Paging technology: Flex, ERMES paging codes Part 3–ERMES takes Europe, and Motorola’s Flex (in one form or another) takes the rest of the world in the contest to roll out a faster paging protocol. Data rates as high as 6,400bps are achieved on a paging c
Motorola’s flexible wide-area synchronous protocol (Flex) is being used in several cities in the United States. Also, many service providers in the Far East are planning to install the Flex protocol, including China, Indonesia, Singapore, Thailand and Japan (RCR-43).
For other companies to have the right to implement the Flex protocol in their products, they must first receive a license from Motorola. At the time of the Flex protocol’s announcement (June 1993), NEC America, Fort Worth, TX, was licensed to build Flex paging products, and Glenayre Technologies, Charlotte, NC, was licensed to manufacture Flex infrastructure.
The Flex protocol offers several advantages. One is a significantly long battery life compared to POCSAG pagers. Extended battery life makes smaller pagers with smaller batteries possible.A user also may notice improved message integrity because of the Flex protocol’s error-correction capability. Other user benefits will come from the expanded services that the Flex protocol will provide, including * nationwide paging. * missed message indication. * efficient dynamic group calling. * information services. * computer file transfers.
The service provider benefits because the Flex protocol can serve more subscribers per transmitter than any other paging protocol–four times the current maximum number of pagers on an RF channel. The protocol’s interleaving of address and message bits provide fade protection. The Flex protocol also can be added to existing POCSAG or Golay systems because it was designed to be compatible. The Flex protocol can be transmitted at three different speeds: 1,600bps, 3,200bps or 6,400bps. Flex pagers can operate at any of the three speeds. In fact, they automatically decode the correct signaling speed based on information from the paging signal. The 2-level frequency-shift keying (FSK) modulation, as used for POCSAG and Golay, is used for the slowest transmission, 1,600 bps. The 6,400bps transmission uses 4-level FSK. The 3,200bps transmission may be transmitted at either 2-level or 4-level FSK.
Figure 1 on page 22 shows the differences between 2-level and 4-level FSK. Note that for 4-level FSK, each two-bit sequence, known as a symbol, is represented by a level. Using the same rate of transmission, the bit rate can be doubled. In other words, if the top example is 1,600bps, then the bottom example is 3,200 bps; nevertheless, the bit rate of the top example is the same as the symbol rate of the bottom example.
The Flex protocol is a synchronous code that assigns pagers to frames. These frames occur at a rate of 128 every four minutes .One Flex cycle equals 128 frames. There are 15 Flex cycles per hour. (See Figure 2 on page 22.) Each pager is assigned to look at a particular frame (or frames) within a Flex cycle. Upon “power on,” the pager determines the time of its assigned frame(s) and only turns on its receiver circuitry during that time. This function improves the pager’s battery life because it eliminates the need for preamble detection. The frame assignment for a pager is configured in the pager or determined from information within the paging signal.
Each frame consists of a sync field and 11 blocks. A frame is 1.875 seconds.The sync information takes 115ms to send and is composed of three parts: * sync 1. * frame information. * sync 2.
The first two parts are always transmitted at 1,600bps using 2-level FSK. Sync 1 provides timing information and indicates the speed of the rest of the frame. The frame information word contains the frame number (0-127), the cycle number (0-14) and other information. Sync 2 provides synchronization at the frame’s block speed so the remainder of the frame can be properly decoded. There are 11 blocks, and each is 160ms long.
The information that is transmitted within a Flex frame is shown in Figure 3 on page 24. The fields are not constrained by block boundaries. The fields that are transmitted in a frame are a block information field, an address field, a vector field, a message field and idle blocks.
The block information field is typically one word long and contains information that indicates where in the frame the address and vector fields start, frame information from the system as discussed above and how many priority addresses are placed at the beginning of the field. Priority addresses are assigned to pages with an “urgent” priority.
The address field contains addresses of the pagers that are being paged during this frame time. The vector field has a one-to-one relationship with the address field, so the pager knows where to look for the vector. The vector points to the start word of the message and indicates the length of the message, again so the pager knows where the message occurs. Of course, the message field contains the message information for the pager.
Any unused blocks in a frame are filled with bits representing idle-alternating ones and zeros at 1,600bps. At higher speeds, the symbols must represent that same pattern at 1,600bps. The Flex protocol’s interleaved block structure can be seen from the 1,600bps example in Figure 4 on page 26.
Eight codewords, arranged in rows, are transmitted by columns forming a 256-bit block. Two bit-error corrections per codeword result in the capability to correct a 16-bit error burst on the channel: 16 bits/1,600bps = 10ms. For a 3,200bps transmission, the block structure consists of 16 codewords. These 16 codewords, arranged in rows, are transmitted by columns forming a 512-bit block. Again, with two bit-error corrections per codeword, the protocol can be used to correct a 32-bit error burst on the channel in 10ms. Similarly, for 6,400bps transmission, the block size is 1,024 bits and the protocol can be used to correct a 64-bit error burst.
ERMES The European Radio Message System (ERMES) is a paging protocol defined in a standard published by the European Telecommunications Standards Institute (ETSI). ERMES is intended to be the European-wide standard for radiopaging services, which means that subscribers can be paged outside their home country. ERMES, as is Motorola Flex protocol, is a more complex format than POCSAG or Golay sequential code.*
ERMES has many advantages over the existing POCSAG systems in Europe. It operates in a frequency range that is common throughout Europe. This commonality allows the use of standardized receivers. Moreover, the network is compatible from country to country, so subscribers may roam throughout Europe and still receive pages.
The use of 4-level FSK modulation allows data transmission at 6,250bps with no trade-off in data integrity. With batch transmission techniques, the battery life of a pager is prolonged, because the pager only receives messages in one of 16 batches. In addition, pagers scan as many as 16 RF channels. This scanning allows a network operator to change system operating frequencies as necessary to meet capacity requirements without having to modify pagers.
The structure of the basic ERMES transmission protocol is shown in Figure 5 on page 28. A sequence of 60 cycles lasts 60 minutes. Each cycle is called a paging cycle and lasts one minute. The paging cycle is divided into five subsequences, each lasting 12 seconds or less. Each subsequence is further divided into 16 batches, labeled A through P. The pager population is divided into 16 groups, each being allocated to one of the 16 batches.
Codeword interleaving occurs in the message partition. Interleaving to a depth of nine codewords is specified. Each message codeword is 30 bits long, including 18 information bits and 12 parity bits.
Next month: How pagers are developed and tested to ensure good performance in the field. *Flex is short for flexible wide-area synchronous protocol. ERMES stands for European radio message system. POCSAG stands for Post Office Code Standardisation Advisory Group. Golay sequential code is named after Marcel Jules Edouard Golay (b. 1902 ), a physicist who constructed the code about 1955.
Wireless communications infrastructures are rapidly moving away from urban centers to serve rural customers. Quite often, rural areas do not have access to utility-grade, grid-connected power, forcing telecommunications system operators to design, install and maintain their own power generation facilities. How will operators power these new wireless networks in remote locations, and how will power affect operating costs and network reliability?
A new generation of renewable energy products and services has been developed as the “wireless energy” solution to the new wave of global wireless networks. By shifting to solar or wind energy as the primary power source, operators can use this technology to satisfy many of the needs of modern telecommunications operations: rapid deployment, high reliability, low maintenance and fuel requirements, and future expansion capabilities. Network planners welcome these technologies as a cost-effective means of deploying transmission and distribution infrastructure in rural areas where utility power access may be too far away or the supply may be unreliable.
The object of wireless energy technologies is to minimize dependence on external resources, such as manpower, equipment and fuel, to support remote sites. By reducing these dependencies, site reliability is dramatically improved, and fuel, maintenance and transportation costs are kept to a minimum. Improved site autonomy is a wise design goal, particularly in developing economies where the potential for natural disasters combined with weak response capabilities jeopardizes network integrity.
Three standard system achitectures represent “wireless energy” solutions: hybrid (combined wind and solar power) for larger wireless applications (>650W); solar power alone for smaller power requirements (<650W); and “cycle charge” for locations where renewable energy sources are unavailable or unpredictable. The advantages of all these remote power solutions are bolstered by remote power monitoring software.
Hybrid systems One particular prepackaged renewable energy-based hybrid system, designed specifically for off-grid wireless applications with average continuous communications loads of 650W to 4,000W, uses wind or solar energy for primary power and a fossil fuel engine generator for backup or supplemental capacity. Figure 1 on page 74 shows the system’s fundamental building blocks.
Deployment is streamlined by pre-assembling and testing systems at the factory to minimize field installation requirements and to enhance reliability. The complete system is integrated within an environmentally controlled, transportable shelter with provision to house additional communications equipment.
This hybrid architecture is inherently more reliable than continuously run fossil fuel generators because of its multisource energy supply, long-term battery autonomy (two-to-three days) and use of industrial-grade microprocessor controls that can be remotely monitored and controlled. Hybrid systems can accommodate load expansion by automatically increasing engine run time to supplement power on demand. Because of the hybrid’s flexible power range, the renewable energy elements do not have to be sized for worst-case conditions (peak communications traffic; heating, ventilating and air conditioning [HVAC] system operation; or long periods of low renewable energy output).
Therefore, to serve large loads, capital costs for hybrid systems are comparatively less than for single-source renewable energy systems.The true savings from the hybrid approach becomes clear from a review of operating costs. Continuously run diesel generators, for example, consume about 6,000-12,000 gallons of fuel per year and require engine overhauls every four years (40,000 hours) of operation, simply to support a 1,500W load. In contrast, a comparatively sized hybrid system burns only 500-1,000 gallons (depending on renewable resources) and accrues less than 1,500 hours per year. For maintenance, the hybrid system typically requires only one man-day of service per year, compared to six man-days for diesel generators.
Lower annual fuel and maintenance costs translate into significant project lifecycle savings. Figure 2 on page 76 shows the relative expenditures in capital, maintenance, fuel, repairs and total life-cycle costs for the hybrid and the dual-diesel systems. By the 10th year of operation, with a fuel cost of $2/gallon, the hybrid system saves the operator about $300,000, after a payback period of only two years. Delivered fuel cost is one of largest determining factors affecting total site life cycle costs for power generation. Depending on the site, delivered-fuel cost can range from $1-3/gallon.
Solar only An alternative is to use a single renewable power source, solar energy, to support communications without the use of a backup engine generator. A single source is ideally suited for wireless communications applications requiring less than 650W average continuous power. Because solar systems do not require fuel, they are often less expensive to operate and to maintain than hybrid or dual-diesel alternatives.
One type of solar system is pre-assembled at the factory with integrated enclosures to simplify deployment. The solar array is often installed on top of the system enclosure to eliminate extra foundation work and to shade the power equipment. Another integral design feature is passive cooling technology that mitigates extreme temperature conditions that affect charging performance and battery life.
The cost to operate and maintain solar systems is minimal. Typical annual maintenance includes cleaning the solar array, checking battery voltage and inspecting the site. Applications for solar power systems encompass almost every facet of the wireless marketplace. Within the cellular network, large solar systems are ideally suited for mountaintop cellular repeaters that could link solar-powered cellular pay phones along highways or within rural villages. The almost unlimited access to remote territories via earth-orbiting communications satellites supports using solar power for remote industrial supervisory control and data acquisition (SCADA) applications and rural phone offices.
Where long-distance fiber links cross extremely remote areas, reliable power must be provided every 20-40km for signal regeneration. Finally, rural telephony projects that use point-to-multipoint radio networks commonly use solar energy at most, if not all, of the repeater and subscriber sites to streamline network deployment.
Cycle charge In the absence of “fuel-saving” renewable resources (wind or solar) at remote sites (higher latitudes, for example), there is one other system architecture worth considering before moving to continuously run diesel generators or miles of expensive grid extension. Cycle charge systems operate a fossil fuel generator intermittently (15-40% run time) to recharge a large battery bank which provides continuous current to the load. By discharging the battery to 40-60% DOD, engine run time, maintenance and fuel use are kept to a minimum. The logic is simple: Fossil fuel has a much higher energy density and cost than the battery plant. By cycling the generator and storing energy in the battery, energy efficiency is maximized. The advantages and disadvantages of cycle charge systems, compared to renewable energy-based systems are quite obvious. The cycle charge unit does not require as much site-specific planning because it is independent of wind or solar resources, and it requires “regular” engine maintenance that is well understood by telecom service workers. The disadvantages of cycle charge include higher fuel usage, battery change-outs and engine maintenance than renewable-based systems.
Field examples * GTE/Codetel, Dominican Republic — In 1991, a wind-solar-diesel hybrid system was installed to provide 1,000W of electricity to a mountaintop microwave repeater station. The system receives about 60% of its energy requirements from the 3kW wind turbine and 6kW solar array, resulting in engine run time of only 1,255 hours per year. Maintenance and fuel requirements were reduced by 80% compared to traditional dual-diesel generators.
* AT&T, Bolivia — AT&T World Services used solar-diesel hybrid systems to power each of its 17 mountaintop repeater sites between Santa Cruz and Quijarro, Bolivia, in 1990. On average, the hybrid systems reduced fuel requirements by 75% where anticipated delivered fuel costs were US$1 per liter. Site service is performed biannually.
* Sprint, Nevada — Sprint installed a stand-alone solar system to operate a remote fiber-optic regeneration site in 1990. The system uses a 5kW solar array to displace precious fuel, which is nearly impossible to deliver across 4km of treacherous road.
Remote power monitoring software The remote power monitoring software allows a personal computer to monitor and control power system performance, to display graphics on a cathode-ray tube (CRT) and to schedule maintenance for a multitude of locations. Remote monitoring software products vary in degree of capabilities and sofistication ranging from simple alarm functions to Windows-based viewing screens to help evaluate discrete functions and histories within each power system in the network.
This type of program is extremely valuable to network operators who want to diagnose power system problems before dispatching service employees to remote locations. Furthermore, the software can notify operators of upcoming maintenance requirements according to actual site conditions, such as the engine run time and fuel reserves.
Wireless energy’s market implications Within the new wave of wireless networks, hybrid and stand-alone solar technology will affect the way communications sites are deployed and serviced. No longer tethered to the electric utility grid, network planners will have greater versatility in site selection. The advent of wireless energy technology will undoubtedly create similar advantages to those found with wireless communications: fewer “out-of-plant” liabilities. reduced site-development requirements. lower risk of service interruption.
The broad market implications are more profitable and more widespread telecommunications services for even the most remote communities in the world.