High priority and/or emergency overload access control system

Details High priority and/or emergency overload access control system High priority and/or emergency overload access control system

1999-05-10

2002-04-16

Maung, Nay (Department: 2681)

Telecommunications

Radiotelephone system

Emergency or alarm communication

C455S434000, C455S450000, C455S509000, C455S512000, C455S515000, C370S329000, C370S348000

Reexamination Certificate

active

06374099

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of The Invention
This invention relates to wireless communications and, more particularly, to a access control system for a cellular communications system.
2. Description of Related Art
FIG. 1
depicts a diagram of a wireless communications system
10
which provide wireless communications service to a number of wireless units (e.g., wireless units
12
a-c
) that are situated within a geographic region. The wireless cellular communications system
10
comprises a number of base stations
14
a-e
, geographically distributed to support transmission and receipt of communication signals to and from the wireless units
12
a-c
, which can be mobile or fixed, in the geographic region. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless units in that cell. Each base station
14
a-e
handles voice and/or data communications over a particular region called a cell, and the overall coverage area for the cellular system is defined by the union of cells for all of the cell sites, where the coverage areas for nearby cell sites overlap to some degree to ensure (if possible) contiguous communications coverage within the outer boundaries of the systems coverage area. As depicted in
FIG. 1
, each cell is schematically represented by one hexagon in a honeycomb pattern; in practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell. One cell site may sometimes provide coverage for several sectors. In this specification, cells and sectors are referred to interchangeably.
The base stations also comprise the transmission equipment that the base station uses to communicate with a mobile switching center (MSC) for the geographic region. An MSC
16
is responsible for, among other things, establishing and maintaining calls between the wireless units and calls between a wireless unit and a wireline unit (e.g., wireline unit
18
). As such, the MSC interconnects the wireless units within the geographic region with a public switched telephone network (PSTN)
20
. Within a geographic region, the MSC switches calls between base stations in real time as the wireless unit moves between cells, referred to as call handoff. The MSC
16
is connected to or integrated with a home location register (HLR)
22
. The HLR
22
contains subscriber information and location information for all wireless units which reside in the geographic region of the MSC
16
.
In a wireless cellular communications system, a base station and a wireless unit communicate voice and/or data over a forward link and a reverse link, wherein the forward link carries communication signals from the base station to the wireless unit and the reverse link carries communication signals from the wireless unit to the base station. There are many different schemes for determining how wireless units and base stations communicate in a cellular communications system. For example, wireless communications links between the wireless units and the base stations can be defined according to different radio protocols, including TDMA (time-division multiple access), FDMA (frequency-division multiple access), and CDMA (code-division multiple access).
In the context of wireless communications systems, subscriber access control allows a service provider to control the availability of communications links to wireless units. In the context of current CDMA systems, a purpose behind subscriber access control was to ensure the availability of communications links if an emergency overload condition should develop. As an example, a technical industry standard that is desirably observed in introducing access controls in a mobile communications system is TIA/EIA-95-A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System (March, 1995) (“IS-95-A”); and ANSI J-STD-008 Personal Station-Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communications Systems, (Corrected Version—Aug. 29, 1995).
A detailed sequence of activities are typically followed before the wireless unit can access the wireless communications system to establish a call to a phone number. As is known in the art, calls between a CDMA mobile station and a base station typically employ several kinds of channels. Initially, a pilot channel is employed as a reference signal for detecting other forward link channels. A sync channel is used to establish time and frame synchronization at the wireless unit. The sync channel message also provides information about another class of channels, the paging channels.
Paging channels are used to broadcast a variety of control information, including access channel information, contained in the access parameter message. This access parameter message contains information such as persistence parameters (
0
-
9
), persistence parameters (
10
-
15
), persistence modifiers MSG_PSIST and REG_PSIST, initial access power requirements, the number of access channels, the number of access attempts, maximum size of access messages, values for various overload classes, access attempt backoff parameters and other information of interest to mobiles seeking access to the base station.
To initiate a call and obtain access to the wireless communications system, the user inputs or dials the number into the wireless unit, and the wireless unit stores the phone number in storage, such as a memory. When the user finishes inputting the phone number, the user typically presses a send button to initiate the call. Before the wireless unit can access the wireless communications system, the wireless unit checks to ensure that the parameters received from the base station, such as those in the access parameter message, are current. After the wireless unit updates the parameters or determines that the parameters are current, the wireless unit commences the access procedure.
Access channel(s) are used by the wireless unit to obtain access to the wireless communications system, for example, to originate a call. The IS-95-A standard cited above provides a detailed sequence of activities that are to be followed in transmitting messages over an access channel to a base station. In systems based on the IS-95-A standard, CDMA wireless units transmit on the access channels according to a random access protocol in which the wireless unit attempts to gain access to the wireless communications system, referred to as an access attempt, by sending an access message and receiving (or failing to receive) an acknowledgment for that access message. As shown in
FIG. 2
, each transmission in the access attempt is called an access probe, and within an access attempt, access probes are grouped into access probe sequences. Each access attempt comprises up to max_req_seq (for a request access) or max_rsp_seq (for a response access) access probe sequences. Each access probe sequence consists of up to 1+NUM_STEP access probes, for example 15, all transmitted on the same Access Channel. The first access probe of each access probe sequence is transmitted at a specified power level relative to a nominal power level, and each subsequent access probe is transmitted at a power level a specified amount higher than the previous access probe.
The timing of access probes and access probe sequences is expressed in terms of Access Channel slots. The transmission of an access probe begins at the start of an Access Channel slot. As shown in
FIG. 3
, each such access probe comprises an access channel preamble and an access channel message capsule. The length of the preamble 1+pam_sz as well as the length of message capsule 3+max_cap_sz are expressed in terms of a number of 20 millisecond frames. Thus, the duration of an access probe (access channel slot) is 4+pam_sz+max_cap_sz frames.
FIGS. 4
a
and
4
b
show an example access procedure according to the IS-95-A standard. In attempting to gain access to the wireless communications

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