles may also be used to facilitate fast cell search. In some implementations, correlations may be performed with only a selected portion of the available legacy preamble PN sequences to reduce the processing time at the mobile station for the cell search. The designs of the preambles and post-ambles for the downlink subframes described in this application retain the legacy preamble sequences. In some implementations, the downlink subframes based on the present designs are backward compatible to IEEE P802.16-REVd/D5-2004.  
         [0039]     Some examples described here add a common preamble after the legacy cell-specific preamble or add a common post-amble every L subframes, where L is an integer number greater than 0, for fast time and frequency synchronization and fast cell search. This common preamble or post-amble may be implemented as repetitions of one sequence common for all base stations, or repetitions of one of N possible sequences each of which is common for a group of base stations.  
         [0040]     In other examples, a common preamble may be added before or after the legacy cell-specific preamble, or add a common post-amble every L subframes, where L is an integer number greater than 0, for fast time and frequency synchronization and fast cell search. This common preamble or post-amble symbol may include the repetitions of the first time sequence that are in the first one half of the FFT symbol, the repetitions of the second time sequence that are in the second half of the FFT symbol, and the CP that is the copy of the last portion of the first time sequence; further there is one first sequence that is common for all base stations and there are N second sequences each of which is common for a mutually exclusive group of base stations.  
         [0041]     In yet other examples, first and second common preamble symbols may be added before or after the legacy cell-specific preamble symbol, or first and second common post-amble symbols may be added every L subframes (L is an integer number greater than 0) for fast time and frequency synchronization and fast cell search. The first common preamble or post-amble may include one of N possible first sequences each of which is common for a mutually exclusive group of base stations; the second common preamble or post-amble may include one of M possible second sequences each of which is common for a mutually exclusive subgroup of those base stations that are from the same group of base stations. The two sequences may be included in two different OFDM symbols, i.e., the first sequence is in the first OFDM symbol and the second sequence is in the second OFDM symbol. The first and second OFDM symbols can be implemented, for example, in the same subframe, wherein the second OFDM symbol follows the first OFDM symbol closely in transmission. The first and second OFDM symbols can also be implemented in two different subframes next to each other in time.  
         [0042]     Hence, in some applications, the mobile station can be controlled to process and use the first sequence for coarse timing and frequency synchronization, and the second sequence for fine timing and frequency synchronization. In other applications, the first sequence, the second sequence, or both the first and second sequences may be used by the mobile stations to reduce the number of possible cell-specific preamble sequence in the legacy preamble for fast cell determination. Also, the first sequence and the second sequence may be further divided into sub-sequences, and the resulting sub-sequences are assigned to different frequency subcarrier sets for different antennas in order to support MIMO-capable base stations.  
         [0043]     Specific examples are now described in detail.  
         [0044]      FIG. 3A  shows one example of a preamble structure that uses a CP as a common preamble symbol  310  which is added before the legacy cell-specific preamble symbol  320 . The new common preamble symbol  310  has the inverse fast Fourier Transform (IFFT) of a PN sequence  314 , which is common to all base station, and a CP  312 , which is a copy of the last time portion  316  of the IFFT of the common preamble sequence  314 . The common preamble sequence in frequency is known to all MSSs, e.g. it is the first PN sequence in Table 1 by the standard default. Therefore, the MSS receiver only needs to test the correlation of the unique time sequence of the common preamble in order to synchronize the timing. The common preamble sequence  314  in frequency is also carefully assigned to frequency subcarriers such that the IFFT of the common preamble sequence  314  has a repeated pattern in time. This may be accomplished by, e.g., assigning the common preamble sequence  314  onto the frequency subcarriers in an interlaced pattern while setting the subcarriers in between to zeros.  
         [0045]     It can be shown that the following IFFT expression can be derived for this example: 
 
 IFFT ([ a   1 ,0, a   2 ,0 , . . . ,a   N/2 ,0]) N   =[IFFT ( a   1   ,a   2   , . . . ,a   N/2 ) N/2   IFFT ( a   1   ,a   2   , . . . ,a   N/2 ) N/2 ]  (1) 
 
 where IFFT([x]) N  represents the IFFT of a sequence x with a size of N, N is an even number, and (a 1 , a 2 , . . . , a N/2 ) is the common preamble sequence in frequency. Therefore, the MSS receiver can utilize the repetition characteristics to synchronize the frequency. After time and frequency synchronization, the MSS receiver can further determine the BS identity by correlating the received legacy cell-specific preamble with 114 candidate preamble sequences as illustrated in Table 1 above. 
 
         [0046]     The added common preamble symbol in  FIG. 3A  can speed the initial cell search and acquisition process for newly arrived MSSs on one hand and can also be, on the other hand, an overhead that is unnecessary for those MSSs who already lock up with the system. Therefore, from system efficiency viewpoint, one may wish to limit the transmission of the common preamble symbol to once every L frames where L is greater than 1.  FIG. 3B  shows an example of the subframe structure. The legacy cell-specific preamble symbol  332  is the second OFDM symbol in the kth subframe as the common preamble symbol  330  is added before the legacy cell-specific preamble symbol  332 . From the (k+1)th subframe to (k+L−1)th subframe, the legacy cell-specific preamble symbols  340  and  350  remain as the first OFDM symbol in each subframe. Therefore, this common preamble design adds unnecessary implementation complexity to the hardware and/or software of the MSS receiver, since the MSS receiver needs to acquire the cell-specific preamble symbols at different time locations for different subframes. The MSS receiver may further use the cell-specific preamble for channel estimation. In some implementations, the noisy channel estimation result can be further smoothed out by a low-pass filter. It is much easier to design a filter when the time between the samples remains constant. However, in the case shown in  FIG. 3B , the time between channel estimation samples may vary between subframes.  
         [0047]      FIG. 4A  shows an improved design where a common post-amble symbol  412  is placed at the end of one downlink subframe. The common post-amble symbol  412  includes a CP and the IFFT of a common PN sequence. Similar to the common preamble symbol  310  shown in  FIG. 3A , the common post-amble sequence is assigned to the frequency subcarriers in an interlaced pattern while setting the subcarriers in between to zeros. As a result, repetition of time waveform is obtained. With this new common post-amble structure, it is relatively easy to detect the symbol time by autocorrelation and to take advantage of the repetition property within the common post-amble symbol. The repeated post-amble patterns can also be used to estimate the frequency offset, which should have better performance than using CP based methods. This mitigates the unreliability issue caused by the coarse initial frequency estimation based on the CP correlation method. Notably, this design of the subframe places the legacy cell-specific preamble sequence as the first OFDM symbol in a subframe.  
         [0048]      FIG. 4B  further shows that, the post-amble  422  does not have to be transmitted every subframe and may be transmitted every L subframes, where L is greater than 1. This common post-amble design is different from the common preamble design shown in  FIG. 3A  in that in the case shown in  FIG. 4B , the legacy cell-specific preamble symbol  420 ,  430 ,  440  is always the first OFDM symbol in a subframe. As an alternative implementation,  FIG. 4C  shows a different common preamble design wherein the common preamble symbol  452  is added after the legacy cell-specific preamble symbol  450  such that the legacy cell-specific preamble symbol  450  is always the first OFDM symbol in a subframe.  
         [0049]     One common feature in the above subframe designs in  FIGS. 4A, 4B  and  4 C is that the legacy cell-specific preamble symbol is always the first OFDM symbol in a subframe. Under these designs, the MSS receiver can be simplified in its hardware and/or software for detecting and processing the legacy cell-specific preamble symbol because the MSS receiver can be controlled to look for the first OFDM symbol for the legacy cell-specific preamble symbol. In comparison, the system shown in  FIG. 3B  is more complex where the legacy cell-specific preamble symbol may be either at the first or the second OFDM symbol position in a downlink subframe and thus the position in time of the cell-specific preamble symbol varies from one subframe to another depending on the specific interlaced pattern for assigning the common preamble symbol to the subcarriers in the frequency domain. Accordingly, the MSS receiver for the system in  FIG. 3B  need be configured to monitor and detect the legacy cell-specific preamble sequence at the two different positions depending and thus require more complex hardware or software.  
         [0050]     In the subframe designs in  FIGS. 4A, 4B  and  4 C, after the common preamble  452  or common post-amble  412  is detected, the MSS tests the received legacy cell-specific preamble symbol  450  or  410  against all the candidate preamble sequences in order to obtain the cell identity. In the legacy cell-specific preamble sequences, different cell segments of different base stations are assigned with hand-crafted and unique sequences, respectively. Hence, a received sequence in a downlink subframe is compared to one of the all sequences at a time. Various implementations described in this application provide group identification (ID) sequences to represent different and mutually exclusive groups of all available sequences in the downlink frame. Hence, instead of blindly performing autocorrelation with all available sequences at the mobile station, the mobile station can be controlled to first identify a group to which a received downlink frame belongs and then to perform autocorrelations with only the sequences in the identified group. Therefore, the autocorrelations with sequences in other groups can be eliminated. In addition, some implementations may further use subgroup classification to divide each group into two or more mutually exclusive subgroups to further reduce the number of autocorrelations with the legacy cell-specific preamble sequences to be performed by the mobile station.  
         [0051]     The following examples illustrate the use of group ID sequences to reduce the amount of the processing of the legacy cell-specific sequences and use a global SYNC symbol common to all base stations and networks for synchronization. In some implementations, the global SYNC symbol can use one common PN sequence for all BSs and networks and the group SYNC symbols may be generated from, e.g., 8 common PN sequences for all BSs and networks. Both group ID sequences and the global SYNC symbol may be designed to have time repetition structures. The global SYNC symbol simplifies the frequency and coarse frame timing synchronization for MSS in multi-sector multi-cell deployment. This feature can be important during the initial cell search after MSS is powered on. The group SYNC symbol can be used for fine timing synchronization and the group identification of the legacy cell-specific preamble, which can shorten the search time and reduce MSS processing power for the legacy preamble search. The global SYNC symbol facilitates the support of simple and fast cell search for neighboring cells. Based on the signal measurements of the global SYNC symbol, the group SYNC symbol can be used to identify the group IDs of the neighboring cells as handover (HO) candidates. The group SYNC symbol can be used to greatly reduce the number of legacy cell-specific preamble PN sequences used for neighboring cell search.  
         [0052]      FIG. 4D  shows an example of a cell structure where 19 adjacent base stations are divided into 8 groups. In this cell structure, a mobile station is capable of communicating with these 19 base stations and may selectively communicate with only one of these 19 base stations as a current serving base station and monitor other base stations for handoff and other operations. Based on the grouping of different base stations shown in  FIG. 4D , the common preamble symbol  310  as shown in  FIG. 3A , or the common post-amble symbol  412  as shown in  FIG. 4A , or the common preamble symbol  452  as shown in  FIG. 4C  may be designed to include long and short preamble or post-amble sections. Each of the long and short preamble or post-amble sections can be designed to have a repetition pattern in time with two or more identical segments to allow for synchronization in time and frequency.  
         [0053]      FIGS. 5A and 5B  illustrate an example for the long and short preambles added to each downlink subframe. Long and short post-ambles can also be similarly constructed.  FIG. 5A  shows that the first section has the IFFT of a common short preamble  514 . This short preamble includes a repetition pattern of 4 in time, and a CP  512  which is the copy of the last portion  516  of the IFFT of short preamble  514  of Section 1. The second section in this example has the IFFT of a common long preamble  520 , which has a repetition pattern of 2 in time. The time duration of 2 repetitions of common long preamble  520  is the same as that of 4 repetitions of short preamble  514 . The OFDM symbol time duration of the common preamble or post-amble shown in  FIG. 5A  is the same as a normal OFDM symbol time duration. In section  1 , only one common short preamble PN sequence is needed for all base stations.  
         [0054]     Equation (1) suggests that a preamble symbol with repetitions in time can be formed in two ways. One way is to first assign the preamble symbol to the frequency subcarriers in an interlaced pattern while setting the subcarriers in between to zeros, as shown in  FIG. 5B , then perform the IFFT with the enlarged FFT size. The other way is to first perform the IFFT of the preamble sequence using a decimated FFT size and without interlacing with null subcarriers, then repeat the output of the IFFT in time. These two methods are mathematically equivalent.  FIG. 5A  is an illustration to visualize the second way.  
         [0055]     With this new common preamble or post-amble symbol structure, the repetition property within the short preamble section can be used to detect the symbol time by autocorrelation. The repeated short preamble patterns can also be used to estimate the frequency offset. After the common preamble or post-amble symbol time is detected and the frequency offset is corrected, the MSS receiver can begin processing the common long preamble or post-amble section.  
         [0056]     The common long preamble or post-amble under this may be used to one or more functions. For example, the long preamble or post-amble may be used to achieve more accurate symbol timing. The special structure of common long preamble section with a longer length in time can be used to tolerate larger timing errors. Therefore, the common long preamble section can be used to calculate channel profile through long preamble or post-amble correlation process to achieve better timing and frequency synchronization. For another example, the long preamble or post-amble may be used to carry additional information with its extra length in time that may be difficult to be included in the short preamble or post-amble. In the specific example illustrated here, the common long preamble or post-amble section is used to carry a group ID sequence which is used by the mobile station to reduce the numbers of candidate cell-specific preamble sequences for detection over the legacy cell-specific preamble symbol  320 ,  410 ,  450 .  
         [0057]     One example of using the group ID sequences in the long preambles or post-ambles are illustrated in  FIGS. 5A and 5B  where an additional OFDM symbol having both long and short preambles is included in a subframe along with a legacy cell-specific preamble sequence symbol as a preamble as in  FIGS. 3A and 4C . Similarly, an additional OFDM symbol having both long and short post-ambles can be included in a subframe as a post-amble as in  FIG. 4A .  
         [0058]     Referring to the preamble example in  FIGS. 5A and 5B , the cell-specific preamble sequences as shown in Table 1 above are first divided into several groups, for example, 8 groups.  FIG. 4D  shows such an example for the group assignment. Each group can be identified with a unique long preamble sequence, which can be treated as a group ID for the legacy cell-specific preambles. In the current example, there are totally 8 long preamble sequences for 8 different groups, respectively. Each BS sends the repetitions of IFFT of its long preamble  520  as a legacy preamble group ID in the second section of the common preamble as shown as  FIG. 5B . Hence, to the base stations in a particular group, the long preamble is common. Long preambles are different for different groups due to the differences in the group ID sequences. The MSS can detect this group ID with much less complexity because of the smaller number of possible long preamble sequences (only 8) and shorter preamble length (only ¼ of the legacy preamble length). Once the MSS knows the preamble group ID, the MSS can start to search the possible cell-specific preamble sequences within that identified group over the received legacy cell-specific preamble symbol. In this example, the number of possible cell-specific preamble sequences in each group is only ⅛ of the total number of cell-specific preamble sequences, which is 15 or 16, instead of 114 in absence of the group classification.  
         [0059]     In operation, the mobile station first processes the CP  512  and short preambles  514  to establish the initial synchronization in time and frequency. Next, the long preambles  520  are processed by the mobile station for (1) fine synchronization in time and frequency and (2) identification of a group of the received subframe. The identification of the group can be carried out by correlation between the received long preambles  512  with the know group identification sequences. After a group is selected, the mobile station proceeds to correlate the received legacy cell-specific preamble sequence in the received subframe with legacy cell-specific preamble sequences within the identified group. Hence, autocorrelations between the received legacy cell-specific preamble sequence and legacy cell-specific preamble sequences outside the identified group are not performed by the mobile station. This reduces the processing time for each subframe at the mobile station.  
         [0060]     In other implementations, the long preambles and short preambles may be placed in different OFDM symbols. Each short preamble is still shorter than each long preamble but can be longer in time than a short preamble in the design in  FIG. 5A  where both long and short preambles are placed within a common OFDM symbol. As such, the short preambles in a separate OFDM symbol can be sufficiently long to include additional information other than just the common sequence, e.g., the group ID sequences while the long preambles include subgroup ID sequences in the examples below.  
         [0061]      FIGS. 6A and 6B  respectively show an example of a short preamble OFDM symbol with multiple identical preambles  602  and one CP before the leading short preamble in time, and an example of a separate long preamble OFDM symbol with multiple long preambles  604  and one CP before the leading long preamble in time. The short and long preamble symbols are designed for use in a single subframe or two different adjacent subframes. This use of the common short preamble sequences and common long preamble sequences in separate OFDM symbols can further enhance the cell search speed and other aspects of the cell search. In  FIG. 6A , the common short preambles are 8 repetitions of the IFFT of a short preamble  602  and one CP. In  FIG. 6B , the common long preamble or long post-amble symbol includes 4 repetitions of the IFFT of a long preamble  604  and one CP. Such separate OFDM symbols for short and long preambles may be used to construct the downlink subframes in various configurations. Examples of downlink subframes based on separate OFDM symbols for short and long preambles are now described with reference to  FIGS. 6C through 6H , respectively.  
         [0062]      FIG. 6C  shows a downlink subframe where the common short preamble symbol  610  and the common long preamble symbol  614  are inserted before the legacy preamble symbols  612  and  616  in alternate downlink subframes, respectively. First, the legacy cell-specific preamble sequences, as shown in Table 1 above, are divided into 4 different groups. Each group can be identified with a unique short preamble PN sequence. This short preamble PN sequence can be used to represent a group ID for the legacy cell-specific preambles within the group. Each preamble group is divided further into 8 different subgroups, and each subgroup can be identified with a unique long preamble PN sequence. This long preamble PN sequence can be used to represent a subgroup ID for the legacy cell-specific preambles. Under this exemplary group and subgroup scheme, there are a total of 8 long preamble PN sequences. All the MSSs know the short and long preamble PN sequences by a predetermined standardized default, e.g. the 8 long preamble sequences in frequency can be the first eight legacy PN sequences from Table 1, truncated from the end if needed in order for its IFFT to fit into the repetition pattern in time. The 4 short preamble PN sequences can be standardized in a similar way.  
         [0063]     As a specific example, the group ID PreambleGroupID i  and subgroup ID PreambleSubGroupID i  for a cell-specific preamble with an index of i in Table 1, can be related to the its corresponding IDcell i  as follows:  
               PreambleGroupID   i     =     [       IDcell   i     8     ]             (   2   )                 PreambleSubGroupID   i     =       mod     ⁢       (     IDcell   i     )     8               (   3   )             
        where [ ] represents the integer operator and mod( ) represents the modulus operator. There are 31 or 32 legacy cell-specific preamble sequences associated with each group ID. The cell-specific preamble sequences sharing the same IDcell number are in the same subgroup of the same group. Each subgroup ID can be associated with 15 or 16 legacy cell-specific preamble PN sequences if the group ID is unknown. If the group ID is also known, each subgroup ID can be associated with 3 or 4 legacy cell-specific preamble PN sequences.        
 
         [0065]     After the detection of common short and long preambles, the IDcell number can be obtained through the combination of the ID numbers that the common short and long preambles represent. For example, the value of the 2 most significant bits (MSBs) of IDcell, which can be a number from 0 to 3, is the group ID known from the detection of the common short preamble PN sequence. And the value of the rest 3 least significant bits (LSBs), which can be a number from 0 to 7, is the subgroup ID known from the detection of long preamble PN sequence.  
         [0066]     In the example shown in  FIG. 6C , the BS sends its short preamble as a group ID for the legacy cell-specific preambles in every even subframe. The MSS can detect this group ID with much less complexity because of the smaller number of possible short preamble sequences (only 4) and shorter preamble length (only ⅛ of the legacy preamble length). Once MSS obtains the preamble group ID, it can start to search the possible cell-specific preamble sequences within that particular group over the legacy cell-specific preamble symbol. The number of possible cell-specific preamble sequences in each group is only ¼ of the total number of cell-specific preamble sequences. Further, the BS sends its long preamble as a subgroup ID for the legacy cell-specific preambles in every odd subframe. The MSS can detect this subgroup ID also with much less complexity because of the smaller number of possible long preamble sequences (only 8) and shorter preamble length (only ¼ of the legacy preamble length).  
         [0067]     There may be two scenarios in identifying the candidates of the cell-specific preamble sequence under this specific circumstance: 
        1. If the MSS has already obtained the group ID from the previous subframe, once MSS knows the legacy preamble subgroup ID through the long preamble correlation, the MSS immediately knows the IDcell number. The MSS can start to search the possible cell-specific preamble sequences with the particular IDcell over the legacy cell-specific preamble symbol. The number of possible cell-specific preamble sequences in each subgroup is only 1/32 of the total number of cell-specific preamble sequences, which is only 3 or 4.     2. If the MSS can successfully obtain the subgroup ID through the long preamble correlation without the knowledge of the group ID, the MSS knows that the candidates of cell-specific preamble sequence can be in the particular subgroup of anyone of 4 groups. Therefore MSS can start to search the possible cell-specific preamble sequences within the particular subgroup of all 4 groups over the legacy preamble symbol. The total number of candidates of cell-specific preamble sequence is ⅛ of the total number of cell-specific preamble sequences, which is 15 or 16.        
 
         [0070]     In addition, with the common short and long preamble sequences, it can be relatively easy to detect the symbol time at the mobile station by using the autocorrelation property of the sequences. The repetitions of short preamble pattern can also be used to estimate the frequency offset, which should have better performance than using CP based methods. The MSS receiver further performs the long preamble correlation process to achieve better timing and frequency synchronization and to reduce the number of candidate cell-specific preamble sequences to search for.  
         [0071]      FIG. 6D  shows an example of an alternative use of long and short preamble symbols separately in two adjacent subframes. In this example, the common short preamble symbol  622  and the common long preamble symbol  626  are the same as shown in  FIGS. 6A and 6B , respectively, except that the common short preamble symbol  622  and the common long preamble symbol  626  are placed after the legacy cell-specific preamble  620  and  624 , respectively. In this symbol arrangement, the legacy cell-specific preamble  620  and  624  will always be the first OFDM symbol in each subframe if the common short preamble symbol  622  or the common long preamble symbol  626  is not needed in every subframe. As a result, the hardware and/or software of the MSS receiver can be simplified, due to similar reasons illustrated in  FIG. 4B  vs.  FIG. 3B .  
         [0072]     For the same reason of reducing implementation complexity,  FIG. 6E  shows another alternative realization, wherein the common short post-amble symbol  632  and the common long post-amble symbol  636  are the same as the common short and long preambles as shown in  FIGS. 6A and 6B , respectively, except that the common short post-amble symbol  632  and the common long post-amble symbol  636  are placed at the end of two different downlink subframe, respectively, so that the legacy cell-specific preamble ( 630  and  634 ) remains as the first OFDM symbols for each subframe.  
         [0073]      FIG. 6F  shows an example of a use of the long and short preamble symbols within one subframe. The common short preamble symbol  640  and the common long preamble symbol  642  are the same as shown in  FIGS. 6A and 6B , respectively and the common short preamble symbol  640  and the common long preamble symbol  642  are inserted before the legacy cell-specific preamble symbol  644  in the same downlink subframe.  
         [0074]      FIG. 6G  shows another example for using the long and short preamble symbols within one subframe. The common short preamble symbol  652  and the common long preamble symbol  654  are the same as shown in  FIGS. 6A and 6B , respectively. The common short preamble symbol  652  and the common long preamble symbol  654  are placed after the legacy cell-specific preamble symbol  650 , so that the legacy cell-specific preamble symbol  650  remains as the first OFDM symbol for each downlink subframe, not matter how frequent the common short preamble symbol  652  and the common long preamble symbol  654  are inserted, therefore simplifying the MSS receiver implementation.  
         [0075]      FIG. 6H  shows yet another example for using the long and short preamble symbols within one subframe realization. The common short post-amble symbol  662  and the common long post-amble symbol  664  are the same as the common short and long preambles shown in  FIGS. 6A and 6B . The common short post-amble symbol  662  and the common long post-amble symbol  664  are placed at the end of the downlink subframe, so that the legacy cell-specific preamble symbol  660  remains as the first OFDM symbol for each downlink subframe, therefore simplifying the MSS receiver implementation.  
         [0076]     The above long and short preambles may also be used to support the BS that is capable with MIMO (Multiple Input Multiple Output) antenna technology. MIMO antenna technology uses two or more transmitter antennas in a transmitter, e.g., the base station, to implement transmission diversity (often referred to as “transmit diversity”) based on the space-time coding (STC) to mitigate signal fading due to various factors including the multipath fading.  
         [0077]     In some implementations, the common short and long preamble PN sequences can be divided into sub-sequences, as shown in the  FIGS. 7A, 7B , and  7 C. Each short preamble PN sequence has 2 sub-sequences, and each long preamble PN sequence has 4 sub-sequences with the subcarrier assignment as follows: 
 
ShortPreambleCarrierSet i   =i+ 2 k   (4) 
 
LongPreambleCarrierSet j   =j+ 4 k   (5) 
 
         [0078]     where i=0, 1 is the number of the short preamble carrier-set, j=0, 1, 2, 3 is the number of the long preamble carrier-set, and k is the running PN sequence bit index 0, 1, 2, 3, 4, . . . . Table 2 lists the examples of carrier-set assignment for the common short and long preambles for BS with MIMO capability supporting up to 4 antennas.  
                       TABLE 2                       Number of   Short Preamble   Long Preamble       Antennas   Carrier-sets   Carrier-sets                   1   Antenna 1: 0 and 1   Antenna 1: 0, 1, 2, and 3       2   Antenna 1: 0   Antenna 1: 1 and 3           Antenna 2: 1   Antenna 2: 0 and 2       3   Antenna 1: 0   Antenna 1: 1           Antenna 2: 1   Antenna 2: 3           Antenna 3: null   Antenna 3: 0 and 2       4   Antenna 1: 1   Antenna 1: 0           Antenna 2: 0   Antenna 2: 1           Antenna 3: 0   Antenna 3: 2           Antenna 4: 1   Antenna 4: 3                  
 
         [0079]      FIG. 7A  visualizes an example of the short preamble bits and subcarriers assignment in the case of 2 antennas configuration as listed in Table 2. One sub-sequence of the short preamble, which consists of the even bits of the short preamble, is assigned to one half set of subcarriers (comparing to the full set of subcarriers shown in  FIG. 5B ), then the subcarriers in between are set to zeros, then an IFFT is performed, then a CP is added, then the resulting waveform is transmitted on Antenna  1 . Meanwhile the other sub-sequence, which consists of the odd bits of the short preamble, is assigned to the other half set of subcarriers, then the subcarriers in between are set to zeros, then an IFFT is performed, then a CP is added, then the resulting waveform is transmitted on Antenna  2 . The MSS receives the composite short preamble signals from Antennas  1  and  2 . After the FFT operation, the MSS receiver can use the entire short preamble sequence for correlation process in order to find out the group ID.  
         [0080]      FIG. 7B  visualizes an example of the long preamble bits and subcarriers assignment in the case of 2 antennas configuration as listed in Table 2. Two out of four sub-sequences of the long preamble, which consist of the odd bits of the long preamble, are assigned to one half set of subcarriers, then the subcarriers in between are set to zeros, then an IFFT is performed, then a CP is added, then the resulting waveform is transmitted on Antenna  1 . Meanwhile the other two sub-sequences, which consist of the even bits of the long preamble, are assigned to the other half set of subcarriers, then the subcarriers in between are set to zeros, then an IFFT is performed, then a CP is added, then the resulting waveform is transmitted on Antenna  2 . After the FFT operation, the MSS receiver can use the entire long preamble sequence for correlation process in order to find out the subgroup ID. Then the MSS can detect the legacy cell-specific preamble for a limited number of possible candidates given the detected group ID and subgroup ID.  
         [0081]      FIG. 7C  visualizes an example of the long preamble bits and subcarriers assignment in the case of 3 antennas configuration as listed in Table 2. Two out of four sub-sequences of the long preamble, which consist of the even bits of the long preamble, are assigned to one half set of subcarriers, then the subcarriers in between are set to zeros, then an IFFT is performed, then a CP is added, then the resulting waveform is transmitted on Antenna  3 . Meanwhile one of the two remaining sub-sequences is assigned to one of the two remaining ¼ set of subcarriers, then the subcarriers in between are set to zeros, then an IFFT is performed, then a CP is added, then the resulting waveform is transmitted on Antenna  1 . Similar process is used on Antenna  2 .  
         [0082]     As shown in Table 2, we try to use the different subcarriers for the sub-sequences of the short and long preambles on the same antenna as much as possible in order to explore frequency diversity.  
         [0083]     In implementations, the above described techniques and their variations may be implemented as computer software instructions or firmware instructions. Such instructions may be stored in an article with one or more machine-readable storage media or stored in one or more machine-readable storage devices connected to one or more computers. In operation, the instructions are executed by, e.g., one or more computer processors, to cause the machine to perform the described functions and operations. For example, the techniques for generating the downlink subframes may be implemented as computer instructions stored in the base stations or a control module that controls base stations. The techniques for processing the downlink subframes may be implemented as computer instructions stored in the mobile stations.  
         [0084]     Only a few examples are described. However, other implementations and enhancements may be made based on what is described and illustrated here.