standards for indoor optical wireless communications ss - IEEE Xplore

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STANDARDS FOR INDOOR OPTICAL WIRELESS COMMUNICATIONS The application of Optical Wireless Communications (OWC) has grown so much in recent years that the whole industry would benefit from common standards to which competitive products comply. Standards are essential, particularly when the market expands into high-volume products like home appliances and other consumer goods. This article reviews the existing OWC standards and points toward future directions A. C. Boucouvalas, Periklis Chatzimisios, Zabih Ghassemlooy, Murat Uysal, and Konstantinos Yiannopoulos

ABSTRACT

Service-specific convergence sublayers (SSCS)

MAC MAC common part sublayer (MCPS)

MAC link management entity (MLME)

PHY PHY layer management entity (PLME)

PHY layer data (PD) PHY-switch

Service access point (SAP) Optical media

Instead, IEEE continued to pursue OWC standardization. The result is the state-of-the-art 802.15 standard on Visible Light Communications (VLC) that use commercial LEDs to provide broadband indoor communications. IrDA, on the other hand, has maintained a comprehensive list of standards that focus mainly on the implementation of OW links for Personal Communication Systems (PCS). OW-based personal communications at 1 Gb/s have been demonstrated and a ten-fold increase in bandwidth is expected in the near future. This article is organized as follows: we offer a short overview of the IEEE 802.11 OW standard and the challenges it faced. We discuss the IrDA protocol stack and unique solutions IrDA adopted for implementing a low-cost, high-bandwidth PCS. We present the IEEE 802.15 VLC standard, focusing on the key requirements of the physical and link layers. Finally, we summarize ongoing efforts for the development of future standards.

The application of Optical Wireless Communications (OWC) has grown in recent years that the PTICAL IRELESS OCAL REA ETWORKS whole industry would benefit from common standards to which competitive products comply. Standards are essential, particularly when the Following the introduction of Wireless Local market expands into high-volume products like Area Networks (WLANs), in 1997, IEEE home appliances and other consumer goods. This released the initial IEEE 802.11 standard for the article offers a timely review of standards-writing physical (PHY) and Medium Access Control activity, as OWCs find their way into diverse (MAC) layers [1]. Three choices are standardized products varying from TV remote in the PHY layer; two utilize controls to satellite links. This radio (Direct Sequence and FreCOMMUNICATIONS article discusses the most popular quency Hopping Spread SpecTA N DA R D standards for optical wireless trum modulation) and one communications. We outline the utilizes IR signals, although a IEEE 802.11 standard for optical wireless local single product can support only one of them. All area networks, and the ongoing standardization three transmission methods provide a basic data effort by IrDA on personal optical wireless sysrate (R) of 1 Mb/s and an optional rate of 2 tems. The article concludes with a discussion of Mb/s. The PHY of 802.11 is split into the Physithe recently announced IEEE 802.15.7 standard cal Layer Convergence Protocol (PLCP) and the on visible light communications. Physical Medium Dependent (PMD) sublayers. PLCP prepares (parses) data units that are transmitted (received), whereas PMD performs the NTRODUCTION data transmission (reception) and modulation (demodulation) under the guidance of PLCP. Optical Wireless Communications (OWC) is a The IR PHY supports two different Pulse Posiniche technology combining the best of both the tion Modulation (PPM) schemes, 16 PPM for 1 wireless and optical worlds, enabling the developMb/s and 4 PPM for 2 Mb/s. The peak-power ment of ultra-broadband, agile and non-obtrusive wavelength of the transmitter (Tx) is between communication systems [1, 2]. OWC is being 850 and 950 nm, while a typical link length L is applied in numerous commercial indoor and outlimited to 10 m. Finally, two main transmission door systems that range in scale from shorttechniques are used: point-to-point and diffused. range/personal-area to access/metro area The MAC layer is based on the CSMA/CA networks. In particular, OWC is compatible with (Carrier Sense Multiple Access with Collision other well-established technologies like optical Avoidance) technique, where the station that has fiber and interior lighting, and can benefit signifia frame ready for transmission first senses the cantly from advances in these industries. As a channel. If the channel is free for a period longer result, OWC during the last decade has seen a sigthan the DIFS (Distributed Inter-Frame Spacnificant increase in the attained bandwidth and an ing) duration, the station transmits. If the chanequally important cost decrease. OWC is expected nel is busy, the station continues sensing until to keep up these trends, closely following the the channel is free for a period longer than advance of fiber optics, and provide even more DIFS. Afterward, the station employs a conbroadband and cost-effective systems in the future. tention-resolution method prior to sending, The potential of OWC for broadband, costnamely Binary Exponential Back-off, so as to effective, safe and non-obtrusive communication minimize the probability of frame collisions due systems has attracted the interest of two major to multiple simultaneous transmissions. standardization bodies, IEEE and IrDA (Fig. 1). IR transmission was initially employed in IEEE included an OWC implementation sceIEEE 802.11 because it provides a number of signario of the original 802.11 standard, which has nificant advantages over radio approaches such been discontinued.

O

S

W

L

A

N

S

I

A. C. Boucouvalas and Konstantinos Yiannopoulos are with the University of Peloponnese. Periklis Chatzimisios is with Alexander TEI of Thessaloniki. Zabih Ghassemlooy is with the University of Northumbria at Newcastle. Murat Uysal is with the Ozyegin University. This work is supported by COST Action IC1101 Optical Wireless Communications-An Emerging Technology (OPTICWISE).

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0163-6804/15/$25.00 © 2015 IEEE

IEEE Communications Magazine — Communications Standards Supplement • March 2015


as unregulated broadband spectrum. It is low cost, offers high security against eavesdropping and a large LAN can be established over separate installations without interference. However, Infrared Links (IRL) exhibit significant drawbacks, including multipath-induced inter-symbol interference, difficulties in detecting effectively optical collisions, interference from ambient lights and shadowing. Additional drawbacks are limited device mobility, exclusively indoor operation (meaning a limited coverage area), as well as dangers to the eye and relatively high power consumption. Because of drawbacks, IR 802.11 was never successfully implemented commercially. As a result, IEEE discontinued the IR version of 802.11 and since 1999 it has released WLAN standards that utilize only radio waves.

PERSONAL AREA COMMUNICATIONS Infrared Data Association (IrDA) produced a set of OWC standards focusing on low cost, short-range, point-to-point links mainly for handheld devices and maintains a comprehensive list of technical documents that define the operation of short-range PCS over IRLs (Fig. 2). Similar to the OSI reference model, IrDA has adopted a layered approach to networking, although there is no exact one-to-one mapping between OSI and IrDA layers. Following the layered approach, different protocols are typically implemented on each layer, especially the upper ones, with each protocol engineered to meet the needs of one or more application profiles. Three application profiles are currently in use, ranging from contact information exchanges to ultra-fast file transfers.

RF

Optical 802.11

10 m Bluetooth v4.0

Bluetooth

UWB

VLC

802.11ad

1m

10GigaIR SIR/FIR/MIR

VFIR

0,1 m Transfer GigaIR jet 1 Mb/s

10 Mb/s

1 Gb/s

100 Mb/s

10 Gb/s

Figure 1. Popular OW/ RF standards for indoor and personal communications.

Applications

IrComm IrLMP (IAS)

OBEX TTP

OBEX

IrUSB

IrSMP

IrUSB

IrLMP (MUX) IrLAP

SIR

MIR

FIR

IrMC

THE IRDA PHYSICAL LAYER (IRPHY) IrPHY provides high-speed and almost error-free connections between mobile devices or mobile devices and fixed stations by means of halfduplex, narrow-cone point-to-point IRLs at wavelengths of 850-900 nm. PHY standards include 2.4–115.2 kb/s Serial Infrared (SIR), 0.576 and 1.152 Mb/s Medium Infrared (MIR), 4 Mb/s Fast Infrared (FIR), 16 Mb/s Very Fast Infrared (VFIR) and 96 Mb/s Ultra Fast Infrared (UFIR). The Gigabit Infrared (Giga-IR) standard currently supports 512 Mb/s and 1.024 Gb/s [4], while a data rate up to 10 Gb/s is being standardized for the next version of Giga-IR. IrPHY provides recommendations for a number of IRL (operating over limited distances and narrow emission and reception angles) parameters to enhance link security, unobtrusive operation, conformance to the eye/skin safety regulations and immunity to background radiation. In SIR-to-UFIR speeds the maximum link length Lmax between devices may not exceed 1 m (or 20 cm for low-power implementations), while the 3-dB half-width angle ranges between 15°–30° and >15° at Tx and receiver (Rx), respectively. Beam alignment is more crucial in Giga-IR and a docking station is typically used. Although beaming techniques are expected to allow for L > 1 m in future standards, the docking station currently limits L max to 6 cm, while the transceiver angles are limited to 5°.

802.11n 802.11ac

VFIR

UFIR

IrSimple

GigaIR

IrUSB

Figure 2. The IrDA protocol stack and application profiles. With respect to link reliability, the achievable Bit-Error-Rate (BER) for all IRLs should be better than 10 –8 (SIR-VFIR) and 10 –10 (GigaIR), which is comparable to BERs of optical fiber networks. IrPHY is responsible for physical-layer framing and appends headers and trailers to the transmitted higher-layer frames. The IrPHY header consists of a repeated padding sequence followed by a start character to indicate the arrival of a new frame and ensure that Rx electronics have adequate stabilization time. The trailer contains a Cyclic Redundancy Check (CRC) field that protects frames against errors plus a stop character to indicate the end of frame transmission.

THE IRDA LINK ACCESS PROTOCOL The key functions of the IrDA link-layer are to establish, maintain and terminate connections over IrPHY [5]. This is accomplished by the IrDA Link Access Protocol (IrLAP), an HDLC variant augmented with features that compensate for the volatile nature of IrPHY. IrLAP operates in two distinct modes: • Normal Disconnected Mode (NDM) when no link-layer connection is present — for


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Whenever set to exclusive mode, LM-MUX does not provide multiplexing but is fully commanded by a single upper-layer application. Exclusive mode is primarily aimed for low-latency applications or ones that require self-control over the link.

26

instance when a device boots or after a previously established connection has been terminated • Normal Response Mode (NRM), which provides all the procedures for establishing a new connection, including device initialization, device discovery and link negotiation. During initialization, IrLAP generates a random 32-bit device address (similar to the Ethernet MAC address) and initializes the link-layer parameters to default values, common to all IrDA-compliant devices. Default values must be used in NDM procedures so that all devices may participate in the subsequent device discovery and link negotiation/establishment processes regardless of their capabilities. Device discovery is initiated when no other IR communication takes place. To this end, the device that initiates the discovery process monitors the link for 500 msec. If no ongoing communications are perceived, the initiator broadcasts an unnumbered U-frame to notify nearby peers of its availability. Nearby devices respond to the U-frame and if more than one device is present users are prompted to select the device they wish to connect to. Finally, link negotiation follows a successful device discovery, where the initiator and responder devices agree upon link parameter values that will be used during NRM. Negotiated link parameters include the transmission speed, the frame payload size, the protocol window size, the minimum and maximum link turnaround times (TLT), and the the link disconnect threshold (3–40 sec). Following a successful link negotiation IrLAP switches to NRM, which revolves around the communication of information I-frames that carry the application data. The payload size of I-frames ranges between 64 and 2048 bytes at SIR-VFIR, and is extended to 32 kbytes (UFIR) or 64 kbytes (Giga-IR). Due to the half-duplex link (HDL) nature of IR, a key responsibility of IrLAP is to supervise the link operation and ensure that both communicating devices take their fair share of time transmitting frames. Thus, each device is allowed to transmit its own I-frames until a predefined amount of time (up to 500 msec) has elapsed. After a full transmission or if the device has no more frames to send, it informs the remote side about the imminent link turnaround time using a special field on the header of the last I-frame, called the Poll/Final bit. The link direction is reversed after a short pause (0.01–10 msec) required for transceivers to stabilize for reverse operation. Apart from supervising HDL operation, IrLAP is also responsible for the correct and inorder-delivery of I-frames to upper layers. To this end, IrLAP sequences I-frames and utilizes the error-detection mechanism provided by IrPHY CRC to isolate transmission errors. When an Iframe error is detected, Rx provides negative feedback and requests retransmission(s). At SIR-UFIR speeds, the retransmissions take place in a Go-Back-N fashion so as to simplify Rx; a higher throughput Selective-Repeat mode of operation is standardized for contemporary Giga-IR devices with extended reception capabilities [6]. In both cases, a protocol window of up to 7 (SIR-MIR) or 127 (VFIR and higher) frames is utilized so as to limit the number of pending frames at Tx, as well.

THE IRDA LINK MANAGEMENT PROTOCOL (IRLMP) IrLMP is responsible for providing upper layers with a multiplexing scheme over IrLAP, a Link Control (LCon) mechanism and a service discovery mechanism. IrLMP is separated in two logical entities: the Link Management Multiplexer (LMMUX) and the Information Access Service (IAS) [7]. The LM-MUX entity implements frame multiplexing by means of Logical Service Access Points (LSAPs), which operate similarly to TCP ports. Each upper layer application is assigned a unique LSAP identifier (i.e., a LSAP-selector, or LSAP-sel). Applications utilize their LSAP-sels to establish IrLMP connections. A source-destination LSAP-sel pair defines a unique IrLMP connection and creates communication endpoints required for demultiplexing frames inside LSAPs. IrLMP frames carry the source-destination LSAP-sels on their headers to indicate the IrLMP connection they belong to. LM-MUX also supports an exclusive mode of operation to facilitate LCon. Whenever set to exclusive mode, LM-MUX does not provide multiplexing but is fully commanded by a single upper-layer application. Exclusive mode is primarily aimed for lowlatency applications or ones that require self-control over the link. Service discovery is performed in the IAS entity. This consists of two separate components: a local database that stores the associations between applications with LSAP-sels, and an Information Access Protocol (IAP) that communicates the database entries between IrLMP peers. The local database stores registered applications in the form of objects associated with a class (application) name, a unique identifier and several attributes, including LSAP-sel. Remote applications may access the database and retrieve these objects through the command/response capabilities provided by IAP. IAP utilizes its own operation frames encapsulated in the payloads of IrLMP frames, to communicate queries and responses. For implementation purposes, LM-MUX regards IAS as an upper-layer application with a “well-known” LSAPsel value of (0×00), so that all remote connections have a common access reference.

APPLICATION PROFILES The first application profile standardized by IrDA was the Infrared Mobile Communications (IrMC) [8]. IrMC defines methods for exchanging objects typically found in mobile phones (like contact cards, phonebooks, text messages and call logs). IrMC-oriented applications utilize two core protocols, the Infrared Object Exchange (OBEX) [9] and the Tiny Transport Protocol (TTP). OBEX is a session layer protocol that provides an API for transferring application objects. OBEX follows a client/server requestresponse model (similar to HTTP) and applications may request the transfer of objects using PUT/GET commands. Requests and responses are transferred over OBEX frames in the form of headers, which typically include information about the communicated objects such as the size, name or the object itself. TTP, on the other hand, performs higher level flow control as well as segmentation and re-assembly. Flow control, required to avoid flooding the OBEX server



with an excessive number of requests, is implemented using a credit-based mechanism. TTP peers use up a credit upon sending a TTP frame and receive a new credit with each TTP frame. Peers reaching zero credits must wait for credits to be issued from the other side before sending additional frames. Segmentation and re-assembly is also a critical TTP function because the maximum size of OBEX frames is 64 kbytes and cannot be accommodated by the standard IrLAP payload in SIR-UFIR speeds. IrMC provides a generic framework for transferring data over IRLs. But it has been enhanced to better suit “point-and-shoot”(P&S) applications that require short-lived connections — for example, a camera sending a digital picture to a printer. The key requirement in P&S applications is the minimization of the interaction time between devices and the IrSimple application profile achieves this by modifying time-consuming procedures in IrLAP and TTP. The three primary reasons for delay in IrLAP are device discovery, the TLT, and automaticrepeat-requests, especially those taking place in a Go-back-N fashion. IrSimple-compliant devices completely ignore the 500 msec listening period during device discovery, assuming that no IRL is present. Moreover, link turnarounds occur less often in IrSimple, with the maximum turnaround time raised to 1 sec from 500 msec. Data transfers are also considered uni-directional and the receiving side immediately reverses the link direction without sending any data of its own. Finally, application data are sent over unnumbered U-frames instead of information I-frames, alleviating the requirements for frame sequencing, conforming to a transmission window and providing feedback about erroneous frames. The Infrared Sequence Management Protocol (IrSMP), which completely ignores flow control, assuming that the receiving side may not be overwhelmed by a simple P&S application, replaces TTP. IrSMP maintains the segmentation and re-assembly function of TTP and also provides a frame-sequencing mechanism for Automatic Repeat-reQuest (ARQ) purposes, since ARQ is not implemented in IrLAP. To these ends, IrSMP transfers data blocks segmented into frames before they are passed to IrLMP and IrLAP. Erroneous frames inside a block are re-transmitted by IrSMP; the transfer of a new block begins only after all frames in a preceding block have been delivered correctly. Another application framework provided by IrDA is Infrared Financial Messaging (IrFM) that supports wireless digital financial transactions (known as Point-and-Pay) [10]. In IrFM, a user who wants to pay for services and merchandise, utilizes a device (also called a Personal Trusted Device, or PTD) and interacts with a point of sale terminal. IrFM addresses many aspects of the requirements of a desirable digital payment system by achieving a fast transaction due to the P&S nature of IrDA. Moreover, IrFM maintains simplicity, security (it’s difficult to eavesdrop), interoperability and reliability for IR financial transactions. Within the same rationale of minimization of the interaction time between devices, the IrBurst profile replaces OBEX with a more efficient session protocol, without altering the operation of

TTP and IrLAP [11]. The main drawbacks of OBEX are • Its stop-and-wait method for transferring objects • A limited frame size of 64 kbytes To transfer a large-size object with OBEX requires multiple sequential GET commands; none of them, however, has sufficient data to fully occupy an IrLAP window at high R (UFIR or higher), while the response to each command is additionally delayed by one tlt due to the HDL operation. As a result, the complete object transmission is delayed by the number of OBEX frames that the object consists of, times the tlt. IrBurst alleviates this delay by organizing data in streams that need not be acknowledged in a stop-and-wait fashion. Moreover, IrBurst can create multiple data streams that run over parallel TTP connections to bypass the TTP flow-control mechanism. The net result is a fully utilized IrLAP transmission window and a drastic decrease in the number of turnarounds required to transmit the object. Finally, IrDA provides standards for using IRLs as “wire-replacements.” The initial “wirereplacement” standard, known as IrComm, is part of the IrMC profile [12]. IrComm provides an interface that emulates the operation of legacy serial and parallel ports over IRLs and takes into account three things: that these ports are bidirectional, typically involve data transmission over more than one wire and are sensitive to timing. To this end, IrComm creates multiple data and control channels that emulate the electrical operation of wires, and utilizes the flow control mechanisms provided by IrLAP and TTP to emulate the software and hardware flow control of serial and parallel ports. A more recent application profile for “wire-replacement” is IrUSB now under development and not officially accepted by IrDA [13]. IrUSB aims to provide applications with a USB 2.0 emulation interface that runs solely over Giga-IR, since USB 2.0 supports R of up to 480 Mb/s.

Current OWC systems mainly use the IR band due to the availability of Tx and Rx devices originally developed for fiber-optic communication systems. However, recently, a growing number of academic and industrial interests are looking toward the potential deployment of the visible band for various OWC applications.

VISIBLE LIGHT COMMUNICATIONS Current OWC systems mainly use the IR band due to the availability of Tx and Rx devices originally developed for fiber-optic communication systems. However, recently, a growing number of academic and industrial interests are looking toward the potential deployment of the visible band (380–780 nm) for various OWC applications. This has been triggered by recent advances in solid-state technologies that have led to the development of highly-efficient (up to 60% energy efficiency) light emitting diodes (LEDs) [14]. In line with government plans worldwide to phase out incandescent light bulbs in favor of more energy-efficient lighting, it is predicted that LEDs will gradually replace both incandescent and fluorescent lights and be the ultimate light source in the near future. Visible LEDs are already widely employed for indoor and outdoor illumination. The main drivers for this emerging green technology include: longer lifetime of highbrightness LEDs compared to other light sources, higher bandwidth/data rates, higher tolerance to humidity, higher data security, mer-


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Infrastructure

Mobile

Vehicle

Yes

No

No

Power supply

Ample

Limited

Moderate

Form factor

Unconstrained

Constrained

Unconstrained

Light source

Intense

Weak

Intense

No

Yes

Yes

Range

Short/long

Short

Long

Data rates

High/low

High

Low

Fixed coordinator

Physical mobility

Table 1. VLC device classification.

Network and application layers

Logical link control (LLC) layer

Service-specific convergence sublayers (SSCS) Device management entity (DME) MAC MAC common part sublayer (MCPS)

MAC link management entity (MLME)

VLC DEVICE CLASSES AND OVERALL ARCHITECTURE

PHY PHY layer management entity (PLME)

PHY layer data (PD) PHY-switch

Dimmer Service access point (SAP) Optical media

Cell 1, 1

Cell 2, 1

Figure 3. VLC device architecture.

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cury-free manufacture, extreme power efficiency, compactness and, most importantly, fast switching characteristics [15]. The expected wide availability of LEDs in the near future will open the door to so-called “Visible Light Communications (VLCs).” Colors of commercially available and highly efficient blue, green and red LEDs can be mixed to produce a white light. These LEDs can be modulated individually to provide high-bandwidth communications. A more conventional way is to directly modulate a single LED at tens or hundreds of megahertz, without noticeable effect on the lighting output and human eyes. The dual functionality (i.e., illumination and data communication) of high-brightness visible LEDs has created a wide range of short- and medium-range communication applications. These include wireless local, personal and body area networks (WLAN, WPAN, and WBANs), vehicular networks and machine-to-machine, airplane cabin, train, ship and traffic-light communications among many others. In comparison to RF technologies, VLC offers inherent advantages, such as immunity to electromagnetic interference, unlicensed bands, physical security and a high degree of spatial confinement that allows for a much higher reuse factor. Recognizing the potential of this technology, IEEE produced the 802.15.7 standard, approved in 2011 [16, 17]. This standard defines PHY and MAC layers for short-range VLC in an optically transparent media to support audio and video multimedia services. This new standard was required due to the unique challenges evident in VLC usage profiles that render inapplicable the existing 802.11 and IrDA standards . VLCs, for example, are designed to provide permanent or semi-permanent data connections that also enable non-trivial user mobility. This is in stark contrast to IrDA P&S systems that are optimized for short interaction times and very high speeds at the expense of users remaining practically stationary. In a sense, VLCs are closer to 802.11 in terms of user mobility and connection life-time. Still, VLCs must also provide high-quality illumination that is simply beyond the scope of an evolving 802.11.

Cell #n, 1

The IEEE 802.15.7 standard considers three classes of VLC devices, namely “infrastructure,” “mobile” and “vehicle,” see Table 1 for their main features. The VLC device is comprised of a PHY layer, which contains the transceiver with intensity modulation/direct detection, and a MAC layer that provides access to the physical channel for all types of transfers. A logical LCon layer accesses the MAC layer through the service-specific convergence sub-layer. The Device Management Entity (DME) has access to certain dimmer-related attributes and provides dimming information to both MAC and PHY layers. DME also controls the PHY switch for selecting optical sources/photodetectors in devices comprised of multiple transmitters/receivers. The PHY switch interfaces to the optical Service Access Point (SAP) and connects to the optical media, which may consist of one or more transmitter/receivers. The upper layers consist of the network and application layers (Fig. 3).



THE PHYSICAL LAYER Modulation

RLL code

Manchester

Optical clock rate

200 kHz

PHY I

OOK

VPPM

4B6B

Outer code (RS)

Inner code (CC)

Data rate

(15,7)

1/4

11.67 kb/s

(15,11)

1/3

24.44 kb/s

(15,11)

2/3

48.89 kb/s

(15,11)

—

73.3 kb/s

—

—

100 kb/s

(15,2)

—

35.56 kb/s

(15,4)

—

71.11 kb/s

(15,7)

—

124.4 kb/s

—

—

266.6 kb/s

400 kHz

RS(64,32)

1.25 Mb/s

RS(160,128)

2 Mb/s

RS(64,32)

2.5 Mb/s

RS(160,128)

4 Mb/s

—

5 Mb/s

RS(64,32)

6 Mb/s

RS(160,128)

9.6 Mb/s

RS(64,32)

12 Mb/s

RS(160,128)

19.2 Mb/s

RS(64,32)

24 Mb/s

RS(160,128)

38.4 Mb/s

RS(64,32)

48 Mb/s

RS(160,128)

76.8 Mb/s

—

96 Mb/s

RS(64,32)

12 Mb/s

8-CSK

RS(64,32)

18 Mb/s

4-CSK

RS(64,32)

24 Mb/s

RS(64,32)

36 Mb/s

RS(64,32)

48 Mb/s

8-CSK

—

72 Mb/s

16-CSK

—

96 Mb/s

3.75 MHz

VPPM

4B6B 7.5 MHz

PHY II

15 MHz

30 MHz OOK

8B10B 60 MHz

120 MHz

4-CSK 12 MHz

PHY III

The standard supports the following three PHY types (Table 2 for a complete summary): • PHY I — mainly for lower R (11.6 to 266.6 kb/s) outdoor applications. It employs OnOff Keying (OOK) and Variable PPM, and also supports concatenated coding with Reed-Solomon (RS) and Convolutional Codes. • PHY II — for higher R outdoor/indoor applications (1.25 to 96 Mb/s). It also employs OOK and VPPM, but supports only RS coding. • PHY III — for applications with multiple light sources/detectors at different frequencies (colors). It uses a particular modulation format, called Color-Shift Keying (CSK), and RS coding to achieve R in the range of 12 to 96 Mb/s. The above PHYs may co-exist but do not interoperate. An IEEE 802.15.7-compliant device must implement at least one of the PHY I or PHY II types. A device implementing the PHY III type must also implement PHY II mode for co-existence. Note that multiple optical rates are provided for all PHY types in order to support a broad class of LEDs for various applications. The choice of the optical rate for communications is determined by the MAC layer during device discovery. Other details of the main PHY functionalities include: Modulation: OOK and VPPM provide a trade-off between R and the dimming range. PHYs I and II also support a run length limited coding to provide a DC balance, clock recovery and flicker mitigation. Type I utilizes Manchester DC-balancing and 4B6B encoding, while PHY II utilizes 8B10B/4B6B encoding. In PHY III, CSK modulation is generated by using 3color light sources out of the 7-color bands defined in the standard. Error-correction coding: In PHY I (outdoor applications), path losses due to long link span and the interference due to ambient and artificial light sources require more powerful concatenated coding with a combination of convolutional outer codes and RS inner codes. The RS encoder output is padded with zeros to form an interleaved boundary, which is then punctured and the result sent to the inner convolutional encoder. On the other hand, RS codes are used for indoor applications (PHYs II and III), since the coding requirements are less stringent. Channel selection: The IEEE 802.15.7 PHY can perform Clear Channel Assessment (CCA) according to one of the following three methods: • CCA Mode 1 (energy E above threshold E th ): In this mode, CCA reports a busy medium if it detects any E above Eth. • CCA Mode-2 (carrier sense only): In this mode, CCA reports a busy medium if it detects a signal with the modulation characteristics of IEEE 802.15.7. This signal may be above or below Eth. Finally, there is • CCA Mode-3 (carrier sense with E above Eth): In this mode, a busy medium is reported if a signal with the modulation characteristics of IEEE 802.15.7 exists and its E is above Eth.

FEC

8-CSK

—

16-CSK

24 MHz

Table 1. VLC PHY operating.


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The MAC layer provides two services accessed through two SAPs. The MAC management is accessed through the MAC sublayer management entity SAP (MLME-SAP), while MAC data is accessed through the MAC common part sub-layer SAP (MCPS-SAP).

Wavelength Quality Indication (WQI): WQI measurements characterize the strength and/or quality of a received frame, with the result reported to the MAC sublayer. At least seven unique values of WQI are required in a standard-compliant device. Neither its measurement nor its use by the network or application layers are specified in the standard; they are left vendor specific. In practice, WQI may be implemented using an energy detector, a signal-to-noise ratio estimator or a combination of these methods. Synchronization: A preamble field in the PHY frame is used by the transceiver to synchronize an optical clock with an incoming message. The standard defines one fast locking pattern followed by the choice of four topology-dependent patterns to distinguish between different PHY topologies. The preamble is sent at a clock rate chosen by the Tx and supported by the Rx. A time-domain sequence the preamble does not incorporate any channel coding or line coding. Dimming mechanism: Light dimming is defined as controlling the perceived brightness of the light source according to the user’s requirement, and is a cross layer function between the PHY and MAC layers. There are several schemes for dimming. It can be achieved by either redefining the “on” or “off” levels of the OOK symbol to have a lower intensity or by keeping the same levels and only changing the average duty cycle by inserting a “compensation” time in the modulation symbol [2]. Pulse Width Modulation (PWM): Another option that offers the advantage of linear dimming control. A PWM dimming scheme does not suffer from the wavelength shift caused by the current variation in amplitude-modulated dimming techniques. In PWM, the pulse duration is used to control the light-source drive current, thus adjusting the dimming level (brightness). Frequency of the PWM dimming signal is typically above 100 Hz and human eyes cannot see the current switching [2].

MAC LAYER The MAC layer provides two services accessed through two SAPs. The MAC management is accessed through the MAC sublayer Management Entity SAP (MLME-SAP), while MAC data is accessed through the MAC Common Part Sub-layer SAP (MCPS-SAP). The IEEE 802.15.7 standard supports three MAC topologies: • Star topology: communication is established between devices and a single central controller, referred to as the coordinator • Peer-to-peer topology: each device can communicate with any other device within its coverage range, with one of the two devices in an association acting as a coordinator • Broadcast topology: each device can transmit (in unidirectional mode) a signal to other devices without being associated with any device or having any devices associated with it. The standard allows the optional use of a Super-Frame (SF) structure with its format defined by the coordinator and its limits bounded by a Beacon Signal (BS) initiated by the coor-

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dinator. Beacons are used in networks that either require synchronization or support for low-latency devices. Networks not requiring synchronization or supporting low-latency devices can elect to turn off BS for normal transfers. However, BS is still required for network discovery. The standard has adopted four types of channel-access mechanisms, depending on the network configuration. The non-Beacon-Enabled Network (non-BEN) uses an unslotted random channel-access mechanism, with or without CSMA/CA. When a device needs to transmit data frames or MAC commands, it first waits for a random back-off period and then transmits its data frame. If the optional carrier sense mechanism is active and the channel is found to be busy, the device waits for another random period before trying again to access the channel. On the other hand, BEN uses a slotted random channel access mechanism, with or without CSMA/CA, where the back-off slots are aligned with the start of BS transmission. When a device wishes to start data transmission, it first locates the boundary of the next back-off slot and then waits for a random number of back-off slots. If the channel is idle, the device begins transmitting on the next available back-off slot boundary. A successful reception and validation of a data or MAC command frame can be optionally confirmed with an acknowledgment command. Acknowledgment and beacon frames are sent without using a random-access mechanism. The MAC layer, which also handles all access to the PHY, is responsible for the following main tasks [16]: Procedures for initiating/maintaining a VLC network: VLC devices perform channel scanning to assess the current state of a channel (or channels), locate all beacons within its operating space or locate a particular beacon if synchronization has been lost. Results of a channel scan are used to select an appropriate logical channel along with a VLC network identifier not being used by any other VLC networks in the same area. Association/disassociation procedures: outline the conditions under which a device may join a VLC network and the conditions for a coordinator to permit devices to join in. Similarly, a disassociation procedure exists, which can be initiated by the associated device or its coordinator. Color-function support mechanism: provides device information to the human eye via color. Various device states such as the device discovery (scan, association and disassociation), file transfer status, wavelength quality indication and acknowledgments can be visually indicated to the user to help with device alignment for communications. Illumination and dimming support mechanisms: feature extended preamble mode for providing illumination with improved synchronization performance, dimming overrides, adjusting the MAC layer transmission schedule to accommodate dimming, association and link adaptation in the presence of dimming. Mobility support mechanism: supports mobility of the device under an infrastructure that supports multiple optical elements over a wide coverage area (“cells”).



Color stabilization: used to stabilize the optical color emitted by the Tx. The color-visibilitydimming frames are used to estimate the change in color and this information can be provided as feedback to the Tx to stabilize its color.

CONCLUSIONS AND FUTURE DIRECTIONS We have presented the three most important families of standards for OWCs maintained by IEEE and IrDA. IEEE standards focus mainly on the end-user access, providing permanent connections, moderate line data rate and acceptable user mobility. VLCs, in particular, can also provide high-quality illumination without additional equipment. On the other hand, IrDA systems target “P&S” applications and are optimized for short interaction times and very high capacities. With respect to future standardization endeavors, the main evolutionary drive in IrDA standards at the time of writing this article is the migration to link speeds of over 10 Gb/s. Currently, a Special-Interest-Group (SIG) is working on 5 Gb/s and 10 Gb/s IrPHY and IrLAP standard extensions, aiming to achieve link speeds currently unattainable by competing RF technologies. We must also consider the future evolution of 5G technology offering possibly a cellular optical wireless PHY option. Such a cellular architecture could borrow from the IrDA protocol stack experience, especially for the Device-to-Device mode as well as from VLC technology developments. It is likely therefore that new versions of standards in such cases would emerge. An interesting evolution of IrDA would be the development of ‘zero delay’ links. Such links would be important in very fast connection and payment situations [18] where large crowds are involved using low cost short-range links, in large volume applications (i.e. ticket payment, shopping) within mobile devices. The SIG is also working on alleviating some of the cumbersome features of Giga-IR, such as docking. To this end, future standards are being designed to support longer L (>1 m), hand-held operation of devices and broader coverage by implementing optical beaming, spotting and Multiple Input Multiple Output techniques. Finally, future IrDA links will be more efficient networking-wise and will support full-duplex mode operation, where devices are able to transmit and receive data simultaneously.

REFERENCES [1] S. Arnon et al., Advanced Optical Wireless Communication Systems, Cambridge University Press, 2012. [2] Z. Ghassemlooy, W. O. Popoola, and S. Rajbhandari, Optical Wireless Communications — System and Channel Modelling with Matlab, CRC Press, 2012. [3] IEEE 802.11 — Standard for Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, June 1997. [4] IrDA, Serial Infrared Physical Layer Specification — Giga-IR Addition Version 1.0, Infrared Data Association, 2009. [5] IrDA, IrDA Serial Infrared Link Access Protocol Specification for Giga-IR Addition, 2009. [6] K. Yiannopoulos and A. C. Boucouvalas, “Link Layer Protocol Analysis of Future 10 Gb/s Infrared Links,” IET Communications, vol. 8, issue 12, 2014, pp. 2114–20. [7] IrDA, Link Management Protocol (IrLMP) — Version 1.1, Infrared Data Association, (1996). [8] IrDA, Specifications for Ir Mobile Communications (IrMC) — Version 1.1, Infrared Data Association, (1999). [9] IrDA, Object Exchange Protocol (OBEXTM) — Version 1.5, Infrared Data Association, (2009).

[10] IrDA, Infrared Financial Messaging (IrFM™) Point and Pay Profile — Version 1.0, Infrared Data Association, 2002. [11] IrDA, IrBurst™ High-speed Information Transmission Profile — Version 1.0, Infrared Data Association, (2007). [12] IrDA, “IrCOMM”: Serial and Parallel Port Emulation over IR (Wire Replacement) — Version 1.0, Infrared Data Association, (1995). [13] IrDA, Infrared Universal Bus Specification — Version 0.11, Infrared Data Association, (2010). [14] R. D. Dupuis, and M. R. Krames, “History, Development, and Applications of High-Brightness Visible Light-Emitting Diodes,” IEEE J. Lightwave Tech., vol. 26, no. 9, May 2008, p. 1154–70. [15] P. R. Boyce, Human Factors in Lighting, 3rd ed., CRC Press, 2014. [16] IEEE 802.15.7 — Standard for Short-Range Wireless Optical Communication Using Visible Light, 2011. [17] S. Hranilovic, L. Lampe, S. Hosur and R.D. Roberts, “Visible Light Communications: The Road to Standardization and Commercialization (Part 2),” IEEE Commun. Mag., vol. 52, no. 7, July 2014. [18] P. Huang and A. C. Boucouvalas, “Future personal “e-payment”: IrFM,” IEEE Wireless Commun. Mag., vol. 13, no. 1, Feb. 2006, pp. 60-66.

BIOGRAPHIES A. C. BOUCOUVALAS ([email protected]) is a professor of communication networks and applications at the University of Peloponnese in Tripoli, Greece. He has been involved with research in various aspects of fiber-optic communications, wireless communications and multimedia, and has accumulated 25 years of experience working in well-known academic and industrial research centers. He was a professor of multimedia communications and director of the Microelectronics and Multimedia Communications Research Centre of Bournemouth University, in the UK. He has worked as a project manager for Hewlett-Packard Laboratories and as group leader and divisional chief scientist at GEC Hirst Research Laboratories, both in the UK. His work, supported by numerous research grants and contracts from European Union and industrial organizations, has been published in over 300 publications. He is a Fellow of the Institution of Electrical Engineers (UK) and of the Institute of Electrical and Electronic Engineers (USA). He has also been a Member of the Architectures Council of the Infrared Data Association (IrDA) since 2003.

Currently, a SpecialInterest-Group (SIG) is working on 5 Gb/s and 10 Gb/s IrPHY and IrLAP standard extensions, aiming to achieve link speeds currently unattainable by competing RF technologies. The SIG is also working on alleviating some of the cumbersome features of Giga-IR, such as docking.

PERIKLIS CHATZIMISIOS ([email protected]) is an associate professor with the Computing Systems, Security and Networks (CSSN) Research Lab at Alexander TEI of Thessaloniki, Greece. He is involved in several standardization activities and serves on the Standards Development Board of the IEEE Communication Society (ComSoc) (from 2010 until today). He is acting as the director for the ELetter of the IEEE Technical Committee on Multimedia Communications (MMTC). He is the author/editor of eight books and more than 80 peerreviewed papers on the topics of performance evaluation and standardization activities of mobile/wireless communications and networked multimedia. His published research work has received more than 1300 citations. ZABIH GHASSEMLOOY ([email protected]) is a professor in optical communications on the Faculty of Engineering and Environment, Northumbria University at Newcastle, UK. He heads the Northumbria Communications Research Laboratories within the Faculty. He is the editor-in-chief of the International Journal of Optics and Applications and The Mediterranean Journal of Electronics and Communications. His research interests are on photonics switching, optical wireless and wired communications, free-space optics, visible light communications, radio over FSO/fiber, and mobile communications. He has received a number of research grants from a number of UK Research Councils, the UK government, the European Union and industry and has supervised almost 50 Ph.D. students and published over 520 papers ( in 200 journals) and presented several keynote and invited talks. He is a coauthor of a CRC book, Optical Wireless Communications — Systems and Channel Modelling with Matlab (2012). From 2004–2006 he was the IEEE UK/IR Communications Chapter secretary and its vice-chairman (2004–2008) and chairman (2008–2011), and chairman of the IET Northumbria Network (Oct 2011–present). MURAT UYSAL ([email protected]) is a professor at Ozyegin University, Istanbul, Turkey, where he leads the Communication Theory and Technologies (CT&T) Research Group. Before joining Ozyegin University, he was a tenured associate professor at the University of Waterloo, Canada, where he still holds an adjunct faculty position. His research interests are in the broad areas of communications theory and signal processing with particular emphasis on the physical layer aspects of wireless communication systems in radio-, acoustic-, and optical-frequency bands. He has authored more than 200 journal and conference papers on these topics and received more than 3400 citations. KONSTANTINOS YIANNOPOULOS ([email protected]) is a lecturer at the University of Peloponnese, Greece. He was a member of the research teams of the Photonics Communications Research Laboratory at the National Technical University of Athens, Greece (2000–2004), and the Computer Networks Laboratory at the University of Patras, Greece (2005–2010). At present, his research focuses on optical wireless networks (system architectures and protocol analysis). He has more than 40 published papers in international journals and conferences. His research work was granted the IEEE/LEOS Graduate Student Fellowship Program Award for 2004 and he has received more than 600 independent citations.


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