A space-time coding approach for RFID MIMO systems

This paper discusses the space-time coding (STC) problem for RFID MIMO systems. First, a mathematical model for this kind of system is developed from the viewpoint of signal processing, which makes it easy to design the STC schemes. Then two STC schemes, namely Scheme I and Scheme II, are proposed. Simulation results illustrate that the proposed approaches can greatly improve the symbol-error rate (SER) or bit-error rate (BER) performance of RFID systems, compared to the non space-time encoded RFID system. The SER/BER performance for Scheme I and Scheme II is thoroughly compared. It is found that Scheme II with the innate real-symbol constellation yields better SER/BER performance than Scheme I. Some design guidelines for RFID-MIMO systems are pointed out. Introduction Radio frequency identification (RFID) is a contactless, usually short distance, wireless data transmission and reception technique for identification of objects. It is believed that RFID can substitute, in the not-far future, the widely used optical barcode technology due to the limitations of the latter in i) the barcode cannot read nonline-of-sight (NLOS) tag; ii) each barcode needs personal care to be read; and iii) limited information-carrying ability of the barcode. Currently, a single antenna is usually used at the reader and tag of RFID in the market. However, RFID research community recently started to pay attention on using multiple antennas at either the reader side or the tag side [1,2]. The reason is that using multiple antennas is an efficient approach to increasing the coverage of RFID, solving the NLOS problem, improving the reliability of data communications between the reader and tag, and thus further extending the information-carrying ability of RFID. Besides, some advanced technology in multiple transmit and receive antennas (MIMO) can be used to solve the problem of detecting multiple objects simultaneously, see e.g., [3]. There have been several studies about RFID-MIMO. In general, these studies are somehow scattered in different topics. It is difficult to find the logical relationship among these studies. Therefore, the state of the art of the *Correspondence: [email protected] Institute of Digital Signal Processing, University of Duisburg-Essen, 47057 Duisburg, Germany studies will be reviewed in a large degree in a chronological order. The work [4] first showed the idea of using multiple antennas at the reader for both transmission and reception. In [1], the authors first proposed to use multiple antennas at the tag and showed the performance gain by equipping multiple antennas at the reader (for both transmission and reception) and the tag. In [5], the multipath fading for both single-antenna based RFID channel and RFID-MIMO channel was measured and compared. The improvement on the fading depth by using MIMO can be clearly seen from the measured power distribution (see, e.g., Figure Ten therein). In [6], the authors first proposed to apply the Alamouti space-time coding (STC) technique, which is now popularly used in wireless communication systems, to the RFID systems. The reference [6] presented a closed-form expression for the bit-error rate (BER) of the RFID system with the nonecoherent frequency shift keying modulation and multiple transmit antennas at the tag and single transmit/receive antenna at the reader, where the double Rayleigh fading is assumed at the forward and backward links. In [7], the interrogation range of ultrahigh-frequency-band (UHFband) RFID with multiple transmit/receive antennas at the reader and single antenna at the tag was analyzed, where the forward and backward channels are assumed to take the Nakagami-m distribution. In [3], the blind source separation technique in antenna array was used to solve the multiple tag identification problem, where the reader is equipped with multiple antennas. The work © 2012 Zheng and Kaiser; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Zheng and Kaiser EURASIP Journal on Embedded Systems 2012, 2012:9 Page 2 of 10 http://jes.eurasipjournals.com/content/2012/1/9 [8] applied the maximal ratio combining technique to the RFID receiver, where the channel of the whole chain, including forward link, backscattering coefficient, and backward link, was estimated and used as the weighting coefficient for the combining branches. In [9], a prototype for the RFID-MIMO in the UHF-band was reported. In [10], both MIMO-based zero-forcing and minimummean-square-error receivers were used to deal with the multiple-tag identification problem, where the channel of the whole chain was estimated, similar to the approach in [8]. It is reported in [11] that four antennas are fabricated in a given fixed surface at the reader. The measurement results showed that an increase of 83% in area gave a 300% increase in available power to turn on a given tag load and the operational distance of the powered device is increased to 100 cm by the four-antenna setup from roughly 40 cm for the single-antenna setup. The result in [11] suggests that the MIMO technique can be very promising to the RFID technology. In the aforementioned reports, the Alamouti STC technique has been shown to be able to extend to RFID-MIMO systems. However, it can only apply to the case where the tag has two antennas. Since implementing four antennas at the tag have been shown to be possible in experiments, it is necessary to investigate the possibility of applying other STC techniques to RFID-MIMO systems. In this paper, we will study how to apply the real orthogonal design (ROD) technique, proposed by Tarokh et al. in [12], to RFID-MIMO systems. This technique is suitable for the case where the tag is equipped up to eight antennas, which should be sufficient for the RFID technology in the near future. The paper is organized as follows. A modified MIMORFID channel model will be developed in Section “Channel Modeling of RFID MIMO Wireless Systems”. The ROD in [12] and the companion of the ROD (CROD) proposed in [13] are briefly introduced in Section “A Space-Time Coding Scheme for RFIDMIMO Systems”. Two space-time decoding approaches for RFID MIMO systems will be discussed in Section “Two Space-Time Decoding Approaches for RFID MIMO Systems”. Section “Simulation Results” presents the simulation results and discussions, and Section “Conclusions” concludes the paper. Channel Modeling of RFID MIMOWireless Systems In this paper our discussion is confined only on narrowband RFID systems. The block diagram of the RFID MIMO system is illustrated in Figure 1, where both the reader and tag are equipped with multiple antennas. In terms of equation (1) of [1], the narrowband RFID MIMO wireless channel can be expressed as y(t) = HbS(t)Hfx(t)+ n(t), (1) Figure 1 A block diagram of the RFIDMIMO system. where the reader and tag are equipped with Nrd and Ntag antennas, respectively, x (an Nrd × 1 vector) is the transmitted signal at the reader, y (an Nrd × 1 vector) is the received signal at the reader, n is the receiver noise, Hf (an Ntag × Nrd matrix) is the channel matrix from the reader to the tag, Hb (anNrd ×Ntag matrix) is the channel matrix from the tag to the reader, and S is the backscattering matrix, which is also called signaling matrix. It is assumed that the Nrd antennas at the reader are used for both reception and transmission. This assumption is just for brevity of the notation. It is straightforward to extend the approach presented in this paper to the case where the reader has different numbers of antennas for reception and transmission. The channels Hf and Hb are assumed to be complex Gaussian distributed,Hf and Hb are mutually independent, and all the entries of eitherHf orHb are independent of each other. It is also assumed that Re(Hf), Im(Hf), Re(Hb), Im(Hb) are mutually independent and of the same distribution. In most general case where the modulated backscatter signals at the tag are transferred between the antennas, the signaling matrix S is a full matrix [1]. However no application of the full signalling matrix has been identified up to now [1]. Therefore, we will consider the situation where the RF tag antennas modulate backscatter with different signals and no signals are transferred between the antennas. In this case, the signaling matrix is a diagonal matrix [1] S(t)=diag { 1(t), 2(t) . . . , Ntag(t)} with | i(t)|≤1, where i(t) is the backscattering coefficient of ith antenna at the tag. The ith tag identity (ID) is contained in the coefficient i(t). Note that in the RFID system, the transmitted signal x is mainly used to carry the transmit power, while the information data (i.e., tag ID) is carried out by S. Therefore, the Zheng and Kaiser EURASIP Journal on Embedded Systems 2012, 2012:9 Page 3 of 10 http://jes.eurasipjournals.com/content/2012/1/9 central issue for the RFID is to decode 1, . . ., Ntag from the received signal. Next we transform equation (1) to the conventional form in signal processing. Let us define γ (t) = ⎡ ⎢⎢⎣ 1(t) 2(t) .. Ntag (t) ⎤ ⎥⎥⎦ , Hf = ⎡ ⎢⎢⎢⎣ H1 H2 .. HNtag ⎤ ⎥⎥⎥⎦ . (2) Then equation (1) can be rewritten as y(t) = Hbdiag { 1(t), 2(t) . . . , Ntag(t)}Hfx(t)+ n(t) = Hbdiag {1, 0, . . . , 0}Hfx(t) 1(t) +Hbdiag {0, 1, . . . , 0}Hfx(t) 2(t)+ · · · +Hbdiag {0, 0, . . . , 1}Hfx(t) Ntag (t)+ n(t) = Hb ⎡ ⎢⎢⎣ H1 0 .. 0 ⎤ ⎥⎥⎦ x(t) 1(t)+H ⎡ ⎢⎢⎣ 0 H2 .. 0 ⎤ ⎥⎥⎦ x(t) 2(t)+ · · · +Hb ⎡ ⎢⎢⎣ 0 0 .. HNtag ⎤ ⎥⎥⎦ x(t) Ntag (t)+ n(t) = Hb ⎡ ⎢⎢⎢⎣ H1x(t) 0 · · · 0 0 H2x(t) · · · 0 .. .. . . . .. 0 0 · · · HNtagx(t) ⎤ ⎥⎥⎥⎦ × ⎡ ⎢⎢⎣ 1(t) 2(t) .. Ntag (t) ⎤ ⎥⎥⎦+ n(t) = HbH̆(t)γ (t)+ n(t), (3)