# A Novel Channel Coding Scheme for RFID Generation-2 Systems
## 1 Introduction  
In recent years, systems employing radio frequency identification (RFID) have been growing exponentially [1] as industry realized the potential of this technology when applied to complex manufacturing processes. RFID proved to be an effective and low-cost solution for many key aspects of the so-called \"smart factory\", such as asset tracking, supply chain monitoring, inventory control, and access management. As the technology becomes cheaper, it is becoming increasingly prevalent in many applications, and it is widely adopted in other contexts, from mobile payment solutions and passports to virtually any application using the Internet-of-Things (IoT) paradigm [2]. RFID technology consists of multiple tags and at least one reader which exchange information under a specific communication protocol. Depending on the sources of the operating power, tags are usually divided into three categories: passive, semipassive, and active [2; 3; 4]. Among them, low cost and long life make passive tags an attractive solution despite their limited link budget. In fact, passive tags have no internal power source: the electromagnetic energy transmitted by the reader powers the internal circuit of the tag, and the same energy is also used to reply to the reader. Under this perspective, simple algorithms in the field of RFID technology become fundamental to the development of both low-cost and energy-saving solutions. In particular, energy-efficient solutions could be implemented in RFID systems where tags are made by recyclable materials to be integrated in goods, clothes, and packing. This can be realized by using \"green electronics\" with recyclable, possibly organic, materials [5]. However, despite the advantages of RFID, there are still some significant issues to be solved, such as the resolution of collisions. The RFID air-interface protocol defines the physical layer, line coding, structure of messages, operating modes, and algorithms for preventing, detecting, and resolving the collisions during communications [6]. Considering a couple of passive tags deployed in the environment, the reader issues a query command and the desired tag replies, ideally leading toa successful identification. Actually, multiple tags are scattered in the range of the reader, increasing the chance that several of them will respond simultaneously, generating a collision. Hence, one of the current problems with RFID systems is related to the insufficient performance of the anti-collision algorithm, since the wireless RFID environment (e.g., a marketplace) is usually affected by fading noise [7; 8], and collisions can significantly degrade the channel efficiency. RFID anti-collision protocols can be classified into Aloha-based protocols and tree-based protocols. Aloha-based protocols reduce the probability of tag collisions by spreading out tag responses over time, while the latter family of protocols guarantees on the time required to read all tags in the range. Due to their simplicity, Aloha-based protocols were quickly adopted in early RFID systems [9]. A variation of the frame slotted Aloha (FSA) protocol--referred to as Q-algorithm--is used in the widespread UHF EPC Class 1 Generation-2 protocol (hereafter Gen-2 for brevity), which became a standard in 2006 as part of ISO/IEC 18000-6 as amendment 18000-6C [10]. However, the Q-algorithm still shows some drawbacks. For example, all the messages exchanged add a significant overhead on the channel, and its dynamic slot updates require time to converge to the best configuration. Several studies have been carried out in order to improve the performance of Aloha-based protocols in the presence of collisions [11; 12; 13; 14]. However, most of these works depend on the estimation of the tag population in the reader's range, leveraging on results obtained in the previous time slots. In addition, the authors usually consider ideal air interface conditions, while anti-collision performance highly depends on the physical level of channel noise [15].