Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-11-28
2004-06-29
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C370S441000
Reexamination Certificate
active
06757319
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to spread-spectrum communications, and more particularly to code-division-multiple-access (CDMA) cellular, packet-switched communication systems. The inventive concepts involve using a Common Packet Channel (CPCH) as an uplink transport channel to perform closed-loop power control of a common or shared downlink transport channel, such as the Forward Access Channel (FACH) or the Downlink Shared Channel (DSCH).
BACKGROUND
Recent developments in wireless communications technologies have allowed expansion of service offerings from the original voice telephone service model to include a number of services supporting packet data communications. As customers become increasingly familiar with data services offered through landline networks, they are increasingly demanding comparable data communications in the wireless domain, for example to maintain service while mobile subscribers roam freely or to provide remote service in locations where wireless loops are preferable to landline subscriber loops. A number of technologies support packet data communications in the wireless domain. Increasingly, these technologies rely on direct sequence spread spectrum communication over the air-link.
In a conventional Code Division Multiple Access (CDMA) communication system, information transmitted between stations is modulated using a spreading code. All stations transmit over one common frequency band. Different spreading codes are used to distinguish information associated with different stations. Logically speaking, the “channels” in such a communication system correspond to the different codes.
Since all stations transmit information using the common frequency band, the transmission from each station interferes with the communications of the other stations. In any such direct sequence spread spectrum communication system, when a particular device is attempting to receive a transmission, any transmissions of other stations using codes not recognized by or assigned to the particular device appear as background noise. If the noise level is too high at the receiver, that device can not accurately distinguish transmissions directed to it even though the signals were spread using the assigned code(s). For example, signals received by a base station from a mobile station close to the base station are much stronger than signals received from other mobile stations located at the base station's cell boundary. As a result, distant mobile communications are overshadowed and dominated by closein mobile stations. This condition is sometimes referred as the “near-far effect.”
To increase capacity it becomes necessary decrease interference. To that end, most conventional CDMA communication techniques involve some form of transmission power control. In a CDMA system, all mobile-transmitted signals should arrive at the base station with about the same average power irrespective of their distance from the base station. Typically, the mobile station attempts to control its transmit (uplink) power within a relatively small tolerance based on power control messages from the base station. Conventional dedicated channel communications in CDMA utilize a similar closed-loop power control for the downlink (or forward) transmit channel, except during hand-off wherein each base station varies the transmit power using an open-loop control algorithm.
These direct sequence spread spectrum communication techniques have been extensively developed and deployed to offer circuit switched communication services, particularly for voice telephone grade communications of mobile customers. These systems offer some limited data communications capabilities. However, customers increasingly are demanding higher-rate packet data communications in the wireless domain. Considerable technological effort is under way in the wireless industry to develop a third generation (3G) digital communication system, which among other services, will support higher-rate packet data communications.
Currently, there are two modes of operation for packet data transport in the 3GPP UMTS standard for wideband code division multiple access (W-CDMA), circuit mode and packet mode. A Dedicated Channel (DCH) is used to transfer packet data in a circuit mode of operation in both directions. In the packet mode, the Common Packet Channel (CPCH), the Forward Access Channel (FACH), and the Downlink Shared Channel (DSCH) offer a common packet data mechanism for bursty data Internet traffic, such as:
E-Commerce
Unified Messaging
Video Clips
Video Monitoring
Web Browsing
Chatting
Picture and File Transfers
As presently envisage for packet services, there are two or more “common/shared downlink transport channels,” of which the Forward Access Channel FACH is one and the Downlink Shared Channel (DSCH) is the other. In the current standards, the operation of the downlink common transport channels does not support closed loop power control (CLPC). In the case of DSCH, a pair of dedicated channels, one of which for the uplink and the other of which for the downlink, can be used to enable CLPC. However, this method is very inefficient and wastes valuable system capacity.
FIG. 1
shows an example of the current operation of a FACH type common downlink channel. As shown, the base station listens to a signaling message periodically sent by the mobile station indicating the level at which the mobile station can listen (lower line). Then, the base station transmits the packet through the FACH (top line). The base station can signal to the MS to initiate a downlink packet transfer through one of the available mechanisms, such as the FACH, the Broadcast Channel (BCH) or the Paging Indicator.
A basic common-packet channel (CPCH) provides an uplink transport channel for transmitting variable size packets from a mobile station (MS) to a base station (BS) without the need for direct resource allocation. The channel resource allocation is contention based. A number of mobile stations could at any time contend for the same resources, as found in ALOHA systems.
FIG. 2
shows the operation of a mobile station over a CPCH channel together with the related indicator channel and downlink channel from the base station. In operation, a mobile station (MS), having all the necessary information from a nearby BS, starts transmitting a series of access preambles as shown at AP in the lower line in FIG.
2
. Each particular access preamble in the series is the same preamble, selected from a set of predefined access preambles corresponding to CPCH channels operating through the base station. The MS transmits each particular access preamble (AP) at predefined time intervals and at increasing power levels, preferably in a step-wise manner. The transmitted power during each access preamble transmission is constant.
When the base station receives and identifies one of the access preambles, it responds by transmitting a corresponding acknowledgment AP-ACK over the AP indicator channel (AP-AICH), as shown in the middle line in FIG.
2
. With this ALOHA type access technique, there is a good possibility that two or more MSs may try to access the BS using the same access preamble at substantially the same time. In such a case, the AP-ACK acknowledgement signal could be successfully received by more than one MSs. If these MSs are allowed to transmit data, the transmissions will collide. In case of collision, none of the data from either of the transmitting MSs will be received correctly. To resolve this problem, the MSs need to undergo a collision detection or “CD” phase.
In the CD phase, upon receiving an AP-ACK acknowledgement, the MS randomly selects a collision detection (CD) signature from a predetermined set of possible CD signatures. The MS transmits a CD preamble containing the coded CD signature (shown as a CD packet in the lower line of FIG.
2
). If the base station successfully receives a CD preamble, it sends back a CD acknowledgement (CD-ACK), which is the same as or otherwise corresponds to the CD signature transmitted by the MS, over a downlink in
Kanterakis Emmanuel
Parsa Kourosh
Bocure Tesfaldet
Golden Bridge Technology Inc.
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