Improved key generation method for XL-MIMO systems

Abstract

A key generation method for securing communication between at least an access point (100) comprising at least a large scale antenna array (110) having N number of AP antenna elements (111) and at least a user equipment (200) comprising at least a UE antenna element (210). The method comprises a probing step where some of the AP antenna elements are (111) probed in each repetition of probing step in order to generate key bits for generating a key using said key bits.

Description

[0001] IMPROVED KEY GENERATION METHOD FOR XL-MIMO SYSTEMS

[0002] TECHNICAL FIELD

[0003] Invention relates to a key generation method for securing communication between at least an access point comprising at least a large scale antenna array and at least a user equipment.

[0004] BACKGROUND

[0005] Security is one of the critical requirements in wireless networks. Cryptographic keys have been widely used to secure the message bits being communicated. Traditional cryptograph-based security approaches are mainly based on the Diffie-Hellman key exchange mechanisms which provides security by relying on the hardness of the key-computation problem at the adversaries’ node.

[0006] Disclosed traditional approaches can be costly and insufficient in most of the modern wireless network settings. Furthermore, once the computational capability of the adversaries is improved, the integrity of these approaches is compromised. J. M. Hamamreh, H. M. Furqan, and H. Arslan, “Classifications and applications of physical layer security techniques for confidentiality: A comprehensive survey,” IEEE Commun. Surveys Tuts., vol. 21 , no. 2, pp. 1773-1828, 2018 emphasizes above mentioned shortcoming.

[0007] Therefore, Physical layer security (PLS) has been proposed as an alternative approach for complementing the traditional security key management techniques. Physical layer-based key generation relies on exploiting the random and reciprocal aspects of the wireless channel as a means of generating an inherently shared cryptographic key. In a multipath or mobile environment, channel characteristics fluctuate randomly due to fading. In the absence of interference and hardware impairments, the bidirectional channel fluctuations are identical between two transceivers at a given instant of time and frequency band.

[0008] A typical physical layer based key generation process involves 5 steps:

[0009] Channel probing: the communicating nodes exchange channel sounding signals with each other during channel coherence time to measure correlated bidirectional channel responses. The measurement can be in terms of channel envelope, channel state information (CSI) (i.e., channel impulse response (CIR), channel frequency response (CFR), etc.), phase, directions of arrivals, etc.

[0010] Randomness extraction: pre-processing of the measured channel samples to remove the deterministic aspects of the channel that can be inferred by the adversaries. Deterministic parts of the channel can be the slowly varying large scale fading effects (path loss and shadowing), etc.

[0011] Quantization: transforming the extracted random channel samples into binary bits.

[0012] Information reconciliation: Correcting any mismatch between the keys generated separately at the legitimate nodes. This step is realized through error correction mechanisms between legitimate nodes.

[0013] Privacy amplification: Eliminating portions of generated key bit streams that might have been exposed to the attacker during the information reconciliation step.

[0014] In the above steps of the key generation process, channel randomness, reciprocity, and coherence time are critical factors that determine efficiency and successfulness of the key generation process. Coherence time generally determines the channel probing rate in the first step. In order to reduce temporal correlation in the key bit stream, it is necessary to perform the bidirectional channel probing once in each coherence window. In the static or low mobility propagation environment, coherence times tend to be very long which dictates low channel probing rate, leading to high latency and low key generation rate. This problem is usually tackled by resorting to exploiting channel randomness in other domains, such as spatial domain.

[0015] Multiple antenna transceiver systems are usually required in order to be able to exploit spatial domain randomness of the channel. The recent advent of the massive multiple-input multipleoutput (mMIMO) transceiver architectures have made exploitation of the spatial domain characteristics of the channel possible. In the literature, techniques that leverage the spatial channel characteristics for channel-based key generation using mMIMO have been proposed in several studies.

[0016] A straightforward channel-based key generation approach using mMIMO is by generating the key from the NxN CSI matrix observed by each of the legitimate transceiver nodes. This approach can generate a high entropy security key. However, this requires equal number of antenna arrays in both nodes which is not feasible for most user equipment (UE). Furthermore, although the antenna elements across the array may observe spatially uncorrelated channel responses, the channel observed by each element can remain the same over long period of time (i.e., long coherence time) in the absence of mobility in the scenario.

[0017] To eliminate the necessity of having equal antenna array sizes at both legitimate nodes, beams-pace channel processing has been employed. The key bits are extracted from the random channel parameters associated with the virtual beam-space channel in this case, where the virtual angle of departure (AoD) - angle of arrival (AoA) pairs, gains, and / or phases of the observed principal components are used as the common source of randomness for a key generation. [2] L. Jiao, J. Tang, and K. Zeng, “Physical layer key generation using virtual AoA and AoD of mmWave massive MIMO channel,” in Proc. IEEE Conf. Commun. Netw. Security (CNS), Beijing, China, 2018, pp. 1-9 discloses such approach. This approach, however, is only applicable in sparse channels as beam-space channel processing fails in rich scattering scenarios. Beam-space channel parameters are also robust against channel mobility which makes them vary very slowly. This can result in low key generation rates in case of very sparse (with only a few principal components) and static propagation environment.

[0018] It is also possible to harvest spatial channel randomness by using the beamforming ability facilitated by mMIMO systems. Beam forming provides two parameters that can be used to control channel randomness and correlation for high entropy key generation. These include beam width and beam direction. Different beam patterns (beam width and direction) effectuate different multipath components and thus different fading coefficients. Therefore, some studies such as “L. Jiao et al., “Physical layer key generation in 5G wireless networks,” IEEE Wireless Commun. Mag., vol. 26, no. 5, pp. 48-54, 2019.” dynamically change the beam pattern during channel probing to induce temporal decorrelation between successive probing in order to improve entropy of the key. The main drawback of this approach is that a full knowledge of the scatter locations within the propagation environment is required by at least of the node in order to optimize directions of the probing beams.

[0019] All the problems mentioned above have made it necessary to make an innovation in the relevant technical field as a result.

[0020] BRIEF DESCRIPTION OF THE INVENTION

[0021] The present invention relates to a method and a system to eliminate the above-mentioned disadvantages and bring new advantages to the relevant technical field. An object of the invention is to generate a high entropy key even under static or slowly timevarying channels.

[0022] Another object of the invention is to realize a method in order to generate secret key with increased randomness and suitable to be realized with access point and user equipment that are having unequal antenna sizes.

[0023] To achieve all the objects mentioned above and that will emerge from the following detailed description, the present invention relates to a key generation method for securing communication between at least an access point comprising at least a large scale antenna array having N number of AP antenna elements and at least a user equipment comprising at least one UE antenna element. Accordingly, it is characterized by comprising a probing step, where said probing step is repeated predetermined number of times, having steps of;

[0024] - by access point selecting plurality AP antenna elements where M is lower than N;

[0025] - by access point activating selected AP antenna elements;

[0026] - by access point transmitting channel sounding signal from activated AP antenna elements;

[0027] - by user equipment receiving channel sounding signals and measuring at least a channel parameter of received sounding signals;

[0028] - by user equipment generating a key bit using measurements;

[0029] - by user equipment transmitting channel sounding signals to access point;

[0030] - by access point receiving channel probing signals using activated AP antenna elements and measuring said channel parameter;

[0031] - by access point generating a key bit using measurements; comprising a key generation step having step of;

[0032] - generating at least a key using generated key bits.

[0033] Thus, a high-entropy keys can be generated even under static or slowly time-varying channel. Further, the temporal randomness is achieved by activating different sets of antennas. It also solves the problem of the unequal array sizes at the access point and the user equipment sides without resorting to beamforming or beam-space channel representation and their associated drawbacks as explained in prior art section.

[0034] A possible embodiment of the invention is characterized in that wherein said AP antenna elements are selected randomly. Another possible embodiment of the invention is characterized in that determining visibility regions between access point and user equipment; selecting AP antenna elements in such way at least two of the selected AP antenna elements belongs to different visibility regions.

[0035] Another possible embodiment of the invention is characterized in that wherein at least consecutive subarrays where subarray defines the selected probing AP antenna elements, are different than each other on each repeat.

[0036] Another possible embodiment of the invention is characterized in that wherein selected antenna number Ms is changed in each repetition.

[0037] Another possible embodiment of the invention is characterized in that wherein selected antenna number Ms is randomly changed.

[0038] The invention is also a key generation system comprising at least an access point comprising at least a large scale antenna array having N number of AP antenna elements; a baseband processing unit and plurality of AP RF chains for selectively connecting baseband processing unit with AP antenna elements; and at least a user equipment having at least a UE antenna element; at least a UE baseband processing unit and plurality of UE RF chains connected between baseband unit and UE antenna elements characterized in that said access point comprising switching unit for selectively connecting AP RF chains at with AP antenna elements; access point is configured to realize steps of:

[0039] - selecting Ms number of AP antenna elements where Ms is lower than N;

[0040] - activating selected AP antenna elements using switching unit;

[0041] - transmitting channel sounding signal from activated AP antenna elements;

[0042] - receiving channel probing signals using activated AP antenna elements and measuring at least a channel parameter;

[0043] - generating a key bit using measurements; user equipment is configured to realize steps of:

[0044] - receiving channel sounding signals using and measuring at least said channel parameter of received sounding signals;

[0045] - generating a key bit using measurements;

[0046] - transmitting channel sounding signals to access point; user equipment and access point are configured to generate matching keys using generated key bits. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 is a drawing illustrating top schematic view of the system.

[0048] Figure 2 is a drawing illustrating visibility windows having scatter entities between access point and user equipment.

[0049] REFERENCE NUMBERS GIVEN IN THE FIGURE

[0050] 100 Access Point

[0051] 110 Large Scale Antenna Array

[0052] 11 1 AP Antenna elements

[0053] 120 AP Baseband processing unit

[0054] 130 AP RF Chain

[0055] 140 AP Control Unit

[0056] 150 Switching unit

[0057] 200 User Equipment

[0058] 210 UE Antenna element

[0059] 220 UE RF Chains

[0060] 230 UE Control unit

[0061] 240 UE baseband processing unit

[0062] 300 Scattering Entity

[0063] 310 Visibility region

[0064] DETAILED DESCRIPTION OF THE INVENTION

[0065] In this detailed description, the subject matter is explained with references to examples without forming any restrictive effect only in order to make the subject more understandable.

[0066] Reference is made to Third Generation Partnership Project (3GPP) system, in accordance with embodiments of the present disclosure. The present application employs abbreviations, terms and technology defined in accord with Third Generation Partnership Project (3GPP) technology standards, including the following standards and definitions. 3GPP technical specifications (TS) and technical reports (TR) incorporated in their entirety by reference herein, define the related terms and architecture reference models that follow. AP: Access Point

[0067] UE: User equipment gNB: gNodeB I Next Generation Node B

[0068] UL: uplink

[0069] DL: downlink

[0070] XL- Ml MO: Extra Large Multiple-Input Multiple-Output

[0071] ELAA: Extra Large Antenna Array

[0072] AoA: Angle of arrival

[0073] RF: Radio Frequency mMIMO: massive Multiple-Input Multiple-Output

[0074] Referring to figure 1 , present invention is a system that realizes an improved channel probing step of key generation in XL-MIMO systems. System activates different sets of antennas in each probing session and generates key bits. Generated key bits then utilized in order to generate a key on each side.

[0075] System comprises an access point (100) and a user equipment (200). Access point (100) and user equipment (200) realizes channel probing in order to generate keys for communication.

[0076] Access point (100) comprises a large scale antenna array (110) having plurality of AP antenna elements (1 11 ). Large scale antenna array (1 10) is suitable for XL-MIMO communication. AP Antenna elements (1 11 ) may be provided in number of N.

[0077] Access point (100) in this description refers to a device or system that has high cost and / or large scale antenna elements in order to realize telecommunication. For example, but not limited to, access point (100) may be a Node B, g Node B, radio head, a New Radio, a 5G node B etc.

[0078] Access point (100) comprises a switching unit (150) and plurality of RF chains. Switching unit (150) is connected AP antenna elements (111 ) in large scale antenna array (110). Access point (100) further comprises an AP baseband processing unit (120) connected to RF chains. Switching unit (150) switches connections between RF chains and AP antenna elements (1 11 ). In other words, switching controls which RF chains connects to which AP antenna elements (11 1 ). RF chains are well known in the art. RF chains may comprise, amplifiers, mixers, filters etc. AP baseband processing unit (120) is also well known in the art, so further details is not disclosed herein. RF chains may be provided in number of (100) NRF where NRF « N. Switching unit (150) may be implemented by using multiple pole multiple throw switches or any other switching mechanism capable of multiple inputs and multiple outputs (100).

[0079] Access point (100) comprises an AP control unit (140) for controlling switching unit (150) and AP baseband processing unit (120). AP control unit (140) may be a microprocessor or any other suitable processing unit (100).

[0080] User equipment (200) comprises UE antenna elements (210). UE antenna elements (210) may be in number of M. User equipment (200) comprises UE RF chains (220) connected to UE antenna elements (210). User equipment (200) comprises a UE baseband processing unit (240). UE RF chains (220) are connected between UE baseband processing unit (240) and UE antenna elements (210). User equipment (200) may comprise UE control unit (230) in order to control components of user equipment (200).

[0081] Scattering entities (300) are present between user equipment (200) and AP (100) With large scale antenna array (110) architecture, most of the user equipment (200) and scattering entities (300) fall within the near-field of the AP antenna elements (1 11 ). The near-field of an antenna array is defined as the region where the signal propagates in a distance smaller than the Fraunhofer distance of the array. The Fraunhofer distance is defined (100) as , 2D dF- ~2where D is the array aperture size and A is the wavelength of the signal. In the near-field, a non-stationary channel is observed across the array, in the technical area it is referred to as array non-stationarity. The array non-stationarity is a by-product of two fundamental phenomena. The first one is the spherical-wavefront (SW) propagation. In the near-field, electromagnetic waves propagate with SW instead of the traditional planar wavefront assumption which is only applicable in far-field scenarios. The curvature of the SW induces an extra phase shift to each AP antenna element (11 1 ) (as a function of the element’s index) across large scale antenna array (1 10). It also induces different angle of arrivals (AoA) to each element which makes the received signal at each element experience different effective aperture areas and polarization mismatch losses. The second is the cluster visibility region (310) (VR) across the array. Specifically, in a complex propagation environment, different portions of the large scale antenna array (110) may be exposed to distinct sets of blockages and scattering entities (300) and thus experience different channel characteristics as shown in Figure 2. Therefore, these two phenomena, i.e., SW and VR, cause significant variation of the channel parameters, in terms of received power, path gain, AoA, K-factor, coherence time, etc., across the array elements even in the poor scattering environment. Therefore, by activating different set of AP antenna elements (1 11 ) across the large scale antenna array

[0082] (110) during each probing instant effectuates different channel realization due to the channel non-stationarity property discussed above. In this way, a temporal and spatial decorrelation can be enhanced, and a high entropy key can be generated even under a slowly varying propagation condition.

[0083] Selected set of AP antenna elements are defined as subarrays.

[0084] Access point (100) and user equipment (200) are configured to realize below steps in order to realize subject matter key generation method:

[0085] A channel probing step is realized where, said probing step is repeated in a predetermined number of times. Said channel probing step comprises below sub-steps:

[0086] - Access point (100) selects Ms number of AP antenna elements (11 1 ) where Ms is lower than N.

[0087] - Access point (100) activates selected AP antenna elements (1 11 ). Activation may be realized by connecting AP RF chains (130) to selected AP antenna elements (1 11 ) by switching unit (150).

[0088] - Access point (100) transmits channel sounding signal from activated AP antenna elements

[0089] (11 1 ). Channel sounding signal may be generated by baseband processing unit.

[0090] - User equipment (200) receives channel sounding signals and measures at least one channel parameter from the received sounding signals.

[0091] - User equipment (200) transmits channel sounding signals to access point (100). Access point (100) receives channel probing signals using activated AP antenna elements (1 11 ) and measures said channel parameter.

[0092] - User equipment (200) generating a key bit using measurements.

[0093] - Access point (100) generates a key bit using measurements.

[0094] Therefore, on each repetition, since channel characteristics are the same during probing a same key bit is generated both on user equipment (200) side and access point (100) side. The method further comprises a key generation step where key bits are used to generate a key in order to secure communication between user equipment (200) and access point (100).

[0095] In a possible embodiment, access point (100) may select AP antenna elements (11 1 ) in a random manner in each repetition.

[0096] Number of repetitions may be predetermined, based on key’s requirements. Key’s requirements may be based on a known standard. The selection can be based on correlation between the channel coefficients. In specific, by measuring the correlation amount between the channel coefficients, the coefficients which has the lowest correlation are selected.

[0097] In a possible embodiment, visibility regions (310) between user equipment (200) and access point (100) are determined. AP antenna elements (11 1 ) are selected in such way at least two of the selected AP antenna elements (1 1 1 ) belongs to different visibility region (310). For slow- varying channels, visibility regions can be learned by long-term observation of the channel across the array (110). This can be done by observing, for example, the received power distribution across the array.

[0098] In another possible embodiment, at least consecutive selected antenna groups are different than each other on each repeat.

[0099] Above mentioned channel parameter may be any suitable parameter that can be extracted during channel probing known in the art. Channel parameters ,for instance, may be received signal strength (RSS), channel impulse response (CIR), channel frequency response (CFR), angle-of-arrival / departure (AoA / AoD), path gain and / or phase, path delay, Doppler characteristics, channel sparsity level, etc.

[0100] During the downlink probing, all subarray elements transmit the same channel sounding signal simultaneously which are superposed at the UE antenna elements (210) such that the user equipment (200) observe similar effective channel due to all selected AP antenna elements (11 1 ). In this way, the reciprocity of the channels measured by both nodes is preserved.

[0101] In order to effectuate a different channel realization and induce randomness, a different set of subarrays where subarray is defined by selected AP antenna elements (1 1 1 ), are activated ELAA during each channel probing window.

[0102] It is also possible that, during the downlink probing, the subarray elements transmit orthogonal pilot symbols as the probing signal and the UE uses these pilots to estimate the desired channel parameter for each individual channel links between AP and UE. Similarly for the uplink probing. The above description of the invention considers poor scattering environment and exploits array non-stationarity due to ELAA. However, the method is not restricted to this setup alone. It can easily work with the standard mMIMO setup and rich-scattering propagation environment as well. In rich scattering environment, channel is assumed to de-correlate at half wavelength distance in space and thus all antenna elements in mMIMO are assumed to observe completely different / independent / uncorrelated channels which can be considered as the special case of VR. Since it is possible to have rich scattering but static or slowly varying propagation condition, the proposed channel probing method can be used in such scenarios to enhance key generation rate (independent of the channel’s coherence time) improve temporal randomness of the generated key bit stream.

[0103] The method is also applicable for scenarios with single-antenna UEs, i.e., M=1. In this case, the subarray size in each probing can be reduced to M=1. It is also possible to use (M>1 )- sized subarray at the access point (100) side in each probing. In this case, all subarray elements are connected to the same RF-chain through the switching mechanism. In this way, only an effective channel due to all subarray elements (summation of their channel responses) is observed in the baseband domain during uplink channel measurement / probing. During the downlink probing, all subarray elements transmit the same channel sounding signal simultaneously which are superposed at the UE’s antenna such that the UE observe similar effective channel due to all subarray elements. In this way, the reciprocity of the channels measured by both nodes is preserved. Note that, in this case, the subarray size can be changed in every probing instant which can increase the number of random sets of subarrays and thus number of ways of effectuating channel randomness in each probing.

[0104] The above description of the invention considers poor scattering environment and exploits array non-stationarity due to ELAA. However, the method is not restricted to this setup alone. It can easily work with the standard mMIMO setup and rich-scattering propagation environment as well. In rich scattering environment, channel is assumed to de-correlate at half wavelength distance in space and thus all antenna elements in mMIMO are assumed to observe completely different / independent / uncorrelated channels which can be considered as the special case of VR. Since it possible to have rich scattering but static or slowly varying propagation condition, the proposed channel probing method can be used in such scenarios to enhance key generation rate (independent of the channel’s coherence time) improve temporal randomness of the generated key bit stream. The scope of protection of the invention is specified in the attached claims and cannot be limited to those explained for sampling purposes in this detailed description. It is evident that a person skilled in the art may exhibit similar embodiments in light of the above-mentioned facts without drifting apart from the main theme of the invention.

Claims

CLAIMS1 . A key generation method for securing communication between at least an access point (100) comprising at least a large scale antenna array (1 10) having N number of AP antenna elements (11 1 ) and at least a user equipment (200) comprising at least a UE antenna element (210) characterized in that comprising a probing step, where said probing step is repeated predetermined number of times, having steps of;- by access point (100) selecting plurality AP antenna elements (1 11 ) where Ms is lower than N;- by access point (100) activating selected AP antenna elements (11 1 );- by access point (100) transmitting channel sounding signal from activated AP antenna elements (11 1 );- by user equipment (200) receiving channel sounding signals and measuring at least a channel parameter of received sounding signals;- by user equipment (200) generating a key bit using measurements;- by user equipment (200) transmitting channel sounding signals to access point;- by (100) access point (100) receiving channel probing signals using activated AP antenna elements (11 1 ) and measuring said channel parameter;- by access point (100) generating a key bit using measurements; comprising a key generation step having step of;- generating at least a key using generated key bits.

2. The key generation method according to claim 1 , characterized in that wherein said AP antenna elements (1 11 ) are selected randomly.

3. The key generation method according to claim 1 , characterized in that determining visibility regions (310) between access point (100) and user equipment (200); selecting AP antenna elements (11 1 ) in such way at least two of the selected AP antenna elements (11 1 ) belongs to different visibility region (310).

4. The key generation method according to claim 1 , characterized in that wherein at least consecutive subarrays, where subarray is defined by selected AP antenna elements (11 1 ), are different than each other on each repeat.

5. The key generation method according to claim 1 , characterized in that wherein selected antenna number Ms is changed in each repetition.

6. The key generation method according to claim 5, characterized in that wherein selected antenna number Ms is randomly changed.

7. A key generation system comprising at least an access point (100) comprising at least a large scale antenna array (1 10) having N number of AP antenna elements (1 1 1 ); a baseband processing unit and plurality of AP RF chains (130) for selectively connecting baseband processing unit with AP antenna elements (11 1 ); and at least a user equipment (200) having at least a UE antenna element (210); at least a UE baseband processing unit (240) and plurality of UE RF chains (220) connected between baseband unit and UE antenna elements (210) characterized in that said access point (100) comprising switching unit (150) for selectively connecting AP RF chains (130) at with AP antenna elements (1 11 ); access point (100) is configured to realize steps of:- selecting Ms number of AP antenna elements (111 ) where Ms is lower than N;- activating selected AP antenna elements (11 1 ) using switching unit (150);- transmitting channel sounding signal from activated AP antenna elements (11 1 );- receiving channel probing signals using activated AP antenna elements (1 11 ) and measuring at least a channel parameter;- generating a key bit using measurements; user equipment is configured to realize steps of:- receiving channel sounding signals using and measuring at least said channel parameter of received sounding signals;- generating a key bit using measurements;- transmitting channel sounding signals to access point; user (100) equipment and access point (100) are configured to generate matching keys using generated key bits.

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