U.S. patent application number 10/401115 was filed with the patent office on 2004-01-15 for reverse link initial power setting using effective radiated power message to compute path loss.
This patent application is currently assigned to Tantivy Communications, Inc.. Invention is credited to Hoffmann, John E., Nelson, George R. JR., Proctor, James A. JR., Rouphael, Antoine J..
Application Number | 20040009785 10/401115 |
Document ID | / |
Family ID | 22676039 |
Filed Date | 2004-01-15 |
United States Patent
Application |
20040009785 |
Kind Code |
A1 |
Nelson, George R. JR. ; et
al. |
January 15, 2004 |
Reverse link initial power setting using effective radiated power
message to compute path loss
Abstract
In an illustrative embodiment of the present invention, a
reference signal including pilot information is transmitted from a
base station to one or multiple field units over a pilot channel. A
message is also sent to the field units over a paging channel to
indicate an effective radiated power level at which the reference
signal is transmitted on the pilot channel. Based on a received
power level of the reference signal at a field unit and the
effective radiated power level of the reference signal, a forward
path loss is estimated at the field unit for the forward link
between the base station and field unit. Assuming the path loss in
the reverse link is approximately the same as the estimated forward
link path loss, the field unit can transmit, a reply message in the
reverse link so that the base station generally receives a message
at-a desired power level.
Inventors: |
Nelson, George R. JR.;
(Merritt Island, FL) ; Proctor, James A. JR.;
(Indialantic, FL) ; Hoffmann, John E.;
(Indialantic, FL) ; Rouphael, Antoine J.;
(Escondido, CA) |
Correspondence
Address: |
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
530 VIRGINIA ROAD
P.O. BOX 9133
CONCORD
MA
01742-9133
US
|
Assignee: |
Tantivy Communications,
Inc.
Melbourne
FL
|
Family ID: |
22676039 |
Appl. No.: |
10/401115 |
Filed: |
March 26, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10401115 |
Mar 26, 2003 |
|
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|
09792870 |
Feb 23, 2001 |
|
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60184223 |
Feb 23, 2000 |
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Current U.S.
Class: |
455/522 ;
370/318; 370/320; 455/69 |
Current CPC
Class: |
H04W 74/0858 20130101;
H04L 12/5602 20130101; H04W 8/04 20130101; H04L 1/1607 20130101;
H04W 74/0825 20130101; H04L 47/26 20130101; H04W 52/242 20130101;
H04W 24/00 20130101; H04W 52/50 20130101; H04W 52/48 20130101; H04L
2001/0093 20130101; H04W 52/228 20130101 |
Class at
Publication: |
455/522 ; 455/69;
370/320; 370/318 |
International
Class: |
H04Q 007/20; H04B
007/00; H04B 001/00 |
Claims
What is claimed is:
1. A method for supporting wireless communications, the method
comprising the steps of: detecting that a collision occurs at a
first transceiver when two or more second transceivers attempt to
send messages over a first channel, a collision occurring when each
of the two or more second transceivers transmit messages on the
first channel, at least a portion of each message transmitted in a
time slot overlap, a common encoded sequence of the messages being
detectable at a transceiver even though there is a collision;
notifying the two or more second transceivers of a collision on the
first channel by sending a message from the first transceiver to
the two or more second transceivers over a feedback channel;
adjusting a power output level of a corresponding one of the two or
more second transceivers attempting to send a message over the
first channel depending on whether a collision occurs on the first
channel, the step of adjusting further comprising: transmitting a
first message from the first transceiver, the first message
including effective radiated power level information at which the
first transceiver transmits a reference signal; measuring a
received power level of the reference signal received at the
corresponding second transceiver; decoding the first message at the
second transceiver to determine the effective radiated power level
information; and estimating a path loss between the first and
second transceivers by comparing the received power level and the
effective radiated power level information.
2. A method as in claim 1 further comprising the step of:
transmitting a second message from the first transceiver to the
second transceiver indicating a desired power level at which
subsequent messages shall be received at the first transceiver.
3. A method as in claim 2 further comprising the step of:
transmitting a reply message from the second transceiver to the
first transceiver at a power level so that the first transceiver
receives the reply message at the desired power level.
4. A method as in claim 2 further comprising the step of:
determining a power output setting at the second transceiver based
on an estimated path loss so that a message is received at the
first transceiver at the desired power level.
5. A method as in claim 1, wherein the first message is transmitted
over a paging channel of a CDMA (Code Division Multiple Access)
communication system.
6. A method as in claim 3, wherein the reply message is transmitted
over an access channel of a CDMA communication system.
7. A method as in claim 1, wherein the second transceiver transmits
an access request message to the first transceiver in a time slot
of an allocated reverse channel.
8. A method as in claim 7, wherein the access request message from
the second transceiver to the first transceiver includes encoded
information for measuring timing alignment of the second
transceiver.
9. A method as in claim 1, wherein the second transceiver, a mobile
transceiver, sends a reply message to the first transceiver, a base
station transceiver, the reply message including a reference marker
that is monitored for generating feedback messages to synchronize
the mobile transceiver to the base station transceiver.
10. A method as in claim 1, wherein the first transceiver transmits
the first message over a dedicated channel to multiple
transceivers.
11. A method as in claim 1 further comprising the step of:
measuring a power level at which a reply message transmitted from
the second transceiver is received at the first transceiver.
12. A method in claim 11 further comprising the steps of: comparing
a power level at which the reply message is received at the first
transceiver with a desired power level at which messages should be
received; and transmitting a power adjustment message to the second
transceiver for subsequent message transmissions.
13. A method as in claim 1, wherein the reference signal includes
pilot symbols.
14. A method as in claim 1, wherein the reference signal is
transmitted over a pilot channel.
15. A method as in claim 1, wherein a path loss is calculated based
on the difference between an effective radiated power level of the
reference signal and power level of one or multiple received
reference signals.
16. A method for supporting wireless communications, the method
comprising the steps of: detecting that a collision occurs at a
first transceiver when two or more second transceivers attempt to
send messages over a first channel, a collision occurring when each
of the two or more second transceivers transmit messages on the
first channel, at least a portion of each message transmitted in a
time slot overlap, a common encoded sequence of the messages being
detectable at a transceiver even though there is a collision;
notifying the two or more second transceivers of a collision on the
first channel by sending a message from the first transceiver to
the two or more second transceivers over a feedback channel;
adjusting a power output level of a corresponding one of the two or
more second transceivers attempting to send a message over the
first channel depending on whether a collision occurs on the first
channel, the step of adjusting further comprising: transmitting a
first message from the first transceiver, the first message
including power level information at which the first transceiver
transmits a reference signal; measuring a received power level of
the reference signal received at the corresponding second
transceiver; decoding the first message at the second transceiver
to determine the effective radiated power level information;
estimating a path loss between the first and second transceivers by
comparing the received power level and the effective radiated power
level information; measuring a power level at which a reply message
transmitted from the second transceiver is received at the first
transceiver; and providing feedback information to the second
transceiver how to adjust its power output level.
17. A method in claim 16 further comprising the steps of: comparing
a power level at which the reply message is received at the first
transceiver with a desired power level at which messages should be
received; and transmitting a power adjustment message to the second
transceiver for subsequent message transmissions.
18. A method in claim 1 further comprising the step of:
transmitting a second message from the first transceiver to the
second transceiver indicating a desired power level at which
subsequent messages shall be received at the first transceiver.
19. A method in claim 18 further comprising the step of:
transmitting a reply message from the second transceiver to the
first transceiver at a power level so that the first transceiver
receives the reply message at the desired power level.
20. A method as in claim 18 further comprising the step of:
determining a power output setting at the second transceiver based
on an estimated path loss so that a message is received at the
first transceiver at the desired power level.
21. A method as in claim 16, wherein the first message is
transmitted over a paging channel of a CDMA (Code Division Multiple
Access) communication system.
22. A method as in claim 19, wherein the reply message is
transmitted over an access channel of a CDMA communication
system.
23. A method as in claim 16, wherein the second transceiver
transmits an access request message to the first transceiver in a
time slot of an allocated reverse channel.
24. A method as in claim 23, wherein the access request message
from the second transceiver to the first transceiver includes
encoded information for measuring timing alignment of the second
transceiver.
25. A method as in claim 16, wherein the second transceiver sends a
reply message to the first transceiver, the reply message including
a reference marker that is monitored for timing alignment.
26. A method as in claim 16, wherein the first transceiver
transmits the first message over a dedicated channel to multiple
transceivers.
27. A method as in claim 16, wherein the, reference signal includes
pilot symbols.
28. A method as in claim 16, wherein the reference signal is
transmitted over a pilot channel.
29. A method as in claim 16, wherein a path loss is calculated
based on the difference between an effective radiated power level
of the reference signal and power level of one or multiple received
reference signals.
Description
RELATED APPLICATION(S)
[0001] This application is a continuation application of U.S.
application Ser. No. 09/792,870 filed on Feb. 23, 2001 which claims
the benefit of U.S. Provisional Application No. 60/184,223 filed on
Feb. 23, 2000, the entire teachings of which is incorporated herein
by this reference.
BACKGROUND OF THE INVENTION
[0002] 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).
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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
[0010] 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, a
reference signal is transmitted from a first transceiver to a
second transceiver or group of target transceivers. The first
transceiver also transmits a first message, which includes
information indicating an effective radiated power level at which
the first transceiver transmits the reference signal. The received
power level of the reference signal is then measured at the second
transceiver to estimate a path loss between the first transceiver
and second transceiver. More specifically, a path loss can be
calculated by comparing the received power level of the reference
signal with the effective radiated power level information as
indicated by the first message.
[0011] A second message is optionally sent from the first
transceiver to the second transceiver. This second message can
include information indicating a desired power level at which
subsequent messages in a reverse direction should be received at
the first transceiver.
[0012] In certain applications, the forward path loss between the
first transceiver and second transceiver is approximately the same
as a reverse path loss for message transmissions from the second
transceiver back to the first transceiver. Consequently, the second
transceiver can adjust its power output level so that the first
transceiver receives a message at the desired power level taking
into account the estimated path loss as previously discussed. Of
course, the path loss can be different in the reverse link
direction than that of the forward link and an estimated power
setting at which the second transceiver transmits a message can be
a starting point for transmitting subsequent messages. For example,
the power output level of the second transceiver can be increased
for subsequent message transmissions until the first transceiver
detects the message.
[0013] In a specific application of the present invention, the
reference signal is transmitted over a pilot channel of a CDMA
(Code Division Multiple Access) communication system. The reference
signal itself optionally includes a marker such as pilot symbols
that are monitored at a second transceiver. The first message as
transmitted by the first transceiver can be transmitted over a
paging channel, while a reply message from the second transceiver
to the first transceiver can be transmitted over an access channel
in which multiple transceivers compete to transmit messages to the
first transceiver. The access channel is optionally divided into
time slots in which a transceiver sends a message to the first
transceiver.
[0014] One message type that can be transmitted on the access
channel is an access request message. Such a message is an
indication to the first transceiver that a more formal
communication link should be established between the first
transceiver and second transceiver. Consequently, the more formal
communication link can be allocated to support more efficient,
on-demand data transfers.
[0015] An access request message is optionally encoded so that it
includes timing alignment information. For example, the first
transceiver can analyze a reply message such as an access request
message including a timing marker and provide feedback indicating
whether the reply is appropriately transmitted within a time slot.
Other types of reply messages can also include a reference marker
for timing alignment. In one application, a reference marker is a
string including pilot information such as one or multiple pilot
symbols.
[0016] In addition to monitoring the timing of a reply message, the
received power level of a reply message can be monitored to
determine whether a transceiver is transmitting a message so that
it is received at a desired power level. This can be achieved by
comparing a power level at which a reply message is received to a
desired power level at which the reply message should be received.
Based on this comparison, a power adjustment message is optionally
transmitted to a corresponding target transceiver. Thus, subsequent
message transmissions from a second transceiver to the first
transceiver can be optimally adjusted based on operating conditions
of the wireless communication system at an instant in time.
[0017] Certain aspects of the present invention reduce co-channel
interference and generally increase the throughput capability of a
wireless communication system. As previously discussed, an initial
power output level of a field unit can be adjusted so that it
minimally interferes with others when it initially transmits
messages or transmits subsequent messages. When message
transmissions on an allocated channel such as an access channel are
minimized, more wireless resources can otherwise be allocated for
supporting higher speed data transfers in a wireless communication
system.
[0018] Of course, the initial power output level of a field unit
can be so low that a message transmission is not detected at a
target device such as a base station. In this case, a power output
level can be increased accordingly for subsequent message
transmissions until the message is detected at a target
transceiver. This process of incrementally increasing the power
level can be a time consuming process, especially if the power
output of the transceiver were to start transmitting at a lowest
possible power level. Thus, it can take a considerable amount of
time to successfully transmit a message to a target receiver such
as a base station.
[0019] The principles of the present invention can be used to
simultaneously reduce the effective time it takes to transmit a
message to a base station while minimally interfering with other
channels of the wireless communication system. This is achieved, at
least in part, by approximating a path loss between a field unit
and base station and transmitting a message from the field unit so
that a message is received at a desired power level at the base
station. Since the initial power setting of the field unit is
approximately set to a detectable power level for data
transmissions to a target receiver, it typically requires less time
to transmit an initial message to a target receiver when power is
incrementally increased so that a message is eventually
detected.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a bock diagram of an illustrative wireless
communication system according to certain principles of the present
invention.
[0021] FIG. 2 is a timing diagram illustrating multiple channels on
which messages are transmitted according to certain principles of
the present invention.
[0022] FIG. 3 is a timing diagram illustrating a use of data fields
within a time-slotted channel according to certain principles of
the present invention.
[0023] FIG. 4 is a pictorial diagram illustrating details of a
message according to certain principles of the present
invention.
[0024] FIG. 5 is a flow chart for processing messages at a target
receiver according to certain principles of the present
invention.
[0025] FIG. 6 is a flow chart for transmitting messages to a target
receiver according to certain principles of the present
invention.
[0026] FIG. 7 is a timing diagram illustrating multiple channels on
which messages are transmitted according to certain principles of
the present invention.
[0027] FIG. 8 is a diagram of a monitored reference signal
according to certain principles of the present invention.
[0028] FIG. 9 is a flow chart for setting an initial power output
level of a transmitter device according to certain principles of
the present invention.
[0029] The foregoing and other objects, features and advantages of
the invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0030] A description of preferred embodiments of the invention
follows.
[0031] FIG. 1 is a block diagram illustrating a wireless
communication system supporting the transmission of data
information over multiple allocated communication channels
according to certain aspects of the present invention. As in many
wireless communication systems, users compete for wireless
bandwidth allocation. Hence, it is desirable that the wireless
communication 10 is optimized for data throughput and, in certain
applications, hi-speed bursts of data throughput.
[0032] Certain aspects of the present invention are based on the
recognition that the power output of a field unit transmitting over
a wireless channel can be controlled so that it minimally
interferes with other field units using the same general wireless
airspace. In particular, a power output level of a newly
transmitting field unit is initially set so low that a base station
may not detect messages transmitted by the field unit. This
initially low power setting of a field unit reduces co-channel
interference because the transmitter device is not transmitting at
excessive power levels. During subsequent communication attempts
with the base station, the power output of a field unit is then
ramped up until messages are acknowledged at the base station.
[0033] In one application, one or multiple field units randomly
transmit messages on a first allocated channel. When two field
units transmit a message simultaneously on this first allocated
channel, there can be a message collision at the base station. The
base station may be able to detect that a message was transmitted
by a field unit and there was a message collision. However, the
base station may not be able to decipher the contents of the
message and determine from which field unit transmitted a message.
Thus, in certain situations, the base station can not transmit a
message directed specifically to a particular field unit indicating
that a collision occurred for a previous message transmission.
[0034] One aspect of the present invention involves providing
general feedback information to the field units indicating that a
collision was detected. Consequently, a previous undetected message
can be re-transmitted by a field unit. If there was no collision
detected and no acknowledgment received by the filed unit, the
field unit can successively ramp up its power output setting for
subsequent message transmission attempts to ensure that a message
will eventually be acknowledged by the base station.
[0035] According to the following description of a preferred
embodiment, communication system 10 is described as a wireless
communication link such as a CDMA radio channel utilizing shared
channel resources. However, it should be noted that the techniques
described herein can be applied in other applications supporting
shared access. For example, the principles of the present invention
can be applied to other general applications such as telephone
connections, computer network connections, cable connections, or
other physical media to which allocation of resources such as data
channels are granted on an as-needed basis.
[0036] As shown, communication system 10 includes a number of
Personal Computer (PC) devices 12-1, 12-2, . . . 12-h, . . . 12-m,
corresponding field units or terminals 14-1, 14-2, . . . 14-h, . .
. 14-m, and associated directional antenna devices 16-1, 16-2, . .
. 16-h, 16-m. Centrally located equipment includes a base station
antenna 18, and a corresponding base station 20 that includes high
speed processing capability.
[0037] Base station 20 and related infrastructure provides
connections to and from a network gateway 22, network 24 such as
the Internet, and network file server 30. Communication system 10
is preferably a demand access, point to multi-point wireless
communication system such that the PC devices 12 can transmit data
to and receive data from network server 30 based on a logical
connection including bi-directional wireless connections
implemented over forward links 40 and reverse links 50. That is, in
the point to multi-point multiple access wireless communication
system 10 as shown, a given base station 20 typically supports
communication with a number of different field units 14 in a manner
which is similar to a cellular telephone communication network.
Accordingly, system 10 can provide a framework for a CDMA wireless
communication system where digital information is relayed on-demand
between multiple mobile cellular users and a hardwired network 24
such as the Internet.
[0038] PC devices 12 are typically laptop computers, handheld
units, Internet-enabled cellular telephones, Personal Digital
Assistant (PDA)-type computers, digital processors or other end
user devices, although almost any type of processing device can be
used in place of PC devices 12. One or multiple PC devices 12 are
each connected to a respective subscriber unit 14 through a
suitable hard wired connection such as an Ethernet-type connection
via cable 13.
[0039] Each field unit 14 permits its associated PC device 12
access to network file server 30. In the reverse link 50 direction,
that is, for data traffic transmitted from the PC 12 towards the
server 30, the PC device 12 transmits information to field unit 14
based on, for example, an Internet Protocol (IP) level network
packets. The field unit 14 then encapsulates the wired framing,
i.e., Ethernet framing, with appropriate wireless connection
framing so that data packets can be transmitted over the wireless
link of communication system 10. Based on a selected wireless
protocol, the appropriately formatted wireless data packet then
travels over one of the radio channels that comprise the reverse
link 50 through field unit antenna 16 to base station antenna 18.
At the central base station location, the base station 20 then
extracts the radio link framed data packets and reformats the
packets into an IP format. The packets are then routed through
gateway 22 and any number or type of networks 24 to an ultimate
destination such as a network file server 30.
[0040] In one application, information generated by PC device 12 is
based on a TCP/IP protocol. Consequently, a PC device 12 has access
to digital information such as web pages available on the Internet.
It should be noted that other types of digital information can be
transmitted over channels of communication system 10 based on the
principles of the present invention.
[0041] Data information can also be transferred from the network
file server 30 to PCs 12 on forward link 40. In this instance,
network data such as IP (Internet Protocol) packets originating at
file server 30 travel on network 24 through gateway 22 to
eventually arrive at base station 20. As previously discussed for
reverse link data transmissions, appropriate wireless protocol
framing is then added to raw data such as IP packets for
communication of the packets over wireless forward link 40. The
newly framed packets then travel via an RF signal through base
station antenna 18 and field unit antenna 16 to the intended target
field unit 14. An appropriate target field unit 14 decodes the
wireless packet protocol layer, and forwards the packet or data
packets to the intended PC device 12 that performs further
processing such as IP layer processing.
[0042] A given PC device 12 and file server 30 can therefore be
viewed as the end points of a logical connection at the IP level.
Once a connection is established between the base station processor
20 and corresponding field unit 14, a user at the PC device 12 can
then transmit data to and receive data from file server 30 on an
as-needed basis.
[0043] Reverse link 50 optionally includes different types of
logical and/or physical radio channels such as an access channel
51, multiple traffic channels 52-1, . . . 52-m, and a maintenance
channel 53. The reverse link access channel 51 is typically used by
the subscriber units 14 to request an allocation of traffic
channels by the base station 20. For example, traffic channels 52
can be assigned to users on an as-needed basis. The assigned
traffic channels 52 in the reverse link 50 then carry payload data
from field unit 14 to base station 20.
[0044] Notably, a given link between base station 20 and field unit
14 can have more than one traffic channel 52 assigned to it at a
given instant in time. This enables the transfer of information at
higher rates.
[0045] Maintenance channel 53 can be used to carry maintenance
information such as synchronization and power control messages to
further support transmission of digital information over both
reverse link 50 and forward link 40.
[0046] Forward link 40 can include a paging channel 41, which is
used by base station 20 to inform a field unit 14 of general
information such as that one or multiple forward link traffic
channels 42 have been allocated to it for forward link data
transmissions. Traffic channels 42-1 . . . 42-n on the forward link
40 are used to carry payload information from base station 20 to a
corresponding target subscriber unit 14.
[0047] Maintenance channel 43 can be used to transmit
synchronization and power control information on forward link 40
from base station processor 20 to field units 14. Additionally,
paging channel 41 can be used to inform a field unit 14 of
allocated traffic channels 52 in the reverse link 50 direction.
[0048] Traffic channels 42 of the forward link 40 can be shared
among multiple subscriber units 14 based on a Time Division
Multiplexing scheme. Specifically, a forward link traffic, channel
42 is optionally partitioned into a predetermined number of
periodically repeating time slots for transmission of data packets
from the base station 20 to multiple subscriber units 14. It should
be understood that a given subscriber unit 14 can, at any instant
in time, have multiple time slots or no time slots assigned to it
for use. In certain applications, an entire time-slotted forward or
reverse link traffic channel can also be assigned for use by a
particular field unit 14 on a continuous basis.
[0049] FIG. 2 is a timing diagram illustrating multiple channels on
which messages are transmitted between a base station 20 and field
units 14 according to certain principles of the present invention.
As shown, field units 14 can transmit messages to base station 20
over a dedicated channel such as access channel 51. Access channel
51 supports a transmission of access request messages from a field
unit 14 to base station 20. An access request message can indicate
a request by field unit 14 for a high speed bi-directional
communication link.
[0050] Message transmissions over access channel 51 need not be
limited to access request type messages. For example, access
channel 51 can be structured to support other types of
messages.
[0051] In the illustrative timing diagram as shown in FIG. 2,
access channel 51 is optionally partitioned into periodically
repeating time slots 210 in which messages are transmitted from a
field unit 14 to base station 20. More specifically, an Epoch on
the order of 26.6 mS in duration can be partitioned to include time
slot #0 and time slot #1 as shown. In this application, a field
unit or multiple field units 14 can randomly send messages to base
station 20 in either time slot of an Epoch. Providing multiple time
slots 210 in which a field unit 20 can transmit a message renders
it less likely that two randomly transmitting field units 14 will
transmit a message in a same time slot 210. Depending on an
application, a field unit 14 can be synchronized with base station
20 using pilot channel 44 so that messages can be transmitted in a
particular time slot 210.
[0052] When a collision occurs, i.e., two field units 14 transmit a
message in the same time slot 210, a device monitoring access
channel 51 for such messages may not be able to properly decode or
decipher the content of either transmitted message. An indication
of this condition is transmitted to field units 14.
[0053] Feedback channel 45 is provided so that base station 20 can
send feedback messages 230 to field units 14. At least a portion of
the feedback channel 45 as shown is reserved for transmitting
general messages to the collective field units 14 whether or not a
message collision occurred on access channel 51 in a previous Epoch
and, more particularly, that a message occurred in a particular
time slot 210.
[0054] A feedback message 230 is optionally a sequence of encoded
information or single bit indicating whether a collision occurred
for a message transmitted to base station 20 in a time slot 210. As
shown, multiple feedback messages 230 can be generated during a
given time duration such as an Epoch or half Epoch. For example,
feedback information such as feedback messages 230 can be
transmitted in duration A of Epoch N+1 to indicate that a collision
occurred for the reception of a message 210 in time slot #0 of
Epoch N at base station 20. More specifically, a logic 1
transmitted in each of three feedback messages 230 of duration A
can indicate that a collision was detected while a logic 0 setting
can indicate that no collision was detected.
[0055] Transmitting multiple, spaced feedback messages 230 as shown
for duration A provides redundancy to some extent. For example,
multiple messages 230 transmitted in a given duration, such as
duration A, can be part of a redundantly transmitted message to
increase the chance that a field unit 14 will be properly notified
whether a collision occurs. Hopefully, at least one of the multiple
feedback messages 230 can be identified at a field unit 14 even if
a message transmission failure occurs for some of the messages 230
in, for example, duration A.
[0056] In a similar manner as previously discussed, feedback
messages 230 of feedback channel 45 transmitted in duration B of
Epoch N+1 can indicate whether a message collision is detected in
time slot #1 of Epoch N as monitored by base station 20.
[0057] In a reverse link direction, a message transmitted by a
field unit 14 to base station 20 on access channel 51 includes
unique information such as the identification number of the field
unit 14 transmitting the message. In forward link direction, paging
channel 41 supports message transmissions from base station 20 to
field units 14, where the message transmissions are typically
directed to a specific field unit 14. Thus, base station 20 can
respond to a field unit 14 that sent a message on access channel 51
by transmitting a reply message to the field unit 14 over paging
channel 41. Other link information forwarded on the paging channel
41 can be forwarded to a field unit 14 to establish a formal
bidirectional link between base station 20 and field unit 14.
[0058] One type of message transmitted on paging channel 41 is an
ACK (acknowledgment) message 240. ACK messages 240 are sent by base
station 20 to indicate that a message received in a time slot 210
of access channel 51 was properly received. Similar to the feedback
messages 230 on feedback channel 45, ACK messages transmitted to a
field unit 14 are also feedback messages. However, an ACK message
240 indicates that a previous access request message transmitted by
a field unit 14 was properly received. ACK message 240 can also
indicate that a formal communication link will be established with
the access requesting field unit 14. For instance, traffic channels
can be assigned to field unit 14 for transmitting or receiving a
data payload.
[0059] It should be noted that field units 14 optionally transmit
at such a low power output level that messages transmitted in a
particular time slot 210 are not detected at base station 20. In
this instance, a field unit 14 can adjust its power output level
for a subsequent message transmission based on feedback information
received from base station 20. More specifically, a field unit 14
can adjust its power output level depending on a feedback message
230 or ACK message 240 received on feedback channel 45 or paging
channel 41 respectively. Accordingly, the power output level of a
field unit 14 can be optimized so that it minimally interferes with
other field units 14 transmitting information over a common radio
frequency.
[0060] Although other message types can be supported, the messages
transmitted in a time slot 210 of access channel 51 are typically
access request messages indicating that a field unit 14 would like
to receive or transmit data payload information on reverse link
traffic channels 52 or forward traffic channels 42.
[0061] One aspect of adjusting the power output of a field unit 14
is to determine whether or not a collision was detected for a
previous message transmission from the field unit 14 to base
station 20. For example, if a collision is not detected for a
previously transmitted message from a particular field unit 14, the
power output level for a subsequent message transmission from the
field unit 14, if any, can be increased so that base station 20 is
more likely to detect the message transmission. More specifically,
the power output level of a field unit 14 can be increased a
predetermined amount such as +0.5 dB for a subsequent message
transmission if no collision was detected for a previous message
transmission.
[0062] In the event that a message transmission by a field unit 14
does result in a collision due to multiple field units 14
transmitting in a same time slot 210, the power output level of the
field unit 14 can be unchanged or potentially reduced for a
subsequent message transmission since it is not known whether the
message transmission by a field unit 14 would have otherwise been
detected at base station 20 if only one field unit 14 transmitted a
message within a particular time slot 210. Hence, one aspect of the
present invention involves adjusting the power output level of a
field unit 14 so that it minimally interferes with others using the
same radio channel.
[0063] This method of transmitting messages can be particularly
useful when a user first powers a field unit 14 and must
communicate with base station 20. For example, it is undesirable in
certain situations to transmit a message at such a high power
output level that such message transmissions cause other data
transmissions on the radio channel to become corrupted due to
excessive noise.
[0064] In a similar manner as previously discussed, the power
output level of a field unit 14 can be adjusted depending on
whether base station 20 acknowledges receipt of a message on access
channel 51. Thus, if field unit 14 does not detect a reply ACK
message 240, the power output level of field unit 14 can be
increased for subsequent message transmissions.
[0065] A maximum power adjustment level such as 60 dBm can be
selected in which the field unit 14 will discontinue transmitting
if no ACK message is received even at this level.
[0066] A more sophisticated application of the present invention
can involve adjusting a power output level of a field unit 14
depending on both a feedback message 230 indicating whether a
collision was detected and an ACK message 240 indicating that an
access message was acknowledged by base station 20. More
specifically, a field unit 14 can adjust its power output level for
subsequent message transmissions if no collision is detected and no
ACK message is received for a previously transmitted message from a
field unit 14. Otherwise, the field unit 14 can re-transmit at a
previous power output level.
[0067] Regardless of which method is used to adjust the power
output level, the power level setting at which base station 20
acknowledges receipt of a message from a field unit 14 can be used
to determine power level settings at which the field unit 14 must
transmit other information to base station 20. For example, a
message from a field unit 14 can be transmitted using a particular
modulation rate during the initial message transmission. The power
output level of subsequent transmissions from the field unit 14 can
be adjusted to accommodate transmitting messages at different
modulation rates. For example, it can be determined at what power
level a field unit should transmit on allocated traffic channels
using a different modulation rate. A history of the power level
output adjustments are optionally maintained to track power
adjustments and determine at what power level a monitoring device
such as base station 20 detects the transmission of a message.
[0068] As previously discussed, one aspect of the present invention
involves re-transmitting a message from a field unit 14 so that it
can be detected at base station 20. A re-transmission is optionally
based on a random back off time so that a collision is less likely
to occur on a subsequent attempt to transmit a message.
[0069] Consider a situation where two or more field units 14
transmit a message over access channel 51 and a collision is
detected at base station 20. As previously discussed, a feedback
message will be transmitted to the field units 14 indicating that a
collision occurred. Both field units 14 must then re-transmit their
corresponding messages to base station 20.
[0070] To avoid another collision, the field units 14 randomly
choose a back off time relative to the previous message
transmission in which the collision occurred and transmit in
another time slot 210. For example, if field unit A and field unit
B transmit a message in time slot #0 of Epoch N, field unit A will
choose a back off time such as 3 Epochs and re-transmit a message
to base station 20 in time slot #1 of Epoch N+3 while field unit B
re-transmits a message based on a random back off time in time slot
#0 of Epoch N+2. Accordingly, field units A and B are less likely
to cause another collision for a message re-transmission.
[0071] FIG. 3 is a timing diagram illustrating another embodiment
of the present invention for transmitting feedback messages to
field units 14. An access channel 51 is partitioned so that a field
unit 14 can transmit an access probe or other message in a time
slot 345. As shown, feedback channel 355 is partitioned to include
64 time slots TS#0, TS#1, TS#2 . . . TS#63 that repeat every
Epoch.
[0072] Each time slot 315 of feedback channel 355 preferably
includes a data field supporting 16 bits of information. In the
specific application as shown, ten bits of information are reserved
for a general message, one bit is reserved as a collision detect
bit 325 and five bits are reserved for CRC (Cyclical Redundancy
Check) data 328. General message 320 is optionally a message
directed to a particular field unit 14. For example, each of
multiple field units 14 can be assigned use of a particular time
slot 315 for receiving feedback information from base station 20 to
field units 14. When assigned, a corresponding field unit 14
monitors an appropriate time slot 315 to receive messages from base
station 20. One type of specific message in a time slot 315 is
feedback information to a field unit 14 indicating how its timing
or power should be adjusted so that messages transmitted from a
field unit 14 are properly received at base station 20.
[0073] Time slots 315 are optionally unassigned and the message
itself can include an address to which field unit 14 a message is
directed. Thus, in a modified embodiment, feedback messages can be
transmitted asynchronously to a field unit 14.
[0074] Collision detect bit 325 in a time slot 315 is a single bit
indicating whether a collision occurred in a monitored time slot
345. More specifically, collision detect bits 325 of time slot
TS#0, TS#1 . . . TS#31 of Epoch M can be used to indicate that a
collision occurred in access probe slot #0 of Epoch M-1.
Accordingly, this string of individual collision detect bits 328
over multiple time slots can be set to a same logic state
indicating that a collision was detected.
[0075] In a similar manner, TS#32, TS#33 . . . TS#63 of Epoch M can
be set appropriately to indicate whether a collision occurred on
access probe slot #1 of Epoch M-1. Thus, a monitoring field unit 14
can determine whether a collision occurred at base station 20 based
on a single bit, a sequence of multiple bits, or a sequence of
spaced bits.
[0076] CRC data 328 is also provided in a feedback message 360. The
CRC data 328 is optionally decoded at the field unit 14 to ensure
that a message 360 is properly received at a field unit 14 and,
more specifically, that a particular collision detect bit 325 is
properly received. Other methods can also be used to ensure and
verify that a message and data is properly received at field unit
14. For example, a message can be transmitted based on an FEC
(Forward Error Correction) code.
[0077] FIG. 4 is a diagram illustrating a format for transmitting
messages over the access channel from a field unit to a target
receiver according to the principles of the present invention.
[0078] In one application, message 410 is transmitted by a field
unit 14 over access channel 51 and includes two parts. As shown, a
first part or preamble 415 of message 410 is a coded message
indicating a request by the field unit 14 for a communication link.
Each field unit 14 can transmit a message 410 having a commonly
coded preamble 415. Thus, if two field units 14 transmit a message
including the same preamble message 415, base station 20 can
determine that at least a preamble message 415 was sent by at least
one field unit 14. That is, the preamble message 415 as transmitted
by one field unit can overlap with the preamble message 415 as
transmitted by another field unit 14 when multiple messages 410 are
transmitted in the same time slot.
[0079] Message 410 optionally includes a data payload 420 that is
transmitted to base station 20. In one application, data payload
420 includes the serial number of the field unit 14 transmitting
message 410. Typically, some form of redundancy check information
such as CRC data is included with message 410 so that base station
20 can determine whether message 410 is properly received without
errors.
[0080] If message 410 is received without errors, base station 20
can respond accordingly to establish a link with a field unit 14
and transmit a "non-collision" message on feedback channel 45 to
the field units 14. Alternatively, if message 410 includes an error
free preamble 415 but improperly received data payload information
420, base station 20 can deduce that two or more transmitters sent
a message at the same time. A collision detection message is then
transmitted over feedback channel 45 indicating that a collision
occurred. Thus, a target receiver such as base station 20
monitoring messages 410 can provide valuable feedback to multiple
transmitting field units 14 whether a message collision occurs.
[0081] Another aspect of the present invention involves coding a
preamble 415 using pilot block 53 and Barker code block 54. Based
on this coding or use of a sequence of symbols, a field unit 14 can
transmit a message 410 to base station 20.
[0082] As shown, a preamble message 415 can include four pilot
blocks 53 and four Barker code blocks 54. The Barker code blocks 54
assist base station 20 identify a point where preamble 415 of a
message 410 starts. In other words, the information in the preamble
415 can be used for timing purposes at the base station 20 to
asynchronously receive a message. Thus, it is not necessary that a
field unit 14 transmit a message 410 in a time slot 210 because
base station 20 can be modified to receive asynchronous
messages.
[0083] However, in an application where messages 410 include Barker
code blocks 54 that are transmitted in a time slot 210, base
station 20 can identify a received message 410 even if a collision
occurs because the preamble 415 of a message 410 simultaneously
transmitted by multiple field units 14 in a time slot will overlap
and, thus, will be detectable at base station 20.
[0084] Each pilot block 53 includes a number of repeating pilot
symbols. Preferably, a pilot block includes 48 symbols that are
used by a target receiver to decode message 410.
[0085] The second portion of a message 410 can include a data
payload 420 that is sent to base station 20. Preferably, pilot
symbols are also inserted in the data payload 420 portion of
message 410 for assisting in coherent demodulation of-data at a
target receiver. Pilot symbols typically include a series of
positive data bits and therefore do not themselves inherently
include timing information.
[0086] A Barker code block 54 as shown includes a predetermined
pattern of bit information. Use of BPSK (Binary Phase Shift Keying)
can be used to generate a positive barker sequence 450, +B, such as
three positive bits, followed by three negative bits, a positive
bit, a pair of negative bits, a positive bit and then a negative
bit respectively. A Barker code sequence can alternatively be
negative such as a negative Barker sequence, -B, further assisting
in message processing at a monitoring device. Further details for
processing message 410 including pilot blocks 53 and Barker code
blocks 54 can be found in co-pending U.S. patent application Ser.
No. 09/766,875 filed on Jan. 19, 2001 entitled "Access Channel
Structure for Wireless Communication System," the entire teachings
of which are incorporated herein by reference.
[0087] FIG. 5 is a flow chart illustrating a process for monitoring
a channel for messages according to the principles of the present
invention.
[0088] Step 500 generally indicates an entry point into the flow
chart. In step 510, an access channel 51 is monitored for message
transmissions such as access request messages transmitted by a
field unit 14. It is then determined in step 520 whether the
message includes a Barker code or an appropriately received
preamble 415 of a message 410. If no Barker code or preamble 415 is
detected in step 520, process flow resumes at step 510 again.
Alternatively, if a Barker code is detected in step 520, the
message 410 is further analyzed to determine if a collision occurs
in a time slot. That is, it is determined whether at least a
portion of data in a received message 410 is corrupted.
[0089] One way to determine if a message collision occurs is to
verify that data in a message 410 was properly received. This can
be achieved by analyzing the received message 410 according to
redundancy check information. If the data in a message 410 is not
properly received at base station 20, a feedback message is
transmitted by base station 20 over feedback channel 45 indicating
that a collision was detected for a previous access request message
in step 540. Following step 540, process flow resumes again at step
510.
[0090] If a collision is not detected for a particular message in
step 530, the message 410 is analyzed to determine which of
multiple field units 14 sent the message. Following in step 550, an
ACK message 240 is sent to the requesting field unit 14 over the
paging channel 41. Also, a message is sent over the feedback
channel 45 indicating that no message collision occurred for the
corresponding previous time slot 210 of access channel 51. Finally,
a more formal link is established with the access requesting field
unit in step 560.
[0091] FIG. 6 is a flow chart illustrating a process flow at a
field unit for transmitting a message to a target receiver
according to the principles of the present invention. Step 600
generally indicates an entry point into the flow chart.
[0092] In step 620, the status of field unit 14 is monitored for an
input by a user indicating that the field unit 14 desires to
establish a communication link with a target receiver such as base
station 20. It is then determined in step 620 whether the input
indicates that a field unit 14 desires to establish a communication
link. If not, process flow again resumes at step 610. If so, field
unit 14 transmits an access request message on the access channel
51 in step 630. Thereafter, the feedback channel 45 is monitored by
a field unit 14 in step 640 for feedback information such as
collision detection messages.
[0093] If a collision is detected for a previous transmission by a
field unit 14 in step 650, process flow continues at step 660 where
the power output level is not adjusted for the field unit 14 and a
message 410 is subsequently re-transmitted in step 630. If a
collision is not detected in step 650 as indicated by a collision
feedback message, it is determined in step 670 whether an ACK
message 240 is received at the field unit 14 over paging channel
41. If so, a link is established between the field unit 14 and base
station 20 in step 690. If not, the power output level of the field
unit 14 is increased in step 680 and process flow continues at step
630 to re-transmit a message from the field unit 14 to base station
20.
[0094] FIG. 7 is a timing diagram illustrating multiple channels on
which messages are transmitted among transceivers according to the
principles of the present invention.
[0095] As previously discussed, one aspect of the present invention
involves setting a field unit 14 to an initial power level so that
it minimally interferes with other users during a message
transmission. Since power is ramped up based on whether an access
message is detected at base station 20, it is preferable that the
initial power level of the field unit is reasonably near a power
level at which base station 20 will receive a message at a desired
power level. Consequently, a field unit 14 will be able to transmit
a message to base station 20 and establish a more formal
communication link in less time since a power level output of field
unit 14 will need only minimal adjustments so that a message is
received at base station 20.
[0096] One method for initially setting a power output level of a
field unit 14 involves transmitting a reference signal 710 on pilot
channel 44 from base station 20. Preferably, the reference signal
710 is transmitted at an appropriate power level so that multiple
field units 14 in a wireless airspace monitoring the pilot channel
44 can identify the reference signal 710 and measure a power level
at which it is received. In one application, reference signal
includes pilot information such as a sequence of pilot symbols,
where the pilot symbols are defined by PN (Pseudo Noise) codes. One
or multiple pilot correlation filters in field unit 14 is used to
detect the pilot symbols.
[0097] Each field unit 14 monitoring the pilot channel 44 typically
includes a power detector circuit to measure a power level of the
received reference signal 710. For example, the power detector is
used to measure the strongest pilot path of the received reference
signal 710. This measurement is used to estimate a forward path
loss between base station 20 and field unit 14.
[0098] The total received signal power level of the reference
signal 710 can be computed based on the sum of the magnitude
squared of the I and Q channel. Power measurements are optionally
filtered for providing a better estimate of a received power level
under fading conditions.
[0099] As shown in FIG. 7, messages are transmitted on paging
channel 41 from base station 20 to field units 14. One such message
is message A that includes information indicating a power level at
which reference signal 710 is transmitted from base station 20.
This value can be expressed in dBm that already takes into account
the gain of the base station antenna. Thus, message A can include
effective radiated power level information at which base station 20
transmits reference signal 710. In harmony with the principles of
the present invention, additional messages such as antenna gain
information, offset information, correction information and general
information can be transmitted to a field unit 14.
[0100] Field unit 14 decodes message A to determine a power level
at which reference signal 710 is transmitted. The forward path loss
between base station 20 and field unit 14 is then determined by
comparing the received power level of the reference signal 710 at
field unit 14 with the effective radiated power level as indicated
by message A.
[0101] The calculated forward path loss can then be used to
estimate a reverse path loss between field unit 14 and base station
20. For example, the reverse path loss is estimated to be about the
same as the forward path loss, although it is probably at least
slightly different. This estimated path loss is used to determine
an initial setpoint at which messages can be transmitted from field
unit 14 to base station 20.
[0102] Consider a case where base station 20 transmits a reference
signal 710 at an effective radiated power level of 55 dBm. As
discussed, this information is sent to field units 14 via message A
generally broadcasted on the paging channel 41. If the received
power level of the reference signal 710 is 22 dBm, the forward path
loss is calculated as 55-22 dBm, or forward path loss=33 dBm. Based
on this path loss, a field unit 14 can estimate a reasonable power
output level for a subsequent attempt to transmit a message to base
station 20.
[0103] Additional messages can be sent on paging channel 41 from
base station 20 to field units 14. For example, message B is also
generally transmitted to field units over paging channel 41.
Message B preferably includes encoded information indicating a
desired power level at which base station 20 will receive
subsequent messages from a field unit 14. This information can also
be a specific message directed transmitted to a particular field
unit 14. Thus, a field unit 14 can use the information to estimate
at what level a message should be transmitted so that a message is
received at the desired power level. In a case where message B
indicates a desired power level of 12 dBm and the forward path loss
is approximately 33 dB as discussed, field unit 14 can attempt to
transmit a message at 33+12 dBm, or 45 dBm, to base station 20.
[0104] Notably, the reverse path loss may be much more than 33 dBm
as estimated. In such a situation, base station would not
necessarily detect a message transmitted by field unit 14. As
previously discussed, however, the power output setpoint of 45 dBm
can be a starting point at which messages such as access request
messages 750 are transmitted over access channel 51. If a collision
is not detected at base station 20 and no ACK message 240 is
received over paging channel 41, the power output of field unit 14
can be increased by 1 dBm to 46 dBm for a subsequent attempt to
transmit a message. This procedure of adjusting the power output
level of a field unit 14 can be repeated until a message is
detected at base station 20.
[0105] Messages transmitted to base station 20 can also be
monitored to determine a power level at which a message is received
from a field unit 14. To achieve this end, a message such as
message C can include pilot information such as a pilot symbol or
sequence of pilot symbols. Pilot correlation filters are then used
to identify the strongest diversity path and side paths as shown in
FIG. 8. One or multiple paths are then used to determine a power
level at which the message is received at base station 20 on access
channel 51. To ensure that a message C is properly received, the
message is analyzed for errors using error detection information
such as CRC check bits. These and other aspects of invention were
previously discussed in this specification.
[0106] After message C is properly received at base station 20, a
power adjustment message is generated at base station 20 to
indicate how the field unit 14 should be adjusted so that
subsequent messages to base station 20 are received at a desired
power level. For example, if base station 20 determines that a
message is received at 23 dBm, base station 20 can send a message
over paging channel 41 indicating that the field unit should reduce
its power output level for subsequent message transmissions so that
a message from a field unit is received at a lower power level such
as 12 dBm.
[0107] FIG. 8 is a graph illustrating a received diversity string
for a pilot symbol according to the principles of the present
invention. A received message such as reference signal 710, message
A, message B or message C can include a marker such as one or
multiple pilot symbols that are monitored at a receiver to
determine a power level of a received message.
[0108] Both base station 20 and field units 14 include pilot
correlation filters for identifying a marker such as one or
multiple pilot symbols in a transmitted message. This marker aids
in analyzing both timing alignment and a received power level of a
message. Incidentally, the diversity string illustrates the receipt
of a marker in a message as a result of multipath fading. That is,
a signal from a transmitter is received at a target at different
times due to varying times it takes for the signal to reach the
target over different paths between a transmitter and receiver.
[0109] Preferably, the strongest received diversity path will be
designated as the time alignment string at base station 20 field
unit 14 for analyzing the timing of a received message. Likewise,
the single strongest path is preferably chosen to calculate a power
level at which a message is received. However, additional paths are
optionally used to determine a received power level of a
message.
[0110] Timing alignment and a received power level of a message is
determined using the correlation profile of the strongest pilot in
a particular string, which is analyzed as mentioned using
correlation filters. The output of the correlation filters
typically consist of 256 samples, which represent 64 lags at 4
samples per lag. The 256 sample output-window represents the total
correlation time span of a receiver device. This can vary depending
on the application. Preferably, the time alignment point is sample
number 80 which allows 20 lags for precursor and 44 lags for post
cursor channel information.
[0111] Generally, the computation of the time alignment error is
based on a determination of where the centroid or peak lies in a
given sample string. For example, each field unit 14 transmitting
in a time slot includes a marker, i.e., the peak signal, located at
a predetermined position within a time slot. The strongest pilot
path for the channel and 2 samples on either side of the main path,
i.e., 1 and 1/4 chips, is statistically analyzed to determine the
centroid or peak of a marker within a time slot. Location of the
centroid, L, of the samples in FIG. 6 is calculated based on the
following equation: 1 L = [ t x Q ( + ) ] Q ( t )
[0112] where t=sample time and Q(t) is the magnitude of a sample at
a given time. For example, L is calculated based on the results as
shown in FIG. 6: 2 L = ( .25 * 76 ) + ( .5 * 77 ) + ( 1.0 * 78 ) +
( .8 * 79 ) + ( .6 * 80 ) .25 + .5 + 1.0 + .8 + .6 L = 78.317
[0113] Again, the timing alignment error is determined by comparing
the timing of the computed centroid to the desired time set point
of 80, which is chosen as the reference point for timing alignment
within a time slot. Since the centroid in the example above is
estimated to be 78.317, timing is early relative to the set point
of 80. An appropriate message can be sent to field unit 14
indicating how its timing should be finely tuned so that messages
from field unit 14 are received at the appropriate time at base
station 20. In a similar manner, the diversity string of FIG. 8 can
be analyzed to determine a power level at which a message is
received. Thus, an appropriate message can be sent to field unit 14
indicating how its power output level should be adjusted so that a
message is received at a desired power level.
[0114] As mentioned, this technique can be used to detect a
received power level of reference signal 710 at base station
20.
[0115] More details regarding timing alignment and power control
between a base station 20 and each of multiple field units 14 can
be found in co-pending U.S. Application No. (2479.1067-001)
entitled "Minimal Maintenance Link to Support Synchronization"
filed on Feb. 7, 2001, and co-pending U.S. application Ser. No.
09/775,305 entitled "Maintenance Link Using Active/Standby Request
Channels" filed on Feb. 1, 2001, the entire teachings of both of
which are incorporated herein by reference.
[0116] FIG. 9 is a flow chart illustrating a method for setting a
power level output of a field unit based on an estimated path loss
according to the principles of the present invention.
[0117] Step 900 indicates an entry point into the flow chart.
Following in step 910, field unit 14 monitors pilot channel 44 for
reference signals 710. As previously discussed, the field unit
determines a power level at which reference signals 710 are
received using a power detector circuit and pilot correlation
filters.
[0118] In step 920, paging channel 41 is monitored by a field unit
14 for messages transmitted from base station 20. As previously
discussed, message A is received on paging channel 41 and decoded
to determine an effective radiated power level at which reference
signals 710 are being transmitted from base station 20.
[0119] Based on the detected power level of reference signal 710 as
received at field unit 14 and corresponding effective radiated
power level at which the reference signal 710 is transmitted from
base station 20, a path loss is estimated between the base station
and field unit 14 in step 930. Preferably, the path loss is
estimated by computing the difference between the power level at
which reference signal 710 is transmitted from base station 20 and
a power level at which reference signal 710 is received at field
unit 14.
[0120] Message B is subsequently received at field unit 14 in step
940. This message preferably includes information indicating a
desired power level at which messages are to be received at base
station 20.
[0121] Based on the desired power level at which messages are to be
received at base station 20 and the estimated path loss in step
930, field unit 14 determines a power output setting for field unit
14 so that a message is received at the desired power level at base
station 20 in step 950. More specifically, it is presumed that an
actual path loss from the field unit 14 to base station is
approximately the same as the calculated path loss between the base
station and field unit 14 based on measurements of reference signal
710. Thus, an appropriate power output level of field unit 14 can
be determined by adding the estimated path loss to the desired
power level setting to determine a power output setting for field
unit 14. Consequently, this power output setting of a field unit 14
should be a reasonable starting point for attempting to transmit an
initial message to base station 20.
[0122] Also in step 950, field unit 14 transmits message C such as
an access request message to base station 20 over access channel
51. Upon receipt, base station 20 measures a received power level
of message C at base station 20. This received power level is then
compared to the desired power level so that feedback can be
provided to field unit 14 indicating how to adjust its power output
level so that subsequent messages are received at the desired power
level.
[0123] Following in step 960, field unit 14 monitors paging channel
41 for an ACK message indicating that base station 20 properly
received message C. If an ACK is not received in step 970, the
power output level of a field unit 14 is increased in step 975 and
the message is subsequently re-transmitted in step 960. This loop
of increasing power generally repeats until base station 20
acknowledges receipt of the message.
[0124] When an ACK is received in step 970, process flow continues
at step 980 in which additional messages are received from base
station 20 indicating whether the message transmitted in step 950
was received at the desired power level. As discussed, information
can be transmitted to a field unit 14 indicating how to adjust its
power output level so that subsequent messages are received at the
desired power level at base station 20. Consequently, the output
power level of the field unit 14 is adjusted accordingly for
subsequent message transmissions.
[0125] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
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