## 3 Multiple-Access Reservation Receiver Initiation MAC (MAR-RiMAC)
### \"Request to Transmit\" (RtT) Signaling
The y-axis represents the sliding window value attained by the correlator for each received symbol. The horizontal line represents a possible threshold, which is configurable and balances between false positive (too low) and false negative (too high) detections, thus determining the robustness of the CSS. Figure 4a demonstrates the robustness of the correlation process. As can be seen in the figure, the reception of each CSS results in a correlated spike that is clearly identifiable, indicating that CSSs can be successfully received even under low SINRs (\(-6\) dB). Figure 4b depicts an overlap of the outcome at the receiver of eleven different correlators (correlation with eleven different CSSs, including the genuine one). Again, it can be seen that the mechanism is highly robust and the receiver can easily detect the transmitted CSSs, while the other correlators did not produce any false negatives, despite the very low SINR (i.e., low cross-correlation between the set of CSSs). Note that the noise only looks visually denser due to overlapping plots. The utilization of a CSS detection process via cross-correlation as our signaling mechanism enjoys three fundamental advantages over the ordinary contention mechanism: (i) It enables a reliable signaling mechanism between the reporting devices and the report accumulator even at low SINR. (ii) Short-duration signals (transmission times) convey the signaling device's ID. Note that the duration of a signal CSS transmission, overlooking the simultaneous transmission gain, is much shorter than a typical control message duration, especially since a typical control message is proceeded by a CSS. Furthermore, the detection is almost instantaneous, as no decoding is needed. (iii) The benefit from CSSs being transmitted simultaneously (multiple CSSs at a time) is two-fold; first, simultaneous--rather than sequential--transmission saves valuable time. Second, the fact that collisions are avoided not only saves the wasted air-time due to unsuccessful transmissions, but also enables the signaling devices to transmit right after the RtR beacon without any contention delay. The sequence length determines the transmission duration of each CSS as well as the number of supported sequences, which, in turn, determines the probability of misdetection and the number of correlators required at each report aggregator. It is important to note that CSS correlators require a limited amount of resources (they require sign-flip adjustments and several summations on the received samples). Moreover, since correlators are already realized in common wireless cards, CSS enhancement only requires replication of components (all the more so since, basically, only the sink needs to attain many correlators). Obviously, there is a tradeoff between the number of supported sequences and the probability of misdetection, i.e., the length of the sequences in conjunction with the number of supported CSSs determines the magnitude of the cross-correlation between any CSS pair, which, in turn, influences the probability of false detection. PN sequences have been widely explored in the literature; each is characterized by the sequence length (i.e., number of symbols), the number of sequences that can be generated (in our context, how many different signals or different IDs can be supported), and the cross-correlation between any sequence pair, which is an indication of accuracy (misdetection). For example, our suggested scheme can utilize a Gold code with a 127-symbol sequence length, which can  
Figure 3: A device utilizes Correlatable Symbol Sequences (CSSs) to convey its identity. The receiver (sink) detects the specific CSS by cross-correlating it with the incoming samples. (**a**) CSS- predefined binary codeword (sequence). (**b**) CSS+Interference as received by the receiver (sink). (**c**) CSS detection process via cross-correlation. The receiver (sink) cross-correlates the incoming samples with the pre-defined CSSs. Cross-correlating with a matched CSS obtains a spike value.