Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
1999-09-15
2004-03-23
Urban, Edward F. (Department: 2685)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S513000, C455S550100, C370S431000
Reexamination Certificate
active
06711420
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a code division multiple access (“CDMA”) communication system and specifically to a finger assignment algorithm in a CDMA system which matches demodulating fingers in a mobile station (“MS”) with signal paths from a base station (“BS”) or base stations wherein signals received from base stations transmitting on supplemental channels are biased to insure that the MS assigns demodulating fingers to those base stations.
2. Description of the Related Art
The next generation of wireless networks will provide multiple services requiring high data rate transmission and uninterrupted connections. This next generation is often referred to as the “third generation” of CDMA wireless systems. The range of services include text paging, two-way radio connections, internet connectivity using microbrowsers, two-way wireless e-mail capability and wireless modem functionality. The CDMA cellular telephone system offers the capability to provide reliable radio links between a wireless communications device such as a MS and a BS with a much higher data capacity than conventional networks that only support voice service. As an example, in the third generation CDMA wireless systems, radio links supporting high rate (up to 2 Mbps) data transmissions will be established between the MS and the BS to provide multimedia services such as Internet access.
One particularly important feature of CDMA systems for effective third generation wireless communication is the soft handoff, which allows the MS to move smoothly from the coverage of one cell to another without interruption of service to the user. The soft handoff is accomplished by establishing simultaneous communications between the MS and multiple base stations or BS sectors. In a soft handoff, a MS passes to the edge of the coverage area of a serving BS into a new coverage area of a receiving BS. Momentarily, both BS sectors simultaneously communicate with the MS. As the MS passes further into the coverage area of the receiving BS, the server BS stops communicating with the MS. In this manner, there is uninterrupted communication for the user of the MS as the he or she passes from the server cell to the receiving cell. An efficient soft handoff algorithm plays an important role in maintaining the link quality as well as conserving the capacity-related network resources. As the demand to support high rate data services increases, the need to improve the efficiency of the handoff algorithm becomes more critical.
For a third generation system based on CDMA technologies, a highly efficient handoff algorithm is essential to successfully provide the infrastructure to support the new range of services. A conventional protocol for soft handoffs in a CDMA system has been adopted by the Telecommunications Industry Association in the industry standards IS-95, IS-95 A or IS-95 B (collectively “IS-95 A/B”) for implementing a CDMA cellular system. A new feature in the IS-95 B standard not found in IS-95 A is the inclusion of Supplemental Code Channels, or supplemental channels within the traffic channels. The traffic channels are the communication path between the MS and the BS used for user voice and signaling traffic. The term traffic channel includes the forward channel from the BS to the MS and the reverse channel from the MS to the BS.
In a code division multiple access (CDMA) cellular telephone system, a common frequency band is used for communication with all base stations in a system. The common frequency band allows simultaneous communication between a MS and more than one BS. Signals occupying the common frequency band are discriminated at the receiving station through the spread spectrum CDMA waveform properties based on the use of a high speed pseudonoise (PN) code. The high speed PN code is used to modulate signals transmitted from the base stations and the mobile stations. Transmitter stations using different PN codes or PN codes that are offset in time produce signals that can be separately received at the receiving station. The high speed PN modulation also allows the receiving station to receive a signal from a single transmitting station where the signal has traveled over several distinct propagation paths.
A signal having traveled several distinct propagation paths is generated by the multipath characteristics of the cellular channel. One characteristic of a multipath channel is the time spread introduced in a signal that is transmitted through the channel. For example, if an ideal impulse is transmitted over a multipath channel, the received signal appears as a stream of pulses. Another characteristic of the multipath channel is that each path through the channel may cause a different attenuation factor. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different signal strength than other received pulses. Yet another characteristic of the multipath channel is that each path through the channel may cause a different phase on the signal. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different phase than other received pulses.
In the mobile radio channel, the multipath is created by reflection of the signal from obstacles in the environment, such as buildings, trees, cars and people. In general the mobile radio channel is a time varying multipath channel due to the relative motion of the structures that create the multipath. Therefore, if an ideal impulse is transmitted over the time varying multipath channel, the received stream of pulses would change in time location, attenuation, and phase as a function of the time that the ideal impulse was transmitted.
The multipath characteristic of a channel can result in signal fading. Fading is the result of the phasing characteristics of the multipath channel. A fade occurs when multipath vectors are added destructively, yielding a received signal that is smaller than either individual vector. For example if a sine wave is transmitted through a multipath channel having two paths where the first path has an attenuation factor of X dB, a time delay of &dgr; with a phase shift of &THgr; radians, and the second path has an attenuation factor of X dB, a time delay of &dgr; with a phase shift of &THgr;+Π radians, no signal would be received at the output of the channel.
In narrow band modulation systems such as the analog FM modulation employed by conventional radio telephone systems, the existence of multiple paths in the radio channel results in severe multipath fading. As noted above with a wideband CDMA, however, the different paths may be discriminated in the demodulation process. This discrimination not only greatly reduces the severity of multipath fading but provides an advantage to the CDMA system.
The deleterious effects of fading can be mitigated by controlling transmitter power in the CDMA system. A system for BS and MS power control is disclosed in U.S. Pat. No. 5,056,109 entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM,” issued Oct. 8, 1991, assigned to the Assignee of the present invention. Furthermore the effect of multipath fading can be reduced by communication with multiple base stations using a soft handoff process. A handoff process is disclosed in U.S. Pat. No. 5,101,501 entitled “SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,” issued Oct. 8, 1991, and assigned to the Assignee of the present invention. The disclosure of U.S. Pat. Nos. 5,056,109 and 5,101,501 are incorporated herein by reference.
A method of assigning multiple demodulation elements or fingers in a spread spectrum system is disclosed in U.S. Pat. No. 5,490,165 (“the '165 patent”), which disclosure is incorporated as if fully set forth herein. Accordingly, background information and familiarity with the '165 patent are presumed for the present invention. The '165 patent is a
Achour Baaziz
Amerga Messay
Glazko Serguei A.
Brown Charles D.
Nguyen Thuan T.
Seo Howard H.
Urban Edward F.
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