Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-09-09
2001-11-20
Nguyen, Chau (Department: 2663)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S335000, C455S436000
Reexamination Certificate
active
06320855
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to wireless communication devices in general, and to a system and method for initiating idle handoffs between base stations in particular.
II. Description
Communication systems have been developed that allow transmission of information signals from a base station location to a physically distinct user or subscriber location. Both analog and digital methods have been used to transmit these information signals over communication channels linking the base station 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 in either direction 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 recipient 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 during message transmission.
Modulation facilitates multiplexing, i.e., the simultaneous transmission of several signals over a common channel. Multiplexed communication systems generally include a plurality of remote subscriber units or mobile stations requiring intermittent service rather than continuous access to the communication channel. Systems designed to enable communication with a selected subset of a full set of subscriber units are called multiple access communication systems. A particular type of multiple access communications system, known as a code division multiple access (CDMA) modulation system, may be realized in accordance with spread spectrum techniques. In spread spectrum systems, the modulation technique utilized results in spreading of the transmitted signal over a wide frequency band within the communication channel. Other multiple access communication system techniques include, for example, time division multiple access (TDMA) and frequency division multiple access (FDMA). CDMA techniques however, offer significant advantages over other multiple access communication system techniques. 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 and incorporated herein by reference.
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. CDMA modulation in turn allows the frequency spectrum dedicated to cellular telephony to be reused multiple times, resulting in a significant increase in system user capacity. In fact, the same frequency band is used in each cell or sector of a cell within the cellular geographic serving area (CGSA) of the CDMA system. Thus, the use of CDMA results in a much higher spectral efficiency than can be achieved using other multiple access techniques.
In a wireless communications channel, the presence of obstacles in the environment such as buildings, trees, mountains, cars and the like often results in the reflection of wireless communications signals transmitted by either mobile or base stations. This phenomenon is referred to as a multipath propagation environment because any particular wireless communications receiver (such as a mobile station) may receive a plurality of signals corresponding to a single signal transmitted by a particular wireless communications transmitter (such as one or more base stations), each of the plurality of received signals having traveled a different path to the receiver. Typically, the mobile radio channel also is a time varying multipath channel. In other words, the stream of pulses that would be received following the transmission of an ideal pulse over a mobile radio channel would change in time location, attenuation and phase depending on when the ideal pulse were transmitted. This is due in part to relative motion between the wireless transmitters and the environmental obstacles. It also due in part to fading, which occurs when multipath signals are phase shifted to such a degree that destructive interference with one another occurs, and path loss, which is a result of atmospheric effects on wireless communications signals.
In narrowband modulation systems the multipath characteristics of wireless communications channels often results in significantly impaired system performance. In CDMA systems though, the high speed PN code modulation allows a receiving station to receive and discriminate among signals from a single transmitting station that have traveled over several distinct propagation paths. This ability to discriminate between multipath signal transmissions markedly reduces the severity of signal fading in such systems. Indeed, the ability to discriminate between multipath signal transmissions actually provides significant advantages in CDMA systems because each multipath signal typically exhibits independent fading characteristics and may be exploited without regard to the signals arriving via other paths. A receiver design allowing one to exploit these multipath signals is described in U.S. Pat. No. 5,109,390 entitled “Diversity Receiver in a CDMA Cellular Telephone System,” assigned to the assignee of the present invention and incorporated herein by reference. Provided the phase offset (i.e., the arrival time) of the multipath signals is 1 &mgr;s or more, the diversity receiver design referred to above allows independent tracking and demodulation of the diverse signals. Once demodulated, the signals may then be used independently or combined to form composite signals.
An exemplary cellular system is depicted in FIG.
1
. Such systems generally include a plurality of mobile stations
10
, a plurality of base stations
12
, a base station controller (BSC)
14
, and a mobile switching center (MSC)
16
. The MSC
16
is configured to interface with a conventional public switch telephone network (PSTN)
18
. The MSC
16
is also configured to interface with the BSC
14
. The BSC
14
is coupled to each base station
12
. The base stations
12
may also be known as base station transceiver subsystems (BTSs)
12
. Alternatively, “base station” may refer collectively to a BSC
14
and one or more BTSs
12
, BTSs
12
may be referred to as “cell sites”
12
, or sectors of a given BTS
12
may be referred to as cell sites. The mobile stations
10
are typically cellular telephones
10
, and the cellular telephone system is advantageously a spread spectrum CDMA system configured for use in accordance with the IS-95 standard. Each of the base stations
12
and the mobile stations
10
typically include at least one transceiver (not shown), at least one integrated chip (not shown) and software (not shown) executed by those chips for carrying out the numerous functions and operations required of each in a wireless communications system. For instance, and by way of example only, many of the operations involved in the modulation and demodulation of wireless communications signals are controlled by software executed by integrated chips. Integrated chips running software also control many of the operations involved in the transfer of mobile station control from one base station to another, typically referred to as handoff. One skilled in the art will however appreciate
Brown Charles D.
Hyum Soon-Dong
Nguyen Chau
Qualcom Incorporated
Streeter Thomas
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