Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-11-19
2004-02-17
Pham, Chi (Department: 2667)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C455S446000
Reexamination Certificate
active
06693884
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to cellular telephone systems and, more particularly, to processes for quantifying the performance of CDMA 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 selected so that signals on any channel do not interfere with signals on another channel used by that base station.
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 an area covered by another base station, communication with the mobile unit is transferred (handed off from one base station to another base station in an area where the coverage from different cells overlaps.
It is axiomatic that cellular telephone transmissions between the mobile units and the cells should be as free from interference as possible. The manner in which this is accomplished differs depending on the characteristics of the particular cellular system.
In the most prevalent American Mobile Phone System (AMPS) system, channels are defined by frequency. A frequency band providing approximately four hundred different adjoining FM frequency channels is allotted to each cellular system operator. In a typical AMPS system, each channel uses a fixed FM frequency band for downlink transmission from a base station to a mobile unit and another fixed FM frequency band for uplink transmission from a mobile unit to a cell. Typically, the frequencies assigned to the downlink transmissions for an entire AMPS cellular system immediately adjoin one another and are widely separated from the frequencies assigned to the uplink transmissions which also immediately adjoin one another.
Since channels are defined by frequency in an AMPS system, interference with any particular transmission is essentially due to transmissions on the same or immediately adjacent channels. To reduce this interference, an operator assigns channels to any single base station which are separated from one another in frequency sufficiently to eliminate interference between those channels. For example, an operator may allot to 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.
Moreover, since a mobile unit in an AMPS system moving from an area covered by one base station to that covered by another base station must be transferred from one base station to the other in an area in which cell coverage overlaps, interference with base stations having overlapping cell coverage must also be eliminated. To do this, the channels allotted to the adjoining cells are carefully selected to eliminate the same frequencies. This is sometimes 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 assignment by some number for each sequential cell surrounding the central cell. This produces what may be visualized as a honeycomb pattern of cells having a central cell surrounded by a number of overlapping cells transmitting on different frequencies. The same honeycomb pattern extends outward throughout the system with each cell surrounding the central cell functioning as a central cell surrounded by its own overlapping cells producing what is referred to as a reuse pattern. In such a pattern, interference on the same channel usually comes from cells at some distance from the cell carrying the useful information.
In most cellular systems, especially those with cells in urban areas carrying heavy traffic, a position at which a cell is situated includes two or three individual transceiving stations (referred to as “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. The terms cells, sectors, and base stations are normally used interchangeably in this specification unless the context indicates otherwise. If an AMPS system includes significant numbers of sectored cells, six cells arranged in a honeycomb pattern surrounding a central cell may all be assigned different and theoretically non-interfering channels. However, outside the initial central cell and its immediately surrounding cells, the frequency reuse pattern requires that channels be replicated at much closer ranges than in a non-sectored system.
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 using these intervals and assuring that the group of frequencies assigned to any individual base station differ from one another and from the frequencies assigned to base stations surrounding each individual base station, a channel reuse pattern is established which allows substantially greater use of the frequency spectrum because of the time division process.
A newer type of mobile system called Code Division Multiple Access (CDMA) uses encoded digital signals to transmit data. All of the base stations and mobile units of a CDMA system presently use the same “spread spectrum” frequency band of 1.25 megacycles to transmit the encoded digital signals although other band widths are presently proposed. The information bits of each transmission are expanded using coding information called a pseudo noise (PN) code. Each sector throughout a system uses the same PN code to encode the information transferred. Then each sector identifies itself by using a time offset (generally referred to as a pseudo noise (PN) offset) from some repeating initial time in the expanded transmission. Thus, one sector may begin an encoded transmission at the initial time, a second sector at an offset of one unit from the initial time, a third at an offset of two units, and so on up to a total of 512 offset units. Each transmission with a sector is placed on what is effectively a separate channel by further encoding the expanded transmission with one of a plurality of Walsh codes. A Walsh code is a mask used to encode and decode transmissions which eliminates transmissions sent using other Walsh codes. A transmission on a particular channel is decoded by applying a mask including the Walsh and PN codes to the received pattern of information bits commencing at the PN offset designated for the particular channel.
The CDMA system of transmission offers a number of advantages. One of these advantages is that a mobile unit may be receiving the same information from a number of different cells or sectors at the same instant. Since all transmissions take place on the same frequency band, a mobile unit actually receives all of the information which is available within its range. However, it only decodes information on channels which are directed to it. A CDMA mobile unit uses a receiver which is able to apply a number of decoding masks at the same instant to the entire spectrum of information which it receives. By knowing the
Boakye Alexander O.
Duane Morris LLP
Pham Chi
Scoreboard, Inc.
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