Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
2001-11-05
2004-11-30
Duong, Frank (Department: 2666)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S404000, C370S537000, C398S116000, C455S422100
Reexamination Certificate
active
06826163
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to cellular mobile telecommunication systems, and more particularly to a shared network to distribute base station antenna points and the associated base station transceiver hardware.
2. Description of Related Art
A conventional cellular telecommunications system has a fixed number of frequency channel sets distributed among base stations that serve a plurality of cells that are usually arranged in a predetermined reusable pattern. Typical cell areas range from 1 to 300 square miles. The larger cells can cover rural areas and smaller cells cover urban areas. Cell antenna sites utilizing the same channel sets are spaced by a sufficient distance to assure that co-channel interference is held to an acceptably low level.
A basic cellular network is comprised of mobile units, base stations, and a mobile switching center or mobile telecommunications switching office (MTSO). The mobile unit has radio telephone transceiver equipment that communicates over a radio station relays telephone signals between mobile units and an MTSO by way of communication lines. The cell site and the MTSO are typically connected by T1 lines, which carry telephone and control signals. The MTSO is also connected through paths to a switched telephone network.
An MTSO can include a switching network for establishing call connections between the public switched telephone network and mobile units located in cell sites and for switching call connections from one cell site to another. Additionally, the MTSO can include control systems for use in switching a call connection from one cell site to another. Various handoff criteria are known in the art, such as using received signal strength to indicate the potential desirability of a handoff. Also included in the MTSO is a central processing unit for processing data received from the cell sites and supervisory signals obtained from the network to control the operation of setting up and taking down call connections.
A conventional base station includes a radio controller unit that provides the interface between the T1 lines from the MTSO and the base station radio equipment. It also includes one or more transceivers, which perform radio transmit and receive functionality, and are in turn connected to antennas. A single transceiver radio often supports one channel or frequency allocation. The focus of this invention lies in placing a network between the transceiver radio and the antenna. Generally, the radio transmitter signals are then passed to a separate power amplifier for each channel, or the signals may be combined and applied to a single power amplifier. The output of the power amplifier is applied through a duplexer to an antenna, to be broadcast into the cellular area serviced by the base station.
Signals received in an antenna are applied through a duplexer to a filter. The filter isolates the entire cellular band signal from adjacent bands and applies it to receivers, one for each channel. The base station may optionally include a diversity antenna and corresponding diversity filters and a plurality of diversity receivers, one for each associated main receiver. Where implemented, the outputs of diversity receivers are applied to circuits include circuitry for selecting the strongest signal using known techniques. In densely populated urban areas, the capacity of a conventional system is limited by the relatively small number of channels available in each cell. Moreover, the coverage of urban cellular phone systems is limited by blockage, attenuation and shadowing of the RF signals by high rises and other structures. This can also be a problem with respect to suburban office buildings and complexes.
To increase capacity and coverage, a cell area can be subdivided and assigned frequencies reused in closer proximities at lower power levels. Subdivision can be accomplished by dividing the geographic territory of a cell, or for example by assigning cells to buildings or floors within a building. While such “microcell” systems are a viable solution to capacity and coverage problems, it can be difficult to find space at a reasonable cost to install conventional base station equipment in each microcell, especially in densely populated urban areas. Furthermore, maintaining a large number of base stations spread throughout a densely populated urban area can be time consuming and uneconomical.
A generic solution to this problem is to separate some components of the base station from the antenna node, and connect them with a link. The smaller footprint antenna node is located at the desired coverage location, while the rest of the base station is placed at a more accessible location. The link is generally fiber optic. The related art has approached this problem from two distinct positions: single link fiber fed repeaters and distributed base station architectures. Fiber fed repeaters generally separate the base station at the radio output to the antenna, employing a broadband transparent link which carries the RF uplink and downlink signals across the entire communication band, as distinct from a single channel or frequency allocation (FA). The broadband link can be analog or digital, but if digital, the digital signal transparently repeats the entire band, for example, the 12.5 MHz US Cellular A band. The link is point-to-point, one radio to one antenna. U.S. Pat. Nos. 5,627,879, 5,642,405, 5,644,622, 5,657,374 and 5,852,651 form a group which teach the implementation of cellular point-to-point links by employing a digital solution transparent to the communication protocol being employed.
The distributed base station solution, unlike the repeater solution, builds multi-link solutions. EP 0 391 597 discloses a simulcast network over optical fiber using analog carriers. In the network envisioned by this patent, multiple carriers are combined in the RF domain and then optically transported for simulcast transmission/reception out of a fiber-fed antenna array. The optical carrier is analog modulated with the RF signal. Dedicated fiber lines are used rather than optically multiplexed signals between remote antennas and the centralized base station, and the signals are not multiplexed between multiple base station radios and multiple antennas.
A distributed cellular network is disclosed in U.S. Pat. No. 5,519,691 in which radios are pooled at a common location and communication links connect the radios to distributed antenna units. A multiplexing method is provided for multiple channels on a cable or single optical carrier network, in which frequency division multiplexing in the RF domain is combined with analog signal transmission. The network is closely integrated with the base station, with channel allocation and manipulation at both host and remote ends of the network involving base station control. Provision is also made for time division multiplexing in the signal domain.
Another distributed cellular network is disclosed in U.S. Pat. No. 5,761,619. This network is closely integrated with the base station architecture. The base station radios are placed at a different point than the antennas, and the radio is assumed to be a digital unit which either performs a wideband digitization of the cellular band or filtering and a narrowband channel digitization. In this architecture, an optical network transports these digitized signals using a dynamic synchronous protocol. In this protocol, circuit paths are dynamically set up between remote antenna nodes and base stations using this protocol. A synchronous TDM protocol is used for signal multiplexing.
U.S. Pat. No. 6,205,133 B1 discloses a digital architecture that is similar to the one disclosed in U.S. Pat. No. 5,761,619. In this disclosed architecture, the concept of a software radio is used to build a distributed antenna system by modifying the base station architecture. The software radio transceivers are remotely located, and convert the RF signals into digital signals, which are transported over a digital link to a central hub stat
Cutrer David
Mani Sanjay
Davis Paul
Duong Frank
Heller Ehrman White & McAulife
NEXTG Networks
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