Survey on coexistence of heterogeneous wireless networks in 2.4 GHz and TV white spaces <s> Exposed terminal problem: <s> Frequency overlap across wireless networks with different radio technologies can cause severe interference and reduce communication reliability. The circumstances are particularly unfavorable for ZigBee networks that share the 2.4 GHz ISM band with WiFi senders capable of 10 to 100 times higher transmission power. Our work first examines the interference patterns between ZigBee and WiFi networks at the bit-level granularity. Under certain conditions, ZigBee activities can trigger a nearby WiFi transmitter to back off, in which case the header is often the only part of the Zig-Bee packet being corrupted. We call this the symmetric interference regions, in comparison to the asymmetric regions where the ZigBee signal is too weak to be detected by WiFi senders, but WiFi activity can uniformly corrupt any bit in a ZigBee packet. With these observations, we design BuzzBuzz to mitigate WiFi interference through header and payload redundancy. Multi-Headers provides header redundancy giving ZigBee nodes multiple opportunities to detect incoming packets. Then, TinyRS, a full-featured Reed Solomon library for resource-constrained devices, helps decoding polluted packet payload. On a medium-sized testbed, BuzzBuzz improves the ZigBee network delivery rate by 70%. Furthermore, BuzzBuzz reduces ZigBee retransmissions by a factor of three, which increases the WiFi throughput by 10%. <s> BIB001 </s> Survey on coexistence of heterogeneous wireless networks in 2.4 GHz and TV white spaces <s> Exposed terminal problem: <s> As the ISM spectrum becomes crowded by various network devices, such as WiFi and ZigBee, the coexistence between them poses a critical challenge, due to their heterogeneous MAC/PHY layers. Recent measurement studies have shown moderate to high WiFi traffic to severely impair coexisting ZigBee's performance. We propose a new mechanism, called cooperative busy tone (CBT), that exploits the inherent cooperation among ZigBee nodes to harmonize their coexistence with WiFi devices. CBT employs a separate ZigBee node to emit a busy-tone concurrently with the desired data transmission, thereby improving the visibility of ZigBee devices to WiFi. It adopts an innovative way to concurrently schedule the busy-tone and a data packet without causing interference between them. To optimize CBT, we establish a theoretical framework that models the performance of coexisting ZigBee and WiFi networks, and analyzes the impact of key design parameters in CBT. Our analytical results and testbed implementation demonstrate CBT's significant throughput improvement over the legacy ZigBee protocol, with negligible performance loss to WiFi. <s> BIB002 </s> Survey on coexistence of heterogeneous wireless networks in 2.4 GHz and TV white spaces <s> Exposed terminal problem: <s> The ISM spectrum is becoming increasingly populated by emerging wireless networks. Spectrum sharing among the same network of devices can be arbitrated by MAC protocols (e.g., CSMA), but the coexistence between heterogeneous networks remains a challenge. The disparate power levels, asynchronous time slots, and incompatible PHY layers of heterogeneous networks severely degrade the effectiveness of traditional MAC. In this paper, we propose a new mechanism, called the Cooperative Busy Tone (CBT), that enables the reliable coexistence between two such networks, ZigBee and WiFi. CBT allows a separate ZigBee node to schedule a busy tone concurrently with the desired transmission, thereby improving the visibility of ZigBee devices to WiFi. Its core components include a frequency flip scheme that prevents the mutual interference between cooperative ZigBee nodes, and a busy tone scheduler that minimizes the interference to WiFi, for both CSMA and TDMA packets. To optimize CBT, we establish an analytical framework that relates its key design parameters to performance and cost. Both the analytical and detailed simulation results demonstrate CBT's significant throughput improvement over the legacy ZigBee protocol, with negligible performance loss to WiFi. The results are validated further by implementing CBT on sensor motes and software radios. <s> BIB003 </s> Survey on coexistence of heterogeneous wireless networks in 2.4 GHz and TV white spaces <s> Exposed terminal problem: <s> The unlicensed ISM spectrum is getting crowded by wireless local area network (WLAN) and wireless personal area network (WPAN) users and devices. Spectrum sharing within the same network of devices can be arbitrated by existing MAC protocols, but the coexistence between WPAN and WLAN (e.g., ZigBee and WiFi) remains a challenging problem. The traditional MAC protocols are ineffective in dealing with the disparate transmit-power levels, asynchronous time-slots, and incompatible PHY layers of such heterogeneous networks. Recent measurement studies have shown moderate-to-high WiFi traffic to severely impair the performance of coexisting ZigBee. We propose a novel mechanism, called cooperative carrier signaling (CCS), that exploits the inherent cooperation among ZigBee nodes to harmonize their coexistence with WiFi WLANs. CCS employs a separate ZigBee node to emit a carrier signal (busy tone) concurrently with the desired ZigBee's data transmission, thereby enhancing the ZigBee's visibility to WiFi. It employs an innovative way to concurrently schedule a busy tone and a data transmission without causing interference between them. We have implemented and evaluated CCS on the TinyOS/MICAz and GNURadio/USRP platforms. Our extensive experimental evaluation has shown that CCS reduces collision between ZigBee and WiFi by 50% for most cases, and by up to 90% in the presence of a high-level interference, all at negligible WiFi performance loss. <s> BIB004 </s> Survey on coexistence of heterogeneous wireless networks in 2.4 GHz and TV white spaces <s> Exposed terminal problem: <s> Coordination of co-located wireless devices is a fundamental function/requirement for reducing interference. However, different devices cannot directly coordinate with one another as they often use incompatible modulation schemes. Even for the same type (e.g., WiFi) of devices, their coordination is infeasible when neighboring transmitters adopt different spectrum widths. Such an incompatibility between heterogeneous devices may severely degrade the network performance. In this paper, we introduce Gap Sense (GSense), a novel mechanism that can coordinate heterogeneous devices without modifying their PHYlayer modulation schemes or spectrum widths. GSense prepends legacy packets with a customized preamble, which piggy-backs information to enhance inter-device coordination. The preamble leverages the quiet period between signal pulses to convey such information, and can be detected by neighboring nodes even when they have incompatible PHY layers. We have implemented and evaluated GSense on a software radio platform, demonstrating its significance and utility in three popular protocols. GSense is shown to deliver coordination information with close to 100% accuracy within practical SNR regions. It can also reduce the energy consumption by around 44%, and the collision rate by more than 88% in networks of heterogeneous transmitters and receivers. <s> BIB005
The ZigBee sender is in the interference range of WiFi nodes, while the receiver is not. Blind terminal problem: The ZigBee sender and receiver are both in the interference range of WiFi nodes but WiFi nodes cannot sense them. Both hidden terminal problem and blind terminal problem can lead to data collision. WISE addresses the blind terminal problem. WISE consists of two main components, white space modelling and frame adaptation: The first component predicts the white spaces duration. According to experiment, WiFi frames form clusters with inter-frame durations typically less than 1 ms within clusters. ZigBee packet headers being more than 544 ms at least, interframe durations within clusters cannot be used by ZigBee nodes. The real white spaces which can be used are intervals between clusters. Given that the WiFi traffic arrival process exists selfsimilarity, Pareto model is applied to fit the arrival process of WiFi frame clusters and to predict WiFi white spaces duration. The second component calculates the size of ZigBee frames based on the outcome of the first component. The ZigBee frames are then divided into sub-frames which are suitable for transmission in white spaces without collision. Session IDs and delimiter mechanism are also applied for frame reconstruction and correction. BIB001 designed a novel approach called BuzzBuzz to enhance the robustness of ZigBee packets such that they can be successfully decoded at the receiver side even under the presence of WiFi signals. To guide the design of BuzzBuzz, the authors conducted a series of experiments to measure the interference between WiFi and ZigBee on bit level. The experimented scenarios can be divided into two types, which can be analogized to symmetric and asymmetric scenario as shown in Figure 1 . The following observation is drawn: In the symmetric scenario, the bit errors of ZigBee packets due to collision are distributed at the beginning of ZigBee packets since CSMA mechanism can work and WiFi transmissions are suspended shortly after detecting ZigBee transmissions. In the asymmetric scenario, which fails to sense of ZigBee signals, WiFi transmission cannot stop when ZigBee data are transmitted. Consequently, bit errors are distributed across the whole ZigBee packet. Based on the above observation, BuzzBuzz contains two components, each addressing a scenario: In the symmetric scenario, the starting bits of ZigBee packets are corrupted by WiFi signals, thus leading to CRC check failure at the receiver side, despite the remaining bits are received correctly. If there exists some approach that can make ZigBee packets pass CRC check, data transmission can be completed successfully. Motivated by this argument, the Multi-Headers (MH) mechanism is proposed, which puts several headers into the beginning of data part in ZigBee frame such that the remaining headers can pass CRC check when some headers are collided by WiFi signals. In the asymmetric scenario, due to the even distribution of bit errors across the whole packet, the MH mechanism cannot work any more. The forward error correction (FEC) approach is introduced to recover the ZigBee packets using Reed-Solomon (RS) code, a block-based linear error-correction code widely used in digital communication. The authors developed a full-featured TinyOS-compatible RS library (TinyRS) to decode the ZigBee packets. In conclusion, the advantages of BuzzBuzz include the low cost, the ease of using and the low consumption. The major disadvantage is the overhead created by the MH and the FEC components in terms of packet size and extra processing overhead. Enabling WiFi-ZigBee coordination. Another research strand on WiFi-ZigBee coexistence is to devise coordination protocols between them. BIB002 BIB003 BIB004 BIB005 The main challenges of the coordination mechanism design are the asymmetry in transmission power, operation bandwidth and PHY layer protocols. Moreover, for ZigBee networks in CSMA mode, the fastest clear channel assessment (CCA) operation takes 128 ms and the rx/tx switching time 192 ms, while the CCA duration of WiFi networks lasts 20 or 9 ms. As a result, WiFi nodes may pre-empt transmission channel when ZigBee switches from receive mode to transmit mode and may cause collision. To address these design challenges, Zhang and Shin BIB002 BIB003 BIB004 provided an approach called Cooperative Busy Tone, whose core idea is illustrated in Figure 7: A ZigBee signaller is deployed to solve the problem of WiFi-ZigBee power asymmetry. The signaller is a separate ZigBee node, which is close to the WiFi network or has more power than ordinary ZigBee nodes. It is designed to emit busy tone to inform WiFi nodes when the ZigBee network has data to transmit. Note that busy tone can be sensed without decoding. As busy tone can interfere ZigBee network's communication too if they are working in the same channel, and channel bandwidth asymmetry (one WiFi channel bandwidth covers four ZigBee channels) is utilized to solve the problem. A frequency flipping mechanism is developed such that when ZigBee nodes are transmitting data, the signaller will transmit busy tone on adjacent channel. To avoid collisions caused by asymmetric CCA duration, two different approaches are further developed for ZigBee CSMA and TDMA modes: -In CSMA mode, ZigBee devices transmit data after sensing and contention. In CBT mechanism, every data transmission must be initiated by the signaller, who broadcasts a notification message (referred to as CTS) on original channel and then switches to adjacent channel to start emitting busy tone. Meanwhile, ZigBee transmitter contends for channel access normally. Under such mechanism, WiFi nodes can sense busy tone before ZigBee transmission. When ZigBee devices complete communication, an ACK packet is transmitted. When hearing the ACK packet, the signaller stops broadcasting busy tone. -In TDMA mode, each data transmission is also initiated by the signaller, who performs CCA for at most K m times before transmission. If the K j th CCA is idle, the signaller hops to adjacent channel to broadcast busy tone until the end of the transmission. Hence, busy tone broadcasting is (K m 2 K j )CCA earlier than data transmission. In this case, WiFi nodes can also sense busy tone before data transmission and avoid collision. To summarize, the CBT mechanism can coordinate coexistence between ZigBee and WiFi networks effectively and avoids collisions. Such advantage comes with the price of deploying an extra ZigBee channel and an extra device (the signaller). Moreover, fairness is another concern when long duration busy tone is broadcast. Zhang and Shin BIB005 have also developed another mechanism called Gap Sense to coordinate heterogeneous wireless networks using a preamble construction and detection algorithm. The core idea of GSense is to send a number of energy pulses in the preamble of each packet and encode information using the time gap between two consecutive pulses. Heterogeneous networks then utilize such control channel to exchange control information and coordinate with each other. The disadvantages of GSense are as follows. First, the preambles extend the duration of data packets and decrease the network throughput. Second, the sensing and interference ranges of different networks may be asymmetric, meaning that the pulses need to be amplified to be sensed by other networks, leading to extra energy consumption. Third, for TDMA-based networks, it is expensive or even impractical to monitor channels continuously.