Multiplex communications – Generalized orthogonal or special mathematical techniques – Particular set of orthogonal functions
Reexamination Certificate
1997-09-08
2001-09-04
Chin, Wellington (Department: 2664)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Particular set of orthogonal functions
C370S335000, C370S342000, C375S130000, C375S134000
Reexamination Certificate
active
06285655
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a method and apparatus for providing orthogonal spot beams, sectors, and picocells.
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. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the present invention and incorporated by reference herein. The CDMA system can be designed to conform to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, hereinafter referred to as the IS-95 standard.
The CDMA system is a spread spectrum communication system. The benefits of spread spectrum communication are well known in the art and can be appreciated by reference to the above cited references. 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 to a mobile user or remote station through two or more base stations. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing signals arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled “METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM,” and U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention and incorporated by reference herein.
In a CDMA system, the forward link refers to a transmission from a base station to a remote station. In the exemplary CDMA communication system which conforms to the IS-95 standard, forward link data and voice transmissions occur over orthogonal code channels. In accordance with the IS-95 standard, each orthogonal code channel is covered with a unique Walsh sequence which is 64 chips in duration. The orthogonality minimizes the interference between the code channels and improves performance.
CDMA systems offer higher system capacity, as measured by the number of supportable users, through several design features. First, the transmit frequency of adjacent cells can be reused. Second, increased capacity can be achieved by using more directive antennas for the transmission to some areas or to some remote stations. In the CDMA system, the coverage area (or cell) can be divided into several (e.g., three) sectors using directive antennas. The method and apparatus for providing sectors in a CDMA communication system is described in U.S. Pat. No. 5,621,752, entitled “ADAPTIVE SECTORIZATION IN A SPREAD SPECTRUM COMMUNICATION SYSTEM”, assigned to the assignee of the present invention and incorporated by reference herein. Each sector or cell can be further divided into more directive spot beams. Alternatively, spot beams can be assigned to selected remote stations or a set of remote stations within a sector or a cell. A picocell is a localized coverage area within a sector or a cell. The picocell can be embedded within a sector or a cell to improve capacity and provide additional services.
In the exemplary CDMA system, the forward link transmissions in different sectors typically use different short PN spreading sequences (or different offsets of a common set of short PN spreading sequences). Thus, when a remote station is in overlapping sector coverage areas and demodulating the signal from one sector, the signals from other sectors are spread and appear as wideband interference. However, the signals from other sectors or cells are not orthogonal to each other. The non-orthogonal interference from adjacent sectors or cells can degrade the performance of the communication system.
In an IS-95 CDMA communication system, a pilot channel is transmitted on the forward link to assist the remote station perform coherent demodulation of the received signal. Coherent demodulation results in improved performance. For each beam, a pilot channel is utilized. In accordance with the IS-95 standard, the pilot channel is covered with Walsh sequence zero.
A number of challenges arise when attempting to increase the capacity of the CDMA system. First, the Walsh sequences available for covering the code channel is defined by the IS-95 standard and limited to 64. Second, a method is desired to allow the remote stations to distinguish the different beams, sectors, or picocells in CDMA systems with minimal signal processing. And third, maintaining conformance to the IS-95 standard is a desirable condition. The present invention addresses these challenges.
SUMMARY OF THE INVENTION
The present invention is a novel and improved method and apparatus for providing orthogonal spot beams, sectors, and picocells. The transmissions can be made orthogonal by using orthogonal auxiliary pilots and different Walsh traffic channels in adjacent areas. In accordance with the IS-95 standard, the pilot signal is covered with the 64-chip all zeros Walsh sequence. In the exemplary embodiment, the 64-chip all zeros Walsh sequence is designated as P and the 64-chip all ones sequence is designated as M. In the present invention, additional pilot signals can be provided by concatenating the 64-chip all zeros P and all ones M sequences. For two pilot signals, pilot Walsh sequences of PP and PM can be used. For four pilot signals, pilot Walsh sequences of PPPP, PMPM. PPMM, and PMMP can be used. The present invention can be extended such that K pilot Walsh sequences can be generated by substituting each bit in an K-bit Walsh sequence with the 64-chip all zeros P or all ones M sequence depending on the value of that bit. Using this method, K pilot Walsh sequences can be generated from the basic all zeros P and all ones M sequences, where K is a number which is a power of two.
It is an object of the present invention to provide orthogonal spot beams, sectors, and picocells. In the exemplary embodiment, the traffic channels in a transmission area are covered with Walsh sequences which are orthogonal to those of adjacent areas. In addition, the pilot for each transmission area is covered with pilot Walsh sequence which is derived from Walsh sequence zero. Orthogonal traffic channels and pilots minimize interference and improve capacity.
It is another object of the present invention to provide additional orthogonal pilot channels without reducing the number of orthogonal Walsh channels available for traffic and control channels. In accordance with the IS-95 standard, 64 Walsh sequences are available for covering 64 code channels. Walsh sequence zero is reserved for the pilot channel and the remaining 63 Walsh sequences can be used for other code channels, such as traffic channels and control channels. In the present invention, the additional pilot signals are generated using concatenated combinations of the all zeros and all ones sequen
Lundby Stein A.
Odenwalder Joseph P.
Tiedemann Jr. Edward G.
Chin Wellington
Kalousek Pavel
Nguyen Steven
Qualcomm Inc.
Wadsworth Philip
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