Abstract:
According to some embodiments, a method of synchronizing a wireless device with a network node comprises receiving a radio subframe transmitted from the network node. The MBSFN radio subframe comprises a first Primary Synchronization Signal (PSS) associated with a first Orthogonal Frequency Division Multiplexing (OFDM) symbol and paired with a first Secondary Synchronization Signal (SSS) associated with a second OFDM symbol. The method further comprises detecting the first PSS and detecting the first SSS within the radio subframe. The method determines system information associated with the network node based on the detected PSS and SSS. The radio subframe is transmitted from the network node as a Multimedia Broadcast Multicast Services Single-Frequency Network (MBSFN) subframe.

Description:
TECHNICAL FIELD 
       [0001]    Particular embodiments relate generally to synchronization signals in wireless communications, and more particularly to frame formats for cell search procedure synchronization signals. 
       BACKGROUND 
       [0002]    When a wireless device powers on or moves between cells in a wireless network, the wireless device receives and synchronizes to downlink signals in a cell search procedure. The cell search procedure identifies a preferable cell and performs time and frequency synchronization to the network in downlink (e.g., from a base station to a user equipment). 
         [0003]    A user equipment (UE) may use primary and secondary synchronization signals (PSS and SSS), such as those described in Section 6.11 of Third Generation Partnership Project (3GPP) TS 36.211, version 11.2.0, for performing a cell search procedure, such as the cell search procedure described in Section 4.1 of 3GPP TS 36.213, version 12.1.0. 
         [0004]      FIG. 1  illustrates an example radio frame and locations of OFDM symbols used for PSS and SSS, according to a 3GPP specification. The X-axis represents orthogonal frequency division multiplexing (OFDM) symbols  112  and the Y-axis represents subcarriers  114 . Each resource element  113  corresponds to one OFDM subcarrier  114  during one OFDM symbol interval  112 . Particular OFDM symbols  112  include PSS  116  and SSS  118 . 
         [0005]    3GPP specifies that for frequency division duplex (FDD) PSS  116  is transmitted in the last OFDM symbol of slots  0  and  10  within a radio frame and that SSS  118  is transmitted in the OFDM symbol preceding PSS, such as illustrated in  FIG. 1 . 
         [0006]    3GPP specifies that for time division duplex (TDD) PSS is transmitted in the third OFDM symbol of slots  3  and  13  within a frame and that SSS is transmitted in the last OFDM symbol of slots  2  and  12  (i.e., three symbols ahead of the PSS). 
         [0007]      FIG. 2  illustrates an example of an initial cell search procedure. A UE typically may have a frequency error of 2 to 20 ppm (part per million) at power on. This corresponds to 40 to 400 kHz frequency error at a carrier frequency of 2 GHz. The UE then tries to detect a PSS. From the detected PSS, the UE may derive the cell id within a cell-identity group, which consists of three different cell identities corresponding to three different PSS. To perform the detection, the UE searches for all of the three possible cell identities. The UE may also achieve OFDM symbol synchronization and determine a coarse frequency offset estimation with an accuracy of about 1 kHz. The UE estimates the latter by evaluating several hypotheses of the frequency error. 
         [0008]    The UE then detects the SSS. From the detected SSS, the UE acquires the physical cell id and achieves radio frame synchronization. The UE also detects whether the cyclic prefix length is normal or extended. A UE that is not preconfigured for a particular duplex mode (e.g., TDD or FDD) may detect the duplex mode by the frame position of the detected SSS in relation to the detected PSS. The UE may estimate fine frequency offset by correlating PSS and SSS. Alternatively, the UE may use cell-specific reference signals (CRS) to estimate fine frequency offset. 
         [0009]    After synchronizing with the PSS and the SSS, the UE may receive and decode cell system information, which contains cell configuration parameters such as the Physical Broadcast Channel (PBCH). The number of OFDM symbols used for PDCCH (Physical Downlink Control Channel) is signaled by PCFICH (Physical Control Format Indicator Channel) according to Section 6.7 of 3GPP TS 36.211, version 11.2.0. The PCFICH is decoded before the UE receives PDCCH. The number of OFDM symbols signaled by PCFICH may be 1, 2 or 3 for large bandwidth allocations (e.g., more than 10 resource blocks) and 2, 3 or 4 OFDM symbols for small bandwidths (e.g., less than or equal to 10 resource blocks). The first OFDM symbols of a sub-frame are used for PDCCH. 
         [0010]    Multimedia Broadcast Multicast Services Single-Frequency Network (MBSFN) is an LTE format for transmitting multicast/broadcast information (i.e., transmissions that are of interest to many users in the system). The LTE standard, however, does not require that MBSFN subframes contain broadcast information. The MBSFN subframe only contains broadcast information if the Physical Multicast Channel (PMCH) is mapped accordingly. 
       SUMMARY 
       [0011]    According to some embodiments, a method of synchronizing a wireless device with a network node comprises receiving a radio subframe transmitted from the network node. The radio subframe comprises a first Primary Synchronization Signal (PSS) associated with a first Orthogonal Frequency Division Multiplexing (OFDM) symbol and paired with a first Secondary Synchronization Signal (SSS) associated with a second OFDM symbol. The method further comprises detecting the first PSS within the radio subframe and detecting the first SSS within the radio subframe. The method determines system information associated with the network node based on the detected PSS and the detected SSS. The radio subframe is transmitted from the network node as a Multimedia Broadcast Multicast Services Single-Frequency Network (MBSFN) subframe. 
         [0012]    In particular embodiments, the radio subframe further comprises a second PSS associated with a third OFDM symbol and paired with a second SSS associated with a fourth OFDM symbol. 
         [0013]    In particular embodiments, the method comprises detecting both the first PSS and the second PSS within the radio subframe and accumulating the first PSS and the second PSS. The method further comprises determining system information associated with the network node based on the accumulated first PSS and second PSS. 
         [0014]    Particular embodiments may exhibit some of the following technical advantages. Particular embodiments may include a PSS and SSS cell search frame format that is backward compatible such that legacy UEs will not detect these cell search signals or need to be aware of their existence. Particular embodiments use synchronization sequences other than those specified in LTE release 12. In particular embodiments, cell search signals are placed in MBSFN subframes so that UEs are not scheduled at the time-frequency resources used for PSS/SSS transmission. In particular embodiments, PSS and SSS frame formats may use a large fraction of the reserved resource blocks which results in low overhead. Particular embodiments may allocate PSS/SSS pairs in subsequent OFDM symbols such that a high resolution frequency offset estimate can be calculated with low computational complexity. 
         [0015]    In particular embodiments, a PSS/SSS pair is not transmitted such that PSS is transmitted in one slot and SSS in the next, or vice versa. The first symbol of each slot has a longer cyclic prefix than the other OFDM symbols of the slot. In embodiments that transmit a PSS/SSS pair in the same slot, the timing between PSS and SSS within each pair is constant, such that the phase rotation between PSS and SSS may be used for a fine granularity frequency offset estimator. In particular embodiments, a UE may use coherent accumulation to improve the received SINR. In particular embodiments, a base station may use beamforming or repetition of PSS and SSS to increase successful cell detection rate and reduce cell detection latency. Other technical advantages will be readily apparent to one skilled in the art from the following figures, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    For a more complete understanding of the present invention and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
           [0017]      FIG. 1  illustrates an example radio frame and locations of OFDM symbols used for PSS and SSS, according to a 3GPP specification; 
           [0018]      FIG. 2  is a flowchart of an example method for performing cell search synchronization, according to a particular embodiment; 
           [0019]      FIG. 3  is a block diagram illustrating an example of a network, according to a particular embodiment; 
           [0020]      FIGS. 4-13  illustrate example locations of primary and secondary search signals within a MBSFN subframe, according to particular embodiments; 
           [0021]      FIG. 14  is a flowchart of an example method of transmitting PSS and SSS, according to particular embodiments; 
           [0022]      FIG. 15  is a flowchart of an example method of detecting PSS and SSS, according to particular embodiments; 
           [0023]      FIG. 16  is a block diagram illustrating an example embodiment of a wireless device; and 
           [0024]      FIG. 17  is a block diagram illustrating an example embodiment of a radio network node. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    In particular networks, a UE might receive cell search signals at a low signal to interference plus noise ratio (SINR), which results in degraded or impossible cell attachment. 3GPP specifies that the same synchronization signals are transmitted each 5 ms (e.g., as illustrated in  FIG. 1 ). A UE might attempt to accumulate several occasions of these signals; however, fading radio channel and frequency errors negatively impact this possibility. A fading radio channel exhibits time variations both in amplitude and phase. The speed of these variations depends on both the speed of the UE and how the radio propagation environment is changing. In both cases, these variations may result in received signals that cannot be accumulated coherently in order to increase SINR. The phase variations may lead to a destructive superposition at this accumulation. 
         [0026]    An alternative may be for a UE to use a non-coherent accumulation in its receiver. Non-coherent accumulation, however, does not increase the SINR. It only improves the statistics of the receiver (i.e., the sensitivity to variations in individual noise samples). 
         [0027]    A particular technique to improve coverage of cell search signals uses several antenna elements and beamforming to improve the SINR. A directional cell search procedure is proposed by C. Nicolas Barati et al. in “Directional Cell Search for Millimeter Wave Cellular Systems”, Cornell University Library. In this procedure a base station periodically transmits synchronization signals in random directions to scan the angular space. The need for synchronization and broadcast signals that can be used in the initial cell search for scanning over a range of angles is discussed by Sundeep Rangan et al. in “Millimeter-Wave Cellular Wireless Networks: Potentials and Challenges”, Proceedings of the IEEE, Volume: 102, Issue 3, 2014, pages 366-385. Another technique to improve coverage of cell search signals is described in U.S. patent application Ser. No. 14/541,922 filed on Nov. 14, 2014, titled “Cell Search Procedure Frame Format.” 
         [0028]    An object of the present disclosure is to obviate at least these disadvantages and provide an improved method to transmit synchronization signals with a density and directionality that enables successful cell search in low SINR environments. As discussed above, the MBSFN subframe only contains broadcast information if the Physical Multicast Channel (PMCH) is mapped accordingly. Thus, the MBSFN subframe format may also be used as a flexible format for DL transmissions. In particular embodiments, more PSS/SSS pairs may be included in a MBSFN subframe than other subframe types. For example, the MBSFN subframe does not include CRS (among other signals) and thus more OFDM symbols may be available for PSS/SSS pairs. Particular embodiments are described with reference to  FIGS. 1-17  of the drawings, like numerals being used for like and corresponding parts of the various drawings. LTE is used throughout this disclosure as an example cellular system, but the ideas presented herein apply to other wireless communication systems as well. 
         [0029]      FIG. 3  is a block diagram illustrating an example of a network, according to a particular embodiment. Network  100  includes radio network node  120  (such as a base station or eNodeB) and wireless devices  110  (such as mobile phones, smart phones, laptop computers, tablet computers, or any other devices that can provide wireless communication). In general, wireless devices  110  that are within coverage of radio network node  120  communicate with radio network node  120  by transmitting and receiving wireless signals  130 . For example, wireless devices  110  and radio network node  120  may communicate wireless signals  130  containing voice traffic, data traffic, and/or control signals. Wireless signals  130  may include both downlink transmissions (from radio network node  120  to wireless devices  110 ) and uplink transmissions (from wireless devices  110  to radio network node  120 ). Wireless signals  130  may include synchronization signals, such PSS and SSS. Wireless device  110  may detect the synchronization signals to determine system information for network  100 . 
         [0030]    Wireless signals  130  comprise radio frames which in turn comprise subframes. Different subframes may comprise different formats. A particular subframe format is the MBSFN subframe format. A MBSFN subframe may contain multicast/broadcast information, or it may contain other information as described in more detail herein. Particular example formats for MBSFN radio subframes are illustrated in  FIGS. 4-13  described below. 
         [0031]    Radio network node  120  transmits and receives wireless signals  130  using antenna  140 . In particular embodiments, radio network node  120  may comprise multiple antennas  140 . For example, radio network node  120  may comprise a multi-input multi-output (MIMO) system with two, four, or eight antennas  140 . 
         [0032]    In network  100 , each radio network node  120  may use any suitable radio access technology, such as long term evolution (LTE), LTE-Advanced, UMTS, HSPA, GSM, cdma2000, WiMax, WiFi, and/or other suitable radio access technology. Network  100  may include any suitable combination of one or more radio access technologies. For purposes of example, various embodiments may be described within the context of certain radio access technologies. However, the scope of the disclosure is not limited to the examples and other embodiments could use different radio access technologies. 
         [0033]    As described above, embodiments of a network may include one or more wireless devices and one or more different types of radio network nodes capable of communicating with the wireless devices. The network may also include any additional elements suitable to support communication between wireless devices or between a wireless device and another communication device (such as a landline telephone). A wireless device may include any suitable combination of hardware and/or software. For example, in particular embodiments, a wireless device, such as wireless device  110 , may include the components described with respect to  FIG. 16  below. Similarly, a radio network node may include any suitable combination of hardware and/or software. For example, in particular embodiments, a radio network node, such as radio network node  120 , may include the components described with respect to  FIG. 17  below. 
         [0034]    This disclosure describes several frame formats for transmitting and receiving synchronization signals using LTE as an example. Particular embodiments may be applicable to both FDD and TDD subframes that can be scheduled as an MBSFN subframe (i.e., all downlink subframes except  0 ,  4 ,  5  and  9  in FDD and  0 ,  1 ,  5 , and  6  in TDD). According to 3GPP 36.211 Section 6.10.2, if PMCH is not mapped to the MBSFN subframe, then no MBSFN-reference signals are transmitted. The subframe will then also use the same cyclic prefix length as subframe number  0  (see 36.211 Section 6.1). If PMCH is mapped to the MBSFN-region of the subframe, then this part of the subframe will use extended cyclic prefix. 
         [0035]    In legacy systems, SSS and PSS are placed at subcarriers centered on the DC carrier. Such a configuration enables a UE to detect them without knowing the total system bandwidth. In particular embodiments, SSS and PSS are also placed at subcarriers centered on the DC carrier, but in a different subframe than legacy SSS and PSS.  FIGS. 4-13  illustrate example locations of primary and secondary search signals within a subframe, according to particular embodiments. 
         [0036]      FIG. 4  illustrates an example MBSFN cell search subframe format for FDD using normal cyclic prefix.  FIG. 4  illustrates two slots of an example subframe. Each slot comprises a plurality of OFDM symbols  412 , illustrated on the horizontal axis, and a plurality of subcarriers  414 , illustrated on the vertical axis. The first two OFDM symbols of the first slot include PDCCH  420 . The remaining OFDM symbols may include pairs of PSS  416  and SSS  418 . 
         [0037]    In the illustrated embodiment, the first slot includes a PSS/SSS pair in symbols  2  and  3  and another PSS/SSS pair in symbols  4  and  5 . The second slot includes PSS/SSS pairs in symbols  1  and  2 ,  3  and  4 , and  5  and  6 . In particular embodiments, the PSS/SSS pairs in the first slot may include symbols  3  and  4 , and  5  and  6 . 
         [0038]    In the illustrated embodiment, the PSS/SSS pairs are shown in adjacent OFDM symbols with the SSS transmitted before the PSS. A particular advantage of this ordering is that the order matches the legacy PSS/SSS ordering, which may enable reuse of some existing hardware or software components. In particular embodiments, the PSS/SSS pairs may include OFDM symbols that are not adjacent. In particular embodiments, PSS may be transmitted before SSS. Particular embodiments may include any suitable combination. For example, some PSS/SSS pairs within the subframe may be adjacent and some may not. Some PSS/SSS pairs within the subframe may transmit SSS before PSS and others may transmit after. 
         [0039]    In particular embodiments, the PSS sequence may comprise the same sequence for some or all PSS/SSS pairs. In particular embodiments, the SSS sequence may comprise a different sequence for some or all PSS/SSS pairs. 
         [0040]    The example in  FIG. 4  illustrates an embodiment where the MBSFN subframe is only used for PSS/SSS transmission.  FIG. 5  illustrates an example embodiment with unicast data included in the resource elements. DMRS is mapped to the MBSFN subframe to support demodulation of the unicast data. 
         [0041]      FIG. 5  illustrates another example MBSFN cell search subframe format for FDD using normal cyclic prefix.  FIG. 5  illustrates two slots of an example subframe. Each slot comprises a plurality of OFDM symbols  512 , illustrated on the horizontal axis, and a plurality of subcarriers  514 , illustrated on the vertical axis. The first two OFDM symbols of the first slot include PDCCH  520 . Symbols  5  and  6  of both the first and second slots include DMRS  522 . The remaining OFDM symbols may include pairs of PSS  516  and SSS  518 . 
         [0042]    In the illustrated embodiment, the first slot includes a PSS/SSS pair in symbols  2  and  3 . The second slot includes PSS/SSS pairs in symbols  1  and  2 , and  3  and  4 . In particular embodiments, the PSS/SSS pairs may include any suitable combination of symbols not occupied by PDCCH  520  and DMRS  522 . 
         [0043]    DMRS  522  is only present in the MBSFN subframe when data is scheduled for a UE. In particular embodiments, an eNodeB may switch between the formats described with reference to  FIGS. 4 and 5  based on load situation. 
         [0044]    Particular embodiments may apply to extended cyclic prefix.  FIGS. 6 and 7  are similar to  FIGS. 4 and 5 , except they illustrate examples of extended cyclic prefix. 
         [0045]      FIG. 6  illustrates an example MBSFN cell search subframe format for FDD using extended cyclic prefix.  FIG. 6  illustrates two slots of an example subframe. Each slot comprises a plurality of OFDM symbols  612 , illustrated on the horizontal axis, and a plurality of subcarriers  614 , illustrated on the vertical axis. The first two OFDM symbols of the first slot include PDCCH  620 . The remaining OFDM symbols may include pairs of PSS  616  and SSS  618 . Each slot of an extended cyclic prefix subframe includes fewer OFDM symbols than slots of a normal cyclic prefix subframe. 
         [0046]    In the illustrated embodiment, the first slot includes a PSS/SSS pair in symbols  2  and  3  and another PSS/SSS pair in symbols  4  and  5 . The second slot includes PSS/SSS pairs in symbols  0  and  1 ,  2  and  3 , and  4  and  5 . 
         [0047]      FIG. 7  illustrates another example MBSFN cell search subframe format for FDD using extended cyclic prefix.  FIG. 7  illustrates two slots of an example subframe. Each slot comprises a plurality of OFDM symbols  712 , illustrated on the horizontal axis, and a plurality of subcarriers  714 , illustrated on the vertical axis. The first two OFDM symbols of the first slot include PDCCH  720 . Symbols  4  and  5  of both the first and second slots include DMRS  722 . The remaining OFDM symbols may include pairs of PSS  716  and SSS  718 . 
         [0048]    In the illustrated embodiment, the first slot includes a PSS/SSS pair in symbols  2  and  3 . The second slot includes PSS/SSS pairs in symbols  0  and  1 , and  2  and  3 . 
         [0049]      FIG. 8  illustrates an example MBSFN cell search subframe format for FDD using extended cyclic prefix and PMCH.  FIG. 8  illustrates two slots of an example subframe. Each slot comprises a plurality of OFDM symbols  812 , illustrated on the horizontal axis, and a plurality of subcarriers  814 , illustrated on the vertical axis. The first two OFDM symbols of the first slot include PDCCH  820 . PMCH  824  is mapped to the MBSFN subframe and thus OFDM symbol  2  in the first slot and OFDM symbols  0  and  4  in the second slot include MBSFN reference signals. In this example the entire subframe uses extended cyclic prefix because the MBSFN reference signals are only defined for extended cyclic prefix. The remaining OFDM symbols may include pairs of PSS  716  and SSS  718 . 
         [0050]    In the illustrated embodiment, the first slot includes a PSS/SSS pair in symbols  4  and  5 . The second slot includes a PSS/SSS pair in symbols  2  and  3 . 
         [0051]    Particular embodiments may apply to TDD.  FIGS. 9-13  illustrate TDD examples. 
         [0052]      FIG. 9  illustrates an example MBSFN cell search subframe format for TDD using normal cyclic prefix.  FIG. 9  illustrates two slots of an example subframe. Each slot comprises a plurality of OFDM symbols  912 , illustrated on the horizontal axis, and a plurality of subcarriers  914 , illustrated on the vertical axis. The first two OFDM symbols of the first slot include PDCCH  920 . The remaining OFDM symbols may include pairs of PSS  9416  and SSS  918 . 
         [0053]    In the illustrated embodiment, the first slot includes a PSS/SSS pair in symbols  2  and  5  and another PSS/SSS pair in symbols  3  and  6 . The second slot includes PSS/SSS pairs in symbols  1  and  4 ,  2  and  5 , and  3  and  6 . 
         [0054]    In the illustrated embodiment, the PSS/SSS pairs are shown separated by two OFDM symbols with the SSS transmitted before the PSS. In particular embodiments, a UE may distinguish between duplex modes (i.e., FDD or TDD) based on the separation. 
         [0055]    In particular embodiments, the PSS/SSS pairs may include any suitable separation. In particular embodiments, PSS may be transmitted before SSS. Particular embodiments may include any suitable combination. For example, some PSS/SSS pairs within the subframe may transmit SSS before PSS and others may transmit after. 
         [0056]    The example in  FIG. 9  illustrates an embodiment where the MBSFN subframe is only used for PSS/SSS transmission.  FIG. 10  illustrates an example embodiment with unicast data included in the resource elements. DMRS is mapped to the MBSFN subframe to support demodulation of the unicast data. 
         [0057]      FIG. 10  illustrates another example MBSFN cell search subframe format for TDD using normal cyclic prefix.  FIG. 10  illustrates two slots of an example subframe. Each slot comprises a plurality of OFDM symbols  1012 , illustrated on the horizontal axis, and a plurality of subcarriers  1014 , illustrated on the vertical axis. The first two OFDM symbols of the first slot include PDCCH  1020 . Symbols  5  and  6  of both the first and second slots include DMRS  1022 . The remaining OFDM symbols may include pairs of PSS  1016  and SSS  1018 . 
         [0058]    In the illustrated embodiment, the second slot includes PSS/SSS pairs in symbols  0  and  3 , and  1  and  4 . In particular embodiments, the PSS/SSS pairs may include any suitable combination of symbols not occupied by PDCCH  1020  and DMRS  1022 . 
         [0059]    DMRS  1022  is only present in the MBSFN subframe when data is scheduled for a UE. In particular embodiments, an eNodeB may switch between the formats described with reference to  FIGS. 9 and 10  based on load situation. 
         [0060]    Particular PSS/SSS configurations may apply to extended cyclic prefix.  FIGS. 11 and 12  are similar to  FIGS. 9 and 10 , except they illustrate examples of extended cyclic prefix. 
         [0061]      FIG. 11  illustrates an example MBSFN cell search subframe format for TDD using extended cyclic prefix.  FIG. 11  illustrates two slots of an example subframe. Each slot comprises a plurality of OFDM symbols  1112 , illustrated on the horizontal axis, and a plurality of subcarriers  1114 , illustrated on the vertical axis. The first two OFDM symbols of the first slot include PDCCH  1120 . The remaining OFDM symbols may include pairs of PSS  1116  and SSS  1118 . Each slot of an extended cyclic prefix subframe includes fewer OFDM symbols than slots of a normal cyclic prefix subframe. 
         [0062]    In the illustrated embodiment, the first slot includes a PSS/SSS pair in symbols  2  and  5 . The second slot includes PSS/SSS pairs in symbols  0  and  3 ,  1  and  4 , and  3  and  5 . 
         [0063]      FIG. 12  illustrates another example MBSFN cell search subframe format for TDD using extended cyclic prefix.  FIG. 12  illustrates two slots of an example subframe. Each slot comprises a plurality of OFDM symbols  1212 , illustrated on the horizontal axis, and a plurality of subcarriers  1214 , illustrated on the vertical axis. The first two OFDM symbols of the first slot include PDCCH  1220 . Symbols  4  and  5  of both the first and second slots include DMRS  1222 . The remaining OFDM symbols may include pairs of PSS  1216  and SSS  1218 . In the illustrated embodiment, the second slot includes a PSS/SSS pair in symbols  0  and  3 . 
         [0064]      FIG. 13  illustrates an example MBSFN cell search subframe format for TDD using extended cyclic prefix and PMCH.  FIG. 13  illustrates two slots of an example subframe. Each slot comprises a plurality of OFDM symbols  1312 , illustrated on the horizontal axis, and a plurality of subcarriers  1314 , illustrated on the vertical axis. The first two OFDM symbols of the first slot include PDCCH  1320 . PMCH  1324  is mapped to the MBSFN subframe and thus OFDM symbol  2  in the first slot and OFDM symbols  0  and  4  in the second slot include MBSFN reference signals. In this example the entire subframe uses extended cyclic prefix because the MBSFN reference signals are only defined for extended cyclic prefix. The remaining OFDM symbols may include pairs of PSS  1316  and SSS  1318 . In the illustrated embodiment, the second slot includes a PSS/SSS pair in symbols  2  and  5 . 
         [0065]    Particular embodiments may recognize an advantage of additional PSS/SSS pairs that outweighs any disadvantage of locating the corresponding PSS and SSS in different slots. In such embodiments, additional PSS/SSS pairs may be added. For example, with respect to  FIG. 4 or 6 , an additional PSS may be included in OFDM symbol  6  in the first slot and a corresponding SSS may be included in OFDM symbol  0  in the second slot. As another example, with respect to  FIG. 13 , an additional PSS may be included in OFDM symbol  4  in the first slot and a corresponding SSS may be included in OFDM symbol  1  in the second slot. 
         [0066]    Although particular PSS/SSS patterns are illustrated above, additional patterns will be apparent to those skilled in the art. Furthermore, any of the patterns described above, or combination of patterns, may be repeated in other subframes within the frame. 
         [0067]      FIG. 14  is a flowchart of an example method of transmitting PSS and SSS, according to particular embodiments. In particular embodiments, one or more steps of the method may be performed by components of network  100  described with reference to  FIGS. 1-17 . 
         [0068]    The method begins at step  1410 , where a network node generates synchronization signals. For example, radio network node  120  may generate a plurality of PSS sequences and SSS sequences. Each PSS sequence is paired with an SSS sequence to form a PSS/SSS pair. In particular embodiments, a first PSS sequence and a second PSS sequence may comprise identical sequences. In particular embodiments, a first SSS sequence and a second SSS sequence may comprise different sequences. 
         [0069]    At step  1412 , the network node maps the synchronization signals to MBSFN radio subframes. For example, radio network node  120  may map the plurality of PSS/SSS pairs to a subframe according to any one of the frame formats described above, such as those described with respect to  FIGS. 4-13 . In particular embodiments, radio network node  120  may map PSS/SSS pairs to more than one subframe or repeat a mapping or combination of mappings in multiple subframes. In particular embodiments, radio network node  120  may map a legacy PSS/SSS pair in a particular subframe and additional PSS/SSS pairs in MBSFN subframes. 
         [0070]    At step  1414 , the network node transmits the synchronization signals. For example, radio network node  120  transmits the radio frame comprising the MBSFN subframes with the mapping of PSS/SSS pairs. In particular embodiments, radio network node  120  may perform directional signal transmission. For example, radio network node  120  may transmit a first PSS/SSS pair in a first direction and a second PSS/SSS pair in a second direction. In particular embodiments, radio network node  120  may transmit a first PSS/SSS pair in different directions over time. 
         [0071]    Modifications, additions, or omissions may be made to the method of  FIG. 14 . Additionally, one or more steps in the method of  FIG. 14  may be performed in parallel or in any suitable order. The method may be repeated as necessary over time. 
         [0072]      FIG. 15  is a flowchart of an example method of detecting PSS and SSS, according to particular embodiments. In particular embodiments, one or more steps of the method may be performed by components of network  100  described with reference to  FIGS. 1-17 . 
         [0073]    The method begins at step  1510 , where a wireless device receives signals transmitted from a radio network node. For example, wireless device  110  may receive wireless signal  130  from radio network node  120 . Wireless signal  130  may comprise an MBSFN subframe including primary and secondary cell search signals. For example, wireless signal  130  may comprise a plurality of PSS/SSS pairs according to any one of the frame formats described above with respect to  FIGS. 4-13 . 
         [0074]    At step  1512 , the wireless device tries to detect a PSS sequence, such as a PSS sequence according to one of the formats described herein. In particular embodiments, wireless device  110  may accumulate multiple PSS received within a MBSFN subframe or received across multiple subframes. A particular advantage is that wireless device  110  may combine signals to create a stronger signal. 
         [0075]    In particular embodiments, radio network node  120  may transmit a first PSS in a first direction and a second PSS in a second direction. A particular advantage of this transmission method is that wireless device  110  may receive a stronger PSS when radio network node  120  transmits the PSS in the direction of wireless device  110 . 
         [0076]    If, at step  1514 , wireless device  110  successfully detects PSS, then the method continues to step  1516 . If wireless device  110  does not successfully detect PSS, then the method returns to step  1512  where wireless device  110  may try to detect another PSS sequence in the received MBSFN subframe. In other embodiments, if wireless device  110  does not successfully detect PSS, then the method returns to step  1512  and continues to receive signals from network node  120 . 
         [0077]    At step  1516 , the wireless device tries to detect an SSS sequence according to one of the formats described herein. Similar to detecting the PSS, the wireless device may accumulate multiple SSS and the radio network node may transmit different SSS in different directions. After detecting both primary and secondary cell search signals, the method is complete. 
         [0078]    Modifications, additions, or omissions may be made to the method of  FIG. 15 . Additionally, one or more steps in the method of  FIG. 15  may be performed in parallel or in any suitable order. The method may be repeated as necessary over time, such as when a wireless device travels into a new cell coverage area. 
         [0079]      FIG. 16  is a block diagram illustrating an example embodiment of a wireless device. The wireless device is an example of the wireless devices  110  illustrated in  FIG. 3 . Particular examples include a mobile phone, a smart phone, a PDA (Personal Digital Assistant), a portable computer (e.g., laptop, tablet), a sensor, a modem, a machine type (MTC) device/machine to machine (M2M) device, laptop embedded equipment (LEE), laptop mounted equipment (LME), USB dongles, a device-to-device capable device, or any other device that can provide wireless communication. The wireless device includes transceiver  1610 , processor  1620 , and memory  1630 . In some embodiments, transceiver  1610  facilitates transmitting wireless signals to and receiving wireless signals from wireless network node  120  (e.g., via an antenna), processor  1620  executes instructions to provide some or all of the functionality described herein as provided by the wireless device, and memory  1630  stores the instructions executed by processor  1620 . 
         [0080]    Processor  1620  includes any suitable combination of hardware and software implemented in one or more integrated circuits or modules to execute instructions and manipulate data to perform some or all of the described functions of the wireless device. Memory  1630  is generally operable to store computer executable code and data. Examples of memory  1630  include computer memory (e.g., Random Access Memory (RAM) or Read Only Memory (ROM)), mass storage media (e.g., a hard disk), removable storage media (e.g., a Compact Disk (CD) or a Digital Video Disk (DVD)), and/or or any other volatile or non-volatile, non-transitory computer-readable and/or computer-executable memory devices that store information. 
         [0081]    In particular embodiments, processor  1620  in communication with transceiver  1610  receives cell search signals from radio network node  120 . Other embodiments of the wireless device may include additional components (beyond those shown in  FIG. 16 ) responsible for providing certain aspects of the wireless device&#39;s functionality, including any of the functionality described above and/or any additional functionality (including any functionality necessary to support the solution described above). 
         [0082]      FIG. 17  is a block diagram illustrating an example embodiment of a radio network node. Radio network node  120  can be an eNodeB, a node B, a base station, a wireless access point (e.g., a Wi-Fi access point), a low power node, a base transceiver station (BTS), a transmission point or node, a remote RF unit (RRU), a remote radio head (RRH), or other radio access node. Radio network node  120  includes at least one transceiver  1710 , at least one processor  1720 , at least one memory  1730 , and at least one network interface  1740 . Transceiver  1710  facilitates transmitting wireless signals to and receiving wireless signals from a wireless device, such as wireless devices  110  (e.g., via an antenna); processor  1720  executes instructions to provide some or all of the functionality described above as being provided by a radio network node  120 ; memory  1730  stores the instructions executed by processor  1720 ; and network interface  1740  communicates signals to backend network components, such as a gateway, switch, router, Internet, Public Switched Telephone Network (PSTN), controller, and/or other radio network nodes  120 . Processor  1720  and memory  1730  can be of the same types as described with respect to processor  1620  and memory  1630  of  FIG. 16  above. 
         [0083]    In some embodiments, network interface  1740  is communicatively coupled to processor  1720  and refers to any suitable device operable to receive input for radio network node  120 , send output from radio network node  120 , perform suitable processing of the input or output or both, communicate to other devices, or any combination of the preceding. Network interface  1740  includes appropriate hardware (e.g., port, modem, network interface card, etc.) and software, including protocol conversion and data processing capabilities, to communicate through a network. 
         [0084]    In particular embodiments, processor  1720  in communication with transceiver  1710  transmits, to wireless device  110 , cell search signals. In particular embodiments, processor  1720  in communication with transceiver  1710  transmits cell search signals such as the PSS and SSS described above to wireless device  110 . 
         [0085]    Other embodiments of radio network node  120  include additional components (beyond those shown in  FIG. 17 ) responsible for providing certain aspects of the radio network node&#39;s functionality, including any of the functionality described above and/or any additional functionality (including any functionality necessary to support the solution described above). The various different types of radio network nodes may include components having the same physical hardware but configured (e.g., via programming) to support different radio access technologies, or may represent partly or entirely different physical components. 
         [0086]    Some embodiments of the disclosure may provide one or more technical advantages. As an example, in some embodiments, the methods and apparatus disclosed herein may facilitate detecting synchronization signals in a low SINR environment. Cell search procedure may be performed more efficiently to improve overall system performance. 
         [0087]    Some embodiments may benefit from some, none, or all of these advantages. Other technical advantages may be readily ascertained by one of ordinary skill in the art. 
         [0088]    Modifications, additions, or omissions may be made to the systems and apparatuses disclosed herein without departing from the scope of the invention. The components of the systems and apparatuses may be integrated or separated. Moreover, the operations of the systems and apparatuses may be performed by more, fewer, or other components. Additionally, operations of the systems and apparatuses may be performed using any suitable logic comprising software, hardware, and/or other logic. As used in this document, “each” refers to each member of a set or each member of a subset of a set. 
         [0089]    Modifications, additions, or omissions may be made to the methods disclosed herein without departing from the scope of the invention. The methods may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order. 
         [0090]    Although this disclosure has been described in terms of certain embodiments, alterations and permutations of the embodiments will be apparent to those skilled in the art. Accordingly, the above description of the embodiments does not constrain this disclosure. Other changes, substitutions, and alterations are possible without departing from the spirit and scope of this disclosure, as defined by the claims below. 
         [0091]    Abbreviations used in the preceding description include: 
         [0092]    3GPP 3rd Generation Partnership Project 
         [0093]    CP Cyclic Prefix 
         [0094]    CRS Cell-Specific Reference Signal 
         [0095]    DMRS Demodulation Reference Signal 
         [0096]    eNB Enhanced Node-B 
         [0097]    eNodeB Enhanced Node-B 
         [0098]    ePDCCH Enhance Physical Downlink Control Channel 
         [0099]    FDD Frequency Division Duplex 
         [0100]    LTE Long Term Evolution 
         [0101]    MBSFN Multimedia Broadcast Single-Frequency Network 
         [0102]    OFDM Orthogonal Frequency-Division Multiplexing 
         [0103]    PBCH Physical Broadcast Channel 
         [0104]    PCFICH Physical Control Format Indicator Channel 
         [0105]    PDCCH Physical Downlink Control Channel 
         [0106]    PDSCH Physical Downlink Shared Channel 
         [0107]    PMCH Physical Multicast Channel 
         [0108]    ppm Part Per Million 
         [0109]    PSS Primary Synchronization Signal 
         [0110]    RE Resource Element 
         [0111]    RB Resource Block 
         [0112]    SINR Signal to Interference and Noise Ratio 
         [0113]    SNR Signal to Noise Ratio 
         [0114]    SSS Secondary Synchronization Signal 
         [0115]    TDD Time Division Duplexing 
         [0116]    UE User Equipment