Abstract:
A method is disclosed for generating data encoded RF (radio frequency) waveform without a separate memory device/chip. The hardware in the proposed method consists of entities performing the operations of time-delay (B), phase shifting (C), attenuation, power dividing (D) and power combining (E). An integral part of the invention is the application of the the method in designing radio frequency identification devices or RFID-tags.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a method for generation of an encoded waveform and more precisely to a simplified method for producing radio frequency identification (RFID) tags.  
       BACKGROUND  
       [0002]     Two widely used methods for identifying objects today are barcodes and magnetic strips. Bar codes are commonly used for identifying objects in shops and supermarkets. An application of magnetic strips is the credit card. The main reason for the popularity of barcodes and magnetic strips is that they are inexpensive. One drawback of barcodes and magnetic strips is the distance range in which they can be used. The reader has to have a physical contact or has to be very close, say a few centimetres. If there is no physical contact, then the space between the code and the reader should not have any obstruction. In addition, the reader and the code have to be properly aligned for correct readability. This demands concentration from the part of the human operator and therefore is inconvenient.  
         [0003]     RFID (radio frequency identification) tag (or RFID-tag) is another technology used for identifying the identity of an object. In an RFID system, the interrogator or the reader and the tag can be separated by a larger distance compared to that of the magnetic strip technology or the bar-code technology. Once interrogated by the reader, RFID-tags will return an encoded radio signal that contains the identity of the object. RFID-tag devices can be broadly divided based on the criteria weather they contain an integrated memory chip or not. Those that contain a memory chip, e.g. U.S. Pat. No. 5,874,902, in general have more memory capacity than those of chip-less tags, e.g. U.S. Pat. No. 6,708,881. However, chip based tags have a significantly higher cost compared to that of the chip-less tags. RFID-tags can also be divided based on the criteria weather they contain a battery or not, active and passive tags. In general active tags, which are the most commonly available tags in market today, have a larger operational distance range when compared to the passive tags, e.g. U.S. Pat. No. 6,621,417.  
       SUMMARY OF THE INVENTION  
       [0004]     The invention relates to the method of generating a RF waveform containing a code at a remote point but not encoded using a memory chip. The underlying assumption in the above method is that there is the availability of a set of finite duration RF waveforms of different frequencies. These waveforms can be generated prior locally or can be made available through antennas wirelessly at the remote point. These finite duration signal bursts can be conveniently called hereafter as the mother signals. In order to generate the coded RF waveform, the mother signals are processed in time, frequency, phase and amplitude domain, whereby a set of child signals are produced and further processing of child signals or the mother signals are directly manipulated in time, frequency, phase and amplitude domain. The processing steps are achieved by means of RF passive devices such as time delays, phase shifters attenuators, power combiners and power dividers. The main difference in characteristics between the mother signals and child signals are their power and spectral characteristics. One of the main application area of the above method is in the RFID (Radio Frequency Identification) arena for generation of the tags. In RFID applications, the duration and shape of the finite duration signals are chosen such that the final encoded waveform will obey the power and bandwidth criteria imposed by the regulatory bodies. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     The invention, together with further objects and advantages thereof, may best be understood by referring to the following detailed description taken together with the accompanying drawings, in which:  
         [0006]      FIG. 1  is an example of a finite duration mother signal;  
         [0007]      FIG. 2  illustrates generation of a child signal from a mother signal;  
         [0008]      FIG. 3  illustrates generation of an encoded PSK or ASK or PPM modulated signal string from a child signal;  
         [0009]      FIG. 4  illustrates generation of an encoded FSK signal string from two different child signals;  
         [0010]      FIG. 5  illustrates an alternative realization of the generation of ASK or PPM signal from a mother signal; and  
         [0011]      FIG. 6  illustrates an alternative realization of the generation of ASK or PPM signal from a mother signal. 
     
    
     DETAILED DESCRIPTION  
       [0012]     In a preferred embodiment the invention can be described as follows:  
         [0013]     Assume the presence of K finite duration RF (radio frequency) signals S i   M  the mother signals of different frequencies locally for given duration T i , 1≦i≦K. A binary on-off keyed waveform, for example a sinusoidal wave of fixed duration when ON and zero signal when OFF is an example of a mother signal, see  FIG. 1 . However the mother signals may also attain other shapes, but with the fixed duration similar to that of  FIG. 1 .  
         [0014]     The invention relates to the generation of further K signals, S j   C  referred to as child signals with duration T j , 1≦j≦K. using RF time-delays, phase shifters power dividers and power combiners. It is to be noted that T j&lt;T   i .  FIG. 2  is an example of generating one child signal from a mother signal of one particular frequency and type shown in  FIG. 1 . As can be seen in  FIG. 1 , first a two way power divider (see  FIG. 2 , block A) splits the mother signal S i   M  into two branches with equal power and are subsequently passed through two unequal delays TD 1  and TD 2  (see  FIG. 2 , blocks B). In one branch, in addition to the time-delay there is phase shifter of angle Φ° (see  FIG. 2 , block C). Φ is adjusted such that phase difference between the signals in the two branches at the input of the power combiner (see  FIG. 2 , block E) is 180°. The combination of the signals in the two branches using the two way power combiner (see  FIG. 2 , block E) result in the child signal S 1   C  of duration |TD 2 −TD 1 | together with a tail signal of duration |TD 2 −TD 1 |.  
         [0015]     As can be seen in  FIG. 2 , the appearance of the tail is marked by a gap in time of (T M −|TD 2 −TD 1 |) from the disappearance of child signal. It is to be noted that the delays and the duration of the mother signal are selected such that the encoded waveform is produced according to the operations described as follows.  
         [0016]     Each child signal from a particular mother signal can be multiplied or cloned into several child signals of same shape by means of a power divider. By subjecting each such child signal to time-delays and (or) phase shifting and (or) attenuation and then combining, we can produce a signal string having the properties of an encoded RF signal. First we demonstrate this idea by generating an encoded signal string containing an N bit data. Next we demonstrate the generation of signal string from two different child signals (two different frequencies), containing N-1 bits of data.  
         [0017]      FIG. 3  shows a general block diagram showing the generation of a signal string from a single child signal produced from a mother signal of one particular frequency containing an N bit data having the properties of either PSK (phase shift keying), ASK (amplitude shift keying) or PPM (pulse position modulation).  FIG. 3  consists of an N way power divider (see  FIG. 3 , block A), N time-delays (see  FIG. 3 , blocks B), N phase shifters (see  FIG. 3 , blocks C), N attenuators (see  FIG. 3 , blocks D) and an N way power combiner (see  FIG. 3 , block E). The various signal string formats can be derived as follows.  
         [0018]     PSK signal string: Choose attenuators (see  FIG. 3 , blocks D) to be either absent or attenuation values to be zero. Choose TD j =j×T C , j=1 to N, where T C  is the duration of child signal S 1   C  at the input of the power divider (see  FIG. 3 , block A). Choose the phase shifters ( 101 j °, see  FIG. 3 , blocks C), such that the total phase shift is 0° if b j =1 and total phase shift is 180° if b j =0. Then the signal string S 1   MOD  resembles a PSK waveform encoded with N bits of data, b 1 , b 2  . . . b N .  
         [0019]     ASK signal string: Choose phase shifting to be zero for the phase shifters (see  FIG. 3 , blocks C) or phase-shifters to be excluded. Choose TD j =j×T C , j=1 to N, where T C  is the duration of child signal S 1   C  at the input of the power divider (see  FIG. 3 , block A). Choose attenuation of attenuators (see  FIG. 3 , blocks D) to be A j =0 if bit b j =1 and A j =infinity if b j =0. Then the signal string S 1   MOD  resembles an ASK waveform encoded with N bits of data, b 1 , b 2  . . . b N .  
         [0020]     PPM signal string: Choose phase shifting to be zero for the phase shifters (see  FIG. 3 , block C) or phase shifters to be excluded. Choose for the attenuators (see  FIG. 3 , block D) the attenuation values to be zero. Choose TD j −TD j-1 =t 0  for bit b j =0 and TD j −TD j-1 =t 1  for bit b j =1 where t 0 ≠t 1  and j=2 to N. Then the signal string S 1   MOD  resembles a PPM waveform encoded with N-1 bits of data, b 1 , b 2  . . . b N-1 .  
         [0021]     FSK signal string: A FSK signal string is formed using a different method compared to that of  FIG. 3  because of the requirement of two different child signals from different mothers having different frequencies.  FIG. 4  shows in part the method to produce an FSK signal string. This assumes the presence of two different child signals of different frequencies s 1   c  or s 2   c , see  FIG. 4 . The input to the time-delays, see  FIG. 4 , depends on the bit to be programmed. If bit b j =0 choose s 1   c  as the input else choose s 2   c . Choose TD j=j×T   C , j=1 to N, where T C  is the duration of child signal s 1   c  or s 2   c , (see  FIG. 4 , block B). A FSK signal string will result when the delayed versions of the child signals are combined using an N-way divider, (see  FIG. 4 , block E). Let the number of s 1   c  in the signal string be m. We can derive the s 1   c  by using an m way power divider after S 1   C  in  FIG. 2 . In a similar way we can derive s 2   c  from S 2   C .  
         [0022]     In another embodiment, certain modulated signal string formats can be generated by not explicitly generating a child signal and dividing the child signal, as shown in  FIG. 3 . This idea can be explained with respect to  FIG. 5 . In this embodiment, the mother signal of one particular frequency S 1   M  is directly divided by means of N-way power divider where N is an even number, (see  FIG. 5  block A). Outputs of the power divider are passed through branches which contain time delays T ij , phase shifters Φ ij  and attenuators A ij  in arbitrary order where 1≦i≦M, 1≦j≦2 and N=2M. The operations of time delays, phase shifting and attenuation, which the signals undergo in each branch is such that when the signals pairwise meet they are 180 degree out of phase. As shown in  FIG. 5  the signal pairs 180 degree out of phase are u i1 , u i2 , where 1≦i≦M. By imposing the additional constraint on the values of the delays that T 11 &lt;T 12 &lt;T 21  . . . &lt;T M1 &lt;T M2 , the modulated signal format S 1   MOD  generated in this embodiment are amplitude modulated or ASK and position modulated or PPM signals.  
         [0023]     In another embodiment, see  FIG. 6 , the reciprocity property of RF power dividers, attenuators, time delays and phase shifters will be used to exclude the need of power combiner in  FIG. 5 . As shown in  FIG. 6 , instead of power combiner in  FIG. 5 , each branch following the divider/combiner (block A in  FIG. 6 ) is terminated with an open circuit, where the incident signals will get reflected. The reflected signals from each branch therefore will pass through the reciprocal delays, phase shifters and attenuators in the reverse direction and finally through block A, which acts like a combiner in the reverse direction. An example of block A in  FIG. 6  is the Wilkinson&#39;s divider/combiner. The delayed signal reflected out at the point where the signal enters at block A will have same property as in S 1   MOD  of  FIG. 5  if the values of the delays, attenuation and phase shifting in  FIG. 6  are half that of  FIG. 5 . Therefore the embodiment described in  FIG. 6  will have same functionality as that of  FIG. 5  with the additional advantage of reduced size.  
         [0000]     Application to RFID Systems  
         [0024]     The method described above can be used to construct an RFID-tag and a reader to decode the encoded RF waveform emitted by the RFID-tag. The RFID-tag can be considered as an interconnection of the following building blocks.  
         [0025]     1. An antenna to receive mother signals from the reader or the mother signals is generated locally using an oscillator. The DC power to the oscillator can be derived using rectification of RF power from the reader or via a battery.  
         [0026]     2. Building blocks to generate the data encoded RF waveform is based on a given modulation format as described above.  
         [0027]     3. Optional building blocks such as amplifiers are used for increasing the power of the encoded waveform.  
         [0028]     4. An antenna is used to transmit the data encoded RF waveform.  
         [0029]     The reader is designed in such a way to decode the above encoded waveform.  
         [0030]     It will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departure from the scope thereof, which is defined by the appended claims.