A balancing act
Land-mobile radio systems that utilize base stations and repeaters can be engineered to save electrical power. By using link-budget engineering, and by balancing the uplink and downlink radio paths, the base station transmit power in the downlink radio path sometimes can be lowered to match the uplink transmitters from the mobile and portable devices.
As discussed in previous articles in this series on resource management, base stations and repeaters consume a large portion of the electrical power in a wireless communications system. The efficiency ratio of the transmitted or radiated power to electrical power in most base stations is 50% or less. Every 1-watt decrease in transmitted power reduces electrical power by 2 watts.
Most wireless systems attempt to maximize the transmitted power from the base station to the subscriber device in the downlink radio path. While this practice may be appropriate for one-way paging, and television and radio over-the-air broadcast systems to maximize the coverage, there always are FCC-mandated limitations placed upon radiated output power, based on antenna height above average terrain, proximity to co-channel users and the reduction of potential interference.
Consequently, designing wireless systems for two-way communications requires link-budget balancing, in order to ensure that the transmitted downlink path to the mobile or portable device’s receiver will match the transmitted uplink path from the mobile or portable device to the base station receiver. There are the same limitations of radiated output power from the FCC.
Now let’s examine base station systems. The base station consists of one or more transmitters and receivers, electrical power management, control electronics and the antenna transmission system. The transmitter can be analog or digital, and is configured to repeat the signals from its receiver or receivers on separate frequencies, or is controlled by local or remote signaling.
The efficiency of the base station or repeater transmitter does come into play, especially when backup power or alternative power sources are required for the system. Many of the newer radio transmitters can be set to operate with a lower transmitter power level when the unit senses that it is running on backup power due to a primary power outage. A design engineer or maintenance technician should incorporate this feature into any critical communications system, in order to insure that the system remains operational in the event of a long-lasting primary power outage. This is when the communications systems are needed most.
The mobiles, portables and fixed subscriber units have a very important role in the balancing of the link budgets and in the potential lowering of the base station power levels to save electricity. If the design goal of the two-way radio system is portable-at-the-hip in-building coverage, and there is an imbalance of weaker uplink to stronger downlink range, then certain measures can be taken to maintain the design goals but also achieve energy savings. If allowed, tower-top amplifiers can be added to help offset the imbalance. Distributed antenna systems can be used in buildings, including tunnels and large structures such as sports facilities, to address coverage challenges. A combination of these methods can be used so that the minimum number of sites will cover the entire area under the worst of link-balancing conditions. If the objective is electrical power savings at the base station sites, these methods also can be used to lower transmitted power in the downlink.
For mobile and fixed client systems, the transmitted power level is relatively close to the base station; as a result, the link budgets are easier to balance. If there are portables, mobile and fixed clients in the same system and the goal is portable-at-the-hip in-building coverage, then the mobile and fixed client powers will need to be lowered to equalize the range of the system.
The downlink radio path, which consists of the base station transmitter; jumper cables at the bottom of the radio site and at the top at the antenna; filtering, such as duplexing and combining; and the antenna. The remainder of the path consists of the subscriber device’s receiver. It can be a mobile, portable or fixed device, and each has an antenna system to be considered.
A portable radio that receives in the downlink radio path has no transmission line or filtering, but does have body losses to consider, which vary based on whether the device is worn at the hip or held at head level. When calculating the effective receiver sensitivity of portable devices, the antenna de-coupling to the body changes the gain from the value specified and must be modified in the link budget.
A mobile radio that receives in the downlink path has a roof-mounted antenna and transmission line. The mounting of the antenna is important when determining the link budget. If the mobile antenna is mounted squarely on the roof of the vehicle, then an ideal ground plane can be assumed and the specified gain of the antenna can be used in the link budget. If the mobile antenna is not mounted with a solid ground plane, then a modified gain should be factored into the link budget.
In a point-to-point or point-to-multipoint system, the downlink receiver is normally fixed in location and high-gain external antennas at the receiver end can be used, even if the downlink transmitter has an omnidirectional antenna. This added gain of the receiving antenna can be translated to lower power for the downlink transmitter, depending on the uplink path.
The uplink radio path consists of the portable, mobile or fixed unit transmitting its signal back to the fixed transmitter location. If a voting receiver network is part of the system, you generally can have lower-power subscriber units connecting to the fixed equipment, or have much greater range on the uplink because of the distributed network of receivers for a single transmitter. A radio system operator that only has to cover five square miles for its service area does not have to worry much about these issues. But let’s consider a large operator that covers a thousand square miles and has to deal with terrain issues, multistory buildings with multiple basement levels, and multiple groups that have different missions when using the radio system. In such a circumstance, the operator has to consider all aspects of the system.
As with the downlink radio path, portable radios in the uplink radio path have no transmission line or filtering but do have body losses to consider; again, these depend on whether the device is worn either at the hip or held at head level. Mobile radios transmitting in the uplink path typically have a roof-mounted antenna and transmission line, just as in the downlink.
In a point-to-point or point-to-multipoint system, the uplink transmitter is fixed in location and high-gain external antennas are used. It should be noted that when the transmitting antenna is below the level of the surrounding trees and buildings, then effective radiated power in the uplink may not be as advertised; this is because multipath losses may occur that rob the link budget of its full potential. An extra margin — 1 dB or 2 dB — should be added to the uplink path to ensure that localized multipath is not limiting the radio’s ability to communicate.
Often, the police department of a large community claims that there are no coverage holes in their system, while the fire department using the very same system claims that it does not work at all. This is because the police generally work at or above ground level, while firefighters and EMS workers often toil below ground level in sub-basements that were not part of the system’s design criteria. Consequently, design engineers and maintenance technicians working to provide the required coverage of a radio system, should consider end-user needs when configuring the system’s design and operating parameters.
A two-way radio system that has a 250-watt transmitter talking to 5-watt portable subscriber units truly will be lopsided in the downlink path. If the system is designed for portable-at-the-hip coverage inside buildings with an average penetration loss of 15 dB, then the uplink and downlink should be balanced. One thing that you do not want in a public-safety radio system is mobile-only coverage, as the portable radios usually are needed most when the first responders are out of their vehicles. In addition, when balancing the link budget, do not limit the range of the base station receiver to match the range of the mobile unit transmitters, as that will keep the portable units from using the system.
When calculating the link budget, you must first know the noise floor of the base station site, as the effective receiver sensitivity must be used and not the published specification. There are many systems in operation today that are not satisfying the needs of the users because the engineers and technicians have not taken this parameter into consideration. The receiver’s effective sensitivity is either measured or calculated in two different ways, as follows:
In the downlink, start with the thermal noise formula, which represents the noise bandwidth of the mobile or portable receiver’s radio channel, and then add to that the noise figure of the device. The faded channel is calculated using the digital audio quality (DAQ) value from TSB-88.
In the uplink, the base station’s 5% bit error rate (BER) specification is used as the starting point. The static design value is used to find the static sensitivity. Once the static sensitivity is found, the faded channel again is calculated using the DAQ value from TSB-88.
There are many methods to balance the links once all of the design parameters are realized. As the base station, portable, mobile and fixed client subscriber parameters are found from the methods discussed in this article, a spreadsheet can be used to analyze the balancing mobiles versus portables, and uplink versus downlink. The most important aspect of the link budget balancing is the need to meet coverage requirements. Also important is minimizing the number of sites to meet these goals, and minimizing operating expenses such as electrical power at the base stations.
A good propagation-prediction program will tell the system designer whether a site will meet the needs of the system users. In past years, the equipment did not have particularly good receiver sensitivity or selectivity, and brute force power was the way that most engineers would satisfy the needs of the users. In today’s environment, the electricity to power the stations, the extreme crowding of the radio spectrum and the better specifications of all of the radios on the market has made it necessary to keep the power at a level that satisfies the users of the system, but does not overwhelm the channels.
Other factors also can result in better-performing systems without resorting to the high-power transmitters. Directional antennas, eliminating shadows from the towers, antennas using the gain in the right manner, and placing the transmitters in the right location all will have a major impact on how much power you will need to serve the needs of the users of the system.
Managing the radio-frequency power from the base stations of a land-mobile-radio system can save electrical power and help reduce costs to maintain the system. And by properly balancing the link budgets for mobiles and portables, the base station transmitter power can be set to a level that meets the coverage requirements and saves energy.
Ira Wiesenfeld, P.E., is a consulting engineer who has been involved in the radio communications business since 1966. He is a senior member of the IEEE and has been a licensed amateur radio operator since 1963. He can be reached at [email protected].
Robert C. Shapiro, P.E, is a consulting engineer who has been in land-mobile radio since 1984. He serves on the TIA TR8 committee (TSB-88) as vice chair and is a senior member of the IEEE. He can be reached at [email protected].
LMR 200 Series
Part 1: School’s back in session — LMR in real-world applications
Part 2: Where it all begins — Pros, cons of primary Part 90 spectrum bands
Part 3: Spectrum redux — Part 90 spectrum bands in real-world applications
Part 4: The power of reduced power — Smart technologies optimize radio performance
LMR 100 Series
Part 1: Class is in session: Basic LMR and FCC definitions
Part 2: Start at the beginning: Understanding LMR user needs
Part 3: The devil’s in the details: Conducting a user-needs survey
Part 4: Decisions, decisions: Understanding the LRM procurement process
Part 5: Let’s get started: System engineering begins with RF planning
Part 6: The lynchpin: Receiver planning and noise interference
Part 7: Connecting the dots: How to connect LMR sites
Part 8: The next piece of the puzzle: Understanding dispatch communications
Part 9: Now the real work begins: How to select a suitable LMR site
Part 10: The bane of your existence: How to deal with RF interference
Part 11: Winning the battle: More causes of RF interference
Part 12: Now the fun begins: Installing the LMR system
Part 13: Dotting Is and crossing Ts: Choosing the LMR project, program managers
Part 14: Now you’re done: Maintaining the LMR system