Patent Description:
An example of industrial communication related to the present invention is communications for the monitoring and control of electric power grids, wherein wired communications, compliant with IEC <NUM>, may be used that provide time slots of some micro-seconds or lower. In, for example, electric substation automation for the control and monitoring of electric power distribution, the requirements for reliability and low latency are comparably high. On the other hand, such communications will often have lower requirements on data transmission capacity, or bandwidth, than for example entertainment, such as film and television.

Using wired communications has drawbacks in cost, and wireless alternatives are sought after for in industrial automation. However, wireless communication standards like IEEE <NUM>, e.g. IEEE <NUM>, introduces latencies, since such standards are not optimized for communication that require low latency, and therefore do not provide an alternative that can compete with wired communications in terms of latency. Also, wireless communication may in general be more vulnerable for intrusive attacks, such as e.g. spoofing, than wired communications that are protected for example by the use of fiber optics that makes it more difficult for an intruder to interfere.

An example of known authentication methods performed in the PHY-layer (physical layer) is authentication based on CSI (Channel State Information). CSI based authentication is beneficial compared to cryptographic methods using key encryption, since, when using an encryption key, an intruder may perform repeated attempts to reveal the encryption key, basically until the attack succeeds.

A CSI based method may use estimates of CIR (Channel Impulse Response) to authenticate communication. The channel estimations made for channel decoding in IEEE <NUM> are also used for packet authentication and, thus, the preambles are also used for counteracting intrusion. To provide reliable decoding, IEEE <NUM> uses a preamble of five OFDM symbols for each data packet, which preamble is used for channel estimation purposes.

<CIT> describes a method and a system using the physical layer for security operations, e.g. using CIR (abstract, §<NUM>). An aim is to enhance security, since the security methods of wired communication is not considered sufficient for wireless communication, such as WLANs like IEEE <NUM> (§<NUM>, <NUM>, <NUM>).

The paper <NPL>, discloses simulations performed in four different industrial sites where CIR (channel impulse response) is used as CSI (abstract, §II). The simulations show that CSI can be used for authentication and intrusion detection in industrial automation environments also for detecting intruders that acts close to a legitimate node. CSI can be especially effective in machine to machine communication in industrial automation appliances with low mobility, such as electric power substation where all, or almost all, equipment remains stationary. The paper by Pan et al. discloses a method where differences in channel response is determined between a legitimate, or reference, sender and communication received from a sender to be authenticated (§III-A). The method uses a predetermined threshold for determining an illegitimate sender (§III-B).

As previously noted, in order to provide reliable decoding, IEEE <NUM> uses a preamble of five OFDM symbols for each data packet. The preamble is used for channel estimation purposes, and can be used for CSI based authentication. However, the latency of such communication is too large for many industrial appliances.

A licentiate thesis by <NPL>), Stockholm, discloses an example of wireless communication with comparable low latency that has been suggested for industrial communications called WirelessHP. WirelessHP has been presented as an alternative to IEEE <NUM> and uses a short preamble of only one OFDM symbol.

<CIT> is a further document that suggests comparably short preambles in wired, or wireless, OFDM communication. The document suggests using as short a preamble as possible that still satisfies, for example, synchronization requirements (§<NUM>). A general dilemma with preambles is expressed: a longer preamble can be used to enhance communication quality, however longer preambles negatively affect the data rate (§<NUM> ). The '<NUM> document discloses performing channel estimations where training patterns are compared to received preambles (e.g. §<NUM>-<NUM>, <NUM>-<NUM> and claim <NUM> therein).

Using a preamble for each data packet consisting of only one OFDM symbol, like in Jiang's thesis, reduces latency to become of similar size as wired communication. However, a CSI based authentication process that needs a preamble of each data packet that comprises for example five OFDM to perform an estimation of the CIR cannot be used.

Thus, there is a need for an alternative method for authenticating wireless communication, which authentication method are not based on CIR estimations that requires large preambles that introduce latency, e.g. preambles comprising five OFDM symbols, but instead can be performed in wireless communications using shorter preambles for the data packets, especially, for wireless communications where each data packet comprises a short preamble in order to avoid latency, preferably a preamble consisting of only one, or two, OFDM symbols.

An aim of the present invention is to provide a method for wireless communication suitable for industrial appliances, such as industrial automation, which wireless communication method utilize an authentication with security performance similar to the previous physical layer CSI authentication methods of the prior art, however without the drawback of latency of the previous methods.

For this purpose, in accordance with a first aspect, the present invention provides a communication method according to claim <NUM>.

By utilizing preambles from a plurality of consecutive packets, each packet can have a comparably short preamble consisting of few OFDM symbols, e.g. one OFDM symbol, in order to ensure for a comparably short latency. At the same time, the number of OFDM symbols used for the CSI based authentication can be larger since the preambles of more than one packet is used for this authentication. Because a packet comprises one or more OFDM symbols, it may be referred to as an OFDM packet.

Thus, the method utilizes one preamble from each packet, and the authentication is based on CSI derived from a group consisting of a plurality of consecutive packets received from the second node. Preferably the CSI of the group is compared to a corresponding reference CSI for communication with the second node.

In an embodiment of the first aspect, each preamble consists of not more than two OFDM symbols, preferably one single OFDM symbol.

In an embodiment of the first aspect, the authentication comprises comparing the CSI of the preamble of each packet of the group to a reference CSI, and accumulating the result of the comparisons from all packets of the group.

In a further embodiment of the first aspect, the method comprises comparing the accumulated results to a threshold, and setting an alarm indicating intrusion in case the accumulated results exceed the threshold.

In a further embodiment of the first aspect, the method comprises obtaining training sequences from the at least one second node, and set the reference CSI and/or the authentication threshold in view of the CSI of the received training sequences. Thus, an embodiment of the first aspect comprises evaluating the training sequences and setting the threshold based on the evaluation, and an embodiment of the first aspect comprises analyzing, preferably statistically, the training sequences and setting the reference CSI based on analysis of the training sequences.

In an embodiment of the first aspect, the method uses a CIR (Channel Impulse Response) as CSI. Thus, the method includes estimating the CIR from the received packets and the estimation of the CIR concerning communication received from the second node is used as CSI.

In accordance with a second aspect, the present invention provides a communication node according to claim <NUM>.

In accordance with a third aspect, the present invention provides a computer program and computer program product.

Embodiments of the invention will now be described with reference to the accompanying drawings, on which:.

<FIG> illustrates a wireless communication network <NUM> having a star configuration where a network controller, central node, first node or master node <NUM> is connected to a plurality of second nodes, or slave nodes, <NUM>. The wireless communication network <NUM> exemplifies a typical industrial communications network for monitoring and controlling of equipment, such as communication in a production facility, a manufacturing process or an electric power system. Such industrial wireless communication may also be referred to as wireless machine-to-machine communication. Each slave node <NUM> has an RF (radio-frequency) front-end that enables wireless communication with the central master node <NUM>, in order to control and monitor the actual equipment of the slave node. In a substation, the equipment may for example constitute switches, breakers, transformers, electric lines, controllable capacitors or machines, or an energy storage. <FIG> also exemplifies an intruder <NUM>. A wireless network relying on cryptography or passwords is vulnerable to intruders that can utilize computing power and time to crack this defense. Using a physical layer protection, such as CSI, enhances security since the spatial distance between a slave node <NUM>, i.e. a legitimate node, and an intruder <NUM> will influence the channel characteristics so that an intrusion is detected. In wireless networks <NUM> where the mobility of the nodes and the environment are small, monitoring channel characteristics, such as CSI, can be especially effective and reliable for detecting intrusion. In for example electric power substations, the slave nodes <NUM>, such as all the nodes, may be stationary, which makes CSI protection especially suitable. In many other industrial communication networks used in industrial automation, such as manufacturing or processing systems, the mobility is small and slow making CSI based protection beneficial.

<FIG> illustrate data packets <NUM>. <FIG> illustrates one data packet <NUM> in an OFDM communication system. The data packet <NUM> comprises OFDM symbols <NUM> and may therefore be referred to as an OFDM packet. The OFDM symbols <NUM> are divided into two groups: a preamble <NUM> on the one hand and data <NUM>, or payload data, on the other hand. The preamble <NUM> of the data packet in <FIG> comprises five OFDM symbols. The preambles 22a-n of the data packets 22a-n exemplified in <FIG> comprises only one single OFDM symbol <NUM>. Although data packets 20a-n with short preambles 22a-n consisting of only one or two OFDM symbols <NUM> are preferred, the present invention can be used in communication systems utilizing longer preambles <NUM>, e.g. such as five as illustrated in <FIG>. The present invention uses preambles 22a, 22b,. , 22n from a plurality of consecutive data packets 20a-n, as illustrated in <FIG>.

<FIG> illustrates a communication method according to an embodiment of the present invention. The communication method <NUM> includes receiving <NUM> packets from a node (such as a second node or slave node <NUM> in <FIG>) and authenticating <NUM> the sending node <NUM>. The communication method <NUM> may typically be performed by a master node, or controller, <NUM>, or by a second node, or slave node <NUM> that acts as a receiving node. The main example given here is wherein a master node <NUM> authenticates a slave node <NUM>, however, any slave node <NUM> may authenticate the master node <NUM> in the same way. The receiving node <NUM>, <NUM> receives <NUM> a plurality of consecutive packets 20a, 20b,. , 20n and authenticates <NUM> the sender node <NUM>, <NUM> based on these received packets 20a-n as a group. The authentication <NUM> is based on a plurality of preambles 22a-n extracted from the packets 20a-n of the group of packets. The number of packets in a group will depend on the number of OFDM symbols <NUM> needed for a reliable CSI based authentication, and the number can be determined in the same way as when performing CSI based authentication of one single packet, but instead of all OFDM symbols belonging to the same packet <NUM>, the preambles belong to a number of consecutive packets 20a-n.

The authentication <NUM> of the sender node includes comparing CSI of the received preambles with a CSI reference. If the CSI, such as the CIR, of the received packets differs more than a threshold from the CSI reference, the method may suitably include setting <NUM> an alarm, for example indicating intrusion in a wired communication network to which the node is connected at the industrial facility, such as electric power substation or other industrial automation facility such as in an automated manufacturing process. Thus, the method may include determining <NUM> whether the threshold is violated, and setting <NUM> an alarm if so, i.e. if the result of the authentication <NUM> indicates intrusion.

The communication method may include setting <NUM> the CSI reference. The communication method <NUM> may include setting <NUM> the authentication threshold. The setting <NUM> of CSI reference and the setting <NUM> of the authentication threshold may suitable be based on analysis of received training signals. Thus, the process may include obtaining <NUM> training sequences from the sender node, and setting <NUM> the CSI reference based on a statistical analysis of the CSI of the received training sequences. Also, the setting <NUM> of the authentication threshold may be based on an evaluation or analysis of the received training sequences.

The CSI reference and the authentication threshold may be updated, suitably when an update time has lapsed, and the communication method <NUM> may include one or more steps for determining <NUM> whether the CSI reference and/or the authentication threshold should be updated. Such an updating time should suitably be set in view of the industrial process in question, and stationary nodes in an environment with small or no mobility of radio frequency interfering equipment can be expected to have a longer time between such updates than nodes arranged in an environment that affects radio transmission and thus affects the CSI, such as the CIR.

<FIG> illustrate an embodiment similar to the embodiment of <FIG>, were the setting <NUM> of CSI reference, the setting <NUM> of authentication threshold and the authentication <NUM> are illustrated in more detail.

The setting <NUM> of CSI reference may be based on a statistical analysis <NUM> of a training sequence. Also, the setting <NUM> of authentication threshold may be based on evaluation <NUM> of a training sequence, suitably the same training sequence as used for setting <NUM> the CSI reference. The threshold may be set so that an expected variation of the CSI, such as the CIR, should no result in an alarm, and therefore a variation determined when evaluating the training sequence should not violate the authentication threshold. Thus, the setting <NUM> of reference and the setting <NUM> of threshold may include obtaining <NUM> a training sequence, suitable the same training sequence, which training sequence may include a hundred to some thousands, or about one thousand, of OFDM symbols. The authentication <NUM> of the sender node may be based on the group of received packets and include comparing <NUM> each packet to the CSI reference. The method <NUM>, especially the authentication <NUM> of the node, may also include accumulating <NUM> determined differences between the CSI of received packets and the CSI reference, and subsequently comparing <NUM> the accumulated deviations from the CSI reference with the authentication threshold. The comparing <NUM> of accumulated differences may be based on a suitable metric, such as an Euclidean norm. When setting <NUM> the authentication threshold based on the training sequence obtained (in <NUM>), the same metric may suitably be used as when comparing <NUM> accumulated deviations of the CSI of the received packets from the CSI reference.

<FIG> illustrate some parts of a communication device, or communication node <NUM>, <NUM> of the present invention. The communication node <NUM>, <NUM> is configured to perform any of the disclosed methods, including the embodiments described in relation to <FIG> and <FIG>. The communication node <NUM>, <NUM> consists of a combination of software and hardware and <FIG> illustrate some functionality, especially functionality related to the present invention, that is achieved by the communication node <NUM>, <NUM>. The functionality is shown as functional blocks.

<FIG> is a simplified illustration of the construction of the communication node <NUM>, <NUM> and focuses on illustrating the communication functionality of the communication node <NUM>, <NUM>, and especially focuses on the functionality utilized for authentication.

The communication node <NUM>, <NUM> comprises a communication interface <NUM> and a main controller <NUM>. The main controller <NUM> can be configured for monitoring and controlling equipment in an industrial setting. Such equipment may in an electric power system, or in a power substation, include for example switches, breakers, transformers, generators, controllable capacitors, or voltage source converters. The communication interface <NUM> includes a transmitter and receiver and is configured to communicate wirelessly in a communication system <NUM> with other communication nodes <NUM>, <NUM>, such as a master node <NUM> and slave nodes <NUM>. The communication node <NUM>, <NUM> comprises an authenticator <NUM> configured to authenticate another communication node <NUM>, <NUM>, especially perform the authentication <NUM> based on a number of preambles <NUM>, 22a-n from consecutive packets <NUM>, 20a-n received from the communication node <NUM>, <NUM> that should be authenticated. The authenticator <NUM> preferably comprises respective function units for performing the different measures <NUM>, <NUM>, <NUM>, <NUM> of the previously disclosed wireless communication method of <FIG>.

The authenticator <NUM> comprises a CSI estimator <NUM> and a CSI comparator by means of which the communication node is configured to estimate the CSI of the preamble of each packet and compare the CSI to a CSI reference (see the comparing <NUM> in <FIG>). The authenticator further includes a deviation accumulator <NUM> configured to accumulate (see <NUM> of <FIG>) the deviations from the CSI reference, wherein the communication node <NUM>, <NUM> is adapted to accumulate all the deviations used for the authentication, including the deviations from at least two consecutive packets 20a-n. The authenticator <NUM> further includes an alarm unit <NUM> configured to compare said accumulated deviations to a threshold and configured to set an alarm when the threshold is exceeded and the received packets violates the authentication criteria, corresponding to the comparing <NUM> of accumulated CSI deviation to the threshold and setting <NUM> of an alarm in <FIG>, and the determining of an authentication violation <NUM> and setting of alarm <NUM> in <FIG>.

The communication node <NUM>, <NUM> is also configured to set <NUM>, or update, a CSI reference, and to set <NUM>, or update, the authentication threshold. For these purposes <NUM>, <NUM>, the communication node is provided with a training sequence obtainer <NUM> configured to obtaining <NUM> a training sequence, e.g. including to initiate and receive training sequences from one or more communication nodes <NUM>, <NUM> that should be authenticated. The communication node <NUM>, <NUM>, preferably the CSI reference setter <NUM>, also includes a training sequence analyzer for effectuating the analyzing of the training sequence when performing the previously disclosed setting <NUM> of the CSI reference. In similar fashion, the communication node <NUM>, <NUM>, preferably the threshold setter <NUM>, also includes a training sequence evaluator for effectuating the analyzing of the training sequence when performing the previously disclosed setting <NUM> of the authentication threshold. The CSI reference setter <NUM> and/or the threshold setter <NUM> may preferably include a timer configured to determining <NUM> whether the CSI reference or the authentication threshold, respectively, should be updated.

Embodiments of the communication node <NUM>, <NUM> of the invention is configured to perform the methods of <FIG> and <FIG>; and embodiments of the communication method of the present invention include the methods and functionalities provided with the communication node <NUM>, <NUM> described in relation to <FIG>.

A communication method <NUM> comprising transmitting and receiving packets <NUM>, 20a-n between a first node <NUM> and at least one second node <NUM> has been described in embodiments, together with a communication node <NUM>, <NUM> configured to perform the method including said embodiments. In these embodiments, the packet comprises a preamble <NUM>, 22a-n and payload data <NUM>, 23a-n. The method is performed by the first node <NUM> and comprises:.

<FIG> shows a computer-readable optical medium <NUM>. The methods of <FIG> and <FIG> may be implemented as a computer program <NUM> (software) comprising instructions which, when the program is executed by a programmable computer, cause the computer to carry out any of these methods. In particular, the instructions may be such as to cause a communication node <NUM>, <NUM>, which functions as first node, master node or controller in the sense described above, to carry out the method of <FIG> or <FIG>. The computer program <NUM> may be stored or distributed on a computer-readable medium <NUM> like the one shown in <FIG>. Further computer-readable media include non-volatile (including permanent and non-permanent storage) and volatile media, such as random access memory, magnetic, optical or solid-state memory, fixed and movable memory drives. Computer-readable media may further be systematized as non-transitory media, including storage media, and transitory media, such as a modulated electromagnetic or optical wave carrying information.

Claim 1:
A communication method (<NUM>) using OFDM (Orthogonal Frequency Division Multiplexing) comprising transmitting and receiving packets (<NUM>, 20a-n) between a first node (<NUM>, <NUM>) and at least one second node (<NUM>, <NUM>), where each packet comprises a preamble (<NUM>, 22a-n) and payload data (<NUM>, 23a-n), said method being performed by the first node (<NUM>, <NUM>) and comprising:
- receiving (<NUM>) packets (<NUM>, 20a-n) from the at least one second node, and
- authenticating (<NUM>) the at least one second node based on CSI (Channel State Information),
wherein
the authenticating (<NUM>) of the at least one second node (<NUM>, <NUM>) is based on a plurality of preambles (22a-n), which are extracted from a group of consecutively received packets (20a-n), wherein the authenticating (<NUM>) comprising:
- comparing (<NUM>) the CSI of the preamble (22a-n) of each packet (20a-n) of the group of consecutively received packets (20a-n) to a reference CSI, and
- accumulating (<NUM>) the result of the comparisons from all packets (20a-n) of the group,
the communication method further comprising the step of:
- comparing (<NUM>) the accumulated results to a threshold.