WO2023107075A2

WO2023107075A2 - Distributed carrier frequency offset compensation for secure wireless communication

Info

Publication number: WO2023107075A2
Application number: PCT/TR2022/051438
Authority: WO
Inventors: Abu Bakari KIHERO; Muhammad Sohaib Jamal SOLAIJA; Hüseyin ARSLAN
Original assignee: Istanbul Medipol Universitesi Teknoloji Transfer Ofisi Anonim Sirketi
Priority date: 2021-12-09
Filing date: 2022-12-07
Publication date: 2023-06-15
Also published as: TR2021019472A2; WO2023107075A3

Abstract

In this invention, a method to protect wireless communication signals from being overheard/intercepted by malicious eavesdroppers is proposed.

Description

DISTRIBUTED CARRIER FREQUENCY OFFSET COMPENSATION FOR SECURE

WIRELESS COMMUNICATION

Technical Field

Prior Art

At present the following approaches are being used for physical layer security (PLS) in multicarrier systems such as OFDM:

Extraction of Secret Sequences from Wireless Channels

The basic idea in these types of PLS techniques is to extract a random sequence from the wireless channel. This sequence is then used as a key for encrypting the information to be exchanged between the transceivers. However, the key generation approach is very sensitive to imperfect channel estimation and channel reciprocity mismatch.

Addition of Interfering (Noise/Jamming) Signals along with the Transmitted Signals

In this approach the legitimate node adds an intentional interfering signal by exploiting the nullspace of the legitimate user's channel in such a way that it degrades the performance at illegitimate receiver without affecting the performance at the legitimate receiver. However, it may sacrifice some power and throughput.

Channel-Based Adaptation Transmission

In these techniques the transmission parameters of are adjusted specifically based on the channel between the legitimate transceivers to fulfill their quality of service (QoS) requirements. Some examples include adaptive modulation and coding with Automatic Repeat Request (ARQ) fading based sub carrier activation technique, optimal power allocation based techniques, channel shortening and OFDM-subcarrier index selection. However, the problem of these approaches is that they fail if the eavesdropper has a better channel as compared to the legitimate receiver.

Coordinated Multipoint (CoMP)-Based Security

Apart from the above-mentioned general approaches of facilitating PLS, some works have attempted to exploit CoMP concept for securing wireless systems.

CoMP has been leveraged in prior art to address the limitation of directional modulation as it fails to secure the communication if the eavesdropper is in the same direction as the legitimate receiver. The coordinating transmission points (TPs) transmit copies of the same signal using directional modulation, such that these are correctly received only at the legitimate receiver’s location, providing location-based security. However, this approach also fails to provide the desired level of security if the attacker is in the vicinity of the legitimate receiver.

CoMP is also used for sparse radio environments such that data is only decodable at the intersection of information beams while a distorted constellation is observed at other locations. This approach also does not guarantee secure communication if the attacker is close to the legitimate receiver.

CoMP -based transmissions from distributed antenna elements in an underwater scenario is proposed in prior art, where the power and schedule of transmissions are manipulated such that, the messages are non-overlapping at the legitimate receiver while being overlapped (and interfering) at the eavesdropper.

A dynamic CoMP scheme is proposed in prior art to enhance secured coverage. The proposed scheme is based on the received signal power for the legitimate users where TPs far from the legitimate user are blocked for security and energy consumption concerns. A CoMP scheme is also being used in unmanned aerial vehicle (UAV) systems to achieve secrecy where multiple ground nodes cooperatively detect the legitimate information sent from the UAV to enhance the legitimate communication performance in eavesdropper’s presence.

Carrier Frequency Offset (CFO) Compensation- Based Security

A CFO pre-compensation based PLS technique is proposed in prior art. The technique considers a point-to-point transmission in which only one CFO exists between the communicating legitimate nodes. Due to the uniqueness of the CFO incurred by any pair of the communicating nodes, the authors consider pre-compensating the CFO at the transmitter with respect to the legitimate receiver as a means of facilitating secure communication. However, due to the presence of many robust single CFO estimation techniques in the literature, security provided by such approach can be easily disrupted. To this end, the authors complemented their approach with channel pre-equalization in order to enhance the achieved quality of security. The channel pre-equalization, however, has an obvious drawback of the elevated peak-to-average power ratio (PAPR) of the signal.

Aims of the Invention and Brief Description

The present invention is related to a method for protecting wireless communication signals from being overheard/intercepted by malicious eavesdroppers in order to overcome the disadvantages mentioned above and to bring new advantages to the related technical field.

The broadcast nature of wireless communication renders it prone to various security threats. One of these threats is the violation of confidentiality of communication, also referred to as eavesdropping. In this case, a malicious node/device tries to intercept and interpret the communication going on between two legitimate nodes. Conventionally, cryptography is used for securing the communication, but with the increasing heterogeneity of future wireless networks this approach faces challenges related to key sharing and management. This motivates the use of physical layer security (PLS) approaches, which exploits the properties of the wireless channel and hardware to secure wireless transmissions. Keeping this in mind, PLS mechanisms such as the one mentioned in this invention have become increasingly popular in recent years. The invention disclosed in this document provides a PLS mechanism where multiple transmitting nodes under the coordinated multipoint (CoMP) concept cooperate to provide secure communication for multicarrier systems such as orthogonal frequency division multiplexing (OFDM) which is widely used in wireless communication standards such as Long-Term Evolution (LTE), 5G New Radio (NR), and IEEE 802.11 based Wi-Fi networks.

Multicarrier systems such as OFDM are popular because they mitigate the frequency selective fading issue by dividing the channel into several narrow subchannels (or subcarriers) where each is smaller than the coherence bandwidth, converting the channel into a flat fading one. However, in this case the system relies on the orthogonality of the subcarriers in frequency domain to ensure reliable communication. Any loss of orthogonality can lead to inter-carrier interference which causes severe performance degradation. This renders OFDM and other multicarrier systems susceptible to any mismatch in frequency synchronization which may occur due to impairments such as Doppler and carrier frequency offset (CFO), resulting in inter-carrier interference (ICI).

In this invention, this limitation of OFDM is exploited to secure wireless transmissions. If CFO is known at the transmitter, it can be pre-compensated before the transmission is carried out. Since the CFO experienced by different user equipment (UE)- transmission point (TP) links are different, the proposed invention leverages the different CFOs between the different TPs provided by CoMP network and the desired user to secure the signal. The pre-compensation for the desired user results in no degradation for it, however, on the other hand, this compensated offset appears as an undesired interference for the illegitimate node. That is, the multiple, precompensated CFOs provided by cooperating TPs causes the signal received by the illegitimate node to spread in frequency, thereby inducing an ICI problem which makes interception much more challenging.

Existing PLS method leveraging cooperative networks, CoMP in particular, can only provide the region-based security. That is, the confidentiality of the communication is ensured only if the attacker is not in the vicinity of the legitimate UE or along the direction of signal transmission. In addition to these, the CFO pre-compensation based PLS approach proposed in prior art relies on the channel pre-equalization step to strengthen the quality of security in case the attacker can successfully estimate and compensate the introduced single frequency shift. However, channel pre-equalization can potentially cause a PAPR problem that imposes some strict hardware requirements in order to maintain the desired performance.

The proposed invention solves both the above-mentioned problems by leveraging the CFOs of the geographically separated TPs in cooperative network topologies. Since the proposed security method relies on the CFOs pre-compensation with respect to a specific, legitimate UE, quality of the attacker’s signal will be degraded even if the attacker is co-located with the legitimate node. The fact that we now have multiple, individually pre-compensated CFOs in the considered system, will cause the attacker to receive a signal with multiple independent frequency shifts that are much more challenging to deal with as compared to the single frequency shift observed in the case of the point-to-point transmission proposed in prior art.

Furthermore, in case the attacker is not in the vicinity of the legitimate receiver, but it is in the coverage area of any of the coordinating TPs, it is possible that it can intercept the signal. In that case, the proposed invention can be used in conjunction with channel pre-equalization with some modification to circumvent the PAPR problem demonstrated. We again leverage the presence of multiple, geographically separated TPs experiencing independent channels with the legitimate node. In this case, subcarriers falling into the deep fades of one TP-legitimate-UE link can be turned off for that particular TP and transmitted from the rest of the cooperating TPs. This removes the necessity of performing channel pre-equalization at the locations of the deep fades for each TP, thereby solving the PAPR problem.

The invented scheme constitutes a hybrid joint transmission (JT) and dynamic point selection (DPS) (H-JT-DPS) mechanism with channel pre-equalization and multiple CFOs precompensation to facilitate PLS in the whole coverage area. The multiple CFO pre-compensation ensures confidentiality of the communication even if the attacker is co-located with the legitimate node, whereas the channel pre-equalization provides security when the attacker is in any other location within the coverage area. The H-JT-DPS avoids the potential PAPR problem that may arise due to the channel pre-equalization in the deep fade locations. Another potential approach that can be used in conjunction with the multiple CFOs pre- compensation to facilitate security against attacker that in the proximity of one of the cooperating TPs or along the direction of transmission of any of the TPs is data splitting. With data splitting, each of the cooperating APs transmit different portion of the data intended for the legitimate node such that it is necessary to receive the signal from all the TPs to be able to decode it properly.

In fact, the multiple CFOs pre-compensation scheme invented herein can also be leveraged by any other cooperating TPs based security schemes, to extend their region-based security to a full security within the coverage area.

Any wireless communication technology can utilize this invention to provide protection against eavesdroppers. However, standards like 3GPP -based cellular and IEEE 802.11 based Wi-Fi networks are particularly relevant to the invention due to the support of multipoint coordination and use of multicarrier schemes such as OFDM in both standards.

Advantages of Invention:

Technical Merit

The proposed invention provides a low-complexity method for future wireless networks that uses the availability of multiple geographically distributed transmission points to provide secure and confidential communication. This is achieved by sending the data through different links between the cooperating TPs and the legitimate receiver, where each link has its independent CFO. The independence of these CFOs, as well as the ones experienced by the illegitimate receiver, ensures that while the pre-compensation ensures no ICI is experienced at the legitimate receiver, this is not the case for the illegitimate receiver. The illegitimate receiver experiences a spread in frequency domain, which causes ICI and degrades the interception probability.

Definition of the Figures of the Invention

The figures have been used in order to further disclose a method of distributed carrier frequency offset compensation for secure wireless communication developed by the present invention as described below: Figure 1 : A flow chart of basic operations for wireless communication in accordance with certain aspects of the present disclosure.

Figure 2: An alternative implementation option of the present disclosure

Figure 3: Another alternative implementation option of the present disclosure.

Figure 4: Illustration of CFO pre-compensation with respect to the legitimate node and its effect on the received signal at the legitimate and attacker nodes

Figure 5: Example of the cooperative network applying the proposed invention to facilitate confidentiality of the downlink transmission

Figure 6: Illustrating the multiple CFOs pre-compensation with an enhanced channel preequalization technique to ensure security across the whole coverage area.

Definition of Elements and Steps in Figures:

TP: Transmission Point

CFO: Carrier Frequency Offset

UE: User Equipment

Steps:

110 is selection of the coordinating TPs.

120 each TP estimates the CFO between itself and the desired user.

130 Depending on the CFO estimated in step (120), the necessary compensation is calculated in this step.

140 is the transmission of user data from the coordinating TPs

210 is selection of the coordinating TPs.

220 each TP estimates the CFO between itself and the desired user.

230 is the pre-equalization of channel experienced at all links between the TPs and the user.

240 is the pre-compensation of the estimated CFO obtained in step (220) 250 is the transmission of user data from the coordinating TPs

310 is selection of the coordinating TPs.

320 each TP estimates the CFO between itself and the desired user.

330 the user data is split into different parts where each coordinating TPs sends a part of the split data.

340 is the pre-compensation of the estimated CFO obtained in step (220)

350 is the transmission of user data from the coordinating TPs

410 is the baseband signal spectrum prior to CFO pre-compensation.

420 is the CFO pre-compensation process.

430 is the baseband signal spectrum after CFO pre-compensation.

440 is the legitimate TP.

450 is the legitimate receiver node.

460 shows the correctly frequency-synchronized baseband signal as received by the legitimate node.

470 is the attacker node.

480 is the signal received by the attacker with imperfect frequency synchronization.

510 is the CFO pre-compensation step.

520 are the geographically separated TPs cooperatively serving the UE.

530 is the legitimate receiver node (UE)

540 is the frequency-synchronized signal received by the legitimate receiver node. 550 is the attacker’s node that wishes to intercept the communication.

560 is the signal received with distortion (frequency spreading) at the attacker’s node.

610 represents a frequency domain one-tap channel pre-equalization process at each of the cooperating TPs

620 is the CFO pre-compensation step at each of the cooperating TPs

630 represents the geographically separated TPs cooperatively serving a given legitimate UE

640 represents the independent frequency channel responses (CFR) experienced by each TP with respect to the legitimate UE

650 represents deep fades based scheduled subcarriers carrying the data to be transmitted to the legitimate UE. 660 independently occurring deep fades

Detailed Description of the Invention

The novelty of the invention has been described with examples that shall not limit the scope of the invention and which have been intended to only clarify the subject matter of the invention. The present invention has been described in detail below.

The invention is a method to protect wireless communication signals from being overheard/intercepted by malicious eavesdroppers.

Certain aspects of the present disclosure:

One embodiment of the invention is directed to a method for preventing interception of a wireless communication means by malicious eavesdroppers wherein;

• The first step (110) of the proposed invention consists of selecting the coordinating transmission points (TPs). This selection can be based on different criteria, such as the received signal strength indicator (RSSI), received power relative to the serving TP, or a combination of both. The selection of coordination set can also be done for different objectives/constraints such as capacity maximization, spectral efficiency, energy efficiency etc. In addition to this, in line with this invention, it is possible to consider carrier frequency offset (CFO) between the TP and legitimate receiver as a criterion for coordinating TP selection.

• In the second step (120), each TP estimates the CFO between itself and the desired user. The CFO estimation may be done using preambles or in a blind manner.

• Depending on the CFO estimated in step (120), the necessary compensation is calculated in step (130).

• Step (140) then commences the transmission from all or some selected TPs. Here the transmissions from the different TPs may carry the same data, or different parts of the data.

Fig. 2 is a flowchart that describes an alternate implementation of the present disclosure, thus another embodiment of the invention is directed to a method for preventing interception of a wireless communication means by malicious eavesdroppers wherein: • The first step (210) of the proposed invention consists of selecting the coordinating TPs. This selection can be based on different criteria, such as the received signal strength indicator (RSSI), received power relative to the serving TP, or a combination of both. The selection of coordination set can also be done for different objectives/constraints such as capacity maximization, spectral efficiency, energy efficiency etc. In addition to this, in line with this invention, it is possible to consider CFO between the TP and legitimate receiver as a criterion for coordinating TP selection.

• In the second step (220), each TP estimates the CFO between itself and the desired user. The CFO estimation may be done using preambles or in a blind manner.

• Depending on the CFO estimated in step (220), the necessary compensation is performed in step (230).

• Step (240) performs the channel pre-equalization for each TP-UE links.

• Step (250) then commences the transmission from all or some selected TPs. In order to avoid issues such as high PAPR, dynamic point selection (DPS) can be used such that the subcarriers which are in deep fade are not used for that particular link.

Fig. 3 is a flowchart that describes an alternate implementation of the present disclosure thus another embodiment of the invention is directed to a method for preventing interception of a wireless communication means by malicious eavesdroppers wherein:

• The first step (310) of the proposed invention consists of selecting the coordinating TPs. This selection can be based on different criteria, such as the received signal strength indicator (RSSI), received power relative to the serving TP, or a combination of both. The selection of coordination set can also be done for different objectives/constraints such as capacity maximization, spectral efficiency, energy efficiency etc. In addition to this, in line with this invention, it is possible to consider CFO between the TP and legitimate receiver as a criterion for coordinating TP selection.

• In the second step (320), each TP estimates the CFO between itself and the desired user. The CFO estimation may be done using preambles or in a blind manner

• Step (330) encompasses splitting of the data to be transmitted from the different TPs. This splitting can be on an intra-symbol level (e.g., one TP may transmit real part of the symbol while the imaginary part of the same symbol is transmitted by a same signal), or even on a subcarrierlevel (where different TPs transmit different subcarriers).

• Depending on the CFO estimated in step (320), the necessary compensation is performed in step (340).

• Step (350) then commences the transmission from all or some selected TPs.

Here, it should be kept in mind that Fig. 2 and Fig. 3 (and the steps within) are not mutually exclusive, i.e., the different implementations can be combined in different ways to form even more alternative implementations.

The method disclosed in Figure 4 is directed to an illustration of CFO pre-compensation with respect to the legitimate node and its effect on the received signal at the legitimate node and the attacker’s node.

Explanation of Fig. 4:

At step (420) the frequency offset f -u_E is introduced to the baseband signal 410 intended for the legitimate UE receiver (450). The frequency offset

i^s equal to the expected CFO between the legitimate transmitter (440) and legitimate receiver (450). The step (420) results into a frequency shifted signal (430) which is then transmitted from legitimate transmitter (440). Upon reception, the frequency-shift on frequency shifted signal (430) cancels out with the CFO between legitimate transmitter (440) and legitimate receiver (450), resulting into a perfectly received signal (460).

At the attacker’s node (470), the frequency-shift on frequency shifted signal (430) does not cancel out with CFO between legitimate transmitter (440) and the attacker node (470), rather, a composite random frequency shift is suffered by the received signal which is the signal received by the attacker with imperfect frequency synchronization (480).

Explanation of Fig. 5:

Extension of Fig. 4 to multiple transmitting TPs (520). The frequency shift step is performed at each TP at step (510) which is the CFO pre-compensation step. The frequency shift induced to the baseband signal at each TP (520) is determined by the CFO between the respective TP and the legitimate receiver node (530). As it is the case with Fig. 4, each of the frequency shifts introduced at the CFO pre-compensation step (510) cancels out with the respective CFO upon arriving at the legitimate receiver node (530), resulting into the perfectly frequency-synchronized received signal (540). Such cancellations do not occur at the attacker’s node (550), leading to a frequency spread signal (560) which is Id-infested and thus difficult to decode.

The method disclosed in Figure 5 is directed to an example of the cooperative network applying the proposed invention to facilitate confidentiality of the downlink transmission.

Fig. 6 shows the implementation of the joint channel pre-equalization and multiple CFOs precompensation to facilitate security over the whole coverage area. The enhanced pre-equalization approach avoids pre-equalization at the locations of the deep fades by proposing a hybrid jointtransmission and dynamic -points-selection (H-JT-DPS) technique.

In figure 6, (660) represents deep fades based scheduled subcarriers carrying the data to be transmitted to the legitimate UE. Based on the CFR of each TP, some subcarriers are turned off for that particular TP and transmitted by the other TP with relatively better CFR at that location (i.e., subcarrier based DPS). As such, each TP does not have to pre-equalize the channel at the locations of the deep fades, thereby circumventing the high PAPR problem.

Explanation of Fig. 6:

This figure demonstrates implementation of the H-JT-DPS concept that facilitates channel preequalization without causing the PAPR problem. Each of the cooperating TPs avoid transmitting at the deep fade locations (660) and thus can ignore to pre-equalize the channel response at those locations at step (610). Note that, while some of the subcarriers are turned off for some TPs due to the deep fades in their respective locations, other subcarriers that experience relatively good channel condition in all TPs are transmitted from all of them. This is tantamount to applying DPS to some subcarriers and JT to the others. Step (620) introduces frequency shift to the signal based on the expected CFO of a particular TP (630) with the legitimate UE (not shown in the figure). Process steps;

A. Selecting the coordinating transmission points (110, 210, 310, 520, 630)

While the prior art has different mechanisms for coordination TP selection (as mentioned in the previous section), the invention also discloses a novel alternate, i.e., selection of the coordinating TPs depending on the CFO between the TP and the legitimates receiver.

B. Multiple/distributed CFO estimation and compensation (120, 130, 220, 230, 320, 340, 420, 510, 620)

CFO estimation and compensation in general is well studied. However, exploiting the multiple TPs and CFOs in a CoMP setting to provide secure communication is a novel proposition.

C. The CFO pre-compensation can be applied in conjunction with channel pre-equalization to enhance the secured coverage region. The optional channel pre-equalization step is featured in Fig. 6 step 610.

D. Hybrid DPS and JT to avoid the subcarriers in deep fades for the different TP-user links. This facilitates a smooth channel pre-equalization at each TP without elevating the PAPR problem (610).

E. Data splitting before transmission can also be applied in conjunction with the multiple CFO instead of the channel pre-equalization. Other techniques like CoMP based directional modulations can be used as well (330).

The invention is a method to protect wireless communication signals from being overheard/intercepted by malicious eavesdroppers. As such, it is applicable to industry which is interested in security of the communication.

Depending on the all information above, a method of distributed carrier frequency offset compensation for secure wireless communication comprising the steps of; • Selecting the coordinating transmission points,

• Estimating carrier frequency offset between each coordinating transmission point and the desired user,

• Compensating of the estimated carrier frequency offset,

• Starting transmission from the coordinating transmission points.

A method of distributed carrier frequency offset compensation for secure wireless communication comprising the steps of;

• Selecting the coordinating transmission points,

• Pre-equalizing the channel between each coordinating transmission point and the user,

• Compensating of the estimated carrier frequency offset at each transmission points,

• Starting transmission from the coordinating transmission points

• Selecting the coordinating transmission points,

• Splitting of the data to be transmitted between coordinating transmission point,

• Starting transmission from the coordinating transmission points.

Around these basic concepts, it is possible to develop several embodiments regarding the subject matter of the invention; therefore the invention cannot be limited to the examples disclosed herein, and the invention is essentially as defined in the claims. It is obvious that a person skilled in the art can convey the novelty of the invention using similar embodiments and/or that such embodiments can be applied to other fields similar to those used in the related art. Therefore it is also obvious that these kinds of embodiments are void of the novelty criteria and the criteria of exceeding the known state of the art.

Industrial Application of the Invention The invention enhances privacy of the communication links against eavesdropping attacks. Unlike most of the existing physical layer security techniques that fail to provide confidentiality of the communication when eavesdropper is located very close to the legitimate user due to channel correlation, the invented method ensures communication privacy even against such eavesdropper (i.e., eavesdroppers that are collocated with the legitimate user).

Claims

CLAIMS A method of distributed carrier frequency offset pre-compensation for secure wireless communication comprising the steps of;

• Selecting the coordinating transmission points,

• Optionally, splitting of the data to be transmitted between coordinating transmission point and/or pre-equalizing the channel between each coordinating transmission point and the user

• Pre-compensating the estimated carrier frequency offset,

• Starting transmission from the coordinating transmission points.

2. A method according to claim 1 characterized in that selection of the coordinating transmission points can be based on received signal strength indicator (RSSI), received power relative to the serving TP, or a combination thereof.

3. A method according to claim 1 or 2, characterized in that selection of coordination set is done for capacity maximization, spectral efficiency or energy efficiency.

4. A method according to claim 1 or 2 characterized in that selection of the coordinating transmission points can be based on carrier frequency offset (CFO) between the TP and legitimate receiver.

5. A method according to any one of claims 1-4 characterized in that estimation of the carrier frequency offset between each coordinating transmission point and the desired user is done by using preambles or in a blind manner.

6. A method according to any one of claim 1-5 characterized in that dynamic point selection (DPS), wherein the subcarriers which are in deep fade are not used for that particular link, is used to avoid issues such as noise enhancements in the case of deep fades during transmission from all or selected TPs.

7. A method according to any one of claims 1-6 characterized in that splitting of the data to be transmitted from different TPs is • on an intra-symbol level, wherein one TP transmits real part of the symbol while a different TP transmits the imaginary part of the same symbol, or

• on a sample level, wherein different TPs transmit different samples of the same signal, or

• on a subcarrier-level, wherein where different TPs transmit different subcarriers