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Claims  |
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We claim:
1. A method of direct-sequence spread-spectrum digital communication
between a transmitter and a receiver, said method at the transmitter
comprising the steps of:
(a) providing a plurality of frames of digital information, each of said
frames having a predetermined number of time slots,
(b) assigning one of said predetermined number of time slots in each frame
as an acquisition time slot,
(c) providing an acquisition direct-sequence spreading code,
(d) spreading the acquisition time slot with the provided acquisition
direct-sequence code in each of said frames,
(e) transmitting said plurality of frames, each of the transmitted frames
containing the acquisition time slot spread by the acquisition
direct-sequence code,
said method at the receiver comprising the steps of:
(a) receiving the spread transmitted plurality of frames from the
transmitter,
(b) providing the identical acquisition direct-sequence spreading code as
provided by said transmitter,
(c) despreading the acquisition time slot with the receiver provided
acquisition direct-sequence code,
(d) acquiring synchronization with the spread transmitted frames by
measuring the signal strength in the step of despreading in each of said
predetermined number of time slots in a given number of successively
received time frames by incrementally adjusting the chip position of the
receiver provided acquisition direct-sequence code until a maximum signal
strength is detected thereby locating the assigned acquisition time slot.
2. The method of claim 1 wherein the step of acquiring synchronization
comprises the steps of:
(a) performing a major sweep of a predetermined number of successive spread
transmitted frames to approximately locate the chip position of the
maximum signal strength,
(b) performing a refinement sweep of a predetermined number of successive
spread transmitted frames to precisely locate the chip position of maximum
signal strength so as to acquire synchronization.
3. The method of claim 2 wherein the major sweeps and refinement sweeps
adjust the chip position in at least one-half chip increments.
4. The method of claim 2 wherein the major sweep scans a group of a
predetermined number of chips, N, in less than one time slot and repeats
the scan for each group a fixed number of times per frame so as to scan
each time slot in the frame wherein the time required to scan the fixed
number of groups is greater than the time of one frame.
5. The method of claim 4 wherein the major sweep compensates for drift by
starting the sweep of each successive frame at the N.sub.F +1 +DA chip
position, where N.sub.F =the last chip position measured during the prior
frame and DA=the number of chip positions necessary to compensate for the
drift.
6. The method of claim 2 further comprising the step at the receiver of
tracking acquired synchronization in the transmitted time slots by
measuring the signal strength in the header bytes of a given time slot in
successive frames.
7. The method of claim 6 in which the step of tracking further comprises
the step of measuring the signal strength of a set number of locations
within a header field by varying the chip positioning so as to compensate
for drift in order to track the acquired synchronization.
8. The method of claim 7 wherein the step of tracking varies the chip
positions by at least one-quarter chip increments.
9. The method of claim 6 wherein the step of tracking further comprises the
steps of:
(a) determining the drift between the header bytes in the given time slot
between successive frames,
(b) moving the chip position of the receiver provided acquisition
direct-sequence code for the next successive frame in response to the
determined value of drift in order to minimize the value of drift.
10. The method of claim 9 wherein the step of tracking further comprises
the step of controlling the timing of the acquisition direct-sequence code
for the next successive frame in response to the determined value of drift
in order to minimize the value of drift.
11. The method of claim 1 further comprising the steps of providing a delay
after each signal strength measurement so as to allow time for equipment
at the receiving location to adjust.
12. The method of claim 1 further comprising the steps of the transmitter
of assigning header bytes in each time slot.
13. A method of acquiring and tracking direct-sequence spread-spectrum
frames of digital information, said method comprising the steps of:
(a) spreading at a first location digital acquisition data in an assigned
acquisition time slot in each frame of said digital information with a
direct-sequence acquisition spreading code, each frame having a
predetermined number of time slots,
(b) spreading at the first location digital communication data in the
remaining time slots of each frame with a direct-sequence communication
code,
(c) acquiring synchronization at a second location with the spread
direct-sequence acquisition spreading code by measuring the signal
strength in each time slot in a given number of successive frames until
the assigned time slot having the largest signal strength is located,
(d) tracking synchronization of the received digital communication data by
selecting the largest signal strength in a header field provided in each
remaining time slot of each successive frame after acquisition so as to
compensate for drift between the first and second locations,
(e) despreading at the second location the digital communication data in
the remaining time slots in each frame with the direct-sequence
communication spreading code.
14. A method of acquiring direct-sequence spread-spectrum frames of digital
information, said method comprising the steps of:
(a) spreading at a first location digital acquisition data in an assigned
acquisition time slot in each frame of said digital information with a
direct-sequence acquisition spreading code, each frame having a
predetermined number of time slots,
(b) spreading at the first location digital communication data in the
remaining time slots of each frame with a direct-sequence communication
code,
(c) acquiring synchronization at a second location with the spread
direct-sequence acquisition spreading code by measuring the signal
strength in each time slot in a given number of successive frames until
the assigned time slot having the largest signal strength is located, and
(d) despreading after the steps of acquiring and tracking at the second
location the digital communication data in the remaining time slots in
each frame with the direct-sequence communication spreading code.
15. The method of claim 14 wherein the step of acquiring synchronization
comprises the steps of:
(a) performing a major sweep of a predetermined number of successive spread
transmitted frames to approximately locate the chip position of the
maximum signal strength,
(b) performing a refinement sweep of a predetermined number of successive
spread transmitted frames to precisely locate the chip position of maximum
signal strength so as to acquire synchronization.
16. The method of claim 15 wherein the major sweeps and refinement sweeps
adjust the chip position in at lease one-half chip increments.
17. The method of claim 15 wherein the major sweep scans a group of a
predetermined number of chips, N, in less than one time slot and repeats
the scan for each group a fixed number of times per frame so as to scan
each time slot in the frame wherein the time required to scan the fixed
number of groups is greater than the time of one frame.
18. The method of claim 17 wherein the major sweep compensates for drift by
starting the sweep of each successive frame at the N.sub.F +1+DA chip
position, where N.sub.F =the last chip position measured during the prior
frame and DA=the number of chip positions necessary to compensate for the
drift.
19. The method of claim 14 further comprising the step of providing a delay
after each signal strength measurement so as to allow time for equipment
at the receiving location to adjust.
20. A method of tracking direct-sequence spread-spectrum frames of digital
information, said method comprising the steps of:
(a) spreading at a first location said digital information with a
direct-sequence acquisition spreading code, each frame having a
predetermined number of time slots with each time slot having header
bytes,
(b) acquiring synchronization at a second location with the spread
direct-sequence acquisition spreading code,
(c) tracking synchronization by selecting the largest signal strength in
the header field provided in a given time slot of each successive frame
after acquisition so as to compensate for drift between the first and
second locations,
(d) despreading after the step of tracking at the second location the
digital information in each frame with the direct-sequence communication
spreading code.
21. The method of claim 20 further comprising the steps of the transmitter
of assigning header bytes in each time slot.
22. The method of claim 21 further comprising the step at the receiver of
tracking acquired synchronization in the transmitted time slots by
measuring the signal strength in the header bytes of a given time slot in
successive frames.
23. The method of claim 22 in which the step of tracking further comprises
the step of measuring the signal strength of a set number of locations
within a header field by varying the chip positioning so as to compensate
for drift in order to track the acquired synchronization.
24. The method of claim 23 wherein the step of tracking varies the chip
positions by at least one-quarter chip increments.
25. The method of claim 22 wherein the step of tracking further comprises
the steps of:
(a) determining the drift between the given time slots in successive
frames,
(b) moving the chip position of the receiver provided acquisition
direct-sequence code for the next successive frame in response to the
determined value of drift in order to minimize the value of drift.
26. A spread-spectrum communication system for transferring digital
information from a transmitter to a receiver, said system comprising:
said transmitter comprising:
(a) means (40) for providing a plurality of frames of digital data, each
frame having a predetermined number of time slots, one of said time slots
in each frame being assigned to carry acquisition digital data, the time
slots in each frame being assigned to carry communication digital data,
each of said time slots having a header field containing tracking digital
information,
(b) means (10, 20) for generating first and second direct-sequence
spreading digital codes having a higher frequency than the frequency of
said digital data,
(c) means (30, 50) connected to said providing means and to said generating
means for spreading said frames of digital data, said acquisition digital
data in said one time slot being spread by said first direct-sequence
spreading digital code and said communication digital data in said
remaining time slots being spread by said second direct-sequence spreading
digital code,
said receiver having:
(a) means (320) for generating said first and second direct-sequence
spreading digital codes,
(b) means (310, 350) receptive of said spread frames of digital data and
connected to said receiver generating means for despreading said spread
frames of digital data,
(c) means (340, 360, 370) connected to said receiver generating means and
to said despreading means for adjusting said generating means until said
receiver generated first and second direct-sequence spreading digital
codes are in synchronization with said first and second transmitter
generated direct-sequence codes used to spread said frames of digital
data, said adjusting means acquiring synchronization with said acquisition
digital data in said one time slot in each of said spread frames; after
acquiring synchronization said adjusting means tracking synchronization
with the tracking digital information in each of said remaining time slots
in each of said spread frames.
27. A spread-spectrum communication system for despreading digital
information, said system comprising:
said transmitter comprising:
(a) means (40) for providing a plurality of frames of digital data, each
frame having a predetermined number of time slots, one of said time slots
in each frame being assigned to carry acquisition digital data, the time
slots in each frame being assigned to carry communication digital data,
each of said time slots having a header field containing tracking digital
information,
(b) means (10, 20, 30, 50) connected to said providing means and to said
generating means for spreading said frames of digital data,
said receiver having:
(a) means (310, 320, 350) receptive of said spread frames of digital data
and connected to said receiver generating means for despreading said
spread frames of digital data,
(b) means (340, 360, 370) connected to said despreading means for adjusting
said generating means until said receiver acquires said spread frames by
determining the location of said time slot carrying said acquisition data,
said adjusting means acquiring synchronization with said acquisition
digital data in said one time slot in each of said spread frames; after
acquiring synchronization said adjusting means tracking synchronization by
further adjusting said despreading means only during the presence of the
tracking digital information in each of said remaining time slots in each
of said spread frames.
28. A spread-spectrum communication system for transferring digital
information from a plurality of transmitters to a receiver, said system
comprising:
each of said transmitters comprising:
(a) means (40) for providing a plurality of frames of digital data, each
frame having a predetermined number of time slots, one of said time slots
in each frame being assigned to carry acquisition digital data,
(b) means (10, 20) for generating a direct-sequence spreading digital
acquisition code having a higher frequency than the frequency of said
digital data,
(c) means (30, 50) connected to said providing means and to said generating
means for spreading said frames of digital data, said acquisition digital
data in said one time slot being spread by said direct-sequence spreading
digital acquisition code,
said receiver having:
(a) means (320) for generating said direct-sequence spreading digital
acquisition code,
(b) means (310, 350) receptive of said spread frames of digital data from
each of said transmitters and connected to said receiver generating means
for despreading said spread frames of digital data from each of said
transmitters,
(c) means (340, 360, 370) connected to said receiver generating means and
to said despreading means for adjusting said generating means until said
receiver generated direct-sequence spreading digital acquisition code is
in synchronization with said generated direct-sequence code used to spread
said frames of digital data from each of said transmitter, said adjusting
means acquiring synchronization with the acquisition digital data in said
one time slot in each of said spread frames from the transmitter providing
the highest signal strength. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to spread-spectrum signal acquisition and
tracking and, in particular, to direct-sequence spread-spectrum digital
signal acquisition having predictive tracking with clock drift adjustment.
2. Statement of the Problem
A recognized problem with spread-spectrum communication is the acquisition
of the spread signal by a receiver and, after acquisition, the continued
tracking of the spread signal for proper data reception. The term
"spread-spectrum" is defined as any of a group of modulation formats in
which an RF bandwidth much greater than necessary is used to transmit an
information signal so that a signal-to-interference improvement may be
gained in the process. Dixon, "Spread-Spectrum Systems" (second edition,
1984) by John Wiley & Sons, Inc.
Spread-spectrum communication may be based upon direct-sequence,
frequency-hopping, or hybrid modulation formats. The term
"direct-sequence" is defined as a form of spread spectrum modulation
wherein a code sequence is used to directly modulate a carrier. Id. In
direct-sequence spread-spectrum communication, the modulation of the
carrier occurs with a digital code sequence whose bit rate is much higher
than the information signal bandwidth. Typically, the frequency of the
carrier is in the bandwidth range of tens to hundreds of megahertz with
the information signals occurring in the bandwidth range of tens to
hundreds kilohertz.
The information being spread, in some applications, is digital. A need
exists to spread time division multiple access (TDMA) or time division
duplex (TDD) data.
When a direct-sequence spread-spectrum signal is broadcast, a receiver must
be capable of first acquiring the signal and then tracking the signal in
order to accurately despread the information being carried. This problem
is compounded in a TDMA spread-spectrum system because the transmitted
signal is only present in bursts for short periods of time. This
represents two separate design problems--i.e., a problem of acquisition
and a problem of tracking. Most spread-spectrum systems divide the
incoming signal into three hardware paths. One for acquisition, one for
tracking and one for data demodulation. The tracking loop is active even
during data reception and is continually updating the pseudo random
sequence generator (PRSG) of the data demodulation loop. Because of the
non-continuous nature of TDMA or TDD signals, these conventional
approaches are not suitable.
Conventionally, acquisition of direct-sequence spread-spectrum signals can
occur in one of several ways. These approaches are summarized in Rappaport
and Grieco, "Spread-spectrum Signal Acquisition: Methods and Technology",
IEEE Communications Magazine, June 1984, Volume 22, No. 6 (pp. 6-21). One
approach is the utilization of matched filters and active correlators. A
second uses a serial search technique. Other approaches include variable
dwell time schemes, estimation methods, and two-level schemes.
One problem plaguing spread-spectrum communication systems is the length of
time it takes to acquire the transmitted signal by an individual receiver.
The Rappaport and Grieco article specifically recognizes that this problem
in spread-spectrum communications usually consumes a significant amount of
time. A need critically exists in telephony applications to quickly
acquire and track the spread digital data with small and inexpensive
circuitry located in a portable telephone (PT). This problem of
acquisition is caused by the lack of synchronization between the spread
digital signal and the locally generated code in the portable telephone
which is used to de-spread the incoming signal. For proper despreading to
occur, the locally generated direct-sequence code must align or
synchronize with the transmitted direct-sequence code in order to
successfully despread the digital information carried. Receivers can power
up at any given time and, therefore, the locally generated direct-sequence
code can arbitrarily start at any time with respect to the spread signal.
A second problem pertains to the fact that the clocks of the transmitter
and the receiver having normal operating tolerances are typically
misaligned. For example, the clock in the receiver may be faster or slower
than the clock at the transmitter. This must be accounted for during the
step of acquiring, but becomes critical during tracking of the incoming
signal. If the receiver and transmitter clocks are extremely accurate, the
problems of acquisition and tracking become less of a concern. Extremely
accurate clocks are not low power, low cost, or small in size and,
therefore, not suitable for a large number of highly portable and compact
receivers. A need exists to obtain acquisition and tracking in receiver
environment using low cost, low power small clocks in circuits.
Hence, the twofold nature of the problem. First, the desire to quickly and
with inexpensive circuitry acquire the spread TDMA/TDD digital signal and,
secondly, the continued successful tracking after acquisition despite
drift in the clocks between the transmitter and the receiver.
3. Results of Patentability Search
A search of issued patents resulted in the following:
1. U.S. Pat. No. 4,587,662--Langewellpott
2. U.S. Pat. No. 4,984,247--Kaufmann, et al.
3. U.S. Pat. No. 4,841,544--Nuytkens
The Langewellpott patent sets forth a TDMA spread-spectrum receiver with
coherent detection. The Langewellpott system utilizes indirect-path
signals in a receiver for fixed and mobil transmitter-receiver stations of
a TDMA spread-spectrum digital radio system utilizing coherent detection.
At the beginning of each time slot, a synchronization preamble is sent
(once in a preferred embodiment and twice in a second preferred
embodiment). The sync preamble is a specific pseudo random sequence that
is the same for each slot. A correlator is used such as a matched filter.
Sixteen other matched filters detect a specific pseudo random sequence
that corresponds to sixteen four-bit characters. The sync-tracking
correlator therefore serves to synchronize the time slots in the event of
drift between the clock of the transmitter and of the receiver.
Langewellpott requires a synchronization preamble every time slot and the
use of two series-connected correlators. The first correlator serves as a
delay line for the second correlator. The result of the first correlator
is fed to an envelope detector which is followed by a peak detector and a
reducing stage. The result of the second correlator is fed to an envelope
despreader. The peak detector detects the absolute maximum of the
correlation peaks while the second peak detector determines the times of
arrival of the peaks exceeding the threshold value. Hence, synchronization
and continuous tracking is obtained by this. Langewellpott requires the
use of separate correlators which significantly adds to the cost of each
receiver.
The Kaufmann approach provides a digital radio transmission system for a
cellular network using the spread-spectrum method utilizing frequency
division duplex information. The Kaufmann approach is not suited for TDMA
acquisition and tracking.
The patent to Nuytkens sets forth a digital direct-sequence spread-spectrum
receiver. This approach is not applicable to acquiring and tracking TDMA
digital information.
4. Solution to the Problem
The present invention provides a novel system and method for quickly
acquiring a spread digital signal containing TDMA or TDD digital
information which is spread by direct-sequence codes. Once acquired, the
present invention, through a novel and unique process, tracks the incoming
spread signal in order to continually despread the TDMA or TDD digital
information.
A pseudo random sequence generator (PRSG) is designed so that its random
sequence output can be moved in time to match the random sequence in the
received spread signal. The received spread spectrum signal is then
acquired, under the teachings of the present invention, by adjusting the
PRSG in such a manner as to insure that the incoming signal strength of
the desired signal has been sampled at a point in time when the
transmitted signal is present. The PRSG is then adjusted to the point in
the sequence that corresponded with the maximum signal strength. Two types
of sweeps are required to acquire the signal. The major acquisition sweep
approximately locates the peak within a frame and the refinement sweep
precisely locates the peak and identifies the frame and time slot
boundaries.
Tracking is similar to the acquisition process. The amount that the PRSG is
to be adjusted is smaller in tracking because it must adjust only for the
amount of drift between the transmitter and receiver clock that has
occurred since the prior frame. To accomplish tracking, header bytes are
used at the beginning of each time slot. These header bytes allow the
tracking to be completed at a point in time prior to the data portion
being received. The number of header bytes are further minimized by having
a tunable clock which is adjusted to run faster or slower depending upon
how the PRSG was adjusted to find the signal peak.
The present invention accomplishes acquisition, tracking, and data
demodulation with shared circuitry thereby resulting in an inexpensive
receiver. Unlike Langewellpott, the same circuitry is used for
acquisition, tracking and despreading of the data.
SUMMARY OF THE INVENTION
A system and method of direct-sequence spread-spectrum TDMA (or TDD)
digital communication is disclosed. The system and method of the present
invention acquires and tracks a plurality of frames of digital information
wherein each of the frames has a predetermined number of time slots. One
of the predetermined number of time slots in each frame is assigned for
acquisition purposes and carries acquisition and sync digital information.
The remaining time slots contain communication data including header bytes
used for tracking of the acquired signal. The assigned acquisition time
slot is spread with an acquisition direct-sequence spreading code. The
remaining time slots are spread with communication direct-sequence
spreading codes.
The receiver of the present invention receives the transmitted spread
frames of digital information. During acquisition, the signal strength in
each successive time slot for each frame is measured for a given number of
time frames. The peak is located within the given number of time frames
through a major acquisition sweep and a refinement sweep. The major
acquisition sweep and refinement acquisition sweep locate the frame and
time slot boundaries of the transmitted acquisition signal.
Once acquired, tracking occurs during the header bytes found in each
successive time slot. Tracking occurs under two approaches of the present
invention. The first approach is to predictively adjust the pseudo random
signal generator at the receiver for the next frame by measuring the drift
between the prior two successive frames. Tracking is also accomplished by
adjusting the main oscillator of the receiver for drift down to a
predetermined incremental amount. The predictive tracking and the
oscillator adjustment procedures combine together into a "hybrid" tracking
technique which rapidly and inexpensively performs tracking during the
header bytes of each time slot. Acquisition and tracking do not occur
during data demodulation and, therefore, the same receiver circuitry is
used for acquisition, tracking and demodulation.
DESCRIPTION OF THE DRAWING
FIG. 1 is a block diagram of the electronic components of the transmitter
of the present invention;
FIG. 2 sets forth the formats for the frame, time slots, and acquisition
coded sequence of the present invention;
FIG. 3 sets forth the block diagram components of the receiver of the
present invention;
FIG. 4 sets forth the flow chart for the major acquisition sweep of the
present invention;
FIG. 5 sets forth the frame by frame analysis performed by the present
invention during the major acquisition sweep of FIG. 4;
FIG. 6 illustrates the advancement of the pseudo random sequence generator
by one-half chip intervals during the active correlation of the major
acquisition sweep of FIG. 4;
FIGS. 7a-7b illustrate an example of the refinement sweep of the present
invention;
FIGS. 8a-8b set forth the flow chart of the refinement sweep of the present
invention;
FIG. 9 illustrates the portable telephone of the present invention
receiving three transmitted signals;
FIG. 10 sets forth the predictive tracking feature of the present
invention;
FIG. 11 sets forth the block diagram components of the PRSG of the present
invention;
FIG. 12 illustrates the digital timing generation from the chips of the
pseudo random sequence; and
FIG. 13 illustrates the digital bit clock, nibble clock, and byte clock
generation for the despread digital information.
DETAILED SPECIFICATION
1. Spreading
In FIG. 1, the transmission by a transmitter system (or a remote cell unit
RCU 100) of the direct-sequence spread spectrum carrying time division
duplex (TDD) or time division multiple access (TDMA) digital information
is shown. A conventional CB-X clock 10 drives a pseudo random sequence
generator (PRSG) 20 and a 1.times. clock 14 over lines 12. In the
preferred embodiment, the clock (or oscillator) operates at a frequency of
12.288 MHz. The present invention is not limited by the frequency of the
clock 10 or by the amount of drift present within clock 10. In fact, the
present invention using time frames of 10 milliseconds operates with
inexpensive clocks having high drift such as 25 ppm. As will be explained
with reference to FIG. 1, the 1.times. clock 14 provides the bit clock
signals over lines 16 to the digital data 40 and to the PRSG 20.
The CB-X clock provides timing for the chips of the pseudo random sequence
and, in the preferred embodiment, CB=32 chips per bit and clock 10 is a
32.times. clock. The drift of the clock 10 is relative and based upon a
number of factors. For example, the drift of the transmitter clock 10 and
the receiver clock is relative to each other--e.g. the receiver may have
greater drift than the transmitter or vice-versa. The drift is also a
function of the length of the time frame. The shorter the time frame, the
greater the drift tolerated. The drift is also a function of the length of
the pseudo random sequence. The shorter the length, the greater the drift
that can be tolerated. The ensuing invention seeks to use inexpensive
clocks with high drifts. However, and as will be further discussed, if the
sequence is too long and/or if the time frame is too long, then
acquisition will become too long. In the preferred embodiment, a chip
sequence of 128 chips, a time frame of 10 milliseconds and a clock drift
of 25 ppm is used and provides only one design choice of many.
The PRSG 20 is also conventional and provides the direct-sequence codes to
spread the digital information. One code produced by the PRSG generator 20
is an acquisition code. The acquisition code is illustrated in FIG. 2 as a
partial sequence 200. The output of generator 20 is delivered over line 22
into a modulo-2 addition circuit 30. TDMA digital data is provided by
circuit 40 over lines 42 into the addition circuit 30. It is to be
expressly understood that the individual electronic components of the RCU
100 of FIG. 1 are conventional and that a number of different hardware
designs could be utilized to spread TDMA digital data by direct-sequence
codes under the teachings of the present invention. The assembly of
digital data to be spread is well known and may be accomplished by a
number of different approaches. The relationships of the chips in the
direct-sequence code to the bit timing and the methods of acquisition and
tracking are fully explained in the following with reference to a
preferred embodiment.
In FIG. 2, the TDD or TDMA data is delivered one frame 210 at a time at a
preferred frequency, f.sub.F, such as at a 100 Hz rate. Each frame 210 has
a predetermined number of time slots, N.sub.ts, such as twelve time slots,
TS1 through TS12. The time slots are delivered at a frequency of F.sub.ts
=N.sub.ts * f.sub.F or 1.2 KHz in the preferred embodiment. The first time
slot, as shown at 220, is dedicated for acquisition purposes. Slot 1 of
each frame contains fixed and variable digital data spread according to an
acquisition code sequence 200.
While the first slot is used in the following discussion, it is to be
understood that the acquisition time slot could be located anywhere in the
frame. Furthermore, any number or configuration of time slots TS could be
provided. In the embodiment shown in FIG. 2, each time slot TS contains
forty bytes of formatted information. In the case of the acquisition time
slot TS1, the data is formatted as follows:
TABLE I
G=Guard Band (1 byte)
H=Header (6 bytes)
SYNC1=First Sync (3 bytes)
D=Data (27 bytes)
SYNC2=Second Sync (3 bytes)
The G byte is dead time which allows for propagation delays and for turning
on and off the receiver and transmitter. The H bytes are used for tracking
and will be discussed later. The SYNC1 and SYNC2 bytes are used for
acquisition. Each sync field contains a synch word followed by two bytes
identifying the beginning or end of the acquisition time slot TS1. The D
bytes are used to carry communication data. It is to be expressly
understood that the acquisition and tracking features of the present
invention are not to be limited by the format of Table I or by the
inclusion of the D bytes.
This specific data format 220 for the acquisition of time slot TS1 shown in
FIG. 2 is designed for transmission of digital communication data. It is
to be expressly understood that a frame 210 may be configured with any
number of time slots, N.sub.ts. Each time slot 220 can have any number of
bytes, N.sub.bytes, and can be suitably formatted with any arrangement of
digital data necessary for a specific application. Each time slot has a
predetermined number of bits, B.sub.ts. This invention, therefore, is not
to be limited to the data format 220 of FIG. 2. The drawings herein are
for a preferred system embodiment and serve to illustrate the operation of
the present invention.
One bit of data in time slot 220 is shown at 230. With reference back to
FIG. 1, each bit of data on line 42 is delivered into the adder 30 where
it is combined with the direct-sequence acquisition code 200 to output on
line 32 the spread TDMA digital signal shown in FIG. 2 at 240 which is
spread transmitted by transmitter 50 over airwaves 52. In FIG. 2, a solid
curve 230A represents a digital "zero" whereas the dashed curve 230B
represents a digital "one." The transmitted signal 240 corresponds to the
digital "zero" bit 230A, whereas the inverse of curve 240 would correspond
to a transmitted digital "one" bit 230B.
The frames 210 are repeatedly delivered at a 100 Hz rate in bursts. Each
time slot 230 is delivered at a frequency of f.sub.ts =1.2 KHz. Hence, the
acquisition time slot TS1 is repeatedly delivered in each frame of the
transmission. Each digital bit 230 is delivered at a frequency of
f.sub.bit =8 * N.sub.bytes * f.sub.ts or at a 384 KHz rate (8 * 40 * 1.2
KHz) in the preferred embodiment.
The direct-sequence acquisition code is designed to have a fixed number of
chips precisely aligned and synchronized with each digital bit,
N.sub.chips/bit, which in the preferred embodiment, is 32 chips per
digital bit as provided by line 16 from the 1.times. clock. This is
clearly shown in FIG. 2 with the initial chip (i.e., chip 1) commencing
with the start of the bit 230 as shown by line 204 and with the final chip
(i.e., chip 32) terminating with the bit 230 as shown by line 208). Any
suitable chip rate per bit more or less than 32, could also be utilized
under the teachings of the present invention. When 32 chips per bit (i.e.,
CB=32) are utilized, then the transmitted data would be a spread-spectrum
signal of 12.28 MHz or f.sub.chips =CB * f.sub.bit. The acquisition
spreading code 200 is M chips long (M=128 chips in the preferred
embodiment). Therefore, in the acquisition time slot TS1, the spreading
code is repeated precisely 80 times (i.e., 8 bits.times.40 bytes=320
bits/time slot; 320 bits/time slot.times.32 chips/bit=10,240 chips/time
slot; 10,240 chips/time slot.div.128 chips/sequence=80 sequences/time
slot). This can be expressed as B.sub.ts * CBM which has an integer value
under the teachings of this invention.
In summary, each frame 210 of digital information carries the acquisition
time slot which in turn provides digital acquisition information spread by
an acquisition code sequence continually repeating within the acquisition
time slot. It is to be expressly understood that the chip length of the
coded sequence can be more or less than N.sub.chips =128 chips and that
the teachings of this invention are not to be limited by the chip length
of the repeating acquisition code. Furthermore, more or less than 32 chips
per bit CB could be utilized. The ratio between N.sub.CHIPS and CB will be
discussed in the section on digital timing.
Only the acquisition time slot TS1 is spread by the direct-sequence
acquisition code, the remaining time slots TS2-TS12 contain communication
digital data and are spread by the same or by different direct-sequence
communication codes depending upon system implementation. All spreading
codes are M chips in length and have the same CB value. The teachings of
the present invention are not limited by the specific combination of
direct-access pseudo random sequence codes used to spread the digital
information.
2. Receiving System
In FIG. 3, a remotely located portable telephone PT 300 contains a receiver
310, a pseudo random sequence generator (PRSG) 320, a mixer 330, a clock
340, a despreader 350, a microprocessor 360, and a read circuit 370. FIG.
3 sets forth a design for "despreading" received TDMA data. Only a portion
of the circuitry necessary to implement the present invention is shown,
the remaining circuitry needed to operate the portable telephone 300 is
not shown.
With reference to FIG. 2, the transmitted spread data 52 is received by
receiver 310 and delivered over lines 312 into a mixer 330 of despreader
350. Mixer 330 receives the direct-sequence acquisition code over lines
322 from the PRSG 320 and despreads the time division duplex data 230 on
lines 312 from the transmitted data and delivers a 122 MHz DPSK encoded
signal (i.e., a despread RF signal carrying the despread digital data) as
shown by curve 230 on lines 332. The delivered direct-sequence acquisition
code is identical to the acquisition code used to spread the digital
content of the acquisition time slot.
In the preferred embodiment, the despread RF signal on lines 332 is
delivered into a first bandpass circuit 352 which has a bandpass range of
750 KHz and a center frequency of 122 MHz. The output 353 of bandpass
circuit 352 is delivered into a mixer 354 which is mixed which a 112 MHz
signal from oscillator 356 delivered on line 357. The output 355 of mixer
354 is delivered into a second bandpass filter 358 having a bandpass range
of 750 KHz and a center frequency of 10.7 MHz. The output 359 of the
second bandpass circuit is delivered into an amplifier 380 which delivers
the despread 10.7 MHz DPSK encoded data on lines 382 into the portable
telephone 300 for processing. A signal strength output 384 is delivered
into the microprocessor 360.
A main oscillator 340 delivers clock signals over line 342 to the
microprocessor 360. The main oscillator 340 is accurate within plus or
minus twenty-five parts per million. The microprocessor 360 can control
the speed of the main oscillator over lines 344. The microprocessor 360 is
also connected to the PRSG 320 over lines 362 and 364 which retard and
advance, respectively, the PRSG 320 in chip, one-half chip or in
one-quarter chip intervals. The individual components of the receiving
system 310 are of known design and configuration. The teachings of the
present invention are not to be limited by the circuit configuration of
FIG. 3.
As will be explained in the following, the circuitry presented in FIG. 3 is
utilized first, to acquire the spread transmitted signal 52. Once
acquired, the circuitry is used to continually track and to deliver the
despread digital data on lines 382 into the portable telephone 300. In
this case, the demodulated signal on lines 382 is a 10.7 MHz differential
phase shift key (DPSK) signal. The circuitry is used for all three
functions of acquisition, tracking, and despreading. The techniques of
acquisition and tracking are not limited to the environment of telephony
receivers and could find application in any suitable spread spectrum
system of communication in which TDMA or TDD data is transferred.
3. Acquisition of the Spread Signal
The acquisition process is divided into two major steps. The first step is
called the major acquisition sweep and the second step is termed the
refinement sweep. The object of the major acquisition sweep is to align
the receiver's PRSG 320 with the transmitted acquisition time slot signal
240. When alignment (i.e., alignment of the receiver's acquisition code
with the transmitter's acquisition code) occurs, a maximum signal strength
on line 384 results. The object of the refinement sweep is to locate the
frame and slot timing boundaries of the transmitted digital data. Each of
these steps are discussed in the following.
a. Major Acquisition Sweep
The remote cell unit RCU 100 transmits the acquisition time slot TS1 at the
beginning of each frame. Hence, in the preferred embodiment, every 10
milliseconds, the acquisition time slot TS1 is broadcast by the RCU 100.
When a portable telephone PT300 awakens, the receiving system 300 is
powered and the receiver 310 commences to receive the burst of transmitted
frames 210 from the remote cell unit 100 with each frame transmitted every
ten milliseconds. The portable telephone 300 must become synchronized with
the acquisition code 200 in time slot 1 before output digital data can be
delivered on lines 382. Since the portable telephone 300 can awaken at any
given time, system 300 simply does not know where the acquisition time
slot S1, in time, is. Hence, the pseudo random acquisition sequence of the
transmitted signal on line 312 must be interrogated in all possible time
slots.
In FIGS. 4-6, the operation of the major acquisition sweep of the present
invention is set forth and is conventionally implemented as software in
the microprocessor 360 of FIG. 3. Major acquisition is accomplished by
checking all possible alignments of the PRSG 320 against all possible
positions of the acquisition slot. In order to minimize the length of time
for the major acquisition sweep, the number of chips scanned in a group
per frame is maximized. In the preferred embodiment, the number of chips
per group scanned is 21 chips and is shown in FIG. 5. It is to be
expressly understood that this number could be more or less than 21. The
PRSG 320 is incremented in one-half chip increments when scanning so that
42 individual scans are performed. The scanning of all chips in a group
must take less than one slot time. If the scanning of the group of 21
chips takes more than one time slot in time to perform, then chips in the
acquisition code will be missed. Hence, it is optimum to configure the
scanning of the group of 21 chips to occur in a time slightly less than
one time slot. Thus, 13 groups of 21 chips are scanned in a frame since 12
time slots are used. This insures that each of 128 chips in the
acquisition sequence will be scanned in the major sweep. Likewise, the 13
groups of 21 chip scans will be optimally performed in a time slightly
greater than the time of a frame. The 21-chip group scan is repeated until
just greater than one time frame elapses and then the next set of
21-chip/groups are evaluated. Of course, the number of groups scanned
depends on the number of time slots.
As set forth in FIG. 4, the PT 300 can awaken at any time, and therefore,
has an arbitrary start 400. The PRSG 320 of FIG. 3 is at an arbitrary
start position 400 and the mixer 330 attempts to despread the signal 240.
The signal strength on line 384 of the despread signal is me | | |
