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  • Abstract

A Robust Radio Frequency Identification System Enhanced With Spread Spectrum Technique

A robust passive UHF RFID backscatter system enhanced with spread spectrum technique is presented in this paper. Due to the weak signal energy of the backscatter chain, traditional RFID backscatter communication is easily affected by the noises, interferences, and interceptions from the environment. To solve the problem, spread spectrum technique is introduced into the backscatter link of RFID system. Simulated results show that this approach largely reduces the Bit Error Rate and improves the system's reliability and security. For hardware realization, spectrum spreading operation is implemented in a RFID tag baseband processor, which is finally applied into a complete RFID tag and fabricated using 0.18 um 1P6M CMOS technology. Furthermore, de-spreading operation including the PN acquisition is integrated into a FPGA implementation of RFID reader. Thus, a complete robust RFID backscatter system enhanced with spread spectrum technique is constructed.

SECTION I

INTRODUCTION

Radio Frequency Identification (RFID) systems are gaining increasing popularity in numbers of applications involving the tracking of items [1]. RFID system is typically constituted of the forward link and the backscatter link entailing RFID reader and RFID tag. In the forward link, the reader performs as the interrogation, transmitting a Radio Frequency (RF) wave to the tag. In the backscatter link communication, passive tag generates the reverse RF wave just by reflecting back a portion of RF wave in a process known as backscatter [2].

Passive tag does not include any energy storage device and is powered solely by the RF signal that it receives. The operating energy that the tag recovers from the RF signal is usually low and it enervates with the increase of read range; signal energy in the backscatter link from tag to reader is even weaker. Ideally, RFID reader should receive exactly what the tag transmits. But in reality, the signal received by reader is easily distorted for various reasons such as noises, interferences and interceptions. It is hence important to enhance the anti-interference capacity of the RFID backscatter communication system [3]. On the other hand, the mass usage of RFID has raised concerns regarding security and privacy which also need more consideration [4].

To solve the problems mentioned above, Direct Sequence Spread Spectrum (DSSS) technique is introduced into the backscatter link of RFID system. Simulations are carried out in Matlab/Simulink environment, results of which show that compared with traditional RFID backscatter system, this novel art largely reduces the Bit Error Rate (BER) under the same communication conditions. A digital baseband processor with the spread spectrum encoding circuit is presented, which is applied into the ASIC implementation of a complete RFID tag. Besides, a FPGA based RFID reader integrated with de-spreading circuit is implemented. Thus, a whole RFID backscatter communication system enhanced with DSSS technique is constructed.

This paper is organized as follows. Section II presents the architecture of the proposed spread spectrum enhanced RFID backscatter system and the simulation results of its BER performance under harsh environments. Section III more specifically describes the hardware design of the novel system, including the spectrum spreading operation in RFID tag and de-spreading operation in RFID reader. Conclusion and further development are eventually drawn in Section IV.

SECTION II

PERFORMANCE SIMULATION OF DSSS ENHANCED RFID BACKSCATTER SYSTEM

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 [2], the system architecture of which is demonstrated in Fig. 1.

Figure 1
Fig. 1. Conventional RFID system architecture.

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 [2]. 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 [5], 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 [6], 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.

Figure 2
Fig. 2. DSSS enhancd RFID backscatter system simulation model.

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.

Figure 3
Fig. 3. Power spectrum of the backscatter signal.

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.

Figure 4
Fig. 4. BER performances for rician fading channels.

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.

Figure 5
Fig. 5. BER performances for AWGN channels with different SNRs.

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.

SECTION III

HARDWARE IMPLEMENTATION OF DSSS ENHANCED RFID BACKSCATTER SYSTEM

A. ASIC Design of DSSS Encoding Circuit in RFID Tag

Spectrum spreading operation is implemented in the RFID tag baseband processor design, by replacing FM0/Miller encoding circuit with the DSSS encoding one. DSSS encoding is realized by attaching a DSSS key into the backscattering baseband data. In practical communications, the DSSS key is a digital sequence that is as long and as random as possible to appear as “noise like”. But in any case, they must remain reproducible. Otherwise, the corresponding RFID reader will be unable to extract the message that has been sent. Thus, the sequence is “nearly random”. Such sequence is called a pseudo-random sequence (PN sequence). M-sequence is a perfect PN sequence for its good performances of auto-correlation, cross-correlation, orthogonality, and bits balancing [5].

So in RFID tag baseband processor design, a DSSS key of 6-bit M-Sequence pseudo-random number based on the feedback shift register is injected in the backscattered data, which is shown in Fig. 6. That injection is just called the “DSSS encoding”. The polynomial of the M-Sequence here is f(x)=x6 + x + 1 the initial value is [1 0 0 0 0 0], and the M-Sequence is 10000011111101010110011011101101001001110 001011 1100101000110000.

Figure 6
Fig. 6. RFID tag baseband processor with spectrum spreading circuit.

The digital core designed above is integrated into a complete RFID tag. Besides, the tag contains modules of modulation, demodulation, oscillator, power rectifier, and reset circuit [7]. The complete RFID tag was then fabricated as a full digital ASIC using 0.18 um 1P6M standard CMOS technology.

B. Implementation of De-Spreading Operation in the RFID Reader receiver

For the recovery of the original backscatter data, de-spreading operation is integrated into the digital baseband of RFID reader receiver, the structure of which is shown in Fig. 7.

Figure 7
Fig. 7. RFID reader baseband integrated with de-spreading operation.

Before de-spreading operated, 40 MHz ADC output is followed by a programmable cascaded integrator-comb (CIC) decimation filter, which has the reconfigurable decimating rate to produce different sampling frequencies for backscatter data rate ranging from 40 kbps to 640 kbps. In this way, the following channel filtering could have variable bandwidths whiling using the same set of filter coefficients [8]. The digital channel filtering is a combination of a 36 taps finite impulse response (FIR) filter and a second order infinite impulse response (IIR) filter. The FIR filter is an equiripple low-pass filter and the IIR filter has very low cutoff frequency to estimate the DC, which is then subtracted from the FIR filter output [9].

Decimation filtering and channel filtering reduce the sampling frequency to 32 times of the backscatter data rate and remove both noise-shaped and out-of-band interferences. Signals here are ready for de-spreading operation, which is based on PN acquisition. PN acquisition is accomplished by the matched filter with its prominent advantage of acquiring the PN code quickly [10]. During the processing, the filter correlates the received signal with the known DSSS key (PN sequence) continuously, and compares the correlation with the threshold at the sampling rate. Consequently, the acquisition can be accomplished with one PN sequence cycle. However, according to the protocol, the received data from RFID tag is 40 k–640 kbps continuously variable with maximum frequency deviation of ±22%. Such a large frequency deviation adds a lot of difficulties on the PN acquisition, compared with other common spread spectrum receivers. To solve the problem, parallel correlation operation is introduced into the de-spreading process to estimate the most approximate frequency [11]. For realization, 5 matched filters are involved in the PN acquisition, and all of the filters set the coefficients as the PN sequence symbols but different in sampling rate per symbol, which covers the whole frequency deviation ranging from −22% to +22%. Five matched filters here will produce five correlation values synchronously and the maximum one determines the frequency of the received data. As a bit symbol contains a complete PN sequence cycle, so the parallel correlator can also do the job of both time&frame synchronization and bit decoding without the need of the dedicated circuits. Besides, received signal strength information (RSSI) is derived from amplitude information and provides the real time threshold value for both the correlators and the bit-decision. Finally, the whole de-spreading operation integrated RFID reader transceiver is implemented on a single Xilinx Spartan3 xc3s400 FPGA.

Thus, a complete DSSS enhanced RFID backscatter communication system is constructed with the combination of the ASIC RFID tag and the FPGA based RFID reader designed above, which is shown in Fig. 8.

Figure 8
Fig. 8. DSSS enhanced RFID reader and RFID tag.
SECTION IV

CONCLUSION

To ensure the reliable and secure RFID communication, spread spectrum technique is applied into the backscatter chain of RFID system. Compared with the conventional RFID system which is FM0/miller encoded in the backscatter link, DSSS enhanced RFID system has much better BER performances under noises, interferences, and interceptions. A RFID tag baseband processor with the spread spectrum encoding circuit is designed, and it is integrated into a complete RFID tag ASIC design. In addition, to construct a complete DSSS RFID backscatter system hardware, the spread spectrum receiver is implemented in a FPGA prototype of RFID reader. For reducing the sampling rare and removing the out-of-band noises, decimation filtering and channel filtering are executed before the de-spreading operation. To acquire the PN sequence quickly and deal with the frequency deviation which exists in the system, parallel matched filters/correlators are proposed.

RFID system must be operated within applicable local regulation. The application of DSSS technique in RFID system will not violate the current local regulation because the signal strength in backscattered link is getting weaker after spectrum spread operation, which seems like noise and would produce little interference to the adjacent channel.

Future works include the testing and performance analyzing of the current DSSS enhanced RFID backscatter communication system hardware.

Acknowledgment

This research is supported by National High Technology Research and Development Program of China (863 Program, No. 2006AA04A109).

Footnotes

Z. Qiuling, Z. Chun, L. Zhongqi, W. Jingchao, L. Fule, W. Zhihua are with the Institute of Microelectronics, Tsinghua University Beijing, 100084, P.R. ChinaEmail: zhuql07@mails.tsinghua.edu.cn.

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Authors

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Zhu Qiuling

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Zhang Chun

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Liu Zhongqi

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Wang Jingchao

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Li Fule

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Wang Zhihua

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