# Backscatter Communications: Inception of the Battery-Free Era--A Comprehensive Survey
## 7 BackCom: Range Extension and Reliability
### Coverage and Range Improvement
#### 7.1.3 Multi-Antenna and MIMO Systems  
The construction of antennas with any shape of radiation pattern is possible [13]. Multiple antennas can be placed in the reader or tag to enhance the communication range and throughput of backscatter devices. MIMO technology embeds various antennas at the source tag or the destination tag. MIMO systems in wireless communications can also improve the throughput and coverage area. [PERSON] et al. suggested a Micro-Mo design that is considered the first design of multi-antenna BackCom tags [32]. Micro-Mo utilize ambient signals to communicate between tags.  
[PERSON] et al. proposed a system that can convert bistatic modulated backscatter to an ambient bistatic modulated re-scatter model in [171]. This system also enables the transmission of data from many antenna transmitters to multiple antenna receivers. [PERSON] et al. studied the physical layer of a BackCom system and designed a protocol that focuses on the use of various antennas at the tag and the receiver [172]. BackFi also utilizes multiple antennas to improve the range and throughput of WiFi backscattered signals [173]. Figure 36 illustrates the multi-antenna reader design to collect backscattered signals of a single antenna tag, coming from different paths [91]. The letters \(f,g,h\) represent different channels, and \(M\) shows the number of antennas in the reader. [PERSON] et al. presented a dyadic backscatter channel (DBC) that is especially designed for BackCom [74].  
A signal detection algorithm was proposed for a multi-antenna Amb-BackCom model based on information of the channel state [91]. This algorithm shows that the increased number of receiving antennas reduces the bit error rate. However, if the number of receiving antenna rises above a certain level, an error floor can be observed. The work in [74] investigated RFID MIMO systems for BackCom to improve throughput. Likewise, a unitary query scheme for \(M\times L\times N\) MIMO backscatter RFID channels was proposed for the data rate [174]. This scheme consists of three components: (i) a query component, (ii) a tag component, and (iii) a receiver component. The query component includes \(M\) number of antennas in the reader for the transmission of data, the tag component has \(L\) antennas,  
Figure 36: Multi-antenna in BackCom.  
and the receiver component is embedded with \(N\) receiving antennas for the reader. The unitary query scheme enables time diversity in coherent time of channel, while displaying performance improvements of 5-10 dB in mid-range SNR regimes.  
[PERSON] et al. considered a MIMO wireless broadcast system for three nodes [175]. This system demonstrates all transmitters and receivers with multiple antennas. One receiver can harvest energy, while other receivers decode information received from the same transmitter. This work is based on two practical designs, namely (a) time stitching and (b) power splitting for the co-located receiver. Afterwards, a cooperative receiver was suggested that could recover signals from two directions, i.e., from Amb-BackCom devices and from RF sources [89].