DATA COMMUNICATION THROUGH SPREAD SPECTRUM TECHNOLOGY ON POWER LINES

Transcription

1 Noise Level (dbv) Latest Trends in Information Technology DATA COMMUNICATION THROUGH SPREAD SPECTRUM TECHNOLOGY ON POWER LINES Marium Jalal Chaudhry*, Azhar Ul Haq**, Farhat Saleemi* **University of Jyvaskyla, Finland *Lahore College for women University Lahore, Pakistan 1. ABSTRACT: Data transmission through power lines has received increasing attention in recent years. In fact the electrical energy distribution system is a big back bone that can be use for data communication. The interest for this medium includes automatic reading of energy meter, management of electrical distribution system, automation of buildings, and high speed data rate connection within the home and so on. The main advantages of data transmission on power line communication is the facts that the physical networks is almost already installed, making it a very attractive option. Beside this, physical drawback of the network, it become necessary to install repeaters, if distance matters bridges over distribution transmission, use of technology that reduce the noise level of the power lines, after that we will be able to achieve our goals on power lines. The data flow over power line through high voltage, low voltage, through cooper and other medium has different characteristics. DSSS, OFDM are the main applicable techniques to get QoS over not very good environment for data communication on power lines and this QoS is the point of study of this paper and we will discuss how we can achieve our goal to make our service better at consumer premises. 2. CHANNEL CHARACTERISTICS OF PLC: The power line as a communications channel has specific characteristics that must be considered. This includes the dominant and widely varying noise, impedance changes and multi-path effects. Noise sources are electronics, electro-mechanical and even included by the power lines themselves. Some noise is harmonically related to the 50 or 60Hz. Light dimmers and related products that uses triacs create impulse designed switching supplies, conduct quite a bit of noise onto the power lines. These noises sources have both time domain and frequency domain characteristics. The easiest way to get a first order understanding of signal to Noise ratio SNR on the power line is to look at noise amplitude as a function of frequency, as shown in fig no Frequency (MHz) Fig-1 Max noise level. In PLC, the physical carrier medium for digital data is the ISBN:

2 conventional power wiring that acts therefore as a transmission line, upon proper de-coupling of transmit and receive equipments from the 220 V or 110V power network voltage. It is fairly evident that the characteristics of power wiring are well far from optimal as far as transmission is concerned. Uncertain network topology: The PLC wired channels are time-varying since there are lots of plugged in electrical devices that are turned on and off all the time. In addition, the delicate applications that want to communicate with each other may stay in different phases like computer, routers etc. Attenuation: There are many factors that cause attenuation, such as the phase coupling losses, the device impedance and insertion losses due to the appliances that are plugged in the power line. A very large induction at the main transformer that attenuate the high frequency signal normally used for the data communication is also a major factor. The frequency response exhibits deep attenuation peaks at some random frequencies and this attenuation can reach up to some higher extent, which is also a main source of noise. Noise of interference: Inherently noisy medium is a problem of PLC, like electric appliances are the main sources of noise that are plugged into the power line and cause impulses that reveal very harmful for the data signal. Like; low quality power cords, low quality power supplies, are major causes, dimming of lighting system, the starting of an air compressor, inductive load causes harmonics same as in the switching of battery charger. Line impedance: Experimental measurements reported that a typical power lines exhibits low value impedance. 3. PLCs MODULATION TECHNIQUES FOR DIGITAL TRANSMISISON: The transmission techniques adopted for PLC must be capable of enhancing signal robustness with respect to distortions and interferences experienced during the propagation in power lines. There are several obstacles to be overcome in attempting to transmit high bit rate data signals over the electrical power network. Its bus architecture in heterogeneous and impedance is not well defined. In addition, the noisy nature of the network between the transformer and the customer premises, and inside the home, requires sophisticated but feasible techniques. Among the most promising is coded orthogonal frequency division multiplexing (COFDM), spread spectrum techniques like direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS). But the most popular approach, addressed to as spread spectrum consists in artificially expanding the spectral content of the signal before transmitting it. There are three techniques used nowadays for PLC system X-10, SSC and LONworks; 3.1 The X-10 system: The X-10 is relatively long established technique for PLC communication. It has a simple protocol, commands set and a low implementation cost. In essence it can be described as an on-off keying modulation that uses a fixed single carrier frequency of 120KHz. The X-10 system sends the carrier at every zero crossing of the fundamental frequency AC line voltages. The X-10 protocol represents a bit one by 1ms of carrier ISBN:

3 presence and a bit zero by absence of the carrier. To increase communication reliability every bit of transmission is sent twice, requiring a full line cycle, which limits the transmission rate to 60bps. There is no mechanism to know if a particular command has been received correctly. The only concession towards error detection is a bi-phase bit transmission coupled with duplication of each transmission. The synchronization with the zero-crossing points limits the bit rate of the transmission data at 60bps. AC voltages between 120V positive and 120V negative, 60 cycles per second, the resulting 60Hz power, curve reveals a point in every cycle where the voltage is zero, so called zero crossing. The signal itself is 5 volts, 120 KHz pulse at a zero crossing. The voltages on the line do not exceed 5 to 6 volts. Information is sent as a coded digital sequence that contains information for identifying the receiver and for commanding its operation. This is a low bit rate transmission at 60Hz but working for higher speed as well as for 50Hz. Another disadvantage is low noise immunity. Fig 2 show signal propagation on a 3 phase power line. Fig-2 X-10 signal for 3 phase system. 3.2 The LONworks: It is a technology developed by Echelon corp. and subsequently standardized by the EIA (electronics industries Association). This system provides a peer-to-peer communications protocol implementing carrier sense multiple access (CSMA) techniques. It is a narrow band spread spectrum modulations scheme (125 to 140 KHz) that uses a multi-bit correlation intended to preserve data in the presence of interference noise, with a potential impulse noise cancellation. Physical communication occurs over power lines inside and outsides of homes over 125 KHz to 140 KHz and comm. At 10Kp/s by using SS technology. The standard support different electrical configuration and call out a narrow-band power lines signaling technology that meets regulatory requirement for North America and EU. 3.3 The Spread spectrum Technology (CEBus): Spread spectrum (SS) technology is a method of signal modulation where the transmitted signal occupies a bandwidth considerably greater than the minimum necessary to send the information. Other functions than the information being sent is used to increase this bandwidth. Since its applies to board frequencies, the SS can maintain strong immunity in hostile environment and let itself suitable for power line medium which has the undesired characteristics previously described. For example, narrowband attenuation or impedance notch will affect only a small portion of the transmitted signal, so the remainder signal has sufficient information for the transmitted data to be reconstructed properly. Table-1 summarized the characteristics of the above mentioned three technologies, its comparative parameters among three of them Characteristic X-10 SSC LONworks Cost Low High Medium ISBN:

4 Access control Bad Good Medium Simplicity God Bad Bad Effective comm. Rate (bps) Noise interference B G G Market diffusion H M M Freq. allocation (FCC/CENELEC) Both FCC Both Table-1 Comparison sheet of PLC techniques Transmission of high data rate is achievable through SSC with good control on noise interference and ease to handle a data transmission through this technology. It is robust and secures communication, can allow us to use it for PLC system. 4.1 PLC DATA TRANSMISSION THROUGH SPREAD SPECTRUM CHIRPING (SSc): This standard defines all the communication and network functions required to develop interoperable products for use in home or building control networks. Like Ethernet, it allows connectionless, peer-to-peer communication over a common bus utilizing a carrier sense multiple access/ collision-detect, contention resolution protocol. The CEBus standard defines only those functions required to facilitate communications; it does not describe the specific implementation, design or application interfaces. Those are left to the innovation of the system developer. However, a full range of multi-vendor interoperability issues are addressed by the standard including such items, as message timing and structure, connectors and signaling formats. At the physical layer, the CEBus message packets format is shown in Fig-3. The packets begin with an eight bit ON OFF keyed preamble during which channel access and contentions are resolved. When the channel is secured the packet body containing addressing, and control or data elements is sent using Phase Shift Keying. The packet body is followed by an end of packet symbol and 16 bit CRC for error control. In a CEBus packet, data is encoded utilizing pulse width encoding to represent the symbols 1, 0, end of field (EOF), and end of Packet (EOP). The minimum symbol period is defined as the unit symbol time (UST). 114µs Preamble 8-16 UST Up to 1824 µs 100µss Preamble EOF 8 USTs 400µs 800µs 100ms 1600µs typ 14µs typ Body 100USTs typ EOP USTs Fig-3 CEBus Packet Timing The duration of one UST is 100µs during the packet body. The four CEBus encoded symbols have the periods shown in Table-2. This type of symbol encoding allows for the efficient delivery of a large number of short messages from numerous nodes in a communication network. A more thorough treatment of the basic elements of the CEBus standard can be found. Symbol Preamble Packet Body UST s Duration UST s Duration µs 1 100µs µs 2 200µs EOF N/A N/A 3 300µs Preamble EOF 8 800µs N/A N/A EOP N/A N/A 4 400µs Table-2 CEBus Symbol Format CRC 10USTs 4 3 In this protocol, the data is transmitted in short format, the protocol can use the response mode, it means that ISBN:

5 the target modem/receiver will answer back to the source modem when it receives a request. 4.2 MODULATION OF MULTI- CARRIER SPREAD SPECTRUM: Below fig-4 represent the model of MC-SS and its specifications; by non-linear distortion. The above said model is a combination of DS/SS system and OFDM modulation through which we can obtain data transfer speed more than or equal to 100kb/s. Multi carrierspread spectrum is a guarantee of high speed communication on power lines. Modulation: DQPSK for MC-SS Spreading sequence: M sequence Code length L: 7 Sub-carrier N: 224 Chirp duration Tc: 143µs Symbol duration Ts: 1ms Freq. of symbol: KHz Tc=1(LR/2), number of sequence of data M=32 are used, N= ML which is a number of sub-carriers can be employed. Then the bandwidth will be (N+1)/Ts where Ts= N/(R/2). Data BPF Carrier QPSK P/S IFFT Diff. Encoder S/P Figure-4 Model of MC-SS for PLC system This modulation system can achieve stable BER performance for the power line including the burst noise using limiter. Limiter can be applying at first level in order to suppress the burst noise, and limiting level can be set at 10dB. BER of MC-SS is better then all other systems. This is due to the facts that symbol duration is long and that freq. diversity provides significant improvement against inter-modulation PN 5. SSC SINGAL ON POWER LINES: The SSC power lines signals is a swept frequency pulse, or chirp that is very short direct sequence pattern. As in traditional direct sequence systems, the chirp effectively spreads the signaling energy over a board frequency range. In this case of the EIA s CEBus standard the signal is spread over a frequency range of 100 KHz to 400KHz, with an effective bit rate of 10 Kb/s, while systems develop in Europe spread the signal over a frequency range of 20Khz to 80Khz, while lowering the effective communication bit rate to 2Kb/s, thus, preserving the processing gain. Theoretically the signal could be generated by simply sweeping the pulse frequency from 100 KHz to 400 KHz, or 20Khz to 80Khz, Transmission begins and ends the frequency sweep at 200Khz with a transition from 400 to 100Khz in the middle. One reason for doing this was to limit the amount of energy radiated into the AM radio band. The FCC regulations are quite stringent in this area; therefore, the signal design was optimized to allow design of output filtering that provides the roll-off characteristics necessary to satisfy the FCC guidelines. This technique also allows a smooth transition between data bits further reducing the amount of harmonic energy produced. As mentioned earlier this signal can be swept over the 20 to 80 KHz ISBN:

6 range. It can also be seen that the signal amplitude varies noticeably as the frequency of the signal changes. Theoretically the processing gain, provided by the spread spectrum signal is equal to the spread bandwidth divided by the data bandwidth. In the two cases mentioned above, this ratio is 30:1, which equates to a processing gain of 14dB. 6.1 ISSUES LINKED WITH PLC SYSTEM: High voltage transmission line may carry one or two analog voice circuits or telemetry and control circuits with an equivalent data rate of a few hundred bits per second, however, these circuits may be many Km long. High data rates generally imply shorter length. Power lines are inherently a very noisy environment, and Spread spectrum is a well suited technology to control that issue while sending data on broad band spectrum. Signal strength and operating frequency is also an issue while transmitting a data on unshielded wire which will act as antennas for the signals they carry, and they have potential to interface with short wave communication. SSC is an authenticated secure communication can resolve that issue of chance of interference of data. 6.2 APPLICATIONS Power line communication can also be used to interconnect home computer, networked consumer peripherals, household electrical power wirings, as transmission medium. We can connect a modem at receiver end in order to use that signal for Bluetooth appliance. In this way a secure PLC communication system can be use at the end as high data rate in home networking with a help of Bluetooth technology too. Utility companies are using special coupling capacitors to connect medium-frequency transmitters to the power frequency AC conductors, with transmitter power level up to 100s of watts. Filtering devices are applied at substation to prevent the carrier frequency current from being by passed through the station apparatus to ensure that distant fault do not affect the isolated segment of the PLC systems. These circuits can also be used for controlling of switchgears, breaker, power transformers and protection of transmission lines. A protective relay can use a PLC channel to trip a line if a fault is detected between its two terminals, but to leave the line in operation if the fault is elsewhere on the system. More advanced, microwave and now fiber optics for their primary system communication needs, the power line carrier apparatus may still be useful as a backup channel. This communication system was brought into for automatic meter reading can also be sometime used for load management applications. PLC is one of the technologies used in automatic meter reading industry; both one way and two way system have been successfully used for decades. Utility companies are not earning on reading of meters at consumer side so no need to invest for mobile signal or other special purpose communication way for this requirement. Already installed system can be used for meter reading at consumer side, Grid to Grid data transferring. That why utility companies can also use this system for advanced metering infrastructure systems. ISBN:

7 REFERNCE: Shin'ichi TACHIKAWA, Masahiko NANRI, Masanori HAMAMURA: Power Line Data Transmission Using OFDM and DS/SS Systems, Proceedings 6th International Symposium on Power-line Communications and its Applications (ISPLC'02), Athens, Greece, pp.19-23, (2002.3). L.T.Tang at all, Characteristics of Power distribution lines for highspeed Data transmission IEEE international conference on Power system technology (Power comm. 2000) vol.1, pp , 2000 M.H. Shwehdi and A.Z. Khan, A power line data communincation interface using spread SPECTRUM TECHNOLOGY IN HOME AUTOMATION IEEE Transection Power delivery, vol.11, no.3, pp , July K.M Dostert Frequency hopping spread-spectrum modulation for digital communication on electrical power lines IEEE journal on selected area in communications, vol.8, no.4, pp , may M.Nanri, K.Nakayasu, M.Hamamura and S.Tachikawa, On power line data transmission using OFDM an DS/SS system IEICE Technology report SST2000-4, tpn2000, pp P.Sutterlin, A Power line communication tutorial-challenges and technologies PLC 98, March 1998, pp ISBN: