Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-12-07
2001-08-21
Chin, Wellington (Department: 2664)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S337000
Reexamination Certificate
active
06278703
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to wireless communications. More specifically, this invention relates to pilot-assisted digital wireless communications.
2. Description of Related Art and General Background Call and Standby Modes
Wireless mobile communications involves communications between mobile units and/or between a mobile unit and a base unit. Mobile units typically operate in one of two principal modes. In call mode, the mobile unit is actively engaged in a communication with another unit. In standby mode, the mobile unit is not in use but is ready to receive incoming calls. Although a mobile unit in standby mode is usually not transmitting, it must still remain sufficiently active to detect and respond to signals that are directed to it, such as notifications of incoming calls.
It is desirable to reduce power consumption by the mobile units, thereby enabling them to operate for longer periods between battery replacement or recharging. One method of reducing power consumption is to interrupt the supply of power to circuits in the mobile unit that are not currently in use. Because a mobile unit in standby mode is not being used, for example, it would normally be advantageous to power down the display circuitry until a display is needed again.
Slotted Paging
In a typical wireless telephone application, the mobile unit receives information about incoming calls by monitoring a paging channel, which is a one-way link for communications from the base unit to the mobile units. When the base unit receives notice of a call destined for a particular mobile unit, it pages the mobile unit by broadcasting a paging signal over the paging channel. Included in the paging signal is an identifier associated with the mobile unit. When the mobile unit receives the paging signal and recognizes the identifier, it responds to the base unit in an appropriate manner on another channel (commonly called an access channel or access request channel) and the connection is initiated.
It has been recognized that a considerable portion of the power consumed by a mobile unit in standby mode is due to the RF circuitry, which receives the radio signal and outputs a data signal at baseband. Power consumption may be greatly reduced, and the time between battery recharges for a mobile unit in standby mode may therefore be significantly extended, by implementing discontinuous reception on the paging channel (a technique which is also called ‘slotted paging’). In one version of slotted paging, time is divided into consecutive slots of equal duration which are numbered in chronological order from 1 to N (where N is a counting number, and the slot numbering is restarted at 1 after N is reached). At least one slot number is assigned to each mobile unit, and the base unit is constrained to broadcast a paging signal to any particular mobile unit only during a slot whose number has been assigned to that mobile unit. As the mobile unit's RF circuitry may be powered down (i.e. the RF circuitry may be in an unpowered state) during most of the other slots, a significant power savings is thereby achieved.
In a slotted paging system, it is usually desirable to power up the RF circuitry at some moment prior to the start of an assigned paging slot, thus giving the circuitry a chance to stabilize by the time it is required to receive and output signals. This power-up moment is indicated in the timeline of
FIG. 1
as point A.
In order for the receiver to receive valid symbols, it may be necessary to perform additional procedures once the RF circuitry has become stable. In a CDMA system, for example, acquisition of at least one (and preferably several) of the multipath instances of the received signal must be performed before the symbols in the signal may be identified. In
FIG. 1
, the start of the acquisition period is indicated by point B.
Digital wireless signals are usually encoded for various reasons (e.g., error correction, redundancy introduction and dispersal, encryption, etc.), and the received signals must therefore be decoded before interpretation may begin. Some of the circuits or algorithms used to decode these signals (such as Viterbi and other maximum likelihood decoders) should be initialized, typically with a preamble of received symbols, before they can begin to process data reliably. Once the acquisition process has completed and the receiver is ready to produce valid symbols, therefore, the decoder initialization period may begin (point C in FIG.
1
).
At some subsequent moment, the mobile unit begins to receive the paging signal. This moment may occur as early as the start of the assigned paging slot (point D in FIG.
1
), or it may be delayed depending, for example, on whether the base unit is occupied with other system activity. Eventually, the mobile unit will receive as much of the paging signal as it requires for proper interpretation, a moment indicated by point E in FIG.
1
. It is possible for this moment to arrive before the entire paging signal is received, as the mobile unit only needs to interpret enough of the paging signal to know that no message is pending and may ignore the remainder. Possibly after a decoding delay, then, the interpretation of this portion of the paging signal will be completed (point F in FIG.
1
).
In anticipation that no incoming message will be reported, it is possible to power down the RF circuitry (which event is indicated as point G in
FIG. 1
) as early as point E. Usually, however, the RF circuitry will be kept in a powered state until after point F (as shown in FIG.
1
), in case interpretation of the paging signal should indicate that a response is required. In order to facilitate system synchronization, it may also be desirable to power the RF circuitry up or down only at moments having a substantially predetermined relation to a system reference time such as the slot boundary.
Existing cellular telephone systems that use the technique of slotted paging include those operating under the GSM and IS-95 standards. The parameters that define slotted paging under the IS-95 standard, for example, are presented in section 6.6.2.1.1 of TR45 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systems (TIA/EIA/SP-3693 [to be published as TIA/EIA-95], TIA [Telecommunications Industry Association], Arlington, Va., 1997). Discontinuous reception in the GSM system is described in GSM Technical Specification 03.13 (version 5.0.0, March 1996, European Telecommunications Standards Institute [ETSI], 06921 Sophia Antipolis Cedex France).
Under the IS-95 standard, the slots in the paging channel are numbered from 0 to 2047 and each paging slot has a duration of 80 ms. A mobile unit has a slot number and a slot cycle index, and the number of slots n between the start of adjacent assigned slots may be expressed as a function of the slot cycle index i as
n=
16×2
i
,
where i is a nonnegative integer. For a mobile unit having a slot cycle index of 0, for example, the starting times of each adjacent pair of assigned slots are separated by 16 slots (i.e. 1.28 seconds). Therefore, a mobile unit having a slot cycle index of 0 and the slot number 3 would be assigned to slots 3,19, 35, and so on up to slots 2019 and 2035.
For slot cycle indices of
1
,
2
, and
3
, the respective periods of separation between the starting times of each adjacent pair of assigned slots are 32, 64, and 128 slots (or 2.56, 5.12, and 10.24 seconds). In North America, cellular telephones typically operate at slot cycle indices of 1, while a slot cycle index of 2 is more common for cellular telephones in Japan. The slot cycle index is a variable quantity, however, and it is also common for a cellular telephone to operate at slot cycle index 0 upon power-up until a good estimate of the relation between local and system oscillator frequencies is obtained. For another type of mobile unit, such as a remote data terminal, operation at a larger slot cycle index may be more appropr
Brown Charles D.
Chin Wellington
Hom Raymond
Pham Brenda H.
Qualcomm Incorporated
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