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Claims  |
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We claim:
1. A spread spectrum diversity receiver, comprising:
searcher means for, receiving multiple pilot signals each travelling upon a
different propagation path and having a resultant time difference with
respect to one another, determining signal strength of each received pilot
signal and corresponding time relationship with respect to one another,
and providing a searcher control signal indicative of received pilot
signals of greatest signal strength and corresponding time relationship;
and
data receiver means for receiving spread spectrum modulated information
signals each corresponding to a different one of said pilot signals, said
data receiver means responsive to said searcher control signal for
demodulating one of said spread spectrum modulated information signals
corresponding to one of said pilot signals of greatest signal strength and
for providing an output signal bearing information.
2. The diversity receiver of claim 1 wherein said data receiver means is
further responsive to said searcher control signal for demodulating at
least one additional one of said spread spectrum modulated information
signals each corresponding to a respective other one of said pilot signals
of next to greatest signal strength, and providing corresponding
additional output signals each bearing said information.
3. The diversity receiver of claim 2 further comprising combiner means for
receiving and coherently combining said output signal and said additional
output signals and for providing a corresponding combined output signal
bearing said information.
4. The diversity receiver of claim 2 further comprising combiner means for
receiving and combining said output signal and each of said additional
output signal and providing a resultant combined output signal.
5. The diversity receiver of claim 4 wherein said combined output signal
bears said information in an error correction coded format and further
comprises decoder means for receiving and error correction decoding said
combined output signal.
6. The diversity receiver of claim 1 wherein each one of said multiple
pilot signals results form a single cell-site transmitted pilot signal
travelling different propagation paths to reception by said searcher
means, each one of said multiple pilot signals is of a same spreading code
offset in time corresponding to its propagation path.
7. The diversity receiver of claim 1 wherein said multiple pilot signals
result form different cell-sites each transmitting a single pilot signal
spread spectrum modulated by a same spreading code with each cell-site
transmitted single pilot signal travelling different propagation paths to
reception by said searcher means, each cell-site transmitting its
respective single pilot signal at a different code phase offset with
respect to each other cell-site transmitted single pilot signal, ones of
said multiple pilot signals resulting from a same cell-site transmitted
single pilot signal being of a same spreading code offset in time
corresponding to its propagation path.
8. In a cellular communication system in which user information signals are
communicated to an intended recipient user by a cell-site using spread
spectrum communication signals, wherein said cell-site transmits a spread
spectrum pilot signal of a predetermined code phase, and wherein said
cell-site transmitted spread spectrum communication signals and pilot
signal are susceptible to multipath propagation, a recipient user spread
spectrum receiver comprising:
searcher means for receiving an input signal which includes multiple path
propagations of a pilot signal transmitted by a cell-site wherein each
multiple path propagation pilot signal travels a different propagation
path and has a corresponding path dependent offset in code phase, for
scanning at different code phases so as to detect a presence of at least
one of said multiple path propagation pilot signals, for measuring signal
strength of each detected multiple path propagation pilot signal, for
determining code phase of each detected multiple path propagation pilot
signal, and for providing searcher control signals representative of
multiple path propagation pilot signals of greatest signal strength and
corresponding code phase; and
receiver means for receiving said searcher control signals, for receiving
said input signal which further includes multiple path propagations of
spread spectrum communication signals transmitted by said cell-site each
corresponding to a respective multiple path propagation pilot signal, for,
in response to said searcher control signals, spread spectrum processing
certain ones of said multiple path propagations of said spread spectrum
communication signals corresponding to said multiple path propagation
pilot signals of greatest signal strength so as to extract corresponding
intended recipient user information signals therefrom, and for providing
corresponding output signals representative of said extracted intended
recipient user information signals.
9. The receiver of claim 8 wherein said receiver means comprises a
plurality of data receiver means each for, receiving a different one of
said searcher control signals, receiving in said input signal said
multiple path propagation of spread spectrum communication signals, spread
spectrum processing a different selected one of said multiple path
propagations of said spread spectrum communication signals in response to
said searcher control signals at a synchronization provided by a
corresponding multiple path propagation pilot signal, and providing a
corresponding one of said output signals.
10. The receiver of claim 8 further comprising combiner means for,
receiving said output signals, coherently combining said received output
signals and providing a corresponding combined output signal.
11. The receiver of claim 8 further comprising input receiver means for,
receiver RF signals in a predetermined frequency band, amplifying said RF
signals, frequency downconverting said amplified RF signals to an
intermediate frequency range so as to produce corresponding IF signals,
filtering said IF signals, digitizing said IF signals, wherein said
digitized IF signals correspond to multiple path propagation of said pilot
signal and corresponding multiple path propagations of said spread
spectrum communication signals, and providing said IF signals to said
searcher means and said receiver means as said input signal.
12. In a cellular communication system in which user information signals
are communicated to an intended recipient user by at least one cell-site
using spread spectrum communication signals, wherein each cell-site
transmits a spread spectrum pilot signal of a same spreading code and
predetermined different code phase, and wherein each cell-site transmitted
spread spectrum communication signals and pilot signal are susceptible to
multipath propagation, a recipient user spread spectrum receiver
comprising:
searcher means for receiving an input signal including multiple path
propagations of at least one pilot signal wherein each pilot signal is
transmitted by a respective one of a plurality of cell-sites and wherein
each multiple path propagation pilot signal travels a different
propagation path and has a corresponding path dependent offset in code
phase, for scanning at different code phases so as to detect a presence of
at least one of said multiple path propagation pilot signals, for
measuring signal strength of each detected multiple path propagation pilot
signal, for determining code phase of each detected multiple path
propagation pilot signal, and for providing searcher control signals
representative of multiple path propagation pilot signals of greatest
signal strength and corresponding code phase; and
receiver means for receiving said searcher control signals, for receiving
said input signal which further includes multiple path propagations of
spread spectrum communication signals transmitted by at least one of said
plurality of cell-sites wherein each multiple path propagation of said
spread spectrum communication signals corresponds to a respective multiple
path propagation pilot signal, for, in response to said searcher control
signals, spread spectrum processing certain ones of said multiple path
propagations of said spread spectrum communication signals corresponding
to said multiple path propagation pilot signals of greatest signal
strength so as to extract corresponding intended recipient user
information signals therefrom, and for providing corresponding output
signals representative of said extracted intended recipient user
information signals.
13. The receiver of claim 12 wherein said receiver means comprises a
plurality of data receiver means each for, receiving a different one of
said searcher control signals, receiving in said input signal and multiple
path propagations of spread spectrum communication signals, spread
spectrum processing a different selected one of said multiple path
propagations of said spread spectrum communication signals in response to
said searcher control signals at a synchronization provided by a
corresponding multiple path propagation pilot signal, and providing a
corresponding one of said output signals.
14. The receiver of claim 12 further comprising combiner means for,
receiving said output signals, coherently combining said received output
signals and providing a corresponding combined output signal.
15. The receiver of claim 12 further comprising input receiver means for,
receiving RF signals in a predetermined frequency band, amplifying said RF
signals, frequency downconverting said amplified RF signals to an
intermediate frequency range so as to produce corresponding IF signals,
filtering said IF signals, digitizing said IF signals, wherein said
digitized IF signals correspond to multiple path propagations of said
pilot signal and corresponding multiple path propagations of said spread
spectrum communication signals, and providing said IF signals to said
searcher means and said receiver means as said input signal.
16. In a cellular communication system in which user information signals
are communicated to an intended recipient user by at least one cell-site
using spread spectrum communication signals, wherein each cell-site
transmits a spread spectrum pilot signal of a same spreading code and
predetermined different code phase, and wherein each cell-site transmitted
spread spectrum communication signals and pilot signal are susceptible to
multipath propagation, a method for acquiring and processing intended
recipient user spread spectrum communication signals comprising the steps
of:
receiving input signals including (a) multiple path propagations of at
least one pilot signal wherein each pilot signal is transmitted by a
respective one of a plurality of cell-site and wherein each multiple path
propagation pilot signal travels a different propagation path and has a
corresponding path dependent offset in code phase and (b) multiple path
propagations of spread spectrum communication signals transmitted by at
least one of said plurality of cell-sites wherein each multiple path
propagation of said spread spectrum communication signals corresponds to a
respective multiple path propagation pilot signal;
scanning said input signals at different code phases so as to detect a
presence of at least one of said multiple path propagation pilot signals;
measuring signal strength of each detected multiple path propagation pilot
signal;
determining code phase of each detected multiple path propagation pilot
signal;
providing a searcher signal representative of multiple path propagation
pilot signals of greatest signal strength and corresponding code phase;
spread spectrum processing, in response to said searcher signal, ones of
said multiple path propagations of said spread spectrum communication
signals corresponding to said multiple path propagation pilot signals of
greatest signal strength so as to extract corresponding intended recipient
user information signals therefrom; and
providing corresponding output signals representative of said extracted
intended recipient user information signals.
17. The method of claim 16 further comprising the steps of:
combining said output signals; and
providing a corresponding combined output signal.
18. The method of claim 16 further comprising the steps of:
receiving RF signals in a predetermined frequency band;
amplifying said RF signals;
frequency downconverting said amplified RF signals to an intermediate
frequency range so as to produce corresponding IF signals;
filtering said IF signals;
digitizing said IF signals; and
providing said IF signals to said searcher means and said receiver means as
said input signals. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to cellular telephone systems. More
specifically, the present invention relates to a novel and improved
receiver design for enhancing the reliability and communications in the
cellular telephone environment.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is
one of several techniques for facilitating communications in which a large
number of system users are present. Although other techniques such as time
division multiple access (TDMA), frequency division multiple access (FDMA)
and AM modulation schemes such as amplitude companded single sideband
(ACSSB) are known, CDMA has significant advantages over these other
techniques. The use of CDMA techniques in a multiple access communication
system is disclosed in U.S. Patent application Ser. No. 06/921,261, filed
Oct. 17, 1986, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION
SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", now U.S. Pat. No.
4,901,307 assigned to the assignee of the present invention, the
disclosure thereof incorporated by reference.
In the just mentioned patent, a multiple access technique is disclosed
where a large number of mobile telephone system users each having a
transceiver communicate through satellite repeaters or terrestrial base
stations (also known as cell-sites stations, or for short cell-sites)
using code division multiple access (CDMA) spread spectrum communication
signals. In using CDMA communications, the frequency spectrum can be
reused multiple times thus permitting an increase in system user capacity.
The use of CDMA results in a much higher spectral efficiency than can be
achieved using other multiple access techniques. In a CDMA system,
increases in system capacity may be realized by controlling the
transmitter power of each mobile user so as to reduce interference to
other system users.
In the satellite application of the CDMA communication techniques, the
mobile unit transceiver measures the power level of a signal received via
a satellite repeater. Using this power measurement, along with knowledge
of the satellite transponder downlink transmit power level and the
sensitivity of the mobile unit receiver, the mobile unit transceiver can
estimate the path loss of the channel between the mobile unit and the
satellite. The mobile unit transceiver then determines the appropriate
transmitter power to be used for signal transmissions between the mobile
unit and the satellite, taking into account the path loss measurement, the
transmitted data rate and the satellite receiver sensitivity.
The signals transmitted by the mobile unit to the satellite are relayed by
the satellite to a Hub control system earth station. The Hub measures the
received signal power from signals transmitted by each active mobile unit
transceiver. The Hub then determines the deviation in the received power
level from that which is necessary to maintain the desired communications.
Preferably the desired power level is a minimum power level necessary to
maintain quality communications so as to result in a reduction in system
interference.
The Hub then transmits a power control command signal to each mobile user
so as to adjust or "fine tune" the transmit power of the mobile unit. This
command signal is used by the mobile unit to change the transmit power
level closer to a minimum level required to maintain the desired
communications. As channel conditions change, typically due to motion of
the mobile unit, both the mobile unit receiver power measurement and the
power control feedback from the Hub continually readjust the transmit
power level so as to maintain a proper power level. The power control
feedback from the Hub is generally quite slow due to round trip delays
through the satellite requiring approximately 1/2 of a second of
propagation time.
One important difference between satellite or terrestrial base stations
systems are the relative distances separating the mobile units and the
satellite or cell-site. Another important different in the satellite
versus the terrestrial system is the type of fading that occurs in these
channels. Thus, these differences require various refinements in the
approach to system power control for the terrestrial system.
In the satellite/mobile unit channel, i.e. the satellite channel, the
satellite repeaters are normally located in a geosynchronous earth orbit.
As such, the mobile units are all at approximately the same distance from
the satellite repeaters and therefore experience nearly the same
propagation loss. Furthermore, the satellite channel has a propagation
loss characteristic that follows approximately the inverse square law,
i.e. the propagation loss is inversely proportional to the square of the
distance between the mobile unit and the satellite repeater in use.
Accordingly, in the satellite channel the variation in path loss due to
distance variation is typically on the order of only 1-2 dB.
In contrast to the satellite channel, the terrestrial/mobile unit channel,
i.e. the terrestrial channel, the distance between the mobile units and
the cell sites can vary considerably. For example, one mobile unit may be
located at a distance of five miles from the cell site while another
mobile unit may be located only a few feet away. The variation in distance
may exceed a factor of one hundred to one. The terrestrial channel
experiences a propagation loss characteristic as did the satellite
channel. However, in the terrestrial channel the propagation loss
characteristic corresponds to an inverse fourth-power law, i.e. the path
loss is proportional to the inverse of the path distance raised to the
fourth power. Accordingly, path loss variations may be encountered which
are on the order of over 80 dB in a cell having a radius of five miles.
The satellite channel typically experiences fading that is characterized as
Rician. Accordingly the received signal consists of a direct component
summed with a multiply reflected component having Rayleigh fading
statistics. The power ratio between the direct and reflected component is
typically on the order of 6-10 dB, depending upon the characteristics of
the mobile unit antenna and the environment about the mobile unit.
Contrasting the satellite channel with the terrestrial channel, the
terrestrial channel experiences signal fading that typically consists of
the Rayleigh faded component without a direct component. Thus, the
terrestrial channel presents a more severe fading environment than the
satellite channel where Rician fading is the dominant fading
characteristic.
The Rayleigh fading characteristics in the terrestrial channel signal is
caused by the signal being reflected from many different features of the
physical environment. As a result, a signal arrives almost simultaneously
at a mobile unit receiver from many directions with different transmission
delays. At the UHF frequency bands usually employed for mobile radio
communications, including those of cellular mobile telephone systems,
significant phase differences in signals traveling on different paths may
occur. The possibility for destructive summation of the signals may
result, with on occasion deep fades occurring.
Terrestrial channel fading is a very strong function of the physical
position of the mobile unit. A small change in position of the mobile unit
changes the physical delays of all the signal propagation paths, which
further results in a different phase for each path. Thus, the motion of
the mobile unit through the environment can result in a quite rapid fading
process. For example, in the 850 MHz cellular radio frequency band, this
fading can typically be as fast as one fade per second per mile per hour
of vehicle speed. Fading on this order can be extremely disruptive to
signals in the terrestrial channel resulting in poor communication
quality. However, additional transmitter power can be used to overcome the
problem of fading.
The terrestrial cellular mobile telephone system typically requires a
full-duplex channel to be provided in order to allow both directions of
the telephone conversation to be simultaneously active such as provided by
the conventional wired telephone system. This full-duplex radio channel is
normally provided by using one frequency band for the outbound link, i.e.
transmissions from the cell-site transmitter to the mobile unit receivers.
A different frequency band is utilized for the inbound link, i.e.
transmissions from the mobile unit transmitters to the cell-site
receivers. According, this frequency band separation allows a mobile unit
transmitter and receiver to be active simultaneously without feedback or
interference from the transmitter into the receiver.
In the conventional cellular telephone system the available frequency band
is divided into channels typically 30 KHz in bandwidth while analog FM
modulation techniques are used. The system service area is divided
geographically into cells of varying size. The available frequency
channels are divided into sets with each set usually containing an equal
number of channels. The frequency sets are assigned to cells in such a way
as to minimize the possibility of co-channel interference. For example,
consider a system in which there are seven frequency sets and the cells
are equal size hexagons. A frequency set used in one cell will not be used
in the six nearest or surrounding neighbors of that cell. Furthermore, the
frequency set in one cell will not be used in the twelve next nearest
neighbors of that cell.
In the conventional cellular telephone system, the handoff scheme
implemented is intended to allow a call to continue when a mobile
telephone crosses the boundary between two cells. The handoff from one
cell to another is initiated when the cell-site receiver handling the call
notices that the received signal strength from the mobile telephone falls
below a predetermined threshold value. A low signal strength indication
implies that the mobile telephone must be near the cell border. When the
signal level falls below the predetermined threshold value, the cell-site
asks the system controller to determine whether a neighboring cell-site
receives the mobile telephone signal with better signal strength than the
current cell-site.
The system controller in response to the current cell-site inquiry sends
messages to the neighboring cell-sites with a handoff request. The
cell-site neighboring the current cell-site employs special scanning
receivers which look for the signal from the mobile unit on the specified
channel. Should one of the neighboring cell-sites report an adequate
signal level to the system controller, then a handoff will be attempted.
Handoff is then initiated when an idle channel from the channel set used in
the new cell-site is selected. A control message is sent to the mobile
telephone commanding it to switch from the current channel to the new
channel. At the same time, the system controller switches the call from
the first cell-site to the second cell-site. In the conventional system a
break-before-make scheme is utilized such that no diversity reception is
possible in overcoming fades.
Furthermore should the mobile telephone fail to hear the command to switch
channels, the handoff will fail. Actual operating experience indicates
that handoff failures occur frequently which questions the reliability of
the system.
In the conventional cellular telephone system, path fading deleteriously
affects communications and can cause disruption in call service. It is
therefore an object of the present invention to provide, in a cellular
telephone system, receiver a design which facilitates reception and
processing of the strongest signals transmitted from one or more
cell-sites, these signals being multipath signals from a single cell-site
or signals transmitted by multiple cell-sites.
SUMMARY OF THE INVENTION
In a CDMA cellular telephone system, the same frequency band is used for
communication in all cells. The CDMA waveform properties that provide
processing gain are also used to discriminate between signals that occupy
the same frequency band. Furthermore the high speed pseudonoise (PN)
modulation allows many different production paths to be separated,
provided the difference in path propagation delays exceed the pN chip
duration, or one/bandwidth. If a PN chip rate of 1 MHz is employed in a
CDMA system, the full spread spectrum processing gain, equal to the ratio
of the spread bandwidth to system data rate, can be employed against paths
that differ by more than one microsecond in path delay from the desired
path. A one microsecond path delay differential corresponds to
differential path distance of 1,000 feet. The urban environment typically
provides differential path delays in excess of one microsecond, and up to
10-20 microseconds are reported in some areas.
In narrow band modulation systems such as the analog FM modulation employed
by conventional telephone systems, the existence of multiple paths results
in severe multipath fading. With wideband CDMA modulation, however, the
different paths may be discriminated against in the demodulation process.
This discrimination greatly reduces the severity of multipath fading.
Multipath fading is not totally eliminated in using CDMA discrimination
techniques because there will occasionally exist paths with delay
differentials of less than the minimum path delay for the particular
system. Signals having path delays on this order cannot be discriminated
against in the demodulator. It is therefor desirable that the system
should provide diversity to further reduce the effects of fading.
The deleterious effects of fading can be controlled somewhat by controlling
transmitter power in the CDMA system. A system for cell-site and mobile
unit power control is disclosed in copending U.S. Patent Application
entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A
CDMA CELLULAR MOBILE TELEPHONE SYSTEM", Ser. No. 07/433,031, filed Nov. 7,
1989, by the inventors hereof and assigned to the Assignee of the present
invention. Furthermore the effect of multipath fading can be reduced in
the handoff mode when the mobile unit is transitioning between cell-site
service area with the mobile unit communicating cell-sites during the
handoff process. The handoff scheme is disclosed in copending U.S. Patent
Application entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM",
Ser. No. 07/433,030, filed Nov. 7, 1989, by the inventors hereof and
assigned to the Assignee of the present invention.
The existence of multipaths can provide path diversity to a wideband PN
CDMA system. If two or more paths are available with greater than one
microsecond differential path delay, two or more PN receivers can be
employed to separately receive these signals. Since these signals will
typically exhibit independence in multipath fading, i.e., they usually do
not fade together, the outputs of the two receivers can be diversity
combined. Therefore a loss in performance only occurs when both receivers
experience fades at the same time. Hence, one aspect of the present
invention is the provision of two or more PN receivers in combination with
a diversity combiner.
Another aspect of the present invention is that as a mobile unit moves
through the physical environment, the number of multiple paths and their
signals strengths constantly vary. The present invention therefore
utilizes a special receiver, called a searcher receiver, which constantly
scans the time domain of the channel to determine the existence, the
location in the time domain, and the relative signal strengths of signals
in the multiple path environment. The searcher receiver provides control
over the data receivers in tracking the best signals available on
differing paths.
In a CDMA cellular telephone system, each cell-site has a plurality of
modulator-demodulator units or spread spectrum modems. Each modem consists
of a digital spread spectrum transmit modulator, at least one digital
spread spectrum data receiver and a searcher receiver. Each modem at the
cell-site is assigned to a mobile unit as needed to facilitate
communications with the assigned mobile unit. Therefore in many instances
many modems are available for use while other ones may be active in
communicating with respective mobile units. A soft handoff scheme is
employed for a CDMA cellular telephone system in which a new cell-site
modem is assigned to a mobile unit while the old cell-site continues to
service the call. When the mobile unit is located in the transition region
between the two cell-sites, the call can be switched back and forth
between cell-sites as signal strength dictates. Since the mobile unit is
always communicating through at least one cell-site, no disrupting effects
to the mobile unit or in service will occur. The present invention
utilizes multiple receivers at the mobile unit which are also used in a
diversity function when in the handoff process or firmly in a single cell.
In the CDMA cellular telephone system, each cell-site transmits a "pilot
carrier" signal. This pilot signal is used by the mobile units to obtain
initial system synchronization and to provide robust time, frequency and
phase tracking of the cell-site transmitted signals.
Each cell-site also transmits a "setup" channel comprised of spread
spectrum modulated information, such as cell-site identification, system
timing, mobile paging information and various other control signals. The
pilot signal transmitted by each cell-site is of the same spreading code
but with a different code phase offset. Phase offset allows the pilot
signals to be distinguished from one another resulting in distinguishment
between cell-sites from which they originate. Use of the same pilot signal
code allows the mobile unit to find system timing synchronization by a
single search through all pilot signal code phases. The strongest pilot
signal, as determined by a correlation process for each code phase, is
readily identifiable. The identified pilot signal corresponds to the pilot
signal transmitted by the nearest cell-site.
Upon acquisition of the strongest pilot signal, i.e. initial
synchronization of the mobile unit with the strongest pilot signal, the
mobile unit searches for the appropriate setup channel of that cell-site.
The setup channel is transmitted by the cell-site using one of a plurality
of different predetermined spread spectrum codes. In an exemplary
embodiment of the present invention, twenty-one different codes are used.
However, it should be understood that more or less codes could be used in
the setup channel as determined by system parameters. The mobile unit then
begins a search through all of the different codes used in the setup
channel.
When the mobile unit identifies the appropriate setup code for that
cell-site, system information is received and processed. The mobile unit
further monitors the setup channel for control messages. One such control
message would indicate a call is waiting for transfer to this mobile unit.
The mobile unit continues to scan the received pilot carrier signal code at
the code offsets corresponding to neighboring cell-site transmitted pilot
signals. This scanning is done in order to determine if the pilot signal
emanating from neighboring cells is becoming stronger than the pilot
signal first determined to be strongest. If, while in this call inactive
mode, a neighbor cell-site pilot signal becomes stronger than that of the
initial cell-site transmitted pilot signal, the mobile unit will acquire
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