Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-10-05
2004-08-31
Nguyen, Steven (Department: 2665)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S331000, C455S067110, C455S436000
Reexamination Certificate
active
06785249
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to digital wireless communication systems, and more particularly to methods for detecting forward and reverse link imbalances in digital wireless communications systems.
2. Description of Related Art
Wireless communication systems facilitate two-way communication between a plurality of subscriber mobile radio stations or “wireless units” and a fixed network infrastructure. Typically, the wireless units communicate with the fixed network infrastructure via a plurality of fixed base stations. Exemplary systems include such mobile cellular telephone systems as Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) systems, and Frequency Division Multiple Access (FDMA) systems. The objective of these digital wireless communication systems is to provide communication channels on demand between the wireless units and the base stations in order to connect the wireless unit users with the fixed network infrastructure (usually a wired-line system).
Wireless units typically communicate with base stations using a duplexing scheme that allows for the exchange of information in both directions of connection. Transmissions from a base station to a wireless unit are commonly referred to as “downlink” transmissions. Transmissions from a wireless unit to a base station are commonly referred to as “uplink” transmissions. In CDMA and FDMA communication systems, the downlink is commonly referred to as the “forward” link and the uplink is commonly referred to as the “reverse” link. A well-known problem in cellular communication systems is system performance degradation caused by signal strength imbalances in the forward and reverse links. To mitigate this problem, cellular communication system designers attempt to ensure that signal path losses tolerated by the reverse links are equal to or approximately equal to those tolerated by the forward links. One important design objective is to balance the forward and reverse links. Unfortunately, due to dynamically changing network conditions such as system loading, antenna pattern mismatches, differences in antenna gains, and other channel variations, imbalances still occur. In cellular communication systems such as CDMA and FDMA, forward and reverse link imbalances often cause degraded system performance.
Therefore, balancing the forward and reverse links is a very important design goal in wireless digital communication systems. Unless the links are balanced, system performance is degraded. For example, under weak reverse link conditions (i.e., the reverse link is weaker than the forward link) wireless units attempt to access their associated base stations by generating multiple access probes until all of the access probes are exhausted. These multiple access attempts result in increased channel interference on the reverse link. Under weak forward link conditions (i.e., the forward link is weaker than the reverse link) wireless units are unable to receive acknowledgment messages on their associated forward links. Consequently, the wireless units will not declare service, initiate calls, nor respond to base station orders.
Unfortunately, link imbalances are indiscernible by the prior art wireless units. Consequently, the prior art wireless units exhibit undesirable behavior in the presence of link imbalances. For example, in a weak reverse link condition, the prior art wireless units can become locked into a digital mode of operation when the digital system is, in fact, unavailable for service. This occurs when the wireless unit receives a strong signal on the forward link paging channel. However, the reverse link is weak.
Although the wireless unit is unable to register or originate calls on the reverse link, it believes that digital service is available due to the strong paging channel signal. Therefore, even though an alternative analog system may be available, the mobile is locked into a useless digital mode of operation. Performance also degrades when the reverse link is stronger than the forward link. Under these conditions, the wireless unit can communicate with the base station. However, because of the relatively weak forward link, the wireless unit cannot decipher the control information transmitted by the base station. In either scenario, calls are disadvantageously lost, and system call delivery rates are reduced. A better understanding of the performance problems created by link imbalances can be obtained by briefly reviewing simple call flow examples in a CDMA communication system.
CDMA Call Flow Examples and CDMA Call Handshake Protocols
Tables 1 and 2 show simple call flow examples as set forth in the Telecommunications Industry Association (TIA) specification governing the operation of CDMA wireless unit and base station equipment. The TIA specification is entitled “Wireless unit-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” TIA/EIA/IS-95-A, was published in May 1995 by the Telecommunications Industry Association, and is referred to hereafter as the IS-95 specification. As set forth in the IS-95 specification, Tables 1 and 2 follow the following conventions:
All messages are received without error.
Receipt of messages is not shown (except in the handoff examples).
Acknowledgements are not shown.
Optional authentication procedures are not shown.
Optional private long code transitions are not shown.
TABLE 1
Simple Call Flow Example - Wireless unit Origination
Wireless unit
Base Station
Detects user-initiated
call
Sends Origination
>
Access
>
Sets up Traffic
Message
Channel
Channel
Begins sending null
Traffic Channel data
Sets up Traffic Channel
<
Paging
<
Sends Channel
Channel
Assignment Message
Receives N
5m
consecutive valid
frames
Begins sending the
Acquires the Reverse
Traffic Channel
Traffic Channel
preamble
Begins transmitting
<
Forward
<
Sends Base Station
null Traffic Channel
Traffic
Acknowledgement
data
Channel
Order
Begins processing
<
Forward
<
Sends Service Option
primary traffic in
Traffic
Response Order
accordance with
Channel
Service Option 1
Optional
Optional
Sends Origination
>
Reverse
>
Continuation Message
Traffic
Channel
Optional
Optional
Applies ring back in
<
Forward
<
Sends Alert With
audio path
Traffic
Information Message
Channel
(ring back tone)
Optional
Optional
Removes ring back
<
Forward
<
Sends Alert With
from
Traffic
Information Message
audio path
Channel
(tones off)
(User conversation)
(User conversation)
TABLE 2
Simple Call Flow Example - Wireless unit Termination
Wireless unit
Base Station
<
Paging
<
Sends Page
Channel
Message or Slotted
Page Message
Sends Page Response
>
Access
>
Sets up Traffic
Message
Channel
Channel
Begins sending
null Traffic
Channel data
Sets up Traffic Channel
<
Paging
<
Sends Channel
Channel
Assignment
Message
Receives N
5m
consecutive valid frames
Begins sending the
Acquires the
Traffic Channel preamble
Reverse Traffic
Channel
Begins transmitting null
<
Forward
<
Sends Base
Traffic Channel data
Traffic
Station
Channel
Acknowledgement
Order
Begins processing
<
Forward
<
Sends Service
primary traffic in
Traffic
Option Response
accordance with Service
Channel
Order
Option 1
Starts ringing
<
Forward
<
Sends Alert With
Traffic
Information
Channel
Message (ring)
User answers call
Stops ringing
Sends Connect Order
>
Reverse
>
Traffic
Channel
Begins sending primary
traffic packets from the
Service Option 1
application
(User conversation)
(User conversation)
Table 1 shows a simple call flow example wherein a wireless unit originates a call. Messages are transmitted from the wireless unit to the base station using the access channel. Messages are transmitted from the base station to the wireless unit using the paging channel. As shown in Table 1, the wireless unit first detects a user-initiated call, and then sends an “origination” message via the CDMA access channel. The access channel is a slotted random access channel. The wireless unit transmits on the access
Kalousek Pavel
Nguyen Steven
Nguyen Thein T.
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