Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MULTI-DETECTION OF HEARTBEAT TO REDUCE ERROR PROBABILITY
This application is a divisional application of co-pending Application Serial
No.
2,670,758 which is a divisional of Application Serial No. 2,450,670, filed
June 13, 2002.
FIELD OF THE INVENTION
This invention relates to the field of wireless user devices and methods of
using
same.
BACKGROUND OF THE INVENTION
Increasing use of wireless telephones and personal computers has led to a
corresponding increase in demand for advanced telecommunication services that
were
once thought practical only for specialized applications. In the 1980s,
wireless voice
communications became widely available through cellular telephone networks.
Such
services were thought at first to be for the exclusive province of businessmen
because of
expected high subscriber costs. The same was also true for access to remotely
distributed
computer networks, whereby until very recently, only business people and large
institutions could afford the necessary computers and wireline access
equipment.
As a result of the widespread availability of affordable new technologies, the
general
population now increasingly desires to have not only wireline access to
networks such as the
Internet and private intranets, but also wireless access as well. Wireless
technology is
particularly useful to users of portable computers, laptop computers, hand-
held personal
digital assistants and the like who prefer access to such networks without
being tethered to a
telephone line.
There still is no widely available satisfactory solution for providing low
cost, high
speed access to the Internet, private intranets, and other networks using the
existing
wireless infrastructure. This is most likely an artifact of several
unfortunate
circumstances. First, the typical manner of providing high speed data service
in the
business environment over a wireline network is not readily adaptable to the
voice grade
service available in most homes or offices. For example, such standard high
speed data
services do not necessarily lend themselves to efficient transmission over
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standard cellular wireless handsets because wireless networks were originally
designed only to provide voice services. As a result, present day digital
wireless
communications systems are optimized for voice transmissions, although certain
schemes such as CDMA do provide some measure of asymmetrical behavior for the
accommodation of data transmissions. For example, the data rate specified by
the
Telecommunication Industry Association (TIA) for IS-95 on the forward traffic
channel is adjustable in increments from 1.2 kbps up to 9.6 kbps for so-called
Rate
Set 1, and increments from 1.8 kbps up to 14.4 kbps for Rate Set 2. On the
reverse
link traffic channel, however, the data rate is fixed at 4.8 kbps.
At best, existing wireless systems therefore typically provide a radio channel
that can accommodate maximum data rate transfers of 14.4 kilobits per second
(kbps) over a forward link direction. Such a low data rate channel does not
lend
itself directly to transmitting data at rates of 28.8 or even 56.6 kbps that
are now
commonly available using inexpensive wireline modems, not to mention even
higher
rates such as the 128 kbps that are available with Integrated Services Digital
Network (ISDN) type equipment. Data rates at these levels are rapidly becoming
the
minimum acceptable rates for activities such as browsing web pages.
Although wireline networks were known at the time when cellular systems
were initially developed, for the most part, there was no provision made for
such
wireless systems to provide higher speed ISDN- or ADSL-grade data services
over
cellular network topologies.
In most wireless systems, there are many more potential users than radio
channel resources. Some type of demand-based multiple access system is
therefore
required.
Whether the multiple access is provided by the traditional Frequency
Division Multiple Access (FDMA) using analog modulation on a group of radio
frequency carrier signals, or by schemes that permit sharing of a radio
carrier
frequency using Time Division Multiple Access (TDMA), or Code Division
Multiple Access (CDMA), the nature of the radio spectrum is such that it is
expected
to be shared. This is quite dissimilar to the traditional environment
supporting data
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transmissions in which the wireline medium is relatively inexpensive and is
not
typically intended to be shared.
Other factors to consider in the design of a wireless system are the
characteristics of the data itself. For example, consider that access to web
pages
generally is burst-oriented, with asymmetrical data rate transmission
requirements in
a reverse and forward direction. In a conunon application, a user of a remote
client
computer first specifies the address of a web page to a browser program. The
browser program then sends the web page address data, which is usually 100
bytes
or less in length, over the network to a server computer. The server computer
then
responds with the content of the requested web page, which may include
anywhere
from 10 kilobytes to several megabytes of text, image, audio, or even video
data.
The user thereafter may spend several seconds or even several minutes reading
the
content of the page before downloading another web page.
In an office environment, the nature of most employees' computer work
habits is typically to check a few web pages and then to do something else for
an
extended period of time, such as accessing locally stored data or even
terminating
use of the computer altogether. Therefore, even though such users may remain
connected to the hitemet or private intranet continuously during an entire
day, actual
use of the high speed data link is usually quite sporadic.
If wireless data transfer services supporting Internet connectivity are to
coexist with wireless voice communication, it is becoming increasingly
important to
optimize the use of available resources in wireless CDMA systems. Frequency re-
use and dynamic traffic channel allocation address some aspects of increasing
the
efficiency of high performance wireless CDMA conuntmication systems, but there
is
still a need for more efficient utilization of available resources.
SUMMARY OF THE INVENTION
One way of making more efficient utilization of available resources is to
ensure the resources are allocated in an error-free manner. For example, a
base
station should not allocate traffic channels to a field unit when a request
for traffic
channels has not been made. Similarly, the base station should allocate
traffic
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channels to a field unit when a request has been made. Such a request is made
by
the field unit when the field unit is employed by a user to send traffic data
to a
remote network node.
In one application, a transmission of a marker in a time slot over one channel
indicates a request by the corresponding field unit to go active. That is,
transmission
of a marker in an assigned time slot indicates that the field unit is
requesting that
reverse link traffic channels be assigned to the user for transmitting a data
payload
from the field unit to the base station. This presumes that the field unit is
presently
in the standby mode. Alternatively, a field unit transmits a marker over a
second
channel of the pair of reverse link channels to indicate that the field unit
is not
requesting to be placed in the active mode. For example, the field unit does
not
want to transmit data on a reverse link channel. Rather, the field unit
requests to
remain inactive but synchronized with the base station so that the field unit
can
immediately go active again at any moment.
=
In either case, the present invention improves performance for detecting a
signal having a marker, or indication, of a request to change communications
states,
for example, by making a measurement of the indications to determine that a
request
to change communications states has been made. In one particular embodiment,
the
measurement includes at least two positive identifications of the request in a
given
time span. The system may further improve performance by applying a difference
in
power levels for a non-request state (i.e., steady state or 'control hold'
state) versus a
request state (i.e., 'request to change' communications state). The result may
include a reduced number of erroneous communications states, such as
erroneously
assigned or allocated traffic channels.
In one particular application, a subscriber unit provides a heartbeat channel
using a first code in a CDMA system in a heartbeat with request channel using
a
second code in the reverse link to a base station. The subscriber unit
provides the
signal(s) with a repetition and, optionally, different power levels in a
manner that a
base station employing the principles of the present invention determines a
request
to change communications states with a reasonably high probability of
detection and
a reasonably low probability of false detection.
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The teachings of the present invention are compatible with 1xEV-DV
systems and I-CDMA systems, but general enough to support systems
employing various other communications protocols used in wired and
wireless communications systems. Code Division Multiple Access
5 (CDMA) systems, such as IS-2000, and Orthogonal Frequency Division
Multiplexing (OFDM) systems, such as IEEE 802.11 a wireless local area
network (LAN), may employ an embodiment of the present invention.
Accordingly, in one aspect, the present invention provides a wireless user
device comprising: circuitry configured to transmit signals having different
formats in time intervals to a base station; wherein the transmitted signals
are
transmitted in the time intervals that the wireless user device is not
transmitting
user data; and wherein each time interval includes at least one slot; wherein
at
least one of the different formats is configured to include an indication that
the
wireless user device is requesting resources to transmit user data; wherein at
least two of the different formats are associated with different power levels;
wherein the circuitry is configured to receive information indicating
resources
to transmit user data in response to transmitting the indication requesting
resources.
In a further aspect, the present invention provides a method implemented
in a wireless user device, the method comprising: transmitting signals having
different formats in time intervals to a base station; wherein each time
interval
includes at least one slot; wherein the transmitted signals are transmitted in
the
time intervals that the wireless user device is not transmitting user data;
wherein at least one of the different formats include an indication that the
wireless user device is requesting resources to transmit user data; wherein at
least two of the different formats are associated with different power levels;
and receiving information indicating resources to transmit user data in
response to transmitting the indication requesting resources.
In a further aspect, the present invention provides a wireless user
device comprising: circuitry configured to transmit signals having different
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formats in time intervals to a base station; wherein the transmitted signals
are transmitted
in the time intervals that the wireless user device is not transmitting data;
and wherein
each time interval includes at least one slot; wherein at least one of the
different formats
is configured to include an indication that the wireless user device is
requesting resources
to transmit data; wherein at least two of the different formats are associated
with different
power levels; wherein a first power level is applied for a non-request state
and a second
power level is applied for a request state; wherein the circuitry is
configured to receive
information indicating resources to transmit data in response to transmitting
the
indication requesting resources; wherein the wireless user device is assigned
an
orthogonal sequence and the transmitted signals are derived from the
orthogonal
sequence; wherein signals for the non-request state and signals for the
request state are
transmitted in a mutually exclusive code channel in time slots when a
heartbeat signal is
at a logical zero.
In a still further aspect, the present invention provides a method implemented
in a
wireless user device, the method comprising: transmitting signals having
different formats in
time intervals to a base station; wherein each time interval includes at least
one slot; wherein
the transmitted signals are transmitted in the time intervals that the
wireless user device is
not transmitting data; wherein at least one of the different formats include
an indication that
the wireless user device is requesting resources to transmit data; wherein in
the time intervals
that the signals are transmitted no data is transmitted; wherein at least two
of the different
formats are associated with different power levels; wherein a first power
level is applied for
a non-request state and a second power level is applied for a request state;
wherein signals
for the non-request state and signals for the request state are transmitted in
a mutually
exclusive code channel in time slots when a heartbeat signal is at a logical
zero; and
receiving information indicating resources to transmit data in response to
transmitting the
indication requesting resources; wherein the wireless user device is assigned
an orthogonal
sequence and the transmitted signals are derived from the orthogonal sequence.
In a further aspect, the present invention provides a wireless user device
comprising:
circuitry configured to transmit signals having different formats in time
intervals to a base
station; wherein each time interval includes at least one slot; wherein at
least one of the
different formats is configured to include an indication that the wireless
user device is
requesting resources to transmit user data; wherein in the time intervals that
the signals are
transmitted no user data is transmitted; wherein at least two of the different
formats are
associated with a different power offsets; wherein the circuitry is configured
to receive
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information indicating resources to transmit user data in response to
transmitting the
indication requesting resources; and wherein the wireless user device is
assigned an
orthogonal sequence and the transmitted signals are derived at least from the
orthogonal
sequence.
In a further aspect, the present invention provides a method of wireless
communications comprising: transmitting signals having different formats in
time intervals
to a base station; wherein each time interval includes at least one slot;
wherein at least one of
the different formats is configured to include an indication that the wireless
user device is
requesting resources to transmit user data; wherein in the time intervals that
the signals are
transmitted no user data is transmitted; wherein at least two of the different
formats are
associated with a different power offset; wherein the wireless user device is
configured to
receive information indicating resources to transmit user data in a response
to transmitting
the indication requesting resources; and wherein the wireless user device is
assigned an
orthogonal sequence and the transmitted signals are derived at least from the
orthogonal
sequence.
In yet a further aspect, the present invention provides an apparatus for
wireless
communications comprising: means for transmitting signals having different
formats in time
intervals to a base station; wherein each time interval includes at least one
slot; wherein at
least one of the different formats is configured to include an indication that
the wireless user
device is requesting resources to transmit user data; wherein in the time
intervals that the
signals are transmitted no user data is transmitted; wherein at least two of
the different
formats are associated with a different power offset; wherein the wireless
user device is
configured to receive information indicating resources to transmit user data
in a response to
transmitting the indication requesting resources; and wherein the wireless
user device is
assigned an orthogonal sequence and the transmitted signals are derived at
least from the
orthogonal sequence.
Further aspects of the invention will become apparent upon reading the
following
detailed description and drawings, which illustrate the invention and
preferred embodiments
of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
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
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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.
Fig. 1 is a schematic diagram of a communications system in which an
embodiment
of the present invention may be deployed;
Fig. 2A is a schematic diagram of a subsystem employed by a base station in
the
communications system of Fig. 1 used to determine whether a reverse link
signal includes
an indication for a request to change communications states;
Fig. 2B is a flow diagram of a process executed by a state machine in the
subsystem
of Fig. 2A;
Fig. 3A is a signal diagram of alxEV-DV signal with a first marker indicating
'control hold' and a second marker indicating a 'request to go active';
Fig. 3B is a signal diagram of a code division multiple access (CDMA) set of
code
channels having a marker in an assigned time slot that indicates that the
field unit is
requesting a change in communications states;
Fig. 3C is a signal diagram of an alternative embodiment of a reverse link
signal
having the indications; and
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Fig. 4 is a plot of signal-to-noise ratio versus probability of detection that
may be used in determining energy levels of the indications in the signals of
Figs.
3A-3C.
DETAILED DESCRIPTION OF THE INVENTION
A description of preferred embodiments of the invention follows.
The cost of missed or erroneous detection of Heartbeat (11E) and Heartbeat
with Request to Go Active (11B/RQST) signals is costly. If a false detection
occurs
for HB, power control commands and timing coramands used between a base
station
and field terminal may be generated based on a received code phase that is not
correct. Thus, the power control can be erroneous and not based on the actual
received power from the temninaL For the request message, resources will be
assigned to a user when the resources are not needed, which results in wasted
capacity.
Traditionally, if a very low probability of false detection is important, a
requirement of a very high Eb/No (i. e., energy-per-bit per noise density)
threshold
at the Base Transceiver Station (BTS) is imposed. As an alternative, if the
speed of
detection is less important, as in the case of the HE signal, multiple
successive
detections may be useful. This allows the probability of false detection to be
greatly
reduced.
For instance, if the P(fd) = 0.01 and if three detections in a row is
specified
to be made before a "Valid Detection" is determined, the overall P(fd) .=
(0.01)3 or
0.000001. This is less costly for detection as the probability is much higher
to start
with. For instance, if the single detection probability is 0.9, requiring
three
detections lowers the detection probability to 0.93 or 0.72, which is only a
slight
reduction. This technique is known in radar systems, but has not been used in
this
application for detecting HB and I-IB/RQST signals and other communications
systems and applications. It should be understood that the HB and HB/RQST
signals
are examples of signals to which the teachings of the present invention may be
applied and are not intended to be limiting in any way.
=
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The signals to be detected and counted (i) may be successive-either in time
or by a user allocated slot in a TDMA system, for example-or (ii) may have
breaks
between the signals but have a given number of pulses, bits, or other
indicators in a
given time interval. For a CDMA reverse link, multiple serial detections or
non-
serial detections may be used to qualify as a system level detection. Further,
the
system may set a different power control target versus a detection target,
which
means that for a. lower transmission power, integration time is increased to
increase
energy for detection. For a system that uses time slots, the system may
include
intelligence to monitor successive or non-successive time slots for the given
user. In
addition, the system worlcs on gated and non-gated signals.
The interference level of the heartbeats is derived as a classical RADAR
detection problem. To this end, the benefits are made possible based on the
heartbeat
pulses being "detected" rather than being demodulated as in the case with the
Dedicated Control Channel (DCCH) and Slotted Control Hold Mode (DCHM) in
CDMA technology.
Fig. 1 is a diagram of an example communications system 100, similar to the
system described above, employing an embodiment of the present invention. A
base
transceiver station (BTS) 25 with antenna tower 23 maintains wireless
communications links with each of a plurality of field units 42a, 42b, 42b
(collectively, 5e1d units 42) as shown. Such wireless links are established
based
upon assignment of resources on a forward link 70 and a reverse link 65
between the
base station 25 and field units 42. Each link 65 or 70 is typically made up of
several
logical reverse link channels 55 and several logical forward link channels 60,
respectively.
As sh.own, the communications system 100 supports wireless
communications between an interface 50 and a network 20. Typically, the
network
20 is a Public Switched Telephone Network (PSTN) or computer network, such as
the Internet or intranet. The interface 50 is preferably coupled to a digital
processing
device, Stith as a portable computer 12, sometimes referred to as an access
unit, to
provide wireless access to the network 20. Consequently, the portable
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computer 12 has access to the network 20 based on communications over a
combination of both hard-wired and wireless data links.
In a preferred embodiment, the forward link channels 60 and reverse link
channels 55 are defined in the communications system 100 as Code Division
Multiple Access (CDMA) channels. That is, each CDMA channel is preferably
defined by encoding and transmitting data over the channel with an augmented
pseudo random noise (PN) code sequence. The PN coded data is then modulated
onto a radio frequency carrier. This enables a receiver to decipher one CDMA
channel from another knowing only the particular augmented PN code assigned
for a
given channel. In accordance with an embodiment, each channel preferably
occupies a 1.25 MHZ band consistent with the IS-95 CDMA standard or 1xEV-DV
standard and is capable of transmitting at 38.4 kbps.
A forward link 70 includes at least four, logical, forward link channels 60.
As shown, this includes a Pilot Channel 60PL, Link Quality Management (LQM)
channel 60L, paging channel 60PG and multiple traffic channels 60T.
The reverse link 65 includes at least five logical channels 55. As shown, this
includes a heartbeat standby channel 55HS, heartbeat request active channel
55HRA, access channel 55A and multiple traffic channels 55T. Generally, the
reverse link channels 55 are similar to the forward link channels 60 except
that each
reverse link traffic channel 60T can support variable data rates from 2.4 kbps
to a
maximum of 160 kbps.
Data transmitted between the base station 25 and field unit 42a typically
include encoded digital information, such as web page data. Based on the
allocation
of multiple traffic channels in the reverse link 65 or forward link 70, higher
data
transfer rates can be achieved in a particular link between the base station
25 and
field unit 42a. However, since the field units 42 compete for bandwidth
allocation, a
field unit 42a may have to wait until resources are free to be assigned
traffic
channels to transmit a data payload.
Before discussing an example detector system (Fig. 2) that can be used to
distinguish a heartbeat from a heartbeat-with-request signal, a brief
discussion of
example signals will be discussed in reference to Figs. 3A-3C.
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In Fig. 3A, a 1xEV-DV signal 160 that may be transmitted by the field unit is
shown having three distinct states: a 'control hold' state 165, a 'request to
go active'
state 170, and a data traffic state 175. In the 'control hold' state 165, the
signal 160
does not include a 'request to go active' indication. In other words, the
signal 160
remains in an 'idle' or 'control hold' state, which indicates that the field
unit 42a is
not requesting traffic channels. The 'request to go active' state 170 is an
indication
that the field unit is requesting to transmit data on a traffic channel over a
reverse
link to the BTS 25. In the traffic state 175, traffic data is transmitted by
the field
unit to the BTS. Following transmission of the traffic data over the reverse
link, the
signal 160 reverts back to the 'control hold' state 165 following a
transmission of a
'data transmission complete' state (not shown).
Although shown as a single signal 160, it should be understood that the
signal may be multiple signals, optionally coded with orthogonal or non-
orthogonal
codes into mutually exclusive channels. For example, the 'control hold' state
165
may be transmitted on a different channel from the 'request to go active'
state 170.
Similarly, the traffic data transmitted in a transmit state 175 may be on a
separate
channel from the other two states 165, 170. An example of multiple channel is
discussed in reference to Figs. 3B and 3C.
Fig. 3B is an example of an Internet code division multiple access (I-CDMA)
signaling diagram that has assigned time slots for users l, 2, 3, ... , N
repeating in
epoch i 177a, epoch i+/ 177b, and so forth. The channels are composed of the
heartbeat channel 55H, request channel 55R, and traffic channels 55T. Each of
these
channels has an associated code CI, C2, C3, C4, CN, which allow
signals to be
transmitted on mutually exclusive code channels. Both the transmitting and
receiving systems process the information in the channels by using the codes
to
separate the information respectively included therein in a typical CDMA
mariner.
In the example shown, users 1, 2, 4, 5, 6, ..., N are requesting to remain in
an
idle state, indicated by the presence of a signal 180 in the heartbeat channel
55H.
User 3, however, is requesting to transmit data over a reverse link based on a
signal
185 in the request channel 55R in the first epoch'177a, a signal 185b in the
request
channel 55R in the second epoch 177b, and possibly additional epochs. In the
third
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epoch 177c, the BTS 25 has detected the request to transmit data based on the
two
consecutive indications I85a and 185b. Following receipt of an acknowledgment,
user 3 begins to transmit traffic data 190 in an associated traffic channel
using code
C5. In an alternative embodiment, the BTS 25 may require three consecutive
indications 185a through 185c before determining that a request is being made
and
acknowledging same.
Fig. 3C is a more detailed signal diagram of the 1xEV-DV signal of Fig. 3A
that is used to indicate a 'request to go active' to the base station 25 from
the field
unit 42a. In this embodiment, the 1xEV-DV signal is composed of multiple
signals
on different logical channels: a heartbeat channel 55H and a request channel
55R.
The heartbeat channel 55H provides continuous timing and other information
(e.g.,
power level, synchronization, etc.) from the field unit 42a to the base
station 25.
The field unit 42a uses the request channel 55R to make a request (e.g.,
digital "1")
of the base station 25 to request a traffic channel on the reverse link 65 for
transmitting data.
Sampling times 195a, 195b, ..., 195f (collectively 195) denoted by arrows
indicate times or intervals at which the BTS 25 samples the time slots of the
request
signal 55R and, optionally, the heartbeat channel 55H to determine whether a
request for a traffic channel is being made. It should be understood that the
sampling may occur over the entire time slot or a subset thereof. Also, the
heartbeat
channel 55H and request channel 55R use mutually exclusive codes, in this
particular embodiment, so the sampling is performed on their mutually
exclusive
code channels 55H, 55R in all or a subset of time slots. In one particular
embodiment, the base station 25 samples mutually exclusive code channels 55H,
55R in time slots designated for request indications, such as in time slots at
sampling
times 195b, 195d, and 195f. During these time slots, the heartbeat channel 55H
is
"inactive", but the request channel 55R is "active".
As discussed above, the signals in the "active" request time slots may be
modulated messages or simply coded pilot signals with no "bits". Thus,
detection
may be based solely on the respective energy levels of the heartbeat and
heartbeat-
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with-request signals in respective time slots over a given time interval or
spanning
several time intervals.
In one particular embodiment, the 'control hold' state 165 indication has a
first energy level, and the 'request to go active' state 170 has a second
energy level.
The base station 25 may take advantage of the difference in power levels in
addition
to the repetition of the pulses used to indicate a request to go active. For
example, in
this particular embodiment, distinguishing the two states may be a matter of
measuring energy levels of the signals(s) and (i) comparing the energy levels
against
at least one threshold or (ii) determining that a request is present,
optionally in a
mutually exclusive code channel in time slots when the heartbeat signal is at
a
logical zero. The different energy levels of the indications may be provided
by the
duty cycle of the signals, frequency of the signals, power of the signals,
signaling
structure, and so forth.
To understand how the energy levels of the sigaals can be used to improve
system performance, one can refer to Fig. 4, which provid.es a chart for
selecting
signaling requirements based on the foU.ovving parameters or factors: (i)
probability
of detection, P(d) (x-axis), (ii) signal-to-noise ratio in decibels (y-axis),
and (iii)
probability of false detection, P(fd) (curves in the chart). This chart shows
a required
sigial-to-noise ratio at the input terminals of a linear-rectifier detector as
a function
of probability of detection for a single pulse, with the false-alarm
probability P (fd)
as a parameter, calculated for a non-fluctuating signal. It should be
understood that
alternative parameters or factors may be used to establish or define the
'transmitted
power levels of the indications.
At the circled point 200, the signal-to-noise ratio is 3 dB, P(d) = 20%, and
P(fd) 1 %. To increase the probability of detection for the same probability
of false
detection, one simply needs to slide the circled point 200 upward along the
same
probability of faLse detection curve, which suggests that an increase in the
signal-to-
noise ratio is used to improve system performance and, thus, improving the
likelihood that the request sigaal will be detected quickly..
Before providing an example model and discussion regarding example
Heartbeat standby 5511S and Heartbeat Request Active 55ERA energy levels for
the
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example communications system 100 (Fig. 1), a brief discussion of a processor
and
detector that may be used in the system is now provided.
Fig. 2A is a schematic diagram of a request detection processor 110 used to
determine whether the field unit 42a has requested to send data to the BTS 25.
The
receiver Rx 35 receives signals 55, which includes the maintenance channel
55M,
traffic channels 55T, access channel 55A, heartbeat standby channel 551-IS,
and
heartbeat request active channel 55HRA. The signal 55 is processed such that a
heartbeat channel processor 112 receives the heartbeat standby charnel 55HS
and a
request ehannel processor 114 receives the Heartbeat Request Active channel
55HRA.
The heartbeat channel processor 112 and request channel processor 114
include the same processing elements, in this particular embodiment, so a
discussion
ofjust the heartbeat channel processor 112 will be provided for brevity.
The heartbeat channel processor 112 receives the heartbeat standby channel
55HS. A correlator 115 uses a despreader 120 to despread the heartbeat standby
channel 55HS. An integrator 125 is used to coherently combine the heartbeat
signal.
By coherently combining the signal, an integration of I, Q and its phase
causes the
phase of the signal to be removed and output the power of the signal.
Following the correlator 115, a rectifier 130 (i. e., absolute value of the
signal squared) rectifies the power of the signal, which is then integrated by
a second
integrator 135 to calculate the energy of the received heartbeat signal. The
second
integrator 135 provides a non-coherent combination of the signal, which is
calculated over short time intervals. The non-coherent integration provides
just
magnitudes if the terminal is moving too fast, thus causing a cross-over of
the 180-
degree phase point, which can cause ambiguities in determining the energy of
the
signal in the absence of the non-coherent combination.
The output from the heartbeat channel processor 112 is a heartbeat energy
level, and the output from the request channel processor 114 is a request
energy
level. Each of these energy levels, in this particular embodiment, is fed to a
hypothesis detector 140, which determines whether a heartbeat signal, request
signal, or neither signal is in the signal 55 received by the base station 25.
CA 02882928 2015-02-24
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The output from the hypothesis detector 140 is provided to a state machine
145. The state machine is used to determine whether the field unit is making a
'request to go active' according to a given criteria, where, in one particular
embodiment, is a measurement of the output from the hypothesis detector 140.
Example measurements include counting the number of consecutive request
signals,
measuring a ratio of heartbeat standby channel signals and heartbeat request
active
channel signals, counting heartbeat request active signals in a given time
span, and
so forth. Further, the hypothesis detector 140 and the difference in energy
levels of
the indications improves system performance, but are not required for the
present
invention. In other words, the heartbeat standby channel 55HS and heartbeat
request
active channel 551IRA may be processed directly by the state machine 145 to
determine whether the field unit 42a is requesting to go active. More detail
is
provided following a description of an embodiment of the state machine 145.
In this particular embodiment, the state machine 145 outputs a Boolean true
or false signal. An example of a process executed by the state machine is
depicted
in Fig. 2B.
Fig. 2B is an example flow diagram of the state machine 145. The example
state machine 145 starts in step 205 when the detection processor 110 "boots
up." In
step 210, the state machine 145 initializes counters that are used to
determine if a
detection has occurred. In step 215, the state machine 145 receives the output
from
the hypothesis detector 140. After boot up, the state machine 145 may act as
an
'interrupt service routine', beginning in step 215, upon receipt of any output
from
the hypothesis detector 140. The counters are cleared (i.e., set to zero) upon
a
determination of a detection or a non-detection to reset the measurement
process
without a re-boot of the detection processor 110, as discussed below.
Following receipt of the output from the hypothesis detector 140 in step 215,
the state machine 145 determines whether the output of the hypothesis detector
145
is a request (i.e., 'request to go active'). If yes, then the state machine
145 continues
in step 240 in which a detection counter is incremented. In step 245, the
detection
counter is compared against a threshold. If the detection counter exceeds the
threshold, then, in step 250, the state machine 145 reports a detection of a
'request to
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go active' from the field unit 42a. If the detection counter does not exceed
the
threshold, then the state machine 145 returns to step 215 and waits to receive
another
output from the hypothesis detector 140.
Continuing to refer to Fig. 2B, if, in step 220, the output of the hypothesis
detector 140 is determined not to be a 'request', then the state machine 145
continues in step 225. In step 225, the state machine 145 increments a non-
detection
counter. In step 230, a determination is made as to whether the non-detection
counter exceeds a threshold. If yes, then the state machine 145 continues in
step
235, in which the state machine 145 reports a non-detection of a 'request to
go
active' by the field unit 42a. If the non-detection counter does not exceed
the
threshold, then the state machine 145 continues in step 215.
Following steps 235 and 250, the state machine 145 clears the counters in
step 255, allowing the state machine 145 to detect future 'requests to go
active' by
the field unit 42a. In step 260, the state machine 145 ends.
The detection counter is used by the state machine 145 to determine how
many indications of a 'request to go active' have been received by the
detection
processor 110 according to a given criteria. The criteria can be of any form,
including a given number of consecutive detections, a given number of
detections in
a given time span, or a ratio of detections to non-detections. Alternative non-
counting based measurements may be employed to determine whether a request is
being made to go active, such as measuring the phase of the request signals.
It should be understood that alternative embodiments of using counters or
other criteria may be used by the state machine 145. For example, the state
machine
145 may use other process flows, non-counter variables, or other standard or
non-
standard techniques for determining a detection. Further, rather than
receiving the
output from the hypothesis detector 140, the input to the state machine 145
may be
raw data from the heartbeat channel processor 112 or request channel processor
114.
Further, in an alternative embodiment, the state machine 145 may be included
in
combination with the hypothesis detector 140.
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Referring again to Fig. 2A, in addition to using the state machine 145 to
ascertain with high probability whether the field unit 42a is malcing a
'request to go
active', the hypothesis detector 140 is also employed.
To determine which signal (s) is/are present, the hypothesis detector 140
includes logical functions. For example, in this particular embodiment, the
hypothesis detector 140 compares a first energy level threshold against the
first
energy level (i, e., heartbeat energy level) and compares a second energy
level
threshold against the second energy level (i. e., request energy level).
Example energy level thresholds against which to compare the heartbeat
energy level and the request energy level are 9 dB and 11 c1B, respectively.
The
energy level thresholds may be dynamically selected, predetermined, or applied
in
another manner, such as based on a transmitted power level, which may be
reported
by the field unit to the base station over the heartbeat standby channel 55HS,
for
instance. In the case of the energy level calculation and comparison, the
first and
second energy levels may be dependent on occupancy of time slots in the
signaling
channel(s) used by the signal 55, so the energy level thresholds can be based
on an
expected or specified number of "1" bits used to indicate a 'request to go
active' or to
indicate a request to remain in idle mode. Use of the energy level thresholds
is
discussed in related U.S. Patent No. 7,221,664 entitled "Transmittal of
Heartbeat Signal
At A Lower Than Heartbeat Request," by Proctor, J.
As discussed above, the output of the hypothesis detector 140 is measured by
the state machine 145 to determine whether to change the state of the
communications system, which is the state of reverse link traffic chaimels
between
the field unit 42a and the base station 2$. For example, if the hypothesis
detector 140
determines that a 'request to go active' (i. e., send a data transmission on
the reverse
link) is being made by the field unit 42a, then the state machine 145 outputs
a signal
to a processor (not shown) in the BTS 25 that is responsible for providing the
portable computer 12 with a traffic channel 55T. In one particular embodiment,
the
BTS 25 allocates the traffic channel 55T if the number of consecutive request
signals is determined to be two or more consecutively. Alternative criteria
have been
discussed above.
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As described in reference to Fig. 3C, the heartbeat channel processor 112,
request channel processor 114, and hypothesis detector 140 may be configured
or
designed in a manner that monitors an occupancy of time slots used to indicate
the
request to change communications states. In one embodiment, the detecting
includes monitoring occupancy of mutually exclusive code channels, such as
shown
in Figs. 3B and 3C.
A feedback loop (not shown) may be employed to cause the heartbeat
channel processor 112 and request channel processor 114 to be "adaptive". For
example, based on the received energy level of the heartbeat channel 55H, the
integration time of the integrators 125, 135 may be adjusted, and the energy
level
thresholds used by the hypothesis detector 140 for comparison of the energy
levels
of the heartbeat and request signals may also be adjusted by the feedback
loop.
Other feedback may cause (i) the number of consecutive pulses required for a
detection to be increased or decreased or (ii) the number of transmitted
request
signals to be increased or decreased. Such a feedback loop may use a command
or
message to transfer information between the BTS 25 and field unit 42a that
includes
information regarding the pulse repetitions or power levels of the heartbeat
and
heartbeat-with-request signals transmitted by the field unit 42a.
As discussed above, the first communications state may be a standby state
and the second communications state may be a payload state. In other systems
or
even the same system, the communications states may refer to other
communications
states, such as a request to change base stations, power control signaling,
and so
forth. The use of different signal repetitions or energy levels in signaling
as
described herein is applicable to wireless, wired, or optical communications
systems.
In either case, the communications states may be used in voice or data
conununications systems.
As also discussed above, the second energy level may be based on a target
probability of detection, false detection, or combination of both as discussed
in
reference to Fig. 4. In other words, the field unit may transmit the request
signal at a
given power level or a given number of pulses per given time period to achieve
a
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corresponding signal-to-noise ratio for a given target probability of
detection, false
detection, or both as discussed in reference to Fig. 4.
An analysis may be used to set the transmission power or number of
transmitted indications, or the feedback mechanism discussed above may be
employed in the communications system for causing the field unit to change its
behavior so as to have the received energy levels of the indications achieve a
predetermined signal-to-noise ratio, thus providing the desired probability of
detection and false detection parameters.
SIMULATION
1.0 A simulation for a reverse link was conducted where the reverse link
is
assumed to have power control and a heartbeat channel of any of the example
types
shown in Figs. 3A-3C or another type of communications link signaling.
First, there are two assumptions that have been made for this simulation.
First, power control is used on a combination of detected paths or in a single
path.
Power control is perfomied even when a positive detection is not achieved.
Second,
the probability of detection was set to achieve detection at a high enough
rate to
ensure that power control is performed on the comet signal. To clarify,
detection is
required to track the received signal.
Table 1 shows the rate of detection required for a single path channel from a
vehicle moving away from the base station at 60mph. This table requires that
there is
at least one detection per slew of a chip due to movement.
TABLE 1
Slew Distance for 1 chip 814 ft
Handset Velocity 60 mph
Handset Velocity 88 ft/s
Chip Slew Rate9.2 chips/s
________________________________________ ' __________________
Heartbeat Rate 50 1113/s
Heartbeats/Td 462
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In Table 1, the time period Td is defined as the period over which a single
heartbeat pulse must be detected to ensure the signal is tracked as the time
of arrival
of the signal is skewed due to movement of the vehicle. Table I shows that one
out
of every 462 pulses must be received with a very high probability or there is
a risk of
losing the tracking of the signal.
Based on this calculation, the threshold of detection was set from a table of
detection/false detection probabilities (e. g., Fig. 4). While Table 1 is
defined for
Additive White Gaussian Noise (AWGN), it is expected that the probabilities of
detection are not greatly affected over a relatively short arn.ount of time.
This is due
to the statistical independence of the fading from heartbeat pulse to
heartbeat pulse.
While the individual pulse probabilities of detection varied significantly,
the
overall results were not seen to vary significantly by more than a factor of
roughly
50% in the latency of detection. Specifically, the average detection latency
for the
request message in AWGN was 1 lms as compared to rougbly 15ms for 30Ian/hr.
Again, this result is due to requiring a detection process rather than a more
difficult
demodulation process.
= Based on this analysis, a probability of detection of 20% and false
detection
of 1% was selected. This requires an average Eb/No of 3dIa. This is shown and
discussed in reference to Fig. 4.
Table 2 shows a calculation of the probability of detection and false
= detection during the time Td defined above.
TABLE 2 =
Target EA (entire energy/interference density) - 3dB
Probability of detection 0.2
Probability of false detection 0.01
Probability of detection for 3 Sequential HB 8.00E-03
Number of trials in Td 462
Probability of no detection in Td 2,44E-02
Probability of false detection for 3 sequential 1.00E-06
Required no false detection trials 462
Probability of false detection for Td 4.62E-04
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To reduce the probability of false detection, three sequential detections were
required to validate a single detection. Since the probability of false
detections is
multiplicative in this case, the probability of a single false detection is
cubed.
Table 3 below calculates the average Ecao (energy-per-chip per the
interference density, which is the SNR integrated over the entire chip)
required to
achieve the statistics of Table 2.
TABLE 3
Target Rao 3 dB
Processing Gain 256
Burst Ec/lo -21.08 dB
Average Ecilo -40.9 dB I
Since the heartbeat channel is time division multiplexed (TD/vI) in structure,
the interference to all other users due to heartbeat users increases as
follows:
Effective average Ec/lo (all H33 users) = 10* loglO(N)-40. 9, where N is the
number of users.
Thus, for 96 users of a given base station, the average total interference
will
equal the burst Ealo or-21.08dB.
While this invention has been particularly shown and described with
references to prefen-ed 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.
