Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
2001-02-23
2003-04-08
Hsu, Alpus H. (Department: 2662)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S345000, C370S347000, C370S441000, C455S452200, C455S522000, C714S036000, C714S750000
Reexamination Certificate
active
06545994
ABSTRACT:
BACKGROUND OF THE INVENTION
A specific protocol has been developed for allowing multiple users to transmit over a shared radio channel. For example, the IEEE (Institute of Electrical and Electronics Engineers) 802.11 Standard generally supports access to radio channels based on a method known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).
In simple terms, this method is based on a “listen before talk” scheme. A transmitter device monitors traffic on a shared radio channel to determine if another transmitter device is presently transmitting on the same channel. If the radio channel is in use, the transmitter device will continue to monitor the channel until it is clear. When the radio channel is finally clear, the transmitter will then transmit over the radio channel.
Ideally, another transmitter device will not simultaneously transmit during the same time. However, a collision can occur on the radio channel when two or more transmitter devices transmit on the radio channel simultaneously. Consequently, neither message transmission would be intelligible and both transmitter devices must re-transmit their messages again to a corresponding target device.
Based on this CSMA/CA scheme, re-transmission of data due to a collision cannot occur before a minimum time gap. After the minimum time gap has passed, the transmitter device selects a random “backoff interval,” which is the wait time before the radio channel is again monitored to determine whether the radio channel is clear to transmit. If the channel is still busy, another shorter “backoff interval,” is selected for a subsequent message transmission. This process is repeated until the transmitter device is able to transmit data.
Another standard for transmitting data on a shared radio channel is based on IS-95, in which multiple field units can transmit at the same time.
The IS-95 standard suggests a method of ramping RF power of a field unit until a message from the field unit is transmitted at a power level that is detectable at a base station. According to this method, a field unit transmits an access request message to a base station for the assignment of wireless resources on a reverse link.
After transmitting an access request message on an access channel, the field unit monitors a paging channel for an acknowledgment message from the base station indicating that the access request message was properly received. If no acknowledgment message is sent to the requesting field unit, it is presumed that the message from the field unit was not transmitted at an appropriate power level. That is, the power output level of the field unit is so low that the base station did not detect a previously transmitted access request message. The access request message is then re-transmitted over the access channel at a higher power level.
This process is subsequently repeated until the field unit transmits a message at a power level that is high enough for the base station to properly receive the message. Similar to the IEEE 802.11 standard, a collision can occur on the shared radio channel when two or more field units simultaneously transmit a message.
SUMMARY OF THE INVENTION
The present invention is generally directed towards an apparatus and method for enhancing the utilization of resources in a wireless communication system. In an illustrative embodiment, messages are transmitted over a shared channel to a target receiver. The shared channel is monitored for collisions that can occur when two or more transmitter devices attempt to send a message at or about the same time. Feedback is provided to notify one or multiple transmitters when a collision is detected.
One method of notifying the transmitter devices of a collision is to provide feedback in a reverse channel to the transmitter devices. More specifically, a device such as a base station monitoring the shared channel for collisions can transmit messages indicating that a collision occurred on the first channel for a previous message transmission.
In a specific application, the transmitter device is a field unit transmitting a message such as an access request message to a target receiver monitoring a channel for messages. For example, the shared channel can be a reverse link radio channel defined by codes such as those used in wireless CDMA (Code Division Multiple Access) communication systems. More specifically, the shared reverse link channel can be an access channel. The second channel for notifying field units of message collisions can be a forward link radio channel defined by another unique PN (pseudnoise) code.
To support communications between a transmitter and target receiver device such as a base station, the transmitter can synchronize itself prior to transmitting messages on the shared channel. Thus, there can be at least some structure as to when a transmitter device sends a message over the shared channel. Alternatively, a transmitter device can send a message asynchronously over the shared channel.
In an application where the transmitter is at least grossly synchronized with the target receiver, the transmitter can then transmit in a time slot or data field of the shared channel. Consequently, a device monitoring the shared channel for message collisions can monitor time slots of the shared channel for messages. One way to determine whether a message is properly received is to provide redundancy information in the message itself, where a check can be performed to verify that a message is properly received.
If a transmitter device sends a message over the shared channel and no collision is detected at the target receiver, a message such as an ACK (Acknowledgment) message can be fed back to the transmitter device indicating that a message from the transmitter was successfully received at the target receiver without detecting a collision. This ACK message is optionally fed back to a transmitter device over a third channel such as a paging channel of a CDMA communication system. Thereafter, a more formal communication link is optionally established between the transmitter and target receiver for future communications.
A more formal communication link can include a feedback loop for synchronizing a transmitter device to a target receiver when no data payload is being transmitted from the transmitter device target receiver. For example, a target receiver can analyze received messages and generate power and timing adjustment messages on a feedback channel to provide synchronization and power control. Since the feedback loop can be used to provide more sophisticated synchronization between two communication devices, wireless resources can be more quicky allocated for on-demand data payload transfers.
In one application, the second channel or feedback channel is time-slotted or partitioned so that transmitter devices can be notified via feedback messages in time slots similar to the first channel. Consequently, each of the multiple transmitters can monitor designated time slots of the second channel for feedback messages.
A feedback message can be as simple as a single bit indicating whether or not a collision occurred on the shared channel for a previous access request message transmission. Likewise, a sequence of bit information or multiple spaced bits can also be sent over the second channel indicating that a collision occurred. When used, multiple bits can provide redundancy to some extent so that a message can still be conveyed even if part of a message is otherwise corrupted due to a failed data transfer.
In an application where the shared channel is time-slotted, each of the multiple transmitter devices is preferably synchronized so that it can send a message to a target receiver in generally any time slot. When transmitter devices sporadically transmit messages in a time slot to a target receiver, there is an increased likelihood that a collision will occur with another transmitter sending a message in the same time slot.
Although a message format can vary depending on a particular application, one embodiment of the present invention involves se
Hoffmann John E.
Nelson, Jr. George R.
Proctor, Jr. James A.
Rouphael Antoine J.
Hamilton Brook Smith & Reynolds P.C.
Hsu Alpus H.
Qureshi Afsar M.
Tantivy Communications Inc.
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