Jia Zhao Simon Fraser University BC, Canada zhaojiaz@sfu.ca X-Tandem: Towards Multi-hop Backscatter Communication with Commodity WiFi Jia Zhao Simon Fraser University BC, Canada zhaojiaz@sfu.ca Wei Gong University of Science and Technology of China, China Simon Fraser University, Canada weigong@ustc.edu.cn Jiangchuan Liu Simon Fraser University BC, Canada jcliu@cs.sfu.ca MobiCom18

Motivation Pioneer works of backscatter all in single-hop mode restricted connectivity limit the robustness and scalability Multi-hop backscatter Path diversity and robustness Multipath routing and transport

Challenge A series of intermediate backscattered signals A series of operations for tag hardware configuration assigning a route synchronizing multiple tags information to specific data fields Out-of-order reflection 发射一个wifi信号，第一个tag接收到把自己信息调制到上面，第二个接收第一个的backscatter信号，信息也调制到上面 接收端全解出

X-Tandem Design Modular overview of the system WIFI发射端，产生控制信号 tag前端模拟电路，激活命令 预定义的数据包持续时间模式 FPGA模块，确定协议类型 为多tag分配路由和数据段 原始信号，进入tag调制模块，codebook保证调制过的数据依然可以由wifi星座图解出 频移保证和之前的信号占用信道不同，不冲突 WIFI接收端解码

Tag Relay Multiple Frequency Shifts fc激励信号的中心频率 f1,f2方波频率 𝛼base(t)激励信号的基带波形 𝛽(𝑡)多跳信号 m,n = 1 频移避免干扰 25MHZ 2.417到2.467 w1 = w2 = wt

Tag Modulation Embedding data in a single WiFi packet Codeword translation method Separating the data fields and allocate them to different tags The tag modulation makes phase change on the original signal’s data fields 原始信号数据域 相位改变 仍然在星座图上的点 图中相位改90度 the latter only needs the last hop backscattered signal, and tags are mutually independent

Data fields Allocation Data fields separation Packet length Calculation RF power detector voltage comparator 802.11n wifi packet PLCP Includes a PLCP preamble field, a MAC header field, and a PLCP Protocol Data Unit (PPDU) data fields PPDU数据域包括可变数量的OFDM symbol，每个symbol包含编码数据的相同比特数并具有固定的符号持续时间 Tag前端电路 an RF power detector to convert the incoming RF power signal to an equivalent DC voltage(等效直流电压) 输出v正比信号强度 a voltage comparator, which converts the detector’s output voltage to a binary-level voltage, to eliminate the noise voltage 阈值过滤

Tag Data Decoding A packet filter XOR operation decoder Search the tag bits from OFDM symbols for 802.11g/n 过滤去除乱序的信号

Implementation Frontend Analog Circuits FPGA Modules of Control Signal Detection and Data Fields Allocation FPGA Modules for Tag Modulation Synchronization

Frontend Analog Circuits An RF signal power detector convert an RF signal to an equivalent DC voltage A voltage comparator converts the detector’s output voltage to a binary-level voltage 获取包长

FPGA Modules of Control Signal Detection and Data Fields Allocation 6 long packets switch on tag relay 6 short packets switch off tag relay 3 long + 3 short packets tag ‘01’ as the first hop tag 3 short + 3 long packets tag ‘10’ as the first hop tag 决定是否中继 以及中继顺序

FPGA Modules of Control Signal Detection and Data Fields Allocation After the control signals, the two tags will receive the predefined packets with fixed bits of payload data These packets are used to identify protocols and calculate the modulation range allocated in the packet data fields 控制信号后，还会收到预定义的包 确定wifi协议 802.11b/g/n 同时计算tag的调制区域

FPGA Modules for Tag Modulation 获取到了调制区域 tag可以进行调制 同步到开始调制的位置 改变OFDM symbols的相位 如果使用中继 用25MHz平移 否则使用50MHz The tag’s frontend circuitry outputs a positive edge to trigger tag modulation when it detects WiFi signal power data fields allocation module outputs the start position and the length of the allocated data fields Tag modulation’s frequency shift uses a 25MHz clock if tag relay is on, and 50MHz otherwise

Synchronization WiFi TX sends a specific pattern of consecutive packets to synchronize with the tags. Skip the PLCP preamble and MAC header 802.11b signals, the PLCP preamble is 192μs, the MAC header is 192μs 1.什么时候开始调制 检测到pattern 后开始调制 2.跳过preamble和header 使用FPGA’s internal clock

Evaluation Communication Ranges Tag-to-tag Distance Obstacles

Communication Ranges 移动接收端 RSSI,throughput,BER For all the three different protocols, the BER values are less than 0.1 when the distance is less than 0.5m

Tag-to-tag Distance 移动第二个tag 0.05m到0.4m rssi >70dBm到<80dBm 0.05到0.1下降最为严重 Throughput BER表现均下降 距离增加

Obstacles 实验场景 a) over 90% packets have signal levels below 85dBm and throughputs below 10 bps with communication ranges within 0.15m b) throughput is improved to 135 bps with communication ranges up to 1.5m

Contribution To relay among multiple tags, we introduce a Multiple Frequency Shifts (MFS) scheme To coordinate the multi-hop transmission, we design a smart data field allocation mechanism We also design a packet verification scheme that eliminates out-of- order backscattered packets using RSSI measurements while keeping in-order backscattered packets intact tag前后信号频率偏移 避免干扰 控制信号决定tag传输顺序 为每个tag分配不同的调制区域

Thank you ~