Survey on coexistence of heterogeneous wireless networks in 2.4 GHz and TV white spaces <s> WiFi-WiFi coexistence <s> Emerging WLAN standards have been incorporating a variety of channel widths ranging from 5MHz to 160MHz, in order to match the diverse traffic demands on different networks. Unfortunately, the current 802.11 MAC/PHY is not designed for the coexistence of variable-width channels. Overlapping narrowband channels may block an entire wide-band channel, resulting in severe spectrum underutilization and even starvation of WLANs on the wide-band. A similar peril exists when a WLAN partially overlaps its channel with multiple orthogonal WLANs. In this paper, we propose to solve the problem of partial spectrum sharing using Adaptive Subcarrier Nulling (ASN). ASN builds on the 802.11 OFDM PHY, but allows the radios to sense, transmit, detect, and decode packets through spectrum fragments, or subbands. An ASN transmitter can adapt its spectrum usage on a per-packet basis, by nulling the subbands used by neighboring WLANs, and sending packets through the remaining idle subbands. ASN preserves the 802.11 CSMA/CA primitives while allowing users to contend for access to each subband, and can opportunistically exploit the merits of wide-band channels via spectrum aggregation. We have implemented and evaluated ASN on the GNURadio/USRP platform. Our experimental results have shown ASN to achieve detection and decoding performance comparable to the legacy 802.11. Our detailed simulation in ns-2 further shows that ASN substantially improves the efficiency and fairness of spectrum sharing for multi-cell WLANs. <s> BIB001 </s> Survey on coexistence of heterogeneous wireless networks in 2.4 GHz and TV white spaces <s> WiFi-WiFi coexistence <s> Unlike their cellular counterparts, Wi-Fi networks do not have the luxury of a dedicated control plane that is decoupled from the data plane. Consequently, Wi-Fi struggles to provide many of the capabilities that are taken for granted in cellular networks, including efficient and fair resource allocation, QoS and handoffs. The reason for the lack of a control plane with designated spectrum is that it would impose significant overhead. This is at odds with Wi-Fi's goal of providing a simple, plug-and-play network. In this paper we present Flashback, a novel technique that provides a decoupled low overhead control plane for wireless networks that retains the simplicity of Wi-Fi's distributed asynchronous operation. Flashback allows nodes to reliably send short control messages concurrently with data transmissions, while ensuring that data packets are decoded correctly without harming throughput. We utilize Flashback's novel messaging capability to design, implement and experimentally evaluate a reliable control plane for Wi-Fi with rates from 175Kbps to 400Kbps depending on the environment. Moreover, to demonstrate its broad applicability, we design and implement a novel resource allocation mechanism that utilizes Flashback to provide efficient, QoS-aware and fair medium access, while eliminating control overheads including data plane contention, RTS/CTS and random back offs. <s> BIB002
We start with WiFi-WiFi coexistence. Figure 3 illustrates the default spectrum operation setting of most WiFi networks today. Specifically, 14 channels of 22 MHz each are available on the 2.4-GHz ISM band, resulting a total bandwidth of 83.5 MHz. Among the 14 channels, there are only three non-overlapping channels, namely, the channels 1, 6 and 11 according to the IEEE 802.11 standard. Given the high density of the deployment of WiFi hot-spots today, it is inevitable that a WiFi network shares part or all of its operating spectrum band with neighbour WiFi networks. Consequently, designing coexistence mechanisms among WiFi networks becomes an urgent research problem. The problem of WiFi-WiFi coexistence becomes even more pressing as IEEE 802.11ac  further envisions to increase the channel bandwidth to 80 and 160 MHz. Coexistence via partial spectrum sharing. Current WiFi networks rely on CSMA/CA to coordinate transmissions on the same channel. However, the CSMA/CA cannot coordinate the transmissions of different WiFi networks operating on partially overlapping channels. In response to the problem of coexistence of WiFi networks on partially overlapping spectrum channels, Zhang and Shin 20 developed a solution called adaptive subcarrier nulling. The main purpose of ASN is to mitigate collisions due to partial overlapping by enabling wireless local area networks (WLANs) sharing overlapped spectrum. To motivate the design of ASN, a comprehensive measurement about partial overlapping interference in IEEE802.11b/g networks is conducted. BIB001 Particularly, due to orthogonal frequency division multiplexing (OFDM) PHY layer of IEEE802.11g, partial overlapping may cause more destructive damage to data transmission in IEEE802.11g than in IEEE802.11b since full overlapping and partial overlapping may both lead to all packets loss in IEEE802.11g network. Specifically, this damage may cause two types of problem as shown in Figure 4 , 'A', 'B', 'C' represent different networks nodes' transmission channel width, and (a), (b), (c) in Figure 4 are different overlapping scenarios: The first type, illustrated in Figure 4 (a), is called partial channel blocking. As CSMA/CA mechanism is employed by IEEE 802.11, when part of channel is occupied by co-located narrow band WLANs, the entire channel should be suspended. The second, illustrated in Figure 4 (b) and (c), is called channel starvation. In these two scenarios, node A can transmit only if nodes B and C are both idle. It is rare that both B and C complete transmission at the same time and lose contention with A. So, A would remain starvation with large probability. ASN is proposed to address the above problem. In ASN, a WiFi channel is split into several subbands. As shown in Figure 3 , the minimum overlapping spectrum bandwidth between adjacent channels is 5 MHz in 802.11 g, each channel of 20 MHz is thus split into four subbands. When mutual interference is detected, a node can adjust its bandwidth on the basis on subbands so as to share the spectrum with other nodes. In ASN, since the transmission spectrum may change, receivers cannot detect and decode packets as in original IEEE 802.11 networks. Hence, a suite of mechanisms is proposed in ASN in this context, including packet detection, synchronization and decoding. Moreover, adjacent subbands may generate interference even though they are orthogonal. To mitigate interference between subbands, narrow guard bands are introduced in ASN. Specifically, two adjacent subbands should be separated by three subcarriers. In summary, ASN is easy to implement in existing network devices without any other extra equipment and can increase the overall throughput of heterogeneous networks and decreases collisions among them by facilitating spectrum sharing. However, due to adjacent channel interference, guard bands should be configured at the subband level, which may decrease the overall spectrum efficiency. Coexistence via control channel. However, an additional in-band control channel mechanism Flashback is proposed by Cidon et al. BIB002 to coordinate heterogeneous WiFi networks coexistence. In current WiFi networks communication, ordinary extra control channel construction is unappropriate to apply, since the ISM band is overcrowded, no extra spectrum can be equipped as control channel. For example, a 2-MHz control channel is employed in a WiFi channel, and an extra 1.875-MHz guard band must be added to eliminate the mutual interference between control channel and data plane. Hence, control channel would occupy 25% bandwidth of one channel in a 20-MHz WiFi channel at least, collapse of throughput cannot be avoided. This extra cost is too high for WiFi networks communication. While another solution of this problem, orthogonal frequency division multiple access (OFDMA) protocol, can realize the data and control packets transmitting at the same time on different frequency without extra spectrum requirement. But OFDMA protocol requires tight clock synchronization among all network nodes which is not suitable for WiFi network. And other solutions like RTS (request to send) and CTS (clear to send) packets are also unappropriate for WiFi network due to extra consumption of transmission duration which lead to decrease in throughput too. So, using in-band control channel approach without consumption on throughput is the best idea to solve this problem. To realize this idea, there are two challenges, link margin and flashes decoding. First is link margin; before its explanation, we should present the concept of flash at first. As shown in Figure 5 , flash is a type of simple high-power sinusoid and has same frequency and duration to a OFDM subcarrier since WiFi networks have the OFDM PHY layer. In essential, flash is a kind of noise to normal data. However, link margin is a kind of statistic data to estimate current channel quality. It is the difference between the instantaneous channel signal-to-noise ratio (SNR) and the minimal SNR which must be guaranteed to decode packet. Therefore, link margin is designed to measure the interference from flash to ordinary data transmission. The flash emitting should not surpass the link margin constraint so that flashes and ordinary data both can be decoded correctly in receiver, and in-band control channel can be constructed. Second is flashes decoding, which is easy to be understood. As the literal meaning, flashes are only a simple sinusoid signal which is impossible to bring complex message in a single signal, and this challenge is to design a method to transmit and decode the control messages in flashes. Consequently, to address these problems and challenges, Flashback technique is proposed. First, flashes transmission is important. By applying subcarriers of OFDM PHY, each flash can be transmitted in one OFDM symbol, which only occupied 1/64 of channel and last for 4 ms in WiFi network. Hence, compared to whole WiFi packet transmission, the transmission of flashes is rare, which would not cause important impact on data transmission in WiFi network. And for one period of data packet transmission, flashes may appear at different subcarrier in different time slot, so these dynamical transmission pattern can be abstracted into a time-frequency coordinate matrix, which is convenient for locating and encoding flashes of Flashback, so that control messages can be encoded in these matrix. Second, for each flash decoding, erasure mechanism is designed. For each packet receiving, flashes and packets are received simultaneously by detecting flashes first, and the error bit which is flashed would be erased since correcting it would cost much more than erasure. Due to the burst feature of flash, the duration of decoding flash is much shorter than data packet transmission, receiver can easily use algorithm to decode whole packet without the erased error bits, so control message and data can be received completely. Third, due to asynchronous WiFi devices, ordinary timefrequency grid cannot be applied to encode and decode control messages directly. Hence, a distance detecting mechanism is employed. It is easy to understand this mechanism, for instance, two flashes can be sent at the same time at subcarriers 2 and 27, so that the distance between them is 25, which can be the code of control messages. Using distances, control messages can be sent on an average 175 Kbps bitrate, meanwhile, an 8-bit cyclic redundancy check (CRC) is also added into control packets for error correction. Finally, the link margin is very easy to be guaranteed using an algorithm to calculate the appropriate transmitting power before each flash transmission can eliminate this problem perfectly. In general, by detecting control messages, coordination between WiFi nodes can be completed well, the interference can be mitigated without any extra cost and an extra control channel is constructed to transmit information. But this solution only focuses on nodes coexistence in same WiFi network, which is not suitable for heterogeneous IEEE 802.11 standard networks coexistence.