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 C1-C6. 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 C1-C6, 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 C1 and C6; while a second residential user sector spans the coverage area C4. Similarly, a user sector including primarily rural areas is associated with the coverage areas C2 and C3, while business users are concentrated within the coverage area C5.
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 C5 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 C5 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 C2-C3.
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 C5 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 further object of the invention to tailor the size of specific user sectors within a CDMA communication to the traffic channel demand within the sector. Such efficient traffic channel allocation would enable optimum utilization of communication system resources, thereby minimizing the cost per user.
In addition to addressing the need for flexible traffic channel allocation as a consequence of the short-term changes in user demand described above, it is a further object of the invention to accommodate long-term changes in user demand. Such long-term variation in demand could arise from, for example, shifts in population distribution and building patterns within a given geographic area.
A further disadvantage of conventional fixed-beam systems, such as the system of FIG. 1A, is that relatively accurate estimates of user demand must typically be available prior to system installation. That is, system designers generally must be supplied with detailed information related to expected demand patterns in order that the fixed-beam base station be configured to provide the requisite traffic channel capacity to each user sector. Changes in usage patterns occurring proximate the installation period thus tend to prevent optimal utilization of the available traffic channels. It is therefore yet another object of the present invention to provide a communication system capable of being tailored, upon installation, in accordance with existing patterns of traffic channel demand.