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Custom CD of patents similar to US5416797 : System and method for generating signal waveforms in a CDMA cellular
telephone system - $19.95 |
| United States Patent | 5416797 |
| Link to this page | http://www.wikipatents.com/5416797.html |
| Inventor(s) | Gilhousen; Klein S. (San Diego, CA);
Jacobs; Irwin M. (La Jolla, CA);
Padovani; Roberto (San Diego, CA);
Weaver, Jr.; Lindsay A. (San Diego, CA);
Viterbi; Andrew J. (La Jolla, CA) |
| Abstract | A system and method for communicating information signals using spread
spectrum communication techniques. PN sequences are constructed that
provide orthogonality between the users so that mutual interference will
be reduced, allowing higher capacity and better link performance. With
orthogonal PN codes, the cross-correlation is zero over a predetermined
time interval, resulting in no interference between the orthogonal codes,
provided only that the code time frames are time aligned with each other.
In an exemplary embodiment, signals are communicated between a cell-site
and mobile units using direct sequence spread spectrum communication
signals. In the cell-to-mobile link, pilot, sync, paging and voice
channels are defined. Information communicated on the cell-to-mobile link
channels are, in general, encoded, interleaved, hi-phase shift key (BPSK)
modulated with orthogonal covering of each BPSK symbol along with
quadrature phase shift key (QPSK) spreading of the covered symbols. In the
mobile-to-cell link, access and voice channels are defined. Information
communicated on the mobile-to-cell link channels are, in general, encoded,
interleaved, orthogonal signalling along with QPSK spreading. |
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Title Information  |
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Drawing from US Patent 5416797 |
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System and method for generating signal waveforms in a CDMA cellular
telephone system |
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| Publication Date |
May 16, 1995 |
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| Filing Date |
January 24, 1992 |
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| Parent Case |
This is a continuation of application Ser. No. 07/543,496, filed Jun. 25,
1990, now U.S. Pat. No. 5,103,459. |
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Title Information |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to cellular telephone systems. More
specifically, the present invention relates to a novel and improved system
and method for communicating information, in a mobile cellular telephone
system or satellite mobile telephone system, using spread spectrum
communication signals.
2. 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. Other multiple access communication
system 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 in the art. However
the spread spectrum modulation technique of CDMA has significant
advantages over these modulation techniques for multiple access
communication systems. The use of CDMA techniques in a multiple access
communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb.
13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM
USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the
present invention, of which the disclosure thereof is incorporated by
reference.
In the just mentioned patent, a multiple access technique is disclosed
where a large number of mobile telephone system users each PG,3 having a
transceiver communicate through satellite repeaters or terrestrial base
stations (also referred to as cell-sites stations, cell-sites or for
short, cells) 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.
The satellite channel typically experiences fading that is characterized as
Rician. Accordingly the received signal consists of a direct component
summed with a multiple 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 with the satellite 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 in which Rician
fading is the dominant fading characteristic.
The Rayleigh fading characteristic 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 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 this severe can be extremely disruptive to
signals in the terrestrial channel resulting in poor communication
quality. Additional transmitter power can be used to overcome the problem
of fading. However, such power increases effect both the user, in
excessive power consumption, and the system by increased interference.
The CDMA modulation techniques disclosed in U.S. Pat. No. 4,901,307 offer
many advantages over narrow band modulation techniques used in
communication systems employing satellite or terrestrial repeaters. The
terrestrial channel poses special problems to any communication system
particularly with respect to multipath signals. The use of CDMA techniques
permit the special problems of the terrestrial channel to be overcome by
mitigating the adverse effect of multipath, e.g. fading, while also
exploiting the advantages thereof.
In a CDMA cellular telephone system, the same frequency band can be 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 propagation paths to be separated,
provided the difference in path delays exceed the PN chip duration, i.e.
1/bandwidth. If a PN chip rate of approximately 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 approximately 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 wide band 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 delayed
differentials of less than the PN chip duration for the particular system.
Signals having path delays on this order cannot be discriminated against
in the demodulator, resulting in some degree of fading.
It is therefore desirable that some form of diversity be provided which
would permit a system to reduce fading. Diversity is one approach for
mitigating the deleterious effects of fading. Three major types of
diversity exist: time diversity, frequency diversity and space diversity.
Time diversity can best be obtained by the use of repetition, time
interleaving, and error detection and coding which is a form of
repetition. The present invention employes each of these techniques as a
form of time diversity.
CDMA by its inherent nature of being a wideband signal offers a form of
frequency diversity by spreading the signal energy over a wide bandwidth.
Therefore, frequency selective fading affects only a small part of the
CDMA signal bandwidth.
Space or path diversity is obtained by providing multiple signal paths
through simultaneous links from a mobile user through two or more
cell-sites. Furthermore, path diversity may be obtained by exploiting the
multipath environment through spread spectrum processing by allowing a
signal arriving with different propagation delays to be received and
processed separately. Examples of path diversity are illustrated in
copending U.S. Patent application entitled "SOFT HANDOFF IN A CDMA
CELLULAR TELEPHONE SYSTEM", Ser. No. 07/433,030, filed Nov. 7, 1989, now
U.S. Pat. No. 5,101,501, and copending U.S. Patent application entitled
"DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", Ser. No.
07/432,552, also filed Nov. 7, 1989, now U.S. Pat. No. 5,109,390, both
assigned to the assignee of the present invention.
The deleterious effects of fading can be further controlled to a certain
extent in a CDMA system by controlling transmitter power. 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, also assigned to the assignee of the
present invention, now U.S. Pat. No. 5,056,109.
The CDMA techniques as disclosed in U.S. Pat. No. 4,901,307 contemplated
the use of coherent modulation and demodulation for both directions of the
link in mobile-satellite communications. Accordingly, disclosed therein is
the use of a pilot carrier signal as a coherent phase reference for the
satellite-to-mobile link and the cell-to-mobile link. In the terrestrial
cellular environment, however, the severity of multipath fading, with the
resulting phase disruption of the channel, precludes usage of coherent
demodulation technique for the mobile-to-cell link. The present invention
provides a means for overcoming the adverse effects of multipath in the
mobile-to-cell link by using noncoherent modulation and demodulation
techniques.
The CDMA techniques as disclosed in U.S. Pat. No. 4,901,307 further
contemplated the use of relatively long PN sequences with each user
channel being assigned a different PN sequence. The cross-correlation
between different PN sequences and the autocorrelation of a PN sequence
for all time shifts other than zero both have a zero average value which
allows the different user signals to be discriminated upon reception.
However, such PN signals are not orthogonal. Although the
cross-correlations average to zero, for a short time interval such as an
information bit time the cross-correlation follows a binomial
distribution. As such, the signals interfere with each other much the same
as if they were wide bandwidth Gaussian noise at the same power spectral
density. Thus the other user signals, or mutual interference noise,
ultimately limits the achievable capacity.
The existence of multipath 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. In order to exploit the existence of multipath
signals, to overcome fading, it is necessary to utilize a waveform that
permits path diversity combining operations to be performed.
It is therefore an object of the present invention to provide for the
generation of PN sequences which are orthogonal so as to reduce mutual
interference, thereby permitting greater user capacity, and support path
diversity thereby overcoming fading.
SUMMARY OF THE INVENTION
The implementation of spread spectrum communication techniques,
particularly CDMA techniques, in the mobile cellular telephone environment
therefore provides features which vastly enhance system reliability and
capacity over other communication system techniques. CDMA techniques as
previously mentioned further enable problems such as fading and
interference to be readily overcome. Accordingly, CDMA techniques further
promote greater frequency reuse, thus enabling a substantial increase in
the number of system users.
The present invention is a novel and improved method and system for
constructing PN sequences that provide orthogonality between the users so
that mutual interference will be reduced, allowing higher capacity and
better link performance. With orthogonal PN codes, the cross-correlation
is zero over a predetermined time interval, resulting in no interference
between the orthogonal codes, provided only that the code time frames are
time aligned with each other.
In an exemplary embodiment, signals are communicated between a cell-site
and mobile units using direct sequence spread spectrum communication
signals. In the cell-to-mobile link, pilot, sync, paging and voice
channels are defined. Information communicated on the cell-to-mobile link
channels are, in general, encoded, interleaved, bi-phase shift key (BPSK)
modulated with orthogonal covering of each BPSK symbol along with
quadrature phase shift key (QPSK) spreading of the covered symbols.
In the mobile-to-cell link, access and voice channels are defined.
Information communicated on the mobile-to-cell link channels are, in
general, encoded, interleaved, orthogonal signalling along with QPSK
spreading.
BRIEF DESCRIPTION OF THE DRAWINGS
The features, objects, and advantages of the present invention will become
more apparent from the detailed description set forth below when taken in
conjunction with the drawings in which like reference characters identify
correspondingly throughout and wherein:
FIG. 1 is a schematic overview of an exemplary CDMA cellular telephone
system;
FIG. 2 is a block diagram of the cell-site equipment as implemented in the
CDMA cellular telephone system;
FIG. 3 is a block diagram of the cell-site receiver;
FIGS. 4A-4C illustrate a block diagram of the cell-site transmit modulator;
and
FIG. 5 is an exemplary timing diagram of sync channel symbol
synchronization;
FIG. 6 is an exemplary timing diagram of sync channel timing with
orthogonal covering;
FIG. 7 is an exemplary timing diagram of the overall cell-to-mobile link
timing;
FIG. 8 is a block diagram of the mobile telephone switching office
equipment;
FIG. 9 is a block diagram of the mobile unit telephone configured for CDMA
communications in the CDMA cellular telephone system;
FIG. 10 is a block diagram of the mobile unit receiver; and
FIG. 11 is a block diagram of the mobile unit transmit modulator;
FIG. 12 is an exemplary timing diagram of the mobile-to-cell link for the
variable data rate with burst transmission; and
FIG. 13 is an exemplary timing diagram of the overall mobile-to-cell link
timing.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
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.
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 modem 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 modem, fewer disrupting effects to the mobile unit or in service
will occur. The mobile unit thus utilizes multiple receivers for assisting
in the handoff process in addition to a diversity function for mitigating
the effects of fading.
In the CDMA cellular telephone system, each cell-site transmits a "pilot
carrier" signal. Should the cell be divided into sectors, each sector has
an associated distinct pilot signal within the cell. 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 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 sector of each cell 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 thus distinguishing
originating cell-sites or sectors. 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 strongest pilot signal generally corresponds
to the pilot signal transmitted by the nearest cell-site. However, the
strongest pilot signal is used whether or not it is transmitted by the
closest 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 another carrier intended to be received by all
system users in the cell. This carrier, called the synchronization
channel, transmits a broadcast message containing system information for
use by the mobiles in the system. The system information identifies the
cell-site and the system in addition to conveying information which allows
the long PN codes, interleaver frames, vocoders and other system timing
information used by the mobile mobile unit to be synchronized without
additional searching. Another channel, called the paging channel may also
be provided to transmit messages to mobiles indicating that a call has
arrived for them, and to respond with channel assignments when a mobile
initiates a call.
The mobile unit continues to scan the received pilot carrier signal code at
the code offsets corresponding to cell-site neighboring sector or
neighboring transmitted pilot signals. This scanning is done in order to
determine if a pilot signal emanating from a neighboring sector or cell is
becoming stronger than the pilot signal first determined to be strongest.
If, while in this call inactive mode, a neighbor sector or neighbor
cell-site pilot signal becomes stronger than that of the initial cell-site
sector or cell-site transmitted pilot signal, the mobile unit will acquire
the stronger pilot signals and corresponding sync and paging channel of
the new sector or cell-site.
When a call is initiated, a pseudonoise (PN) code address is determined for
use during the course of this call. The code address may be either
assigned by the cell-site or be determined by prearrangement based upon
the identity of the mobile unit. After a call is initiated the mobile unit
continues to scan the pilot signal transmitted by the cell-site through
which communications are established in addition to pilot signal of
neighboring sectors or cells. Pilot signal scanning continues in order to
determine if one of the neighboring sector or cell transmitted pilot
signals becomes stronger than the pilot signal transmitted by the
cell-site the mobile unit is in communication with. When the pilot signal
associated with a neighboring cell or cell sector becomes stronger than
the pilot signal of the current cell or cell sector, it is an indication
to the mobile unit that a new cell or cell sector has been entered and
that a handoff should be initiated.
An exemplary telephone system in which the present invention is embodied is
illustrated in FIG. 1. The system illustrated in FIG. 1 utilizes spread
spectrum modulation techniques in communication between the system mobile
units or mobile telephones, and the cell-sites. Cellular systems in large
cities may have hundreds of cell-site stations serving hundreds of
thousands of mobile telephones. The use of spread spectrum techniques, in
particular CDMA, readily facilitates increases in user capacity in systems
of this size as compared to conventional FM modulation cellular systems.
In FIG. 1, system controller and switch 10, also referred to as mobile
telephone switching office (MTSO), typically includes interface and
processing circuitry for providing system control to the cell-sites.
Controller 10 also controls the routing of telephone calls from the public
switched telephone network (PSTN) to the appropriate cell-site for
transmission to the appropriate mobile unit. Controller 10 also controls
the routing of calls from the mobile units, via at least one cell-site, to
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