Liwen Qiu, Qinglin Zhao, Lianbo Zhang, Shumin Yao, Jing Zhao, and Li Feng
Abstract In the Internet of things, cross-technology communication (CTC) enables direct communication between heterogeneous devices without a gateway. CTC is receiving growing attention, because it can significantly reduce the hardware cost and the complexity of network deployment for communication between heterogeneous devices, as well as improve the network performance. At present, there are two types of CTC techniques, namely packet-level CTC and physical-level CTC. In this paper, we first provide an overview of the two types of techniques. Then, we detail the packet-level CTC via two typical protocols: Freebee and ZigFi, and the physical-level CTC via a typical protocol: WeBee. Finally, we compare the two CTC techniques in terms of hardware cost, communication direction, throughput, spectrum efficiency, and parallelism.
9.1 Introduction
In recent years, the extensive application of the Internet of things (IoT) has led to the widespread deployment of dense heterogeneous networks (e.g., Wi-Fi, ZigBee, and Bluetooth). In this scenario, since the devices use the same SIM band, inter- device interference is becoming more and more serious. But it also provides an opportunity for communication between the devices with different protocols. Recent research shows that cross-technology communication (CTC) has many benefits such as reducing hardware overhead, reducing the complexity of network deployment, and improving network performance. Traditionally, we use a gateway to build communi- cation among heterogeneous devices by converting the protocols in high layer. How- ever, this will introduce additional hardware overhead, increase network deployment complexity, and decrease network performance. In addition, if heterogeneous devices want to communicate with each other, we need to deploy gateways in advance. To overcome these drawbacks, some researchers propose packet-level CTC technique L. Qiu (
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)·Q. Zhao·L. Zhang·S. Yao·J. Zhao·L. FengMacau University of Science and Technology, Macau, China e-mail:[email protected]
© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021
Y.-D. Zhang et al. (eds.),Smart Trends in Computing and Communications: Proceedings of SmartCom 2020, Smart Innovation, Systems and Technologies 182,
https://doi.org/10.1007/978-981-15-5224-3_9
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which utilizes some features like packet energy, packet interval, and packet dura- tion to transmit information to other heterogeneous devices. For example in [1], a sender transmits a sequence of Wi-Fi packets by shifting the beacon frame position to emulate symbol message. These packets can be decoded by the ZigBee receiver through sensing the energy of emulated signal. In [2], the transmitter encodes CTC bits by mapping them to a unique packet duration which is different from the nor- mal communication packet length. In [3], the sender will encode CTC bits in two different packet energy levels to transmit CTC information. Packet-level CTC only can carry limited bits in one packet and therefore obtain limited system throughput.
Because of its low throughput, it is not suitable for deployment in high-speed and dense networks.
In recent years, some researchers proposed physical-level CTC techniques which embed CTC bits in the payload to communicate among heterogeneous devices by emulating the signal of heterogeneous devices. The receiver will consider the emu- lated signals as a legal signal and demodulate it correctly. For example, in [4], the Wi-Fi sender will emulate ZigBee signals by selecting payload bit pattern carefully.
ZigBee receiver cannot identify the difference between the emulated signals and nor- mal ZigBee signals. Since a packet can transmit thousands of bits, the physical-level CTC can achieve high throughput.
In this survey paper, we focus on packet-level CTC and physical-level CTC tech- niques. And we also consider some common features like cost, communication direction, throughput, data rate, spectrum efficiency, and parallelism.
The rest of the paper is organized as follows. First, we introduce two packet- level CTC techniques named FreeBee and ZigFi, respectively, in Sect. 9.2. Next, we present one representative physical-level CTC WeBee in Sect.9.3. Finally, we summarize the common features of these two kinds of CTC in Sect.9.4.
9.2 Packet-Level CTC
In this section, we introduce FreeBee and ZigFi in details. FreeBee realizes com- munication from Wi-Fi to ZigBee by utilizing the beacon position to encode CTC bits. ZigFi can make ZigBee communicate to Wi-Fi through different CSI in Wi-Fi packet.
9.2.1 FreeBee
FreeBee modulates the messages by shifting beacon position which is different from normal beacon position in wireless standard to encode CTC bits. In Fig. 9.1, we consider a standard beacon position is at t position where the modulating range of beacon interval ist. According to FreeBee, we shift the beacon from its standard position to another position that in the range of [−T/2,T/2] to show the symbol is
9 Packet-Level and Physical-Level Cross-Technology … 97
Time t(standard position)
T
Fig. 9.1 The beacon position
encoded. As we know, the number of bits that can be embedded is determined by the shift unit. According to the 802.11 standard, we can set the shift unit as 1.024 ms.
And the range of intervals is equal to 100. So the beacon shift can express 6 bits.
If we want to transmit 0 bit and 1 bit, we sett−2as 0 bit andt−2as 1 bit.
To promise the realizable of communication, FreeBee needs to retransmit the same beacon pattern many times which is depended on the channel noise.
Because of the incompatible physical protocol between Wi-Fi and ZigBee, the ZigBee receiver needs to adopt a folding algorithm to get the beacon position from collected RSSI samples. Next, we need to compare the collected beacon position with the standard beacon position. If it is in front of the standard beacon position, we decode it into 1 bit, otherwise, we decode it as 0 bit.
9.2.2 ZigFi
In [5], ZigFi realizes ZigBee communicates to Wi-Fi by different channel state information (CSI) values to encode CTC bits with 1 or 0. At the receiver, it pro- posed a receiver-initiated protocol which translates the decoding problem into CSI classification problem with support vector machine (SVM).
As shown in Fig.9.2, when the Wi-Fi sender is transmitting Wi-Fi packets, we control the ZigBee packet to overlap with the Wi-Fi packet. It will make different CSI value that is overlapped by ZigBee packet from normal CSI value. We set the interfered CSI as 0 bit and the non-interfered CSI as 1 bit. As we can see in Fig.9.2, the packet length will interfere the CTC communication. If the ZigBee packet is too short, the collision probability of ZigBee and Wi-Fi is low and the variation of CSI value is so small that the receiver cannot identify the overlapped CSI from normal CSI.
TDZ>2TDW+TIW (9.1)
Wi-Fi packet length and ZigBee packet length must satisfy Eq. (9.1) to ensure discriminative CSI value. WhereTDZis the transmission time of the ZigBee packet.
TDWis the transmission time of the Wi-Fi packet andTIWis the transmission interval between two adjacent Wi-Fi packets.
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ZigBee TX
WiFi TX WiFi RX
1 0 1
CSI sample not interfered by ZigBee CSI sample interfered by ZigBee
Fig. 9.2 Illustration of ZigFi
The Wi-Fi receiver will receive Wi-Fi packets and collect CSI values in each packet. Then, CSI classifier will classify CSI values by trained SVM classifiers.
Next, the receiver demodulates the CSI values into 1 bit or 0 bit according to the decoding rule.
9.3 Physical-Level CTC
In this section, we introduce a representative physical-level CTC named WeBee in details. Because of the low throughput of packet-level CTC, the researcher pro- posed physical-level CTC to greatly improve throughput. WeBee makes Wi-Fi communicate to ZigBee by emulating ZigBee signals in Wi-Fi device.
As shows in Fig.9.3, Wi-Fi sender will embed the ZigBee message in the payload portion and send emulated ZigBee signals to the receiver. ZigBee receiver considers
ZigBee Device WiFi Device
CTC
WiFi Frame
Emulated ZigBee Signals
Ignore
ZigBee Preamble Detection
ZigBee Demodulation Fig. 9.3 Process of WeBee
9 Packet-Level and Physical-Level Cross-Technology … 99 the emulated signal as a normal ZigBee signal and ignores the Wi-Fi header and trailer. If the receiver detects the ZigBee preamble, it will demodulate emulated ZigBee signals.
However, we need to modify some Wi-Fi processes to emulate ZigBee signal perfectly. The sender sends an emulated ZigBee signal followed by follow steps.
First, get the constellation points of the emulated ZigBee signal by reversing the Wi- Fi transmission procedure. Then, we quantify constellation points of the emulated ZigBee signal into predefined points according to the minimum Euclidean distance.
Since the pilot signal affects the emulated ZigBee signal, we need to carefully select the transmitter central frequency to avoid overlapping the pilot frequency position with the emulated ZigBee signal frequency range. Because of the interfered by cyclic prefix (CP) which only exists in Wi-Fi transmission process, the sender will flip the packet header and trailer to reduce error. Finally, Wi-Fi sender sends the emulated signals.
When the ZigBee receiver detects the preamble of emulated ZigBee signal, it first converts the analog signal into the digital signal. Then, the receiver will get the phrase shift according to ar tan(s(n)∗s∗(n−1)). ZigBee outputs the chip value
“1” if the phase shift is bigger than 0 and otherwise outputs the chip value “0”. After collecting 32 chips, the receiver will map these chips into four bits symbol according to the predefined symbol-to-chips table.
9.4 Conclusion
In this section, we compare three CTC techniques in Table9.1.
From Table 9.1, CTC realizes direct communication among heterogeneous devices without a gateway. So it has a low-cost feature. The physical-level CTC can transmit more bits than the packet-level CTC depending on the number of bits transmitted by a single packet. So the former has the characteristics of high through- put. Because the Wi-Fi band is larger than the ZigBee band, WeBee can embed two different CTC messages at different Wi-Fi frequencies. WeBee has higher spec- trum utilization than the other two packet-level CTC techniques and supports parallel Table 9.1 Comparison of three CTC techniques
Cost Communication direction
Throughput Spectrum efficiency
Data rate
Parallelism Packet
layer CTC
FreeBee Low Wi-Fi to ZigBee
Low Medium Low Not
support ZigFi Low ZigBee to
Wi-Fi
Low Low Low Not
support Physical
layer CTC
WeBee Low Wi-Fi to ZigBee
High High High Support
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CTC. Compared with ZigFi that can only transmit one bit in one packet, FreeBee can transmit more bits in one packet. In addition, FreeBee has higher spectrum efficiency than ZigFi.
Acknowledgements This work is funded in part by the National Nature Science Foundation of China (File no. 61872451 and 61872452) and in part by the Science and Technology Development Fund, Macau SAR (File no. 0098/2018/A3 and 0076/2019/A2). Li Feng is the corresponding author.
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