WO2001035318A2

WO2001035318A2 - Bandwidth efficient pulse processor for rfid data communication

Info

Publication number: WO2001035318A2
Application number: PCT/US2000/041727
Authority: WO
Inventors: Ronald Saleski; Michael K. Freed; William F. Gallagher, Iii; Shinichiro Inui
Original assignee: Checkpoint Systems, Inc.
Priority date: 1999-11-09
Filing date: 2000-11-01
Publication date: 2001-05-17
Also published as: WO2001035318A3; AU2749401A; TW498289B

Abstract

An interrogator is provided for reducing the bandwidth required for transmitting a command or control signals to a radio frequency identification transponder. The interrogator includes a carrier signal generator (RF OSC.), a modulator (10) which modulates the carrier signal with a shaped pulse envelope, and a transmitter (28) which transmits the modulated carrier signal to the transponder. The modulated carrier signal results in a reduced transmission signal bandwidth. The shaped pulse envelope may be a sinusoidal envelope, wherein the modulated carrier signal is a raised cosine modulated signal.

Description

TITLE OF THE INVENTION

BANDWIDTH EFFICIENT PULSE PROCESSOR FOR RFID DATA

COMMUNICATION

BACKGROUND OF THE INVENTION

In radio frequency identification (RFED) systems, data is transmitted to, and stored on, RFID "tags". The transmitter or "interrogator" must send commands and data to a transponder IC resident on a RF tag by broadcasting at a particular carrier frequency, and at a band of frequencies near the carrier frequency. The band of frequencies that is transmitted is commonly referred to as the transmission bandwidth of the system.

Regulatory agencies control the use of the RF spectrum by imposing restrictions on amplitude and frequency of RF transmissions. Ultimately, designers of RFID systems must restrict the bandwidth and level of emissions from an RFID interrogator. The restrictions directly impact the performance of the system, specifically the data rate and transmission range.

The challenge in RFID command signaling is to establish an effective compromise between a limited transmission bandwidth, power requirements of the carrier energized receiver and optimized data throughput.

Accordingly, the present invention improves the data rate and transmission range of RFID systems by applying a signaling technique that uses pulse shaping to minimize harmonics of binary amplitude shift keying (ASK) to more efficiently use available bandwidth.

SUMMARY OF THE INVENTION

The present invention provides an interrogator for reducing the bandwidth required for transmitting a command or control signals to a radio frequency identification transponder. The interrogator comprises a carrier signal generator, a modulator which modulates the carrier signal with a shaped pulse envelope, and a transmitter which transmits the modulated carrier signal to the transponder. The modulated carrier signal results in a reduced transmission signal bandwidth. In one preferred embodiment of the present invention, the shaped pulse envelope is a sinusoidal envelope and the modulated carrier signal is a raised cosine modulated signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:

Fig. 1 A is a time vs. amplitude graph of a prior art pulse position modulation system without inter-symbol guarding;

Fig IB is a time vs. amplitude graph of a prior art pulse position modulation system with inter-symbol guarding;

Fig. 2A is a frequency vs. amplitude graph of a prior art carrier signal;

Fig. 2B is a frequency vs. amplitude graph of a carrier signal in accordance with the present invention;

Fig. 3 is a time vs. amplitude graph of the carrier signal in accordance with the present invention; Fig. 4 is a schematic diagram of an interrogator in accordance with a prefened embodiment of the invention;

Fig. 5 is a schematic diagram of a transmitter of the interrogator of Fig 4; Fig. 6 A is a power spectral density plot of a prior art carrier gap modulation scheme; and

Fig. 6B is a power spectral density plot of a raised cosine envelope modulation scheme in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Certain terminology is used herein for convenience only and is not to be taken as a limitation on the present invention. In the drawings, the same reference letters are employed for designating the same elements throughout the several figures.

The present invention provides a pulse processor circuit for use in pulse position modulation systems to minimize the harmonic content of transmissions by imposing a sinusoidal amplitude modulation response on an radio frequency (RF) carrier that is used to remotely power and signal an RFID transponder.

Low cost RFID tags are powered by electric or magnetic fields from an interrogator or "reader". As can be appreciated, interrogator transmissions are constrained by the power requirements of the RFID transponder circuit. That is, the transmission signal of the interrogator simultaneously powers the transponder circuit and provides data to the circuit.

In RFID systems which can send and store information on RFID tags, the interrogator must send commands and data to a transponder IC of the transponder circuit resident on an RF tag by broadcasting at a particular carrier frequency and at a band of frequencies near that carrier. One common prior art signaling method is "gapping," i.e., turning off the carrier for short periods with a time separated series of gaps used to communicate commands and data from the intenogator to the transponder circuit of the RF tag.

The performance of prior art systems utilizing the gapping method is limited in that the RFID transponder must be able to remain functional during the carrier transmission gap. During the gap, the transponder circuit must operate on internally stored reserves obtained from the RF carrier in the non-gapping interval. As the transmission gap widens, the reliability of transponder circuit operation degrades. Alternatively, the nanower the transmission gap, the higher the frequency content (sidebands) is away from the carrier.

In addition, a guard time between symbols is commonly used so that in the case of two adjacent gaps, the start of each respective gap can be resolved by the transponder circuit. This guard time limits the effective data rate of transmission. Fig. 1A shows a 2 bit symbol sequence { 0,3,0,1 } with PPM (pulse position modulation, i.e., the position of the pulse in one of four equal slots which make up each respective symbol encodes the symbol transmitted). In Fig. 1A, without a guard band, the '3' gap runs into the '0' gap. In Fig. IB, with a guard band, the gaps are distinct. Also, the overall time to transmit these four symbols is greater due to the necessity of the guard band.

The present invention provides intenogator and associated signal processing scheme that overcomes the conflict between power delivery and bandwidth requirements, with the added benefit of eliminating the inter-symbol guard time requirement.

By employing a sinusoidal envelope as a replacement for a signaling gap, the three requirements described above are met. Fig. 2A illustrates a carrier gap as is commonly employed by prior art systems. Figure 2B illustrates a sinusoidal envelope that has the same fundamental frequency as the wide gap, but allows the tag to obtain energy from the field as described herein.

The obtainable power from the RF carrier is directly related to the amplitude of the carrier and the duration energy is obtained from the carrier. For ease of calculation, assume a gap interval of 2π time units. The area under a non-modulated carrier for this time interval is 2*2π = 4π. The area under the gap is 0*2π=0, for no obtainable energy during signaling. The area under the raised cosine modulated carrier is expressed by the following equation: π/ 2 π

|(2 -cost)Jt = 2* j^" (2 -cost)<it + J(2 - cos t)Λ

T nil

- 2π

This equation gives an average over the pulse modulation of 50% of non-modulated carrier for the RFID IC to obtain energy from.

The bandwidth efficiency comes from two factors. Namely, (1) the wider equivalent gap allowed due to energy available during modulation gives a lower frequency fundamental and (2) the high frequency content in the squared edges of the gap envelope is not present in the sinusoidal envelope.

Furthermore, the scheme in the present invention eliminates the need for inter-symbol guard time. Thus, no guard band is needed or used in the present invention. Using the same example symbol sequence from Fig. 1 A, adjacent pulses on the inter- symbol boundary are clearly resolvable. As shown in Fig. 3, a performance boost due to increased symbol throughput per unit time is achieved.

Digital signals are binary, meaning they have distinct logic states of either 1 or 0. The spectral content of continuous binary signals consists of a fundamental frequency, and harmonics of that frequency. These harmonics can extend far out from the fundamental frequency, and can present problems in terms of unwanted emissions. By effectively removing or minimizing harmonic emissions, a binary amplitude shift keying (ASK) system can run at or near the maximum amplitude and bandwidth.

INTERROGATOR HARDWARE

The binary (On/Off) pulses are "smoothed" by a system that replaces sharp transitions in logic state with a synthesized waveform that approximates a sinusoidal response. The sinusoidal response is chosen because of the low (ideally, zero) harmonic frequency content. The present invention numerically adjusts the time domain response of the entire system, including compensation of non-linearities in the modulator, transmitter, and antenna. The end result is low-harmonic content in the frequency domain. Figs. 4 and 5 show one prefened embodiment of the present invention. Fig. 4 is a block diagram of the Pulse Shaper Circuit, and Fig. 5 is a block diagram of the Class E transmitter with appropriate modulation input.

Referring to Fig. 4, one prefened circuit 10 uses a fiber optic input 12 to receive a pulse width modulated (PWM) signal which gets decoded by a Programmable

Logic Device (PLD) 14. The PLD 14 controls the activity of the system. The PWM signal can be used to specify amplitude, frequency, and phase of the modulation sr-nal. The only limitation on such specifications is resources in PLD and ROM space. The invention could also suitably use an infra red (IR), wire, opto-isolated, RF, or other types of inputs. The input is detected with a device that preferably offers noise immunity, such as a Schmitt trigger 16.

The Programmable Logic Device (PLD) 14 accepts input from the pulse receiver. It implements a binary counter to sequence through addresses of a read only memory (ROM) 18 here a set of EEPROMS, when the circuit is "looking up" numbers from the table of the sampled sinusoid values in the ROM. That is, the EEPROMS hold a look-up table for the sinusoidal waveform. For the purposes of electromagnetic compatibility (EMC) and /or electromagnetic interference (EMI), the binary counter may be implemented with a modified gray scale that minimizes the number of bit transitions on successive clock cycles, ideally providing only one bit transition for every clock. This clock may a fixed frequency, or may be modulated with noise or discrete signals for the purpose of reducing peaks of spurious emissions by "spreading" the spectrum of undesired emissions.

The PLD 14 also provides control signals for indexing the ROM(s) 18, and a Digital to Analog Converter (DAC) function in DAC 20. The PLD 12 also selects between at least two different pulse widths for the smoothed output pulses, based on the

Pulse Width Modulated (PWM) input signal.

The EEPROMs 18 used in this embodiment are byte wide, and have a 14 bit address bus; hence they are 16K x 8. As the DAC 20 accepts 12 bit data, the pair of EEPROMs 18 is used to hold the upper 4 and lower 8 bits of the sinusoid value. Other forms of memory other than EEPROM may be used as well, including Dynamic RAM (DRAM), Static RAM (SRAM), EPROM, Flash memory, and Feno-electric type memories such as FRAM or FeRAM. The use of faster volatile memories, such as SRAM, may be employed to change the pulse shape parameters under program control, or to provide faster look-up times, and are within the scope of this invention. For this application, a cunent mode DAC is used to convert the 12 bit integer values from the memory to an analog cunent. The present invention may also use voltage mode output DACs that may be of different number of quantization levels. An op-amp circuit 22 performs a cunent to voltage conversion, and amplifies the signal. Another op-amp 24 is used to lowpass filter the signal, and to remove undesired harmonics present in the sampled sinusoid. A bipolar junction transistor (BJT)

26 of NPN type is used to control the cunent flow to the final stage amplifier. This is accomplished by controlling the base cunent, and in turn, the emitter current. This cunent amplifier is commonly refened to as a "pass transistor," as it will pass all the cunent for the transmitter amplifier. It is also used to gradually cut off current flow to the transmitter final. Although the prefened embodiment uses a BJT, a Field Effect Transistor such as a

Metal Oxide FET (MOSFET), Junction FET (JFET), Insulted Gate Bipolar Transistor (IGBT), or other gated device may also be suitable. The circuit may be run at variable pulse widths, for different data rates.

Fig. 5 shows a block diagram 28 of the RFID transmitter. The preferred embodiment uses a switching transmitter that is a modified E Class transmitter, as described in U.S. Patent No. 5,926,093 (Bowers et al.) entitled "Drive Circuit for Reactive Loads," the contents of which are incorporated by reference herein. In the preferred configuration, the modulation voltage V_mod , is shown to directly drive the MOSFET drains. By varying V_mod , and the current flow to the MOSFET switching amplifier, the present invention produces a numerically controlled smooth transition in the powering and de-powering of the output devices. It is this gradual, sinusoidal transition that allows the system to run at or near the maximum bandwidth allowed by individual regulatory agencies, and at or near the maximum emission level. This is made possible by eliminating harmonics of the digital signal by creating a sinusoidal response in the time domain.

Fig. 6A is a power spectral density plot of a prior art carrier gap modulation scheme, and Fig. 6B is a power spectral density plot of a raised cosine envelope modulation scheme in accordance with the present invention. Referring to Figs. 6A and

6B, the present invention provides significantly less harmonic content, and is thus a more bandwidth efficient scheme.

The scope of the present invention is not limited to the use of sinusoidal envelopes, but also includes other forms of shaped pulse envelopes which are used in place of a carrier gap envelope. A raised cosine modulated carrier is the preferred shaped signal.

A trapezoid shaped signal is one alternative embodiment.

It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention.

We claim:

Claims

1. An intenogator for reducing the bandwidth required for transmitting a command or control signals to a radio frequency identification transponder, the intenogator comprising: a carrier signal generator; a modulator which modulates the carrier signal with a shaped pulse envelope; and a transmitter which transmits the modulated carrier signal to the transponder, the modulated carrier signal resulting in a reduced transmission signal bandwidth.

2. The intenogator of claim 1 wherein the shaped pulse envelope is a sinusoidal envelope and the modulated carrier signal is a raised cosine modulated signal.

3. A method of reducing the bandwidth required for transmitting a command or control signal of an interrogator to a radio frequency identification transponder, comprising: the interrogator generating a carrier signal; modulating the carrier signal with a shaped pulse envelope; and transmitting the modulated carrier signal to the transponder wherein the modulated carrier signal results in a reduced transmission signal bandwidth.

4. The method of claim 3 wherein the shaped pulse envelope is a sinusoidal envelope and the modulated carrier signal is a raised cosine modulated signal.