Pulse or digital communications – Spread spectrum
Reexamination Certificate
1995-06-28
2002-10-29
Luther, William (Department: 2664)
Pulse or digital communications
Spread spectrum
C375S144000, C342S361000
Reexamination Certificate
active
06473447
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communication systems utilizing spread spectrum signals, and, more particularly, to a novel and improved method and apparatus for dynamic channel sectorization within a spread spectrum communication system.
II. Description of the Related Art
Communication systems have been developed to allow transmission of information signals from a source location to a physically distinct user destination. Both analog and digital methods have been used to transmit such information signals over communication channels linking the source and user locations. Digital methods tend to afford several advantages relative to analog techniques, including, for example, improved immunity to channel noise and interference, increased capacity, and improved security of communication through the use of encryption.
In transmitting an information signal from a source location over a communication channel, the information signal is first converted into a form suitable for efficient transmission over the channel. Conversion, or modulation, of the information signal involves varying a parameter of a carrier wave on the basis of the information signal in such a way that the spectrum of the resulting modulated carrier is confined within the channel bandwidth. At the user location the original message signal is replicated from a version of the modulated carrier received subsequent to propagation over the channel. Such replication is generally achieved by using an inverse of the modulation process employed by the source transmitter.
Modulation also facilitates multiplexing, i.e., the simultaneous transmission of several signals over a common channel. Multiplexed communication systems will generally include a plurality of remote subscriber units requiring intermittent service of relatively short duration rather than continuous access to the communication channel. Systems designed to enable communication over brief periods of time with a set of subscriber units have been termed multiple access communication systems.
A particular type of multiple access communication system is known as a spread spectrum system. In spread spectrum systems, the modulation technique utilized results in a spreading of the transmitted signal over a wide frequency band within the communication channel. One type of multiple access spread spectrum system is a code division multiple access (CDMA) modulation system. 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 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.
In the above-referenced U.S. Pat. No. 4,901,307, a multiple access technique is disclosed where a large number of mobile system users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations using 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 particular, cellular CDMA systems communication between a base station and subscriber units within the surrounding cell region is achieved by spreading each transmitted signal over the available channel bandwidth by using a unique user spreading code. In such CDMA systems the code sequences used for spreading the spectrum are constructed from two different types of sequences, each with different properties, to provide different functions. There is an outer code that is shared by all signals in a cell or sector that is used to discriminate between multipath signals. In addition, adjusting the phase of the outer code allows it to be used to discriminate between sets of users grouped into “sectors” within a given cell. For example, the users within a given cell may be partitioned into three sectors by providing three phases of the outer code. There is also an inner code that is used to discriminate between user signals transmitted over a plurality of “traffic channels” associated with each user sector. Specific transmitted signals are extracted from the communication channel by despreading the composite signal energy in the communication channel with the inner code associated with the transmitted signal to be extracted.
Referring to
FIG. 1A
, there is shown a first exemplary cell
10
in which are disposed a plurality of subscriber units
12
and a base station
14
. As is indicated by
FIG. 1A
, the cell
10
is partitioned into six coverage areas C
1
-C
6
. The base station
14
may include a set of six fixed-beam antennas (not shown) dedicated to facilitating communication with subscriber units in the coverage areas C
1
-C
6
, respectively. The subscriber units
12
are grouped into a plurality of user sectors, each of which supports an equivalent number of traffic channels. As is indicated by
FIG. 1A
, a first residential user sector encompasses the coverage areas C
1
and C
6
; while a second residential user sector spans the coverage area C
4
. Similarly, a user sector including primarily rural areas is associated with the coverage areas C
2
and C
3
, while business users are concentrated within the coverage area C
5
.
As is indicated by
FIG. 1A
, it is necessary that certain user sectors be relatively narrow in order to accommodate demand during peak periods of system utilization. For example, the relatively narrow breadth of the business user sector is necessitated by the high concentration of business users within coverage area C
5
desiring to communicate during working hours, e.g., between 8 a.m. and 5 p.m. That is, if the scope of the business user sector were expanded to include regions other than coverage area C
5
it is possible that an insufficient number of traffic channels would be available during business hours to accommodate all those desiring to place calls. In contrast, the diffuse concentration of subscriber units
12
among rural dwellings allows the traffic channels associated with the rural user sector to be allocated among users distributed over two coverage areas C
2
-C
3
.
Unfortunately, during non-working hours a number of the traffic channels dedicated to the business user sector will likely go unused, since at such times there exist significantly fewer business callers and a correspondingly larger number of residential callers. Accordingly, it would be desirable to be able to provide a high concentration of traffic channels to business users within coverage area C
5
during business hours, and to provide a relatively lower traffic channel concentration during non-working hours.
Although there exist antenna arrays capable of adaptively shaping a projected beam in response to changing user demand, implementation of such antenna arrays within the system of
FIG. 1A
would require corresponding modification of the fixed-beam architecture of the base station
14
. In addition, the relatively sophisticated RF/microwave circuits typically employed in adaptive beam-forming networks result in increased system cost and complexity. Accordingly, it is an object of the invention to provide a cost-effective technique for varying the concentration of traffic channels in response to changes in the distribution of users within a spread spectrum cellular communication system.
In the specific instance of a CDMA communication system, each user sector is capable of supporting a given level of traffic demand. Accordingly, it is a furt
Strich W. Eli
Thompson James H.
Baker Kent D.
Luther William
Miller Russel B.
Qualcomm Incorporated
Wadsworth Phil R.
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