A system of wireless sensor networks and a method for synchronizing thereof

Abstract

A method for synchronizing gateway nodes (2.1, 2.2) and sensor nodes (3) in a wireless sensor network (1), characterized in that the method comprises the steps of: - providing at least one main gateway node (2.1) equipped with at least one transceiver for transmission and reception of direct-sequence spread-spectrum, DSSS, signals; - providing the sensor nodes (3) with transceivers for reception of DSSS signal; and - synchronizing all the sensor nodes (3) with a DSSS signal (TX Sync) transmitted from the at least one main gateway node (2.1). It is also disclosed a system of wireless sensor networks.

Description

[0001] A system of wireless sensor networks and a method for synchronizing thereof

[0002] The invention relates to a system of wireless sensor networks and to a method for synchronizing said system.

[0003] Indoor wireless sensor networks are systems of interconnected, low-power sensor nodes that wirelessly collect and transmit data about environmental or physical conditions within a building, such as temperature, humidity, light, and motion. These networks use radio transmissions to send data to a main gateway node, which then forwards it to a server or cloud for analysis, providing real-time, localized insights without extensive wiring. They are used for applications like smart home automation, security, and optimizing building comfort and energy efficiency.

[0004] In a wireless sensor network, sensor nodes are the distributed devices that collect raw data from the environment and transmit it to a gateway node, while the gateway node aggregates and filters this data, then relays it to a server, the internet, or a user for further processing and action. Sensor nodes are typically low-power devices, often employing a "sense-send- sleep" cycle, whereas gateway nodes are more powerful, enabling communication between the low-power sensor network and higher- level networks or systems.

[0005] The basic synchronization methods designed and proposed for WSNs are discussed in [1, 8] . Methods such as 'Sender-Receiver synchronization', 'Receiver-Receiver synchronization' and 'Reference Broadcast Synchronization' require messages to be sent and received by each sensor node for synchronization.

[0006] Wireless sensor networks can use frequency-hopping technique to improve communication reliability, security, and resistance to interference [1-5] . Instead of transmitting data on a fixed frequency, nodes in the network rapidly switch-or hop-between different frequency channels according to a predefined or adaptive sequence. The time synchronization is one of the main issues in frequency hopping time synchronous wireless sensor networks [6,7] .

[0007] In frequency hopping WSNs, nodes switch frequencies in a coordinated manner to avoid interference and enhance security. This requires precise (time) synchronization across the network. The main goals are: 1) aligning clocks across sensor nodes to coordinate frequency hops and sleep cycles 2) reducing drift and offset between local clocks and 3) maintaining synchronization despite mobility, interference, and energy constraints.

[0008] Consider a WSN where a bunch of sensor nodes in the same environment wants to transmit data to one or multiple gateway nodes. Some sensor nodes transmit directly to the gateway, and other sensor nodes transmit through further other sensor nodes. A gateway node typically serves as the bridge between the sensor nodes and external systems or users. Each sensor is required to send a short packet of data on a predefined time-interval which is the same for all the sensor nodes. The time-interval can be from a few milli-seconds to several seconds. The sensor nodes may transmit data packets on different frequency-channels, which are selected from a fixed or predefined pseudo-random channel sequence. To be able to receive all the nodes' transmissions with one single radio receiver at the gateway node, and to avoid transmission collisions, each node transmits on a pre-defined different time slot in the time-interval.

[0009] Different sensor nodes have different timing references derived from their internal Resistor-Capacitor / Crystal oscillators or their Temperature-Compensated Crystal Oscillators, with random timing errors within a range. This range is usually specified in parts-per-million (ppm) . In an ideal case where all sensor nodes are having the same accurate timing, no transmission collision will occur. However, with a timing error in different sensor nodes, their dedicated timing slots start shifting and they will collide on top of each other occasionally. Therefore, a single radio receiver in the gateway node misses packets of at least one of the sensor nodes during the collision period.

[0010] In the prior art [1, 8] many synchronization methods designed and proposed for WSNs are already disclosed such as 'Sender- Receiver synchronization', 'Receiver-Receiver synchronization' and 'Reference Broadcast Synchronization' which all require messages to be sent and received by each sensor node for synchronization. Furthermore, Global Positioning Sytem, GPS, based synchronization techniques are also used in wireless sensor networks [9,10] . GPS requires a clear view of the sky, which can be a limitation in indoor or underground environments.

[0011] It is an object of the invention to correct the short-comings of the prior art and to provide an alternative, more reliably and stable solution for synchronizing a wireless sensor network in indoor and outdoor setups. This and other objects which will become apparent from the following disclosure, are provided with a system and a method having the features of one or more of the appended claims.

[0012] In a first aspect of the invention, the method for synchronizing gateway nodes and sensor nodes in a wireless sensor network comprises the steps of: providing at least one main gateway node with at least one transceiver for transmission and reception of direct- sequence spread-spectrum, DSSS, signal; providing the sensor nodes with receivers for DSSS signal reception; and synchronizing all the sensor nodes with a DSSS signal transmitted from the at least one main gateway node. One of the advantages of the invention is that there is no need to send and receive multiple messages ( data packets ) between the sensor nodes . All the sensor nodes can be synchroni zed based on the DSSS signal broadcasted from at least one main gateway node . In other words , this invention relies at least one main gateway node as an ' at least one pseudolite ' i . e . pseudo-satellite which transmits DSSS signals and sensor nodes performing the receiver algorithm . Pseudolites are typically ground-based transmitters that emulate the signals of GNSS ( Global Navigation Satellite System) satellites . This overcomes the clear sky requirement of GPS based wireless sensor networks synchroni zation and enables operation in indoor or underground environments .

[0013] To make the wireless sensor network seamles sly extendable , the method may comprise the steps of : providing at least one auxiliary gateway node with at least one receiver for DSSS signal reception; and synchroni zing said at least one auxiliary gateway node with the DSSS signal transmitted from the at least one main gateway node . The network may be extended by adding auxiliary gateway nodes , wherein each auxiliary gateway node may act as a new main gateway node for a new set of sensor nodes , while remaining an auxiliary gateway node for the old set of sensor nodes . The at least one auxiliary gateway node may serve as bridge for the sensor nodes to an external network and may also serve as main gateway node for a new set of sensor nodes .

[0014] Advantageously, the method may comprise the step of providing the wireless sensor network as an indoor wireless sensor network compri sing indoor sensor nodes and at least one indoor gateway node . The main advantage o f instal ling WSN inside buildings is the reduction of cabling, which reduces installation time and cost , and the ease reconfiguration or expansion of the network as building layouts change .

[0015] More advantageously, the method may comprise the step of synchroni zing all indoor sensor nodes with a signal transmitted by a Global Navigation Satellite System, GNSS , by synchroni zing the at least one main gateway node with said GNSS signal before synchroni zing all the sensor nodes with a DSSS signal transmitted by said at least one main gateway node .

[0016] The proposed method can operate indoors . However, i f the at least one main gateway node is synchroni zed with GNSS s ignals , then the sensor nodes can continue their transmi ssion outdoors as well without the need for the at least one main gateway outdoor .

[0017] The same goes for the at least one auxiliary gate way node , wherein the method may comprise the step of synchroni z ing said at least one auxiliary gateway node with a signal transmitted by a Global Navigation Satellite System, GNSS , by synchroni zing the at least one main gateway node with said GNSS signal before synchroni zing al l the at least one auxiliary gateway node with a DSSS signal transmitted by said at least one main gateway node .

[0018] The method may comprise the steps of operating the transceivers of the sensor nodes in a time-division duplexing way, such that transmission and reception occur on the same frequency band but in non-overlapping time slots . Time-Division Duplexing ( TDD) allows the sensor nodes to transmit and receive data over the same frequency channel , but at di f ferent times . Instead of us ing separate frequencies for sending and receiving ( like Frequency- Division Duplexing, or FDD) , TDD splits time into slots . Some slots may be used for uplink ( sending data ) , and others for downlink ( receiving data ) . Since both directions share the same frequency, TDD i s great for environments with limited spectrum availability .

[0019] The method may comprise the step of configuring the transceivers in the sensor nodes to dedicate a data- free time slot for receiving the DSSS signal from the at least one main gateway node . A dedicated data- free i . e . empty time slot is one that is reserved solely for synchronization purposes , not for user data or control signals . This guarantees that the synchroni zation is not hindered by any non-synchroni zation related signals .

[0020] The method comprises the step of configuring the transceivers in the sensor nodes to receive the DSSS signal from the at least one main gateway node once every N time s lots , wherein N is an integer number . This for example be after the transceivers get out o f s leep-mode . Optionally, N can be chosen to optimi ze the energy ef ficiency .

[0021] The method may comprise the steps of : finding a code-phase and / or a frequency error of the DSSS signal transmitted by the at least one main gateway node ; tracking the received DSSS signal by estimating a di f ference between the code-phase of the received DSSS signal and a sequence of pseudo random numbers sequence at the at least one receiver of the sensor nodes and / or of the at least one auxiliary gateway node ; synchroni zing a bit-time interval by synchroni zing a clock of the at least one receiver of the sensor nodes and / or of the at least one auxil iary gateway node with a clock of the at least one main gateway node ; and generating a reference pulsed signal for defining time intervals and time slots at the at least one receiver of the sensor nodes and / or of the at least one auxiliary gateway node . A reference pulsed signal can be any signal composed of short , periodic pulses with known characteristics (amplitude, timing, phase) usable as a reference for other signals. An example of such reference pulsed signal is disclosed in [9] .

[0022] Suitably, the method may comprise the step of initializing the sensor nodes and / or the at least one auxiliary gateway node by starting from an initial state with no information.

[0023] To save on power consumption and to improve energy efficiency, the method may comprise the step of putting the sensor nodes and / or the at least one auxiliary gateway node into sleep-mode by switching OFF tracking of the received DSSS signal.

[0024] The method may comprise the steps of: estimating a maximum possible code-phase error based on a sleep duration and on frequency shifts in an oscillator of the sensor nodes and / or of the at least one auxiliary gateway node; and switching ON tracking of the received DSSS signal when said estimated maximum possible code-phase error is above a predefined threshold.

[0025] During sleep-mode, tracking of the DSSS signal is paused in the sensors and / or in the at least one auxiliary gateway node i.e. the receiver in said sensor nodes and at least one auxiliary gateway node stops actively monitoring the incoming DSSS signal. The internal clocks of said sensor nodes and / or said at least one auxiliary gateway node may slightly deviate (oscillators drift) , causing shifts in code-phase (the alignment between the receiver's internal code and the incoming signal's code) , and in frequency (the rate at which the receiver expects the signal to arrive) . According to the method of the invention, the receivers know how long they were in sleep-mode. Based on the oscillator's stability specs, the receivers can predict the maximum drift in both code-phase and frequency. This means the receivers do not need to search the entire DSSS signal space when waking up, they only need to search a narrow window . Consequently, instead of a full-blown signal acquisition (which is time- and energy-intensive ) , the receivers perform a quick search by scanning the received DSSS signal only in the expected dri ft range . Once the receivers find the DSSS signal again, they restart tracking and lock onto the DSSS signal ' s phase and frequency .

[0026] As indicated above , the reference pulsed signal can be any signal composed of short , periodic pulses with known characteristics ( amplitude , timing, phase ) usable as a reference for other signals . An example of such reference pulsed signal is disclosed in

[0009] .

[0027] Advantageously, the method may comprise the step generating said reference pulsed signal using a pul se generator in the sensor nodes and / or in the at least one auxiliary gateway node , wherein said method comprises the step of correcting a phase of f set in said reference pulsed signal using a processing unit in the sensor nodes and / or in the at least one auxiliary gateway node , wherein said processing unit is configured to evaluate a time deviation of said pulsed signals from at least one time-stamped reference signal .

[0028] More advantageously, the method may optionally comprise the step of operating the pulse generator in at least two operational modes , wherein in a first operational mode the pulse generator generates pulsed signals having a first predefined frequency range , and in a second operational mode the pulse generator generates pulsed signals having a second predefined frequency range , wherein the second frequency range is wider than the first frequency range .

[0029] The method may comprise the step of setting the first frequency range between 0 . 1 Hz and 1 kHz and the second frequency range between 10 Hz and 10 MHz .

[0030] The method may also comprise the steps of : estimating the phase of fset for every pulse of said pulsed signals when the pulse generator is in the first operational mode ; and estimating the phase of fset periodically over N pulses within the pulsed signals ( S ) , wherein N is a predefined integer value , when the pulse generator is in the second operational mode .

[0031] The first operational mode requires relatively simple architecture and features higher pulse accuracy than the second operational mode because in the first operational mode , the errors of every pulse are calculated and compensated . In the current context , pulse accuracy describes how closely a generated or detected pulse matches its intended characteristics in time , amplitude , and shape . It ' s essentially a measure of how "true" the pulse is to its design . The second operational mode requires relatively more complex architecture and features lower pulse accuracy but supports a higher frequency range . Generating the lower frequency pulsed signals with the first operational mode and generating the higher frequency pulsed signal with the second operational mode , leads to the support of higher frequency ranges without much sacrifice on pulse accuracy . Additionally, the processing unit may be used to estimate and correct the phase of fset which leads to a reduction in the silicon area and an increase in the application flexibility .

[0032] In a second aspect of the invention, the system of wireless sensor networks , characteri zed in that the system comprises : at least one main gateway node equipped with at least one transceiver for transmission and reception of direct- sequence spread-spectrum, DSSS , signals ; and sensor nodes equipped with transceivers for reception of DSSS signal , wherein all the sensor nodes are configured to be synchroni zed with a DSSS signal transmitted from the at least one main gateway node .

[0033] One of the advantages of the invention is that there is no need to send and receive multiple messages ( data packets ) between the sensor nodes . All the sensor nodes can be synchroni zed based on the DSSS signal broadcasted from at least one main gateway node . In other words , this invention relies at least one main gateway node as an ' at least one pseudolite ' which transmits DSSS signals and sensor nodes performing the receiver algorithm . Thi s overcomes the clear sky requirement of GPS based wireless sensor networks synchroni zation and enables operation in indoor or underground environments .

[0034] To make the wireless sensor network seamless ly extendable , the system may comprise at least one auxil iary gateway node equipped with at least one transceiver for reception of DSSS signals , wherein said at least one auxil iary gateway node is conf igured to be synchroni zed with the DSSS signal transmitted from the at least one main gateway node .

[0035] Suitably, the system may be an indoor wireless sensor network compri sing indoor sensor nodes and at least one indoor gateway node . The main advantage o f instal ling WSN inside buildings is the reduction of cabling, which reduces installation time and cost , and the ease reconfiguration or expansion of the network as building layouts change .

[0036] The invention will hereinafter be further elucidated with reference to the drawing of an exemplary embodiment of a wireless sensor network according to the invention that i s not l imiting as to the appended claims .

[0037] In the drawing : figure 1 shows a schematic of an exemplary system according to the invention; figure 2 shows a schematic of the method steps according to the invention; figure 3 shows a schematic of an exemplary step of generation the DSSS signal according to the method of the invention figure 4 shows a schematic of an exemplary correlator for code-phase acquisition according to the method of the invention; and figure 5 shows a schematic of an exemplary tracking the code-phase and doppler frequency according to the method of the invention .

[0038] Whenever in the figures the same reference numerals are applied, these numerals refer to the same parts .

[0039] The system 1 and method of the invention are particularly suited for synchroni zing indoor wireless sensor networks , but as already explained above , the invention can also be appl ied outdoor .

[0040] An exemplary system 1 of the invention is shown in figure 1 . To synchroni ze all sensor nodes 3 at once , the synchroni zation signal TX Sync needs to be transmitted from the at least one main gateway node 2 . 1 to al l sensor nodes 3 i . e . each one of the sensor nodes 3 receives and become synchroni zed with the TX sync . Each one of the sensor nodes calculates its base timing reference error once at the beginning to find its reference time slot . Based on uncertainty range of the sensor node 3 timing reference error, each node 3 should again be synchroni zed with the TX Sync from time to time . Thi s way, each node 3 can be guaranteed to transmit data packets only within its allocated time slot . Nodes 3 can receive the TX Sync signal on a dedicated empty i . e . data- free time slot ( e . g . slot #0 ) , which can be once every 1 to N intervals, wherein N is an integer number. To extend the network, at least one auxiliary gateway node 2.2 may be added to the system 1. To synchronize said auxiliary gateway node 2.2 with the at least one main gateway node 2.1 and / or the sensor nodes 3, the same TX Sync should be received by said auxiliary gateway node 2.2 receiver. The receiver RX Sync of the at least one auxiliary gateway node 2.2 can be hardwired or wirelessly connected to the transmitter of the at least one main gateway node 2.1.

[0041] To be able to receive the TX Sync at the sensor nodes 3 with ultra-low power consumption, the TX Sync can be a direct- sequence spread-spectrum (DSSS) signal. Advantageously, the transceivers of the sensor nodes 3 may be configured to receive DSSS signals and to transmit packets in a Time-division Duplexing way, TDD. A DSSS signal with a pseudo-random number (PRN) sequence (for very fine resolution) and a bit sign change or any other pre-defined sequence (for coarse resolution) can be used to mark the beginning of each interval.

[0042] In this invention, a novel architecture for WSN synchronization is presented, wherein the gateway nodes 2.1, 2.2 are equipped with DSSS signal transmission and reception capabilities, and the sensor nodes 3 are equipped with DSSS signal reception capabilities. The direct advantage of the invention is that there is no need to send and receive multiple messages between the sensor nodes 3. All the sensor nodes 3 synchronize based on the DSSS signal broadcasted from the at least one gateway node 2.1. In other words, the system 1 and method disclose a 'pseudolite' in the at least one main gateway node 2.1 which transmits DSSS signals and sensor nodes 3 performing the receiver algorithm for receiving said DSSS signal. This overcomes the clear sky requirement of GPS based WSN synchronization and enables operation in indoor or underground environments . The gateway nodes 2.1, 2.2 and the sensor nodes 3 can also be receiving GPS signals. So, the network can synchronize based on GPS signals in the case of outdoor operation and switch to the DSSS signals transmitted by the gateway nodes in indoor or underground environments.

[0043] A pseudo random number, PRN, epoch (NT) consists of a sequence [cocq ... v-j , with each cnwith a time duration T, as shown below in figure 3. The bit-duration of the bit-sequence can be any number of PRN epochs.

[0044] The system and method of the invention can also work for WSNs which do not use frequency hopping.

[0045] The method of the invention is illustrated in figure 2 which outlines the steps of synchronizing the sensor nodes 3 and the at least one auxiliary gateway node 2.2 with the DSSS signal TX Sync received from the at least one main gateway node 2.1.

[0046] The steps illustrated in figure 2 are typically performed at the sensor nodes 3 and at the receivers RX Sync of the at least one auxiliary gateway node 2.2 to synchronize them with the TX Sync.

[0047] In figure 2 the steps are as follows:

[0048] Cold start: the receivers of the sensor nodes 3 and / or the at least one auxiliary gateway node 2.2 are initialed without any information;

[0049] Acquisition: Finding DSSS code phase and frequency error;

[0050] Tracking: Synchronizing code phase, extracting symbols ; Bit Synchronization: Synchronizing the bit-time interval;

[0051] Generating time pulse and ticking a new synchronization interval;

[0052] Long sleep: To save power; and Re-acquisition / tracking from an estimated state

[0053] Acquisition : The code phase of the DSSS signal, and the carrier frequency shift due to doppler effect or hardware imperfections are acquired in this step. The code phase at the receiver can be changed by cyclically shifting the local copy of the PRN sequence of length N, [cocq ... cw-x] . The code phase which maximizes the correlator output R, as shown in figure 4, is the acquired code-phase. The integration

[0054] -T operation can R = Jor^dtcan be achieved in the discrete-time domain by accumulating the samples. The acquisition of frequency shift can be achieved by any frequency estimator.

[0055] Tracking: The code-phase and the carrier frequency shift due to doppler effect can be tracked by a method shown in figure 5, as an example. This exemplary architecture contains a DLL (delay- locked loop) and an FLL / PLL ( Frequency / Phase locked loop) . The difference between the code-phases of the received DSSS signal and the local PRN sequence at the receiver can be estimated by the 'Timing error detector' . The carrier f requency / phase error can be estimated by Frequency / phase error detector. The loop filters are linear filters used to control the loop dynamics of the DLL and FLL / PLL. The SRC (sample-rate-converter) re-samples the input signals to match the code-phase. The NCO (numerically controlled oscillator) generates frequencies to match the frequency shift.

[0056] Bit Synchronization: This is the process of aligning a receiver's clock with a transmitter's clock to correctly interpret incoming digital signals, ensuring that the receiver knows exactly when each bit begins and ends. This preserves data integrity, allowing the receiver to sample the signal at the optimal point to extract the data accurately and avoid errors. The fundamental goal is to get the receiver's internal clock in sync with the transmitter's clock.

[0057] Bit Timing: This ensures the receiver can correctly identify the duration of each bit in a data stream, rather than just the data values themselves.

[0058] Sampling: The receiver samples the signal at the correct time within each bit's duration to determine if it's a 0 or a 1.

[0059] When the tracking step is done, the receiver can perfectly despread the DSSS signal to obtain the bit sequence. Let the bitduration of the bit-sequence be NbPRN epochs. The receiver has to detect where the transition happens between +1 and -1, in order to synchronize with the bit sequence. This can be achieved by observing the sum of Nbconsecutive received bits. When there is a bit transition, the +ls and -Is cancel out and the sum becomes 0. So, the zero-crossing point of this sum gives the bit transition boundary.

[0060] Time Pulse: An accurate pulse generator may be used to produce a set of reference pulsed signals that defines the time intervals and slots precisely. This is shown as 'Time Pulse' in Figure 2. An example of said reference pulsed signal is disclosed in [9] .

[0061] Re-acquisition / tracking : During the sleep time (when the tracking is switched off) , the code-phase and frequency can drift. As the receiver knows the sleep duration, the maximum possible code-phase and frequency shifts are known. So, the receiver can potentially search over a smaller range of codephases and frequencies in a quick acquisition process. Once this is done, tracking can be started. This invention can be util i zed in indoor locali zation networks and WSNs deployed for healthcare applications such as remote patient monitoring, elderly care and wearable health devices . Industrial automation applications such as production monitoring systems and process control systems are also potential applications of this invention .

[0062] Although the invention has been discussed in the foregoing with reference to an exemplary embodiment of the method of the invention, the invention is not restricted to this particular embodiment which can be varied in many ways without departing from the invention . The discussed exemplary embodiment shall therefore not be used to construe the append-ed claims strictly in accordance therewith . On the contrary the embodiment is merely intended to explain the wording of the appended claims without intent to limit the claims to this exemplary embodiment . The scope of protection of the invention shall therefore be construed in accordance with the appended claims only, wherein a possible ambiguity in the wording of the claims shall be resolved using this exemplary embodiment .

[0063] References :

[0064] 1) Waltenegus Dargie, Christian Poellabauer, Fundamentals of Wireless Sensor Networks: Theory and Practice, Wiley, 2010

[0065] 2) Umer Javed, Frequency hopping in wireless sensor networks, Masters' Thesis, Helsinki University of Technology, 2009.

[0066] 3) Vikram Kulkarni, et.al, A Survey on Interference Avoiding Methods for Wireless Sensor Networks Working in the 2.4 GHz Frequency Band, Journal of engineering science and technology review, 2020.

[0067] 4) Siti Hadijah Binti Ishak, Interference mitigation for wireless sensor networks using frequency hopping, Masters' Thesis, Universiti Teknologi Malaysia 2022.

[0068] 5) Gaurav Sharma, Suman Bala, Security in Wireless Sensor Networks using Frequency Hopping, International Journal of Computer Applications

[0069] 6) Branko Kerkez, Adaptive Time Synchronization and Frequency Channel Hopping for Wireless Sensor Networks, Technical Report, Electrical Engineering and Computer Sciences University of California at Berkeley, 2012.

[0070] 7) Teemu Vanninen, Frequency hopping mobile an-hoc and sensor network synchronization, IEEE Military Communications Conference 2008.

[0071] 8) Fikret Sivrikaya, Bulent Yener, Time Synchronization in Sensor Networks: A Survey, IEEE Network, 2004.

[0072] 9) Vincent Le Cam, Arthur Bouche, David Pallier, Wireless Sensors Synchronization : an accurate and deterministic GPSbased algorithm, IWSHM 2017 - 11th international workshop on structural health monitoring 2017.

[0073] 10) Ki Young Koo, David Hester, and Sehoon Kim, Time Synchronization for Wireless Sensors Using Low-Cost GPS Module and Arduino, Frontiers in Built Environment, 2019

Claims

CLAIMS1. A method for synchronizing gateway nodes (2.1, 2.2) and sensor nodes (3) in a wireless sensor network (1) , characterized in that the method comprises the steps of: providing at least one main gateway node (2.1) equipped with at least one transceiver for transmission and reception of direct-sequence spread-spectrum, DSSS, signals ; providing the sensor nodes (3) with transceivers for reception of a DSSS signal; and synchronizing all the sensor nodes (3) with a DSSS signal (TX Sync) transmitted from the at least one main gateway node (2.1) .

2. The method according to claim 1, characterized in that the method comprises the steps of: providing at least one auxiliary gateway node (2.2) with at least one receiver for reception of DSSS signals; and synchronizing said at least one auxiliary gateway node (2.2) with the DSSS signal (TX Sync) transmitted from the at least one main gateway node (2.2) .

3. The method according to claim 1 or 2, characterized in that the method comprises the step of providing the wireless sensor network (1) as an indoor wireless sensor network (1) comprising indoor sensor nodes (3) and at least one indoor gateway node (2.1, 2.2) .

4. The method according to any one of the preceding claims, characterized in that the method comprises the step of synchronizing all indoor sensor nodes (3) with a signal transmitted by a Global Navigation Satellite System, GNSS, by synchronizing the at least one main gateway node (2.1) with said GNSS signal before synchronizing all the sensor nodes (3) with the DSSS signal (TX Sync) transmitted by said at least one main gateway node (2.1) .

5. The method according to any one of the precedingclaims, characterized in that the method comprises the step of synchronizing the at least one auxiliary gateway node (2.2) with a signal transmitted by a Global Navigation Satellite System, GNSS, by synchronizing the at least one main gateway node (2.1) with said GNSS signal before synchronizing the at least one auxiliary gateway node (2.2) with a DSSS signal transmitted by said at least one main gateway node (2.1) .

6. The method according to any one of the preceding claims, characterized in that the method comprises the step of operating the transceivers of the sensor nodes (3) in a timedivision duplexing way, such that transmission and reception occur on the same frequency band but in non-overlapping time slots .

7. The method according to claim 6, characterized in that the method comprises the step of configuring the transceivers in the sensor nodes (3) to dedicate a data-free time slot for receiving the DSSS signal (TX Sync) from the at least one main gateway node (2.1) .

8. The method according to claim 7, characterized in that the method comprises the step of configuring the transceivers in the sensor nodes (3) to receive the DSSS signal (TX Sync) from the at least one main gateway node (2.1) once every N time slots, wherein N is an integer number.

9. The method according to any one of the preceding claims, characterized in that the method comprises the steps of: finding a code-phase and / or a frequency error of the DSSS signal (TX Sync) transmitted by the at least one main gateway node (2.1) ; tracking the received DSSS signal (TX Sync) by estimating a difference between the code-phase of the received DSSS signal (TX Sync) and a sequence of pseudo random numbers sequence at the at least one receiver of the sensor nodes (3) and / or of the at least one auxiliary gateway node (2.2) ; synchronizing a bit-time interval by synchronizing a clock of the at least one receiver of the sensor nodes (3) and / orof the at least one auxiliary gateway node (2.2) with a clock of the at least one main gateway node (2.1) ; and generating a reference pulsed signal for defining time intervals and time slots at the at least one receiver of the sensor nodes (3) and / or of the at least one auxiliary gateway node (2.2) .

10. The method according to any one of the preceding claims, characterized in that the method comprises the step of initializing the sensor nodes (3) and / or the at least one auxiliary gateway node (2.1) by starting from an initial state with no information.

11. The method according to any one of the preceding claims, characterized in that the method comprises the step of putting the sensor nodes (3) and / or the at least one auxiliary gateway node (2.1) into sleep-mode by switching OFF tracking of the received DSSS signal (TX Sync) .

12. The method according to any one of the preceding claims, characterized in that the method comprises the steps of: estimating a maximum possible code-phase error based on a sleep duration and on frequency shifts in an oscillator of the sensor nodes (3) and / or of the at least one auxiliary gateway node (2.2) ; and switching ON tracking of the received DSSS signal when said estimated maximum possible code-phase error is above a predefined threshold.

13. The method according to any one of the preceding claims, characterized in that the method comprises the step generating a reference pulsed signal using a pulse generator in the sensor nodes (3) and / or in the at least one auxiliary gateway node (2.2) , wherein said method comprises the step of correcting a phase offset in said reference pulsed signal using a processing unit in at least one of the sensor nodes (3) and / or in the at least one auxiliary gateway node (2.2) , wherein said processing unit is configured to evaluate a time deviation of said pulsed signals from at least one time-stamped reference signal.

14. The method according to claim 13, characterized in that the method comprises the step of operating the pulse generator in at least two operational modes, wherein in a first operational mode the pulse generator generates pulsed signals having a first predefined frequency range, and in a second operational mode the pulse generator generates pulsed signals having a second predefined frequency range, wherein the second frequency range is wider than the first frequency range.

15. The method according to any one of the preceding claims, characterized in that the method comprises the step of extending the network by adding at least one auxiliary gateway node, wherein said at least one auxiliary gateway node is configured to act as a main gateway node for a new set of sensor nodes, while remaining an auxiliary gateway node for the old set of sensor nodes (3) .

16. A system (1) of a wireless sensor network (1) comprising gateway nodes (2.1, 2.2) and sensor nodes (3) , characterized in that at least one main gateway node (2.1) is equipped with at least one transceiver for transmission and reception of direct-sequence spread-spectrum, DSSS, signals; and the sensor nodes (3) are equipped with transceivers for reception of a DSSS signal, wherein all the sensor nodes (3) are configured to be synchronized with a DSSS signal (TX Sync) transmitted from the at least one main gateway node (2.1) .

17. The system (1) according to claim 16, characterized in that the system (1) comprises at least one auxiliary gateway node (2.2) equipped with at least one transceiver for reception of DSSS signals, wherein said at least one auxiliary gateway node (2.2) is configured to be synchronized with the DSSS signal (TX Sync) transmitted from the at least one main gateway node (2.2) .

18. The system (1) according to claim 16 or 17, characterized in that the system (1) is an indoor wireless sensornetwork comprising indoor sensor nodes (3) and at least one indoor gateway node (2.1, 2.2) .

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