Design of an Analog Electronic Interface for a Power Line Based Telephony System

Transcription

1 Design of an Analog Electronic Interface for a Power Line Based Telephony System Alejandro Barreras Gutierrez University of Technology, School of Engineering Department of Electrical Engineering Hope Road, Kingston 6, Jamaica [email protected] Audley Darmand University of Technology, School of Engineering Department of Electrical Engineering Hope Road, Kingston 6, Jamaica [email protected] Victor Watt University of Technology, School of Engineering Department of Electrical Engineering Hope Road, Kingston 6, Jamaica [email protected] Lucien Ngalamou University of the West Indies, Department of Physics Electronics and Computer Systems Group Mona Campus, Kingston 7, Jamaica [email protected] Abstract This paper considers the use of in-building electrical wiring in as the communication channel for the deployment of telephony system for homes and small businesses in Jamaica. The paper presents the design considerations and implementation of the analog electronic interface to the power line infrastructure and presents results from actual powerline channel measurements in a laboratory setting as well as in an existing inbuilding electrical wiring application.. The paper also presents some considerations for the integration of the powerline communication system with existing telephony standards, ranging for standard analog phone sets to the emerging Voice Over IP (VOIP) standards such as the Session Initiation Protocol (SIP). on hold among others [13]; The other element is a multifunction power line-to-phone set transceiver which, by design, will allow the standard digital phone sets available in the market to interface with the Private Automatic Branch Exchange (PABX) board. The PABX board communicates with each user so each user must have a user phone set transceiver board. Figure 1 shows the system configuration. Keywords: Power Line Communications, Multi-Carrier Code Division Multiple Access, Private Automatic Branch Exchange. I. INTRODUCTION Communication by telephone for many small companies is a medium by which they interact with their customers and staff. These companies use small telephony systems for their daily activity, but most of the systems that are available are designed to accommodate large numbers of users and they are extremely expensive. This system presents a solution to those who are developing small companies and businesses around the world and are looking for a low cost and reliable way to communicate. The system can be divided in two major elements: the first element is a Peripheral Component Interconnected (PCI- PABX) module, designed for installation in a PCI slot of a PC. It includes an analog unit to interface with the Public Switched Telephony Network (PSTN) and a Signal Processing Module, which will perform the features of a standard switchboard, plus other specific functions such as caller identification, call waiting, call transfer, automatic interactive voice interface, multiple-user account setting, automatic user billing and music Figure 1. System Configuration The design of the entire system is conceived as a combination of three different sub-systems, due to the complexity and the extension of the system design. The block diagram in Figure 2 shows the system design approach divided into the three different sub-systems. Figure 2. The Communication System Design /06/$20.00 c 2006 IEEE. 232

2 This paper presents a design solution for the analog electronic interface of the telephony system. II. STUDY AND MEASUREMENT OF THE POWER LINE CHANNEL Power Lines are not designed to transmit data at high frequencies. High frequency transmission by PLC is noisy. Usually signals above a few KHz experience severe unbalanced attenuation and frequency fading exists when transmitting across power lines [1], [5], [9], [10], [23]. The attenuation increases dramatically with the transmission distance and network structure. This includes the distribution of the connected loads in the network, such as the appliances plugged into the low power grid or sub-circuits in the high power topology as well as the time of the day and the day on which the system is being analyzed [11]. In order to analyze the attenuation characteristics of a given power line, a general measurement system was implemented. A diagram of the entire system is shown in Figure 3. Attenuation (db) 0,0-0,5-1,0-1,5-2,0-2,5-3,0-3,5-4,0-4,5-5,0-5,5-6, Frequency (MHz) Figure 4. Attenuation characteristic of the Isolated Cable It was observed that even for the highest frequency values where the attenuation is higher; a communication system implementation is still possible. This test does not accurately represent a real power line communication system. The second test was run using the real physical power line network present in the lab represented in Figure 5 bellow which simulates a typical household with appliances connected. Figure 3. The Power Line Measurement System The system is composed of a receiver and a transmitter coupling properly connected to the communication channel. The function generator sweeps a signal through the coupling network from 150 KHz to 30 MHz when measuring the attenuation parameters. The experimental test carried out to obtain the attenuation characteristic of the power line was run twice, first over a set of wires with absolutely no loads connected, transmitting no power and similar characteristics to the one used for the electrical installation and the second over the real physical network existing in the lab. The signal at the input of the power line before it was injected into the coupling circuit was set to 4 Volts RMS, recorded, and maintained throughout the test. A variable gain buffer amplifier is placed at the output of the function generator to prevent impedance mismatching and the voltage drop of the generator output signal. The voltage at the output of the receiver coupling circuit changed while the frequency was swept from the minimum of 250 KHz to the maximum value of 30 MHz. This voltage was recorded during the test as well as the frequency values. The attenuation characteristic graph for the first case, where no 50 Hz main power wave was present and no loads where connected to the network, is shown in Figure 4. Figure 5. Laboratory Power Line Network Configuration The diagram above shows the distance between each outlet and the service panel. The total distance between the transmitter and the receiver is 29 feet. The obtained attenuation characteristic is shown on Figure 6. It shows that there are some significant attenuation notches at certain frequency values. The location and intensity of these signal fading points will change depending on where appliances are plugged into the network and on the topology of the network. This behavior may be expected for all power line networks around the world. Recent publications from the developed world reveal similar attenuation characteristics with just a few differences such as the overall attenuation level and the positions of the notches [5], [7],[8],[12]. 233

3 Attenuation (db) 0,0-2,0-4,0-6,0-8,0-10,0-12,0-14, Frequency (MHz) Figure 6. Attenuation Characteristic of the Real Physical Power Network in Lab, Showing Notches The critical points of attenuations and the impedance value of a power line to be used as high frequency data transmission channel cannot be predicted because the configuration and the characteristic of each particular network, is network dependent and time dependent. The characteristic of the test bed showed a transmission frequency of 25 MHz is best selected as it is in the region of least attenuation and is a more stable bandwidth. Depending on the location where the system is tested and the connected loads these parameters may change. One way to get around this complex channel behavior and its effects is through the proper design of band pass filters and coupling circuits. The implementation of suitable transmission schemes and good error correction methods will vastly improve the performance of the PLC system. The use of MC-CDMA over OFDM specifically for this system is justified due to the fact that OFDM applies coding to avoid an excessive BER on sub-carriers that are in a deep frequency fading. Thus the number of needed sub-carriers is larger than the number of bits or symbols transmitted simultaneously. MC-CDMA replaces this encoder by an N x N matrix operation. In other words, MC-CDMA offers better frequency diversity to combat frequency-selective fading, one of the main issues seen in power lines at high frequencies [18]. In MC-CDMA, the number of sub-carriers is increased, creating a denser signal transmission. MC-CDMA then offers grater bandwidth utilization for power lines than OFDM. Considering the fact of bandwidth limitations evidently present in power lines for high speed data transmission, it can be concluded that MC-CDMA is the ideal solution, so far, to transmit over power lines. The power line channel frequency response is perfectly acceptable up to 30 MHz, even though selective fading may be present. A comparison of the overall performance of MC-CDMA and the OFDM systems with the same features, shows that MC-CDMA performs better than OFDM in terms of BER for this frequency range [14],[15],[16],[17]. Figure 7 shows the scheme structure for the transmitter and the receiver of MC-CDMA scheme [4],[6]. III. TRANSMISSION SCHEME SELECTION Both, Orthogonal Frequency Division Multiplexing (OFDM) and Multi-Carrier Code Division Multiple Access (MC-CDMA) schemes promise to be suitable to transmit high data rates in frequency-selective fading channels with multipath effects such as the power line channels. The fact that OFDM converts the data stream to several parallel streams, increasing the symbols duration, makes the system resistant to frequency-selective fading and minimizes the complexity of the architecture. The use of orthogonal codes and frequencies decreases to a minimum the Inter-Symbol Interference (ISI) and thus the Bit Error Rate (BER) at the receiver level [20]. Jamming and interference to the transmitted signal are avoided by the spreading of the spectrum making the transmitted signal appearing as background noise to the system. The use MC- CDMA increases the efficiency of the bandwidth usage and also decreases the effects that may cause frequency-selective fading channels over the transmitted signals when compared with the use of OFDM [3],[18],[19]. The advantages of frequency diversity and the capability to easily recover lost information occurring during the transmission make MC- CDMA a very good option for high speed data transmission over power lines. But this requires a balance among the channel characteristics, the transmission scheme and the application of the system being designed. Figure 7. The MC-CDMA Receiver & Transmitter Architecture In figure 7, the system converts the original data stream with a symbol rate R into P parallel sequences with a new symbol rate p s R equal to the reciprocal of the original symbol rate P 1, similar to what happens in OFDM, and spreads each obtained sequence over a number, N, of orthogonal sub-carriers 234

4 using a given spreading code with length N, thus each chip of the Pseudo-Noise (PN) code modulates one of N orthogonal sub-carriers. This is done by applying the Inverse Fast Fourier Transform (IFFT). The first stage of the design allows only voice signals to be transmitted over the power line channel. Voice signals do not demand very high speeds for transmission. The human voice spectrum goes from about 300 Hz up to 3.5 KHz. The necessary sampling rate, following the Nyquist theorem for the approximately 4 KHz bandwidth of the human voice range, this becomes 8 Kbps. After the digitalization and the logarithmic mu-law compression process, the transmitted signal requires a bandwidth of 64 Kbps to transmit and output in real time. However, higher data speed range is selected to prepare the basis for data transmission [22]. The system is intended to transmit data at 1 Mbps in the form of 10 bits symbols. In this case: N c + 1 G W = N c Ts = = 10.1MHz 1µ S Approximately 10.1 MHz is needed to transmit the information over the power line channel using MC-CDMA with a band guard at a rate of 1Mbsp. Therefore, the central frequency for the transmission scheme is 25 MHz and the necessary bandwidth goes from to MHz. IV. ANALOG INTERFACE CIRCUIT DESIGN The analog interface circuit design is divided into two main blocks, the design of the user transceiver board and the design of the PCI-based PABX board. These blocks are shown in the block diagrams in Figure 8 and Figure 9 below which gives a better idea of the complete design. (7) R s = 1Mbps (1) Then: T s = 1µS (2) The serial to parallel converter is then set to a 10 bits conversion. The new symbol rate per parallel sequence becomes [18]: R p 1 1 = Rs = 1Mbps = 100Kbps P 10 (3) And in terms of bit duration, from Equation 4: T N cts = = Ts P = 1 µ S 10 = 10 S (4) G p µ G The processing gain is selected in such a way that the chip frequency is at least 10 times higher than the symbol rate of the information intended for transmission. Thus, applying the spreading to each parallel sequence with a PN code of length 10, the processing gain will be: Figure 8. PCI PABX Board Diagram G = 10 (5) With this scheme the number of sub-carriers resulting from the N x N matrix structure will be: N PG = = 100 (6) c = The overall necessary bandwidth to transmit 1Mbps can be calculated as follow: Figure 9. User Transceiver Board Block Diagram 235

5 The power line coupling module is common to both boards and blocks frequencies lower than 100 KHz. It is designed with average impedance of output and input of approximately 50 Ohms. The passive band pass filter is designed with phase correction. It is placed in the receiver section of the board to filter signal frequencies which are outside of the transmission bandwidth such as noise and electrical interference caused by different operating devices that are usually connected to the power line network like induction motors, switching power supplies, dimmers and other noisy appliances. The phase correction segment of the filter corrects distortion due to nonlinear phase response when filtering the signal of interest which may increase the bit error rate of the received signal. The signal power stage module is an adjustable gain linear wideband signal amplifier which acts as a buffer and amplifies the signal before injecting it into the channel in the transmitter and after the signal is filtered in the receiver. A block diagram is shown in Figure 10 for a better understanding of these modules. Figure 10. The Power Line Coupling Module The receiver filters and amplifies the signal before it is received by the Digital Signal Processing / Field Programmable Gate Array (DSP/FPGA) module. At the transmitter, once the signal from the DSP/FPGA module is amplified, it is then inserted into the transmission channel through the power line coupling. Another common element to both boards is the hybrid circuit. Each hybrid circuit gives a bi-directional path to the audio signal to be transmitted to and/or received by the PSTN line for the PABX board. In case of the user transceiver board, it links with the user phone set instead of the PSTN. It creates a difference of phase between signals, one received by the user and one sent by the user, thus allowing the bi-directional path of the signal. The supply voltage of the PCI-based PABX board is taken from the PC power supply since the board is already placed in a PCI slot. The power supply of the user transceiver board is a design of a 48-volt unregulated DC source which supplies 12 V, -12 V and 5 volts to be regulated. Most circuits on the board are supplied with 12 and 5 volts. The 48 volts DC present in the user transceiver board will be only used to supply the user phone set when it is in the on-hook state [2]. Interfacing whit the standard phone sets with which the system is intended to work requires some modules to be added to this board. The block diagram in Figure 11 shows a more detailed structure of the analog interface for the user transceiver board. Figure 11. The User Transceiver Interface Module The on/off hook signal detector and phone voltage regulator modules simulate the standard PSTN conditions to create compatibility between the user transceiver and the regular phone sets available in the market that are to be used in the system. When the phone is on hook the voltage across the tip and the ring terminals is approximately 48 volts. If a call is detected by the board, a ring signal is generated and inserted on to the phone line. This ring signal waveform has a very low frequency of approximately 30 Hz, and a peak to peak value of about 90 Volts. This is excessively high and cannot be transmitted through the power line channel. The ring signal in this case is generated on board and once the call has been detected it is inserted onto the phone line through a low pass filter. When the user picks up the phone the on/off hook signal detector senses a current change in the line and instantly 12 volts is introduced on the phone line. The use of high and low pass filters prevent any harm to the audio and digital processing electronic circuits which can be produced by the insertion of the power ring signal. For the PABX integrated board different modules are required. A PSTN incoming line ring detector and an on/off hook status simulator is vital for the PABX operation as shown in the block diagram in Figure 12. Figure 12. PABX Interface Module to PSTN 236

6 The PSTN incoming line interface is very similar to the user phone set transceiver board discussed before but with some slight differences. In this research, the PCI-based PABX board interfaces with the incoming line from the PSTN, the system operator and also with the users within the system. In the first case, it detects an incoming call and simulate the on/off hook status in order to identify the system with the public network and make it compatible with the standard operation. SX micro-controller from the Scenix family of configurable communications controllers. The control software is implemented, improved by stages and tested using the evaluation board provided with the micro-controller SX28AC shown in Figure 14. When an incoming ring signal is identified the off hook state is simulated and the communication starts. Initially, the call is attended by the operator and then transferred to the intended user. An Interactive Voice Response System (IVRS) must be implemented as part of the design of the Graphic User Interface (GUI) and the system drivers [21]. Its function is to initially attend and process any call from the public network and also to control the voice mail options for the system users. The operator communicates directly with the system through an audio interface placed on the board. Therefore the signal source varies depending on what links with the PSTN line at a given time, the operator or a user inside the system. Switching between the multiple sources is carried out by using analog multiplexers and de-multiplexers which are controlled by the DSP/FPGA software shown in Figure 13. Their function is the multiplexing of the signals coming from the audio interface and the hybrid circuit from the PSTN. Figure 13. Operator/System Audio Signal Processing Module The electronic interface uses the 74HC4052 which is a high-speed dual 4-channel analog multiplexer and demultiplexer CMOS device with common select logic. Each multiplexer has four independent inputs/outputs, controlled by three bits for channel select logics, including two digital select inputs (pins S0 and S1) and an active low enable input (pin E). Its supply voltage ranges between 2 to 10 V. The analog inputs/outputs do not exceed 10 V. The system uses the MAX 507 digital to analog converter and the MAX 185 analog to digital converter with 12-bit resolution to standardize digital and analog signals. Excepting the last circuit on Figure 13, all the electronic sub-circuits and peripherals are linked and commanded by the Figure 14. SX28AC Training Kit V. MICROCONTROLLER SOFTWARE DESIGN As shown in Figures 8 and 9, the Scenix SX28AC microcontroller and the virtual peripheral software control the entire analog electronic interface for both the user transceiver board and the PCI-based. The design of the control software is also an important result of this design. The software carries out the following tasks [2],[13]: Serial communication through the UART protocol: serves as communication link between the analog electronic modules and the DSP/FPGA block. Caller ID receiver: detects the caller ID information signal and transmits it to the DSP/FPGA Module through UART communication protocol. Controls read and write operation for the MT8880 DTMF transceiver IC: the IC control is carried out through 3 bits and the data exchange involves a total of 4 bits. The IC has an analog output terminal which delivers the intended DTMF tone and an analog input for reading. Ring detection and on/off hook status simulator: a total of 2 bits are used to interface with the PSTN incoming line. The program uses the Virtual Peripheral concept to increase the performance of the micro-controller making possible the execution of a higher number of tasks utilizing the same resources. In this Virtual Peripheral, a multi-threading concept to execute the Universal Asynchronous Receiver Transmitter (UART) and all necessary modules is applied. Whenever an interrupt occurs, the program jumps into the interruption service routine that contains the interruption multi-tasker. At every occurrence of an interruption, the interruption multi- 237

7 tasker module switches to a different thread following a table order defined in the program memory. Each Virtual Peripheral module is located into one of the available threads. VI. CONCLUSIONS: A general PLC measurement system was proposed in order to analyze the channel characteristics using a prototype of the coupling network. The paper presents measurements of how the attenuation behaves, according to the frequency of the transmitted signal for an ideal power line with no loads and a real in a power line cable connected to the power grid in Jamaica. Similar behaviors of the frequency vs. attenuation curve were found if compared with [5],[7],[8],[12]. The communication channel has proven to be a multi-path, impedance varying and noisy transmission channel, thus modern, powerful and resistant communication schemes are needed. The necessary analog electronic modules that interface an analog telephone system with power line communication technology were discussed. While the paper proposes MC- CDMA as the ideal transmission scheme to use in power line communication, it is acknowledged that most high speed PLC chips now being marketed [24],[25] uses an OFDM infrastructure. Work is ongoing to compare in greater detail the MC-CDMA vs. OFDM performance for practical PLC channels. There is also ongoing work to use the results of this paper to design and develop an integrated PLC VOIP phone using existing high speed PLC chips and VOIP software and hardware. REFERENCES [1] Klaus Dostert, Powerline Communications. Upper Saddle River, NJ, Prentice Hall PTR, [2] Departmental Report, Computerized Local PABX, Department of Electrical Engineering, University of Queensland, Australia [3] Vjay K. Garg, Smolik K, Wilkes J, Applications of CDMA in Wireless/Personal Communications, Prentice Hall [4] B. Sklar. Digital Communications: Fundamentals and Applications. 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