# MIMO Radio Frequency Identification: A Brief Survey
## 3 Anti-Collision Techniques  
An active area of research in MIMO RFID systems is improving the bulk reading capabilities of the reader through improvements in collision recovery [39] from signals of multiple tags. The anti-collision techniques that are reviewed in this work are summarized in Table 2. Various collision recovery algorithms have been used to overcome the collision problem in MIMO RFID systems. It was observed that most of the works used passive tags having EPCglobal Class 1 Gen 2 standard, whereas double Rayleigh fading channel was used in simulations. [PERSON] et al. [40] suggest that considering backscatter link frequency tolerance, which is ignored in traditional collision recovery methods, can be used for enhancing collision recovery. In traditional collision recovery algorithms, backscatter link frequency (BLF) is assumed to be identical for the collided tags. However, since BLF may vary based on manufacturing processes, the tolerance must be considered but it requires perfect channel estimation. Figure 5 details the proposed multi-antenna reader with collision recover. The power spectrum for pilot tones is computed and converted to energy spectrum followed by the proposed metric given in Equation (2). By further simulating BLF tolerance artificially, additional improvements can be achieved. The technique is to successfully decode a single tag in a single slot from a group of multiple colliding tags. The power spectrum for each antenna is calculated and then divided by the BLF to be converted into energy spectrum. The peaks can then be used to compute a new metric called Normalized Maximum Energy (NME) for each antenna and given as:  
\[\text{NME}_{n_{R}}=\frac{\underset{f}{max}\{E_{s,n_{R}}\left(f\right)\}}{ \sum_{f}\{E_{s,n_{R}}\left(f\right)\}} \tag{2}\]  
where \(E_{s,n_{R}}\left(f\right)\) is the energy received on a single antenna \(n_{R}\) having a BLF \(f\). The antenna with a higher NME will have a higher SIR and then a single antenna matched filter receiver is utilized for the selected antenna. The proposed technique can improve the bulk reading time by up to 8% over traditional ML receivers, whereas an improvement of 25% can be achieved through the means of artificial rate tolerance simulation. The proposed tech \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Paper** & **Algorithm** & **Standard** & **Channel** & **Rx Antennas** \\ \hline [PERSON] et al. [40] & NME + MF & EPCglobal C1 Gen-2 & Double Rayleigh & 2, 4 \\ [PERSON] et al. [41] & Two Step Method & - & Noise & 3 \\ [PERSON] et al. [42] & CE-DFSA & EPCglobal C1 Gen-2 & Block Fading & - \\ [PERSON] et al. [43] & WLMMSE & ISO 18000-6C & Quasi-static Rayleigh & 4 \\ [PERSON] et al. [44] & ML & EPCglobal C1 Gen-2 & Double Rayleigh & 2, 4, 8 \\ [PERSON] et al. [45] & DBSCAN & EPCglobal C1 Gen-2 & Uncorrelated Rayleigh & 4 \\ [PERSON] et al. [46] & GBSA & EPCglobal C1 Gen-2 & Noise-less & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of various anti-collision techniques. Figure 5: Multi-antenna RFID Reader with collision recovery. Reproduced from [40] with permission from IEEE. nique can significantly enhance tag reading in multi-antenna systems without additional complexity overhead. Collision occurs when the tag signals from multiple tags are received at the same time [47]. In case the number of readers is lower than the number of tags, a multi-label communication model can enhance the interrogation performance through the use of non-sparse schemes [48]. The authors of [41] consider the case of multiple tags, which requires the establishment of a MIMO RFID model. It is assumed that the reader has M antennas while there are N tags that can communicate with the reader. A new method for identifying multiple tags, using blind separation of the source signals based on label collision followed by recovery, is proposed. For bulk reading of tags, an anti-collision algorithm based on Searching-and-Averaging Method in Time Domain (SAMTD) is developed.