Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
2001-10-16
2004-01-20
Chin, Wellington (Department: 2664)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S331000, C370S332000
Reexamination Certificate
active
06680925
ABSTRACT:
BACKGROUND
1. Field
The present invention generally relates to wireless communication systems and more particularly to a method and system for site selection transmit diversity (SSTD) in a CDMA data communication system.
2. Related Art
The current generation of cellular phone systems offers more services than those of previous generations, such as data services. First and second generation cellular communication systems were typically used mostly for voice services. Second generation systems began adding limited data services, albeit at low data rates. Third generation systems, such as the Code Division Multiple Access (“CDMA”) High Data Rate (“HDR”) system, offer integrated data capabilities with much higher data rates than that of second generation systems, which are capable of offering services such as streamed audio and video.
A cellular network consists of many geographic cells, each of which may contain multiple sectors. Inside each cell, there is a base station. A user typically communicates with the network through the sector that provides the best signal. When a mobile user changes location, the user may communicate with the network through a different sector that provides the most reliable signal. Techniques for hand off in second generation CDMA communication system are known in the art. However, CDMA data communication systems, such as CDMA HDR, present new problems when a mobile unit selects a new sector.
One such problem occurs when a user switches among sectors too quickly. In a conventional CDMA cellular system, data traffic, which includes voice, is routed to each sector that is actively communicating with a mobile unit, possibly using multiple base stations. Consequently, all active sectors in communication with a mobile unit send traffic to the mobile unit. The redundancy in traffic was needed to meet the low-delay requirements of voice data for handoff. This constraint is relaxed in a data network.
In a packet data network, users may tolerate short delays in the data transmission. Since low delay is no longer a constraint on the system, reliability can be more efficiently achieved through re-transmission rather than redundant transmission through all the active sectors all the time in handoff scenario. Thus, in a conventional high rate packet data cellular system, data traffic is typically routed through one sector that maximizes the forward link throughput. To accomplish this routing, the mobile monitors all the active sectors, among which the user selects the best and informs the network of its selection. Such a system exploits the channel dynamics in order to maximize the capacity. The selection of the transmitter to exploit local peaks in the shadowing process is a form of selection diversity. Thus, the selection of the best serving sector is also referred to as site selection transmit diversity (“SSTD”).
FIG. 1
illustrates a typical CDMA data communication system, such as CDMA HDR. Access network
100
contains several access points, of which only access points
110
and
130
are shown. A mobile unit, such as access terminal
114
, communicates with an access point, such as access point
110
, to connect to access network
100
. In general, an access point, such as access point
110
, will have several sectors, such as sectors
116
,
118
, and
120
.
Since access terminal
114
generally communicates with one sector at a time, data going to access terminal
114
from access point
110
must be routed to the specific sector with which access terminal
114
is communicating.
However, a problem emerges when an access terminal is constantly switching among sectors. Suppose sector
116
has the strongest forward link signal at one instance such that access terminal
114
selects sector
116
as the current serving sector. In a next instance, sector
132
of access point
130
has the strongest forward link signal. Just moments later, sector
116
again has the strongest forward link. It is possible that rapid switching between the two or more sectors can occur. Each time a switch occurs, data that was going to be sent to access terminal
114
, must be sent to the corresponding data queue for that sector. Further, the user cannot receive data before the data queue is ready. Such rapid transitions can create a significant amount of overhead for the network, and outage for the user.
A second problem for selecting the best sector is related with the reverse link reliability. On the reverse link, access terminal
114
may send channel state feedback information to the network to assist the network in achieving the highest forward link throughput. In the high data network system, access terminal
114
transmits a data rate control signal (“DRC”) to control the data rate on the forward link. Access terminal
114
also sends an acknowledge signal (“ACK”) to the serving sector when it successfully receives a packet. Access terminal
114
should select a new sector that has a reliable reverse link connection with access terminal
114
. Otherwise, DRC and ACK information can be lost, which reduces the throughput of the system. However, access terminal
114
does not readily know the reliability of a reverse link connection. If access terminal
114
selects a sector with an unreliable reverse link, throughput can suffer due to retransmission.
Ideally, access terminal
114
should select a new sector so that its throughput on the forward link is maximized. Firstly, the site selection should avoid fast toggling. Secondly, the site selection should incorporate the impact of the reverse link reliability on forward link throughput. Thus, there is a need in the art for methods and systems for properly selecting the best serving sector in a CDMA data communication system.
SUMMARY
Embodiments disclosed herein address the above stated needs by using signal level and timing hysteresis and using a DRC reverse link reliability information in site selection transmit diversity in a CDMA data communication system.
The presently disclosed embodiments are directed to method and system for site selection transmit diversity in a CDMA data communication system. According to one aspect of the present invention, signal levels of the active sectors of an access terminal are compared with the signal level of the current serving sector of the access terminal. Next, using a signal level hysteresis, a delta credit is accumulated. When the reverse link reliability information is available, an accumulated total credit is authorized to produce an authorized accumulated total credit. Afterwards, the best serving sector is identified from a pool of candidate sectors based on the signal levels of the active sectors and the authorized accumulated total credits.
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Attar Rashid
Black Peter J.
Sindhushayana Nagabhushana
Vijayan Rajiv
Wu Qiang
Baker Kent
Chin Wellington
Jain Ray
Qualcomm Incorporated
Wadsworth Phillip
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