# Multi-Dimensional Sparse-Coded Ambient Backscatter Communication for Massive IoT Networks
## 1 Introduction  
In ambient backscatter communication (AmBC), batteryless tags utilize ambient radio frequency (RF) signals emitted from various RF sources including TV [1; 2], Wi-Fi [3] or long-term evolution (LTE) signals for harvesting energy and transmitting data to the reader nearby such as smartphones [4]. However, due to lack of coordination among the RF sources, tags and the reader, the AmBC system suffers low connectivity caused by limitations on channel as well as hardware. For example, the backscattered signals from passive tags without active digital-to-analog converter (DAC) [1; 2; 5] are severely attenuated than active RF devices since the signals propagate two-way channels, typically referred to as dyadic backscatter channel (DBC) [6; 7; 8]. Most common methods to combat channel fading in AmBC are to simply increase the symbol period of backscatter symbols to achieve temporal diversity similar to repetition coding [5], or encode tag's data using long spreading sequence [2; 9; 10] which can significantly decrease the data rate of AmBC when the channel condition is not good. The weak ambient backscatter signals are also vulnerable to various kinds of interferences such as direct-link interference (DLI) [2; 11; 12] from strong ambient sources and multiple access interference (MA) [9; 13] when concurrent backscatter transmissions are considered for non-orthogonal multiple access (NOMA). To eliminate the DLI, successive interference cancellation (SIC)-based receiver structure can be considered [3; 11; 12] to resolve error floor problem in bit-error rate (BER) curves caused by the DLI, but they are only feasible for a local area network (LAN) scenario where a single tagtransmits data to the reader in short range. The DLI is a distinguished feature of the 'ambient' backscatter systems in contrast to the 'bistatic' backscatter systems with unmodulated, dedicated carrier where strong leakage signals can be easily removed by direct-current blocking. For this reason, RF techniques such as power-domain NOMA [13], chirp spreading sequence (CSS) [14], space-time code [15; 16], rateless code [9] in the bistatic scenarios may not be feasible for the ambient backscatter scenarios, inevitably limiting the communication range due to the absence of those diversity techniques. Moreover, hardware limitation on RF tags embedded in AmBC hinders high-rate communication and massive connectivity essential for IoT networks. For example, typical backscatter modulators present in these systems are on-off keying (OOK) or phase-shift keying (PSK) modulators [1; 5] which have modulation order of \(M=2\). When the modulation order \(M\) increases, customized RF tags with \(M\) load impedances connected to RF switches (e.g., 4-PSK [17], 4-QAM [18; 19], 16-QAM [20; 21], 16-PSK [3], 32/64-QAM [22]) are required in manufacturing process, eventually increasing installation costs and form-factor of RF tags. In addition, when high-order modulation is considered in AmBC, data from multiple tags are typically uploaded to the reader based on time-division multiple access (TDMA) [3] and detected at the reader using maximal-ratio combining (MRC) of time-domain signals, which suffers low rate compared to NOMA [13; 23]. Therefore, to realize massive IoT networks with long-range and high-rate AmBC, we propose a novel multi-dimensional sparse-coded AmBC (MSC-AmBC) system which successfully extends our previous works on AmBC [24] limited to short-range LAN and lower signal dimension. Our main contributions can be summarized as follows:  
* **Direct-Link Interference:** We consider separated ambient source and reader for long-range AmBC in contrast to the short-range AmBC with colocated source and reader in [24]. In addition, to effectively cancel DLI, we utilize frequency-domain structure of OFDM carriers from multiple ambient sources, which can be an extension of the works [11; 12; 25] with single ambient source.