A. Conventional RFID backscatter system
It is known that ISO/IEC 18000-6 protocol is the latest and most popular standard that accommodates the development of passive RFID technology in the UHF frequency band from 860 MHz to 960 MHz , the system architecture of which is demonstrated in Fig. 1.
Fig. 1. Conventional RFID system architecture.
View All | Next
According to the protocol, RFID tag would encode the backscatter data as either FM0 baseband or Miller subcarrier and communicate with interrogates using ASK and/or PSK backscatter modulation in which the tag switches the reflection coefficients of its antenna between two states in accordance with the data being sent . For the read range of more than 10 m, the available signal power in the backscatter link is usually weaker than −50 dbm. Such weak backscattered signals are vulnerable to impact of noises, jammers and interferences, which can lead to the deterioration of backscatter communication. In addition, it is required for the RFID reader to have very high sensitivity to detect such weak signals from RFID tag. However, conventional RFID system based on the ISO/IEC 18000-6 protocol mentioned above is difficult to meet such requirements.
B. Description and Performance Simulation of DSSS Enhanced RFID Backscatter System
Spread spectrum is rooted in the Shannon and Hartley channel-capacity theorem , the elegant interpretation of which says that one can increase communication performance by allowing more bandwidth, when Signal Noise Ratio (SNR) is very low. Owing to its significant contribution to wireless communication, DSSS technique is adopted in the return link of the RFID system.
According to the DSSS theory , DSSS communication can be achieved by attaching a specific DSSS key in the transmitting chain (spreading operation), and removing the key in the receiving chain (de-spreading operation). As the application in RFID backscatter system, spreading operation is carried out in tag baseband processor and de-spreading operation is implemented in the RFID reader. The architecture of the simulation model of DSSS enhanced RFID backscatter system is shown in Fig. 2. As a comparison, traditional system of which the backscattered data is FM0/Miller encoded is also presented. Simulations of the two systems' performances are carried out in Matlab/Simulink environment.
Interferences and noises of RFID system are simulated by AWGN Channel and Multipath Rician Fading Channel respectively in SIMULINK, which are described in detail as follows.
Fig. 3 shows the power spectrums of the backscatter signals during the DSSS operation. Figs. 3(A) and (B) are the backscatter data before and after spreading operation respectively. From the results, it is seen that the effect of spreading operation in RFID tag is to diffuse the information to a larger bandwidth and appear as noise. When the spread data transmits through channels, all kinds of narrowband or wideband noises will be added in, as shown in Fig. 3(C). However, after the de-spreading operation in the RFID reader receiver, all the added noises are diffused to a large bandwidth while the expected signal gets back to its original bandwidth, which can be seen in Fig. 3(D). The process shows that with DSSS technique, data from RFID tag to reader could be achieved easily even in environments of low SNR and high interference.
One of the common interferences in RFID backscatter channels comes from multiple-path propagation, in which the signal has more than one path from tag to reader. The reflected path (R) can interfere with the direct path (D) in a phenomenon called fading, and the ratio between power D and power R is named K-factor. According to the spread spectrum theory, the DSSS encoded data can be de-spread only when the receiver has the same DSSS key and carries out the de-spreading operation synchronously. So fading can be prevented in DSSS RFID system effectively because the de-spreading process synchronizes to signal D rather than signal R, so the signal in reflected path is rejected even though it contains the same DSSS key. Fig. 4 depicts the Bit Error Rate (BER) performance in different fading channels. The top curve is the miller modulation result, the middle curve is from FM0 encoding, and the bottom one is the result of DSSS operation (SNR = 5 dB). As expected, DSSS based system has much lower BER, which means better performance under the condition of harsh multiple-path interference.
Another attractive characteristic of DSSS is its strong resistance to intentional or un-intentional noise and jamming signals because they do not contain the specific DSSS key. Fig. 5 depicts the BER performances for AWGN channels with different SNRs (K-factor = 2). From the result, the DSSS based system is compared to be the optimum choice for surviving successfully in low SNR conditions. It is seen in the figure that even when the signal power is below the noise floor, DSSS system can also maintain good BER performance.
RFID backscatter system with DSSS technique also has strong resistance to interceptions. Presently there is no agreed security solution for low cost RFID system and a passive tag is easily to be trailed and monitored. ISO/IEC 18000 6C protocol proposes a simple way to enhance privacy: to directly “kill” the tag after purchasing. However, the drawback is that the clients could not use the tag anymore after killing. Fortunately, the problem can also be easily solved here because non-authorized listeners do not have the specific DSSS key used to de-spread the original signal, so the signal appears as noises or as interferers to them. Only the expected reader, who has the right key, is able to detect the right signal.