GB2428940A

GB2428940A - Carrier leakage cancellation in an RFID reader

Info

Publication number: GB2428940A
Application number: GB0603968A
Authority: GB
Inventors: John Domokos
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2005-07-25
Filing date: 2006-02-28
Publication date: 2007-02-07
Also published as: GB0515110D0; GB0603968D0

Abstract

A transceiver apparatus comprises at least one antenna (16), selectively connectable (15) to a receiver portion (14) and a transmitter portion (13) of the apparatus. The apparatus further comprises a carrier cancellation loop comprising a vector phase modulator (22), whereby magnitude and phase of an output continuous wave (CW) signal are scaled and inverted and added to an input signal to cancel CW leakage.

Description

TRANSCEIVER APPARATUS

This invention relates to a transceiver apparatus and a method of carrier leakage cancellation therein, in particular a cancellation loop for RFID interrogators.

Radio Frequcncy Identification Devices (RFID) play an increasingly important role in the supply chain management sector. An RFID system typically consists of a reader (transmitter and receiver), one or more antennas, and a population of tags.

In passive RFID systems, the tag is energized by a continuous wave (CW) radio frequency (RF) field which is transmitted via the antenna of the reader. The reader to tag communication is arranged by modulating the CW signal. The tag decodes this signal and responds by back-scattering the transmitted CW. The back-scattered signal is received by the antenna and is then decoded in the receiver.

In a typical system, the power of the transmitted CW signal is in the order of 100mW to 2W and the back-scattered signal is recovered with a homodyne receiver.

The received signal is very weak, in the order of mW to lOOnW depending on the distance between the reader and the tag.

RFID readers are homodyne, i.e. they have zero IF frequency. The reader has a transmitter, a receiver and at least one antenna which sends an RF signal to a tag and the tag sends a backscattered signal to the receiver. Installation costs make a two antenna solution less desirable, so one can have a circulator with a single antenna for both transmitting and receiving, i.e. to send and receive from the tag.

A problem with using a circulator is that there is parasitic coupling between the transmitter and the receiver, because the circulator is not perfect. One known solution to this problem is to use a switching matrix, where all spare antennas in a multiple antenna system are used as receive antennas, but only one antenna is used to transmit.

However, this is less attractive for a handheld device, so it is desirable to make the circulator work better.

In accordance with a first aspect of the present invention, a transceiver apparatus comprises at least one antenna, selectively connectable to a receiver portion and a transmitter portion of the apparatus; wherein the apparatus further comprises a carrier cancellation ioop comprising a vector phase modulator, whereby magnitude and phase of an output continuous wave (CW) signal are scaled and inverted and added to an Input signal to cancel CW leakage.

Preferably, the apparatus further comprising a memory to store predicted values of CW leakage magnitude and phase.

Preferably, the apparatus comprises a plurality of antennas.

Preferably, the apparatus further comprises a circulator to couple the transmit and receive portions to their respective antennas.

In accordance with a second aspect of the present invention, a method of carrier leakage cancellation in a transceiver apparatus comprises outputting a continuous wave (CW) carrier signal from a transmitter portion to an antenna; sampling the carrier signal in the apparatus; scaling the magnitude of the sample and inverting the phase of the carrier signal; receiving an input signal at the receiver portion; and applying the scaled and inverted signal to the input signal to cancel CW carrier leakage.

Preferably, values of residual CW leakage magnitude and phase at predetermined frequencies are stored in a memory.

Preferably, antenna allocations for the predetermined frequencies are also stored.

Preferably, variations in signal path are monitored and stored in the memory for the predetermined frequency and antenna combinations.

An example of apparatus for and a method of carrier leakage cancellation will now be described with reference to the accompanying drawings in which: Figure 1 is a block diagram of a typical tag reader, with four common transmit/receiver antennas; Figure 2 is a block diagram of a reader having four separate transmit and receiver antennas; Figure 3 shows the reader of Fig. 2, incorporating a switching matrix; Figure 4 illustrates an example of apparatus according to the present invention; and, Figure 5 shows an alternative cancellation loop for use in the apparatus of Fig.4.

Figure 1 shows a block diagram of a typical reader having a transmitter 1, receiver 2, circulator 3 and switch 4. Four antennas A, B, C, D are used for interrogating a tag (not shown). If a path from antenna A to the tag is obstructed, or impaired by multipath propagation, the switch 4 selects antenna B. The switch cycles through all antennas until communication between the reader and the tag has been successfully completed. The drawback of this arrangement is that the receiver 2 is desensitized by a signal transmitted from the transmitter 1. This signal leaks P1 through the circulator 3 because of the finite isolation of a practical device. Furthermore, some of the transmitted signal is reflected back P2 from the antenna A, B, C, D because of return loss limitations of the antenna and its cabling. The receiver transmitter isolation, i.e. the sum ofPl and P2, is in the order of 15 to 25 dB. This limits the reading range of this type of reader to about im to 3m.

An improved arrangement is shown in Fig. 2. In this case, a transmitter 5 and a receiver 6 are coupled via switches 7, 8 to their antennas. There are four antennas AT, BT, CT, D1 for the transmitter and four separate antennas AR, BR, CR, DR for the receiver, so the desensitization is greatly improved. The isolation P3 in this case is typically 30 to 40dB which means that the reading range is now increased to between 3m and I Om. However, the disadvantage of the system in Fig. 2 is that twice as many antennas are required for the reader, as in Fig. 1, so the hardware and the installation costs of this system are higher.

One solution to this is to use a switching matrix as described in our copending UK patent application no.GBO5 10208.2, which reduces the cost of the arrangement in Fig. 2. A block diagram of such a switching matrix is shown in Fig. 3, in which a transmitter 9 and multi-channel receiver 10 are connected via switches 11, 12 to the same antennas A, B, C, D. The main disadvantage of the switching matrix solution is that it always requires more than one antenna and therefore it is not a good choice for a hand-held reader. Another drawback of the switching arrangement is due to the separate transmit and receive paths. Once the system identifies an acceptable path for the transmit operation, the same path cannot be used for reception. This means that there is a chance that the receiver path is blocked even though the tag has been successfully energized and is transmitting back-scatter.

The carrier cancellation ioop of the present invention is able to achieve similar isolation and therefore similar read range to that of the switching matrix. However, it has the advantages that it can be used with one antenna, making it suitable for portable applications and that the receiver and the transmitter paths are the same at any given read cycle, so that both the receiver and the transmitter can operate through the optimum channel simultaneously An example of apparatus according to the present invention is shown in the block diagram of Fig. 4. The reader comprises a transmitter 13, a receiver 14 and a circulator 15 which connects the transmitter and receiver to an antenna 16. In the transmitter 13, reader to tag data 17 is modulated onto a continuous wave (CW) signal from a signal generator 18, in a mulflplier 19 and output through an amplifier 20 and coupler 21 to the circulator 15 and one of the antennas 16. The coupler also provides an input to a vector modulator 22 and forms part of a cancellation loop. The cancellation loop also includes a summing device 23, a memory 24 and digital to analogue converters 25.

The cancellation loop operates as follows. The coupler 21 couples a small sample of the output CW signal from the transmitter amplifier 20 to the vector modulator 22. The vector modulator scales magnitude and rotates phase of the output CW signal so that it is equal in magnitude, but opposite in phase to any incoming CW leakage from the circulator. When the scaled, rotated signal is added to an input signal, received at the antennas, the unwanted CW leakage component is cancelled out.

The vector modulator 22 is controlled by the homodyne receiver 14. The receiver receives an input signal and the output of the vector modulator via the summer 23. In phase and quadrature components of the CW signal are produced in a local oscillator 27 and multiplied 28 with the output of the summer 23, then separately A to D converted in ADCs 29 and input to a demodulator 26. Residual CW is demodulated in the demodulator 26 and magnitude and phase information are stored in the memory 24. The magnitude and phase of the residual CW depend upon the frequency of the signal, the particular antenna selection made and the signal path.

The frequency and the antenna selection parameters are used to address the memory 24. Slow variations in the signal paths are tracked with the cancellation loop and the corresponding magnitude and phase (or I /Q values) are continuously updated in the memory. The ioop applies these complex values to the vector modulator 22 via the D/A converters 25 in the opposite polarity to achieve signal cancellation.

The loop further comprises an integrator - not shown in figure 4. The integrator, or an appropriate loop filter, determines the transfer function of the tracking loop. This integrator can be implemented either digitally or by analogue RC elements.

The cancellation loop may also be implemented without a circulator 15. Fig. 5 shows an example in which a 3dB coupler 30 is used between the receiver 14 and the transmitter 13, in place of the circulator. This arrangement operates in an identical fashion to that of Fig. 4. Should the two anteimas 31, 32 be cross-polarized, they would produce circularly polarized RF waves. The arrangement of Fig. 5 offers an ideal solution for a hand-held reader using co-located antenna elements.

The present invention has the advantages that it has a large reading range due to the elimination of the self blocking, the same transmit and receive paths are used simultaneously and it is suitable for integration and for hand-held applications.

The parasitic path is also dependant on the antenna properties and its inefficient reflection coefficient, so the memory can also include a reference to the relevant antenna to be used for the best result. For readers with frequency hopping, the memory also needs to update when the frequency changes.

Claims

1. A transceiver apparatus comprising at least one antenna, selectively connectable to a receiver portion and a transmitter portion of the apparatus; wherein the apparatus further comprises a carrier cancellation loop comprising a vector phase modulator, whereby magnitude and phase of an output continuous wave (CW) signal are scaled and inverted and added to an input signal to cancel CW leakage.

2. Apparatus according to claim 1, the apparatus further comprising a memory to store predicted values of CW leakage magnitude and phase.

3. Apparatus according to claim 1 or claim 2, wherein the apparatus comprises a plurality of antennas.

4. Apparatus according to claim 3, further comprising a circulator to couple the transmit and receive portions to their respective antennas.

5. A method of carrier leakage cancellation in a transceiver apparatus, the method comprising outputting a continuous wave (CW) carrier signal from a transmitter portion to an antenna; sampling the carrier signal in the apparatus; scaling the magnitude of the sample and inverting the phase of the carrier signal; receiving an input signal at the receiver portion; and applying the scaled and inverted signal to the input signal to cancel CW carrier leakage.

6. A method according to claim 5, wherein values of residual CW leakage magnitude and phase at predetermined frequencies are stored in a memory.

7. A method according to claim 6, wherein antenna allocation for the predetermined frequencies is also stored.

8. A method according to claim 7, wherein variations in signal path are monitored and stored in the memory for the predetermined frequency and antenna combinations.