A physical layer key generation method for FDD communication system based on loop-back mechanism

Abstract

The present invention relates to a method for generating a physical layer key in an FDD communication system based on a loop-back mechanism, which is specifically as follows: Step 1: User A and User B send pilot signals in frequency band 1 and frequency band 2, respectively, while User A and User B receive signals in frequency band 2 and frequency band 1, respectively; Step 2: User A and User B send the signal received in step 1 in frequency band 2 and frequency band 1, respectively, while User A and User B receive signals in frequency band 1 and frequency band 2, respectively; Step 3: User A and User B send the signal received in step 2 in frequency band 2 and frequency band 1, respectively, while User A and User B receive signals in frequency band 1 and frequency band 2, respectively; and so on. The present invention takes into account the differences in hardware fingerprints in different frequency bands. In an FDD system, the present invention can ensure that a key generation system based on a wireless channel has good mutual difference and security without performing hardware calibration compensation.

Description

Technical Field

[0001] The present invention relates to the field of information security, and in particular to a method for generating a physical layer key in an FDD communication system based on a loop-back mechanism. Background Art

[0002] Existing wireless network key generation systems rely on complex key algorithms and PKI public key infrastructure. Key generation, distribution, and management are complex, limiting their application in wireless devices. Key generation methods based on wireless channels, due to their uniqueness, mutuality, and low complexity, offer a solution for key generation in wireless devices.

[0003] Existing mobile communication systems are primarily divided into FDD and TDD systems. In existing loop-back-based key generation methods for FDD communication systems, an unauthorized eavesdropper can obtain a key similar to that of a legitimate party through simple signal processing, resulting in low system security. Furthermore, existing loop-back-based key generation methods for TDD communication systems fail to account for differences in device hardware fingerprints across different frequency bands, resulting in a high rate of key inconsistency. Improving the security and mutual disparity of key generation in FDD systems is a worthy research topic. Therefore, a new solution is urgently needed to address these technical issues. Summary of the Invention

[0004] The present invention addresses the problems existing in the prior art and provides a method for generating physical layer keys for an FDD communication system based on a loop-back mechanism, which can effectively reduce the impact of differences in hardware fingerprints in different frequency bands on key reciprocity and ensure the security of the system.

[0005] The present invention provides a method for generating a physical layer key in an FDD communication system based on a loop-back mechanism, comprising the following steps:

[0006] Step 1: User A and user B send pilot signals in frequency band 1 and frequency band 2 respectively. At the same time, user A and user B receive signals in frequency band 2 and frequency band 1 respectively.

[0007] Step 2: User A and User B transmit the signal received in Step 1 on Band 2 and Band 1, respectively. Meanwhile, User A and User B receive the signal on Band 1 and Band 2, respectively.

[0008] Step 3: User A and user B transmit the signal received in step 2 in frequency band 2 and frequency band 1 respectively. At the same time, user A and user B receive the signal in frequency band 1 and frequency band 2 respectively.

[0009] Step 4: User A and user B transmit the signal received in step 3 in frequency band 1 and frequency band 2, respectively. At the same time, user A and user B receive the signal in frequency band 2 and frequency band 1, respectively. User A and user B perform channel state information (CSI) estimation on the received signals.

[0010] Step 5: User A and user B perform sliding average filtering on the estimated CSI amplitude;

[0011] Step 6: User A and user B normalize the filtered CSI amplitude;

[0012] Step 7: User A and user B quantize the normalized CSI amplitude;

[0013] Step 8: User A and user B downsample the quantized values;

[0014] Step 9: User A and user B perform Gray encoding on the downsampled values ​​to obtain a preliminary key.

[0015] Step 10: User A and user B reconcile the information and amplify the privacy of the obtained preliminary key to obtain the final key.

[0016] Step 1 is as follows: User A and User B send pilot signals in Band 1 and Band 2 respectively, while User A and User B receive signals in Band 2 and Band 1 respectively. From the signal sent by User B, the CSI estimated by User A is:

[0017]

[0018] in and is the amplitude and phase of the RF fingerprint from user B to user A on frequency band 2, is the channel from user B to user A on frequency band 2 at time 1, is the noise from user B to user A on frequency band 2 at time 1.

[0019] From the signal sent by user A, the CSI estimated by user B is

[0020]

[0021] in and is the amplitude and phase of the RF fingerprint from user A to user B on frequency band 1, is the channel from user A to user B on frequency band 1 at time 1, is the noise from user A to user B on frequency band 1 at time 1.

[0022] Step 2 is as follows: User A and User B send the signal received in step 1 in frequency band 2 and frequency band 1 respectively, while User A and User B receive the signal in frequency band 1 and frequency band 2 respectively. The CSI estimated by User A from the signal sent by User B is

[0023]

[0024] in and is the amplitude and phase of the RF fingerprint from user B to user A on frequency band 1, is the CSI estimated by user B in step 1, is the channel from user B to user A on frequency band 1 at time 2, is the noise from user B to user A on frequency band 1 at time 2.

[0025] From the signal sent by user A, the CSI estimated by user B is

[0026]

[0027] in and is the amplitude and phase of the RF fingerprint from user A to user B on frequency band 2, is the CSI estimated by user A in step 1, is the channel from user A to user B on frequency band 2 at time 2, is the noise from user A to user B on frequency band 2 at time 2.

[0028] Step 3 is as follows: User A and User B send the signal received in step 2 in frequency band 2 and frequency band 1 respectively, while User A and User B receive the signal in frequency band 1 and frequency band 2 respectively. The CSI estimated by User A from the signal sent by User B is

[0029]

[0030] in is the CSI estimated by user B in step 2, is the channel from user B to user A on frequency band 1 at time 3, is the noise from user B to user A on frequency band 1 at time 3.

[0031] From the signal sent by user A, the CSI estimated by user B is

[0032]

[0033] in is the CSI estimated by user A in step 2, is the channel from user A to user B on frequency band 2 at time 3, is the noise from user A to user B on frequency band 2 at time 3.

[0034] Step 4 is as follows: User A and User B send the signal received in step 1 in frequency band 1 and frequency band 2 respectively, while User A and User B receive the signal in frequency band 2 and frequency band 1 respectively. The CSI estimated by User A from the signal sent by User B is

[0035]

[0036] in is the CSI estimated by user B in step 3, is the channel from user B to user A on frequency band 2 at time 4, is the noise from user B to user A on frequency band 2 at time 4.

[0037] From the signal sent by user A, the CSI estimated by user B is

[0038]

[0039] in is the CSI estimated by user A in step 3, is the channel from user A to user B on frequency band 1 at time 4, is the noise from user A to user B on frequency band 1 at time 4.

[0040] from and It can be seen from the expression that within the channel coherence time and without considering the noise, user A and user B share the same random source in and are the channels from user A to user B and from user B to user A on frequency band 1, and These are the channels from user A to user B and from user B to user A on frequency band 2, so it can be ensured that the finally generated key has good mutual difference.

[0041] Step 5 is as follows: User A performs a sliding average filter on the CSI amplitude estimated in step 4 to obtain

[0042]

[0043] in is the CSI estimated by user A in step 4, L is the window length of the sliding average filter, and M is the length of the CSI.

[0044] User B performs a sliding average filter on the CSI amplitude estimated in step 4 to obtain

[0045]

[0046] in is the CSI estimated by user B in step 4, L is the window length of the sliding average filter, and M is the length of the CSI.

[0047] Step 6 is as follows: User A normalizes the filtered CSI amplitude to obtain

[0048]

[0049] in is the CSI amplitude obtained after the sliding average filtering of user A in step 5, M is the length of the CSI, min(·) is the minimum value operation, and max(·) is the maximum value operation.

[0050] User B normalizes the filtered CSI amplitude to obtain

[0051]

[0052] in is the CSI amplitude obtained after the sliding average filtering of user B in step 5, M is the length of the CSI, min(·) is the minimum value operation, and max(·) is the maximum value operation.

[0053] Step 7 is as follows: User A quantizes the normalized CSI amplitude to obtain

[0054]

[0055] in is the normalized CSI amplitude of user A in step 6, T1, T2 and T3 are quantization thresholds, and G is the quantization fuzzy interval. The quantized values ​​falling within this interval are eventually discarded.

[0056] User B quantizes the normalized CSI amplitude to obtain

[0057]

[0058] in is the normalized CSI amplitude of user B in step 6. T1, T2, and T3 are quantization thresholds. G is the quantization fuzzy interval. The quantized values ​​falling within this interval are eventually discarded.

[0059] Step 8 is as follows: User A downsamples the quantized value.

[0060]

[0061] in is the value quantized by user A in step 7, M is the length of CSI, and D is the downsampling ratio.

[0062] User B downsamples the quantized value

[0063]

[0064] in is the value quantized by user B in step 7, M is the length of CSI, and D is the downsampling ratio.

[0065] Step 9 is as follows: User A performs Gray coding on the downsampled value to obtain the preliminary key.

[0066]

[0067] in is the value obtained by downsampling user A in step 8, M is the length of CSI, and D is the downsampling ratio.

[0068] User B performs Gray encoding on the downsampled value to obtain the preliminary key

[0069]

[0070] in is the value obtained by downsampling user B in step 8, M is the length of CSI, and D is the downsampling ratio.

[0071] Step 10 is as follows: User A and User B reconcile the obtained preliminary key and discard the bits that are -1 in the obtained preliminary key. At this time, the key obtained by User A is

[0072]

[0073] in is the preliminary key obtained by Gray encoding of user A in step 9, M is the length of CSI, and D is the downsampling ratio.

[0074] The key obtained by user B is

[0075]

[0076] in This is the preliminary key obtained by Gray encoding of user B in step 9. M is the length of the CSI, and D is the downsampling ratio.

[0077] Then the hash function is used to amplify the privacy of the key, and the final key obtained by user A is

[0078]

[0079] Where Hash(·) represents a hash function.

[0080] The final key obtained by user B is

[0081]

[0082] Compared with the existing technology, the present invention has the following advantages. According to the above method, the technical solution uses the loop-back mechanism to process CSI in the FDD system. At the same time, considering the differences in hardware fingerprints in different frequency bands, the wireless channel-based key generation system can have better mutual diversity and security in the FDD system without hardware calibration compensation. BRIEF DESCRIPTION OF THE DRAWINGS

[0083] Figure 1 is the amplitude of CSI obtained after sliding average filtering;

[0084] Figure 2 is the amplitude of the CSI obtained after sliding average filtering and normalization;

[0085] Figure 3 is the key inconsistency rate between user A and user B, and between user A and user E. DETAILED DESCRIPTION

[0086] In order to deepen the understanding of the present invention, this embodiment is described in detail below with reference to the accompanying drawings.

[0087] Example 1: The present invention proposes a method for generating a physical layer key in an FDD communication system based on a loop-back mechanism. The specific technical steps are as follows.

[0088] 1. User A and user B send pilot signals in band 1 and band 2 respectively. At the same time, user A and user B receive signals in band 2 and band 1 respectively. From the signal sent by user B, user A estimates the CSI to be

[0089]

[0090] in and is the amplitude and phase of the RF fingerprint from user B to user A on frequency band 2, is the channel from user B to user A on frequency band 2 at time 1, is the noise from user B to user A on frequency band 2 at time 1.

[0091] From the signal sent by user B, the illegal user E estimates the CSI to be

[0092]

[0093] in and is the amplitude and phase of the RF fingerprint from user B to illegal user E on frequency band 2, is the channel from user B to illegal user E on frequency band 2 at time 1, is the noise from user B to illegal user E on frequency band 2 at time 1.

[0094] From the signal sent by user A, the CSI estimated by user B is

[0095]

[0096] in and is the amplitude and phase of the RF fingerprint from user A to user B on frequency band 1, is the channel from user A to user B on frequency band 1 at time 1, is the noise from user A to user B on frequency band 1 at time 1.

[0097] From the signal sent by user A, the illegal user E estimates the CSI to be

[0098]

[0099] in and are the amplitude and phase of the RF fingerprint from user A to illegal user E on frequency band 1, is the channel from user A to illegal user E on frequency band 1 at time 1, is the noise from user A to illegal user E on frequency band 1 at time 1.

[0100] 2. User A and user B send the signal received in step 1 in frequency band 2 and frequency band 1 respectively. At the same time, user A and user B receive the signal in frequency band 1 and frequency band 2 respectively. From the signal sent by user B, the CSI estimated by user A is

[0101]

[0102] in and is the amplitude and phase of the RF fingerprint from user B to user A on frequency band 1, is the CSI estimated by user B in step 1, is the channel from user B to user A on frequency band 1 at time 2, is the noise from user B to user A on frequency band 1 at time 2.

[0103] From the signal sent by user B, the illegal user E estimates the CSI to be

[0104]

[0105] in and is the amplitude and phase of the RF fingerprint from user B to the illegal user E on frequency band 1, is the channel from user B to illegal user E on frequency band 1 at time 2, is the noise from user B to illegal user E on frequency band 1 at time 2.

[0106] The CSI estimated by user B is:

[0107]

[0108] in and is the amplitude and phase of the RF fingerprint from user A to user B on frequency band 2, is the CSI estimated by user A in step 1, is the channel from user A to user B on frequency band 2 at time 2, is the noise from user A to user B on frequency band 2 at time 2.

[0109] From the signal sent by user A, the illegal user E estimates the CSI to be

[0110]

[0111] in and is the amplitude and phase of the RF fingerprint from user A to illegal user E on frequency band 2, is the channel from user A to illegal user E on frequency band 2 at time 2, is the noise from user A to illegal user E on frequency band 2 at time 2.

[0112] 3. User A and user B send the signal received in step 2 in frequency band 2 and frequency band 1 respectively. At the same time, user A and user B receive the signal in frequency band 1 and frequency band 2 respectively. From the signal sent by user B, the CSI estimated by user A is

[0113]

[0114] in is the CSI estimated by user B in step 2, is the channel from user B to user A on frequency band 1 at time 3, is the noise from user B to user A on frequency band 1 at time 3.

[0115] From the signal sent by user B, the illegal user E estimates the CSI to be

[0116]

[0117] in is the channel from user B to illegal user E on frequency band 1 at time 3, is the noise from user B to illegal user E on frequency band 1 at time 3.

[0118] From the signal sent by user A, the CSI estimated by user B is

[0119]

[0120] in is the CSI estimated by user A in step 2, is the channel from user A to user B on frequency band 2 at time 3, is the noise from user A to user B on frequency band 2 at time 3.

[0121] From the signal sent by user A, the illegal user E estimates the CSI to be

[0122]

[0123] in is the channel from user A to illegal user E on frequency band 2 at time 3, is the noise from user A to illegal user E on frequency band 2 at time 3.

[0124] 4. User A and user B transmit the signal received in step 1 in frequency band 1 and frequency band 2 respectively. At the same time, user A and user B receive the signal in frequency band 2 and frequency band 1 respectively. From the signal sent by user B, the CSI estimated by user A is

[0125]

[0126] in is the CSI estimated by user B in step 3, is the channel from user B to user A on frequency band 2 at time 4, is the noise from user B to user A on frequency band 2 at time 4.

[0127] From the signal sent by user B, the illegal user E estimates the CSI to be

[0128]

[0129] in is the channel from user B to illegal user E on frequency band 2 at time 4, is the noise from user B to illegal user E on frequency band 2 at time 4.

[0130] From the signal sent by user A, the CSI estimated by user B is

[0131]

[0132] in is the CSI estimated by user A in step 3, is the channel from user A to user B on frequency band 1 at time 4, is the noise from user A to user B on frequency band 1 at time 4.

[0133] From the signal sent by user A, the illegal user E estimates the CSI to be

[0134]

[0135] in is the channel from user A to illegal user E on frequency band 1 at time 4, is the noise from user A to illegal user E on frequency band 1 at time 4.

[0136] The CSI estimated by user A and user B can be simplified as in and are the channels from user A to user B and from user A to user B on frequency band 1, and The following are the channels from user A to user B and vice versa on frequency band 2. The loop-back mechanism mitigates the impact of hardware fingerprint differences across different frequency bands. Furthermore, signal processing prevents unauthorized user E from obtaining the same CSI as authorized users, ensuring system security.

[0137] 5. User A performs a sliding average filter on the CSI amplitude estimated in step 4 to obtain

[0138]

[0139] in is the CSI estimated by user A in step 4, L is the window length of the sliding average filter, and M is the length of the CSI.

[0140] User B performs a sliding average filter on the CSI amplitude estimated in step 4 to obtain

[0141]

[0142] in is the CSI estimated by user B in step 4, L is the window length of the sliding average filter, and M is the length of the CSI.

[0143] like Figure 1 (a) and Figure 1 (b) shows the CSI amplitudes of user A and user B after sliding average filtering, where L is 60 and M is 1200. Figure 1 (a) and Figure 1 As can be seen in (b), after sliding average filtering, the amplitudes of the CSI obtained by user A and user B are similar.

[0144] 6. User A normalizes the filtered CSI amplitude to obtain

[0145]

[0146] in is the CSI amplitude obtained after the sliding average filtering of user A in step 5. min(·) is the minimum value operation, and max(·) is the maximum value operation.

[0147] User B normalizes the filtered CSI amplitude to obtain

[0148]

[0149] in is the CSI amplitude obtained after the sliding average filtering of user B in step 5, M is the length of the CSI, min(·) is the minimum value operation, and max(·) is the maximum value operation.

[0150] like Figure 2 (a) and Figure 2 (b) shows the amplitudes obtained by user A and user B after normalization of the filtered CSI amplitudes. This step is mainly to solve the problem that the CSI waveforms obtained by user A and user B are similar but the amplitudes are inconsistent.

[0151] 7. User A quantizes the normalized CSI amplitude to obtain

[0152]

[0153] in This is the normalized CSI amplitude of user A in step 6. T1, T2, and T3 are the quantization thresholds, and G is the quantization fuzziness interval. Quantized values ​​falling within this interval are ultimately discarded. In this example, T1, T2, and T3 are 0.25, 0.5, and 0.75, respectively, and G is 0.025.

[0154] User B quantizes the normalized CSI amplitude to obtain

[0155]

[0156] in is the normalized CSI amplitude of user B in step 6. T1, T2, and T3 are quantization thresholds. G is the quantization fuzzy interval. The quantized values ​​falling within this interval are eventually discarded.

[0157] 8. User A downsamples the quantized value

[0158]

[0159] in is the value obtained by user A in step 7, and D is the downsampling ratio. In this example, D is 8.

[0160] User B downsamples the quantized value

[0161]

[0162] in is the value quantized by user B in step 7, M is the length of CSI, and D is the downsampling ratio.

[0163] 9. User A performs Gray encoding on the downsampled value to obtain the preliminary key

[0164]

[0165] in is the value obtained by downsampling user A in step 8, M is the length of CSI, and D is the downsampling ratio.

[0166] User B performs Gray encoding on the downsampled value to obtain the preliminary key

[0167]

[0168] in is the value obtained by downsampling user B in step 8, M is the length of CSI, and D is the downsampling ratio.

[0169] Figure 3 It shows the key inconsistency rate between user A and user B, and between user A and user E. Figure 3 It can be seen that as the signal-to-noise ratio increases, the key inconsistency rate between user A and user B decreases rapidly, while the key inconsistency rate between user A and user E is high and basically remains unchanged, so the system has better security performance.

[0170] 10. User A and User B reconcile the obtained preliminary key and discard the bits that are -1 in the obtained preliminary key. At this time, the key obtained by User A is

[0171]

[0172] in is the preliminary key obtained by Gray encoding of user A in step 9, M is the length of CSI, and D is the downsampling ratio.

[0173] The key obtained by user B is

[0174]

[0175] in This is the preliminary key obtained by Gray encoding of user B in step 9. M is the length of the CSI, and D is the downsampling ratio.

[0176] Then the hash function is used to amplify the privacy of the key, and the final key obtained by user A is

[0177]

[0178] Where Hash(·) represents a hash function.

[0179] The final key obtained by user B is

[0180]

[0181] It should be noted that the above embodiments are not intended to limit the scope of protection of the present invention, and equivalent changes or substitutions made on the basis of the above technical solutions fall within the scope of protection of the claims of the present invention.

Claims

1. A method for generating a physical layer key in an FDD communication system based on a loop-back mechanism, characterized in that: The method comprises the following steps: Step 1: User A and user B send pilot signals in frequency band 1 and frequency band 2 respectively. At the same time, user A and user B receive signals in frequency band 2 and frequency band 1 respectively. Step 2: User A and User B transmit the signal received in Step 1 on Band 2 and Band 1, respectively. Meanwhile, User A and User B receive the signal on Band 1 and Band 2, respectively. Step 3: User A and user B transmit the signal received in step 2 in frequency band 2 and frequency band 1 respectively. At the same time, user A and user B receive the signal in frequency band 1 and frequency band 2 respectively. Step 4: User A and user B transmit the signal received in step 3 in frequency band 1 and frequency band 2, respectively. At the same time, user A and user B receive the signal in frequency band 2 and frequency band 1, respectively. User A and user B perform channel state information (CSI) estimation on the received signals. Step 5: User A and user B perform sliding average filtering on the estimated CSI amplitude; Step 6: User A and user B normalize the filtered CSI amplitude; Step 7: User A and user B quantize the normalized CSI amplitude; Step 8: User A and user B downsample the quantized values; Step 9: User A and User B perform Gray encoding on the downsampled values ​​to obtain a preliminary key. Step 10: User A and user B reconcile the information and amplify the privacy of the obtained preliminary key to obtain the final key.

2. The method for generating a physical layer key in an FDD communication system based on a loop-back mechanism according to claim 1, wherein: Step 1 is as follows: User A and User B send pilot signals in Band 1 and Band 2 respectively. At the same time, User A and User B receive signals in Band 2 and Band 1 respectively. The CSI estimated by User A is in and is the amplitude and phase of the RF fingerprint from user B to user A on frequency band 2, is the channel from user B to user A on frequency band 2 at time 1, is the noise from user B to user A on frequency band 2 at time 1, The CSI estimated by user B is: in and is the amplitude and phase of the RF fingerprint from user A to user B on frequency band 1, is the channel from user A to user B on frequency band 1 at time 1, is the noise from user A to user B on frequency band 1 at time 1.

3. The method for generating a physical layer key in an FDD communication system based on a loop-back mechanism according to claim 1, wherein: Step 2 is as follows: User A and User B send the signal received in step 1 in frequency band 2 and frequency band 1 respectively. At the same time, User A and User B receive the signal in frequency band 1 and frequency band 2 respectively. The CSI estimated by User A is in and is the amplitude and phase of the RF fingerprint from user B to user A on frequency band 1, is the CSI estimated by user B in step 1, is the channel from user B to user A on frequency band 1 at time 2, is the noise from user B to user A on frequency band 1 at time 2, The CSI estimated by user B is: in and is the amplitude and phase of the RF fingerprint from user A to user B on frequency band 2, is the CSI estimated by user A in step 1, is the channel from user A to user B on frequency band 2 at time 2, is the noise from user A to user B on frequency band 2 at time 2.

4. The method for generating a physical layer key in an FDD communication system based on a loop-back mechanism according to claim 1, wherein: Step 3 is as follows: User A and User B send the signal received in step 2 in Band 2 and Band 1 respectively. At the same time, User A and User B receive the signal in Band 1 and Band 2 respectively. The CSI estimated by User A is in is the CSI estimated by user B in step 2, is the channel from user B to user A on frequency band 1 at time 3, is the noise from user B to user A on frequency band 1 at time 3, The CSI estimated by user B is: in is the CSI estimated by user A in step 2, is the channel from user A to user B on frequency band 2 at time 3, is the noise from user A to user B on frequency band 2 at time 3.

5. The method for generating a physical layer key in an FDD communication system based on a loop-back mechanism according to claim 1, wherein: Step 4 is as follows: User A and User B send the signal received in step 3 in frequency band 1 and frequency band 2 respectively. At the same time, User A and User B receive the signal in frequency band 2 and frequency band 1 respectively. The CSI estimated by User A is in is the CSI estimated by user B in step 3, is the channel from user B to user A on frequency band 2 at time 4, is the noise from user B to user A on frequency band 2 at time 4, The CSI estimated by user B is: in is the CSI estimated by user A in step 3, is the channel from user A to user B on frequency band 1 at time 4, is the noise from user A to user B on frequency band 1 at time 4, from and It can be seen from the expression that within the channel coherence time and without considering the noise, user A and user B share the same random source in and are the channels from user A to user B and from user B to user A on frequency band 1, and These are the channels from user A to user B and from user B to user A on frequency band 2, so it can be ensured that the finally generated key has good mutual difference.

6. The method for generating a physical layer key in an FDD communication system based on a loop-back mechanism according to claim 1, wherein: Step 5 is as follows: User A performs a sliding average filter on the CSI amplitude estimated in step 4 to obtain in is the CSI estimated by user A in step 4, L is the window length of the sliding average filter, M is the length of the CSI, User B performs a sliding average filter on the CSI amplitude estimated in step 4 to obtain in is the CSI estimated by user B in step 4, L is the window length of the sliding average filter, and M is the length of the CSI.

7. The method for generating a physical layer key in an FDD communication system based on a loop-back mechanism according to claim 1, wherein: Step 6 is as follows: User A normalizes the filtered CSI amplitude to obtain in is the CSI amplitude obtained after the sliding average filtering of user A in step 5, M is the length of CSI, min(·) is the minimum value operation, and max(·) is the maximum value operation. User B normalizes the filtered CSI amplitude to obtain in is the CSI amplitude obtained after the sliding average filtering of user B in step 5, M is the length of the CSI, min(·) is the minimum value operation, and max(·) is the maximum value operation.

8. The method for generating a physical layer key in an FDD communication system based on a loop-back mechanism according to claim 1, wherein: Step 7 is as follows: User A quantizes the normalized CSI amplitude to obtain in is the normalized CSI amplitude of user A in step 6, T1, T2 and T3 are quantization thresholds, G is the quantization fuzzy interval, and the quantization value falling in this interval will eventually be discarded. User B quantizes the normalized CSI amplitude to obtain in is the normalized CSI amplitude of user B in step 6. T1, T2, and T3 are quantization thresholds. G is the quantization fuzzy interval. The quantized values ​​falling within this interval are eventually discarded.

9. The method for generating a physical layer key in an FDD communication system based on a loop-back mechanism according to claim 1, wherein: Step 8 is as follows: User A downsamples the quantized value in is the value obtained by user A in step 7, M is the length of CSI, D is the downsampling ratio, User B downsamples the quantized value in is the value quantized by user B in step 7, M is the length of CSI, and D is the downsampling ratio.

10. The method for generating a physical layer key in an FDD communication system based on a loop-back mechanism according to claim 1, wherein: Step 9 is as follows: User A performs Gray coding on the downsampled value to obtain the preliminary key. in is the value obtained by downsampling of user A in step 8, M is the length of CSI, D is the downsampling ratio, User B performs Gray encoding on the downsampled value to obtain the preliminary key in is the value obtained by downsampling user B in step 8, M is the length of CSI, and D is the downsampling ratio; Step 10 is as follows: User A and User B reconcile the obtained preliminary key and discard the bits that are -1 in the obtained preliminary key. At this time, the key obtained by User A is in is the preliminary key obtained by Gray encoding of user A in step 9, M is the length of CSI, D is the downsampling ratio, The key obtained by user B is in is the preliminary key obtained by Gray encoding of user B in step 9, M is the length of CSI, D is the downsampling ratio, Then the hash function is used to amplify the privacy of the key, and the final key obtained by user A is Where Hash(·) represents the hash function, The final key obtained by user B is Where Hash(·) represents a hash function.

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