Pulse or digital communications – Spread spectrum
Reexamination Certificate
1999-01-28
2004-04-13
Ghebretinsae, Temesghen (Department: 2631)
Pulse or digital communications
Spread spectrum
C375S146000
Reexamination Certificate
active
06721349
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for reducing peak to average ratio in code division multiple, access communication systems.
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 channels is covered with a unique Walsh function 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 references 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 function zero.
A number of challenges arise when attempting to increase the capacity of the CDMA system. First, the Walsh functions available for covering the code channels 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 reducing peak to average ratio in a system using auxiliary pilot channels as described in the TIA/EIA TR45.5 “cdma2000 ITU-R RTT Candidate Submission” to the ITU for consideration in the third generation wireless communication systems. The inventors have discovered that use of the proposed auxiliary pilot channels results in a high peak to average ratio that has undesirable effect on system capacity. The problem that arises in using auxiliary pilot channels is that there is no data to modulate the pilot waveforms. Because of the data carried on each of the auxiliary pilots is identical, the pilot waveforms add constructively and result in high energy peaks in portions of the waveforms.
A first method proposed in the present invention to deal with the above stated problem is to alter the phase of the auxiliary pilot channels to prevent the auxiliary pilot channels from adding constructively. A second method proposed in the present invention to deal with the above stated problem is to gate out the portion of the auxiliary pilot channels that add constructively. The present invention also proposes a novel demodulator design for receiving signals using the modified auxiliary pilot signals.
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Holtzman Jack M.
Shanbhag Abhijit
Willenegger Serge
Edwards Christopher
Ghebretinsae Temesghen
Miller Russell B.
Qualcomm Incorporated
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