U.S. patent application number 09/150493 was filed with the patent office on 2001-09-06 for energy based communication rate detection system and method.
Invention is credited to JOU, YU-CHEUN, NICLAS, JUIEN, NICOLAS, JULIEN, SILBERGER, AMNON.
Application Number | 20010019541 09/150493 |
Document ID | / |
Family ID | 22534795 |
Filed Date | 2001-09-06 |
United States Patent
Application |
20010019541 |
Kind Code |
A1 |
JOU, YU-CHEUN ; et
al. |
September 6, 2001 |
ENERGY BASED COMMUNICATION RATE DETECTION SYSTEM AND METHOD
Abstract
A system and method for the detection of zero-rate communication
frames on forward and reverse communication links is described
herein. In a first embodiment, the rate determination process
begins upon receiving pilot and traffic channel signals. The system
then compares the received traffic channel energy with the pilot
signal to obtain a metric. The metric is then compared with a
predetermined threshold to detect the presence of a zero-rate
traffic channel frame. In a second embodiment, the process begins
with the receipt of a reverse communication link frame having first
and second portions. The system then detects the energy contained
in the first and second portions. The system compares the detected
energy in the first portion with the detected energy in the second
portion to obtain a metric. The system then compares the metric
with a predetermined threshold to detect the presence of the
zero-rate frame. The selection of the predetermined threshold in
both embodiments is based on the probability of detection of a
transmitted zero-rate frame and the probability of erroneously
detecting a zero-rate frame.
Inventors: |
JOU, YU-CHEUN; (SAN DIEGO,
CA) ; SILBERGER, AMNON; (LA JOLLA, CA) ;
NICOLAS, JULIEN; (LA JOLLA, CA) ; NICLAS, JUIEN;
(SAN DIEGO, CA) |
Correspondence
Address: |
Qualcomm Incorporated
Patents Department
5775 Morehouse Drive
San Diego
CA
92121-1714
US
|
Family ID: |
22534795 |
Appl. No.: |
09/150493 |
Filed: |
September 9, 1998 |
Current U.S.
Class: |
370/311 ;
455/522 |
Current CPC
Class: |
H04B 7/216 20130101;
H04B 17/318 20150115 |
Class at
Publication: |
370/311 ;
455/522 |
International
Class: |
G08C 017/00 |
Claims
What Is claimed Is:
1. A method for detecting a zero-rate traffic channel, comprising
the steps of: receiving a pilot channel frame and a traffic channel
frame; measuring an energy of said traffic channel frame; measuring
an energy of said pilot channel frame; comparing said pilot channel
frame energy to said traffic channel frame energy to obtain a
metric; comparing said metric with a predetermined threshold to
obtain a result, said predetermined threshold based on the
probability of detection of a transmitted zero-rate traffic channel
frame and the probability of erroneously detecting a zero-rate
traffic channel frame; and indicating whether a zero-rate traffic
channel frame has been received based on said result.
2. The method of claim 1, wherein said step of comparing said pilot
channel frame energy to said traffic channel frame energy comprises
the step of computing a ratio of said traffic channel frame energy
to said pilot channel frame energy.
3. The method of claim 2, wherein said step of comparing said
metric with a predetermined threshold comprises the step of
comparing the magnitude of said metric to the magnitude of said
predetermined threshold.
4. The method of claim 3, wherein said indicating step comprises
the step of indicating that a zero-rate traffic channel frame has
been received when the magnitude of said predetermined threshold
exceeds the magnitude of said metric.
5. A system for detecting a zero-rate traffic channel, comprising:
means for receiving a pilot channel frame and a traffic channel
frame; means for measuring an energy of said traffic channel frame;
means for measuring an energy of said pilot channel frame; means
for comparing said pilot channel frame energy to said traffic
channel frame energy to obtain a metric; means for comparing said
metric with a predetermined threshold to obtain a result, said
predetermined threshold based on the probability of detection of a
transmitted zero-rate traffic channel frame and the probability of
erroneously detecting a zero-rate traffic channel frame; and means
for indicating whether a zero-rate traffic channel frame has been
received based on said result.
6. The system of claim 5, wherein said means for comparing said
pilot channel frame energy to said traffic channel frame energy
comprises means for computing a ratio of said traffic channel frame
energy to said pilot channel frame energy.
7. The system of claim 6, wherein said means for comparing said
metric with a predetermined threshold comprises means for comparing
the magnitude of said metric to the magnitude of said predetermined
threshold.
8. The system of claim 7, wherein said means for indicating
comprises means for indicating that a zero-rate traffic channel
frame has been received when the magnitude of said predetermined
threshold exceeds the magnitude of said metric.
9. A method for detecting a zero-rate traffic channel, comprising
the steps of: receiving a traffic channel frame having first and
second portions; measuring an energy of said first portion;
measuring an energy of said second portion; comparing said energy
of said first portion with said energy of said second portion to
obtain a metric; comparing said metric with a predetermined
threshold to obtain a result, said predetermined threshold based on
the probability of detection of a transmitted zero-rate frame and
the probability of erroneously detecting a zero-rate frame, to
detect the presence of a zero-rate frame; indicating whether a
zero-rate traffic channel frame has been received based on said
result.
10. The method of claim 9, wherein said step of comparing said
energy of said first portion with said energy of said second
portion comprises the step of: computing a weighted difference
between said energy of said first portion said energy of said
second portion.
11. The method of claim 10, wherein said step of comparing said
metric with a predetermined threshold comprises the step of:
comparing said weighted difference to said predetermined
threshold.
12. The method of claim 11, wherein said indicating step comprises
the step of: indicating that a zero-rate traffic channel frame has
been received when said weighted difference exceeds said
predetermined threshold exceeds the magnitude of said metric.
13. A system for detecting a zero-rate traffic channel, comprising:
means for receiving a traffic channel frame having first and second
portions; means for measuring an energy of said first portion;
means for measuring an energy of said second portion; means for
comparing said energy of said first portion with said energy of
said second portion to obtain a metric; means for comparing said
metric with a predetermined threshold to obtain a result, said
predetermined threshold based on the probability of detection of a
transmitted zero-rate frame and the probability of erroneously
detecting a zero-rate frame, to detect the presence of a zero-rate
frame; means for indicating whether a zero-rate traffic channel
frame has been received based on said result.
14. The system of claim 13, wherein said means for comparing said
energy of said first portion with said energy of said second
portion comprises means for computing a weighted difference between
said energy of said first portion said energy of said second
portion.
15. The system of claim 14, wherein said means for comparing said
metric with a predetermined threshold comprises means for comparing
said weighted difference to said predetermined threshold.
16. The system of claim 15, wherein said means for indicating
comprises means for indicating that a zero-rate traffic channel
frame has been received when said weighted difference exceeds said
predetermined threshold exceeds the magnitude of said metric.
Description
BACKGROUND OF THE INVENTION
[0001] I. Field of the Invention
[0002] The present invention relates generally to detection of data
rates in communication signals. More specifically, the present
invention relates to a system and method for detecting when
zero-rate data frames in communication signals are received at a
communications receiver.
[0003] II. Related Art
[0004] A typical satellite-based communications system comprises at
least one terrestrial base station (hereinafter referred to as a
gateway), at least one user terminal (for example, a mobile
telephone), and at least one satellite for relaying communications
signals between the gateway and the user terminal. The gateway
provides links from a user terminal to other user terminals or
communications systems, such as a terrestrial telephone system.
[0005] A variety of multiple access communications systems and
techniques have been developed for transferring information among a
large number of system users. These techniques include time
division multiple access (TDMA), frequency division multiple access
(FDMA), and code division multiple access (CDMA) spread-spectrum
techniques, the basics of which are well known in the art. The use
of CDMA techniques in a multiple access communications system is
disclosed in U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990,
entitled "Spread Spectrum Multiple Access Communication System
Using Satellite or Terrestrial Repeaters," and U.S. Pat. No.
5,691,974, which issued Nov. 25, 1997, entitled "Method and
Apparatus for Using Full Spectrum Transmitted Power in a Spread
Spectrum Communication System for Tracking Individual Recipient
Phase Time and Energy", which are both incorporated herein by
reference.
[0006] The above-mentioned patent documents disclose multiple
access communications systems in which a large number of generally
mobile or remote system users employ user terminals or mobile
stations to communicate with other system users or users of other
connected systems, such as a public telephone switching network.
The user terminals communicate through gateways and satellites
using CDMA spread-spectrum type communications signals.
[0007] Communications satellites form beams which illuminate
"spots" produced by projecting satellite communications signals
onto the Earth's surface. A typical satellite beam pattern
comprises a number of beams arranged in a predetermined coverage
pattern. Typically, a number of CDMA channels (also referred to as
sub-beams), each occupying a different carrier frequency, are
transmitted in a beam.
[0008] In a typical spread-spectrum communications system, a set of
preselected pseudo-random noise (PN) code sequences is used to
spread information signals over a predetermined spectral band prior
to modulation onto a carrier frequency for transmission. PN
spreading, a method of spread-spectrum transmission that is well
known in the art, produces a signal for transmission that has a
bandwidth much greater than that of the underlying data signal
being transmitted. In a forward communications link (that is, in a
communications link originating at a gateway and terminating at a
user terminal), different PN spreading codes or binary sequences
are used to distinguish signals transmitted by one or more gateways
over different beams. These PN codes are shared by all
communications signals within a given sub-beam.
[0009] In a typical CDMA spread-spectrum system, different
channelizing codes are used to discriminate between signals
intended for different user terminals within a sub-beam on the
forward link. That is, a unique orthogonal code channel is provided
for each user terminal on the forward link by using a unique
"channelizing" orthogonal code. Walsh functions, or equivalent
codes, are generally used to implement the channelizing codes.
[0010] Typical CDMA spread-spectrum communications systems, such as
disclosed in U.S. Pat. No. 4,901,307, contemplate the use of
coherent modulation and demodulation. In mobile communications
systems using this approach, a "pilot" carrier signal (hereinafter
referred to as a "pilot signal") is used as a phase reference for
demodulation. When this approach is used on the forward link, a
pilot signal, which typically contains no data modulation, is
transmitted by a gateway throughout a coverage region. A single
pilot signal is typically transmitted by each gateway for each beam
on each frequency (sub-beam) used. These pilot signals are shared
by all user terminals receiving signals from the gateway.
[0011] In the forward link, pilot signals are used by user
terminals to obtain initial system synchronization and as a time,
frequency, and phase reference for demodulating other signals
transmitted by the gateway. Phase information obtained from a pilot
signal is used as a carrier phase reference for coherent
demodulation of other system signals or traffic signals. Since all
forward link signals on a sub-beam are transmitted synchronously,
this technique allows all traffic signals on a sub-beam to share a
common pilot signal as a phase reference, providing for a less
costly and more efficient signal acquisition or tracking
mechanism.
[0012] When a user terminal is not involved in a two-way
communications session (that is, the user terminal is not receiving
and transmitting traffic signals simultaneously), the gateway can
convey information to that particular user terminal using a signal
channel known as a paging channel. For example, when a call has
been placed to a particular mobile station or user terminal, phone,
the gateway alerts the mobile station by means of a message on the
paging channel. Paging channel messages are also used to distribute
information about traffic channel assignments, access channel
assignments, system configuration, and the like.
[0013] A user terminal can respond to a paging channel message by
sending an access channel message over the reverse link (that is,
the communications link originating at the user terminal and
terminating at the gateway). The access channel is also used by a
user terminal when it originates a call, that is requests that a
communication link be established.
[0014] In a preferred embodiment, data on the forward and reverse
links are transmitted at a variable rate. During conversations as
well as data transmissions, there are often pauses and periods of
intermittent activity that generate little information to be
transmitted. On a duplex communication link, such as voice
telephone links, often one party is speaking and the other is
listening. For voice telephone links, therefore, one of the two
links is idle almost half of the time, and can be transmitted using
a lower data rate. Additionally, during pauses or periods of
intermittent activity, data on forward and reverse links are
transmitted at a lower rate. This lower data rate can approach or
be equal to zero, that is, no data, in some situations.
[0015] Variable rate data transmission results in many benefits in
a multiple access system. Detection of the transmitted signal by
the receiver depends on signal energy. By decreasing the rate of
data transmission, signal energy can be maintained with a lower
average transmission power. This is very useful for power limited
applications, such as in mobile transceivers. In such cases,
reducing the average transmitter output power increases the time
before power sources such as batteries must be recharged or
replaced, and overall battery life. Additionally, lower power
reduced rate transmission reduces the interference and background
noise seen by other receivers on the sub-beam. Since transmitters
are power controlled to maintain a constant signal energy to noise
power ratio at the receiver, lower background noise power results
in reduced signal energy requirements. Lower background noise
power, therefore, allows other transmitters on the sub-beam to
transmit at lower power. Reduced interference may also allow
desired additional user capacity in a communication system.
[0016] What is needed, therefore, is a system and method that allow
the transmission and detection of zero-rate data transmission on
the forward and reverse communication links.
SUMMARY OF THE INVENTION
[0017] The present invention satisfies the above-mentioned needs by
providing a system and method for the detection of zero-rate
communication frames on forward and reverse communication links.
The method for detecting a zero-rate traffic channel, comprises the
steps of receiving a pilot channel frame and a traffic channel
frame, measuring an energy of the traffic channel frame, measuring
an energy of the pilot channel frame, and comparing the pilot
channel frame energy to the traffic channel frame energy to obtain
a metric. Generally, the frames are compared by computing a ratio
of the traffic channel to the pilot channel frame energies. The
metric is compared with a predetermined threshold to obtain a
result, with the predetermined threshold being based on the
probability of detection of a transmitted zero-rate traffic channel
frame and the probability of erroneously detecting a zero-rate
traffic channel frame. This is generally accomplished by comparing
the magnitude of the metric to the magnitude of the predetermined
threshold. Whether or not a zero-rate traffic channel frame has
been received is indicated based on the result of the metric
comparison. A zero-rate traffic channel frame is indicated as being
received when the magnitude of the predetermined threshold exceeds
the magnitude of the metric.
[0018] The system for detecting a zero-rate traffic channel
comprises means for receiving the pilot and traffic channel frames,
means for measuring an energy of the traffic channel frame, means
for measuring an energy of the pilot channel frame, and means for
comparing the pilot channel frame energy to the traffic channel
frame energy to obtain a metric. Generally, this is accomplished
using means for computing a ratio of the traffic and pilot channel
frame energies. Means for comparing the metric with a predetermined
threshold is used to obtain a comparison result, with the
predetermined threshold being based on the probability of detection
of a transmitted zero-rate traffic channel frame and the
probability of erroneously detecting a zero-rate traffic channel
frame. This is generally accomplished by using a means for
comparing the magnitude of the metric to the magnitude of the
predetermined threshold. As before, whether or not a zero-rate
traffic channel frame has been received is indicated based on the
result of the metric comparison. Means for indicating whether a
zero-rate traffic channel frame has been received based on the
result is used to make this indication. A zero-rate traffic channel
frame is indicated as being received when the magnitude of the
predetermined threshold exceeds the magnitude of the metric.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The features and advantages of the present invention will
become more apparent from the detailed description set forth below
when taken in conjunction with the drawings in which like reference
numbers indicate identical or functionally similar elements.
Additionally, the left-most digits of a reference number identifies
the drawing in which the reference number first appears.
[0020] FIG. 1 depicts a typical wireless communication system.
[0021] FIG. 2 is a block diagram of an exemplary transceiver for
use in a user terminal.
[0022] FIG. 3 is a block diagram of an exemplary transmission and
reception apparatus for use in a gateway.
[0023] FIG. 4 illustrates the relationship between data
transmission rate and transmission power.
[0024] FIG. 5 is a block diagram of an exemplary forward link
zero-rate frame detector.
[0025] FIG. 6 is a flow chart depicting the method of detecting
zero-rate frames on the forward link.
[0026] FIG. 7 depicts exemplary reverse link zero-rate
communications frames.
[0027] FIG. 8 is a block diagram of an exemplary reverse link
zero-rate frame detector.
[0028] FIG. 9 is a flow chart depicting the method of detecting
zero-rate frames on the forward link.
[0029] FIGS. 10-11 are detailed diagrams of hot and cold zone
energy detectors.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] I. Introduction
[0031] The present invention is a system and method for detection
of zero-rate data transmission in a communication signal. The
present invention can be applied to CDMA or spread spectrum
communications systems, as well as to other types of communications
systems.
[0032] The preferred embodiment of the invention is discussed in
detail below. While specific steps, configurations and arrangements
are discussed, it should be understood that this is done for
illustrative purposes only. Other steps, configurations and
arrangements can be used without departing from the spirit and
scope of the present invention.
[0033] The present invention will be described in three parts.
First, a typical satellite communications system is described.
Second, the zero-rate transmission detection system and method for
a forward link are described. Third, the zero-rate transmission
detection system and method for a reverse link are described. II.
An Exemplary Wireless Communications System
[0034] An exemplary wireless communication system in which the
present invention is useful is illustrated in FIG. 1. It is
contemplated that this communication system uses CDMA type
communication signals, but this is not required by the present
invention. In a portion of a communication system 100 illustrated
in FIG. 1, one base station 112, two satellites 116 and 118, and
two associated gateways or hubs 120 and 122 are shown for effecting
communications with two remote user terminals 124 and 126.
Typically, the base stations and satellites/gateways are components
of separate communication systems, referred to as being
terrestrial- and satellite-based, although this is not necessary.
The total number of base stations, gateways, and satellites in such
systems depend on desired system capacity and other factors well
understood in the art.
[0035] The terms "base station" and "gateway" are sometimes used
interchangeably in the art, with gateways being perceived as
specialized base stations that direct communications through
satellites and have more `functions,` with associated equipment, to
establish and maintain such communication links through moving
repeaters, while base stations use terrestrial antennas to direct
communications within a surrounding geographical region. Central
control centers will also typically have more functions to perform
when interacting with gateways and satellites. User terminals are
also sometimes referred to as subscriber units, mobile units,
mobile stations, or simply "users," "mobiles," or "subscribers" in
some communication systems, depending on preference.
[0036] User terminals 124 and 126 each include a wireless
communication device such as, but not limited to, a cellular
telephone, a data transceiver, or a paging or position
determination receiver, and can be hand-held or vehicle-mounted as
desired. Here, the user terminals are illustrated as hand-held
telephones. However, it is also understood that the teachings of
the invention are applicable to fixed units where remote wireless
service is desired, including "indoor" as well as "open air"
locations.
[0037] Generally, beams from satellites 116 and 118 cover different
geographical areas in predefined patterns. Beams at different
frequencies, also referred to as CDMA channels or "sub-beams," can
be directed to overlap the same region. Beam coverage or service
areas for multiple satellites, or antenna patterns for multiple
base stations, can be designed to overlap completely or partially
in a given region depending on the communication system design and
the type of service being offered, and whether space diversity is
being achieved.
[0038] A variety of multi-satellite communication systems have been
proposed with an exemplary system employing on the order of 48 or
more satellites, traveling in eight different orbital planes in LEO
orbits for servicing a large number of user terminals. However,
those skilled in the art will readily understand how the teachings
of the present invention are applicable to a variety of satellite
system and gateway configurations, including other orbital
distances and constellations. At the same time, the invention is
equally applicable to terrestrial-based systems of various base
station configurations.
[0039] In FIG. 1, some possible signal paths are illustrated for
communications being established between user terminals 124 and 126
and base station 112, or through satellites 116 and 118, with
gateways 120 and 122. The base station-user terminal communication
links are illustrated by lines 130 and 132. The satellite-user
terminal communication links between satellites 116 and 118, and
user terminals 124 and 126 are illustrated by lines 140, 142, and
144. The gateway-satellite communication links, between gateways
120 and 122 and satellites 116 and 118, are illustrated by lines
146, 148, 150, and 152. Gateways 120 and 122, and base station 112,
may be used as part of one- or two-way communication systems or
simply to transfer messages or data to user terminals 124 and
126.
[0040] An exemplary transceiver 200 for use in a user terminal 106
is illustrated in FIG. 2. Transceiver 200 uses at least one antenna
210 for receiving communication signals, which are transferred to
an analog receiver 214, where they are downconverted, amplified,
and digitized. A duplexer element 212 is typically used to allow
the same antenna to serve both transmit and receive functions.
However, some systems employ separate antennas for operating at
different transmit and receive frequencies.
[0041] The digital communication signals output by analog receiver
214 are transferred to at least one digital data receiver 216A and
at least one searcher receiver 218. Additional digital data
receivers 216B-216N can be used to obtain desired levels of signal
diversity, depending on the acceptable level of transceiver
complexity, as would be apparent to one skilled in the relevant
art.
[0042] At least one user terminal control processor 220 is coupled
to digital data receivers 216A-216N and searcher receiver 218.
Control processor 220 provides, among other functions, basic signal
processing, timing, power and handoff control or coordination, and
selection of frequency used for signal carriers. Another basic
control function often performed by control processor 220 is the
selection or manipulation of pseudonoise (PN) code sequences or
orthogonal functions to be used for processing communication signal
waveforms. Signal processing by control processor 220 can include a
determination of relative signal strength and computation of
various related signal parameters. Such computations of signal
parameters, such as timing and frequency may include the use of
additional or separate dedicated circuitry to provide increased
efficiency or speed in measurements or improved allocation of
control processing resources.
[0043] The outputs of digital data receivers 216A-216N are coupled
to digital baseband circuitry 222 within the user terminal. User
digital baseband circuitry 222 comprises processing and
presentation elements used to transfer information to and from a
user of a user terminal. That is, signal or data storage elements,
such as transient or long term digital memory; decoders; input and
output devices such as display screens, speakers, keypad terminals,
and handsets; A/D and D/A elements, vocoders and other voice and
analog signal processing elements; etc., all form parts of the user
digital baseband circuitry 222 using elements well known in the
art. If diversity signal processing is employed, user digital
baseband circuitry 222 can comprise a diversity combiner and
decoder. Some of these elements may also operate under the control
of, or in communication with, control processor 220.
[0044] When voice or other data is prepared as an output message or
communications signal originated by the user terminal, user digital
baseband circuitry 222 is used to receive, store, process, and
otherwise prepare the desired data for transmission. User digital
baseband circuitry 222 provides this data to a transmit modulator
226 operating under the control of control processor 220. The
output of transmit modulator 226, along with the data rate to be
transmitted, is transferred to a power controller 228 which
provides output power control to a transmit power amplifier 230 for
final transmission of the signal from antenna 210 to a gateway.
[0045] Digital receivers 216A-N are configured with signal
correlation elements to demodulate and track specific signals.
Searcher receiver 218 is used to search for pilot signals, or other
relatively fixed pattern strong signals, while digital receivers
216A-N are used to demodulate other signals associated with
detected pilot signals. Therefore, the outputs of these units can
be monitored to determine the energy in, or frequency of, the pilot
signal or other signals. These receivers also employ frequency
tracking elements that can be monitored to provide current
frequency and timing information to control processor 220 for
signals being demodulated.
[0046] An exemplary transmission and reception apparatus 300 for
use in gateways 120 and 122 is illustrated in FIG. 3. The portion
of gateway 120, 122 illustrated in FIG. 3 has one or more analog
receivers 314 connected to an antenna 310 for receiving
communication signals which are then downconverted, amplified, and
digitized using various schemes well known in the art. Multiple
antennas 310 are used in some communication systems. Digitized
signals output by analog receiver 314 are provided as inputs to at
least one digital receiver module, indicated by dashed lines
generally at 324.
[0047] Each digital receiver module 324 corresponds to signal
processing elements used to manage communications between a gateway
120, 122 and one user terminal 124, 126, although certain
variations are known in the art. One analog receiver 314 can
provide inputs for many digital receiver modules 324, and a number
of such modules are typically used in gateways 120, 122 to
accommodate user terminal signals from all of the satellite beams
and possible diversity mode (multipath) signals being handled at
any given time. Each digital receiver module 324 has one or more
digital data receivers 316 and a searcher receiver 318. Searcher
receiver 318 generally searches for appropriate diversity modes of
signals. Where implemented in the communication system, multiple
digital data receivers 316A-316N are used for diversity signal
reception.
[0048] The outputs of digital data receivers 316 are provided to
subsequent baseband processing elements 322 comprising apparatus
well known in the art and not illustrated in further detail here.
Exemplary baseband apparatus includes diversity combiners and
decoders to combine multipath signals into one output for each
subscriber and to decode the information bits. Exemplary baseband
apparatus also includes interface circuits for providing output
data, typically to a digital switch or network. A variety of other
known elements such as, but not limited to, vocoders, data modems,
and digital data switching and storage components may form a part
of baseband processing elements 322. These elements operate to
control or direct the transfer of data signals to one or more
transmit modules 334.
[0049] Signals to be transmitted to user terminals are each coupled
to one or more appropriate transmit modules 334. A typical gateway
uses a number of such transmit modules 334 to provide service to
many user terminals 124, 126 at a time, and for several satellites
and beams at a time. The number of transmission modules 334 used by
gateway 120, 122 is determined by factors well known in the art,
including system complexity, number of satellites in view,
subscriber capacity, degree of diversity chosen, and the like.
[0050] Each transmit module 334 includes a received power unit 323
for detecting the power levels in received signals. Each transmit
module 334 also includes a transmit modulator 326 which
spread-spectrum modulates data for transmission. Transmit modulator
326 has an output coupled to a digital transmit power controller
328, which controls the transmission power used for the outgoing
digital signal. Digital transmit power controller 328 applies a
minimum level of power for purposes of interference reduction and
resource allocation, but applies appropriate levels of power when
needed to compensate for attenuation in the transmission path and
other path transfer characteristics. A PN generator 332 is used by
transmit modulator 326 in spreading the signals. This code
generation can also form a functional part of one or more control
processors or storage elements used in gateway 122, 124.
[0051] The output of transmit power controller 328 is transferred
to a summer 336 where it is summed with the outputs from other
transmit modules. Those outputs are signals for transmission to
other user terminals 124, 126 at the same frequency and within the
same beam as the output of transmit power controller 328. The
output of summer 336 is provided to an analog transmitter 338 for
digital-to-analog conversion, conversion to the appropriate RF
carrier frequency, further amplification and output to one or more
antennas 340 for radiating to user terminals 124, 126. Antennas 310
and 340 may be the same antennas depending on the complexity and
configuration of the system.
[0052] At least one gateway control processor 320 is coupled to
receiver modules 324, transmit modules 334, and baseband circuitry
322; these units may be physically separated from each other.
Control processor 320 provides command and control signals to
effect functions such as, but not limited to, signal processing,
timing signal generation, power control, handoff control, diversity
combining, and system interfacing. In addition, control processor
320 assigns PN spreading codes, code timing, orthogonal code
sequences, and specific transmitters and receivers for use in
subscriber communications.
[0053] Control processor 320 also controls the generation and power
of pilot, sync, and paging channel signals and their coupling to
transmit power controller 328. The pilot channel is simply a signal
that is not modulated by data, and may use a repetitive unchanging
pattern or non-varying frame structure type input (pattern) into
transmit modulator 326. That is, the orthogonal function, here
preferably a Walsh code, used to form the channel for the pilot
signal generally has a constant value, such as all 1's or 0's, or a
well known repetitive pattern, such as a structured pattern of
interspersed 1's and 0's. This effectively results in transmitting
only the PN spreading codes applied from PN generator 332. In
addition, a pilot signal is not power controlled. That is, the
pilot signal is transmitted at a preselected fixed power level
which is not varied so that accurate measurements of signal power
are achieved by user terminals.
[0054] While control processor 320 can be coupled directly to the
elements of a module, such as transmit module 324 or receive module
334, each module generally comprises a module-specific processor,
such as transmit processor 330 or receive processor 321, which
controls the elements of that module. Thus, in a preferred
embodiment, control processor 320 is coupled to transmit processor
330 and receive processor 321, as shown in FIG. 3. In this manner,
a single control processor 320 can control the operations of a
large number of modules and resources more efficiently. Transmit
processor 330 controls generation of, and signal power for, pilot,
synchronization, paging signals, and traffic channel signals, and
their respective coupling to power controller 328. Receiver
processor 321 controls searching, PN spreading codes and timing
used for demodulation, and monitoring received power.
[0055] For certain operations, such as shared resource power
control, gateways 120 and 122 receive information such as received
signal strength, frequency measurements, or other received signal
parameters from user terminals in communication signals. This
information can be derived from the demodulated outputs of data
receivers 316 by receive processors 321. Alternatively, this
information can be detected as occurring at predefined locations in
the signals being monitored by control processor 320, or receive
processors 321, and transferred to control processor 320. Control
processor 320 uses this information to control the timing and
frequency of signals being transmitted and processed using transmit
power controllers 328 and analog transmitter 338.
[0056] A preferred embodiment of the invention is discussed in
detail below. While specific steps, configurations and arrangements
are discussed, it should be understood that this is done for
illustrative purposes only. Other steps, configurations and
arrangements can be used without departing from the spirit and
scope of the present invention. The present invention could find
use in a variety of wireless information and communication systems,
including those intended for position determination, and satellite
and terrestrial cellular telephone systems.
[0057] III. Forward Link Zero-rate Frame Detection
[0058] In communications systems, data is transmitted at various
rates, dependent on demand and communication system capacity. For
example, in voice transmissions, normally only one party talks at a
time. One channel, therefore, will have most of the activity while
the other is inactive. In order to save power and bandwidth, and
reduce interference with other channels, the inactive channel is
transmitted at a much lower rate. In the exemplary environment,
data can be transmitted at 9600 bits per second (bps), 4800 bps,
2400 bps and zero bps, or zero rate. For periods of inactivity on a
channel, the zero-rate transmission is preferred in order to save
power and decrease interference with other channels.
[0059] Two factors that are affected by increased transmission rate
are bit transmission power (P.sub.b) and bit transmission duration
(T.sub.b). The quality of the received signal depends on the ratio
of energy of the received bit (E.sub.b) to noise power spectral
density (N.sub.0) or (Eb/N.sub.0). For the purposes of explanation,
noise power spectral density (N.sub.0) will be assumed to be
constant. The energy of the received bit (E.sub.b) is a function of
transmit power of the transmitted bit (P.sub.b), and the duration
of the transmitted bit (T.sub.b). The relationship between the
energy of the received bit (E.sub.b), transmit power of the
transmitted bit (P.sub.b), and the duration of the transmitted bit
(T.sub.b) is given by:
E.sub.b-P.sub.bT.sub.b
[0060] As the rate of transmission is increased, the duration of
the transmitted bit (T.sub.b) necessarily decreases. Thus, the
width of each bit decreases as the rate of transmission increases.
Consequently, in a 20 millisecond data or signal frame, 192, 96,
and 48 bits can fit into a frame at 9600, 4800 and 2400 bps,
respectively.
[0061] In order to maintain the quality of the received signal
(assuming constant noise power (N.sub.0)), the energy of the
received bit (E.sub.b) must be maintained at a constant level. To
maintain constant energy of the received bit (E.sub.b) for an
increased transmission rate, which results in decreased duration of
the transmitted bit (T.sub.b), transmission power (P.sub.b) must be
increased.
[0062] A forward link is used to transmit user and signaling
information from a gateway to a user terminal during a call. The
forward link comprises a pilot channel and a forward traffic
channel. The forward traffic channel carries user and signaling
traffic from a gateway to a user terminal during a call. The pilot
channel carries timing information that allows a user terminal to
obtain and maintain time, phase and frequency synchronization with
the gateway.
[0063] The forward traffic channel carries data at different
possible rates. In a preferred embodiment, the different possible
rates are 9600 bits per second (bps), 4800 bps, 2400 bps, and 0
bps, or zero rate. The data rate of each frame transmitted on the
forward traffic channel is selected on a frame by frame basis. The
selection of the data rate for each frame can be based upon, among
other things, communication system administration, the amount of
information to be communicated to a user terminal, the amount of
bandwidth available for data transmission at the transmitter, and
so forth.
[0064] In a CDMA system, the pilot channel is a direct-sequence
signal transmitted continuously by a gateway on the forward link.
The pilot channel allows a user terminal to acquire the timing of
the forward link, provides a phase reference for coherent
demodulation, and provides a means for signal strength comparison
between beams, satellites or gateways for determining when to hand
off. Since the pilot channel is transmitted at a constant rate, the
transmit power is not varied as a function of data transmission
rate.
[0065] FIG. 4 illustrates the relationship between data
transmission rate and transmission power. Traffic channel frames
402, 406, 410 and 414 illustrate four 20 millisecond length traffic
channel frames of different transmission rates. Traffic channel
frame 402 illustrates a 9600 bps communication frame. The 9600 bps
traffic channel frame 402 can contain up to 192 bits of
information. Bit 404 is an example of one such bit. For explanatory
purposes, the transmit power of exemplary bit 404 has been
normalized to 1. Traffic channel frame 406 is an example of a 4800
bps traffic channel frame. Traffic channel frame 406 can contain up
to 96 signal bits. Bit 408 is an example of one such bit. Bit 408
has twice the duration (T.sub.b) of bit 404, and the transmit power
is reduced to one-half in order to maintain constant bit energy
(E.sub.b). Communication frame 410 exemplifies the 2400 bps frame.
2400 bps signal bit 412 is four times as wide as the 9600 bps
signal bit 404. Since signal bit 412 has a duration of four times
the signal bit of 404, the transmit power for signal bit 412 is
reduced to one-quarter. Traffic channel frame 414 exemplifies a
forward link zero-rate frame, in which no bits or energy (from
data) are transmitted.
[0066] On the forward link, zero-rate frames are detected by
monitoring the traffic and pilot channel signal energy. If traffic
and pilot channel signal energy are detected, then the frame
received is determined to be a non-zero-rate frame. If no traffic
channel energy is detected, on the other hand, the frame may be a
zero-rate frame or a non-zero-rate frame whose signal energy has
been faded by effects occurring in the signal transfer path or
channel.
[0067] To determine if a traffic channel frame is a zero-rate
frame, the traffic and pilot channel signal energies are compared.
The traffic and pilot channel signals are transmitted in the same
band and the pilot channel is transmitted constantly. Fading on the
forward link, therefore, will result in fading on both the traffic
and pilot channels. If, for example, the traffic channel fades, the
pilot channel will also fade; if noise dominates the traffic
channel, then noise will dominate the pilot channel. If pilot
channel energy is detected and no traffic channel energy is
detected, it can be determined that a zero-rate frame has been
transmitted. If no pilot channel energy is detected and no traffic
channel energy is detected, it can be determined that the forward
link has faded and it is impossible to determine whether a zero or
regular non-zero-rate frame has been transmitted.
[0068] The present invention detects zero-rate frames on the
forward link by generating a metric (M.sub.f) based on the energy
of the traffic channel relative to the pilot channel. The metric
(M.sub.f) is compared to a predetermined threshold to determine
whether or not a zero-rate frame has been received. The metric is
governed by the relationship: 1 M f = traffic channel frame energy
pilot channel frame energy
[0069] When a zero-rate frame is transmitted no data bits are
transmitted on the traffic channel, resulting in no transmitted
traffic channel energy. Since the pilot channel is transmitted
continuously, the pilot channel contains energy even when a
zero-rate frame is transmitted on the traffic channel. In a
noiseless communication system, when a zero-rate frame is
transmitted, no traffic channel frame energy and a normal amount of
pilot channel frame energy both exist on the forward link. Due to
noise, however, the estimated traffic energy during a zero-rate
frame will be non-zero.
[0070] If a zero-rate frame has been transmitted, computation of
the forward link metric (M.sub.f) will result in a small number.
If, on the other hand, the entire channel is faded, both traffic
channel and pilot channel energy will be detected and the forward
link metric (M.sub.f) will be a value closer to 1. By comparing the
forward link metric (M.sub.f) with a predetermined threshold, the
receiver in the forward link can determine if the current frame is
a zero-rate frame.
[0071] Deciding where to set the threshold for the forward link
metric (M.sub.f) to indicate a zero-rate frame is a typical signal
detection problem. When the threshold is set very low, the
probability of erroneously detecting a zero-rate frame decreases.
Setting the predetermined threshold low, however, also decreases
the probability of successfully detecting a zero-rate frame. When
the predetermined threshold is set at a higher level, the
probability of detecting a zero-rate frame increases. With a higher
predetermined threshold, however, the probability of erroneously
detecting a zero-rate frame increases. The predetermined threshold,
therefore, will be chosen by determining a ratio of the probability
of erroneously detecting a zero-rate frame to the probability of
detecting a zero-rate frame for each particular system or
communications network on which the zero rate frame detection
method and system is implemented.
[0072] FIG. 5 illustrates a forward link zero-rate frame detector
500. In a preferred embodiment, the zero-rate frame detector 500 is
incorporated in one of the digital data receivers 216A-216N of user
terminal 200. The pilot and traffic channel signals are input to
the input of the zero-rate frame detector 500 at input 502. The
forward link zero-rate frame detector 500 comprises two processing
branches, one for the pilot channel signal and one for the traffic
channel signal. Correlators 504 and 516 separate the pilot and
traffic channel signals from input signal 502. Pilot filter 506
performs an addition and averaging function with the known pilot
channel signal to reduce the noise energy in the received pilot
signal. The correlated traffic channel signal from traffic channel
correlator 516 and the filtered pilot channel signal from pilot
filter 506 are input into dot product unit 518. Since both pilot
and traffic channel signals are transmitted on the same RF channel,
any multipath phase rotation will appear in both the pilot and
traffic channel signals. The dot product unit 518 effectively
removes phase rotation from the traffic channel signal by producing
a dot product between the traffic channel signal and the pilot
channel signal. The output of dot product unit 518 is a coherent
traffic channel signal with no phase error. Summer 520 sums the
output of dot product unit 518 over an entire frame. The output of
summer 520 is then squared at squaring unit 522 to determine the
energy of the traffic channel signal.
[0073] In the pilot channel signal branch, after the pilot channel
signal has been filtered at pilot filter 506, the pilot channel
signal is then squared at squaring unit 508 to determine the energy
of the pilot channel signal. The pilot channel signal energy is
then summed over an entire frame at summer 510. After the pilot
channel energy has been summed over a frame by summer 510, the
pilot channel signal energy is then scaled at scaler 524 by scaling
factor 512 (.alpha..sub.r). The fixed scaling factor 512
compensates for channel and system conditions that may affect the
traffic channel frame energy to pilot channel frame energy ratio.
Both pilot channel and traffic channel signal energies are then
input into comparison/indication unit 514. The
comparison/indication unit 514 compares the ratio of traffic
channel signal energy to pilot channel signal energy to a threshold
526. If the ratio of traffic channel signal energy to pilot channel
signal energy is lower than threshold 526, the received traffic
channel frame is a zero-rate frame. Alternatively, if the ratio of
traffic channel signal energy to pilot channel signal energy is
higher than threshold 526, the received traffic channel frame is a
non-zero-rate frame. Comparison/indication unit 514 then indicates
the zero or non-zero-rate traffic channel frame to the digital data
receivers 216A-216N.
[0074] FIG. 6 illustrates the method for detecting a zero-rate
frame on the forward link. The method begins at step 602 where the
traffic and pilot channel signals are input to the zero-rate frame
detection unit 500. The traffic and pilot channel signals are
received at step 604 by pilot and traffic channel signal
correlators 504 and 516. The pilot channel signal is then
correlated and filtered in step 606 by pilot correlator 504 and
pilot filter 506. The traffic channel signal is correlated and
de-rotated in step 608 by traffic channel signal correlator 516 and
dot product unit 518. At step 610, the energy is then determined
for the pilot and traffic channel signals by squaring units 508,
522 and summed over a frame by summing units 510, 520. The pilot
channel signal energy is then scaled at step 612 to compensate for
channel and system conditions by scaling factor 512 and scaling
unit 524.
[0075] Forward link metric (M.sub.f) is then calculated and
compared with the predetermined threshold 526 at step 614 by
compare indication unit 514. If the quotient exceeds the threshold
at step 614, then the method indicates a non-zero-rate frame at
step 618. If, on the other hand, the traffic channel signal energy
to pilot channel signal energy quotient is determined to be less
than the pre-determined threshold at step 614, then a zero-rate
frame is indicated at step 616. After the indication of a zero or
non-zero-rate frame at steps 616 or 618, the process ends at step
620.
[0076] IV. Reverse Link Zero-rate Frame Detection
[0077] A reverse link is used to transmit user and signaling
information from a user terminal to a gateway during a call. The
reverse link comprises a reverse traffic channel for carrying the
user and signaling traffic from the user terminal to the gateway
during a call. There is no pilot channel on the reverse link.
[0078] The reverse traffic channel carries information at different
possible rates. In a preferred embodiment, the different possible
rates comprise rates of 9600 bits per second (bps), 4800 bps, 2400
bps, and zero bps. The data rate of each frame transmitted on the
reverse traffic channel is selected on a frame by frame basis. This
selection of the data rate for each frame can be based upon, among
other things, communication system administration, the amount of
information to be communicated to a gateway, the amount of band
width available for data transmission at the transmitter, and so
forth.
[0079] Since the reverse link does not include a pilot channel, the
reverse link relies on the reverse traffic channel to maintain
time, phase and frequency synchronization for coherent demodulation
at the receiver. The reverse traffic channel, therefore, must be
transmitted continuously in order to maintain the reverse link
communications channel. If a gateway loses the reverse traffic
channel, the communications link between the user terminal and the
gateway is lost, resulting in a dropped call.
[0080] The system and method of the invention for detecting
zero-rate frames on the reverse link is different from the
detection of zero-rate frames on the forward link. Since there is
no pilot channel on the reverse link, it is not possible to detect
a zero-rate frame by comparing the energy of the pilot channel with
that of the traffic channel.
[0081] The user terminal transmits zero-rate frames by transmitting
a predetermined code in a pseudo randomly placed portion of the
reverse traffic channel frame. In the remainder of the frame,
nothing is transmitted. The interval in which the predetermined
code is transmitted is called the "hot zone" and the interval in
which nothing is transmitted is called the "cold zone." The hot
zone transmission allows the gateway receiver to maintain time and
frequency synchronization, as well as signal lock in order to
maintain the reverse communications link. In a preferred
embodiment, the 20 millisecond reverse traffic frame is divided
into four intervals, each of 5 millisecond duration. In a preferred
embodiment, the user terminal transmits the preselected code at
2400 bps, at the one-quarter transmission rate power in on one of
the four 5 millisecond intervals in a zero-rate frame that is
transmitted.
[0082] Those skilled in the art will readily recognize that a
variety of codes can be used to implement the preselected or
predetermined code. Basically, any known code that can be readily
correlated with in the given communication system design, and
possesses well known properties may be useful for this application.
However, the preferred embodiment of the invention is for use in
CDMA communication systems. As discussed above, in a typical CDMA
spread-spectrum system unique orthogonal "channelizing" codes are
used to create orthogonal code channels for user terminals on the
forward link. Walsh functions, or equivalent codes, are generally
used to implement the channelizing codes. Therefore a very
efficient or preferred technique to implement the present invention
is to use the Walsh 0 code available in such systems from the pilot
channel being sent on the forward link, especially since
synchronization and timing are being acquired using the pilot
signal and system timing can easily be maintained.
[0083] The system and method of the invention detect the zero-rate
frame on the reverse link by comparing a scaled version of the
signal energy in the cold zone with a scaled version of the signal
energy in the hot zone. Since the location of the hot zone is
predetermined pseudo-randomly, the gateway receiver "knows" when it
is transmitted. The gateway receiver compares the energy received
for preselected orthogonal code, here Walsh code 0, in the hot zone
and the energy received in the cold zone. In the case of a
zero-rate frame, the energy in the hot zone is from the Walsh code
0 (W.sub.0) transmitted in the hot zone. In the case of a
non-zero-rate frame, the energy in the hot zone is from the normal
communication maximum signal transmitted on the channel. In the
case of a zero-rate frame, the energy in the cold zone is comprised
of noise energy, since no signal is transmitted. In a non-zero-rate
frame, the energy in the cold zone is from the normal communication
signal transmitted on the channel. In the cold zone, the energy
will depend on the rate. For each different non-zero rate a
separate metric is used, scaled differently.
[0084] The energy detected in the hot and cold zones are compared
to detect a zero-rate frame. If the difference between the energy
for Walsh code 0 in the hot zone and the energy received in the
cold zone is large, a zero-rate frame is declared on the reverse
link. If the difference is small then a non-zero-rate frame is
declared. Since fading on the channel is usually relatively slow
compared to the 20 millisecond frame interval, zero-rate frames can
be detected on the reverse link.
[0085] In a preferred embodiment, the relationship between hot and
cold zone energy is established by subtracting scaled energy in the
15 millisecond cold zones from the energy in the hot zone. A ratio
or other mathematical relationship between the hot and cold zone
energies would be equivalent to the preferred embodiment. In a
preferred embodiment, the quantity that is compared to the
threshold is given by:
C.sub.1E.sub.C-C.sub.2E.sub.H
[0086] where E.sub.C is the energy in the cold zone, E.sub.H is the
energy in the hot zone, and C.sub.1 and C.sub.2 are constants.
[0087] FIG. 7 illustrates a preferred embodiment of a reverse link
zero-rate frame. Frames 706 and 708 are examples of zero-rate
frames on the reverse link. Portions 710 and 714 of the zero-rate
frames 706, 708 are the cold zones. Note that cold zones 710, 714
contain noise only. The 5 millisecond periods 712 and 716 are the
hot zones during which Walsh symbols 0 (W.sub.0) are transmitted at
the 2400 bps rate at the corresponding one-quarter rate
transmission power. Cold zones 710, 714 and hot zones 712, 716
comprise an entire 20 millisecond frame.
[0088] FIG. 8 illustrates a reverse link zero-rate frame detector
800. In a preferred embodiment, a reverse link zero-rate frame
detector 800 is located in each of digital data receivers 316A-316N
of the gateway transceiver 300. Input 802 is the reverse link
signal input. Hot zone energy detector 804 determines the energy
levels in the frame portions that correspond to hot zones 712 and
716. Cold zone energy detectors 806a, 806b, and 806c determine the
energy levels in the frame portions that correspond to cold zones
710 and 714.
[0089] Since the location of hot zones 712, 716 are determined
pseudo-randomly, the location of the hot zone in each zero-rate
frame is known. In a preferred embodiment, each cold zone energy
detector 806 determines the highest energy in the three 5
millisecond intervals in the cold zone and outputs that particular
energy value. After the hot and cold zone energies are determined
by hot and cold zone energy detectors 804, 806, metric generation
unit 808 scales the cold zone energies and determines the
difference between them and the hot zone energy to generate one or
more metrics. In a preferred embodiment, the reverse link metrics
are defined by:
M.sub.1=M.sub.F=(.alpha..sub.F.multidot.C.sub.F)-E.sub.H
M.sub.2=M.sub.H=(.alpha..sub.H.multidot.C.sub.H)-E.sub.H
M.sub.3=M.sub.Q=(.alpha..sub.Q.multidot.C.sub.Q)-E.sub.H
[0090] where C.sub.F, C.sub.H, and C.sub.Q are the full, half, and
quarter rate cold zone energies, respectively, used to determine
M.sub.F, M.sub.H, M.sub.Q the full, half, and quarter rate metrics,
respectively, and .alpha..sub.F, .alpha..sub.H, .alpha..sub.Q are
the appropriate scaling factors used for each of the corresponding
cold zones.
[0091] After the metrics are generated by metric generation unit
808, the compare/indicate unit 810 compares the metrics with a
predetermined threshold 812. The optimum value of predetermined
threshold 812 depends on the particular system in which the present
invention is used. Thus, the value of predetermined threshold 812
is implementation-specific. The preferred or optimum value of
predetermined threshold 812 is defined to avoid erroneously
detecting a zero-rate frame with high probability while still being
able to detect zero-rate frames reliably. If the threshold is set
too high the probability of erroneously detecting a zero-rate frame
is low, but the probability of detecting a zero-rate frame is also
decreased. Conversely, if the threshold is set too low, the
probability of erroneously detecting a zero-rate frame will be
high. In a preferred embodiment, a zero-rate frame is declared when
all of the metrics exceed the threshold, that is, when the
following equations are satisfied:
M.sub.1>T
M.sub.2>T
M.sub.3>T
[0092] where T is predetermined threshold 812. Other methods of
comparison, however, can be used and are recognized as equivalents
by those of skill in the relevant art.
[0093] If the metrics exceed predetermined threshold 812, then the
compare/indicate unit 810 indicates that a zero-rate frame has been
received. If any metric falls below predetermined threshold 812,
the compare/indicate unit 810 indicates that a non-zero-rate frame
has been received.
[0094] FIG. 9 illustrates the process for detecting a reverse link
zero-rate frame. The process begins at step 902 where the reverse
link channel signal is received. The hot zone signal energy for the
reverse link is estimated at step 904 by hot zone energy detector
804. Next, the cold zone signal energy is estimated at step 906 by
cold zone energy detector 806. After the hot and cold zone signal
energies have been determined in steps 904 and 906, the hot zone
signal energy is compared to the cold zone signal energies to
obtain metrics (M.sub.1, M.sub.2, M.sub.3) at step 908 by metric
generation unit 808. After the metrics are computed in step 908,
the metrics are compared to predetermined threshold 812 at step 910
by compare/indicate unit 810. Compare/indicate unit 810 then
determines whether the metrics determined in step 908 exceed
predetermined threshold 812 at step 912. If the metrics exceed
predetermined threshold 812, then compare/indicate unit 810
indicates a zero-rate frame at step 914. If the metrics do not
exceed the predetermined threshold, then compare/indicate unit 810
indicates a non-zero-rate frame at step 918. After the indication
of a zero or non-zero-rate frame at steps 914 or 918 the process
ends at step 916.
[0095] FIG. 10 illustrates a hot zone energy detector 804 in
further detail. Bits in each of the 5 millisecond intervals
1002-1008 in the 20 millisecond frame are accumulated by
accumulators 1010-1016. Since the location of the hot zone is
determined pseudo-randomly, the gateway receiver "knows" the 5
millisecond interval during which the hot zone is transmitted. The
hot zone interval can be selected, therefore, from one of the four
5 millisecond intervals 1002-1008 in the frame. The accumulated
signals are then input to Fast Hadamard transform units (FHTs)
1018-1024. FHTs 1018-1024 are well known in the spread spectrum
communications art. The FHTs 1018-1024 determine the correlation
between the input signal and all of the Walsh symbols. Since the
hot zone is modulated with the Walsh code 0 (W.sub.0) (or other
preselected code), the Walsh symbol 0 (W.sub.0) is selected in the
FHTs 1018-1024. The correlation value for the selected hot zone
interval is then output from the FHTs 1018-1024 to squaring units
1028-1032 which compute the energy for the selected 5 millisecond
interval. The energy is then output from the squaring units
1028-1032 to a selecting unit 1034 whose output is simply the
energy of the selected hot zone interval. The hot zone interval
energy is then output at element 1034.
[0096] FIG. 11 illustrates the cold zone energy detector 806c
(quarter rate) in more detail. Bits are accumulated from each of
the 5 millisecond intervals 1102-1108 in accumulators 1110-1116.
Since the location of the hot zone is determined pseudo-randomly,
the gateway receiver "knows" the 5 millisecond interval during
which the hot zone is transmitted. Thus, the cold zone intervals
are collected from the rest of the four 5 millisecond intervals
1102-1108 in the frame. The accumulated signals from accumulators
1110-1116 are then input to Fast Hadamard Transform units (FHTs)
1118-1124. Signals from the three cold zone 5 millisecond intervals
are demodulated by FHTs 1118-1124 and the winning correlation
values are output from FHTs 1118-11124 to squaring units 1126-1132.
Squaring units 1126-1132 calculate the cold zone energy from the
winning correlation values produced by FHTs 1118-1124. In a
preferred embodiment, the maximum energy out of the three code zone
5 millisecond intervals is selected by selection unit 1134. This
maximum cold zone energy is then given to metric generation unit
808 for computation of the hot and cold zone energy metrics.
[0097] V. Conclusion
[0098] While various embodiments of the present invention have been
nodescribed above, it should be understood that they have been
presented by way of example only, and not limitation. Thus, the
breadth and scope of the present invention should not be limited by
any of the above-described exemplary embodiments, but should be
defined only in accordance with the following claims and their
equivalents.
* * * * *