Remote - December 2011 - (Page 8)

Feature Article Maximize Range While Minimizing Power Consumption in Wireless Digital Transmission Michael Derby, Chief Technical Officer AvaLAN Wireless Systems, Inc. This article will discuss design considerations that provide the maximum transmission distance for digital wireless radio communication in the FCC unlicensed bands, while minimizing the system power supply requirements, thus facilitating solar or wind powered remote applications. System integrators with radio experience are aware that the FCC imposes rules that limit the transmitted RF power in the non-licensed ISM bands. These rules are different for each of the three bands, 900 MHz, 2.4 GHz and 5.8 GHz. There are also different rules for multi-point vs point-to-point topologies. For simplicity, we will only investigate point-to-point range optimization where the conducted output power and antenna gain must be properly designed to ensure legal operation. The conducted power (power coming out of the radio’s connector) limit is 30 dBm or 1 Watt for all three bands, but may need to be reduced when high gain antennas are used. • At 900 MHz, the user is allowed to use maximum conducted power when used with an antenna and cables whose respective gain and loss sum to 6 dB or less. For example, a 15 dBi gain antenna connected with a cable whose loss is 9 dB meets the legal limit. If the antenna has more gain or lower loss cables are used, the radio’s conducted output power would need to be reduced to meet the 6 dB limit. If a 15 dBi antenna is used with short cables (assume no loss), then the conducted output power would need to be reduced by 9 dB to 21 dBm. • At 2.4GHz, the calculations are similar but the conducted power must be reduced by only 1 dB for every 3 dB that the antenna gain exceeds 6 dBi. • At 5.8Ghz, no reduction in conducted power is required regardless of the antenna gain. considerations we believe that short RF cables are typically the best choice and will focus hereafter on this selection. For maximum range we will consider three combinations of antenna and frequency whilst assuming short loss less cables: a 900 Mhz radio with a 15dBi Yagi, a 2.4 Ghz radio with a 24 dBi parabolic and a 5.8 Ghz radio with a 29 dBi parabolic. Applying the previously described conducted limits the systems would respectively need to operate legally at 21 dBm (1/8 W) , 24 dBm (1/4 W) and 30 dBm (1 W). The current draw of the 2.4 Ghz and 5.8 Ghz radio’s power amplifiers are expected to be three and 12 times higher respectively than the 900 Mhz system. If we choose to have all the radios consume similar current draws by limiting their conducted outputs to 21 dBm, the 900 Mhz radio will achieve a superior “real world” fade margin and thus exhibit the best resistance to environmental variations. The 15 dBi antenna used with the 900 Mhz radio with also be much easier to align due to its wider 30° directional beam width compared to the narrow 8° and 1° beam widths of the 2.4 and 5.8 Ghz antennas. Other decision factors might include potential interference levels or international spectral availabilities and thus the installer may wish to consider 2.4 or 5.8 Ghz radio solutions at shorter ranges. Given these considerations we believe that 900Mhz with a 15 dBi antenna is typically the best choice and will focus hereafter on this selection. 900 Mhz has the lowest frequency and longest wavelength compared to 2.4 and 5.8 Ghz. Lower frequencies have lower attenuation through materials that impair the radio’s transmission path. These materials can typically include trees and small buildings. Higher frequencies require a clear visual path (line of sight) with no visible path impairments. Lower frequencies also have an ability to bend slightly around hard path impairments like the ground or large buildings. Applying wave bending characteristics and fade margins should be done by experienced wireless technicians. Given these considerations, we believe that 900 Mhz is typically the best choice given real world path impairments and will focus hereafter on this selection. Most applications involving low power consumption operate well over serial or modest speed Ethernet. Considerations for using each include: • Ethernet data streams allow for digital error detection/correction and packet retransmission. • Serial can more easily allow for duty cycling the system on and off to save power. • Serial applications can run over Ethernet but not vice versa. • Ethernet is forward trending with more devices using this interface. Given these considerations we believe that Ethernet is typically a better choice and will focus hereafter on this selection. For a radio to achieve higher speed communications typically requires a more powerful CPU to process the higher data rate. Sizing the radio’s data communications rate to match the requirements of the application is one key to reducing unnecessarily wasted current draw. For example, applications that require 9600 baud serial communications would not be efficient to run at a 54 Mbps data rate. Considering this and the atypical demand for higher communication speeds used by solar powered nodes, we will focus on the larger part of the market, modest speed Ethernet (~1 Mbps). There is another very significant reason for selecting 1 Mbps Ethernet communications. The radios can utilize narrower RF channel bandwidths. This will allow longer range and enhanced receiver sensitivity compared to The most important decision when designing long range radio systems is to use short RF cables (<1 ft) from the antenna to the radio. Short cables require less conducted output to overcome cable losses and more importantly the short cables don’t act as an attenuator for the receiver’s “ears”. The lower conducted power allows more range and less current draw from the radio’s power amplifier. The lower power also creates less heat to dissipate, enabling outdoor installations that are often requisite to using short RF cables. The reduction in attenuation on the receiver’s sensitivity is acoustically akin to removing ear plugs from one’s ears. The radio is simply able to “hear” better and thus extends the range with no added current draw. A long range antenna must usually be mounted high in the air and thus short RF cables imply mounting the radio next to it. Modern power over Ethernet techniques allow the radio to be mounted at the antenna with power and data transferred over a single digitally loss less and relatively inexpensive Ethernet cable. Full diagnostic information can still available from the radio mounted at the antenna if it provides a browser interface. Given these 8 www.RemoteMagazine.com http://www.RemoteMagazine.com

Table of Contents for the Digital Edition of Remote - December 2011

Utility Telecom Spending Could Reach $3.2 Billion by the End of 2011
Securing Remote Substations for a Smarter Grid
Maximize Range While Minimizing Power Consumption in Wireless Digital Transmission
CMS Deploys Energy Smart Grid Project
Satellite Technology – The Key to Remote Communications for Government Agencies
American Innovations Releases Bullhorn RM4012 Remote Monitoring System
ProSoft Releases Modbus TCP/IP to IEC 61850 Gateway
Pelco Introduces the New Digital Sentry
CEITEC S.A. Releases RFID Chip for Logistics Application
C&D Technologies Introduces Liberty MSE 2 V VRLA Line
Industry News
Calendar of Events
Washington State Archives Keeps Cool with Environmental Monitoring

Remote - December 2011