Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-03-05
2003-07-08
Nguyen, Duc (Department: 2682)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S337000, C455S436000, C455S442000
Reexamination Certificate
active
06590879
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a system and method for mobile stations to maintain multiple physical channels to basestations within a direct sequence-code division multiple access mobile communications system and more specifically, to the handoff operation of such mobile stations and basestations.
BACKGROUND OF THE INVENTION
Improvements are consistently being made on previous cellular technology standards. Each subsequent standard has benefits over the previous, such as increased bandwidth, reductions in interference, or simply more flexible functionality. Since mobile stations that function within these standards are portable, one key component of all standards is the method by which an active call or session can be transferred between basestations, hereinafter referred to as handoff. Such a procedure switches the current radio link for a mobile station from a first basestation to a second basestation in order to maintain communications when the mobile station experiences changes in the communication conditions. Changes in communication conditions can be caused by crossing boundaries of basestation cells, falling into deep fades, or a rearrangement of radio resources at the basestation level. Adjustments in the techniques used to handoff active sessions between basestations must be made in parallel with fundamental changes to the overall standard.
Direct Spreading (DS)-Code Division Multiple Access (CDMA) technology standards are going through an ongoing process of improvements and updates. The “original” DS-CDMA standard is a second generation CDMA standard defined in Telecommunications Industry Association/Electronic Industry Association/Interim Standard 95-A (TIA/EIA/IS-95-A). This cellular standard operates predominantly for voice services in full duplex mode. For every forward link (FL) CDMA channel that is established, a reverse link (RL) CDMA channel is also initiated. This standard does not support asymmetrical service to be provided to the user.
The handoff between basestations in the IS-95-A standard can be triggered in two ways. First, it can be triggered by the mobile station due to a deterioration in the signal strength or quality at the mobile station. Secondly, it can be triggered by one of the basestations currently communicating with the mobile station, hereinafter referred to as active basestations, due to a deterioration in the signal strength or quality at the basestation or due to bandwidth constraints at the basestation which require the handoff of an active session to another basestation, hereinafter referred to as a target basestation.
The IS-95-A standard supports soft, softer, and hard handoff for voice services from an active basestation to a target basestation. In soft handoff, both the active and target basestations have a forward and reverse CDMA channel established with the mobile station prior to the termination of the forward and reverse CDMA channels between the mobile station and the active basestation and prior to the target basestation's assumption of the role of active basestation. Softer handoff is identical to soft handoff, but the “active” and “target” basestations are actually resources provided by the same basestation. In hard handoff, the target basestation replaces the active basestation by immediately terminating the forward and reverse CDMA channels between the mobile station and the active basestation before the new channels are fully established.
Handoffs triggered by the mobile station are performed on the basis of signal strengths measured by each mobile station for basestations which are in range of the mobile station. A special channel called the “pilot channel” is generated by each basestation for the purpose of this measurement. Certain basestations that are equipped with multiple directional antennae are capable of functioning with multiple pilot channels, each pilot channel corresponding to a particular sector. Basestations capable of operating in multiple frequency bands may generate a pilot channel for each such band.
Within the IS-95-A standard, there are four pilot sets of identifiers defined for use in the normal operation of a mobile station. Each of these sets of identifiers contains information which identifies or permits an identification of a set of basestations resources. For example, each pilot channel includes a code which identifies basestation resources consisting of a basestation and/or a sector of a basestation and/or a frequency band and these codes can be used in the sets of identifiers. These sets include an Active Set (AS), a Candidate Set (CS), a Neighbour Set (NS), and a Remaining Set (RS). The AS contains the set of identifiers which correspond with basestation resources currently assigned to the particular mobile station. The CS contains the set of identifiers for basestations transmitting pilot channels with sufficient signal strength to be successfully demodulated, but that have not been placed in the AS. The NS contains the set of identifiers of basestation resources that are probable candidates for handoff, normally due to the fact that they are geographically near the mobile station. Finally, the RS contains the set of all identifiers for allowable basestation resources within the current frequency assignment excluding those included in the AS, CS, and NS.
In simplistic terms, the handoff procedure is done with the updating of the AS which changes the set of basestation resources assigned to the particular mobile station. Whenever a change in the AS occurs, both the mobile station and the relevant basestations must take the necessary steps to set up or tear down channels such that only basestations which exist in the updated AS have active channels.
There are a number of issues concerning the IS-95-A standard that resulted in another second generation CDMA standard being formed. One of these issues, as discussed previously, is that the IS-95-A standard does not support the use of asymmetrical communications. The enhanced second generation CDMA standard, defined by TIA/EIA/IS-95-B, continues assigning a fundamental channel (FCH) with both a FL CDMA channel and a RL CDMA channel to be used for voice service, but also allows for the assignment of FL and RL supplemental channels (SCHs) separately for use in limited data services. These SCHs allow communication systems defined by the IS-95-B standard to handle asymmetrical data communications in a more efficient method than previously done with the IS-95-A standard.
There are numerous other advances made in the IS-95-B CDMA standard that make it significantly more flexible compared to the IS-95-A standard. For example, the initial AS during the initialization of an active session for the IS-95-B standard is capable of containing multiple pilot channels, thus allowing a session to be started between the mobile station and more than one basestation. As well, there is a significant enhancement with regard to inter-frequency handoff procedures within the IS-95-B standard that was not addressed in the IS-95-A standard. In the IS-95-B standard, a basestation can direct a mobile station to search for pilot channels in different operating frequencies through the temporary termination of normal communications. With use of pilot channel measurement reports sent from the mobile station, the basestation makes inter-frequency handoff decisions. A Candidate Frequency Neighbor Set (CFNS) and a Candidate Frequency Search Set (CFSS) are added to the previously defined AS, CS, NS, and RS in the IS-95-B standard to enable the inter-frequency handoff capability. The CFNS is a set of all pilot channels available on the candidate frequency while the CFSS is a subset of the CFNS that includes all pilot channels on the candidate frequency that are to be searched for suitable signal strength.
Overall, handoff in the IS-95-B standard, defined within the TIA/EIA/SP-3693 standard pages 6-383 to 6-462 and pages 7-167 to 7-181, hereby incorporated by reference, is performed in a similar fashion to that done for the IS-95-A standard, but with additio
Guo Ning
Huang Wei
Nguyen Duc
Nortel Networks Limited
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