## 3 Multiple-Access Reservation Receiver Initiation MAC (MAR-RiMAC)
### \"Request to Transmit\" (RtT) Signaling  
The reservation mechanism suggested above relies on the capability of the receiver of identifying the reserving devices even when it has received several reservations simultaneously. Accordingly, our goal is to introduce a robust transmission mechanism with low communication overhead, which enables the reception of multiple transmitted signals simultaneously. In this subsection, we will present two different mechanisms that can attain this goal. Figure 2: Overview of the Multiple-Access Reservation Receiver Initiation MAC (MAR-RiMAC) protocol. A backlogged device waits silently for its intended receiver beacon to send a reservation transmission. The sink sequentially polls the backlogged devices for transmission. Similarly to RI-MAC, a non-backlogged device wakes up periodically, broadcasts its wakening notification, and, if no device transmits to it, goes back to sleep. (**a**) Under low load. (**b**) Under high load, when multiple devices attempt transmission simultaneously, the sink can identify them and poll them consecutively. Correlatable Symbol Sequences (CSSs)  
The first mechanism utilizes Correlatable Symbol Sequences (CSSs). Correlatable symbol sequences are predefined binary codewords (sequences), which are detected by the receiver via correlators. Accordingly, each codeword is associated with a correlator, which is essentially an array of \(n\) weights, where the size \(n\) depends on the desired number of codewords and the robustness of transmissions. Even though these codewords are deterministically generated, they retain the statistical properties of a sampled white noise, according to their designation as pseudo-random or pseudo-noise (PN) sequences. While these appear random to an ignorant listener that has no prior knowledge of the predefined codewords, PN can be easily detected by the legitimate receiver via cross-correlation with a local copy; cross-correlation of any such sequence with a matching copy obtains a sharp and distinct spike when the input signal to the correlator matches the PN sequence itself. Correlation-based detection with a pseudo-random sequence makes the detection more reliable and robust to noise and interference, i.e., CSSs can be detected even at a low signal to interference and noise ratio (SINR). Figure 3 illustrates the codeword detection process. A detailed discussion of signal correlation can be found in the literature (e.g., [47; 48; 49; 50]). For our CSS mechanism, we utilize sequences with low cross-correlation, which means that any two sequences within the set have bounded small cross-correlations. Accordingly, trying to cross-correlate any sequence with any other but its own corresponding sequence (legitimate or not) will result in a correlation close to zero (e.g., Gold and Kasami codes, which are commonly used; e.g., [48; 51]). CSSs are widely used in communication systems (e.g., customary IEEE802.11 preamble used for packet detection, symbol synchronization, and radio parameter tuning). In order to illustrate the CSS concept and, specifically, the correlation and cross-correlation advantages, we present previous experiments that we have conducted on the Wireless Open-Access Research Platform (WARP) (a comprehensive set of the experimental results that examine different aspects of CSSs and can be found in [52; 53]). Here, we present two examples that are borrowed from there. Figure 4a depicts a representative outcome of a realization in which the sender utilized 127-symbol-length Gold codes for CSSs. It transmitted seven repetitions of a 127-symbol CSS. We evaluated the detection of a CSS codeword under background interference generated by random orthogonal frequency-division multiplexing (OFDM) transmissions of a potential interfering node. For each newly acquired sample (at 40 MHz frequency of the WARP platform analog-to-digital converter (ADC)), the receiver computes the signal's correlation with its local copy of the transmitted CSS. The x-axis in both figures represents the index of the collected samples.