Wireless digital data transmission from a passive transceiver

A wireless network transmits digital data. The network includes an active transceiver to transmit carrier waves at a succession of preselected frequencies and a transponder. The transponder transmits digital data to the active transceiver by partially reflecting the carrier waves.

BACKGROUND OF THE INVENTION

This invention relates to wireless digital data transmission.

Typical wireless digital data communication is affected between radio-frequency (RF) active transceivers contained in each of two communication devices. Each RF transceiver has a separate power source to produce the radio-frequency carrier waves used to transmit data to the other devices.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides a wireless network for transmitting digital data. The network includes an active transceiver to transmit carrier waves at a succession of preselected frequencies and a transponder. The transponder transmits digital data to the active transceiver by partially reflecting the carrier waves.

In a second aspect, the invention provides a transceiver for digital data. The transceiver includes an antenna to transmit radio-frequency (RF) carrier waves and an RF module coupled to drive the antenna to produce the RF carrier waves. The RF module detects reflected portions of the produced RF carrier waves at the same time. The RF carrier waves have a succession of frequencies.

In a third aspect, the invention provides a method for wireless data transmission. The method includes transmitting radio-frequency (RF) carrier waves to a transponder at a succession of frequencies and receiving reflected portions of the RF carrier waves from the transponder. The reflected portions are digitally modulated.

In a fourth aspect, the invention provides a method of wireless digital communication. The method includes receiving a first radio-frequency (RF) carrier wave at a receiver, digitally modulating an RF reflectivity of the receiver, and reflecting a portion of the first RF carrier wave in response to the digitally modulating of the RF reflectivity. The method includes repeating the receiving, digitally modulating, and reflecting for a second RF carrier wave at a new frequency.

DETAILED DESCRIPTION

FIG. 1shows a local wireless radio-frequency (RF) network4for transmitting digital data between a digital device6capable of communicating data and other digital devices8,14capable of communicating data. The digital device6contains an interrogator10, which controls communications between digital device6and the other devices8,14. Each of the other devices8,14includes a passive transponder12,16. The interrogator10is a master of wireless communication over the passive transponders12,16, which are communication slaves.

Though the transponders12,16are communications slaves of the interrogator10, devices8,14may control some functions of the digital device6through the wireless network4. The wireless network4supports half duplex communications of digital data between any of the transponders12,16and the interrogator10.

The interrogator10is an active radio-frequency (RF) transceiver of digital data. The active transceiver can transmit digital data to the transponders12,16on an RF carrier wave, e.g., using differential phase shift keying (DPSK) modulation. The interrogator10can also selectively receive digital data from an RF carrier wave that has been modulated through DPSK by one of the transponders12,16. Though the transponders12,16can both transmit data to and receive data from the interrogator10, the transponders12,16are not the source of the RF carrier waves used to transmit digital data to the interrogator10.

Instead, the transponders12,16transmit digital data by passively reflecting a portion of an unmodulated RF carrier wave, which was transmitted by the interrogator10. The digital data appears as a DPSK modulation on the back reflected portion of the carrier wave. The DPSK modulation is produced by changing the transmitting transponder12,16between RF reflective and non-reflective states. DPSK may be a convenient modulation scheme, because the transponders12,16transmit data through passive reflection. The interrogator10receives a portion of the back reflected RF carrier wave and demodulates the received portion to retrieve the digital data sent by the transmitting transponder12,16.

Since the transponders12,16do not produce the RF carrier wave used to transmit data, they can operate with lower power sources than active RF transceivers. The transponders12,16may use small, inexpensive, and light “button” batteries13or solar cells17as power sources, because they do not have to generate the RF carrier waves. Some embodiments of the transponders12,16can even extract enough energy from the received RF carrier waves, to power their internal circuits (not shown).

Small and lightweight power sources make the transponders12,16convenient for use in embodiments of the devices8,14, which have special functionalities. For example, the devices8,14may be personal identity badges, cellular phones, pagers, personal digital assistants, notebook computers, keyboards, or computer mice. The devices6,8,14may also be heavier objects such as printers and facsimile machines.

Referring now toFIG. 2A, the interrogator10includes an RF module18, transmission and reception antennae20,22, and a processor24. The RF module18generates a variable frequency voltage for driving the antenna20to generate RF carrier waves. The RF module18also provides for variable frequency filtering of RF radiation received by the antenna22. The processor24contains logic for controlling the RF module18during signal transmission and reception. The processor24contains memory25and logic elements27and may perform more complex activities, e.g., database look ups, calculations, printing.

Some embodiments of the interrogator10use the same antenna for both transmitting and receiving RF signals.

Each transponder12,16includes an RF module28,30, an antenna32,34, a switch36,38, and a processor40,42.

The RF modules28,30control data transmission modes of the associated antenna32,34through the associated switch36,38. The RF modules28,30also provide variable frequency filtering of RF radiation received by the associated antenna32,34. The transponders16has separate antennas34,35for transmitting data to and receiving data from the interrogator10. The processors40,42control the associated RF module28,30and contain both memory41,43and logic elements45,47to provide for control of data transmission and reception.

The dimensions of the antennae32,34provide good reflection of the RF radiation transmitted by the interrogator10when in a reflective state. The antennas32,34have two states. In the closed state, the switch36,38shorts an electrical dipole loop through the associated antenna32,34, i.e., forming a closed loop. The dipole loop partially back reflects RF radiation, e.g., an RF carrier wave transmitted by the interrogator10. The interrogator10receives detectable amounts of back reflected RF radiation when the antennae32,34are in the closed state. In the open state, the switch36,38does not close an electrical dipole loop through the associated antenna32,34. Then, the antennae32,34reflect very little RF radiation transmitted by the interrogator10, e.g., the above-mentioned RF carrier wave. The interrogator10does not receive detectable amounts of back reflected RF radiation when the antennae32,34are in the open state.

The switches36,38function at high enough frequencies so that the transponders12,16can transmit data at high bit rates. High frequency switches36,38may be formed by single transistors, which series couple across the associated antenna32,34to form an electrical dipole loop. The opened or closed state of the dipole loops are controlled by the associated RF module28,30through a gate bias or base current of the transistor forming the switch36,38. Opening and closing the switches36,38modulates the reflectivity of the associated transponder12,16to an RF carrier wave received from the interrogator10. Opening and closing one of the switches36,38in rapid succession produces a reflected wave with a binary amplitude modulation at frequencies between tens of kilo-Hertz and about a few mega-Hertz. The modulation phase is detectable by the interrogator10at distances between about 10 centimeters and 10 meters and provides for digital data transmission for network4. The detection distance depends on the transmit power level and the reception gain of the interrogator10.

Though the RF modules28,30power the switches36,38and any internal logic and/or memory, they do not produce the RF carrier waves that carry data transmissions. The high energy costs for producing the RF carrier waves used for data transmissions, in both directions, are born by the interrogator10. Thus, the RF modules28,30use less power to transmit digital data than active RF transmitters (not shown). Lower power consumption to transmit data translates into lower demands on power sources.

The interrogator10also hops to a new RF driving frequency at regular intervals. Frequency hopping reduces interference from background RF sources44,46, because the background RF sources44,46usually do not frequency hop. Between frequency hops, the interrogator10transmits an RF carrier wave in a predetermined member of a set of narrow frequency bands.

FIG. 2Bshows another reflective dipole antenna64for an alternate embodiment of the transponders12,16ofFIGS. 1 and 2A. The antenna64includes two linear segments65,66positioned in a linear end-to-end arrangement. The length of each segment65,66is about equal to ¼ of the wavelength of the carrier wave produced by the interrogator10.

The reflectivity of the dipole antenna64is controlled by a high speed switch67connecting the two segments65,66in a linear arrangement. An RF module68, e.g., one of the RF modules28,30ofFIG. 2A, operates the switch67. In the open state, the switch67is electrically open and the antenna64performs as two separate ¼-wavelength antennae.

In the closed state, the switch67is closed and the antenna64performs as a single ½-wavelength antenna. A pair of ¼-wavelength antennae and a ½ wavelength antenna have substantially different RF reflectivities. Thus, the antenna64has a different reflectivity in the open and closed states.

Some embodiments of the network4comply with protocols of the Bluetooth Special Interest Group, www.bluetooth.com, published Jul. 16, 1999. The protocols of the Bluetooth Special Interest Group are used with spread spectrum technology transmissions occurring in 79 preselected narrow RF bands. The narrow RF bands are one mega-Hertz wide, adjacent and located in the range between about 2.402 and 2.480 giga-Hertz. In this range, the transceivers12,16ofFIGS. 1 and 2Acan transmit about 10−3to 10−1watts of RF by passive reflection of a received RF carrier wave.

In the embodiments implementing the protocols of the Bluetooth Special Interest Group, the devices6,8,14hop to an adjacent narrow RF band each 80 milli-seconds. Each hop increases the transmission frequency until the upper extreme of the frequency range is reached. From the upper extreme, the devices6,8,14return to the lowest narrow RF band of the range, i.e., between 2.402 and 2.403 giga-Hertz.

Other embodiments hop between a pseudo-random succession of frequencies in a predetermined frequency range. The succession of frequencies is communicated to the slave transponders12,16by the interrogator10. The succession of frequencies and/or timing information for the hops may be security coded to maintain privacy using the pseudo-random frequency hopping scheme.

In both types of frequency hopping, the RF modules18,28,30filter out RF carrier frequencies that the interrogator10does not transmit. Each transponder12,16is assigned a temporal sequence of RF carrier frequencies. The temporal sequences for the different RF modules28,30differ so that the interrogator10can communicate with the transponders12,16individually. The interrogator10transmits timing data that enables the RF modules28,30, to synchronize filtering with the assigned RF frequency hopping.

FIG. 3illustrates a wireless method50of receiving data transmissions from a passive transponder of a wireless network. For example, the transponders may be the transponders12,16of the network4ofFIGS. 1 and 2. An active transceiver sends an RF protocol message to a target transponder (step52). InFIG. 2A, the active transceiver is the interrogator10.

The protocol message sets up a protocol for subsequent data transmissions by the targeted transponder. The protocol message may contain transmission parameters that identify the targeted transponder and the calling active transceiver, the RF carrier frequency, frequency hopping data, encrypting codes, and timing data. While the protocol message is being sent, the active transceiver and the target transponder act like an ordinary wireless transmitter-receiver pair. The protocol message may also include data and/or queries to the target transponder that request responses.

After sending the protocol message, the active transceiver transmits an unmodulated RF carrier wave to the transponder via the wireless network (step54). The transponder reflects the unmodulated RF carrier wave to produce a modulated RF wave carrying data back to the active transceiver. The active transceiver receives a portion of the RF carrier wave reflected back by the transponder (step56). The active transceiver bandpass filters and demodulates the received RF carrier wave to retrieve digital data transmitted by the target transponder (step58). The active transceiver determines whether a preselected time has elapsed (step60). The preselected time period may be based on number of data packets or bytes received or on a counted time. If the preselected time has not elapsed, the transceiver continues to transmit the unmodulated carrier wave (step54).

If the preselected time has elapsed, the active transceiver and target transponder reset their RF transmission frequencies to a new value, i.e., a frequency hop (step62). After the frequency hop, the active transceiver transmits an RF carrier wave with the new frequency to the target transponder (step54). In the illustrated embodiment, the active transceiver also transmits a new protocol message to the transponder prior to transmitting the new RF carrier wave (step52). The new protocol message informs the target transponder of the new transmission frequency and/or other information. In some embodiments, several transmission cycles at different frequencies terminate before the transmission of a new protocol message.

FIG. 4shows a method70by which a target transponder transmits digital data to the active transceiver. For example, the transponders and active transceiver may be the transponders12,16and the interrogator10ofFIG. 2A. The target transponder receives a protocol message from the active transceiver (step72). The target transponder demodulates the received protocol message and performs setup procedures in response to data therein (step74). For example, the setup procedures may include determining whether the transponder is the target of the protocol message. The setup procedures may also include setting a passband for frequency filtering and procedures to produce data requested by the active receiver. At a time determined by the protocol message, the transponder receives an unmodulated RF carrier wave from the active transceiver (step76).

The target transponder modulates its own RF reflectivity between RF reflective and non-reflective states to reflect a portion of the RF carrier wave back to the active transceiver (step78). The reflected portion of the RF carrier wave transmits data back to the active transceiver in the form of digital DPSK modulation. To modulate its RF reflectivity, the target transponder opens and closes the RF current loop formed by its receiving antenna32,34as was described above. The target transponder again DPSK modulates its own reflectivity to reflect a portion of another RF carrier wave having a new carrier frequency (step80). The reflected portion of the RF carrier wave at the new frequency transmits additional data back to the active transceiver. A portion of each reflected RF carrier wave is received and demodulated by the active transceiver to retrieve the transmitted data.