Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2001-07-12
2004-11-16
Trost, William (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S506000, C375S148000, C375S349000, C375S347000
Reexamination Certificate
active
06819931
ABSTRACT:
BACKGROUND
I. Field of the Invention
The present invention relates to wireless communications, and particularly to signal reception.
II. Description of the Related Art
A wireless receiver may receive a signal that has traveled several distinct propagation paths as generated by the multipath characteristics of the transmission 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 a mobile radio channel of a system for cellular communications, the multipath may be created by the reflections 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. For example, 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;+&pgr; radians, no signal would be received at the output of the channel.
In a code-division multiple-access (CDMA) cellular telephone system, a common frequency band is used, allowing for simultaneous communication between a mobile station (e.g. a cellular telephone, wireless local loop station, or wireless modem) and more than one base station. 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 PN code is used to modulate signals transmitted between the base stations and the mobile stations. Transmitter stations using different PN codes, or the same PN code offset in time, produce signals that may be distinguished at the receiving station. The 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.
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 may result in severe multipath fading. In a wideband CDMA system, however, instances of a transmitted signal that are received over different paths may be discriminated in the demodulation process. This discrimination not only greatly reduces the severity of multipath fading but also 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 base station and mobile station 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, and assigned to the Assignee of the present invention. Furthermore the effect of multipath fading can be reduced through 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 existence of multipath can provide path diversity to a wideband spread spectrum system. A spread spectrum system generates a spread information signal by modulating an information signal with a pseudonoise (PN) code. Generally the PN code runs at many times the rate of the information signal. The rate at which the PN code is generated is called the chip rate, and the duration of one data bit of the PN code is called the chip time. If two or more paths are available that have a differential path delay greater than one chip time, then two or more processing elements (called demodulation elements or fingers) may be employed to separately demodulate the signal instances received over those paths. Such signals typically exhibit independence in multipath fading—i.e., they do not usually fade together—so that loss of signal occurs only when all of the signal instances being demodulated experience a fade at the same time. Therefore, the output of the two or more demodulation elements can be combined to obtain path diversity. In an ideal system, both the base station and the mobile station employ multiple demodulation elements.
As a mobile station moves through the physical environment, the number and qualities of the signal paths may vary constantly, both as received at the mobile station and as received at the base station. Therefore, a receiver may use a special processing element (called a searcher element) that continually scans the channel in the time domain to determine the existence, time offset, and the signal strength of signals in the multiple path environment. The output of the searcher element provides information that may be used to ensure that the demodulation elements are tracking the most advantageous paths.
In an exemplary CDMA cellular telephone system, each base station transmits a pilot signal that is used by the mobile stations to obtain initial system synchronization and to provide robust time, frequency, and phase tracking of the base station transmitted signals. The pilot signals transmitted by the various base stations in a system may all use the same PN code, with each base station's signal being spread using a different code phase offset (i.e. the PN codes transmitted by neighboring base stations are identical but skewed in time with respect to one another). This code phase offset allows the pilot signals to be distinguished from one another according to the base station from which they originate. A typical base station configuration may also contain multiple sectors, with each sector comprising independent transmit and receive antennas and having its own PN code or code offset. While the mobile station is in the call inactive mode, its searcher element may continue to scan the received signal at the code offsets corresponding to the transmitted pilot signals of nearby base stations. When a call is initiated, a PN code address is determined for use during this call. The code address may be either assigned by the base station or determined by prearrangement based upon the identity of the mobile station. After a call is initiated, the mobile station's searcher element continues to scan the pilot signals transmitted by neighboring base stations, and a mobile station may communicate with several base stations at once.
Diversity combining in the mobile station significantly advances the quality and reliability of communications in a cellular telephone system. Diversity exploits the random nature of radio propagation by finding independen
Amerga Messay
Joshi Abhay A.
Levin Mark
Brown Charles
Kordich Donald
Le Danh C
Qualcomm Incorporated
Trost William
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