Remotely Powered Sensor Network over CATV Infrastructure

Young Cheol Kim, Ick Chang Choi, Byoung-Ju Yun, Hyun Deok Kim School of Electrical Engineering and Computer Science

Kyungpook National University Daegu, South Korea

hyundkim@ee.knu.ac.kr

Abstract— The CATV infrastructure has been deployed in most houses and buildings and the coaxial cables of the CATV infrastructure usually can provide sufficient bandwidth to accommodate other services as well as CATV service. We have proposed and demonstrated a novel remotely powered sensor network over the existing CATV infrastructure composed of coaxial cables, splitters and so on. The proposed sensor network has been implemented by using the wasted bandwidth of the coaxial cable (VHF and UHF bands) of the conventional CATV network without any change of the existing infrastructure. Furthermore, it supports a convenient remote power feeding from a central node to end nodes (sensor nodes) by using coaxial cable. We have developed transceiver for the wired sensor network matched the characteristics impedance of the coaxial cable of the CATV network. We also have developed protocol stack of the transceiver to interface with wired cable instead of air and to support the remote power feeding to the end nodes. We have confirmed the performance of the proposed sensor network by measuring the packet error rate (PER) for various data rates when the end nodes were remotely powered. The maximum transmission distance between the central node and the end node of the wired CATV network was about 265 m when the 128 sensor nodes were joined the network at the operating data rate of 250 kb/s.

Keywords-Sensor Network; CATV; Remote Power Feeding; Sensor Network

The wireless sensor network can be easily deployed for various applications since it does not require wire-based infra-structure. ost applications of sensor network focus on sensing and controlling for military, industrial and indoor applications. The advantage of sensor network is mostly characterized by its low power consumption, low cost based on low data rate and highly reliable networking for sensing and controlling signals [1-3]. However, the wireless sensor network can suffer from the interferences with other wireless networking services, the poor networking security and the relatively higher power consumption compared with wired connection. Especially, when the wireless sensor network is deployed indoor applications in houses or buildings instead of open space applications, the problems become more severe [2]. Furthermore, though the sensor node of the wireless sensor network can be powered from a small battery, one should monitor the state of the battery and need to replace or recharge a worn-out battery to guarantee a reliable operation of the

wireless sensor network. This problem consequentially increases the maintenance cost of the sensor network.

Recently, we had proposed and demonstrated a novel sensor network technology by using the CATV infrastructure in order to overcome the problems of the wireless sensor network used in indoor environment [4]. The coaxial cables have been deployed as the CATV infrastructure in most houses and buildings and the coaxial cables usually can provide sufficient bandwidth to accommodate other services as well as CATV service. The sensor network over the CATV infrastructure does not require any new cable deployment similar to the wireless sensor network. Furthermore, it is inherently free from the interferences with other wireless networking services and provides a reliable networking with higher securities. However, though we can overcome most of the problems of wireless sensor network by deploying the network over the CATV infrastructure, the sensor nodes still requires power of battery management.

In this paper, we demonstrate a novel remotely powered sensor network over the CATV network infrastructure. The direct current (dc) power is distributed from the central node to sensor nodes with the signals of sensor network over existing coaxial cable. Especially since the CATV infrastructure usually configured as a multiple star architecture it is inherently suitable for the power distribution from the central node. The proposed network can reduce the maintenance cost of the sensor network.

I. IMPLEMENTAION OF NETWORK

A. Network Configuration The proposed wired sensor network uses the deployed

coaxial cables for the CATV signal distribution in house or building. The CATV network in house or building is usually configured as a multiple star architecture where the cascaded splitters are used to distribute CATV signal from the central node to rooms or offices. To accommodate the CATV service and future services the coaxial cable and other components including the splitters usually have sufficient bandwidth higher than 1 GHz at least whereas the current usage of the

This work was partially supported by the Ministry of Education, Science and Technology (MEST) under BK21 Program and by the MKE(The Ministry of Knowledge Economy), Korea, under the CITRC(Convergence Information Technology Research Center) support program (NIPA-2011-C6150-1102-0011) supervised by the NIPA(National IT Industry Promotion Agency).

2011 IEEE International Conference on Consumer Electronics - Berlin (ICCE-Berlin)

U.S. Government work not protected by U.S. copyright 79

CATV service is limited to less than 870 MHz. Since the coaxial cable deployed for the CATV service has sufficient bandwidth to accommodate other services, the sensor network can be implemented by using the CATV infrastructure. For example, the coordinate node of the sensor can be connected to the sensor nodes located in rooms or offices through the CATV infrastructure configured coaxial cable, splitter and so on as shown in Fig. 1. The diplexers are three terminal devices and combine and separate the CATV and the sensor signals. They are commonly used multi-frequency RF communication transceiver.

Figure 1. The proposed sensor network architecture over CATV infrastructure.

The Fig. 2 shows the frequency response of the coaxial cable. The characteristics of the coaxial cable has flat frequency response up to 2.4 GHz and the attenuation coefficients of the conventional coaxial were around 0.21 dB/m at 915 MHz and 0.27 dB/m at 2.4 GHz, respectively, which is much lower than that of the air used for wireless transmission media.

Figure 2. The frequency response of the coaxial cable (cable length=20 m). Furthermore, as shown in the Fig. 2, since the attenuation of dc power is negligible, it is suitable for remote powering transfer from the central node to the end nodes (sensor nodes) through the cable. When the length of coaxial cable was about 300 m and the voltage of the power supplied from the central

node was 9 V, the measured dc voltage drop measured at the sensor node was less than 1 V. Thus the deployed coaxial cables of the CATV infrastructure are suitable for both the transmission media of the sense network signal and the remote power transfer.

B. Transceivers Generally, the transceivers of the sensor network such as

coordinate node and sensor nodes are characterized by their small size, low power consumption and low cost. Therefore, currently available transceivers employ one of dual PHY of radios for reasons mentioned above. The simplest alternative operates in a license free band (315/433/868/915 MHz) and has a bandwidth in the range 20-50 kbps. Such transceivers usually offer a simple byte oriented interface that permits software implementations of energy efficient MAC protocols. Another type supports in 2.4 GHz band and offering a 250 kbps bandwidth [6].

Currently, most of the sensor network transceivers are optimized to operate in wireless communication. However, the characteristics of the coaxial cable of the CATV infrastructure are quite different from those of the air. For example the characteristic impedance of the coaxial cable is 75 ohm. Thus we have developed the sensor network transceivers suitable for operating with the 75 ohm coaxial cable. We have designed a BALUN and an impedance transformer circuit operating with a characteristic impedance of 75 ohm. The transceiver also includes a remote power feeding circuit, which enables remote power supply from a central node to sensor nodes. It can provide a remote power feeding and sensor signals at the same time without reducing performance of sensor signals.

The implemented transceiver consists of a BALUN and an impedance transformer and a remote power feeding circuit as shown in Fig. 3. We measured the reflection coefficient of the implemented transceiver by using a Network Analyzer considered the 75 ohm impedance characteristics of conventional coaxial cable. The reflection coefficient of the transceiver measured at the cable-connector interface was about -32 dB at 915 MHz as shown in Fig. 4.

We also have developed protocol stack of the transceiver by just modifying the SimpliciTI from Texas Instruments [7] to interface with coaxial cable instead of air and to support the remote power feeding.

80

Figure 3. The implemented transceiver (The inset shown reflection coefficient of the transceiver measured at the cable-connector interface.)

Figure 4. The reflection coefficient of the transceiver measured at the cable-connector interface.

C. Experimental results In order to confirm the feasibility of the proposed sensor

network we have measured PER for various configurations. The central node (TRX#1) and the sensor node (TRX#2) were connected by using the conventional coaxial cable and components used in CATV infrastructure. The power to the sensor node was provided from the central node as shown in Fig. 5. The sensitivity test of the implemented transceiver was carried out as transmitter and receiver. We used SmartRF studio from TI to measure the Packet Error Rate (PER). With a given packet length and BER, the PER can be calculated using the following formula.

_1 (1 ) packet lengthPER BER= − −

The packet length includes both sync word and CRC in addition to the payload data since a bit error in test fields will result in a packet error [7].

Figure 5. The experimental setup

We used a 9 V dc power supply to provide power to the

central node and the sensor node. The voltage drop measured at the sensor node was less than 1 V for a 200 m long coaxial cable.

We measured the packet error rate (PER) as a function of the received power of the transceiver. The receiver power was measured by using the received signal strength indicator (RSSI) of the transceiver. The operating frequency was 915 MHz, the output power of the transceiver was 10 dBm and the modulation format was GFSK with 1.2 kb/s data rate. We used 200 m long coaxial cable between two transceivers and an attenuator was used to adjust the input power of the transceiver. When the RSSI of the transceiver was higher than about -94 dBm, there was no error in the received packets. However, when the RSSI decreased to -94.7 dBm the PER of 0.3 % was observed which later increased to 23.7% when the RSSI was -98.8 dBm as shown in Table I.

Table I. The measured PER vs. RSSI

Attenuation (dB) RSSI (dBm) PER(%)

35 -87.9 0

40 -94.7 0.3

41 -95.7 0.8

45 -98.8 23.7

We also measured the sensitivity of the transceiver at 1% PER for various date rates as shown in Fig. 6. As the data rate increases the minimum required power to guarantee PER of less than 1 % increases. A PHY and MAC (media access control) layer of the IEEE 802.15.4 standard defines a data rate of 250 kb/s in 2.4 GHz unlicensed band and with the lower bit-rate alternatives in 868 MHz and 915 MHz bands [8]. The sensitivity of the implemented transceiver at the data rate of 250 kb/s was about -72.8 dBm. It is notable that we also confirmed experimentally that there was no sensitivity degradation due to the interference from CATV signals. Although the operating frequency was 915 MHz, it is sufficient to satisfy the data rate of the IEEE 802.15.3 standard requirements compare with 250 kb/s data rate in the 2.4 GHz.

81

Figure 6. Transceiver sensitivities as a function of data rates to guarantee the PER less than 1%

The power margin (transceiver output power – sensitivity)

of the network was about 82.8 dB at the date rate of 250 kb/s. Thus the maximum transmission distance is about 414 m at 915 MHz. However, the CATV network uses cascaded splitter to distribute the signals and it causes additional losses in the network. If the number of the sensor node is 128, which coincide with the splitting loss of 24 dB including 3dB extra loss of the splitter, the maxim transmission distance will be about 265 m without any amplification.

II. CONCLUSION We have demonstrated a novel remotely powered sensor

network over the existing CATV infrastructure. We have

developed the sensor network transceivers matched with the 75 ohm coaxial cable, which enables remote power feeding. The remote powering from the central node to sensor node through coaxial cable is a very efficient solution to solve the power management problem of the sensor. The remotely powered sensor network over the CATV infrastructure can provide robustness against interferences with high scalability. It does not require any new power cable deployment and local power supply at the sensor node, which reduces the size, the cost and the complexity of the sensor node as well simplifies the maintenance of the network.

[1] Z. Yiming, et al, “A Design of Greenhouse Monitoring & Control system Based on ZigBee Wireless sensor Network,” Wireless Communications, Networking and Mobile Computing, 2007.

[2] S. Y. Shin. et al, “Packet Error Rate Analysis of ZigBee Under WLAN and Bluetooth Interferences,” IEEE Wireless Communications, 2007.

[3] Y. Chengbo, et al, “ZigBee Wireless Sensor Network in Environmental Monitoring Applications,” WICOM, 2009.

[4] H. C. Cha, et al, “Sensor Network over CATV Infrastructure,” International Conference on Consumer Electronics, 2011.

[5] G. Schmitt, et al, “ Remote Powering of Broadband Multimedia Service Networks,” Telecommunications Conference, 2005.

[6] P. Baronti et al, “Wireless sensor networks: A survey on the state of the art and the 802.15.4 and ZigBee standards,” computer Communications, May 2007.

[7] Texas Instrument, “Practical Sensitivity Testing,” TI Application Note DN002, 2008.

[8] A. Wheeler et al, “Commercial Applications of Wireless Sensor Networks Using ZigBee,” IEEE communications Magazine, April 2007.

82