ES2549707A1 - Method of data modulation with low frequency signals (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Data modulation method with low frequency signals. A method of modulating data with low-frequency signals, which includes the following steps. Encapsulate a plurality of binary data (3) together with an identifier (4) of at least one LR receiver (8) and a predetermined preamble, in an associated field (16, 17, 18) to generate an LR frame (15) by means of a formatter (2), where the preamble prepares the LR receiver (8) for reception. Encoding said frame (15) by means of a line code to generate a plurality of coded data (20) by means of an encoder (5). Modulate in ASK to generate a stream of frames (22), where the frames (22) are regularly spaced by the encoder (5).

Description

DESCRIPTION

Data modulation method with low frequency signals

Field of the Invention

The present invention falls within the field of wireless telecommunications. Particularly within wireless local area network (WLAN) technologies, governed by IEEE 802.11 standards, and wireless personal area network (WPAN) technologies, governed by IEEE 802.15 standards.

Background of the invention

In the document Mishra, N .; Chebrolu, K .; Raman, B .; Pathak, A., "Wake-on-WLAN", in proc. From WWW Conference 2006, it is proposed to use “sleeper” WLAN stations that include a low power sensor to control activity in the WLAN working channel. After the detection of energy in that channel, the sensor wakes up the WLAN transceiver. While this approach has the advantage that it does not require any specific activation signal transmission functionality, the probability of false alarm alarms is excessively high, even operating in a poorly used ISM (Industrial, Science & Medical) channel.

The same authors tried to overcome this problem later by incorporating patterns in the activation signals published in the document Mishra, N .; Golcha, D .; Bhadauria, A .; Raman, B .; Chebrolu, K., "S-WOW: signature based Wake-on-WLAN", in proc. COMSWARE 2007. In its second proposal, large standard Wi-Fi frames and long periods of silence are used to form and send a simple pattern encoded by non-return-to-zero (NRZ). The resulting low rate signal is too slow to efficiently send data streams other than Wake-up signals (the Wake-up signal may take about 1s). Due to the great periods of silence (~ 20ms) this system is still very sensitive to interference.

In a very similar way, the authors Kondo, Y .; Yomo, H .; Tang, S .; Iwai, M .; Tanaka, T .; Tsutsui, H .; Obana, S., in his article “Energy-efficient WLAN with on-demand AP Wake-up using IEEE 802.11 frame length modulation,” Computer Communications, Vol. 35, No. 14, p. 1725-1735, August 2012, were able to encode a unique identifier for the Wake-up (WuR) receiver by sending IEEE 802.11 frames of different sizes. Although it is more resistant to interference than the previous approach, this option remains ineffective for the transmission of long data flows. In addition, the power consumption of your WuR receiver is high, of the order of mA. The state of the art of Wake-up systems establishes that the receiver consumption must be around 10 µW.

The use of IEEE 802.11 frame size (or other wireless technology) is also proposed in US8259691B2, where the authors adjust the size of the data frames to encode information related to the network configuration required by a wireless device with limited features (and without human interface, such as keyboard / screen). More specifically, the binary data is decoded by comparing the relative difference between the lengths of consecutive frames, that is, a change in the length between consecutive frames may binary bit '0', and if no change is perceived in the length is interpreted as bit '1'. Again, in a random access network based on carrier detection (for example, the IEEE 802.11 or IEEE 802.15 families) this solution is not suitable, since any transmission in the neighborhood will interrupt the operation of this mechanism.

In application US20120120859 A1, it is proposed to use unused OFDM subcarriers located at the ends of the spectrum of an IEEE802.11a / g transmission to embed Wake-up signals. In accordance with IEEE 802.11a / g standards, 52 OFDM subcarriers are used, but the authors propose the use of 4 additional subcarriers. This approach requires profound modifications of an IEEE device.

802.11 commercial, and implies that both transmitter and receiver must be IEEE 802.11 WLAN stations. In addition, it is limited to IEEE 802.11 transmissions based on OFDM, which have a smaller range than those using DSSS (Direct Sequence Spread Spectrum).

According to an embodiment of the system described in application US20060063484 A1, a modulated amplitude signal is used to transmit low speed information. This signal is generated by the transmission of IEEE 802.11 frames; A valid IEEE 802.11 preamble is followed by an amplitude modulated waveform by a gain control mechanism installed in the transmitter. Unlike this solution proposed in US20060063484 A1, the invention detailed in this document does not require any specific modification on commercial IEEE 802.11 equipment since it is based on the emulation of a low frequency carrier generated with completely standard frames (IEEE

802.11 or IEEE 802.15) while US20060063484 A1 uses only a standard preamble followed by a modified signal.

In sum, most of the related literature focuses on the generation and detection of Wake-up signals. There are no known proposals to use IEEE 802.11 or IEEE 802.15 transmissions in order to generate a low frequency carrier. There are other solutions that could be used for the same purpose, but these are either inefficient, or require deep modifications in commercial wireless equipment, which limits their application.

The use of IEEE 802.11 signals to modulate other signals, such as Wake-up signals or low speed data, is known. However, existing proposals are limited in different aspects: energy efficiency, scope and implementability / compatibility. Although IEEE 802.11 compatible devices have already been tested as WuR transmitters, no proposals are known where the IEEE 802.11 or IEEE 802.15 signal itself is used to generate a low frequency carrier that can transport binary data at low speed and that , among other applications, could be used as an encoded WuR signal.

Description of the invention

We present a method capable of generating a low frequency carrier from common wireless devices "off-the-shelf" compliant with the family of IEEE 802.11 or IEEE 802.15 standards. This subcarrier can be modulated to transport any type of digital information directed to low-performance and low-consumption wireless receivers, such as sensor / actuator nodes.

The method of modulating data with low frequency signals, includes encapsulating a plurality of binary data together with an identifier of at least one receiver and a predetermined preamble, in an associated field to generate a low speed frame (low rate or LR) by means of a formatter, where the preamble prepares the LR receiver for reception. It also includes encoding said frame by means of a line code to generate a plurality of data encoded by an encoder. After that, a frame stream is generated, where the frames are regularly spaced by the encoder, to emulate a low frequency carrier that will be modulated with ASK to transport the encoded data.

Optionally, the transmission follows the IEEE 802.11 standard.

Optionally, the transmission follows the IEEE 802.15 standard.

Optionally, the frame stream generated is composed of the smallest frame defined in the standard followed by the smaller frame spacing supported by the standard.

Optionally, the generated frame stream is composed of successive recognition frames (ACK).

Optionally, the frame train generated is composed of beacon type control frames (Beacon).

Optionally, the generated frame stream is composed of empty data frames with broadcast / multicast destination.

Optionally, the frame stream generated is composed of empty data frames with broadcast / multicast destination and aggregated in an A-MPDU, according to the IEEE 802.11-2012 standard.

Optionally, the IEEE 802.11 frames of the frame stream include the duration field (Duration / ID) with a value equal to or greater than the time remaining for the completion of the LR frame. For this, the Duration / ID field has a value large enough to protect the entire LR frame.

Brief description of the drawings

A series of drawings that help to better understand the invention and that expressly relate to an embodiment of said invention which is presented as a non-limiting example thereof is described very briefly below.

Figure 1: Block diagram of LR transmitter and LR receiver.

Figure 2: Binary data encapsulation in an LR frame.

Figure 3: Modulation of a train of frames containing encoded information.

Figure 4: Application example.

Detailed description of the invention

The latest generation Wi-Fi communications are designed to exchange information at very high speeds (up to 600 Mbit / s) in the 2.4 or 5GHz ISM free-use frequency bands. The proposed mechanism allows such signals to be used for low-speed data modulation, in order to communicate with low-power wireless receivers and / or low capacities that do not necessarily meet IEEE 802.11 or IEEE 802.15 standards.

The transmission of low-speed digital signals (Low-Rate -LR) is done from a device (smartphone, laptop, netbook, tablet, etc.) equipped with a wireless network interface (Network Interface Card -NIC) compatible with the IEEE 802.11 or IEEE 802.15 standard as shown in Figure 1. The Low-Rate (LR) 1 transmitter has two basic components: data formatter 2, which is responsible for generating LR frames, and encoder 5 that encode and modulate the plot created.

First, by direct user interaction or through an application, the formatter 2 of the transmitter LR 1 receives the binary data 3 that must be transmitted. In addition, the formatter 2 needs the identifier (or address) 4 of the Low-Rate Receiver (LR) 8 to which this data is to be sent. The identifier 4 is a binary sequence of configurable length that distinguishes the desired receiver or receivers (multicast case) LR 8 from among all potential receivers. The formatter 2 encapsulates the identifier 4 and the data 3 in the identifier fields 17 and data 18, respectively, of a frame LR 15, as shown in Figure 2. The frame LR 15 includes a preamble 16. Said preamble 16 consists of a series default bit that will be used to train comparator module 12 of the LR receiver 8.

Next, the bit sequence of the LR frame 15 is encoded by a line code in the encoder module 5 of the LR transmitter 1. Figure 3 shows the effect of the different stages. The first stage consists in the creation of the LR frame 15 from the data 3 and the identifier 4 by the formatter 2. At the output of the formatter 2, the low-speed binary data 19 is obtained. The second stage, taken to carried out by the encoder 5, results in an encoded data signal 20 as shown in Figure 3, in case of using an NRZ encoding. This coding consists in representing the binary value ‘1’ as a high value during a bit period of duration Tb, and the binary value ‘0’ as a low value during Tb. The proposed mechanism can also be used for other line codes, such as with the Manchester code, where each bit period Tb is divided into two equal intervals; a ‘1’ is transmitted with a high voltage value in the first interval and a low value in the second, and a ‘0’ in reverse.

The encoded data 20 will be used in a third modulation stage, also carried out by the encoder 5. In this stage a low frequency carrier signal (less than 200kHz) is modulated by an amplitude modulation of the Amplitude-Shift Keying type ( ASK). The simplest case of ASK is achieved by modulating On-Off Keying (OOK) 21, with which the high voltage values of the encoded data are represented by the presence of a carrier for a time Tb and the low values by the absence of carrier for a time Tb. To emulate the presence of the low frequency carrier signal, the encoder 5 communicates with the driver 6 of the LR transmitter 1 so that it generates a regularly spaced IEEE 802.11 or IEEE 802.15 frame train, as shown in the Figure 3. That is, during the On state, the encoder 5 of the LR 1 transmitter generates frames and in the Off periods, it remains silent. Driver 6 moves the IEEE 802.11 or IEEE frame creation requests to the physical layer

802.15 received from the encoder 5. The physical layer resides on a wireless interface 7 that complies with the IEEE 802.11 or IEEE 802.15 standard. The wireless interface 7 transmits the IEEE 802.11 or IEEE 802.15 frames generated by the encoder 5 in the 2.4 or 5GHz frequency band. The encoder module 5 of the transmitter LR 1 can add a burst from the carrier at the beginning of the transmission of the frame LR 15 in order to prepare the circuit of the receiver LR 8.

The generation of a regular frame stream 22 according to IEEE 802.11 or IEEE 802.15 is not trivial but requires precise timing, as detailed below.

The wireless interface 7 of the proposed LR transmitter 1 may consist of any commercial IEEE 802.11 or IEEE 802.15 NIC since no hardware modification is required. What this invention proposes is to establish a new method of transmission and transformation of binary data 3 from the user to the wireless interface 7, as set forth for the formatter 2 and encoder 5 components of the LR 1 transmitter.

To successfully generate a low frequency carrier, several problems have been addressed. The first problem to overcome is the generation of frame train 22 from the encoder 5 of the LR transmitter 1, through, for example, a transmitter that complies with Wi-Fi certification. This could be achieved by continuous transmission of the smallest frame defined by the IEEE 802.11 standard, followed by the smallest possible frame spacing. Since not all Wi-Fi card models support such transmissions, the following alternatives have been studied, seeking compatibility of the proposed mechanism with the widest variety of manufacturers and suppliers of commercial Wi-Fi devices:

Successive transmission of recognition frames (ACK), followed by a short space between frames (in English, Short Inter-Frame Space, SIFS) whose value of 10 µs is set by the IEEE 802.11-2012 standard.

Successive transmission of beacon control frames (Beacon) with adjustable values for the interval between consecutive beacons (in English, Beacon Interval), for the frame size (in bytes) and for the transmission speed.

Periodic transmission of empty data frames with broadcast / multicast destination (which do not require response by the receiver via ACK frame) disabling the backoff mechanism and with a time between DIFS frames (in English, DCF Inter-Frame Space) whose value of 28µs is set by the IEEE 802.11-2012 standard.

Disable the access mechanism to the medium defined by IEEE 802.11 (CSMA / CA) to transmit a train of empty data frames with multicast / broadcast destination and with the time between frames conveniently adapted to each case.

Use of the frame aggregation function (A-MPDU) defined in the IEEE 802.11n standard. In this case, empty data frames with broadcast / multicast destination separated by a reduced time between frames (in English, Reduced Inter-Frame Space, RIFS) of 2µs are added, as defined in the IEEE 802.11n standard.

The aforementioned proposed mechanisms allow the generation of a low frequency carrier in the range of 5-40 kHz, which we have successfully used to wake up a Wake-up receiver based on the AS3933 chip (Austria Microsystems).

The second challenge derives from multiple carrier detection access (CSMA / CA) used by the media access control (MAC) layer defined in IEEE 802.11 and IEEE 802.15. A low-speed data transmission based on IEEE 802.11 or IEEE 802.15, as we propose (for example, a Wake-up signal) will be interrupted when the LR 1 transmitter detects the presence of other transmissions on the same frequency channel. Since low-speed transmissions generated with IEEE 802.11 or IEEE 802.15 frames will require long periods of silence (for example, a binary '0' in the NRZ encoding is represented as a silence period of duration Tb, signal 21 in Figure 3 ), together with the fact that the 2.4 GHz ISM band, in which most IEEE 802.11 or IEEE 802.15 devices

operate, is subject to massive use, the system can receive a large number of unwanted signals. Said transmissions cause interruptions in the transmission of the frame train 22, which can be frequent, thus avoiding the normal operation of the proposed method. The simplest approach to overcome this problem is the use of wired connections between the transmitter and the receiver, but you choose to maintain the convenience of wireless transmissions as follows.

To reduce the negative impact of transmissions from external Wi-Fi frames, an alternative use of a mechanism provided by the same IEEE 802.11-2012 standard is proposed. Standard IEEE 802.11 frames include a field called Duration / ID to indicate the period during which the channel will be in use due to the transmission of the current IEEE 802.11 frame. In this way, IEEE stations

802.11 decode that field of the frame header, and postpone its pending transmissions for the time specified in the frame, thus avoiding possible collisions with the current frame. If the IEEE 802.11 frames, which make up the frame stream 22 generated by the LR 1 transmitter, contain this Duration / ID field with its highest possible value, potential interference from other nearby Wi-Fi stations will be minimized during the transmission of The low speed signal.

The stages developed in the LR 8 receiver of Figure 1 correspond to the conventional architecture of the so-called Wake-up receivers. Typically, said Wake-up receiver implements a demodulation stage 11 where, by means of an envelope detector circuit, a low frequency signal is extracted from a higher frequency one. This extracted signal is used as input of a comparator 12 that recovers the binary value 14 of the data modulated in the high frequency carrier. On this common circuit, the present invention requires an additional element, the envelope detector 10. Some commercial circuits, such as AS3933, efficiently implement this strategy for carriers in the order of kHz. To enable its use with LR 1 transmitters operating at frequencies in the order of GHz by the use of a wireless interface 7 IEEE 802.11 or IEEE 802.15, as described in this document, the use of an additional envelope detector 10 is required which It allows a first conversion from GHz to kHz, as well as the corresponding replacement of the receiver antenna 9 optimized to work in this GHz frequency band. In this way, at the output of the envelope detector 10, we will have an OOK 21 modulated signal like the one shown in Figure 3.

When the LR frame 15 (Figure 2) incorporates an identifier of the receiver 17, the addressing correlator 13 of the LR receiver 8 itself will compare the address field of the LR frame 15 with the configured address of the LR receiver 8. Said correlation process barely it requires a current consumption in the order of µA (microAmperes), so the LR 8 receiver will not waste energy in processing the LR 15 frames that are not directed to it.

APPLICATIONS:

For the generation of low frequency signals in IEEE 802.11 transmissions two types of applications are contemplated, illustrated in Figure 4: low speed communications with low performance devices, such as sensors or actuators, and Wake-up Radio systems.

The proposed mechanism allows the wireless transmission 24 of information to electronic devices 25, which do not support the complexity and energy demands that would require the implementation of a receiver compatible with the IEEE 802.11 family of standards; technology that, on the other hand, can be found in a large number of current consumer electronics devices. Thus, the proposed method enables the user to interact with different types of electronic devices 25 such as sensors, actuators, switches, etc., by means of any common Wi-Fi device 23 that has an LR 1 transmitter, and be it a phone, laptop, tablet, etc., thanks to the use of an LR 8 receiver.

A second application of a mechanism of this nature consists in the possibility of waking up a very low consumption LR 8 receiver; In this case, this module is called Wake-up. Such wireless receivers are connected to an electronic device 25 whose energy consumption is intended to be reduced. Thanks to the LR 8 receiver, the electronic device 25 can remain set to "sleeping", or low power, as long as its use is not required. By means of a wireless transmission 24, generated by the mechanism that we propose, the LR 8 receiver can be activated to wake up the electronic device 25. Such a way of operating allows energy savings of several orders of magnitude, being able to pass from mA consumption (milliAmps) to µA (microAmps). For an IEEE 802.11-based system, any common Wi-Fi transmitter 23 with an LR 1 transmitter can generate wireless transmission 24 that serves as a Wake-up signal. This signal will wake up the desired electronic device 25, which has an LR 8 receiver, operating as a WuR tuned in the 2.4 or 5GHz band.

In the case of applying this system to a set of Wi-Fi access points (APs), operating as electronic devices 25, in order to save energy and reduce interference, APs that are not in use will remain in a state of saving power until the LR 8 receiver, operating as a WuR circuit, awakens the required AP. Thereafter, the transmission between the common Wi-Fi transmitter 23 and said AP will follow the normal procedure defined by the IEEE 802.11 standard, without the need for the LR 8 receiver.

Numerical references

1 LR transmitter

2 Formatter

3 User binary data

4 Receiver Identifier (s)

5 Encoder

6 Driver

7 Wireless interface

8 LR Receiver

9 Receiver antenna

10 Envelope Detector

11 Demodulation stage

12 Comparator

13 Addressing correlator

14 Binary data extracted from 18. 15 LR frame. 16 Preamble. 17 Receiver identifier (s), obtained from 4.

5 18 Binary data obtained from 3. 19 Low speed binary data. 20 Coded data. 21 OOK modulated signal. 22 Frame train.

10 23 Common Wi-Fi device. 24 Wireless transmission. 25 Electronic device.

Claims (9)

1.-Method of modulating data with low frequency signals, characterized in that it comprises the following steps: encapsulating a plurality of binary data (3) together with an identifier (4) of at least one LR receiver (8) and a predetermined preamble, in an associated field (16, 17, 18) to generate an LR frame (15) by a formatter (2), where the preamble prepares the LR receiver (8) for reception, encoding said frame (15) by a line code to generate a plurality of encoded data (20) by an encoder (5), Modulate with ASK to generate a frame train (22), where the frames (22) are regularly spaced by the encoder (5). 2. Modulation method according to claim 1, characterized in that the transmission follows the IEEE 802.11 standard. 3. Modulation method according to claim 1, characterized in that the transmission follows the IEEE 802.15 standard. 4. Modulation method according to claim 2, characterized in that the frame train (22) generated is composed of the smallest frame defined in the standard followed by the smaller space between frames admitted by the standard. 5. Modulation method according to claim 2, characterized in that the frame train (22) generated is composed of successive recognition frames (ACK). 6. Modulation method according to claim 2, characterized in that the frame train (22) generated is composed of beacon type control frames (Beacon). 7. Modulation method according to claim 2, characterized in that the frame train (22) generated is composed of empty data frames with broadcast / multicast destination. 8. Modulation method according to claim 2, characterized in that the frame stream (22) generated is composed of empty data frames with broadcast / multicast destination and aggregated in an A-MPDU, according to the IEEE 802.112012 standard. 9. Modulation method according to any one of the preceding claims, characterized in that the IEEE 802.11 frames of the frame stream (22) include the duration field (Duration / ID) with a value equal to or greater than the time remaining for the end of the LR frame (15). FIGURES 10