Abstract:
A method for transmitting an encrypted signal to a wireless transmit/receive unit (WTRU) such that decryption of the encrypted signal depends on a trust zone associated with the WTRU is disclosed. The encryption may be performed using hierarchical modulation, scrambling, authentication, location validation, or a combination thereof. The size of a trust zone may also be adjusted.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/283,017 filed Nov. 18, 2005, which claims priority from U.S. Provisional Application No. 60/630,730 filed Nov. 23, 2004, U.S. Provisional Application No. 60/661,856 filed Mar. 15, 2005, and U.S. Provisional Application No. 60/684,257 filed May 25, 2005, which are incorporated by reference as if fully set forth herein. 
    
    
     FIELD OF INVENTION 
     The present invention relates generally to wireless communications. More specifically, the present invention is related to a method and system for securing such wireless communications by strategically positioning the source and/or the recipient of the communications. 
     BACKGROUND 
     As wireless connectivity becomes more pervasive and reliable, it is expected that all the digital computing, data storage and media storage devices that are in widespread use today will become part of Ad-hoc wireless communication networks. However, such networks are susceptible to data security breaches in many respects. For example, Ad-hoc networks, where individual users communicate with each other directly without using intermediary network nodes, create new susceptibilities to the users and networks. 
     To reduce the susceptibility of wireless networks, techniques such as wired equivalent privacy (WEP), Wi-Fi protected access (WPA), extensible authentication protocol (EAP) and GSM-based encryption have been developed. Although these techniques provide some protection, they are still susceptible to various trusts, rights, identity, privacy and security issues. For example, although a particular wireless communication node may have the correct WEP keys to communicate with a wireless user, that user may not know whether the particular node can be trusted. 
     Additionally, authentication of the user using these keys typically occurs at higher layers of the communication stack. Accordingly, even when these controls are in place, a rogue wireless user or hacker may have some (although limited) access to the communication stack. This access creates vulnerabilities, such as denial of service attacks, among others. 
     The fact that wireless signals degrade with distance introduces a natural measure of security since intercepting a signal requires one to be sufficiently close to the source to detect it. This is particularly true in small networks, where the transmit power is typically low and communications typically occur at highest rates and in an Ad-hoc fashion. In many situations, physical proximity may be the most difficult attribute for a malicious attacker to attain. In fact communication which can only be detected within a very short proximity of the transmitter may not need to be very well protected. 
     Accordingly, it would be desirable to implement a security system for wireless networks which can take advantage of the natural security offered by degradation of wireless signals. Furthermore, it would be desirable to ensure that any information transmitted to a user is accessible only at the location of the user, such that a “eavesdropper” located in the general proximity of the user, but not at the user&#39;s immediate location, is prevented from receiving complete messages transmitted to the user. 
     SUMMARY 
     A method for transmitting an encrypted signal to a wireless transmit/receive unit (WTRU) such that decryption of the encrypted signal depends on a trust zone associated with the WTRU is disclosed. The encryption may be performed using hierarchical modulation, scrambling, authentication, location validation, or a combination thereof. The size of a trust zone may also be adjusted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more detailed understanding of the invention may be had from the following description, given by way of example and to be understood in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a graphical representation showing a relationship between effective input SNR of a receiver&#39;s decoder and the decoder&#39;s output BER; 
         FIG. 2  is a block diagram of a wireless communication system including a transmitter and a receiver used to secure wireless communications in accordance with the present invention; 
         FIG. 3  is a graphical representation showing a relationship between normalized secure proximity radius (NSPR) and known symbols for R=1, γ=2; 
         FIG. 4  is a graphical representation showing a relationship between NSPR and known symbols for, R=1, γ=4; 
         FIG. 5  is a graphical representation showing a relationship between NSPR and known symbols for R=½, γ=2; 
         FIG. 6  is a graphical representation showing a relationship between NSPR and known symbols for and R=½, γ=4; 
         FIG. 7  is a diagram of a security network with multiple trust zones used to secure wireless communications in accordance with one embodiment of the present invention; 
         FIG. 8  is a conventional network in which an eavesdropper may intersect a bit stream transmitted from an AP to a WTRU; 
         FIG. 9  is a network in which each of a plurality of APs transmits PDUs to a WTRU located in a trust zone intersected by the transmission patterns of each of the APs to secure wireless communications in accordance with another embodiment of the present invention; and 
         FIG. 10  shows a QPSK modulation constellation which illustrates how wireless communications are secured in accordance with yet another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereafter, the terminology “wireless transmit/receive unit” (WTRU) includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a station (STA) or any other type of device capable of operating in a wireless environment. When referred to hereafter, the terminology “access point” (AP) includes but is not limited to a base station, a Node-B, a site controller or any other type of interfacing device in a wireless environment. 
     The present invention is based on the fact that most conventional channel codes, (e.g., Turbo codes, low density parity check (LDPC) codes, or the like), are operating close to the Shannon limit in most practical scenarios. As applied to wireless communication systems, (ignoring the effect of fading), the receiver&#39;s ability to demodulate data is almost a binary function of the effective SNR at the input to the receiver&#39;s decoder. 
     The features of the present invention may be incorporated into an integrated circuit (IC) or be configured in a circuit comprising a multitude of interconnecting components. 
       FIG. 1  is a graphical representation showing a relationship between effective decoder input SNR and a decoder output BER. A critical SNR exists such that if the actual effective SNR falls below the critical SNR, the decoder fails completely, (i.e., the decoder&#39;s output BER effectively 1), and data in a wireless communication cannot be read. Conversely, if the actual effective SNR at the decoder input is above the critical SNR, the probability of error at the decoder output is extremely low and the data in the wireless communication can be read with very high probability. 
     Since it is assumed that the channel code approaches the Shannon limit, it can be assumed that the coding is performed at the Shannon capacity rate. Moreover, it is convenient to actually work in terms of spectral efficiency, since this makes the numeric results independent of the bandwidth. For a complex-valued additive white Gaussian noise (AWGN) channel, the Shannon capacity rate is given by:
 
 R =log 2 (1 +SNR );  Equation (1)
 
where SNR is used in the E b /N o  sense. It is assumed that for coding rates above this rate, reliable information decoding is not possible and for coding rates below this rate, reliable information decoding is essentially guaranteed. In fact, with large-block length codes, such as LDPC and Turbo codes, this is a realistic assumption.
 
     The SNR basically depends on the distance between the transmitter and the receiver. The SNR dependency on the distance from the transmitter is given by a power law as follows: 
                       SNR   ⁡     (   d   )       =     E     d   γ         ;           Equation   ⁢           ⁢     (   2   )                 
where E is a nominal SNR at a distance of 1 unit. In free space, the exponent γ is 2, but in practical wireless networks, the exponent γ is somewhere between 3 and 4, depending on the channel topology.
 
     Let SNR c  be the critical SNR for the chosen coding scheme. Then, the distance covered with this critical SNR is determined as follows: 
                     d   =       E     SNR   c       γ       ;           Equation   ⁢           ⁢     (   3   )                 
and it can be rewritten in dBs as follows:
 
     
       
         
           
             
               
                 
                   
                     log 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     d 
                   
                   = 
                   
                     
                       
                         1 
                         γ 
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             log 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             E 
                           
                           - 
                           
                             log 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               SNR 
                               c 
                             
                           
                         
                         ) 
                       
                     
                     = 
                     
                       
                         1 
                         γ 
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               E 
                               
                                 d 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 B 
                               
                             
                             - 
                             
                               SNR 
                               
                                 c 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 d 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 B 
                               
                             
                           
                           ) 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     4 
                     ) 
                   
                 
               
             
           
         
       
     
     The present invention makes d a function of security policy. By dynamically choosing d, a receiver at a distance closer than d can operate with a looser security policy, while a receiver with a distance beyond d will require a stricter security policy. 
     In a typical communication scheme, the channel coding scheme is fixed since it is quite expensive to have “programmable” decoders for completely different coding schemes. Thus, SNR c  is fixed. Then, from Equations (3) and (4), d can be controlled by controlling E and γ in a communication system. In order to achieve this goal, at least one of these controls must vary depending on extrinsic security-related information that a receiver may or may not possess. 
     E is defined as a nominal SNR at a unit distance. In reality, E is a transmit power per information bit intended to a particular receiver. The nominal SNR definition is necessary since the power law model of Equation (2) breaks down for small values of d and leads to infinite SNRs. Thus, controlling E means controlling the output power per information bit. For example, the control of the output power per information bit may be accomplished in any one or combination of the following processes: 
     1) by directly controlling the output power applied to the particular receiver&#39;s data; 
     2) by reducing the output SNR and hence the receiver&#39;s receive SNR by adding an additional noise like signal to the transmitted signal. This has the advantage of maintaining constant output power whilst regulating the SNR to individual receivers. 
     3) by controlling a modulation scheme, (e.g., selecting QPSK/M-quadrature amplitude modulation (QAM)/M-phase-shift keying (PSK)/frequency-shift keying (FSK), or the like); 
     4) by adjusting a bit length (e.g. for UWB systems); 
     5) by controlling jitter and timing of transmission; 
     6) by controlling an effective coding rate for the data to the receiver, which is a preferred one in the present invention. This method offers the ability, in a WLAN system, to maintain constant power level between the APs and WTRU in such a way as to maintain a uniform and regular grid spacing between the various APs in a system without affecting the performance of the CSMA system from fluctuating transmit power levels; 
     7) by changing the rate matching rules so as to introduce puncturing or repetition of symbols and hence the effective bit energy; 
     8) by controlling a modulation index; and 
     9) by controlling the amount of interference the receiver will experience. 
     The interference control can be accomplished by one or combination of the following ways, but is not limited to: 
     1) by applying variable interference management techniques, such as pre-equalization to the desired receiver&#39;s signal and/or the interfering receiver&#39;s signal and varying the degree to which cross-interference is removed or introduced; 
     2) by selective power control, (the power control could be a jointly optimized process with the security policy); 
     3) by time/frequency/code scheduling to control the number of potential interferers; 
     4) by dynamic interference control, (e.g., turned on and off); and 
     5) by signaling through a third party beacon which in turn transmits signals creating additional interference pattern. 
     Additionally, in the presence of multiple receive antennae, the value of E can be made dependent on the angular location of the receiver with respect to the transmitter (Θ), (i.e., E=E(Θ), and consequently d can be made as a function of Θ as well. This introduces another set of control possibilities, which include, but not limited to, the following ways: 
     1) beamforming towards or away from the receiver in azimuth, elevation or both; 
     2) interference management using smart antenna techniques; and 
     3) introduction of transmission patterns. 
     With respect to γ, the value of γ depends on Doppler spread of the received signal, which generally depends on the relative velocity of the receiver with respect to the transmitter and the geography of their environment. However, the transmitter can artificially increase the Doppler spread by internal signal processing. Since the value of γ depends on the geography of the environment, if the transmitter is equipped with a plurality of antennas, it can control γ to some extent by aiming the transmitted signal in an appropriate fashion. 
     The receiver may detect the presence of an adversary actively tampering with the wireless channel in accordance with the present invention. If the receiver is informed through auxiliary means that the receiver should be able to successfully demodulate the data stream, but is in fact unable to do so after a sufficiently large number of attempts, and since the security policy and the communication controls of the receiver are set in such a way as to enable the demodulation of the data stream, the receiver can then assume that the wireless channel is being tampered with. 
     The present invention preferably utilizes a code rate as a parameter depending on the security policy of the receiver. Typically, the ability of the receiver to demodulate a signal depends on geography, (the effective distance), which is more complex than a straight-line distance. If necessary, the transmitter and the receiver can discover the effective distance between them by slowly increasing, (or alternatively decreasing), one or more of the control parameters and detecting the point at which reliable data decoding becomes possible, (or alternatively is no longer possible). 
       FIG. 2  is a block diagram of a communication system  100  including a transmitter  110  and a receiver  120  in accordance with the present invention. The transmitter  110  comprises a protocol stack unit  112 , a channel encoder  114 , a rate matching unit  115 , a multi-layer secure bit (MLSB) scrambler  116  and a physical channel processing unit  118 . The receiver  120  comprises a physical channel processing unit  128 , an MLSB descrambler  126 , a rate de-matching unit  125 , a channel decoder  124  and a protocol stack unit  122 . The protocol stack units  112 ,  122 , the channel encoder  114 , the rate matching unit  115 , the rate de-matching unit  125 , the channel decoder  124  and the physical channel processing units  118 ,  128  are essentially the same components as used in conventional transmitters and receivers. The protocol stack unit  112  generates an information stream and this information stream is encoded for error protection by the channel encoder  114 , and then is further processed to be transmitted via a wireless channel  130 , (i.e., a particular air interface), by the physical channel processing unit  118 . This process is reversed at the receiver  120 . 
     The channel encoder  114  maps a sequence of input data to a sequence of output channel symbols. The MLSB scrambler  116  scrambles the channel symbols. The channel symbols may be bits or higher-order modulation symbols. Not all the symbols need to be scrambled. The MLSB scrambler  116  may take a subset of symbols and scrambles them. Receivers should be aware of which symbol positions are scrambled. 
     Several security layers are defined in accordance with the present invention. The proportion of the scrambled symbols that a MLSB descrambler  126  can descramble depends on the security layer. For any symbol that the MLSB descrambler  126  can descramble, the MLSB descrambler  126  does so. For any symbol that the MLSB descrambler  126  cannot descramble, the MLSB descrambler  126  inserts an erasure, (i.e., a channel observation of 0), for that symbol. Any conventional channel decoder is capable of operating with erasures. Therefore, this does not present a problem to a current system. 
     The effect of the security system in accordance with the present invention on those receivers which are not able to descramble all symbols is an increase in the code rate and a simultaneous reduction in the effective SNR per information bit. The specific amount of code-rate increase and effective SNR reduction depends on the security level, which will be explained hereinafter. 
     The rate matching unit  115  in the transmitter  110  operates in accordance with rate matching rules, which may be changed so as to introduce puncturing or repetition of symbols and hence the effective bit energy. A channel with a code rate R is utilized. R can be greater than 1 bit per channel symbol and the effective rate for security layer n is given by: 
                       R   n     =     R     1   -     θ   ⁡     (     1   -     e   n       )             ;           Equation   ⁢           ⁢     (   5   )                 
where θ denotes the proportion of the scrambled symbols and e n  is the proportion of symbols that a descrambler, (i.e., the rate de-matching unit  125  in the receiver  120 ), with a security layer n can descramble. In all cases, e n ε[0,1], e 1 =0, e N =1. The initial SNR per information bit, (more precisely E b /N o ), is denoted by E 0 . The effective SNR for security layer n is given by:
 
 E   n   =E   0 (1−θ(1 −e   n )).  Equation (6)
 
     Both the rate and the SNR are simply scaled by the proportion of non-scrambled known bits, which is given by:
 
η n =1−θ(1 −e   n ).  Equation (7)
 
Therefore, it is sufficient to formulate the analysis exclusively in terms of this quantity. The SNR dependence on the distance from the transmitter is given by Equation (2).
 
     In accordance with the present invention, it is determined that given a certain proportion of non-erased symbols, (i.e., symbols that the receiver is able to unscramble), the distance from the transmitter to the receiver, in order to be able to demodulate the data can be determined. Equation (2) is substituted into Equation (7) and solved for d to obtain the following equation: 
     
       
         
           
             
               
                 
                   d 
                   = 
                   
                     
                       
                         E 
                         
                           
                             2 
                             R 
                           
                           - 
                           1 
                         
                       
                       γ 
                     
                     . 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     8 
                     ) 
                   
                 
               
             
           
         
       
     
     Next, given that a percentage η of the symbols are not erased, Equations (5) and (6) are substituted into Equation (9) to obtain the following equation: 
     
       
         
           
             
               
                 
                   
                     d 
                     ⁡ 
                     
                       ( 
                       η 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           η 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           E 
                         
                         
                           
                             2 
                             
                               R 
                               / 
                               η 
                             
                           
                           - 
                           1 
                         
                       
                       γ 
                     
                     . 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     9 
                     ) 
                   
                 
               
             
           
         
       
     
     The percentage of distance achievable with a particular security level η can be expressed as a percentage of distance achievable with full security (η=1). This is the NSPR which is defined as follows: 
     
       
         
           
             
               
                 
                   
                     
                       d 
                       _ 
                     
                     ⁡ 
                     
                       ( 
                       η 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         ⅆ 
                         
                           ( 
                           η 
                           ) 
                         
                       
                       
                         ⅆ 
                         
                           ( 
                           1 
                           ) 
                         
                       
                     
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                                   2 
                                   R 
                                 
                                 - 
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                               ) 
                             
                           
                           
                             
                               2 
                               
                                 R 
                                 / 
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                         γ 
                       
                       . 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     10 
                     ) 
                   
                 
               
             
           
         
       
     
     The NSPR does not depend on E, although it does depend on the nominal transmission rate. As an example,  FIGS. 3-6  present plots of NSPR versus percentage of known symbols for 4 different scenarios: R=1, γ=2; R=1, γ=4; R=½, γ=2; and R=½, γ=4, respectively. From the simulation results, it is observed that by revealing only 50% of the channel symbols, receivers located farther than about 60% of the “fully secure” transmission radius can be prevented from demodulating the information. Thus, if a receiver is beyond the effective distance for its security parameter, it is theoretically prohibited from decoding the data with a BER much better then 50%. 
       FIG. 7  shows a security network  700  including a plurality of WTRUs  705 ,  710 ,  715 ,  720  and  725  which operate in a plurality of non-overlapping trust zones  730 ,  740 ,  750  or a “no trust zone” area  760  external to the trust zones. The trust zones  730 ,  740 ,  750  and the “no trust zone”  760  are established as follows: 
     Transmission parameters, such as a code rate scheme, puncturing scheme, power scheme or the like, are chosen such that a receiver, (i.e., a WTRU), outside of the boundary between the trust zone  750  and the “no trust zone”  760  is not capable of decoding the transmission signal, even if the receiver is fully aware of all transmission parameters. Furthermore, a bit scrambling scheme, (to be implemented by the MLSB sub-system), is chosen such that receivers inside the trust zone  730  are able to demodulate the data, even if the receivers do not know any of the scrambled bits. The received power will be high enough such that successful demodulation can occur, even if the scrambled bits are simply taken to be punctured. 
     Receivers in the trust zone  740  are no longer able to demodulate the sent data unless they are aware of some of the scrambling pattern applied by the MLSB. Accordingly, receivers located in trust zone  740  will be forced to go through some kind of authentication procedure with the transmitter so that some necessary portion of the scrambling sequence is revealed to them. 
     Receivers in the trust zone  750  are not able to demodulate the data transmitter, even if they are aware of the portion of the scrambling sequence revealed to the receiver in the trust zone  740 , (e.g., by overhearing the side communication whereby those receivers were allowed access to this sequence). Instead, they are required to request additional information about the scrambling sequence, (e.g., they may need to know the full sequence), and thus must go through a separate, (potentially more demanding), authentication process then receivers in the trust zone  740 . As mentioned before, receivers in the area  760  cannot demodulate the sent data under any circumstances. 
     In accordance with the embodiment of the present invention described above, the distance from a transmitting WTRU  705  to a receiving WTRU is a function of security policy. By dynamically choosing the distance d, (e.g., 50 meters), a receiving WTRU  710  at a distance closer than d can operate with a looser security policy, while receiving WTRU  715 ,  720  and  725  with a distance beyond d will require a stricter security policy. 
       FIG. 8  shows a conventional network  800  which includes an AP  805  and a WTRU  810 . When the AP  805  transmits a bit stream  815  to the WTRU  810 , an eavesdropper  820  within range of the AP  805  is able to receive the entire bit stream, e.g., 111000101. 
       FIG. 9  shows a network  900  including a plurality of access points (APs)  905 ,  910 ,  915 , a WTRU  920  and the eavesdropper  820  of  FIG. 8  in accordance with one embodiment of the present invention. By using a plurality of APs  905 ,  910 ,  915 , rather than only the sole AP  805  in the conventional network  800  of  FIG. 8 , the bit stream  815  is secured from being decrypted by the eavesdropper  820 . The WTRU  920  is located at the intersection  935  of the transmission patterns of the APs  905 ,  910  and  915 , whereby the WTRU  920  will receive a first fragment  930   A  of the bit stream  815 , “111”, from the AP  905 , a second fragment  930   B  of the bit stream  815 , “000”, from the AP  910 , and a third fragment  930   C  of the bit stream  815 , “101”, from the AP  915 . Each fragment  930   A ,  930   B ,  930   C  is referred to as a PDU and the original bit stream “111000101” is referred to as a service data unit (SDU). The WTRU  920  then reassembles the entire encrypted SDU from the three PDUs  930   A ,  930   B  and  930   C . Since the eavesdropper  820  is not physically located at the intersection  935  of the transmission patterns of the APs  905 ,  910  and  915  such that all of the fragments  930   A ,  930   B ,  930   C  are received at an error rate comparable to that of the WTRU  920 , the eavesdropper  820  is unable to decipher the entire bit stream  815 , (even with knowledge of a secret key). 
     In the network  900  of  FIG. 9 , the SDU that is deciphered by the WTRU  920  is 111000101, where PDU A =111, PDU B =000 and PDU C =101. If the eavesdropper  820  manages to decipher two out of the three PDUs, (e.g., 000 and 101), the eavesdropper  820  will have managed to obtain some information which is incomplete but correct. 
     In an alternative embodiment, any PDUs that the eavesdropper  820  does receive are rendered meaningless if incomplete. For example, the SDU that needs to be sent to the WTRU  920  in the network  900  is 111000101. However, three PDUs that are sent by three different APs  905 ,  910  and  915 , (e.g., PDU 1 , PDU 2 , PDU 3 ), are not fragments, as illustrated by  FIG. 9 , but are instead selected such that the SDU=PDU 1  XOR PDU 2  XOR PDU 3  where PDU 1 =100110011, PDU  2 =110000111 and PDU  3 =101110001, such that the SDU=100110011 XOR 110000111 XOR 101110001=111000101, where XOR is an exclusive-or function. Thus, assuming that the WTRU  920  is located at the intersection  935  of the transmission patterns of the APs  905 ,  910  and  915 , the WTRU  935  is able to receive all three PDUs and XOR the PDUs together to decipher the SDU  111000101 . If the eavesdropper  820  captures even two of these three PDUs, they are completely meaningless with respect to deciphering the SDU. Alternative mechanisms other than XOR are also possible such as scrambling the packet and sending different bits from different transmitters in such a manner as to render meaningless the transmissions, unless all transmissions are received successfully. 
     In another embodiment, a location-based authentication mechanism may be incorporated in the network  900  of  FIG. 9 . The WTRU  920  receives transmissions from the APs  905 ,  910  and  915 , and reports its location to each of the APs  905 ,  910  and  915 . Based upon the reported locations of the WTRU  920  and the APs  905 ,  910  and  915 , each of the APs  905 ,  910  and  915  may launch a protocol which transmits a sequence of messages, requesting a positive acknowledgement (ACK) or a negative acknowledgement (NACK) from the WTRU  920 , at varying effective coding rates higher and lower than the coding rate suggested by the nominal distance between each respective AP  905 ,  910 ,  915  and the WTRU  920 . Thus, the protocol establishes a criteria which dictates, based on location of the WTRU  920  with respect to the locations of the APs  905 ,  910  and  915 , whether the WTRU may decode transmissions received from the APs  905 ,  910  and  915 . If the location reported by the WTRU  920  is determined to be correct, the protocol will then verify the authenticity of the location of the WTRU  920  by processing ACK/NACK messages received from the WTRU  920  in response to the sequence of messages. 
     Verification of the authenticity of the WTRU  920  may also be performed such that the WTRU  920 , (or a user of the WTRU  920 ), and the APs  905 ,  910  and  915  share a common secret. For example, if APs  905 ,  910  and  915  require the location indicated by the WTRU  920  to be authenticated, the APs  905 ,  910  and  915  send a “challenge question” via a plurality of PDUs, which may be fragmented or encrypted as described above, such that the “challenge question” would be decipherable by the WTRU  920  only if the WTRU  920  is located as indicated. Thus, the WTRU  920  would not be able to “answer” the “challenge question” unless it was located at a position where the “challenge question” could be deciphered. 
       FIG. 10  shows an example of a hierarchical modulation (HM) scheme, defined by a combination of primary and secondary modulation schemes, which, in this case, are QPSK and BPSK respectively. It is well known that a QPSK modulation scheme is defined by 4 modulation points, which together constitute the QPSK modulation constellation. The modulation points represent carrier phases of π/2, 3π/2, −π/2 and −3π/2 and denote two bits  00 ,  01 ,  10  and  11  respectively. Similarly, it is well known that a BPSK modulation scheme is defined by 2 modulation points, which together constitute the BPSK modulation constellation. The modulation points represent carrier phases of +δ and −δ radians, and denote one bit  0  or  1  respectively. In turn, the HM scheme is defined by 8 modulation points, constructed from the primary and secondary modulation constellations. 
     The HM modulation points represent carrier phases of (π/2−δ), (π/2+δ), (3π/2−δ), (3π/2+δ), (−π/2−δ), (−π/2+δ), (−3π/2−δ), (−3π/2+δ) and denote three bits  000 ,  001 ,  010 ,  011 ,  100 ,  101 ,  110  and  111  respectively. These 8 modulation points constitute four (4) clusters, each including two (2) closely spaced modulation points. For example, the modulation represented by the carrier phases (π/2−δ), (π/2+δ) would constitute a cluster. The transmitter sends a sequence of symbols taken from the HM constellation over a wireless channel, which attenuates and contaminates the signal as it travels farther from the transmitter. A receiver which is close to the transmitter will, in general, receive a signal with good signal strength and signal quality, so that it can detect the carrier phase and hence the 3 bits accurately. However, a receiver which is far from the transmitter will, in general, receive a signal with lower signal strength and signal quality, so that it may not be able to discriminate between the closely spaced modulation points in each cluster, although it can determine which cluster the transmitted symbols belongs to. Thus, such a receiver can detect the primary modulation but not the secondary modulation. Accordingly, the receiver can detect two bits of data but not the third bit. 
     This embodiment of the present invention may be used for implementing a security or trust zone. The data associated with the primary modulation points, that is the first 2 bits, is encoded or encrypted or scrambled with a secret key and the secret key itself is transmitted via the 3 rd  bit of a sequence of symbols. Thus, a receiver within the trust zone can detect the key and use it to decode or decrypt or descramble the primary data. A receiver outside of the trust zone can detect the primary data but not the secret key, and thus cannot decode or decrypt or descramble the primary data. Any modulation scheme may be used for the primary and the secondary modulation schemes of the present invention. Examples include M-ary PSK, M-ary FSK, M-ary QAM, or the like. Furthermore, only selected modulation points in the primary modulation constellation may be superimposed with secondary clusters. Finally, more than two levels of hierarchy may be imposed. For example, QPSK on BPSK on BPSK represents a three-level HM. 
     In another embodiment, a layered HM scheme may be implemented.  FIG. 10  shows a simple two-level scheme where the main waveform is a QPSK signal overlaid with a biphase shift keying (BPSK) HM. When a receiver&#39;s SNR is high, it is possible to distinguish all constellation points. As the SNR decreases, it becomes difficult to distinguish the points of the BPSK hierarchy from the nominal QPSK constellation points and hence the HM data is lost. 
     In accordance with the present invention, scrambled data is modulated in the main waveform and descrambling information is encoded in the HM. When the receiver is located within a zone where the HM is discernable, the descrambling information enables successful reception. When the receiver is too far away and hence unable to extract the HM data, the descrambling information has to be explicitly requested through other channels. By varying the power allocated to the HM waveform, the range can be zone controlled. 
     Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention.