Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1997-07-02
2002-06-11
Hunter, Daniel (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S062000, C455S447000, C455S449000
Reexamination Certificate
active
06405043
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to cellular telephone systems and, more particularly, to processes for designing and improving the performance of cellular telephone systems.
2. History of the Prior Art
Presently available commercial mobile communication systems typically include a plurality of fixed base stations (cells) each of which transmits signals to and receives signals from mobile units within its communication area. Each base station is assigned a plurality of channels over which it can communicate with mobile units. A mobile unit within range of the base station communicates with the external world through the base station using these channels. Typically, the channels used by a base station are separated from one another sufficiently that signals on any channel do not interfere with signals on another channel used by that base station. To accomplish this, an operator typically allots to a base station a group of channels which are each widely separated from the next. So long as a mobile unit is within the area in which the signal from a base station is strong enough and is communicating with only that base station, there is no interference with the communication.
In order to allow mobile units to transmit and receive telephone communications as the units travel over a wide geographic area, each cell is normally physically positioned so that its area of coverage is adjacent to and overlaps the areas of coverage of a number of other cells. When a mobile unit moves from an area covered by one base station to that covered by another, communication with the mobile unit is transferred (handed off) from one base station to another in an area where the coverage from different cells overlaps. Because of this overlapping coverage, the channels allotted to the cells are carefully selected so that adjoining cells do not transmit or receive on the same channels. The channels used by adjoining base stations are also theoretically separated from the channels of each adjoining base station sufficiently that signals from any base station do not interfere with signals from another adjoining base station. This separation is typically accomplished by assigning a group of widely separated non-interfering channels to some central cell and then assigning other groups of widely separated non-interfering channels to the cells surrounding that central cell using a pattern which does not reuse the same channels for the cells surrounding the central cell. The pattern of channel assignments continues similarly in the other cells adjoining the first group of cells. The pattern is often called a channel reuse pattern.
So long as a mobile unit is within the area in which the signal from a base station is strong enough and is communicating with only that base station, there is no interference with the communications. However, when a mobile unit moves from an area covered by one base station to that covered by another base station, the communication must be transferred from one base station to the other in an area. This requires cell coverage to overlap. Because of this overlapping coverage, the channels allotted to the cells are carefully selected so that adjoining cells do not transmit or receive on the same channels.
There are a number of different types of mobile communications systems. Channels are defined in different manners in each of the different systems. In the most prevalent American Mobile Phone System (AMPS) system, channels are defined by frequency. A frequency band of 25 MHz providing approximately four hundred different adjoining FM frequency channels is allotted by the federal government to each cellular operator. In a typical AMPS system, each channel uses a fixed FM frequency band width of 30 KHz. for downlink transmission from a base station to a mobile unit and another fixed FM frequency band width of 30 KHz. for uplink transmission from a mobile unit to a cell. Typically, the frequencies assigned to the downlink transmissions for an entire cellular system immediately adjoin one another and are widely separated from the frequencies assigned to the uplink transmissions which also immediately adjoin one another. In this specification, even though widely separated, the pair of frequencies used for both downlink and uplink transmission are generally intended when reference is made to an AMPS channel unless the context indicates otherwise.
Since channels are defined by frequency in an AMPS system, the channels used by any single base station are separated from one another in frequency sufficiently to eliminate interference between those channels. An operator typically allots a base station a set of channels with frequencies which are each separated from the next by some large number (e.g., twenty-one) channels carrying intermediate frequencies. Thus, in a system with twenty-one channel separation, one base station might use channels 1, 22, 43, 64, 85, and so on up to a total of between five and one hundred individual channels.
When a mobile unit moves from an area covered by one base station to that covered by another base station in an AMPS system, the communication must be transferred from one base station to the other in an area in which cell coverage overlaps. Because of this overlapping coverage, the channels allotted to the cells are carefully selected so that adjoining cells do not transmit or receive on the same frequencies. This is typically accomplished by assigning channels to a central cell which are widely separated in frequency in the manner described above, and then assigning channels to the cells surrounding that central cell using a pattern which increases each channel number by one for each sequential cell surrounding the central cell. Thus, if cells are arranged in a honeycomb pattern in which six cells surround a central cell using the above-described channels, a first cell adjacent to the central cell may have channels 2, 23, 44, 65, 86, and so on while a second cell adjoining the central cell may have channels 3, 24, 45, 66, 87, and so on. The pattern of channel assignments continues similarly in the other cells adjoining the central cell.
In some AMPS systems, especially those with cells in urban areas carrying heavy traffic, each cell may be further divided into two or three sectors each of which may include channels having the above-described frequency allotment of channels. The antennas of each sector are typically arranged to provide 180 or 120 degree coverage. When cells are discussed herein, sectors are normally meant as well unless the context indicates otherwise.
Another type of mobile system called Code Division Multiple Access (CDMA) uses digital signals to transmit data. All of the base stations of a CDMA system use the same “spread spectrum” frequency band of 1.25 megacycles to transmit the digital signals. The transmissions are combined with redundant channel coding information to allow error correction. The encoded signals are then multiplied by one of sixty-four Walsh codes which establish individual channels and increase the bandwidth to 1.25 megacycles. Because of the redundancy of the encoded signals, a receiver may decode a signal from the plethora of coded channels carrying data on the broad frequency band. Since the Walsh codes establish a number of individual channels and the pseudo-noise code assigned to each base station differs from those of other surrounding base stations, adjacent and remote cells may reuse the same frequency bands.
In another common type of mobile system called Time Division Multiple Access (TDMA), frequencies are assigned to the entire system in groups much like they are assigned in an AMPS system. However, within any frequency, each base station sends and receives in bursts during some number of different intervals or time slots. These time intervals within frequency bands then effectively constitute the individual channels. By assuring that the group of frequencies assigned to any individual base station differ from one another and from the frequencies assigned to base
Arpee John E.
Jensen Eric H.
Hunter Daniel
ScoreBoard, Inc.
Sheets Kendal M.
Tran Pablo
Zito Joseph J.
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