# MIMO Radio Frequency Identification: A Brief Survey
## 3 Anti-Collision Techniques
Furthermore, a two-step extraction method is proposed that considers non-sparse RFID signals. The base vector of the source signal was estimated using the sample of the unit interval. Later, a corresponding signal with smallest interference is extracted in order to avoid a mixed matrix. The throughput analysis suggests that the proposed algorithm outperforms traditional blind source separation (BSS) and ALOHA [49] algorithms. With three antennas, a total of six tags could be perfectly separated. The scheme would work well for scenarios where collision identification is required and where the recovery of the signals is non-fully sparse. While existing anti-collision algorithms [50] solely focus on reducing collision probability, they undergo an inherent problem of vast idle slots. [PERSON] et al. [42] propose a collision-tolerant dynamic framed slotted Aloha (CE-DFSA) based technique that is able to identify multiple tags in a slot to decrease the identification time during the identification process. Orthogonal Walsh Sequence (WS) is employed to identify multiple tags that eliminates the need for a spread spectrum. When the RFID reader generates a query, multiple tags would respond having orthogonal WS. Throughput of the system, while considering disparity between the duration of the slots, is not a good estimate of performance evaluation of the anti-collision with regards to the identification time. The authors have thus evaluated their proposed algorithm in terms of time efficiency given as:  
\[\mathrm{n_{time\_effi}}=\frac{\mathrm{n\cdot T_{ID}}}{\mathrm{I\cdot T_{I}+S \cdot T_{S}+C\cdot T_{c}}} \tag{3}\]  
where \(\mathrm{T_{ID}}\) is the time for transmitting tag ID, S is the successful slots, I represents idle slots, and C is the collision slots whereas \(\mathrm{T_{S}}\), \(\mathrm{T_{I}}\), and \(\mathrm{T_{C}}\) represent the time durations. It is shown via simulations that the proposed technique is able to improve and accelerate the tag identification procedure more efficiently compared to traditional anti-collision algorithms. The proposed scheme is compared against various anti-collision algorithms and the gains have been quantified. Multi-antenna RFID schemes are effective in solving the collision problem; however, when employing a quadrature receiver [51], the often-overlooked aspect is that the collided signals can be statistically represented as rectilinear signal with a degree of impropriety. For such improper complex signals, the performance can be improved by processing together the received signal and its complex conjugate using the technique known as widely linear (WL) processing. [PERSON] et al. [43] make a case for a widely linear minimum-mean-square-error (WLMMSE) technique for anti-collision in multi-tag RFID systems. It considers the improper statistics of second order for backscatter tag signals. The proposed technique is effective in separating overlapped tag signals in a single slot and can be extended to multi-antenna RFID systems for throughput enhancements. The simulation and experimental results suggest that the proposed scheme is superior to the traditional signal recovery techniques based on linear models such as linear MMSE (LMMSE). One of the most prolific standards for RFID systems is the EPCglobal Class-1 Gen-2 and finds its uses in several applications [52]. FM0 encoding scheme used in the EPCglobal Class-1 Gen-2 standard is particularly of interest specially in the domain of logistics. The operating frequency is around 900 MHz, and the range is close to 10 m. [PERSON] et al. [44] derive and evaluate the performance of ML decoding for a UHF RFID system with several receive antennas. The channel is simulated realistically as a doubly selective Rayleigh fading model that makes the decoding process more complex. The performance metrics used are symbol error probability (SEP) and the derived results are compared against experimental simulations. The pairwise error probability (PEP) closes in when the number of antennas increase and gives good results even with two antennas.