This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE CCNC 2010 proceedings

Fast and Energy-Efficient Link Recovery in Visible Light Communications Ying Li and Sridhar Rajagopal

Abstract— This paper provides mechanisms to support fast and energy-efficient link recovery for optical communication using visible light spectrum. Visible light communication (VLC) is becoming increasingly important to support low power, Gbps data rate communications due to the recent advances in LED technology and high power consumption needed for sampling and processing Gbps data in baseband for RF communication systems. In VLC, the line of sight and high directional feature make the fast link recovery a key issue. Fast link recovery is a new challenge for reliable VLC. This paper presents fast and energy-efficient link recovery approaches for VLC local area network and peer-to-peer communications, with further enhancements for multiple communication directions and color bands. The proposed approaches can achieve link recovery timely, and save the battery life of VLC devices.

I. INTRODUCTION Optical data communication has been present for a long time for high speed wired networks based on fiber optics and also for wireless systems such as IrDA[1]. There has been a renewed interest in optical communication for high data rates due to multiple reasons. First, LED technology has made significant advances in the last decade to now produce very fast LED switching times. Also, baseband processing for high data rates in traditional RF and 60 GHz communication systems can be very challenging due to high power consumption (Watts) due to need for high-speed, highbandwidth ADCs, DACs, multiple antennas and sophisticated signal processing. In contrast, optical processing can provide Gbps data rates with very low power consumption. In addition, optical communication using visible light can provide lighting in addition to optical communication. Visible light communication (VLC) is a new technology for shortrange optical wireless communications using visible light in optically transparent media. VLC can provide access to several hundred THz of unlicensed spectrum; immunity to electromagnetic interference and noninterference with Radio Frequency (RF) systems; additional security by allowing the user to see the communication channel; and communication augmenting and complementing existing services (such as illumination, display, decoration, etc.) from visible-light infrastructures. VLC has also been proposed for use in Intelligent Transport Systems (ITS) for communicating between vehicles/traffic lights. VLC is currently under consideration for standardization in [2][3]. In the VLC system, LOS (Line of Sight) communication between two VLC transceivers constitutes the majority of the The authors are with Wireless Solutions Labs, Samsung Telecommunications America, Dallas, TX, 75082, USA. Email: {yli2, srajagop}@sta.samsung.com.

applications, since the visible light cannot go through obstacles like a wall. However, the temporal blocking like walking people, can cause burst frame errors, or even the link failure, and can occur frequently. In addition, the poor pointing of the VLC devices may cause the decrease of signal quality or even link disconnection. Since VLC can be highly directional, it is difficult to re-establish link that has been lost due to movement or rotation of one of the devices in the link. Given such, it is necessary and important to have fast link recovery for VLC systems, for both point-to-point and pointto-multipoint connections, yet the fast link recovery is very challenging. Fast link recovery is a key issue in VLC system, while it is full of challenges. In the literature of VLC, how to perform fast link recovery is not well studied. In IrDA system [1], although link recovery mechanism exists, it is not really fast or in a timely manner, nor does it consider the power saving for mobile device. Despite the challenges, this paper makes the following contributions to support fast and energy efficient link recovery for visible light communications. The proposed approach in the paper can also be applied to other optical communication such as IrDA. 1. Propose fast and energy efficient link recovery approaches for VLC, for point to multi-point, and point to point communications. 2. Provide the rationale and advantages of proposed approaches of link recovery. II. FAST AND ENERGY-EFFICIENT LINK RECOVERY A. VLC local area network (LAN) In a VLC local area network, let AP denote the access point, MN denote the mobile node, or MD denote the mobile device. We propose the following approach for fast and energyefficient link recovery: - When certain conditions are met, such as when MN has not received acknowledgement consecutively for a number of predefined times, MN may stop sending data and MN may transmit the signaling for link recovery. - To further reduce MN battery consumption, instead of uplink (UL) signaling for link recovery, MN stops sending data and may wait if there is also downlink (DL) service. When certain conditions are met, such as when AP has not received acknowledgement consecutively for a number of predefined times, AP transmits the signaling for link recovery. The rationale of the above approach is that if there is some blocking, such as walking people, often times, the UL and DL will be both blocked because of the line of sight. Since for a

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This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE CCNC 2010 proceedings

VLC local area network, the AP usually is a device (e.g., infrastructure device) which does not have battery concern, while MN usually has battery concerns, the proposed approach tries to let AP transmit link recovery signal if possible rather than letting MN transmit link recovery signal, to recover the link, and to save the battery consumption of MN. Figure 1 illustrates the approach above. The fast link recovery (FLR) signal can be a signal such as pinging signal, to test the link, or a signal carrying the information of missing packets. In Fig. 1(a), MN self-decides when to stop sending data. If there is only UL data service, MN sends fast link recovery signal to AP. If there is also DL data service, MN waits. In Fig.1(b), AP self-decides when to stop sending data, AP sends fast link recovery signal to MN, for either case of DL service only, or case of DL and UL service.

(a) Uplink triggering fast link recovery

Figure 2. Fast and energy-efficient link recovery in VLC P2P. C. Enhancements We propose further enhancements for previous sections. For a device with multiple LEDs of different angles, once fast link recovery is triggered at one angle, the device may use other angles to send fast link recovery signals to test links. Figure 3 illustrates such approach.

Figure 3. Link recovery with multiple directions. For a device with multiple color bands, once fast link recovery is triggered at one color band, the device uses other color bands to send FLR signals to test links. This approach is useful for recover the links which are suffering the interference on different bands.

Figure 4. Link recovery with multiple color bands. III. CONCLUSION (b) Downlink triggering fast link recovery Figure 1. Fast and energy-efficient link recovery in VLC LAN B. VLC peer-to-peer (P2P) communication The approach proposed for VLC LAN can be extended to peer-to-peer communication. In addition, the batter life may optionally be a parameter to decide who sends fast link recovery signal. The VLC devices may exchange battery life. When the device with lower battery life satisfies the link recovery triggering condition, it may wait for the other device with higher battery life (e.g., a device with power plug on) to send the link recovery signal. Figure 2 illustrates the link recovery in peer-to-peer communication.

This paper presents mechanisms to support fast and energyefficient link recovery in visible light communication systems. We propose fast and energy-efficient link recovery approaches for VLC local area network and peer-to-peer communications. Further enhancements of the approaches are proposed for multiple communication directions and color bands. The proposed approaches can achieve link recovery timely, and save the battery life of VLC devices. REFERENCES [1] Infrared Data Association (IrDA) Specification. [2] IEEE 802.15.7 http://www.ieee802.org/15/pub/TG7.html [3] VLCC standard http://www.vlcc.net