Error Correction on an Insertion/Deletion Channel Applying Codes From RFID Standards

Abstract-This paper 1 investigates how to improve the performance of a passive RFID tag-to-reader communication channel with imperfect timing, by using codes mandated by international RFID standards. I. SHORTCUT This brief section is intended for those who want to skip the practical motivation and jump directly to the theoretical problem setting. Essentially, we have a binary channel which transmits information in terms of the length of runs of identical symbols. The valid runlengths are one or two, and if the receiver can determine exactly the time of each transition, she can also acquire the transmitted information sent. Due to a noisy process and with probability p, a given length-one run is detected as a length-two run, in which case a symbol has been inserted. Vice versa, with probability p, a given lengthtwo run is detected as a length-one run, in which case a symbol has been deleted. Thus, this is a special case of an insertion/deletion channel. The uncoded information is totally vulnerable to the noise of this channel. In order to protect the information, an error correction code is applied. In this paper, the error correcting code is actually a CRC-CCITT code, mandated by many international standard protocols (but intended for error detection). Now, if you also know about cyclic redundancy check (CRC) codes, you can go to Section VI if you want to skip the introduction. II. INTRODUCTION Inductive coupling is a technique by which energy from one circuit is transferred to another without wires. This is a fundamental technology for near-field passive radio frequency identification (RFID) applications as well as lightweight sensor applications. In the passive RFID application, a reader, containing or attached to a power source, controls and powers a communication session with a tag; a device without a separate power source. The purpose of the communication session may be, for examples, object identification, access control, or acquisition of sensor data. The operating range of a reader-tag pair is determined by communications requirements as well as by power transfer requirements. To meet the communications requirements, the reader-to-tag and the tag-to-reader communication channels satisfy specified demands on communication transfer rate and reliability. To meet the power transfer requirements, the received power at the tag must be sufficiently large as to provide operating power at the tag. In [1], a discretized Gaussian shift channel is proposed as a modified bit-shift channel to model synchronization loss. In this paper, we will apply the same model to the tag-toreader channel. In terms of coding, the practical difference is that the tag-to-reader channel allows more elaborate decoding schemes, especially since the volume of data transmitted and the transmission rates are modest. We will investigate the performance of Manchester coding, which is a standardized modulation technique for RFID applications. As a stand-alone code this code was studied i