Spread spectrum carrier technology based on power line and radio frequency communications

    Abstract: Spread spectrum technology based on spread spectrum linear frequency modulation pulses uses low-cost integrated circuits to realize power line carrier and radio frequency communications, so that system development not only has low hardware costs, but also low installation costs. These technologies have also been adopted by EIA as the CEbus (Consumer Electronics Bus) physical layer specification for home automation standards. Moreover, due to the low cost and high performance of this technology, it also has a large number of applications in the field of distributed control and monitoring related to home automation.

    Keywords: spread spectrum carrier CEBus network

introduction

The development of two new spread-spectrum technologies, low-voltage power line and radio frequency signaling technology, makes it possible to assemble reliable and low-cost wireless distributed control networks in residential and commercial applications. "Wireless" in power line communications refers to the fact that AC power lines already exist in existing homes and buildings, so there is no need to install new lines. Previously, typical applications of spread spectrum technologies (such as direct sequence and frequency hopping) were limited to military systems such as secure communications and navigation due to the high expense and complexity of technology implementation. However, with the development of solid-state integrated circuits, another spread spectrum technology has become cost-effective and suitable for ordinary users and commercial applications. This is frequency sweep pulse technology, which uses linear frequency modulation pulses to effectively broaden the information bandwidth many times to form a signal bandwidth. This new technology is also called spread spectrum carrier technology SSC (Spread Spectrum Carrier), which allows the correlator to detect the presence of signals in real time just like a matched filter receiver detecting a narrowband modulated carrier. Therefore, this technology can be used to implement carrier sense multiple access packet networks with many users or nodes sharing a channel. In the United States, the Electronic Industries Association (EIA) has adopted SSC technology as the physical layer specification for the Consumer Electronics Bus (CEBus) standard for power lines and radio frequencies.

Consumer Electronics Bus CEBus

The CEBus standard defines all communications and network functions required for interconnecting products within specialized control networks in homes and buildings. Similar to Ethernet, this standard allows the use of a universal bus based on the Carrier Sense Multiple Access/Collision Detection (CSMA/CD) contention decision protocol to achieve point-to-point connectionless communication. As a non-proprietary standard, CEBus is implemented based on the Open System Interconnection OSI reference model of the International Organization for Standardization ISO, but omits the presentation layer, session layer and transport layer to minimize the cost and complexity of product development. The CEBus standard only defines the functions required to complete communication; it does not describe issues such as implementation, design, and application interfaces. These issues are left to the system developers' own design and application. However, the standard clearly addresses a wide range of multi-user interconnect issues, including issues such as signal timing and structure, connectors, signal formats, etc. CEBus data (see Figure 1) begins with an eight-bit switch-keyed synchronization header signal, which resolves channel access and contention issues. When the channel is determined, phase shift keying PSK technology is used to send the address, control or data contained in the data packet. The message text is followed by an end-of-packet flag and a CRC code for error control. In a CEBus data packet, the data pulse width is encoded to indicate the end of the symbol field and the end of the packet. Within the text of the data message, the duration of a minimum symbol period is 100 microseconds.

SSC power line signals and RF signals

The SSC power line signal is a frequency sweep pulse or a very short direct sequence linear frequency modulated pulse. Similar to traditional direct sequence spread spectrum, linear FM pulses spread the signal energy over a wide frequency band. Specific to the EIA's CEBus standard, the signal is widened to the frequency band between 100KHz and 400KHz, with a relatively bit rate of 10kb/s. The corresponding system developed in Europe widens the signal to the bandwidth of 20KHz to 80KHz, while reducing the effective communication bits. The rate is 2kb/s, thus maintaining the processing gain. Although in theory, the signal can be generated simply by sweeping pulses from 100KHz to 400KHz or 20KHz to 80KHz, the actual product sets a transition point 200KH between 100KHz and 400KHz, and the frequency sweep ends at this frequency and starts again. There are two main ways to do this. This is done for two main reasons: it simplifies filtering to limit the harmonic energy generated by the signal, and it allows for smooth transitions between data bits. Figure 2 shows the waveform of this signal.

As can be seen from Figure 2, the frequency scan is performed within the 100 microsecond time period mentioned earlier. This is for a scan of 100KHz to 400KHz. Scanning in the range of 20KHz to 80KHz can be achieved, and the bit processing time will increase to 500 microseconds, a bit rate of 2Kb/s can be obtained, thus maintaining the processing gain of spread spectrum signals. It can be seen that the signal amplitude changes significantly when the signal frequency changes. This is due to the change in power line impedance with frequency. Theoretically, a spread spectrum signal provides a signal gain (i.e., gain over conventional narrowband techniques) equal to the spread bandwidth divided by the data bandwidth. In both cases, this ratio is 30:1, which equates to a processing gain of 14.8dB.

As mentioned previously, low-cost, high-performance spread spectrum transceivers can be developed using this signaling technology. The main factor leading to low cost is that the frequency sweep signal can be simply realized by a data lookup table stored in the IC ROM area, and since the linear FM pulse maintains the linearity of the known pattern, a very simple correlation method is used when decoding the linear FM pulse. That’s it. Not only that, because the system can use the direct sequence method (i.e., narrow-band noise interference suppression) and the frequency hopping method (i.e., narrow-band blocking interference suppression), the linear FM pulse-based system can achieve good performance under variable noise conditions.

Since the swept compression pulse is generated at the baseband frequency, the SSC RF signal is very similar to the PL phase-locked signal. In RF applications this baseband signal can be upconverted to a suitable transmit frequency, such as 915MHZ, which is unrestricted in North America, or 2.4GHZ in Europe.

application

Currently, a large number of home automation products are being developed in the United States, and developers hope that retail sales to users will bring about the full popularity of CEBus-compatible products. These products include lighting controllers, wireless security systems, central heating and ventilation, handheld RF remote controls, audio and video equipment controls, and more. Moreover, many equipment providers promise users that they can easily install power management systems on existing user products and perform meter reading, load control and provide services such as security monitoring. Some commercial applications now use CEBus phase-locked technology and radio frequency spread spectrum carrier technology, and are considering compatibility with CEBus.

Systems using power line technology and radio frequency technology enable owners of coin machines (such as arcade, gaming and retail machines) to monitor coin flow and type, machine diagnostics, and inventory levels. Communication between reception desk and rooms in the hotel industry uses SSC technology based on power lines or coaxial cables to enable a variety of applications, such as room entrance control, room occupancy monitoring, power management, video on demand, in-room safes, and minibar monitoring. Cashless public transportation systems are also using radio frequency technology to transmit passenger traffic statistics from buses or trains to master control terminals. The DC bus is used in the transportation industry to monitor the performance of braking, refrigeration, lighting and other critical systems such as cars, trailers, trains, etc. Traffic intersection control and monitoring can be achieved using radio frequency technology. Public lighting in at least one city in Europe is controlled by an SSC power line communication system, using linear frequency modulated pulse signals from 20KHz to 80KHz. As these technologies mature and become more cost-effective, their applications are likely to become more widespread.

Since the late 1980s, domestic research on SSC technology for power lines and RF has gradually increased. The research direction is mainly concentrated in the fields of water and electricity remote meter reading, security and security, etc. Wider and more in-depth applications still need to be further developed. At present, one of the more successful applications in China is the street light control system of a residential area in Suzhou, which is implemented using SSC power line communication technology. Operation practice shows that the system is stable and reliable.

The American company INTELLON has integrated all the functions required to implement the above-mentioned new signal technology in a low-cost application-specific integrated circuit (ASIC). In addition to generating signals, the ASIC also provides all timing requirements specified by the EIA CEBus physical layer specification, generating a 16-bit CRC for every packet sent, simply requiring the master to encode or decode 8 data USTs at a time, and monitor the provided indicators state. This enables the production of very cheap communication control products using SSC PL/RF technology. The company claims to have the technology to achieve a 10Mb/s communication bit rate on AC power lines. More America. European companies have industrialized power line communication products with communication bit rates of several K to hundreds of K, such as ANI, ECHELON, NOVELL and other companies.

Conclusion

Two new spread spectrum technologies introduced in this article can enable cheap and reliable power line or radio frequency control networks. This technology is not only supported by the North American CEBus standard for home networks, but is also widely supported in many commercial applications that can benefit from wireless communications. In addition to home automation and building automation, other more common applications include: remote sensing, traffic control and monitoring, building entrance control, remote control points for sales transaction processing, public transportation utilization monitoring, power management, vehicle system management, retail machine monitoring, etc. . As these technologies mature and become more cost-effective, more novel and creative applications will emerge.