Patent Publication Number: US-2021168701-A1

Title: Interleaved deep and shallow search during frequency scan for radio resources

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority to U.S. Provisional Patent Application No. 62/942,050, filed Nov. 29, 2019, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This application relates to wireless communication systems, and more particularly to the acquisition of radio resources with interleaved deep and shallow searches during a frequency scan. 
     INTRODUCTION 
     To meet the growing demands for expanded mobile broadband connectivity, wireless communication technologies have advanced from the long term evolution (LTE) technology to a next generation new radio (NR) technology, which may be referred to as 5 th  Generation (5G). For example, NR is designed to provide a lower latency, a higher bandwidth or a higher throughput, and a higher reliability than LTE. NR is designed to operate over a wide array of spectrum bands, for example, from low-frequency bands below about 1 gigahertz (GHz) and mid-frequency bands from about 1 GHz to about 6 GHz, to high-frequency bands such as millimeter wave (mmWave) bands. NR is also designed to operate across different spectrum types, from licensed spectrum to unlicensed and shared spectrum. Spectrum sharing enables operators to opportunistically aggregate spectrums to dynamically support high-bandwidth services. Spectrum sharing can extend the benefit of NR technologies to operating entities that may not have access to a licensed spectrum. 
     Unlike LTE, there are no persistent wideband signals in NR like LTE&#39;s cell specific reference signals. To search for a cell using a certain frequency band, an NR user equipment (UE) scans across the frequency band at various frequencies of a synchronization raster for the frequency band. Cell search and the associated frequency scan may occur in response to a number of events such as a power-up of the UE or a mobility failure recovery for the UE. The delay from the frequency scan is a critical factor that may impact user experience. 
     SUMMARY 
     The following summarizes some aspects of the present disclosure to provide a basic understanding of the discussed technology. This summary is not an extensive overview of all contemplated features of the disclosure and is intended neither to identify key or critical elements of all aspects of the disclosure nor to delineate the scope of any or all aspects of the disclosure. Its sole purpose is to present some concepts of one or more aspects of the disclosure in summary form as a prelude to the more detailed description that is presented later. 
     For example, in an aspect of the disclosure, a method of wireless communication is provided that includes: performing a first shallow scan at a user equipment (UE) over a frequency band by determining at each frequency of a synchronization raster for the frequency band whether a synchronization signal is received over a repetition period for the synchronization signal with a first signal quality sufficient to permit synchronization with a base station; and in response to the first shallow scan not being successful, performing a deep scan over the frequency band by determining at each frequency of the synchronization raster for the frequency band whether the synchronization signal is received over a series of repetition periods for the synchronization signal with a second signal quality sufficient to permit synchronization the base station 
     In an additional aspect of the disclosure, a UE is provided that includes: a transceiver configured to: perform a first shallow scan over a frequency band by a determination at each frequency of a synchronization raster for the frequency band of whether a synchronization signal is received over a repetition period for the synchronization signal with a first signal quality to permit synchronization with downlink transmissions from a base station; and in response to the first shallow scan not being successful, perform a deep scan over the frequency band by a determination at each frequency of the synchronization raster of whether the synchronization signal is received over a series of repetition periods for the synchronization signal with a second signal quality to permit synchronization with the base station. 
     Finally, a method of wireless communication is provided that includes: incrementing a count; responsive to the count being equal to a first integer, accumulating a received signal quality over multiple synchronization signal periods at each frequency of a synchronization raster to form an accumulated signal quality for each frequency; determining if the accumulated signal quality for each frequency exceeds a first threshold to determine whether a synchronization signal is successfully detected at the frequency; responsive to the count not being equal to the first integer, determining a received signal quality for an additional synchronization period at each frequency of a synchronization raster to form a received signal quality for each frequency; and determining if the received signal quality for each frequency exceeds a second threshold to determine whether the synchronization signal is successfully detected at the frequency. 
     Other aspects, features, and embodiments of the present invention will become apparent to those of ordinary skill in the art, upon reviewing the following description of specific, exemplary embodiments of the present invention in conjunction with the accompanying figures. While features of the present invention may be discussed relative to certain embodiments and figures below, all embodiments of the present invention can include one or more of the advantageous features discussed herein. In other words, while one or more embodiments may be discussed as having certain advantageous features, one or more of such features may also be used in accordance with the various embodiments of the invention discussed herein. In similar fashion, while exemplary embodiments may be discussed below as device, system, or method embodiments it should be understood that such exemplary embodiments can be implemented in various devices, systems, and methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a wireless communication system with enhanced frequency scanning in accordance with an aspect of the disclosure. 
         FIG. 2  is a schematic illustration of an organization of wireless resources utilizing orthogonal frequency divisional multiplexing (OFDM) for the wireless communication system of  FIG. 1 . 
         FIG. 3  illustrates a synchronization signal block (SSB) for a base station in the system of  FIG. 1 . 
         FIG. 4A  is a plot for an initial interleaved shallow/deep frequency scan in accordance with an aspect of the disclosure. 
         FIG. 4B  is a plot for a BPLMN frequency scan in accordance with an aspect of the disclosure. 
         FIG. 4C  is a plot for an additional interleaved shallow/deep frequency scan in accordance with an aspect of the disclosure. 
         FIG. 5  illustrates an architecture for a user equipment in the system of  FIG. 1  in accordance with an aspect of the disclosure. 
         FIG. 6  is a flowchart for an example method of frequency scanning in accordance with an aspect of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     An interleaved frequency scan is disclosed that offers an advantageous balance between performance and delay (the amount of time necessary for a successful frequency scan). To provide a better appreciation of this enhanced frequency scanning, some background principles for NR will be reviewed initially and followed by a detailed discussion of the enhanced frequency scanning. The various concepts presented throughout this disclosure may be implemented across a broad variety of telecommunication systems, network architectures, and communication standards. 
     Referring now to  FIG. 1 , as an illustrative example without limitation, various aspects of the present disclosure are illustrated with reference to a wireless communication system  100 . The wireless communication system  100  includes three interacting domains: a core network  102 , a radio access network (RAN)  104 , and a plurality of user equipment (UE)  106 . By virtue of the wireless communication system  100 , each UE  106  may be enabled to carry out data communication with an external data network  110 , such as (but not limited to) the Internet. 
     The RAN  104  may implement any suitable wireless communication technology or technologies to provide radio access to the UE  106 . As one example, the RAN  104  may operate according to 3 rd  Generation Partnership Project (3GPP) New Radio (NR) specifications, often referred to as 5G. As another example, the RAN  104  may operate under a hybrid of 5G NR and Evolved Universal Terrestrial Radio Access Network (eUTRAN) standards, often referred to as LTE. The 3GPP refers to this hybrid RAN as a next-generation RAN, or NG-RAN. Of course, many other examples may be utilized within the scope of the present disclosure. 
     As illustrated, the RAN  104  includes a plurality of base stations  108 . Broadly, a base station is a network element in a radio access network responsible for radio transmission and reception in one or more cells to or from a UE  106 . In different technologies, standards, or contexts, a base station may variously be referred to by those skilled in the art as a base transceiver station (BTS), a radio base station, a radio transceiver, a transceiver function, a basic service set (BSS), an extended service set (ESS), an access point (AP), a Node B (NB), an eNode B (eNB), a gNode B (gNB), or some other suitable terminology. 
     The radio access network  104  is further illustrated supporting wireless communication for multiple mobile apparatuses. A mobile apparatus may be referred to as user equipment (UE) in 3GPP standards, but may also be referred to by those skilled in the art as a mobile station (MS), a subscriber station, a mobile unit, a subscriber unit, a wireless unit, a remote unit, a mobile device, a wireless device, a wireless communications device, a remote device, a mobile subscriber station, an access terminal (AT), a mobile terminal, a wireless terminal, a remote terminal, a handset, a terminal, a user agent, a mobile client, a client, or some other suitable terminology. A UE  106  may be an apparatus that provides a user with access to network services. 
     Within the present document, a “mobile” apparatus need not necessarily have a capability to move and may be stationary. The term mobile apparatus or mobile device broadly refers to a diverse array of devices and technologies. UEs  106  may include a number of hardware structural components sized, shaped, and arranged to help in communication; such components can include antennas, antenna arrays, RF chains, amplifiers, one or more processors, etc. electrically coupled to each other. For example, some non-limiting examples of a mobile apparatus include a mobile, a cellular (cell) phone, a smart phone, a session initiation protocol (SIP) phone, a laptop, a personal computer (PC), a notebook, a netbook, a smartbook, a tablet, a personal digital assistant (PDA), and a broad array of embedded systems, e.g., corresponding to an “Internet of things” (IoT). A mobile apparatus may additionally be an automotive or other transportation vehicle, a remote sensor or actuator, a robot or robotics device, a satellite radio, a global positioning system (GPS) device, an object tracking device, a drone, a multi-copter, a quad-copter, a remote control device, a consumer and/or wearable device, such as eyewear, a wearable camera, a virtual reality device, a smart watch, a health or fitness tracker, a digital audio player (e.g., MP3 player), a camera, a game console, etc. A mobile apparatus may additionally be a digital home or smart home device such as a home audio, video, and/or multimedia device, an appliance, a vending machine, intelligent lighting, a home security system, a smart meter, etc. A mobile apparatus may additionally be a smart energy device, a security device, a solar panel or solar array, a municipal infrastructure device controlling electric power (e.g., a smart grid), lighting, water, etc.; an industrial automation and enterprise device; a logistics controller; agricultural equipment; military defense equipment, vehicles, aircraft, ships, and weaponry, etc. Still further, a mobile apparatus may provide for connected medicine or telemedicine support, e.g., health care at a distance. Telehealth devices may include telehealth monitoring devices and telehealth administration devices, whose communication may be given preferential treatment or prioritized access over other types of information, e.g., in terms of prioritized access for transport of critical service data, and/or relevant QoS for transport of critical service data. 
     Wireless communication between a RAN  104  and a UE  106  may be described as utilizing an air interface. Transmissions over the air interface from a base station (e.g., base station  108 ) to one or more UEs  106  may be referred to as downlink (DL) transmission. In accordance with certain aspects of the present disclosure, the term downlink may refer to a point-to-multipoint transmission originating at a scheduling entity (described further below; e.g., base station  108 ). Another way to describe this scheme may be to use the term broadcast channel multiplexing. Transmissions from a UE (e.g., UE  106 ) to a base station (e.g., base station  108 ) may be referred to as uplink (UL) transmissions. In accordance with further aspects of the present disclosure, the term uplink may refer to a point-to-point transmission originating at a UE  106 . 
     As illustrated in  FIG. 1 , a base station  108  may broadcast downlink traffic  112  to one or more UEs  106 . Broadly, the base station  108  is a node or device responsible for scheduling traffic in a wireless communication network, including the downlink traffic  112  and, in some examples, uplink traffic  116  from the one or more UEs  106 . On the other hand, each UE  106  is a node or device that receives downlink control information  114 , including but not limited to scheduling information (e.g., a grant), synchronization or timing information, or other control information from another entity in the wireless communication network such as the base station  108 . 
     In general, base stations  108  may include a backhaul interface for communication with a backhaul portion  120  of the wireless communication system. The backhaul  120  may provide a link between a base station  108  and the core network  102 . Further, in some examples, a backhaul network may provide interconnection between the respective base stations  108 . Various types of backhaul interfaces may be employed, such as a direct physical connection, a virtual network, or the like using any suitable transport network. 
     The core network  102  may be a part of the wireless communication system  100  and may be independent of the radio access technology used in the RAN  104 . In some examples, the core network  102  may be configured according to 5G standards (e.g., 5GC). In other examples, the core network  102  may be configured according to a 4G evolved packet core (EPC), or any other suitable standard or configuration. 
     In various implementations, the air interface in the radio access network  104  may utilize licensed spectrum, unlicensed spectrum, or shared spectrum. Licensed spectrum provides for exclusive use of a portion of the spectrum, generally by virtue of a mobile network operator purchasing a license from a government regulatory body. Unlicensed spectrum provides for shared use of a portion of the spectrum without need for a government-granted license. While compliance with some technical rules is generally still required to access unlicensed spectrum, generally, any operator or device may gain access. Shared spectrum may fall between licensed and unlicensed spectrum, wherein technical rules or limitations may be required to access the spectrum, but the spectrum may still be shared by multiple operators and/or multiple RATs. For example, the holder of a license for a portion of licensed spectrum may provide licensed shared access (LSA) to share that spectrum with other parties, e.g., with suitable licensee-determined conditions to gain access. 
     The air interface in the radio access network  104  may utilize one or more duplexing algorithms. Duplex refers to a point-to-point communication link where both endpoints can communicate with one another in both directions. Full duplex means both endpoints can simultaneously communicate with one another. Half duplex means only one endpoint can send information to the other at a time. In a wireless link, a full duplex channel generally relies on physical isolation of a transmitter and receiver, and suitable interference cancellation technologies. Full duplex emulation is frequently implemented for wireless links by utilizing frequency division duplex (FDD) or time division duplex (TDD). In FDD, transmissions in different directions operate at different carrier frequencies. In TDD, transmissions in different directions on a given channel are separated from one another using time division multiplexing. That is, at one time the channel is dedicated for transmissions in one direction, while at other times the channel is dedicated for transmissions in the other direction, where the direction may change very rapidly, e.g., several times per slot. 
     Various aspects of the present disclosure will be described with reference to an OFDM waveform, schematically illustrated in  FIG. 2 . Within the present disclosure, a frame refers to a duration of 10 ms for wireless transmissions, with each frame consisting of 10 subframes of 1 ms each. On a given carrier, there may be one set of frames in the UL, and another set of frames in the DL. An expanded view of an exemplary DL subframe  202  is also illustrated in  FIG. 2 , showing an OFDM resource grid  204 . However, as those skilled in the art will readily appreciate, the PHY transmission structure for any particular application may vary from the example described here, depending on any number of factors. Here, time is in the horizontal direction with units of OFDM symbols; and frequency is in the vertical direction with units of subcarriers or tones. 
     The resource grid  204  may be used to schematically represent time-frequency resources for a given antenna port. That is, in a MIMO implementation with multiple antenna ports available, a corresponding multiple number of resource grids  204  may be available for communication. The resource grid  204  is divided into multiple resource elements (REs)  206 . An RE, which is 1 subcarrier×1 symbol, is the smallest discrete part of the time-frequency grid, and contains a single complex value representing data from a physical channel or signal. A block of twelve consecutive subcarriers defined a resource block (RB)  208 , which has an undefined time duration in the NR standard. In  FIG. 2 , resource block  208  extends over a symbol duration. Within the present disclosure, it is assumed that a single RB such as the RB  208  entirely corresponds to a single direction of communication (either transmission or reception for a given device). A set of contiguous RBs  208  such as shown for resource grid  204  form a bandwidth part (BWP). 
     A UE generally utilizes only a subset of the resource grid  204 . An RB may be the smallest unit of resources that can be allocated to a UE. Thus, the more RBs scheduled for a UE, and the higher the modulation scheme chosen for the air interface, the higher the data rate for the 
     UE. 
     In  FIG. 2 , the RB  208  is shown as occupying less than the entire bandwidth of the subframe  202 , with some subcarriers illustrated above and below the RB  208 . In a given implementation, the subframe  202  may have a bandwidth corresponding to any number of one or more RBs  208 . Further, in this illustration, the RB  208  is shown as occupying less than the entire duration of the subframe  202 , although this is merely one possible example. 
     Each 1 ms subframe  202  may consist of one or multiple adjacent slots. In the example shown in  FIG. 2 , one subframe  202  includes four slots  210 , as an illustrative example. In some examples, a slot may be defined according to a specified number of OFDM symbols with a given cyclic prefix (CP) length. For example, a slot may include 7 or 14 OFDM symbols with a nominal CP. Additional examples may include mini-slots having a shorter duration (e.g., one or two OFDM symbols). These mini-slots may in some cases be transmitted occupying resources scheduled for ongoing slot transmissions for the same or for different UEs. 
     An expanded view of one of the slots  210  illustrates the slot  210  including a control region  212  and a data region  214 . In general, the control region  212  may carry control channels (e.g., PDCCH), and the data region  214  may carry data channels (e.g., PDSCH or PUSCH). Of course, a slot may contain all DL, all UL, or at least one DL portion and at least one UL portion. The simple structure illustrated in  FIG. 2  is merely exemplary in nature, and different slot structures may be utilized, and may include one or more of each of the control region(s) and data region(s). 
     Although not illustrated in  FIG. 2 , the various REs  206  within a RB  208  may be scheduled to carry one or more physical channels, including control channels, shared channels, data channels, etc. Other REs  206  within the RB  208  may also carry pilots or reference signals, including but not limited to a demodulation reference signal (DMRS) a control reference signal (CRS), or a sounding reference signal (SRS). These pilots or reference signals may provide for a receiving device to perform channel estimation of the corresponding channel, which may enable coherent demodulation/detection of the control and/or data channels within the RB  208 . 
     Referring again to  FIG. 1 , each UE  106  establishes a connection with an appropriate base station  108  through an initial access procedure using the enhanced frequency scan, whereby the UE  106  obtains system information associated with the network. In an NR network, each base station  108  sequentially transmits a synchronization signal block (SSB). An example SSB block  300  is shown in  FIG. 3 . SSB  300  extends over four OFDM symbols. The available bandwidth for SSB  300  is 240 subcarriers, which is 20 resource blocks. The first OFDM symbol may include a primary synchronization signal (PSS) that extends across 127 subcarriers within the center of the available bandwidth. A physical broadcast channel (PBCH) occupies all 240 subcarriers in the second OFDM symbol. A secondary synchronization signal (SSS) occupies the center 127 subcarriers within the third OFDM signal. If the 240-subcarrier bandwidth for SSB  300  is deemed to extend from a first resource block to a twentieth resource block, the PBCH occupies the first 4 resource blocks and the final four resource blocks in the third OFDM symbol. The PBCH also occupies all 240 subcarriers in the fourth OFDM symbol. The PBCH provides system information including a master information block (MIB). The MIB identifies parameters so that each corresponding UE  106  in a footprint (within the cell coverage) of a base station  108  may acquire a first SIB (SIB1). The SIB1 (not illustrated) contains information on the scheduling of other SIBs. In some implementations, a SIB such as SIB1 provides a transmit power adjustment command for a UE  106  to adjust the transmit power it uses to transmit its uplink messages. 
     Each base station  108  periodically transmits a burst of SSBs, each SSB being assigned to a specific antenna beam. For example, if a base station  108  has N antenna beams, there would be N different SSBs uniquely assigned to the N corresponding antenna beams. If the beams are deemed to be numbered from one to N, the corresponding SSBs may be numbered accordingly to range from an SSB1 to an SSBN. In such an implementation, the SSB burst would be the N SSBs. The maximum value for the integer N depends upon the frequency band. For example, below 3 GHz, there can be up to four antenna beams and corresponding SSBs. The integer N is increased for higher frequency bands such as up to 64 for the FR2 frequency band. In an SSB, the PBCH provides a block time index that identifies the relative location of the SSB within an SSB burst. From an SSB, a UE  106  receives the information necessary to acquire the corresponding SIB1 that in turn provides the UE  106  with the information necessary to carry out an initial random-access (the initial access procedure) to the corresponding base station  108 . 
     Given this introduction, some exemplary implementations for an enhanced frequency scan will now be discussed in more detail. The following discussion will be directed to the system acquisition by a UE  106  of a standalone NR cell. However, it will be appreciated that the system acquisition discussed herein is applicable to any suitable RAT. As used herein, the terms “system acquisition” and “cell acquisition” are used interchangeably. To acquire a cell, a UE  106  synchronizes itself to the symbol boundaries in the downlink transmissions from the base station  108  to the UE  106 . In addition, the UE  106  synchronizes itself as to the specific carrier frequency used by the base station  108  for the downlink communication. 
     Since the UE  106  does not know a priori what carrier frequency is being used by the base station within a given frequency band, it is conventional for the UE  106  to scan a frequency band according to the synchronization raster for the frequency band as defined by the 3GPP organization. The synchronization raster for a frequency band identifies the frequency positions of potential SSBs in the frequency band. These frequency positions vary from frequency band to frequency band and are identified by a frequency band&#39;s synchronization raster. In a standalone NR system, the UE  106  has no explicit signaling that identifies to the UE the frequency position of the SSBs from a base station  108 . In contrast, a UE  106  in a EUTRA-NR system is informed by the network such as through a RRC reconfiguration message the frequency used by the base station  108 . The UE  106  in a standalone NR system has only the synchronization raster and must thus scan for the SSBs to acquire a cell (synchronize with the base station&#39;s downlink channel). 
     There are at least two ways a UE  106  may scan for the SSBs. In a full frequency scan (FFS), the UE  106  scans each frequency within a frequency band as specified by the synchronization raster for the scanned frequency band. Alternatively, the UE  106  may perform a more limited scan (denoted as a list frequency scan) of certain frequencies within the scanned frequency band. A list frequency scan (LFS) is thus a subset of the frequencies specified in the synchronization raster for a given frequency band. The frequencies specified in an LFS scan would typically be known good rasters that the UE  106  has reason to believe would be utilized by base stations  108  in the vicinity of the UE. 
     The number of global synchronization channel number (GSCN) rasters within a synchronization raster may be rather large. For example, there are 341 candidate rasters in band n78 for a standalone NR system. The frequency scan time for NR may thus be substantial and impact the user experience. To provide improved cell acquisition times, an enhanced (interleaved) frequency scan technique is disclosed herein. The interleaving concerns the number of SSBs used or sampled by the UE  106  for a given candidate raster for the corresponding frequency band&#39;s synchronization raster. 
     The scan technique disclosed herein is applicable to single beam base stations  108  as well as multiple-beam base stations  108 . Regardless of the number of beams used by the base station  108 , each beam has a corresponding SSB that is repeated every 20 ms. In what is denoted herein as a deep scan, a UE  106  accumulates a signal quality parameter such as the signal-to-noise ratio (SNR) or a log likelihood ratio (LLR) for multiple SSBs in the same beam. Should a base station  108  have only one antenna beam, there would then be only one type of SSB that is repeated every 20 ms in the single beam. If a base station  108  has multiple antenna beams, the deep scan would be across multiple SSBs for each beam. A deep scan is useful in weak signal environments in which a detected SSB may actually be derived from noise and thus not correspond to an actual SSB. But due to the random nature of noise, it would be unlikely that such a false SSB detection would have approximately the same SNR across a series of such false SSBs. On the other hand, the SNR for a weak but actual SSB will tend to be the same across a series of such real SSBs. By adding the SNRs for such consecutive SSBs and comparing the sum to a suitable threshold, a UE  106  performing a deep scan may thus expect that the accumulated SNR would increase by 6 dB as compared to the SNR for each individual SSB. The sum (the accumulated SNRs) may then be compared to the suitable threshold. If the accumulated SNR exceeds the threshold, the detected SSBs are deemed to be legitimate and the UE  108  may then proceed with the synchronization so as to acquire the cell accordingly. 
     A second scan technique that may be interleaved with the deep scan is denoted herein as a shallow scan. In a shallow scan, the UE uses just one SSB (or a series of SSBs that is fewer than the series used in a deep scan) and makes a decision based upon its signal quality (e.g, its SNR) whether or not the SSB is trustworthy or not. 
     In both types of scans, the signal quality measurement for the SSB may be of the PSS, the SSS, the PHCH signal, or some sub-combination or combination of these signals. For example, the signal quality measurement may be of the demodulation reference signal (DMRS) in the PBCH in some embodiments. 
     The interleaving of the scans depends upon whether the UE is roaming or within a home NR network. This roaming may be to another service provider&#39;s NR network or other suitable RATs such as LTE. If the UE assumes it is in a home network, an interleaved frequency scan may be triggered at power-up or in response to a mobility failure. The interleaving may be 1 deep scan for every N scans. There are thus N-1 shallow scans that may be interleaved with every deep scan. For example, in one embodiment, N equals four. To keep track of the scans, the UE may then leverage an existing counter such as an existing non-access stratum (NAS) counter. Alternatively, a dedicated counter may be used for the interleaved frequency scans. The scanning begins after power-up of the UE  106  or after mobility failures such as a radio link failure (RLF) or an out-of-synchronization (OOS) failure. In one embodiment, the initial scan is a shallow scan so that if a received SSB is relatively-strong the UE  106  will acquire the corresponding cell quickly. If the SSB is not detected or is too weak to be deemed reliable, the subsequent scan is a deep scan so that the resulting integration of relatively-weak SSBs may still lead to a cell acquisition. The remaining scans in the series of N scans may then be shallow scans. In an embodiment in which N equals four, the resulting interleaving would be shallow, deep, shallow, and shallow for the four scans. It will be appreciated, however, that the number of deep scans that are interleaved with shallow scans in a series of N scans may be varied from one in alternative embodiments. Similarly, the positioning of the at least one deep scan in the series of N scans may be varied in alternative embodiments. In an embodiment in which N equals four and the deep scan positioning is the second scan, a radio resource control (RRC) layer in the UE may monitor a modulo-N counter (e.g., a modulo-4 counter) such as the NAS counter to determine the scan type such that the default scan is a shallow scan unless the count is 2. More generally, if the count equals a positive integer X, the RRC layer may command for a deep scan, where 1≤X≤N. For any other value of the count besides X, the RRC layer may command for a shallow scan. The appropriate scan (deep or shallow) may then be carried by a physical layer software element such as ML1. 
     The interleaved frequency scanning discussed herein occurs when a UE  106  is not camped on any cell such as following power-up of the UE or from a mobility failure. But there are frequency scans that may occur when the UE has acquired a cell. For example, a roaming UE  106  may conduct a scan for its home network. Since this scan occurs in the background while the UE  106  has acquired a cell in the roaming network, the resulting frequency scan for its home network may be denoted as a background public land mobile network (BPLMN) scan. For a BPLMN scan, the default scan may be dedicated solely to a shallow scan as finding a relatively-weak home network in a roaming scenario may not be beneficial. A manual public land mobile network (MPLMN) scan is a user-initiated search that is either initiated by the user or by an application in a UE  106  for non-roaming scenarios. For an MPLMN scan, the default scan may be a deep scan to allow for the acquisition of relatively-weak cells 
     Regardless of whether the scan is an interleaved frequency scan or a BPLMN/MPLMN scan, the selection of a shallow scan or a deep scan may be performed by the RRC layer. The RRC layer may then pass the scan type decision to a software layer in a UE  106  for its implementation. Some example scans will now be discussed beginning with  FIG. 4A , which illustrates an interleaved scan following power up of the UE  106 . The initial scan is a shallow scan as controlled by the NAS scan count being one. The shallow scan would extend across all the rasters for a full frequency scan but may be interleaved with list frequency scans as discussed earlier. Should the initial shallow scan be unsuccessful despite scanning all the rasters, the NAS count increments to two so that the second scan is a deep scan. The deep scan may also be a full frequency scan that is interleaved with list frequency scans. Note that both the shallow scan and the deep scans may also involve scans for alternative RATs. Should the deep scan be unsuccessful, the NAS counter increments to three so that another shallow scan in initiated. In this example, this additional shallow scan is successful so that the NAS counter is decremented to one and the UE  106  begins to camp on the acquired cell. 
     While the UE  106  is camped on the acquired cell, it may have a radio link failure (RLF) as shown in  FIG. 4B . A shallow scan is then initiated since the count equals 1. This initial shallow scan results in a connection (Conn) reestablishment. The UE  106  may then enter an idle mode so that the connection is released. A BPLMN scan may then be triggered. Note that the NAS counter is not incremented for the BPLMN scan, which may be a series of shallow scans as noted earlier. 
     The UE  106  may then move out of a coverage area during an idle state and enter an out-of-synchronization (OOS) state as shown in  FIG. 4C . An interleaved frequency scan then ensues. An initial scan is a shallow scan since the count equals one for this initial scan. If the scan is unsuccessful, the counter is incremented to equal two such that a deep scan is triggered. If the deep scan is unsuccessful, the counter is incremented to equal three so that a shallow scan is triggered. 
     An example user equipment  500  for the enhanced frequency scanning disclosed herein is shown in  FIG. 5 . UE  500  includes a processing system  514  having a bus interface  508 , a bus  502 , memory  505 , a processor  504 , and a computer-readable medium  506 . Furthermore, UE  500  may include a user interface  512  and a transceiver  510 . Transceiver  510  transmits and receives through an array of antennas  560 . 
     Processor  504  is also responsible for managing the bus  502  and general processing, including the execution of software stored on the computer-readable medium  506 . The software, when executed by the processor  504 , causes the UE  500  to perform the enhanced frequency scanning disclosed herein. The counter such as a NAS counter may be implemented by logic executed by processor  504 . The computer-readable medium  506  and the memory  505  may also be used for storing data that is manipulated by the processor  504  when executing software. 
     The bus  502  may include any number of interconnecting buses and bridges depending on the specific application of the processing system  514  and the overall design constraints. The bus  502  communicatively couples together various circuits including one or more processors (represented generally by the processor  504 ), the memory  505 , and computer-readable media (represented generally by the computer-readable medium  506 ). The bus  502  may also link various other circuits such as timing sources, peripherals, voltage regulators, and power management circuits, which are well known in the art, and therefore, will not be described any further. The bus interface  508  provides an interface between the bus  502  and the transceiver  510 . The transceiver  510  provides a communication interface or means for communicating with various other apparatus over a transmission medium. Depending upon the nature of the apparatus, a user interface  512  (e.g., keypad, display, speaker, microphone, joystick) may also be provided. 
     A method of frequency scanning will now be discussed with reference to the flowchart of  FIG. 6 . The method includes an act  600  of performing a first shallow scan at a user equipment (UE) over a frequency band by determining at each frequency of a synchronization raster for the frequency band whether a synchronization signal is received over a repetition period for the synchronization signal with a first signal quality to permit synchronization with downlink transmissions from a base station. The shallow scans discussed with regard to  FIG. 4A or 4C  are an example of act  600 . In addition, the method incudes an act  605  that occurs in response to the first shallow scan not being successful and includes performing a deep scan over the frequency band by determining at each frequency of the synchronization raster for the frequency band whether the synchronization signal is received over a series of repetition periods for the synchronization signal with a second signal quality to permit synchronization with the downlink transmissions from the base station. The deep scans discussed with regard to  FIG. 4A or 4C  are an example of act  605 . 
     The disclosure will now be summarized in a series of clauses: 
     Clause 1. A method of wireless communication, comprising: 
     performing a first shallow scan at a user equipment (UE) over a frequency band by determining at each frequency of a synchronization raster for the frequency band whether a synchronization signal is received over a repetition period for the synchronization signal with a first signal quality to permit synchronization a base station; and in response to the first shallow scan not being successful, performing a deep scan over the frequency band by determining at each frequency of the synchronization raster for the frequency band whether the synchronization signal is received over a series of repetition periods for the synchronization signal with a second signal quality to permit synchronization with the base station. 
     Clause 2. The method of clause 1, further comprising: 
     performing a second shallow scan at the user equipment by determining at each frequency of the synchronization raster whether the synchronization signal is received over the repetition period for the synchronization signal with the first sufficient signal quality to permit synchronization with the base station. 
     Clause 3. The method of clause 2, wherein the second shallow scan is responsive to the deep scan not being successful.
 
Clause 4. The method of clause 2, wherein the second shallow scan is subsequent to the first shallow scan, and wherein the deep scan is further responsive to the second shallow scan not being successful.
 
Clause 5. The method of clause 1, wherein the synchronization signal is a synchronization signal block (SSB) for a new radio (NR) system.
 
Clause 6. The method of any of clauses 1-5, wherein the first sufficient signal quality is a first signal-to-noise ratio, and wherein the second sufficient signal quality is a second signal-to-noise ratio.
 
Clause 7. The method of clause 6, wherein the first signal-to-noise ratio and the second signal-to-noise ratio are both measures of a primary synchronization signal (PSS) for the SSB.
 
Clause 8. The method of clause 6, wherein the first signal-to-noise ratio and the second signal-to-noise ratio are both measures of a secondary synchronization signal (SSS) for the SSB.
 
Clause 9. The method of clause 6, wherein the first signal-to-noise ratio and the second signal-to-noise ratio are both measures of a physical broadcast channel (PBCH) signal for the SSB.
 
Clause 10. The method of clause 9, wherein both measures are of a demodulation reference signal (DMRS) in the PBCH signal.
 
Clause 11. The method of any of clauses 1-10, further comprising:
 
     incrementing a count for the first shallow scan and for the deep scan, wherein the first shallow scan is responsive to the count having a first value and wherein the deep scan is further responsive to the count having a second value. 
     Clause 12. The method of clause 11, wherein incrementing the count comprises incrementing the count in a modulo-N counter.
 
Clause 13. The method of clause 12, wherein the modulo-N counter is a modulo-4 counter.
 
Clause 14. The method of clause 13, wherein the deep scan is further responsive to the count for the modulo-4 counter equaling two.
 
Clause 15. A user equipment (UE), comprising:
 
     a transceiver configured to: 
     perform a first shallow scan over a frequency band by a determination at each frequency of a synchronization raster for the frequency band of whether a synchronization signal is received over a repetition period for the synchronization signal with a first signal quality sufficient to permit synchronization with downlink transmissions from a base station; and 
     in response to the first shallow scan not being successful, perform a deep scan over the frequency band by a determination at each frequency of the synchronization raster of whether the synchronization signal is received over a series of repetition periods for the synchronization signal with a second signal quality sufficient to permit synchronization with the downlink transmissions from the base station. 
     Clause 16. The UE of clause 15, wherein the transceiver is further configured to: 
     perform a second shallow scan over the frequency band by a determination at each frequency of the synchronization raster of whether the synchronization signal is received over a repetition period for the synchronization signal with the first sufficient signal quality to permit the synchronization with the downlink transmissions from the base station. 
     Clause 17. The UE of clause 16, wherein the transceiver is further configured so that the second shallow scan is responsive to the deep scan not being successful.
 
Clause 18. The UE of clause 16, wherein the transceiver is further configured so that the second shallow scan is subsequent to the first shallow scan, and so that the deep scan is further responsive to the second shallow scan not being successful.
 
Clause 19. The UE of any of clauses 15-18, wherein the synchronization signal is a synchronization signal block (SSB) for a new radio (NR) system.
 
Clause 20. The UE of any of clauses 15-19, wherein the first signal quality is a first signal-to-noise ratio, and wherein the second signal quality is a second signal-to-noise ratio.
 
Clause 21. The UE of any of clauses 15-20, wherein the transceiver is further configured to:
 
     increment a count in a counter for the first shallow scan and again for the deep scan, wherein the first shallow scan is responsive to the count having a first value and wherein the deep scan is further responsive to the count having a second value. 
     Clause 22. The UE of clause 21, wherein the counter in a modulo-N counter.
 
Clause 23. The UE of clause 22, wherein the modulo-N counter is a modulo-4 counter.
 
Clause 24. The UE of clause 23, wherein the transceiver is further configured so that the deep scan is further responsive to the count for the modulo-4 counter equaling two.
 
Clause 25. A method of wireless communication, comprising:
 
     incrementing a count; 
     responsive to the count being equal to a first integer, accumulating a received signal quality over multiple synchronization signal periods at each frequency of a synchronization raster to form an accumulated signal quality for each frequency; 
     determining if the accumulated signal quality for each frequency exceeds a first threshold to determine whether a synchronization signal is successfully detected at the frequency; 
     responsive to the count not being equal to the first integer, determining a received signal quality for an additional synchronization period at each frequency of a synchronization raster to form a received signal quality for each frequency; and 
     determining if the received signal quality for each frequency exceeds a second threshold to determine whether the synchronization signal is successfully detected at the frequency. 
     Clause 26. The method of clause 25, wherein incrementing the count comprises incrementing the count in a modulo-N counter.
 
Clause 27. The method of clause 26, wherein incrementing the count comprises incrementing the count in a the modulo-4 counter.
 
Clause 28. The method of any of clauses 25-27, wherein the first integer is two.
 
Clause 29. The method of any of clauses 25-28, wherein the synchronization signal is a synchronization signal block (SSB) for a NR radio system.
 
Clause 30. The method of any of clauses 25-29, wherein the accumulated signal quality and the received signal quality each comprises a signal-to-noise ratio.
 
     The functions described herein may be implemented in hardware, software executed by a processor, firmware, or any combination thereof. If implemented in software executed by a processor, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Other examples and implementations are within the scope of the disclosure and appended claims. For example, due to the nature of software, functions described above can be implemented using software executed by a processor, hardware, firmware, hardwiring, or combinations of any of these. Features implementing functions may also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations. Also, as used herein, including in the claims, “or” as used in a list of items (for example, a list of items prefaced by a phrase such as “at least one of” or “one or more of”) indicates an inclusive list such that, for example, a list of [at least one of A, B, or C] means A or B or C or AB or AC or BC or ABC (i.e., A and B and C). 
     As those of some skill in this art will by now appreciate and depending on the particular application at hand, many modifications, substitutions and variations can be made in and to the materials, apparatus, configurations and methods of use of the devices of the present disclosure without departing from the spirit and scope thereof In light of this, the scope of the present disclosure should not be limited to that of the particular embodiments illustrated and described herein, as they are merely by way of some examples thereof, but rather, should be fully commensurate with that of the claims appended hereafter and their functional equivalents.