Abstract:
Selective deciphering of a received signal, as taught herein, provides a number of advantages, including greater efficiency through the elimination or at least reduction of wasted decoding cycles. The technique, such as practiced in a user equipment or other wireless communication device, capitalizes on the advantageous recognition herein that the demodulation results obtained for at least some types of received data blocks may be inspected or otherwise evaluated for characteristic patterns that are indicative of whether the data block was or was not ciphered for transmission. That evaluation informs the selective deciphering decision.

Description:
TECHNICAL FIELD 
       [0001]    The present invention generally relates to communication systems, and particularly relates to selective deciphering of received communications signals. 
       BACKGROUND 
       [0002]    The use of ciphering to transmit at least some of the information conveyed over the air interfaces in wireless communication networks is well known. For example, networks based on the Global System for Mobile communications (GSM) support a number of different encryption techniques, known as “A5/1”, “A5/3” and “A5/4”, to cipher data at Layer 1 on the radio interface. Such operations are described in the Third Generation Partnership Project (3GPP) Technical Specification identified as TS 43.020 V9.1.0 (2009-12-18), which is incorporated by reference herein. Among the various encryption techniques, A5/1 encryption is the most commonly used encryption technique for GSM, and support for A5/1 has been mandatory for all GSM mobile devices since GSM Release-1999. 
         [0003]    However, it was realized that certain aspects of overall GSM network operation resulted in security risks arising from the transmission of ciphered text. For example, according to the GSM specifications, certain information was transmitted as ciphered text and also as unciphered or “clear” text. 
         [0004]    The document 3GPP GERAN#47 (GP-101243, GP-101242) identifies a particular instance of this problem. In this regard, it may be helpful to note that GSM base stations continuously forward information about their current system configuration and other information needed by wireless devices before such devices are allowed to access the network. These various items of information are organized into six different system information (SI) words containing specific parameters. For example, Type 1 to Type 4 SI words are transmitted within the BCCH (broadcast control channel). Type 5 and Type 6 SI words are only transmitted during an established individual radio link in the downlink direction in a multiplexed service channel—i.e., the Slow Associated Control Channel (SACCH). 
         [0005]    As regards the security problem identified in 3GPP GERAN#47 (GP-101243, GP-101242), it is noted that certain SI is sent in ciphered form on the SACCH and that the encryption key used for ciphering on the SACCH is the same key used for encrypting data on the associated traffic channel carrying voice or data to a network user. The same SI is sent as clear text on a broadcast channel, and thus can be received by any GSM receiver within range of the transmission. 
         [0006]    Consequently, an eavesdropper could receive the ciphered transmission on the SACCH and receive the unciphered transmission on the broadcast channel. Having both transmissions and knowing that the system information in the ciphered transmission was identical to that in the unciphered transmission, the eavesdropper could work backward mathematically to derive the encryption key used for ciphering. The encryption key would then allow the eavesdropper to intercept and decrypt user traffic being sent on the given traffic channel. 
         [0007]    Various parties proposed solutions to the above security problem. See, for example, U.S. Pat. No. 8,165,618 and U.S. Patent Pub. 2012/0213373 A1. However, the solution adopted for Release 10 of the 3GPP specifications is based on “selective” ciphering. The Third Generation Partnership Project (3GPP) Technical Specification (TS) detailing the selective ciphering solution is TS 44.018 V11.1.0 (2012-05), which is incorporated herein by reference. 
         [0008]    In particular, Section 3.4.7a of 3GPP TS 44.018 stipulates that a GSM network may implement selective ciphering. If selective ciphering is implemented, the network will not cipher a SACCH data block—e.g., a block of data comprising a SACCH signaling message—if the SACCH data block contains any of the following Radio Resource (RR) messages: SI Type 5, SI Type 5bis, SI Type 5ter, SI Type 6, SI Type 14, Measurement Information, or an Extended Measurement Order. Conversely, the network will cipher the SACCH data block if it contains service access point identifier (SAPI) value 3 data (indicating Short Messaging Service or SMS data)—see 3GPP TS 44.006. Optionally, the SACCH data block is not ciphered if it carries a CP-ACK message—see 3GPP TS 24.011—which is used to acknowledge reception of a CP-DATA message. 
         [0009]    Unfortunately, the receiving mobile station or other wireless device does not know in advance which type of SACCH data block is being received. Consequently, the wireless device does not know whether the received SACCH data block is ciphered or unciphered. 3GPP TS 44.018 proposes a solution for resolving such ambiguity at the user. The wireless device initially assumes that the received SACCH data block is deciphered, and then repeats decoding without deciphering if the initial decoding fails. However, this solution consumes significant processing resources, which increases power consumption and leaves less time for other tasks, such as making signal quality measurements, repeated message combining, etc. 
       SUMMARY 
       [0010]    An example method of received signal processing at a wireless communication device includes demodulating a received communication signal and thereby obtaining soft bits as demodulation results. The method continues with comparing the demodulation results to a characteristic pattern that is indicative of the communication signal not being ciphered (encrypted) for transmission, and controlling whether the demodulation results are decoded with or without deciphering, based on said step of comparing. 
         [0011]    A corresponding example device includes a wireless communication interface that is configured to receive communication signals transmitted from a wireless communication network, and one or more processing circuits that are associated with the wireless communication interface. The one or more processing circuits, which are simply referred to as “processing circuits” for convenience, are configured to demodulate a communication signal received through the wireless communication interface, and thereby obtain soft bits as demodulation results. The processing circuits are further configured to compare the demodulation results to a characteristic pattern that is indicative of the communication signal not being ciphered for transmission, and to control whether the demodulation results are decoded with or without deciphering, based on said step of comparing. 
         [0012]    Selective deciphering of a received signal, as taught herein, provides a number of advantages, including greater efficiency through the elimination or at least reduction of wasted decoding cycles. The technique, such as practiced in a user equipment or a mobile station or other wireless communication device, capitalizes on the advantageous recognition herein that the demodulation results obtained for at least some types of received data blocks may be inspected or otherwise evaluated for characteristic patterns that are indicative of whether the data block was or was not ciphered for transmission. That evaluation informs the selective deciphering decision. 
         [0013]    Of course, the present invention is not limited to the above features and advantages. Indeed, those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a partial block diagram of one embodiment of a wireless communication network. 
           [0015]      FIG. 2  is a diagram of a known signal structure for a Slow Associated Control Channel (SACCH) message, as transmitted in Global System for Mobile communications (GSM) networks. 
           [0016]      FIG. 3  is a diagram of known fixed data, such as may be included in a SACCH data block conveying System Information. 
           [0017]      FIG. 4  is a block diagram of a wireless communication device, such as may be used in the network of  FIG. 1 . 
           [0018]      FIG. 5  is a logic flow diagram of one embodiment of a method at a wireless communication device of selectively deciphering a received communication signal before decoding, according to an example of the teachings herein. 
           [0019]      FIG. 6  is a logic flow diagram showing one embodiment of known network-side transmit processing in correspondence with another example of the device-side received signal processing taught herein. 
           [0020]      FIG. 7  is a logic flow diagram showing further details for a known network-side transmit processing in correspondence with another example of the device-side received signal processing taught herein. 
           [0021]      FIG. 8  is a diagram of example performance results obtained for example cases of the selective deciphering taught herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  provides a simplified, partial illustration of a wireless communication network  10  that provides communication services within a cell  12 , via a base station  14 . While not intended as a limiting definition, the cell  12  may be regarded as the allocation or use of certain defined air interface (radio) resources within a certain coverage area. Further, the term “base station” should be given broad constructions, and it generally connotes a communication network node operating as a radio frequency transceiver. 
         [0023]    In a non-limiting example, the wireless communication network  10  (hereafter “network  10 ”) comprises a GSM network, and the network  10  may comprise many cells  12  and corresponding base stations  14 . Of course, the network  10  may comprise another type of network, such as a Wideband CDMA network or a Long Term Evolution (LTE) network, and may further comprise other entities not illustrated—such as Core Network (CN) entities for device authorization, mobility management, and gateway services for communicating with devices or systems in other networks. 
         [0024]    The depicted base station  14  transmits downlink signals  16 , which are received by a wireless communication device  18 . The base station  14  or the network  10  in general is configured to use “selective ciphering” for transmitting one or more types of the downlink signals  16 . Thus, a given signal received by the wireless communication device  18  (hereafter “device  18 ”) may or may not be ciphered. In a non-limiting example, the signals in question are Slow Associated Control Channel or SACCH transmissions. 
         [0025]    Certain SACCH data blocks are transmitted without ciphering, e.g., the following system information or SI messages are not ciphered: Type 5, SI Type 5bis, SI Type 5ter, SI Type 6, SI Type 14, Measurement Information, or an Extended Measurement Order. Other types of SI messages are ciphered. Thus, in this example case, the device  18  receives some SI messages that are ciphered and some SI messages that are not ciphered, and does not know in advance which case applies. However, the device  18  includes one or more processing circuits  20  (hereafter “processing circuits  20 ”) that are configured to perform selective deciphering as taught herein. With selective deciphering, the device  18  intelligently decides whether to perform decoding with deciphering or decoding without deciphering, based on evaluating the demodulation results obtained for the received signal in question. The signal may be a SACCH data block in a GSM embodiment, or another type of selectively-ciphered signal in embodiments involving other network types. 
         [0026]    Here, “decoding with deciphering” will be understood to mean that the decoder operates on the demodulation results after deciphering processing has been applied to them. Correspondingly, “decoding without deciphering” means that the decoder operates on the demodulation results without deciphering processing having been applied to them, at least with respect to their use by the decoder. 
         [0027]    Using the GSM embodiment to further this example discussion,  FIG. 2  illustrates the block structure and content defined for 3GPP-compliant SACCH signaling message blocks. The downlink SACCH message generally contains 21 bytes of higher-layer information bits for Layer 2/Layer 3 (L2/L3) signaling, and then 2 bytes of header information added to it in Layer 1, for inserting the L1 specific information like Power control, Timing advance etc. However, the L2/3 information part varies, based on whether the SACCH is carrying SI or carrying SMS data. SMS data always will be transmitted with ciphering enabled, while SI may be transmitted using selective ciphering. The address field format of 8 bits (one octet), as defined in 3GPP TS 44.006 section 6.2, is shown at the bottom of  FIG. 2 . It includes Link Protocol Discriminator (LPD) bits, Command/response bit, Address field Extension (EA) bit and three SAPI bits that correspond to a SAPI field. If the 3 bits of the SAPI field are 000, it corresponds to SAPI value 0 and if the three bits are 011 it corresponds to SAPI value 3. 
         [0028]    As defined in Section 6.3.3 of 3GPP TS 44.006, SMS is transmitted using service access point identifier (SAPI) value 3 (SAPI-3) and SI using SAPI value 0 (SAPI-0). The SAPI identifies a point at which data link layer services are provided by the data link layer to a layer 3 entity. SAPI value 0 is allocated for Call control signaling, mobility management signaling and radio resource management signaling including system information (SI), and SAPI value 3 is allocated for Short message service. 
         [0029]    At the base station  14 , 184 bits are input to the Physical (PHY) layer for physical layer processing. The 184 bits are encoded for transmission—where this encoding should not be confused with the ciphering at issue herein. In the context of physical layer encoding, 40 parity bits (generated using a binary cyclic code or a FIRE code) are added and 4 tail bits (all 4 bits are set to 0) are appended, for a total of 228 bits. These bits are then passed to a ½ rate convolution encoder, to generate 456 bits of encoded SACCH data, as described in Section 4.1.1 of 3GPP TS 45.003. Similarly, if the network supports enhanced the power control feature, the processing of SACCH data will be as described in section 4.11 (SACCH/TP) of 3GPP TS 45.003. 
         [0030]    The above block is then interleaved (channelized for transmission) and ciphered if the block does not contain SI. If the block contains SI, it is not ciphered. Thus, the base station  14  can be understood as selectively ciphering the SACCH data block in question, based on whether the SACCH data block does or does not contain SI (or based on the type of SI contained in the block). Then 8 stealing bits are added, which are constant, resulting in a total of 464 bits for a single SACCH data block. The base station  14  forms four bursts using these 464 generated bits, which are then modulated and transmitted as downlink signals  16  to the device  18 . 
         [0031]    In the GSM example case, the device  18  receives these SACCH bursts and demodulates them to obtain demodulation results comprising 464 soft bits, corresponding to the 464 bits of the received SACCH data block. That is, there are 116 bits received per transmitted burst, and at total of four burst per transmitted SACCH data block. Note, too, as a non-limiting example, a “soft bit” here comprises a Log Likelihood Ratio (LLR) or other such value indicating the probability that the corresponding received bit was a “0” or a “1”. Those skilled in the wireless communication arts will immediately understand the generation of soft bits based on received signal demodulation at the device  18 . 
         [0032]    Now, the positions of the encoded and interleaved bits comprising the SAPI-0 data sequence are known a priori to the device  18 . In particular,  FIG. 3  illustrates the fixed pattern in the SACCH data block received at the device  18  for the SACCH input data string including the 2-bytes of the L1 header] as “-0-00-011-11-XXX-X-XXX-0- . . . ” for the SMS case, (Spare bit: 0, LPD: 00, SAPI: 011, C/R: 1, EA: 1) and the 2-bytes of the L1 header as “-0-00-000-11-000X-00-11- . . . ” for the SI case (Spare bit: 0, LPD: 00, SAPI: 000, C/R: 1, EA: 1). Hence, the bit values in the SAPI field of 011 corresponds to SAPI value 3 and the SAPI bits 000 corresponds to SAPI value 0 as discussed in relation to  FIG. 2 . This fixed bit pattern after encoding will also result in another fixed bit pattern, and this remains true whether or not the network supports the power control feature (SACCH/TP). 
         [0033]    These 15 bits—excluding the don&#39;t care bits—will appear in the demodulation results at known interleaving patterns and, because their values are known and fixed, they represent a characteristic pattern that will be present in the demodulation results obtained from any given SACCH data block received and demodulated at the device  18 , for the case where the SACCH data block is an un-ciphered (not ciphered) SI message. Conversely, demodulation of a ciphered SACCH data block, e.g., a SACCH data block conveying SMS data, will not exhibit this characteristic pattern. In other words, the bit positions known at the device  18  representing the interleaved and encoded SAPI-0 data field will not exhibit the characteristic pattern known for such data and instead will be random. 
         [0034]    With this in mind, in an example configuration, the device  18  is configured to evaluate the demodulation results obtained for a received SACCH data block at bit positions: 44, 130, 144, 197, 244, 297, 346, 395 and 446, within the 464 bit positions of the received SACCH data block. “Evaluation” here means determining whether the soft bit values at those locations match the expected values of the SAPI-0 data field. Here, “match” may connote a perfect, one-for-one matching, or may connote a degree or extent of matching, in which a match is declared if a certain number or percentage of the bit positions include bit soft values that are in agreement with the known values for those positions. 
         [0035]    In this regard, and as will be further detailed, the device  18  may be configured to correlate certain ones of the soft bit values in a given set of demodulation results with a set of bit representing the characteristic pattern. If the correlation level meets at least a defined threshold level, the device  18  declares a match, meaning that the device  18  decides based on the correlation level as to whether the received SACCH data block comprises ciphered or un-ciphered data. Advantageously, if the device  18  decides that the SACCH data block comprises un-ciphered data, it decodes the demodulation results without deciphering—i.e., it does not apply deciphering processing to the demodulation results fed to the decoding processing. Such operation avoids the decoding failure that will invariably arise from decoding received data that has been deciphered after demodulation, when such data in fact was not ciphered by the base station  14  for transmission. 
         [0036]    So, if a network does not support the “selective ciphering” feature, then the SACCH data block will always be ciphered, irrespective of whether it contain SI or SMS. In such case, deciphering will always be needed in the receiver side. The fixed bit pattern in the transmitter side will go through the ciphering unit and so, from the perspective of the receiver, the fixed data will be random and will not match with the soft-bit pattern template stored in the receiver side. As such, the de-ciphering unit will be invoked, according to the decision logic taught herein. 
         [0037]      FIG. 4  depicts an example base station  14  and an example device  18 , which are configured for GSM operation in at least one embodiment, but which additionally or alternatively may be configured for other operation, such as LTE, WCDMA, etc. The depicted base station  14  includes one or more transmit/receive (TX/RX) antennas  30 , a wireless communication interface  32 —e.g., radiofrequency circuitry for modulation of signals for transmission, demodulation of received signals, frequency conversion, amplification, etc. 
         [0038]    The base station  14  further includes control and processing circuits  34 , which are operatively associated with the wireless communication interface  32  and which are configured for overall transmit signal generation, signaling control, received signal processing, etc., and which implement various signaling and control protocols. The control and processing circuits  34  are further associated with one or more network communication interfaces  36 , e.g., for communicating with other base stations  14  and/or for communicating with various Core Network (CN) nodes, which are not illustrated in the diagram. 
         [0039]    In general, the details of the base station  14  are not important with respect to the teachings herein, other than to note that the base station  14  transmits at least some downlink signals  16  using selective ciphering—e.g., given SACCH data block transmissions are or are not ciphered in dependence on the type of data being transmitted. Correspondingly, the device  18  includes the earlier-noted processing circuits  20 , which provide selective deciphering functionality. 
         [0040]    In terms of the overall depiction, which should be understood as a non-limiting example, the device  18  in  FIG. 4  includes one or more TX/RX antennas  40 , a wireless communication interface  42 —e.g., a cellular radio transceiver—and one or more baseband RX/TX processing circuits  44 , including a number of processing units representing one example of the device&#39;s processing circuits  20  shown in  FIG. 1 . While such circuits generally should be understood as providing transmit and receive signal processing and overall communication control and protocol processing with respect to the radio link between the device  18  and the network  10 , only certain functional circuits that are particularly helpful in this discussion are illustrated. 
         [0041]    In this regard, the baseband RX/TX processing circuits  44  are illustrated as receiving a received communication signal  46  from the wireless communication interface  42 . The received communication signal  46  corresponds to antenna-received downlink signals  16 , but may comprise a stream of digital samples obtained by filtering, amplifying, and down-converting the antenna-received downlink signals  16 . Further, it will be understood that signal buffering is used within the device  18 , e.g., the received communication signal  46  may be buffered in working memory for processing, and it will be understood that such buffering may be used for any one or more of the intermediate processing results obtained within the overall received signal processing chain represented by the baseband RX/TX processing circuits  44 . 
         [0042]    As regards those circuits,  FIG. 4  depicts a demodulation unit  48 , which produces demodulation results  50  from the received communication signal  46 . These demodulation results  50  are decoded by a decoding unit  52 , either with deciphering or without deciphering according to the selective deciphering method taught herein. Correspondingly, for decoding with deciphering, a deciphering unit  54  provides the decoding unit  52  with the demodulation results  50  after applying deciphering processing to them. For decoding without deciphering, the deciphering unit  54  passes the demodulation results  50  to the decoding unit  52  without first deciphering them. Equivalently, the decoding without deciphering may be understood as bypassing the deciphering unit  54 , with the decoding unit  52  reading the demodulation results  50  from whatever buffer or store they are placed in by the demodulation unit  48 . 
         [0043]    Control for implementing the decision to decode with deciphering or without deciphering is provided by a deciphering control unit  56 , which receiving characteristic pattern matching information from a pattern evaluation unit  58 . In turn, the pattern evaluation unit  58  has access to a memory  60  or other store holding information representing a characteristic pattern  62  that is indicative of the received communication signal  46  not being ciphered by the base station  14  for transmission. The pattern evaluation unit  58  thus evaluates the demodulation results  50  with respect to the characteristic pattern  62  and provides an indication to the deciphering control unit  56  as to the extent or level of correlation between certain soft bit values in the demodulation results  50  and a set of bits comprising the characteristic pattern  62 , which may comprise a set of bits representing known or fixed information that will be at known or fixed positions within the received communication signal  46  if ciphering was not applied to the received communication signal  46 . 
         [0044]      FIG. 4  also depicts the decoding unit  52  as producing decoding results  64 , which are provided to additional processing circuitry  70 . The additional processing circuitry  70  may include application-level processing circuitry, and may include one or more user interfaces, various input/output circuitry, etc., depending on the intended use of the device  18 . In any case, it should be noted that the decoding results  64  may represent an initial decoding of the demodulation results  50  with or without deciphering, if the initial decoding is successful, or may represent the results of a subsequent, repeated decoding, if the initial decoding was not successful. 
         [0045]    As for deciding whether to perform decoding at least initially with or without deciphering the demodulation results  50 , in an example configuration, a given soft bit value will be considered to match its corresponding bit in the characteristic pattern  62  if, e.g., the sign of the soft bit value represents the correct pattern bit value. Evaluation optionally may further require that the magnitude of the soft bit value be above a defined confidence level. 
         [0046]    Alternatively, the set of bits comprising the characteristic pattern  62  can be converted into the soft domain, for comparison with corresponding soft bits in the demodulation results  50 . Further, the comparison may be made by making hard bit decisions on the certain soft bits in the demodulation results  50  and then comparing them with the corresponding bits in the set of bits representing the characteristic pattern  62 . 
         [0047]    As a further note, the term “unit” as used herein, such as “demodulation unit  48 ,” “decoding unit  52 ,” and so on, should be understood as denoting at least a functional circuit arrangement. For example, the various illustrated units may be implemented using fixed hardware circuits, or implemented in programmable processing circuits, or some combination of both. 
         [0048]    Broadly, the device  18  comprises the aforementioned wireless communication interface which is configured to receive communication signals transmitted from the network  10 , including selectively ciphered downlink signals  16 . The device  18  further includes the baseband RX/TX processing circuits  44 , which may be generally referred herein to as “one or more processing circuits  44 ,” and which are configured to: demodulate a communication signal  46  received through the wireless communication interface  42 , and thereby obtain soft bits as demodulation results  50 ; compare the demodulation results  50  to a characteristic pattern  62  that is indicative of the received communication signal  46  not being ciphered for transmission; and control whether the demodulation results  50  are decoded with or without deciphering, based on the comparison of the demodulation results  50  to the characteristic pattern  62 . 
         [0049]    In some embodiments, the one or more processing circuits  44  are configured to control whether the demodulation results  50  are decoded with or without deciphering, based on being configured to: decode the demodulation results  50  without deciphering, when there is at least a threshold correlation between certain soft bits in the demodulation results  50  and a set of bit values comprising the characteristic pattern  62 ; and otherwise decode the demodulation results  50  with deciphering. Further, in at least one embodiment, the one or more processing circuits  44  are configured to decode the demodulation results  50  with deciphering, if decoding the demodulation results  50  without deciphering fails. 
         [0050]    Further, in at least some embodiments, the one or more processing circuits  44  are configured to obtain the characteristic pattern  62  by demodulating an earlier-received communication signal and saving soft bit values obtained therefrom. For example, the device  18  may receive certain data which it knows is received in un-ciphered form, and certain bits from that un-ciphered data can be saved or otherwise remembered as the characteristic pattern  62 , to be used for later comparison with the demodulation results  50  obtained for a subsequent reception that may comprise the same data, or at least the same fixed information, at least for the case where the subsequent transmission is un-ciphered. 
         [0051]    Additionally, or alternatively, the one or more processing circuits  44  are configured to obtain the characteristic pattern  62  from configuration data stored in the device  18 . For example, the device  18  may be configured to use as its characteristic pattern  62  the fixed bit pattern used for the SAPI-0 data field for the transmission of a SI message in the GSM example case. More broadly, the one or more processing circuits  44  are configured to compare the demodulation results  50  to the characteristic pattern  62 , based on being configured to compare the demodulation results  50  to a set of bit values corresponding to fixed or known data in the received communication signal  46 . Thus, for the GSM example case, the received communication signal  46  comprises, for example, a received SACCH data block and the characteristic pattern  62  comprises one of: a characteristic pattern of bits that is known for a SAPI  0  field of SACCH data blocks that SI; or a characteristic pattern of bits that is known from a like SACCH data block earlier received at the device  18 . 
         [0052]      FIG. 5  depicts an example method  500  that is consistent with the above-described device configuration of  FIG. 4 . The method  500  can be used in essentially any instance where a received signal may or may not comprise ciphered information, and where, for the unciphered case, there will be a characteristic pattern  62  of some sort exhibited by the demodulation results  50  obtained for the received communication signal  46 . 
         [0053]    The example method  500  thus can be understood as depicting advantageous received signal processing at the device  18 , and it includes demodulating a received communication signal  46  and thereby obtaining soft bits as demodulation results  50  (Block  502 ), comparing the demodulation results  50  to a characteristic pattern  62  that is indicative of the communication signal  46  not being ciphered for transmission (Block  504 ); and controlling whether the demodulation results  50  are decoded with or without deciphering (Block  506 ), based on the step of comparing performed in Block  504 . 
         [0054]    The comparing step in Block  504  may comprise determining whether there is a matching between certain soft bits in the demodulation results  50  and a set of bits comprising the characteristic pattern  62 . In some embodiments, for example, such matching determination comprises determining an extent or level of correlation between certain soft bit values in the demodulation results  50  and a set of bits comprising the characteristic pattern  62 , based on correlating respective ones of the certain soft bits in the demodulation results  50  with corresponding ones of the bits in the characteristic pattern  62  (Block  504 A). In the earlier example, certain bit positions in the demodulation results  50  correspond with fixed or known bit values; thus, the soft bits in those certain bit positions can be compared to see how well they match, e.g., in a collective or overall sense, with the known bit values comprising the characteristic pattern  62  (Block  504 B, where “YES” denotes at least a threshold level of correlation). 
         [0055]    If the demodulation results  50  exhibit the characteristic pattern  62  (YES from Block  504 B), deciphering is “Off” (Block  504 C) meaning that the demodulation results  50  are provided for decoding processing without first applying deciphering processing to them. If the demodulation results  50  do not exhibit the characteristic pattern  62  (NO from Block  504 B), deciphering is “On” (Block  504 D), meaning that the demodulation results  50  are provided for decoding processing after first applying deciphering processing to them. Thus, the method  500  controls whether the demodulation results  50  are decoded with or without deciphering by controlling whether the demodulation results  50  are input to a decoding unit  52  with or without processing by a deciphering unit  54 . 
         [0056]      FIG. 6  provides a further example of selective ciphering at the transmitter side and selective deciphering at the receiver side of a network  10  of  FIG. 1 , where the method  600 A comprises steps or actions  602 A,  604 A,  606 A,  608 A and  610 A, occurring at the base station  14  or other such network node transmitter. The method  600 B comprises steps or actions  602 B,  604 B and  606 B, occurring at the device  18 . 
         [0057]    The method  600 A represents known processing on the network node side, whereas the method  600 B can be understood as a further, albeit partial, embodiment of the method  500  of  FIG. 5  of selective deciphering at the device  18 . The method  600 B includes demodulating the received communication signal  46  (Block  602 B), and checking the soft bits at different positions in the demodulation results  50 , which are known to correspond to fixed or known data (Block  604 B). If the soft bits at those positions match the expected bit values comprising the characteristic pattern  62 , then generate an indication of such matching (Block  606 B), e.g., generate a flag or signal to skip deciphering before decoding the demodulation results  50 . 
         [0058]      FIG. 7  presents further example details for a transmit-side method  700 A and a receive-side method  700 B, which again can be considered as comprising example details for the method  500  introduced in  FIG. 5 . 
         [0059]    Transmit-side (network node-side) processing is illustrated as comprising known steps or actions  702 A- 714 A. Of interest, one sees selective ciphering performed in Steps  706 A and  708 A, wherein the SACCH data block is not ciphered if it contains SI information. 
         [0060]    On the receive-side (device-side), the method  700 B includes receiving the transmitted signal (Block  702 B) and performing initial radiofrequency front-end processing (Block  704 B) to obtain the earlier-discussed communication signal  46 , which may comprise in-phase (I) and quadrature (Q) components in a stream of digitized waveform samples. 
         [0061]    Processing further includes channel estimation (Block  706 B) and demodulation ( 708 B), which may comprise equalization processing, as is known in the wireless communication arts. Such processing produces soft bits comprising the demodulation results  50 , which are then evaluated against expected soft bit values at know positions or locations within the demodulation results  50  (Block  710 B), to see whether SI is or is not detected. Such processing can be understood as a specific example of determining whether the demodulation results  50  exhibit a characteristic pattern  62  that is indicative of the communication signal  46  not being ciphered for transmission. More particularly, detecting SI in this case can be understood as detecting the fixed data known for SACCH data blocks that carry SI. 
         [0062]    If SI is detected for the demodulation results  50 , processing continues with decoding without deciphering (Block  712 B), which includes de-interleaving the demodulation results  50 , decoding the de-interleaved demodulation results  50 , and then checking whether decoding is successful, e.g., by performing a Cyclic Redundancy Check (CRC) on the decoding results. If the CRC check fails, the one or more processing circuits  44  assume that deciphering should have been applied to the demodulation results despite those results exhibiting the characteristic pattern  62 . Thus, the one or more processing circuits  44  go back and decipher the demodulation results  50 , then de-interleave the deciphered demodulation results  50 , and then decode the deciphered, de-interleaved demodulation results  50 . 
         [0063]    If SI is not detected, processing continues from Block  710 B with the processing of Block  714 B, which includes deciphering the demodulation results  50 , de-interleaving the deciphered demodulation results  50 , and then decoding the deciphered, de-interleaved demodulation results  50 . Here, if decoding fails, the device  18  assumes that deciphering should not have been applied to the demodulation results  50  for decoding, even though SI was not detected—i.e., even though the one or more processing circuits  44  did not detect the characteristic pattern  62  in the demodulation results  50 . Thus, the one or more processing circuits  44  de-interleave the demodulation results  50 , while skipping deciphering, and decode the de-interleaved demodulation results  50 . 
         [0064]    In a sense then, one or more embodiments of the device  18  implement processing that allows the device  18  to recover from an incorrect decision as to whether the communication signal  46  was or was not ciphered. If the device  18  determines that the communication signal  46  was not ciphered and decodes without deciphering, it will repeat decoding with deciphering if the initial decoding without deciphering fails. Conversely, if the device  18  determines that the communication signal  46  was ciphered and decodes with deciphering, it will repeat decoding without deciphering if the initial decoding with deciphering fails. 
         [0065]    However, the ciphered/not-ciphered decision-making taught herein can easily be configured to be reliable. See, for example, the table shown in  FIG. 8 . Consider only three bits of SAPI-0 and SAPI-3 identification—i.e., taking first two bytes of the L2/3 message information in a SACCH data block conveying SI. This approach leads to nine soft bit positions out of the total 464 bits positions when the demodulation results  50  are for a received SI message. 
         [0066]    The illustrated table in  FIG. 8  indicates that all nine soft bit sign values match 100% of the time for any GSM SI message of Type 5, 5ter, 5bis, or 6, for a static-type propagation channel and a Received Signal Strength Indicator (RSSI) of −105 dBm (SINR=6 dB). Even for RSSI=−110 dBm (SINR=1 dB), the soft bit sign values will match the expected pattern 90% of the time. 
         [0067]    As noted, the expected values comprising the characteristic pattern  62  may comprise known or fixed data used in certain types of transmissions. Further, the device  18  can be configured to store the entire soft bit pattern from decoding earlier-received SI, and use that stored information for evaluating a later-received SI message. The comparison value, e.g., a soft-bit sign correlation value, is checked against a threshold value, and decision about the match is made. If the demodulation results  50  of the subsequent message match the stored soft bit information, the device  18  decides that the subsequent message also includes SI information. The threshold may be set dynamically, e.g., using the incidence of decoding failures to raise or lower the threshold. Alternatively, the threshold may be fixed, e.g., to require an 80% match or a 90% match. Additionally, the threshold may default to a starting value and the device  18  then adjusts the threshold up or down, e.g., based on the frequency or occurrence of decoding failures when the device  18  decides to initially decode without deciphering. 
         [0068]    Also, as noted, the teachings herein can be applied to wireless network types other than GSM. Such systems may transmit some information content sent as clear text on a broadcast channel, and will send the same information as ciphered text on one or more other channels. 
         [0069]    Notably, modifications and other embodiments of the disclosed invention(s) will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention(s) is/are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.