Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
2001-01-26
2004-08-17
Trinh, Sonny (Department: 2685)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
Reexamination Certificate
active
06778844
ABSTRACT:
BACKGROUND OF THE INVENTION
The disclosure relates to wireless communications systems, and, more particularly, to a wireless LAN that includes at an access point a continually varying antenna array as a technique for mitigating the deleterious effects of multipath signal propagation.
1. Field of the Invention
The invention relates to wireless communications systems, and, more particularly, to a wireless LAN that includes at an access point a continually varying antenna array as a technique for mitigating the deleterious effects of multipath signal propagation.
2. Description of the Related Art
A wireless local area network (LAN) provides a flexible data communication system that may be implemented as an extension to, or as an alternative for, a wired LAN. Wireless LANs transmit and receive data using radio frequency (RF) communications techniques to thereby minimize the need for wired connections. In this manner, wireless LANs combine data connectivity with user mobility.
Wireless LANs have gained strong popularity in a number of vertical markets, including the health-care, retail, manufacturing, warehousing, and academia. These and other industries have profited from the productivity gains of using hand-held terminals and notebook computers to transmit real-time information to centralized hosts for processing. Today wireless LANs are becoming more widely recognized as a general-purpose connectivity alternative for use by a broad range of business customers. Observers have predicted a sixfold expansion of the worldwide wireless LAN market by yearend 2000, reaching more than $2 billion in revenues. The widespread reliance on networking in business and the meteoric growth of the Internet and online services are strong testimonies to the benefits of shared data and shared resources. Wireless LANs enable users to access shared information without the need to establish a hard-wired connection. Network managers have the option to create or augment networks without installing or relocating wires. Wireless LANs offer productivity, convenience, and cost advantages over traditional wired networks. Those advantages largely derive from speed, flexibility and simplicity of installation, reduced cost of ownership, and scalability.
For a thorough discussion of wireless LAN technology, see Jim Greer,
Wireless LANs: Implementing Interoperable Networks
, Macmillan Technical Publishing (1999), hereby incorporated by reference. In general, the implementation of wireless LANs may be based on one or more of a wide range of technologies, including:
Narrowband Technology. Narrowband wireless systems transmit and receive data or information on a specific radio frequency or within a specific narrow band of frequencies. Narrowband RF techniques strive to minimize the bandwidth necessary to transmit information. Undesirable crosstalk between communications channels is avoided by carefully coordinating different users on different channel frequencies. From an implementation perspective, a salient drawback of narrowband technology is that, in general, the end-user must obtain an FCC license for each site where the technology is employed.
Spread Spectrum Technology. Wireless LAN systems predominately use spread-spectrum technology, a wideband RF technique developed by the military for use in reliable, secure, mission-critical communication systems. Spread-spectrum techniques offer enhanced reliability, integrity, and security, at the expense of increased bandwidth. In other words, greater bandwidth is required than in the case of narrowband transmission. However, the tradeoff produces a signal that is, in effect, more robust and thus easier to detect, provided that the receiver is informed with the parameters of the spread-spectrum signal that is transmitted. If a receiver is not tuned to the correct frequency, a spread-spectrum signal appears as background noise. There are two fundamental types of spread spectrum technology: frequency hopping and direct sequence.
Freguency-Hopping Spread Spectrum Technology. Frequency-hopping spread-spectrum (FHSS) uses a narrowband carrier that changes frequency in a pattern known to both transmitter and receiver. Properly synchronized, the net effect is to maintain a single logical channel. To an unintended receiver, FHSS appears to be short-duration impulse noise.
Direct-Sequence Spread Spectrum Technology. Direct-sequence spread-spectrum (DSSS) generates a redundant bit pattern for each data bit that is transmitted. The bit pattern is called a chip, or chipping code. The longer the chip, the greater the probability that the original data can be recovered and, concomitantly, the greater the amount of the bandwidth required. Even if one or more bits in the chip are dropped in transmission, statistical techniques embedded at the receiver recover the original data without the need for retransmission. To an unintended receiver, DSSS appears as low-power, wideband noise and is rejected (ignored) by most narrowband receivers.
Infrared Technology. Infrared (IR) represents a third available technology, albeit little used in commercial wireless LANs. IR systems use very high frequencies, immediately below visible light in the electromagnetic spectrum, to carry data. As is the case with light, IR cannot penetrate opaque objects and is, therefore, either directed (line-of-sight) or diffuse technology. Inexpensive directed systems provide very limited range (three feet) and typically are used for personal area networks, but are occasionally used in specific wireless LAN applications. High performance directed IR is impracticable for mobile users and is therefore used only to implement fixed sub-networks. Diffuse (or reflective) IR wireless LAN systems do not require a line-of-sight transmission path, but cells are limited to individual rooms.
In a typical wireless LAN configuration, a transmitter/receiver (transceiver) device, called an access point or, alternatively, a hub, connects to the wired network from a fixed location using standard cabling. At a minimum, the access point receives, buffers, and transmits data between the wireless LAN and the wired network infrastructure. A single access point can support a small group of users and can function within a range of less than one hundred to several hundred feet. The access point, or the antenna attached to the access point, is usually elevated, but may be mounted essentially anywhere that is practicable, as long as the desired transmission coverage is obtained.
End users access the wireless LAN through wireless LAN adapters. Wireless LAN adapters are implemented as PCMCIA cards in notebook or palmtop computers, or as cards in desktop computers, and may be integrated within hand-held computers. Wireless LAN adapters provide an interface between the client network operating system (NOS) and the transmission medium via an antenna. The nature of the wireless connection is transparent to the NOS.
Wireless LANs can range from simple to complex in both design and operation. At its most basic, two PCs equipped with wireless adapter cards can establish an independent network whenever they are within range of one another. Such a network is generally referred to a peer-to-peer network. Ad hoc peer-to-peer “networks” require no administration or preconfiguration. In this case, each client will have access only to the resources of the other client, but not to a network server or host computer.
Installing an access point can extend the range of an ad hoc network, effectively doubling the range at which the devices can communicate. Because the access point is connected to the wired LAN, each client is afforded access to server resources as well as to other clients. Each access point has the capacity to accommodate many clients, the specific number of clients depending on the number and nature of the transmissions involved. Many applications exist in which a single access point services from 15-50 client devices.
Access points have a finite range, on the order of 500 feet indoors and 1000 feet outdoors. In a very large facility, such as a war
Dell Products L.P.
Haynes and Boone LLP
Trinh Sonny
LandOfFree
System for reducing multipath fade of RF signals in a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with System for reducing multipath fade of RF signals in a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and System for reducing multipath fade of RF signals in a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3348562