Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1997-12-17
2003-01-14
Trost, William (Department: 2683)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S456500, C455S018000, C455S015000
Reexamination Certificate
active
06507741
ABSTRACT:
TECHNICAL FIELD
The present invention relates in general to radio frequency (RF) communications systems and, in particular, to an RF repeater with a delay to improve hard handoff performance between cells and method therefor.
BACKGROUND
Throughout the world, certain RF bands have been allocated for various types of communications, including personal communications system (PCS), cellular, and other mobile applications. In the United States, the Federal Communications Commission (FCC) has allocated frequency bands in the range of 824-849 and 869-894 MHz; and 1850-1910 and 1930-1990 MHz for such applications. Currently, the 824-849 and 869-894 MHz bands are used for mobile cellular communications and the 1850-1910 and 1930-1990 MHz bands are used for PCS applications. Other countries utilize their own frequency spectrum for such applications.
Within each service provider's allocated band and geographic area (usually there are two or more service providers in a geographic area using a different portion of the spectrum), the service provider may utilize any type of technology including frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), or combination thereof.
Frequency division multiple access (FDMA) technology utilizes narrow and discrete channels within the frequency band. Different subscriber stations are assigned different frequency channels. Interference to and from adjacent channels is limited by the use of bandpass filters which pass the signal energy within the narrow channels while rejecting signals having other frequencies. The United States cellular system (AMPS) divides the allocated spectrum into 30 KHz bandwidth channels and uses FM modulation.
Time division multiple access (TDMA) technology also utilizes narrow and discrete channels within the frequency band. However, each channel is further divided into time slots in the time domain. This results in multiple users on the same frequency channel and increases the number of users per given channel.
Unlike FDMA or TDMA, code division multiple access (CDMA) involves multiple users simultaneously sharing a channel having a relatively wide bandwidth. United States CDMA standards (IS-95) currently specify a CDMA channel having a bandwidth of 1.25 MHz. In CDMA, a large number of signals share the same frequency spectrum. Each signal consists of a different pseudorandom binary sequence that modulates a carrier signal (at the center frequency of the channel's spectrum). This spreads the spectrum of the waveform over the entire channel bandwidth. Use of CDMA technology allows for a larger number of signals than that used in FDMA or TDMA within the same amount of frequency spectrum and geographic area. Use of CDMA technology by service providers is expected to grow due to its increased traffic capabilities, digital technology and security. The PCS and cellular systems were initially designed and deployed with FDMA or TDMA technology (or a specific standard such as AMPS). Because no additional frequency spectrum has been allocated by the FCC in the cellular band, cellular band service providers desiring to use CDMA technology are now integrating CDMA technology into existing systems and must utilize the same frequencies currently allocated.
Typically, service providers in PCS, cellular and other mobile applications divide the particular geographic region in which they are operating into “cells”. This concept is well-known in the industry. Each cell contains a base station (including a transmitter and receiver) and services subscriber users within the boundaries of the cell. Each service provider is free to design its own coverage system including the locations and sizes of its cells.
In FDMA and TDMA, adjacent cells must use channels having different frequencies to avoid interference (a re-use factor of K=7 is traditionally used). When a subscriber station moves from one cell to an adjacent cell, a new communications channel between the subscriber station and the base station in the adjacent cell must be established. The communications channel between the subscriber station and the base station in the original cell is then terminated. This process is known as a “hand-off”, and is more particularly described as a “hard” hand-off because the process requires switching frequency.
In CDMA technology, however, each cell may use all or any portion of the frequency spectrum allocated to the service provider. Therefore, the same frequency channel (i.e., f
1
) may be used in adjacent cells. This increases the number of users within a particular cell and geographic area.
One of the fundamental features of the CDMA cellular technology is the use of a “soft” hand-off process. A soft hand-off occurs when a subscriber station travels from one cell to an adjacent cell with both cells operating at the same channel frequency. In the soft hand-off process, a subscriber station establishes a communications link to an adjacent cell before breaking off communications with the original cell. In most cases, the subscriber station maintains its link for a period of time to both the base station of the original cell and the base station of the adjacent cell. In some cases, the subscriber station may be in the soft hand-off process and communicate with more than two base stations. For the soft handoff to work, it is necessary that the two base stations (or more) involved use the same channel frequency.
A single CDMA channel frequency may not be sufficient to provide service in some areas of the overall service area. Where the need for service is high, the number of channel frequencies must also increase. If the additional channel frequencies cover all cells of the service area, then the subscriber station will keep the same channel frequency while it moves from cell to cell within the service area. However, adding additional channel frequencies to each cell may not be desirable because some cells may not need the increased capacity. Accordingly, some cells that experience high traffic may require one or more additional channel frequencies. This results in a condition where a subscriber station may be required to switch its channel frequency when moving from one cell (i.e. using channel frequencies f
1
and f
2
) to an adjacent cell (i.e. using only channel frequency f
1
). When the subscriber station is communicating with the cell using channel frequency f
2
and moves to the adjacent cell, the soft hand-off process is no longer viable since the subscriber station cannot transmit simultaneously at the two different frequencies needed to communicate with the two base stations. When the soft hand-off process cannot be used, the subscriber station must break from one base station (operating at f
2
), switch frequency to the new channel frequency (f
1
), and establish the link with a new base station (operating at f
1
) operating within the original cell. This process is known as a “hard” hand-off. After the hard hand-off is completed, then the soft hand-off process may be used when the subscriber station (now using channel frequency f
1
) moves from the original cell to the adjacent cell (using channel frequency f
1
).
The problem with this type of hard hand-off is that the effective coverage area of the CDMA source cell is reduced due to several factors. First, the hard hand-off process may take up to five (5) seconds to complete. Assuming that the subscriber station's velocity may be up to seventy (70) m.p.h. and directly away from the base station of the source cell, then the subscriber station may travel approximately 156 meters from the initiation of the hard hand-off process until the process is completed. Second, adding the round trip delay (RTD) uncertainty of about 244 meters, then the subscriber station must be approximately 400 meters or more away from the edge of the cell coverage area when the hard hand-off process is triggered.
In addition, in the case as described above where the subscriber station switches from the channel f
2
to the channel f
1
withi
Carr Law Firm, L.L.P.
Sobutka Philip J.
Trost William
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