# Novel Sparse-Coded Ambient Backscatter Communication for Massive IoT Connectivity
## 1 Introduction
### Related Works  
Despite of its benefits, AmBC has several challenges that originate from inherent characteristics of RF signals and channels. Firstly, the ambient signals are not dedicated to backscatter communications. Hence, some of the backscatter schemes based on unmodulated excitation signals, such as energy beamforming [10; 11], space-time coding [12; 13], and frequency up/down conversion in radio-frequency identification (RFID) [3; 4], may not be available in AmBC. Without dedicated RF infrastructure (i.e., 915 MHz RFID reader and power beacon), previous works such as LoRa (Long Range) [14], rateless code [15], statistical inference [16], time-hopping spreading spectrum [17] are not directly applied to AmBC scenarios. Since low-rate symbols are modulated on high-rate ambient signals, modulations and detections should be redesigned for applications of AmBC. Second, channel propagation of AmBC is distinguished from that of active radios [12; 13; 18; 19; 20; 21]. Compared to one-way channel between transmitter and receiver [22], backscatter radios experience two-way channels (Here, we assume monostatic ambient backscatter where the reader (e.g., Wi-Fi AP) generates RF signals intended for legacy (Wi-Fi) users (instead of RF tags) and also acts as the receiver for the signals reflected from RF tags [2; 23]. On the other hand, there is bistatic ambient backscatter where the RF signals are generated from the separated TV or FM station, and cellular base station, while the reader received the reflected signals from RF tags [5; 7]): forward channel from reader to tag; backward channel from tag to reader. The cascade of two channels is referred to as dyadic backscatter channel (DBC) and exhibits significant channel attenuations [18] as well as delay spreads [23] compared to the one-way channel. Eventually, the phenomena lead to a low signal-to-noise ratio (SNR) for detection and severe intersymbol interference (ISI). To overcome the inherent limitations in AmBC, various modulation and detection techniques based on diversity combining schemes are proposed. Multiple receive antennas can enhance detection performance in AmBC. For example, interference cancellation schemes using channel differences [7], or zero-forcing [24] are proposed. The multi-antenna configurations are useful for achieving spatial diversity gain and effective as the number of receive antennas increases. On the other hand, modulation and detection based on spreading sequences can also improve reliability achieving temporal diversity gain. In [2; 5], detection SNR of AmBC is reinforced using maximal-ratio combining of backscatter samples. In [25], periodicity of ambient signal is utilized using cyclic prefix (CP) in orthogonal frequency division multiplexing (OFDM) sequences. Orthogonal spreading structure similar to code-division multiple access (CDMA) is proposed in [7]. However, most of the AmBC assumed orthogonality in both modulations and detections. For instance, non-overlapping backscatter symbols are typically represented in two-dimensional Euclidean space (e.g., on-off keying (OOK) [5; 6], phase-shift keying (PSK) [2], ternary encoding [26] with orthogonal multiple access (OMA) such as time-division multiple access (TDMA) [27] for providing connectivity). For detections, conventional AmBC ignored ISI in DBC by assuming a long period of backscatter symbol [23] or simply filtered it, which reduces SNR for signal detections [2; 23]. In AmBC, tag backscatters data in active mode and harvests RF energy in idle mode whose time portion is much larger than the active mode [15]. Consequently, we can utilize the duty-cycling operation by employing sparse coding over ambient RF signals and support non-orthogonal multiple access (NOMA) for massive connectivity. In addition, low-rate backscatter data are delivered over high-rateambient RF signal, which can be equivalently regarded as spreading sequences to provide diversity gain for reliable backscatter communications [25].