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Description  |
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RELATED PATENTS
This invention is related to the disclosures in U.S. patent application No.
07,622,235, having filing date of Dec. 14, 1990, entitled, SPREAD SPECTRUM
CDMA COMMUNICATIONS SYSTEM by Donald L. Schilling; U.S. patent application
No. 07/614,816, having filing date of Nov. 16, 1990, and entitled ADAPTIVE
POWER CONTROL FOR A SPREAD SPECTRUM TRANSMITTER by Donald L. Schilling;
U.S. patent application No. 07/614,827, having filing date of Nov. 16,
1990, and entitled SPREAD SPECTRUM MULTIPATH RECEIVER APPARATUS AND METHOD
by Donald L. Schilling; and, U.S. patent application No. 07/626,109,
having filing date of Dec. 14, 1990, and entitled SPREAD SPECTRUM
COMMUNICATIONS SYSTEM AND METHOD by Donald L. Schilling, which are all
incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention relates to spread-spectrum communications and more
particularly to a system and method for handing off a telephone
conversation from one base station to another when the mobile radio device
is moving from one cell to another cell in a CDMA cellular spread-spectrum
system.
DESCRIPTION OF THE PRIOR ART
Many mobile communications systems employ multiple coverage areas to
accommodate necessary mobile communications over a defined region. Of such
systems, simulcast communication systems and cellular communication
systems are the most common types which provide hand-offs between coverage
areas. In simulcast communication systems, a relatively simple hand-off
technique is used. Simulcast communication systems involve linking
together the respective coverage areas of a plurality of communication
sites to form a large wide area coverage. The system typically employs
communication channels which are common to each individual coverage area.
As a mobile radio exits the coverage area of one site and enters the
coverage area of another site, a conversation on the mobile radio is
maintained because the linking of the multiple sites allows for
simultaneous reception and broadcasting of the conversation at each site
on the same channel.
Unlike simulcast communication systems, cellular communications systems do
not employ common communication channels between the various sites.
Rather, each coverage area employs a base site which includes a number of
base stations for providing radiotelephones within the base site coverage
area with a number of radiotelephone communication channels which are
unique with respect to adjacent base site coverage areas. Each base site
is controlled by the system's central switch controller.
A hand-off between two base sites in the present FDMA cellular
communications system may be accomplished through communication between
the radiotelephone and the radio equipment at the base site from which the
radiotelephone is exiting. The base site equipment periodically measures
the signal strength of the radiotelephone during the conversation, and,
once it reaches a relatively low signal strength threshold, the same base
site equipment sends a message to the adjacent base sites to determine
which base site the radiotelephone is entering. The radiotelephone is then
instructed to communicate on a selected channel from the base site
equipment associated with the coverage area the radiotelephone is
entering.
A cellular spread-spectrum-CDMA system communicates using message data,
which may require continuous, uninterrupted communications. When a mobile
station moves from a first cell to a second cell, the chip-codeword used
for spread-spectrum processing the channel containing the digital data has
to be handed-off so as to not interrupt communications. The power method
used for the cellular voice communications system employing FM channels
may not work as well for a spread-spectrum CDMA system because the time
required to switch, i.e., hand-off, a user may result in the loss of
considerable digital data. In addition, the CDMA system employs cells
which may be placed close to one another, e.g., say 1000 feet apart. In
such a case the switching time is far more critical than when the cells
are 3 miles apart, which is typical for todays FDMA systems.
Accordingly, a system for providing a hand-off between coverage areas in a
spread-spectrum-CDMA system is needed which overcomes the aforementioned
deficiencies.
OBJECTS OF THE INVENTION
It is a general object of the present invention to provide a cellular
direct sequence spread-spectrum-CDMA-communications system which overcomes
the foregoing shortcomings.
It is a more particular object of the present invention to provide a
cellular spread-spectrum-CDMA-communications system which ensures that a
radiotelephone hand-off will be successful without loss of data.
Additional objects of the present invention include providing an improved
radiotelephone, an improved base site and an improved switch controller
which operate in accordance with the cellular
spread-spectrum-CDMA-communications system of the present invention.
SUMMARY OF THE INVENTION
According to the present invention, as embodied and broadly described
herein, a spread spectrum hand-off system for use between two cells in a
cellular spread-spectrum-CDMA-communications system is provided comprising
control means and a plurality of cells with each cell having a base
station for transmitting one or more spread-spectrum-communications
signal. The present invention is illustrated, by way of example, with a
radio device moving from a first cell which has a first base station, to a
neighboring cell which has a second base station. The first cell is
assumed to be surrounded by N-1 cells, and each of the base stations in
these N-1 cells transmits a spread-spectrum generic-chip-code signal which
is different from the other cells and the first cell. There are,
therefore, N generic-chip-code signals in a CDMA cellular system which has
M cells (M>>N). The generic-chip-code signals are repeatedly used in
different cells, such that cells with the same generic-chip-code signal
are a maximum distance apart. This is called "chip-codeword reuse". The
mobile radio device scans the N-1 generic-chip-code signals until the
radio detects a generic-chip-code signal which produces a voltage level at
an output of a detector which is greater than the generic-chip-code signal
of the first cell, and also exceeds a predetermined threshold. The
generic-chip-code-signal which meets this criteria is deemed to originate
from a second cell which has a second base station.
The first base station transmits a first spread-spectrum-communications
signal with a first generic-chip-code signal embedded therein. The second
base station transmits a second spread-spectrum-communications signal with
a second generic-chip-code signal embedded therein. The
spread-spectrum-CDMA-communications system has control means for switching
message and signalling data, which are spread-spectrum processed with a
first set of message-chip-code signals embedded in the first
spread-spectrum-communications signal transmitted from the first base
station, to being spread-spectrum processed with a second set of
message-chip-code signals and embedded in the second
spread-spectrum-communications signal transmitted from the second base
station.
Operating within a cell is a plurality of mobile user hand-held radio
devices with each having a personal-communications-network (PCN) antenna,
first generic-detection means, second generic-detection means, comparator
means, message-spread-spectrum-processing means, message-detection means,
and chip-codeword-synchronization means. The first generic-detection means
is coupled to the PCN antenna and has first
generic-spread-spectrum-processing means. The first generic-detection
means detects the first generic-chip-code signal embedded in the first
spread-spectrum-communications signal communicated from the first PCN-base
station. After detection of the first generic-chip-code signal, the first
generic-detection-means outputs a first detected signal. The second
generic-detection means is coupled to the PCN antenna and includes second
generic-spread-spectrum-processing means. The second generic-detection
means detects the second generic-chip-code signal embedded in the second
spread-spectrum-communications signal communicated from the second
PCN-base station. After detection of the second generic-chip-code signal
the second generic-detection means outputs a second detected signal. The
comparator means generates a comparison signal by comparing the first
detected signal with the second detected signal. By repetitively changing
the generic-chip-code signal used by the second generic-detection means,
the N-1 spread-spectrum-communications signals are effectively scanned.
Thus, the second detected signal can be a voltage level which is
proportioned to the detected N-1 generic-chip-code signals emanating from
the N-1 neighboring base stations.
The message-spread-spectrum-processing means is coupled to the PCN antenna
and despreads the first spread-spectrum-communications signal and/or
second spread-spectrum-communications signal as a modulated-data signal.
When the comparison signal is greater than a threshold, then the
chip-codeword-synchronization means synchronizes the
message-spread-spectrum-processing means and the detection means to the
first generic-chip-code signal. Thus, the
message-spread-spectrum-processing means despreads the first
spread-spectrum-communications signal transmitted from the first base
station. When the comparison signal is less than the threshold, then
hand-off data are sent, as signalling data, through a spread-spectrum
channel from the mobile station to the first base station. The hand-off
data directs the first base station to hand-off the mobile unit to a
second base station. In response to receiving the hand-off signal, the
first base station notifies control means to hand-off the mobile station
to the second base station. The control means sends the first base station
one or more spread-spectrum chip codewords which are relayed to the mobile
station. The chip codewords are communicated through a spread-spectrum
channel from the first base station to the mobile station as signalling
data. The mobile station will use the spread-spectrum chip codewords for
communicating with the second base station. Upon receiving the chip
codewords, the message-spread-spectrum-processing means at the mobile
station breaks communications with the first base station and initiates
spread-spectrum communications with the second base station.
If required, the chip-codeword-synchronization means at the mobile station
synchronizes the message-spread-spectrum-processing means and the
detection means to the second generic-chip-code signal. Resynchronization
may not be required if all the base stations are synchronized to a common
clock or timing signal. Accordingly, the
message-spread-spectrum-processing means at the mobile station despreads
the second spread-spectrum-communications signal transmitted from the
second base station. At this point the second
generic-spread-spectrum-processing means is locked onto the second
generic-chip-code signal and the first generic-spread-spectrum-processing
means is used for repetitively searching for a generic-chip-code signal
emanating from a neighboring cell and meeting the criteria for handoff.
The present invention also includes a method for controlling hand-off in a
spread-spectrum-CDMA-communications system, of a radio device moving from
a first cell having a first base station which transmits a first
spread-spectrum-communications signal with a first generic-chip-code
signal embedded therein, toward a second cell having a second base station
which transmits a second spread-spectrum-communications signal with a
second generic-chip-code signal embedded therein. The
spread-spectrum-CDMA-communications system has a control unit for
switching message data, spread-spectrum processed with a first set of
message-chip-code signals embedded in the first
spread-spectrum-communications signal transmitted from the first base
station, to the second spread-spectrum-communications signal spread
spectrum processed with a second set of message-chip-code signals
transmitted from the second base station.
The method has the step of scanning a plurality of generic-chip-code
signals until a generic-chip-code signal which produces a voltage level at
an output of a detector is greater than the other scanned
generic-chip-code signals, and also exceeds a predetermined threshold. The
generic-chip-code signal that meets this criteria is labeled the second
generic-chip-code signal.
The method includes detecting the first generic-chip-code signal embedded
in the first spread-spectrum-communications signal communicated from the
first base station, detecting the second generic-chip-code signal embedded
in the second spread-spectrum-communications signal communicated from the
second base station and outputting a first detected signal and a second
detected signal, respectively. The method generates a comparison signal by
comparing the first detected signal with the second detected signal. Using
message-spread-spectrum-processing means, the method despreads the
spread-spectrum-communications signal as a modulated-data signal. When the
comparison signal is greater than a threshold, the
message-spread-spectrum-processing means uses the first generic-chip-code
signal for processing the first spread-spectrum-communications signal
transmitted from the first base station. When the comparison signal is
less than the threshold, then first hand-off data is sent as signalling
data from the mobile station to the first base station. The first base
station then notifies control means to hand-off the mobile station to the
second base station. One or more chip codewords are sent from the control
means through the first base station to the mobile station. The mobile
station then breaks communications with the first base station and
initiates spread-spectrum communications with the second base station,
using the one or more chip codewords received from the control means. The
generic-spread-spectrum-processing means at the mobile station uses the
second generic-chip-code signal for receiving the second
spread-spectrum-communications signal transmitted from the second base
station.
Additional objects and advantages of the invention will be set forth in
part in the description which follows, and in part will be obvious from
the description, or may be learned by practice of the invention. The
objects and advantages of the invention also may be realized and attained
by means of the instrumentalities and combinations particularly pointed
out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part
of the specification, illustrate preferred embodiments of the invention,
and together with the description serve to explain the principles of the
invention.
FIG. 1 is a diagram of a cellular, spread-spectrum-CDMA-communications
system including two base stations and a control unit;
FIG. 2 is a diagram of cellular, spread-spectrum CDMA showing chip codeword
reuse;
FIG. 3 shows a synchronous spread-spectrum transmitter at a base station;
FIG. 4A is an expanded diagram of the radio device for the mobile station;
FIG. 4B is an expanded diagram of the radio device for the mobile station;
FIG. 5 is an expanded diagram of a message portion of a radio device;
FIG. 6 is a timing diagram of a protocol; and
FIG. 7 is a flowchart of a method for handing off between two cellular base
stations.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference will now be made in detail to the present preferred embodiments
of the invention, examples of which are illustrated in the accompanying
drawings, wherein like reference numerals indicate like elements
throughout the several views.
The arrangement disclosed in this specification has particular use for
handing-off of radio communication in a mobile radiotelephone unit, from
one cell to another in a cellular infrastructure of a
spread-spectrum-CDMA-communications system, without loss of data bits.
More particularly, the arrangement disclosed herein is directed to
ensuring that an attempted hand-off of a radio unit in such an
infrastructure is successful.
The present invention is illustrated, by way of example, with a radio
device moving from a first cell which has a first base station, to a
neighboring cell. The first cell is assumed to be surrounded by N-1 cells.
Each of the base stations in these N-1 cells transmits a
spread-spectrum-communications signal using a generic-chip-code signal
which is different from the other cells and the first cell. All cells
transmit the spread-spectrum communication signal at the same carrier
frequency. The radio device scans the N-1 generic-chip-code signals of the
neighboring cells. The scanning continues until the output voltage level
of the detector which detects the scanned generic-chip-code signals
exceeds a predetermined threshold and is greater than the output voltage
levels of the other scanned generic-chip-code signals. The
generic-chip-code signal which meets this criteria is deemed to originate
from a second cell which has a second base station.
FIG. 1 illustrates a unique cellular spread-spectrum-CDMA-communications
system which, in simplified form, includes a first base station 124 and a
second base station 126 for two geographic spread-spectrum-CDMA
communications areas (cells) 112, 114, respectively. In a preferred
embodiment six cells are adjacent to a given cell, as shown for the second
cell 114. Each cell adjacent to the given cell, i.e. the second cell 114,
uses a different chip codeword from the second cell 114 and from each
other. This permits reuse of chip codewords.
FIG. 2 illustrates generic chip codeword reuse in a cellular CDMA
environment, where the N-1 cells adjacent to a particular cell using
generic chip codeword A, use N-1 different generic chip codewords,
respectively. There are N generic-chip-code signals used in a CDMA
cellular system which has a total of M cells (M>>N). The generic-chip-code
signals are repeatedly used in the cells, such that the cells with the
same generic-chip-code signals are a maximum distance apart, and no two
adjacent cells use the same generic-chip-code signal. FIG. 2 shows for N=7
there are six adjacent cells using six different generic chip codewords,
B, C, D, E, F and G. This pattern is repeated throughout the cellular
geographic area.
Referring to FIG. 1, for the first cell 112, the first base station 124
includes a spread-spectrum transmitter for transmitting a first
spread-spectrum-communications signal with a first generic-chip-code
signal embedded therein. The first base station 124 also has a
spread-spectrum receiver for receiving a spread-spectrum-communications
signal, with the first generic-chip-code signal embedded therein. For the
second cell 112, the second base station 124 includes a spread-spectrum
transmitter for transmitting a second spread-spectrum-communications
signal with a second generic-chip-code signal embedded therein. The second
base station 124 also includes a spread-spectrum receiver for receiving a
spread-spectrum-communications signal, with the second generic-chip-code
signal embedded therein. The generic-chip-code signals for transmitting
and receiving at a particular base station may be different.
The spread-spectrum-CDMA-communications system has control means, embodied
as a control unit 130, for switching message and signaling data,
spread-spectrum processed with a message-chip-code signal embedded in the
first spread-spectrum-communications signal transmitted from the first
base station, to the second spread-spectrum-communications signal
transmitted from the second base station.
For purposes of exemplifying the hand-off operation of the present
invention, a mobile station 122 which has an improved radio device is
depicted in transition from the first cell 112 to the second cell 114.
Overall control of the first base station 124 and the second base station
126 is provided by a signal processing unit of a cellular switch
controller, located in a control unit 130.
Each base station and mobile station has a transmitter for transmitting the
spread-spectrum-communications signal, which may include a plurality of
spread-spectrum-processed signals for handling a plurality of message and
signalling data. The transmitter is coupled to a plurality of message
means and a plurality of spreading means. Referring to FIG. 3, the
plurality of message means may be embodied as a plurality of
transmitter-message-chip-code generators and the plurality of spreading
means may be embodied as a plurality of EXCLUSIVE-OR gates. The plurality
of transmitter-message-chip-code generators generates a plurality of
message-chip-code signals. The plurality of transmitter-message-chip-code
generators is shown as first transmitter-message-chip-code generator 102
generating first message-chip-code signal, g.sub.1 (t), second
transmitter-message-chip-code generator 172 generating second
message-chip-code signal, g.sub.2 (t), through N.sup.th
transmitter-message-chip-code generator 182 generating N.sup.th
message-chip-code signal, g.sub.N (t). The plurality of EXCLUSIVE-OR gates
is shown as first EXCLUSIVE-OR gate 103, second EXCLUSIVE-OR gate 173,
through N.sup.th EXCLUSIVE-OR gate 183. The plurality of EXCLUSIVE-OR
gates generates a plurality of spread-spectrum-processed signals by
modulo-2 adding the plurality of message and signalling data d.sub.1 (t),
d.sub.2 (t), . . . , d.sub.N (t) with the plurality of message-chip-code
signals g.sub.1 (t), g.sub.2 (t), . . . , g.sub.N (t), respectively. More
particularly, the message data, d.sub.1 (t), are modulo-2 added with the
first message-chip-code signal, g.sub.1 (t), the signalling data, d.sub.2
(t), are modulo-2 added with the second message-chip-code signal, g.sub.2
(t), through the N.sup.th message and/or signalling data, d.sub.N (t),
which are modulo-2 added with the N.sup.th message-chip-code signal,
g.sub.N (t).
The transmitter-generic-chip-code generator 101 is coupled to the plurality
of transmitter-message-chip-code generators and the source for the
plurality of message and signalling data, d.sub.1 (t), d.sub.2 (t), . . .
d.sub.N (t). The generic-chip-code signal g.sub.0 (t), in a preferred
embodiment, provides synchronous timing for the plurality of
message-chip-code signals g.sub.1 (t), g.sub.2 (t), . . . , g.sub.N (t),
and the plurality of message and signalling data d.sub.1 (t), d.sub.2 (t),
. . . , d.sub.N (t).
The combiner 105 combines the generic-chip-code signal and the plurality of
spread-spectrum-processed signals, by adding the generic-chip-code signal
with the plurality of spread-spectrum-processed signals. The combined
signal typically is a multilevel signal, which has the instantaneous
voltage levels of the generic-chip-code signal and the plurality of
spread-spectrum-processed signals.
The modulator 107, as part of the transmitter, modulates the combined
generic-chip-code signal and the plurality of spread-spectrum-processed
signals by a carrier signal, cos w.sub.o t, at a carrier frequency,
f.sub.o. The modulated generic-chip-code signal and the plurality of
spread-spectrum processed signals are transmitted over the communications
channel as a spread-spectrum-communications signal, x.sub.c (t). While the
transmitter may use a linear power amplifier for optimum performance, a
nonlinear power amplifier also may be used without significant degradation
or loss in performance.
For the spread-spectrum-CDMA-communications system, illustrated in FIG. 1,
the first spread-spectrum-communications signal, x.sub.c1 (t), transmitted
from the first base station has the form:
##EQU1##
Thus, the first spread-spectrum-communications signal includes the first
generic-chip-code signal, g.sub.10 (t), and a first plurality of
spread-spectrum-processed signals, for i=1, . . . , N, as if they were
each modulated separately, and synchronously, on separate carrier signals
with the same carrier frequency, f.sub.o, and transmitted over the
communications channel.
Similarly, the second spread-spectrum-communications signal, x.sub.c2 (t),
transmitted from the second base station 126 has the form:
##EQU2##
Thus, the second spread-spectrum-communications signal includes the second
generic-chip-code signal, g.sub.20 (t), and a second plurality of
spread-spectrum-processed signals, for j=1, . . . , M, as if they were
each modulated separately, and synchronously, on separate carrier signals
with the same carrier frequency, f.sub.o, and transmitted over the
communications channel.
The improved mobile-user, hand-held, radio device for the mobile station
122 includes a PCN antenna, first generic-detection means, second
generic-detection means, comparator means, controller means,
message-spread-spectrum-processing means, message-detection means and
synchronization means. In FIG. 4A, elements of the radio device, by way of
example, are shown in expanded form. A PCN antenna 301 is coupled through
a low noise amplifier (LNA) 303, down converter 305 and automatic gain
control (AGC) 307 to a first receiver mixer 308 and a second receiver
mixer 309. The first receiver mixer 308 is coupled to a signal source 311,
and the second receiver mixer 309 is coupled through a 90.degree. phase
shifter 312 to the signal source 311. The first receiver mixer 308
multiplies the local signal from the signal source 311 with a received
signal to generate an in-phase signal. The second mixer 309 multiplies the
90.degree. phase-shifted version of the local signal from the signal
source 311 with the received signal to generate a quadrature-phase signal.
The first generic-detection means and the second generic-detection means as
illustrated in FIG. 4B may be embodied using a correlation receiver such
as a first and second generic-chip-code generator 615, 625, first and
second mixer 617, 627 and first and second bandpass filter 616, 626, or,
as illustrated in FIG. 4A, may be embodied using a first and second
matched filter 317, 325 and a first detector 318, 326. The
generic-chip-code generators and matched filter typically are programmable
or adjustable, for adapting to different generic-chip-code signals
embedded in spread spectrum-communications signals, transmitted from each
base transmitter in different cells.
The controller means, embodied as a chip-code controller 340, sets which
generic-chip-code signal the correlation receiver or matched filter is
using. The chip-code controller 340 can repetitively change the
generic-chip-code signal used by the correlations receiver or matched
filter, so as to effectively scan through a plurality of generic-chip-code
signals. In the cellular architecture of FIG. 1, the scanning would move
through N-1=5 generic-chip-code signals which correspond to the five
neighboring cells.
The first generic-detection means is shown in FIG. 4A, by way of example,
embodied as including at least a first matched filter 315 and a first
detector 316, and may further include a third matched filter 317 and a
third detector 318. The first matched filter 315 is coupled between the
first receiver mixer 308 and the first detector 316, and the third matched
filter 317 is coupled between the second receiver mixer 309 and the third
detector 318. The outputs of the first detector 316 and the third detector
317 are combined by first combiner 319. The first matched filter 315 and
the third matched filter 317 have an impulse response matched to the first
generic-chip-code signal. For the particular combination shown, the first
matched filter 315 and first detector 316 detect the in-phase component of
the received, first generic-chip-code signal. The third matched filter 317
and third detector 318 detect the quadrature-phase component of the
received, first generic-chip-code signal.
A spread spectrum correlator may be used in place of a matched filter. The
first generic-detection means is shown in FIG. 4B as a spread-spectrum
correlator includes a first generic-chip-code generator 615, first generic
mixer 617, first generic-bandpass filter 616 and first generic detector
618.
The second generic-detection means is shown in FIG. 4A embodied as
including at least a second matched filter 325 and a second detector 326,
and may further include a fourth matched filter 327 and a fourth detector
328. The second matched filter 325 is coupled between the first receiver
mixer 308 and the second detector 326, and the fourth matched filter 327
is coupled between the second receiver mixer 309 and the fourth detector
328. The outputs of the second detector 326 and the fourth detector 328
are combined by second combiner 329. The second matched filter 325 and the
fourth matched filter 327 have impulse responses matched to the second
generic-chip-code signal. For the particular combination shown the second
matched filter 325 and second detector 326 detect the in-phase component
of the received, second generic-chip-code signal. The fourth matched
filter 327 and the fourth detector 328 detect the quadrature-phase
component of the received, second generic-chip-code signal.
A spread spectrum correlator may be used in place of a matched filter.
Thus, the first generic-detection means is shown in FIG. 4B as a
spread-spectrum correlator including a second generic-chip-code generator
625, second generic mixer 627, second generic-bandpass fi | | |
