Low power wireless data communications systems are typically used in environments for which it is either impractical to install cabling to link a communications network to fixed terminal sites or there exists a need to support communications between the communications network and mobile terminals. Fixed communications environments include railyards and other facilities spread over a large area. For mobile communications environments, a typical application is the use of a wireless data communications system within a warehouse for stocking of goods for shipment or for manufacture.
A typical low power wireless data communications system includes multiple access points for communicating with remote terminals and a host computer. An access point, also described as a base station, generally includes an RF transceiver for communicating with the remote terminals and a network interface for communicating with the host computer. The communications link between the access points and the remote terminals are often implemented by a wireless radio frequency (RF) communications link, whereas the access points are typically connected by coaxial or optical cabling to the host computer. The remote terminals are low power transceivers, typically 100 milliwatts to 1 watt radiated power, having a receiver for receiving data to the access points and a transmitter for transmitting data from the access points. The host computer, which communicates with the access points, is responsible for processing incoming data received by the access points and for generating outgoing data to be transmitted by the access points.
A typical business application for a low power wireless data communications system is the support of communications within a building, such as warehouse, for facilitating the taking of inventory, filling orders, directing employees as to the placement of inbound products in the warehouse, and the like. To supply data for transmission by a remote terminal, each device can include an input device for inputting data and display for displaying data. Input devices include keyboards or keypads, light pens, touch screen displays, and bar code readers and output device include liquid crystal displays, pixel-based output devices, and printers. For mobile communications, the remote terminals can be implemented as a hand-held unit or as a unit mounted on a vehicle, such as a forklift or truck.
Largely by regulatory constraint in the United States of America, wireless data communications system for business applications are typically operated at a low power level because authorized radio transmissions occur within shared portions of the electromagnetic spectrum under the regulatory scheme implemented by the United States Federal Communications Commission (FCC). Since 1985, the FCC has approved the use of low power non-licensed Business Radio Systems regulated under sub part D of Part 15 of Title 47 of the Code of Federal Regulations. Three bands are authorized for such use: 902-928 MHz, 2400-2483.5 MHz, and 5725-5850 MHz. Currently, there are a number of constraints on the operation of such business communications systems, including a maximum radiated power of 1 watt.
Recently, advanced applications of business radio systems have required significantly increased rates of data transmission from mobile remote terminals to access points. While many business radio systems at present operate in the frequency band near approximately 900 MHz, as discussed above, those systems are increasingly unable to support the high data rate capabilities required by new business applications. This limitation on data rate capacity of the 900 MHz systems is a result of the relatively limited frequency bandwidth available in that band. Consequently, many new business radio systems are designed to operate in the frequency band near 2400 MHz to take advantage of the wider frequency bandwidth available in that band. Future systems are expected to migrate to the frequency band near 5800 MHz, as data rate requirements for business communications applications continue to increase.
As newer business radio systems migrate to the higher frequency bands, certain performance limitations become more pronounced. Most particularly, the propagation of RF energy throughout a building or a covered area becomes significantly more directional in nature. In addition, the RF energy within these higher frequency bands is affected more severely by the presence of walls, inventory, ceilings, and other objects and structures within the operating environment. As a result, there may exist certain locations within an application site for which the RF link between an access point and a mobile remote terminal is no longer adequate to effectively maintain the communications link between them. In effect, the RF coverage of a business radio system employing higher frequency access points may be significantly reduced relative to the coverage of a system employing access points at the same physical location and operating at lower frequencies.
In addition to propagation effects, which degrade communications link performance at higher frequencies, higher data rate applications require receivers with wider bandwidths. As receiver bandwidth increases, receiver noise also increases and less link margin is available. Link margin degradation on the order of 30 dB is typical when a data communications system moves from lower to higher frequency bands. This reduced link margin is manifested in a smaller RF coverage area relative to the coverage area that would correspond to a narrow band receiver.
In view of the foregoing, it will be understood that increasing the data rate capacity of a business radio system generally requires an increase in operating frequency to take advantage of the higher bandwidths at these higher frequencies. However, the RF coverage available from a given access point to mobile remote terminals within a site can be reduced significantly by propagation and noise level effects. To compensate for this reduced RF coverage, it is well known to simply increase the number of access points so that RF coverage at all points within the site is adequate to maintain communications links.
Unfortunately, the solution of increasing RF coverage area by increasing the number of access points at a site typically requires the user to incur a significant increase in cost relative to a lower frequency system. This increase in cost arises from two problems. First, the cost of RF components and other devices to operate at higher frequencies with similar power level increases rapidly with increased frequency. As a result, systems with increased numbers of access points have hardware costs significantly greater than the hardware costs for lower frequency systems. In addition, retrofitting existing low frequency sites to operate at higher RF frequencies can entail significant cost when a given set of lower frequency access points must be replaced by an increased number of higher frequency access points. Existing cabling and structure must be removed, while new cabling and structure to accommodate the new access points must be installed at the site.
A need exists for a data communications system which can provide the requisite ubiquitous RF coverage without the expense associated with an increased number of access points. There is a further need for a data communications system that provides an increased data rate associated with operation at a higher RF range, as well as an increased RF coverage area, without the significant cost associated with the addition of more access points to the system. The present invention addresses these needs and other problems of the prior art by the use of a distributed communications architecture employing repeaters to extend the coverage of the data communications system.