TECHNICAL PAPER |
Excitation Frequency: The reader-tag system is based on a transfer of energy between L-C resonant antenna coils in the reader and the tag. Magnetic (inductive) tag-reader coupling is viable at any frequency from under 100 kHz up to approximately 50 MHz. Regulatory Restrictions: The frequency and power of RF emissions are subject to worldwide regulation. International regulations limit commercially usable (unlicensed) radiation to specific frequency, bandwidth and field strength limits. Reader Field Generation Pattern: The reader may emit a continuous or a pulsed field, usually at a fixed frequency. In "full-duplex" systems the reader emits a continuous RF field at a constant frequency and the tag produces a modulation signal while energized by the reader field. Continuous field emission allows tags to be activated at any time they enter the reading volume and to be decoded in the minimum possible time. In "half duplex" systems the reader emits a pulsed field to send energy to the tag, and the tag sends back its message in the "quiet" interval between reader field bursts. This produces a quantized time window for tag and reader to interact, and could theoretically slow the minimum reading time as a result of this quantization. For "read-write" or "query-response" systems the reader will emit either a pulsed or modulated field to send a data signal as well as activation energy to the tag, creating quantized data transaction windows similar to half-duplex systems. Tag Modulation method: The tag modulation method is the pattern with which the tag absorbs power from the reading system or otherwise produces a signal in order to transmit information back to the reader. Some of the modulation patterns presently in use are:
In certain HDX (half-duplex) tags [5] which transmit the ID signal by directly coupling an RF signal to the antenna coil from an active circuit that has been previously charged up by the reader field, the FSK or PSK signals are not superimposed patterns on an ASK signal. Each type of modulation has advantages and disadvantages in terms of signal transmission rate, noise immunity and system complexity. Bit Period: All "full duplex" systems currently in use derive the tag timing from the frequency of the excitation field of the reader. By counting cycles of the excitation field, the modulation periods are obtained, as well as the time length for a transmitted "bit" of information. The fewer cycles per bit (i.e. shorter time length), the faster the message transmission will be. The more cycles per bit, the more reliable the message transmission will be. Data Structure: The Data Structure is the system of organization to transmit a coherent and reliable and information sequence between a tag and a reader. RFID tags generally transmit a message consisting of "PREAMBLE" bits to indicate the beginning of the message, "DATA" bits to transmit the ID information, and "CHECKSUM" bits to insure the reliability of the transmitted data. Similar data structures are utilized by the reader to transmit information to the tag. Data Transaction Length: The total length (in bits and time) of the information in a particular type of data transaction. The transaction length multiplied by the TIME PER BIT equals the transaction time. Error Checking: The CHECKSUM is calculated from the other data in the transaction. When the reader receives a tag code, it re-calculates the checksum and compares it with the data sequence. If the data transmission is correct, the calculated checksum will equal the received checksum. Tag Design Considerations Coil Size: For a given tag size, the coil size should be maximized within the tag volume to maximize the tag's ability to receive and modulate energy by means of its coded information. Coil Resonance: Increased coil resonance also leads to higher energy transfer. A combination of a coil and a capacitor will generally form a more highly resonant circuit than a coil alone. Operating Power Level: The power consumption level at which the IC in the tag begins reliable functioning is a prime determinant of the quality of tag performance. A tag IC that operates at a reduced power level will communicate within a weaker reader energizing field, yielding a greater potential reading distance. Modulation Strength: The intensity with which the tag varies the loading of its antenna coil while maintaining reliable operation determines its "signal strength" to the reader. Higher signal strength contributes to greater reading distance [3, 4]. Reader Design Considerations Activation Field Geometry: The first function of the reader system is to activate the tags in its reading volume. Optimally, the reader should produce an energizing magnetic field appropriate to the geometry of the reading volume and the most probable orientation of tags passing through the volume. For large reading volumes, designing a field generation system with sufficient strength, size and consistency is a primary issue in RFID research and development. Power output: Power output of a reader's magnetic field generator may vary by orders of magnitude from the smallest hand held systems to large fixed-point installations. The requirements for constructing large and powerful magnetic field demand very efficient low-distortion electronics and resonant electro-magnetic networks. Shielding: The power output of field generators sufficient to meet reading requirements for the largest systems may exceed regulatory agency specifications for RF emissions. In this case, electromagnetic shielding is necessary to reduce RF emissions outside the reading volume to acceptable levels. Tag Signal Sensing: The tag signal is sensed at the reader either by sensing variations in the reader field coil signal caused by inductive coupling to the tag signal or by sensing the tag signal with a separate receiving coil(s) in the reader. Analog Signal Processing: The analog signal processing section of the reader performs detection of a very weak perturbation signal from the tag in the presence of a strong energizing field signal. It transforms the signal by filtering and amplification to a level appropriate to digitization and further processing in the digital domain. Digital Signal Processing: The amplified and filtered signal from the tag modulation of the reader field is digitized. Various DSP methods utilizing one-bit (comparator) and multi-bit (ADC) digitization may be more appropriate than analog processing for increasing tag signal-to-noise ratio and other signal attributes which make the tag more readable. Decoding and Event Transmission: The digitized signal is analyzed to detect modulation patterns indicating a valid tag signal. Ideally this processing should occur simultaneously with the tag passing through the reader field. The decoded ID tag data transactions are stored, displayed, and/or transmitted to a central location for utilization. [Continued...] |
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