Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-07-13
2001-04-10
Urban, Edward F. (Department: 2746)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S458000, C455S434000, C370S331000
Reexamination Certificate
active
06216004
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to digital cellular communication systems, and more particularly, to soft handoff in spread spectrum communication systems.
2. Description of the Related Art
In a code division multiple access (CDMA) spread spectrum communication system, a shared frequency band is used for communication with all base stations within that system. An example of such a system is described in the TIA/EIA Standard TIA/EIA-95-B entitled “Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System”, incorporated herein by reference. The generation and receipt of CDMA signals is disclosed in U.S. Pat. No. 4,401,307 entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS USING SATELLITES OR TERRESTRIAL REPEATERS” and in U.S. Pat. No. 5,103,459 entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both of which are assigned to the assignee of the present invention and incorporated herein by reference.
Radio Frequency (RF) signals are exchanged between a respective mobile unit and one or more base stations. Mobile units do not communicate directly with one another. Base stations communicate with a base station cellular or personal communication system controller, referred to herein as a base station controller (BSC) using various media such as ground based wires or a microwave link, for example. The BSC can route calls to a public switching telephone network (PSTN) or can route packets to a packet switched network, such as the Internet. The base station also coordinates the operation of base stations within the system during soft handoff for example.
TIA/EIA-95 is one example of a CDMA communication system. Communication from a mobile unit to one or more base stations in a TIA/EIA-95 CDMA system takes place over shared frequency channels each of which occupies approximately 1.25 MHz of radio frequency bandwidth. More specifically, communication signals occupying a given frequency band are discriminated at a receiving station through the spread spectrum CDMA waveform properties based on the use of a high rate pseudonoise (PN) code. A PN code is used to modulate signals transmitted from the base stations and mobile units. Signals from different base stations can be separately received at a given mobile unit by discrimination of different PN codes. For the TIA/EIA-95 standard, these codes are constructed from a single code, but each base station has a unique time offset of the PN code. The high rate PN spreading also allows a receiving station to receive a signal from a single transmission station where the signal has traveled over distinct propagation paths. Demodulation of multiple signals is disclosed in U.S. Pat. No. 5,490,165 entitled “DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS” and in U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, both of which are assigned to the assignee of the present invention and incorporated herein by reference.
The various channels within a given “forward” (base station to mobile unit) TIA/EIA-95 CDMA channel include data channels, a synchronization channel, a pilot channel, and a set of paging channels, all transmitted from the base station to mobile units. The pilot channel carries a pilot signal that is a regularly repeated pattern that is differently phase offset for each base station. The pilot provides for time reference and for amplitude and phase tracking. The pilot signal allows mobile units to identify and become synchronized with the various base stations that are within range of their communication capability. The synchronization channel carries additional synchronization information for use by mobile units. The set of data channels carry the data associated with the various communication sessions (usually phone calls) and are directed to individual mobile units. These data channels are called traffic channels in TIA/EIA-95. The paging channels are used by the base stations to notify mobile units when a request to communicate has been received.
The protocol for paging a subscriber unit typically is defined so as to evenly distribute power transmitted by the base station over time, and to reduce power consumption in the mobile unit. In some CDMA systems, the mobile unit power consumption while monitoring the paging channel is reduced by dividing each base station's paging channel into a set of paging “slots” or time windows during which paging signals directed to mobile units may be generated, and by assigning a particular CDMA frequency channel, paging channel and paging slot to each mobile unit in an evenly distributed fashion. This assignment generally may be performed, for example, via the use of a hashing function applied to the International Mobile Station Identity (IMSI) assigned to each mobile unit, although other unique numbers could be utilized. The hashing function of set of hashing functions yields a value that corresponds to a particular set of paging parameters including a CDMA channel, paging channel, and paging slot. Mobile units are then configured to “listen” for a paging message over their assigned CDMA frequency channel and paging channel during the corresponding paging slot. Since listening for a page requires a certain amount of power, limiting the time a mobile unit performs the listening function to a particular paging slot reduces the overall power consumption of that mobile unit and therefore increases the life of any battery or other power storage system utilized by that mobile unit.
During a mode of operation called “soft handoff”, the mobile station simultaneously exchanges identical communications traffic on a CDMA traffic channel between two or more different base stations. In the case of (forward link) reception by a mobile station, the signals from the multiple base stations can be coherently combined to improve performance, as with multipath combining. In fact, the second base station signal can be regarded as a delayed version of the first, generated actively and purposely, rather than as a delayed reflection of the first caused by the environment. See,
CDMA, Principles of Spread Spectrum Communication,
by Andrew J. Viterbi, Addison-Wesley Pub. Co., 1995, pp. 181, 183-184, 198-199 and 222-224. Soft handoff is further disclosed in U.S. Pat. No. 5,101,501 entitled “SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM” and U.S. Pat. No. 5,267,261 entitled MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM”, both assigned to the assignee of the present invention and incorporated herein by reference. Similarly, a mobile unit can simultaneously communicate with two sectors of the same base station, known as softer handoff, as disclosed in copending U.S. Pat. No. 5,625,876 entitled “METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION”, assigned to the assignee of the present invention and incorporated herein by reference. Handoffs are described as soft and softer because they make the new connection before breaking the existing one.
In a typical CDMA communication system, mobile unit assisted soft handoff operates based on the pilot signal strength of several sets of base stations as measured by the mobile unit. In the TIA/EIA-95 CDMA communication system, for example, an Active Set is the set of base stations through which active communication is established. A Neighbor Set is a set of base stations surrounding an active base station comprising base stations that have a high probability of having a pilot signal strength of sufficient level to establish communication. A Candidate Set is a set of base stations having a pilot signal strength of sufficient level to establish communication.
In at least one earlier CDMA system, for example, soft handoff can be set up between a mobile unit and two or more base stations in the mobile unit's Active Set. For instance, in such an earlier CDMA system, when communications are i
Sarkar Sandip
Tiedemann Jr. Edward G.
Beladi S. Hossain
Gesesse Tilamun
Martin Roger W.
Qualcomm Incorporated
Urban Edward F.
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