Pulse or digital communications – Spread spectrum
Reexamination Certificate
2001-01-12
2001-11-06
Vo, Don N. (Department: 2631)
Pulse or digital communications
Spread spectrum
C370S332000, C455S436000, C455S442000, C375S141000
Reexamination Certificate
active
06314126
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to packetized, code-division-multiple-access communications, and more particularly to handoff of a remote station, between base stations.
DESCRIPTION OF THE RELEVANT ART
In a wireless, direct-sequence, spread-spectrum, packetized, code-division-multiple-access (CDMA) communications system, a remote station (RS) transmits to a base station (BS), and a base station transmits to a remote station. The selection of base station may be determined from the power level received at the base station from the remote station, and/or from the power level received at the remote station from the base station. During communications between the remote station and the base station, if either the power level received at the remote station, or the power level received at the base station, is too small, then unacceptable communications results, and a change of base station is required or the communications channel is “dropped”. The change of base station is referred in the art as a handover or handoff.
In the prior art, a base station may initiate a handoff, or the remote station may initiate a handoff. The handoff may be a hard handoff, when communications with the remote station is stopped for a short period of time, until handoff to a new base station is complete. The hard handoff may result in loss of data. To avoid the loss of data, the data that might be lost can be stored, and when the handoff is complete, the stored data can be transmitted at an increased data rate and an increased power level. This sometimes is referred to as store and forward. This store and forward of data during the hard handoff can prevent the loss of data.
For a soft handoff, the remote station, at the same time, receives data from, and transmits data to, the old base station and the new base station. The simultaneous transmission results in an increase in received signal power, by as much as 3 dB, since two base stations are transmitting to the remote station.
For the hard handoff and the soft handoff, initially the remote station and/or base station determine that a handoff is required, and then as to which of several base stations the remote station finally will communicate. For the handoff, an initial remote station transmitter power is determined, then the necessary overhead operations take place to effect the handoff. During this process, the hard handoff results in data loss, which is unacceptable for data communications, and the soft handoff results in a decrease in capacity, since two base stations are simultaneously transmitting and/or receiving the same data. In addition, the handoff procedures currently in use were for circuit-switched systems.
SUMMARY OF THE INVENTION
A general object of the invention is a handoff, for a direct-sequence, spread-spectrum, CDMA packet-switched system, between base stations, without loss of capacity or loss of data.
Another object of the invention is to provide continuous operation of a remote station between base stations, without the large overhead operation currently required of a handoff.
According to the present invention, as embodied and broadly described herein, an improvement for a method and system is provided to a spread-spectrum, code-division-multiple-access (CDMA), system. Assume that a remote station (RS) is communicating with a first base station (BS) within a spread-spectrum, CDMA network. The spread-spectrum, CDMA network may be a star network with the same overhead information which is typically used in current cellular systems, or a distributed network. The first base station transmits, using radio waves, a first BS-packet signal to the remote station. The first BS-packet signal is spread by a first BS-chip-sequence signal at a first frequency f
1
.
The remote station receives the first BS-packet signal. A replica of the first BS-chip-sequence signal, or equivalently a matched filter matched to the first BS-chip-sequence signal, is used by the remote station for despreading first BS-packet signals arriving from the first base station. Each BS-packet signal, or certain BS-packet signals, contain capacity availability data, and which BS-chip-sequence signals are available, for the respective base station.
The remote station transmits, using radio waves, a first RS-packet signal to the first base station. The first RS-packet signal is spread by a first RS-chip-sequence signal at a second frequency f
2
. A replica of the first RS-chip-sequence signal, or equivalently a chip-sequence generator matched to the first RS-chip-sequence signal, is used by the remote station for spreading the first RS-packet signal. The first RS-packet signal includes a source address from the remote station.
The first base station receives the first RS-packet signal at the second frequency f
2
. The first base station normally would despread the first RS-packet signal, and then send the despread first RS-packet signal to a central office. The despread first RS-packet signal includes a source address from the remote station, and the source address from the first base station.
While the remote station communicates with the first base station, the remote station also monitors spread-spectrum signals from other base stations, which are in geographic proximity to the remote station. The remote station may continuously monitor, periodically monitor, or monitor when the received power level at the remote station falls below a threshold or when the capacity availability in the first base station falls below a given threshold. The remote station may monitor the control and packet transmission channels, or other channels, of the other base stations. The control and packet transmission channels refer to channels that are continuously operating, or nearly continuously operating, from the other base stations.
Each of the base stations transmits, using radio waves, a BS-packet signal spread by a second BS-chip-sequence signal at the first frequency f
1
. The remote station monitors a signal metric of the control and packet transmission channels from the first base station, and monitors the same signal metric of the control and packet transmission channels from the other base stations. The remote station determines when the signal metric of the first BS-packet signal falls below a threshold, and that the signal metric of a BS-packet signal from one base station, or several BS-packet signals from several base stations, is above the threshold. The remote station also determines available capacity of the one or more base stations. Upon determining the foregoing threshold crossings, and capacity levels, the remote station determines to handoff to a second base station. The second base station is defined herein to be the particular base station, out of the one or more base stations monitored by the remote station, to which the remote station decides to handoff. The second base station, as set by engineering design criteria, has sufficiently high signal level and sufficient capacity, for communicating with the remote station. Upon meeting these criteria, the remote station changes communications from the first base station to the second base station.
A packet switched system makes use of the fact that a packet contains a finite number of bits and that the remote station transmits nothing between packets. Even in voice communications, the dead time, i.e., the time between spoken words, including pauses, is about sixty percent. When a remote station transmits a packet there is a dead time, the time between the remote station transmission of the next packet. This dead time is unknown a priori at the base station and often unknown by the remote station.
Thus, after transmission of a packet i, the base station will use the chip-sequence signal and the available capacity in the base station, given to a first remote station, for the next remote station requiring its use. Such operation is required to ensure, maximum throughput through the system. Thus, the original remote station desiring to send packet i+1, may find that the base station has no capacity
Garodnick Joseph
Schilling Donald L.
Chartered David Newman
Linex Technologies Inc.
Vo Don N.
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