Report, TSKS03 Wireless Systems

Transcription

1 1 Report, TSKS03 Wireless Systems Infrared Communication, kasma064 May 9 th, 2016

2 2 Contents Acronyms, Tables and Figures Abstract 5 2 Introduction IR Configurations IR Communication Channel 6 3 IR Transceiver Design Noise Sources, BER and Sensitivity Optical Sources 9 4 Modulation Schemes Multiple Access Techniques 13 5 IrDA Protocols 14 6 Limitations 17 7 Conclusion 18 8 References 19

3 3 Acronyms FOV LOS Non-LOS IrDA AWGN D-PIM PPM IM/DD PIN CMU CDR RMA OOK DPPM MLSD WDMA SDMA BER SIR MIR FIR VFIR UFIR GigaIR ISI Field of View Line of Sight Non-Line of Sight Infrared Data Association Additive White Gaussian Noise Digital Pulse Interval Modulation Pulse Position Modulation Intensity Modulation/Direct Detection Positive-Intrinsic-Negative Diode Clock Multiplier Unit Clock and Data Recovery Receiver Main Amplifier On-Off Keying Differential Pulse Position Modulation Maximum Likelihood Sequence Detection Wavelength Division Multiple Access Space Division Multiple Access Bit Error Rate Standard Infrared Medium Infrared Fast Infrared Very fast Infrared Ultra fast Infrared Infrared in Gigabits Intersymbol Interference

4 4 EGC MLC SD MRC TDMA CDMA Equal Gain Combining Maximum Likelihood Combining Selection Diversity Maximum Ratio Combining Time Division Multiple Access Code Division Multiple Access List of Figures Figure 1: Various Possible IR Configurations 6 Figure 2: A Simple Channel Model 7 Figure 3: Block of an optical Wireless Link 8 Figure 4: Transceiver Schematic 9 Figure 5: Sources of Noise in a Receiver 10 Figure 6: Pulse Position Modulation Scheme 10 Figure 7: 4-PPM and 4-DPMM Scheme 11 Figure 8: L-PPM Schemes 12 Figure 9: OOK with NRZ and RZ Pulse Figure 10: SDMA Technique 13 Figure 11: IrDA Protocol Stack 14 List of Tables Table 4.1: Mapping of source into 4-PPM and 4-PIM 12

5 5 1 Abstract This report describes about infrared communication technology in the context of wireless communication systems. The sections below provide an overview of infrared communication including a brief introduction followed by an explanation of transceiver design, modulation schemes, IrDA protocols and a brief discussion on some of the limitations of infrared communication technology. 2 Introduction Infrared communication is a form of optical communication that has been around for a long time now. One of the earliest use of infrared technology in wireless communication was the remote control for television and Video Cassette Recorders (VCRs). The Infrared Data Association (IrDA) cites that the application of infrared communication for television remote control was explored in the 1980s. But the first experiment of examining the feasibility of IR communication was performed in the 1970s when some computer terminals were connected with an IR transceiver over a short distance. Most of the remote controls for DVD players, Satellite to home boxes for TVs today are based on infrared communication technology. Since IR communication involves transmitting data in a wireless manner, the technology as a whole, is now bracketed under wireless communication systems. As will be seen in further sections, the application of IR communication technology is not restricted to just a remote control, but extends to various indoor and outdoor applications; specifically, in many entertainment devices and in the field of health care monitoring as well. Within the domain of wireless communication systems, the key advantages offered by IR are: ease of use, small size, low cost implementation and capability to offer huge bandwidths which is unregulated worldwide. IR is also immune to electromagnetic interference and offers a certain level of inherent data security as IR does not go through the walls. 2.1 IR Configurations Two important parameters define the sort of wireless IR link that can be configured in an IR communication system as illustrated in Figure 1. They are: Line of Sight (LOS) or Non Line of Sight Configuration. Directed, non-directed or hybrid configuration in the transmitter and receiver. The transmitter/receiver are unobstructed and view each other directly in the LOS configuration. A Non-LOS configuration takes into account the possibility of an obstructive path between the transmitter and the receiver, thus reflective surfaces are widely used to reduce multipath distortion.

6 6 Figure 1: Various Possible IR Configurations [1] If the transmitter and the Field of View (FOV) of the receiver are restricted, then the configuration is direct. When both the transmitter and receiver have a broad emission range and wide FOV respectively, they are classified as non-directed systems, as illustrated in Figure 1-c. When either the transmitter or receiver is restrictive, then the configuration is termed hybrid, shown in Figure 1-f. A popular configuration is the diffuse configuration, which means that it is non-directed and non- LOS. In the diffuse configuration the transmitter and the receiver employ wider beam emission and the field of view respectively. If the wall surface and ceiling within the room are reflective, then a uniform spread of the IR energy, with lesser reflection loss can be ensured. This configuration results in a more portable and robust IR system. Figure 1 shows the two popular diffuse and quasi diffuse IR configurations. 2.2 IR Communication Channel Intensity modulation with direct detection is the method used for data transmission and reception in

7 7 a vast majority of applications which use IR communication. With the help of a Light Emitting Diodes (LEDs) or a Laser diodes (LDs) the electrical signal data can be modulated onto an optical IR carrier. The detector usually employs a photodetector which operates by converting the received optical power to an equivalent current. Background illumination is the deciding factor for modeling the optical channel. Signal at the receiver is modeled as Poisson process for low background illumination and as Additive White Gaussian Noise (AWGN) for higher background illumination. Some of the common sources of background illumination are fluorescent lamps in indoor systems and sun light in the case of outdoor systems. Figure 2: A simple channel model [1] The channel model for a IM/DD wireless system can also be described by the equation: where x(τ) is the transmitter optical signal, n(t) is the Gaussian noise, h(t) is the multipath impulse response. If the channel is to be defined in terms of its frequency response then the corresponding transfer function is given by: 3 IR Transceiver Design The design of a transmitter involves several constraints. The key constraint being eye safety. This

8 8 results in restriction in the amount of power that can be transmitted. The key issue of background illumination has to be considered while designing the receiver section. The block diagram of the transmitter/receiver section is shown below in Figure 3. Figure 3: Block of an Optical Wireless Link [1] The electrical signal is converted to an optical signal at the transmitter side. This task is accomplished by the drive circuit in the transmitter with the help of a suitable light source such as a laser diode or a light emitting diode. The receiver end usually consists of a photodetector which uses either a positive-intrinsic-negative(pin) diode or the avalanche photodiode (APD). Both the transmitter and receiver are implemented on an integrated circuit, as a transceiver. The block diagram of such a system is shown below in Figure 4. A MUX combines the digitized data into a single data stream. A clock multiplication unit (CMU) is then used to combine a slower word clock to a faster bit-rate-clock for improving the data timing. This is followed by an optical drive portion which consists of the laser driver and a modulator driver which work towards data modulation. Design of the receiver is dependent on the modulation scheme implemented on the transmitter side. The receiver block performs the reverse action. When the photodiode receives the optical information, it converts the information into a current. The preamplifier section amplifies and converts the current to a suitable voltage. Further voltage amplification is performed by receiver main amplifiers Figure 4: Transceiver Schematic [1]

9 9 (RMA). The received data information is timed again by the clock and data recovery circuit (CDR) and fed into a demux, which converts serial data into parallel data for further processing. 3.1 Noise sources, sensitivity and BER Noise is an inevitable component and plays a role in IR communication as well. The sensitivity of an IR receiver is dependent on the noise. The concepts of noise figure/factor used in RF communication can be conveniently applied in IR communication. Noise at the receiver end could either be signal dependent or independent. Figure-5 below shows the various sources of noise. The key sources of noise are the following three: Noise contribution from the optical source. Noise contribution due to atmospheric turbulence and background illumination. Noise contribution from the receiver block. Within IR communication links, sensitivity and bit-error rate are considered to be inter dependent. Sensitivity is an indicator of reliability of the receiver to detect optical data in spite of atmospheric attenuation on the optical data. Usually, bit error rate is performance measure in digital communication. BER plot is usually employed to study the performance. The BER is plotted either as a function of the received power or the amplifier input current. The bandwidth of a receiver is fixed at around 65% of the bit rate. 3.2 Optical Sources Figure 5: Sources of noise in a receiver [1] Optical source forms an essential part of an optical communication system. High intensity and good monochromaticity are the two factors which decide the type of light source for communication. Laser diodes and LEDs are the most commonly used light sources. The choice of LDs or LEDs are application dependent. When speed is deemed necessary, LDs are preferred over LEDs. In applications where temperatures could play a crucial role, LEDs are preferred over LDs.

10 10 4 Modulation Schemes The overall system performance is paramount in the choice of modulation technique. The various modulation techniques are characterized by a certain bandwidth and efficiency(high speed transmission). Pulse Position Modulation (PPM) and on-off keying are two most popular modulation techniques employed in infrared communication systems. The modulation technique generally involves converting the analog signal into samples and these samples are transmitted with the help of pulses that are modulated in terms of the position with reference to the analog signal amplitude. Three variants of pulse modulation schemes have been tried. Figure 6: Pulse Position Modulation Scheme L-PPM, where there are 'L' slots in a single symbol, called chips. Only one chip slot carries the optical power, whereas the other L-1 chips will not transmit power. It has been seen that L-PPM technique has some drawbacks due to Intersymbol Interference (ISI) and as such Maximum Likelihood Sequence detection, equalization, rate 2/3 trellis coded 8-PPM, rate 3/4 trellis coded 16- PPM techniques are used to overcome ISI problem. The different schemes are shown in Figure 8. D-PPM, also called Differential PPM is a technique used when multipath distortion is expected to Figure 7: a) 4-PPM Scheme b) 4-DPPM Scheme [5] be low. This is typically used in direct LOS configurations. Consider a 4-PPM symbol with four chips. Usually, one of the chip is high and the remaining three chips are low. A D-PPM can be obtained by removing the low chips that follow a high chip. MLSD technique is used for symbol decoding as the symbol boundaries are not known previously. The schemes are shown in Figure 7.

11 11 Figure 8: L-PPM Schemes [1] D-PIM, also known as digital pulse interval modulation is another technique where the input signal amplitude determines the intervals between the narrow pulses and each successive time frame commences immediately after the previous pulse. Based on the content of the symbol, the length of the symbol varies. It is possible to achieve higher bit rates using D-PIM than using L-PPM. The table below shows the mapping of source bits and chips for transmission with 4-PPM and 4-PIM.

12 12 Source Bits 4-PPM Chips 4-PIM Chips (0) (0) (0) (0)000 Table 4.1: Mapping of source into 4-PPM and 4-PIM [3] On-off Keying(OOK) is another popular technique which is used for modulation in IR communication due to the ease of implementation. OOK also offers good bandwidth efficiency. In principle, OOK is a form of Amplitude Shift Keying (ASK) where the carrier amplitude is increased for '1' transmission and the carrier amplitude is decreased for '0' transmission. Since OOK is a variant of ASK, the additional feature is that the carrier amplitude being completely OFF for a '0' transmission. Some authors have discussed using both return-to-zero(rz) pulses and non-return-tozero(nrz) pulses for representation; but with a higher preference for the latter technique. Duty cycle for the pulses could change. When the value of 'gamma' is < 1, then average power is decreased and bandwidth factor is increased by 1/gamma. OOK receiver could either be a continuous time matched filter, or a whitened matched filter based on whether multipath distortion is taken into consideration or not. The waveforms for the NRZ and RZ schemes are shown in the two equations and in Figure-9 shown below. where p Tb (t) = 1 for 0 t T b and p Tb (t) = 0 elsewhere, and a belongs to {0, 1}. Here, the peak optical power P peak is related to the average optical power P avg as follows: P peak = 2P avg. Figure 9: a) OOK with NRZ pulse b) RZ pulse [6]

13 Multiple Access Techniques The need for multiple access techniques arise due to constraints like limited hardware in a transceiver system and limited spectrum availability. In the context of IR communication, multiple access techniques describe the ways of allocating access to the channel for different users. These techniques are mostly defined at the physical layer of IR communication system. Two popular methods for multiple access are: 1) Electrical Multiple Access and 2) Optical Multiple Access. Optical features of IR are managed to allocate access to the users. Two methods of optical multiple access techniques that have been used in IR are Wavelength Division Multiple Access (WDMA) and Space Division Multiple Access (SDMA). Figure 10: SDMA Technique implementation [4] In the WDMA scheme, users transmit information at different wavelengths. This is similar to Frequency Division Multiple Access (FDMA), except that WDMA relates to optical signals. Realizing a WDMA transceiver is relatively simple, with the use of just a laser diode with narrow line-width and multiple photodetectors at the receiver end to detect wavelengths. In complex applications, using a WDMA system results in higher cost as the number of terminals operating at various wavelengths increases. The equation for interference model is shown below. X j (t), j= 1,2,, N is the transmissions that are incident on the receiver. h j is the channel impulse response between the jth transmiter and the receiver. Y(t) denotes the photocurrent. Studies also show that performance of WDMA scheme degrades because of Optical Beat Interference (OBI). This phenomenon occurs when the two or more of the nearly similar wavelengths are incident on

14 14 the receiver. Some solutions to overcome this problem are: Over-modulation of the laser transmitter. Using Forward Error Correction (FEC) with out-of-band clipping tones. Modulating every laser transmission with large CDMA signal. When using WDMA system with multiple terminals, the overall cost and complexity of the system increases. This is because of the need for tunable lasers at the transmitter end and high precision tunable band pass filters at the receiver end to detect the various wavelengths. Space Division Multiple Access (SDMA) offers an alternative. SDMA technique involves using an Angle Diversity Receiver (ADR) and detects signals from multiple directions. In combination with a variety of quasi-diffuse transmitters, ADRs can be configured for a specific application. ADRs use a number of techniques like Equal Gain Combining (EGC), Maximum Likelihood Combining (MLC) Selection Diversity (SD) and Maximum Ratio Combining (MRC) for detection and processing the received signals. SDMA techniques using LOS transmitters and quasi-diffuse transmitters are shown in Figure 10. Among the electrical multiple access techniques, Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) are mostly used in IR communication. Transmission in TDMA occurs in various time slots. Better power efficiency is achieved due to low duty cycle. The disadvantage with TDMA is system complexity as a consequence of high level of coordination required for good synchronization. In CDMA there is simultaneous transmission since orthogonal or quasi-orthogonal codes are used by multiple users. Various multiple access schemes in combination with modulation techniques have been studied. These include TDMA with 2-PPM, TDMA with 4- PPM, TDMA with OOK, CDMA with Optical Orthogonal Codes (OOC), CDMA with m- sequences, and FDMA with BPSK. 5 IrDA Protocols There is a variety of protocols available for wireless communication. Protocols exist for both RF as well as infrared communication, the popular ones for RF communication being the IEEE protocol and Bluetooth. Protocols for infrared communication have been developed and specified by the Infrared Data Association, also called IrDA. IrDA works towards a common interoperability standard between the various components, peripherals, hardware and software for using infrared communication. The association is composed of: the architectural council and Special Interest Groups (SIGs) which are involved in protocol development and specifications; and various committees which look into exploring new market opportunities and provide servicing.

15 15 Figure 11: IrDA Protocol Layers [2] The protocol stack is shown in the Figure 11. The stack arrangement is in a layered manner with the bottom most tier being the physical layer. The various stack layers are explained below. Physical Layer: This is the bottom most layer in the protocol and provides definitions for the physical components used in IR communication. Manufacturers are to follow compliance with certain optical, physical and electrical characteristics for both the transmitter and receiver design. Generally, the characteristics would be operation wavelengths, data rates, FOVs for emission and detection, rise and fall times, bit error rates (BERs), pulse duration and range. The April 1994 spec release supported kbps and the current release aims at 5-10 Gbps. The different transceivers in the physical layer are listed below: SIR: kbit/s, asynchronous, RZI, UART-like, 3/16 pulse MIR: Mbit/s, RZI, 1/4 pulse, HDLC bit stuffing FIR: 4 Mbit/s, 4PPM VFIR: 16 Mbit/s, NRZ, HHH(1,13) UFIR: 96 Mbit/s, NRZI, 8b/10b GigaIR: 512 Mbit/s 1 Gbit/s, NRZI, 2-ASK, 4-ASK, 8b/10b 5/10GigaIR: seems to be a new IrPHY coming soon

16 16 Some further characteristics are mentioned below. Range: standard 1 m; low-power to low-power 0.2 m; standard to low-power 0.3 m. The 10 GigaIR also define new usage models that supports higher link distances up to several meters. Angle: minimum cone ±15 Speed: 2.4 kbit/s to 1 Gbit/s Modulation: baseband, no carrier Wavelength: nm Frame and Driver: This is the software layer in the protocol stack. The components which are used in the physical layer would interact with the upper layers with the help of software and hardware which is defined within this layer. The framer converts data into frames for transmission purpose. It consists of boundary bytes, synchronization bytes and error detection bytes. The controller is defined at both the transmitter and receiver. At the transmitter, it drives the optical source based on the data from the stack. At the receiver side, it converts the data into an understandable form for the layers in the stack. The driver is responsible for controller initialization and information transfer. IrLAP: This layer is similar to the data link layer in OSI architecture. Some functions of this layer are access control, discovery of potential communication partners, establishing a bidirectional connection, distribution of the primary/secondary device roles, negotiation of QoS parameters Link Management Protocol: With the help of IrLAP layer, this layer is responsible for access to various application and services. Information Access Protocol: This layer is responsible for discovery, registration and service access. Tiny TP Protocol: This layer manages flow control at link management protocol layer for segmentation and reassembly (SAR) for efficient channel use. Session and Application Protocol: This layer speceifies rles of exchange. The protocols include IrOBEX, IrComm, IrWW, IrTran-P, and IrLAN.

17 17 6 Limitations Some of the limitations faced by IR communication technology are listed below. Factor such as whether the system is installed for indoor or outdoor application goes on to introduce certain limitations in the data rate. They are as follows: Atmospheric Disturbances: The effects of atmospheric disturbances are more pronounced when the system is used in an outdoor environment. Certain conditions such as fog and haze can greatly hinder the performance. Attenuation from fog and haze could reach upto 300dB/km. A similar effect is seen when there is smoke in the surroundings. Even communication between a distance of 100m becomes extremely difficult in the above situations. Studies have shown that rain and snow do not have much effect on the data communication. In addition to all the above effects, scintillation and air absorption are known to cause problems for long distance transmissions. Use of novel adaptive optical techniques, coding and equalization have been studied and when applied, have shown to mitigate the atmospheric disturbances. Eye Safety: While it is true that higher optical power at the transmitter end would improve the system SNR, eye safety considerations are paramount, hence there is a limitation in the maximum transmitted power. Medical studies confirm that near-ir radiation above a threshold could cause retinal burns and medium/far-ir radiation could cause corneal burns. This is one of the major constraints in IR communication. Noise due to Ambient Illumination: This is one of the unavoidable contributor to introducing data rate limitations since the IR transmitter could be exposed to light from lamps in the indoors or from the sun in the outdoors which introduce noise in the detector at the receiver end of the system and result in a diminished SNR. Two techniques are commonly employed to overcome the issue of noise due to ambient light source. Firstly, a narrow FOV receiver followed by the use of optical filters (either bandpass or longpass). Multipath distortion: This effect is mainly seen in diffuse configuration. Since this configuration employs an expanded receiver FOV, there are now multiple paths for the energy. Further a phenomenon called temporal dispersion also leads to ISI.

18 18 7 Conclusion This work has explained the concept of infrared communication within the domain of wireless communication systems. Infrared Communication along with radiofrequency communication can play an important role in wireless systems. With features like higher bandwidths, low cost, better security features, smaller size and low power consumption, infrared communication offers some advantages compared to radio communication. This is even more pronounced in a hospital setting where interference of RF communication with medical equipments such as a pacemaker could lead to fatal consequences. So in a hospital setting IR communication is clearly advantageous. But IR communication is not without drawbacks. As seen in the previous section, issues such as attenuation due to blocking/obstruction, disruption in the link, background illumination noise and other atmospheric limitations, eye safety affect the quality of data communication using infrared technology. This leads to the question of whether IR communication could take the place of RF communication. This is both debatable and doubtful. IR communication can only complement RF communication in certain settings. Currently, studies are underway to develop an integrated hybrid system incorporating both RF as well as IR communication. Such a hybrid would exploit the bandwidth benefits of IR. Such a hybrid system would also include a handover mechanism between RF and IR for throughput optimization. A benefit of this design is compatibility with existing RF devices and improved scalability. Such systems are presently being developed for indoor environments prone to data congestion. NASA has developed a two way Lunar Laser Communication Demostration (LLCD) using near infrared for interplanetary probes. LLCD is a space terminal that transmits and receives data between the moon and the ground station. Two way communication has also been developed in Satellite to Home boxes and its remotes. The implementation is in a way such that data is received by the remote from the boxes, which can then used for updating the software. The same channel could be used for communication between TV, box and the remote. Further, IrDA is also working towards improving and compensating certain drawbacks in IR technology. The biggest factor that works in favor of IR communication is economic feasibility, so with a number of research experiments being carried out in the field of IR technology, we would certainly see IR communication introduced in additional settings in the near future.

19 19 References [1] Roberto Ramirez-Iniguez, Sevia M. Idrus, and Ziran Sun, Optical wireless communication for IR for wireless connectivity, Boca Raton,CRC Press, 2008 [2] [3] Marijan Herceg, Tomislav Svedek, and Tomislav Matic, "Pulse Interval Modulation for Ultra- High Speed IR-UWB Communications Systems", EURASIP Journal on Advances in Signal Processing, 2010, Volume 2010, Number 1, Page 1. [4] Joseph M. Kahn, Roy You, Pouyan Djahani, Amy G. Weisbin, Beh Kian Teik, and Andrew Tang, "Imaging Diversity Receivers for High-Speed Infrared Wireless Communication", IEEE Communications Magazine, pp , December [5] Da-shan Shiu, and Joseph M. Kahn, "Differential Pulse-Position Modulation for Power-Efficient Optical Communication", IEEE Transactions on Communication, Vol. 47, No. 8, pp , August [6] J.M. Kahn and J.R. Barry, Wireless Infrared Communications, Proceedings of the IEEE, 85(2), , 1997 [7] [8] Rahaim, M. B., Vegni, A. M., and Little, T. D. C., A hybrid radio frequency and broadcast visible light communication system, in IEEE Global Communications Conference (GLOBECOM 2011) Workshops, Dec. 5-9, 2011, pp