This invention relates to tranceivers and has particular but not exclusive

applications to transceivers which transmit signals and also receives the

reflected signals therefrom or receives signals wherein the transmitted

signal has been modulated by a passive object such as a tag.

Radio frequency identification devices (RFIDs) are passive devices, also

known as tags, which are used for identification purposes. They may be

located on items such as consumer goods. RFID systems typically

comprise a reader which comprises a transmitter and receiver, and one or

more antennas to transmit and receive signals from the tag.

In passive RFID systems the tag is energised by the reader, typically by a

continuous wave (CW) RF field transmitted via an antenna of the reader.

The reader to tag communication is arranged by modulating the CW

signal. The tag decodes this and responds by back scattering the

transmitted CW. The back scattered signals is received by an antenna and

decoded in the receiver.

In a typical system the power of the transmitted signal is in the order of

10OmW - 2W and the back scattered signal is recovered with a homodyne

receiver. The received signal is very weak, in the order of InW - 10OnW

depending on the distance between the reader and the tag.

Figure 1 shows a prior art arrangement of a typical reader. In this example

four antenna A, B, C and D are used to interrogate the tag. It includes a

transmitter 3 and receiver 2. If the path from antenna A to the tag is

obstructed (or impaired by multi-path propagation) the switch selects

antenna B. For example if the reader is located on a fork lift truck the

antennae may be located at various points on the truck and antennae

proximal to the tag may have better transmission/reception than those

antenna distal thereto. The switch 1 cycles through all antennae until the

communication between the reader and the tag has been successfully

completed.

The problem with this arrangement is that the receiver 2 is desensitised by

the transmitted (own) signal. This signal leaks through the circulator Pl

because of the finite isolation of a practical device. Furthermore some of

the transmitted signal is reflected back from the antenna because of the

return loss limitation of the antenna and cabling. The transmitter receiver

isolation (sum of Pl and P2) is in the order of 15-25 dB; this limits the

reading range of this type of reader to about l-3m.

A known improvement is to isolate the transmitter and receiver. Such an

arrangement is shown schematically in figure 2. In this example there are

separate antennae for the transmitter and the receiver and thus the

aforementioned desensitisation problem is greatly reduced. The isolation

(P3) is typically 30 - 40 dB which means that typically

the reading range is increased to 3 to 10 m. The disadvantage with such a

system is that twice as many antennae are required. This means that the

hardware and the installation costs of this system are higher.

It is an object of the invention to provide for transceiver means having

effective isolation and which reduces the inherent cost of prior art

solutions.

- A -

The invention will now be described by way of example.

Figure 3 shows a basic embodiment of a reader 1 comprising a transmitter

2 and a receiver 3 both if which are connectable to a plurality of antennae

A, B C D by means of a switching matrix 5. In general there are N

antennae, where N=4, all of which can be used as a transmitting or

receiving antenna.

In a basic embodiment of the operation of such a system, one antenna at a

time is used for transmission and the switch matrix permits all the

remaining antennas to be used as receivers. Having separate antenna for

transmitting and receiving at any one time ensures high isolation, in a

similar way to the figure 2 prior art embodiment. Thus if antenna A is

being used to transmit, any of the antenna B C or D may be used to

receive.

In a further variation in methodology more than one antenna may be used

to receive the signal. If in the apparatus and example of figure 3a, antenna

A is being used as the transmitting antenna, then antenna B is used to

receive.

Any combination of transmitters B, C or D may be used. For example B+C

or B+D or C+ D. Additional all the antenna B+ C + D may be used to

receive the signal. The signals coming into the receiving unit from all

antennae B C & D are effectively summed; this is shown schematically in

figure 3 a. Figure 3 b shows such an enhanced system of the invention

where the receiving antennas are summed before being fed into the

receiving unit.

Figure 3c shows a further refined embodiment of the invention. A

drawback of the figure 3b arrangement is that although unlikely, it may be

that occasionally the signals received at the antennae may be out of phase

and/or have different strength. This may lead to the signals cancelling each

other out to a certain extent. In a preferred embodiment the signals of the

antenna used for receiving are combined coherently. By is meant the

received signals from the antenna are rotated in phase so as to make sure

they are all in the same phase by phase rectification units 4. Additionally

the magnitudes of the signals are amplified by an appropriate scaling

factors (amplifiers 5) to ensure that they each have equal weighting.