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Claims  |
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What I claim is:
1. A method of time aligning signals for reception in a code-division
multiple access (CDMA) communication system, the method comprising the
steps of:
receiving, at a base-station, signals transmitted by a plurality of
subscriber units;
determining a time position of each signal received based on estimates of
the power of a plurality of delayed rays representative of each signal;
and
transmitting, from the base-station, an alignment signal, based on the step
of determining, to each of the plurality of subscriber units such that
subsequent signals transmitted by the plurality of subscriber units are
received by the base-station substantially in time alignment.
2. The method of claim 1 wherein the subsequent signals transmitted by the
plurality of subscriber units and received by the base-station
substantially in time alignment are modulated with spreading codes which
are orthogonal to one another.
3. The method of claim 2 wherein reception and a demodulation of one of the
subsequently transmitted signals provides a cross-correlation of
substantially zero with respect to the remaining subsequently transmitted
signals.
4. The method of claim 1 wherein the step of determining a time position of
each signal received based on estimates of the power of a plurality of
delayed rays representative of the signal further comprises the step of
determining a time position of each signal received based on the time of
arrival of the strongest of the estimates of the power of a plurality of
delayed rays representative of the signal.
5. The method of claim 1 wherein the step of determining a time position of
each signal received based on estimates of the power of a plurality of
delayed rays representative of the signal further comprises the step of
determining a time position of each signal received based on the mean of
the times of arrival of the plurality of delayed rays representative of
the signal.
6. A method of time aligning signals for reception in a code-division
multiple access (CDMA) communication system, the method comprising the
steps of:
receiving, at a base-station, signals transmitted from a plurality of
subscriber units, each signal modulated with a per symbol spreading
function orthogonal to one another;
determining a time position of each signal received based on estimates of
the power of a plurality of delayed rays representative of each signal;
and
transmitting, from the base-station, an alignment signal, based on the step
of determining, to each of the plurality of subscriber units such that
subsequent signals transmitted by the plurality of subscriber units are
received by the base-station substantially within a fraction of a chip of
one another.
7. The method of claim 6 wherein reception and a demodulation of one of the
subsequently transmitted signals provides a cross-correlation of
substantially zero with respect to the remaining subsequently transmitted
signals.
8. The method of claim 6 wherein the step of transmitting an alignment
signal further comprises the step of transmitting a distinct alignment
signal to each of the plurality of subscriber units.
9. A base-station for time aligning signals for reception in a
code-division multiple access (CDMA) communication system, the
base-station comprising:
means for receiving signals transmitted by a plurality of subscriber units;
means, coupled to the means for receiving, for determining a time position
of each signal received based on estimates of the power of a plurality of
delayed rays representative of each signal; and
means, coupled to the means for determining, for transmitting an alignment
signal to each of the plurality of subscriber units such that subsequent
signals transmitted by the plurality of subscriber units are received by
the base-station substantially in time alignment.
10. The base-station of claim 9 wherein the means for determining further
comprises a comparator for comparing a time of arrival of each signal
received with an expected time of arrival for each signal to determine a
time position for each signal.
11. The base-station of claim 9 wherein the means for determining a time
position of each signal received based on estimates of the power of a
plurality of delayed rays representative of the signal further comprises
means for determining a time position of each signal received based on the
time of arrival of the strongest of the estimates of the power of a
plurality of delayed rays representative of the signal.
12. The base-station of claim 9 wherein the means for determining a time
position of each signal received based on estimates of the power of a
plurality of delayed rays representative of the signal further comprises
means for determining a time position of each signal received based on the
mean of the times of arrival of the plurality of delayed rays
representative of the signal
13. A base-station for time aligning signals for reception in a
code-division multiple access (CDMA) communication system, the
base-station comprising:
a receiver for receiving signals transmitted by a plurality of subscriber
units;
a comparator, coupled to the receiver, for determining a time position of
each signal received based on estimates of the power of a plurality of
delayed rays representative of each signal; and
a transmitter, coupled to the means for determining, for transmitting an
alignment signal to each of the plurality of subscriber units such that
subsequent signals transmitted by the plurality of subscriber units are
received by the base-station substantially in time alignment.
14. The base-station of claim 13 wherein the alignment signal further
comprises an advance/retard bit periodically transmitted to each of the
plurality of subscriber units.
15. The base-station of claim 13 wherein the alignment signal further
comprises a message containing at least a timing value field and a
polarity field.
16. A method of time aligning signals for reception in a microcellular
code-division multiple access (CDMA) communication system, the method
comprising the steps of:
receiving, at base-stations within first and second microcells, signals
transmitted by first and second groups of subscriber units;
determining, for each group of signals, a time position of the signals
within the group received relative to one another;
comparing the time position for each group; and
transmitting, from either the first or second base-station, an alignment
signal, based on the step of comparing, to the corresponding first or
second group of subscriber units such that subsequent signals transmitted
by the group of subscriber units are received by the base-station within
the first or second microcell substantially in time alignment with signals
received by the basestation within the other microcell.
17. The method of claim 16 wherein said first and second microcells are
adjacent.
18. The method of claim 16 wherein the step of comparing the time position
for each group further comprises the step of comparing the time position
for :each group to a reference value. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The invention relates generally to communication systems, and more
particularly to time aligning signals for reception in code-division
multiple access communication systems.
BACKGROUND OF THE INVENTION
In code-division multiple access (CDMA) communication systems,
communication between a base-station and a subscriber unit is accomplished
by spreading each transmitted signal over the frequency band of the
communication channel with a unique user spreading code. As a result,
transmitted signals are in the same frequency band of the communication
channel and are separated only by the unique user spreading codes. These
unique user spreading codes preferably are orthogonal to one another such
that the cross-correlation between the spreading codes is substantially
zero.
Particular transmitted signals can be retrieved from the communication
channel by despreading a signal representative of the sum of signals in
the communication channel with a user spreading code related to the
particular transmitted signal which is to be retrieved from the
communication channel. Further, when the user spreading codes are
orthogonal to one another, the received signal can be correlated with a
particular user spreading code such that only the desired user signal
related to the particular spreading code is enhanced while the other
signals for all of the other users are significantly de-emphasized.
To maintain orthogonality among all channels, the channels must be
substantially time aligned on a per-chip basis. For forward channel
transmission (from base-station to subscriber unit), this time alignment
is easily obtained via a synchronized process of combining the traffic
channels together at each basestation. However, time alignment of reverse
channel signals (from subscriber units to a particular base-station) is
not as simple due to the time-randomness of transmissions by subscriber
units. Attempts have been made to synchronize the reverse channel signals
at a base-station, but such attempts require extra circuitry at the
base-station, which in turn leads to both increased complexity and added
cost to the base-station.
Thus, a need exists for a simple and cost effective method and apparatus
for time aligning signals for reception in a CDMA communication system.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 generally depicts, in block diagram form, a communication system in
accordance with the invention.
FIG. 2 generally depicts a forward channel frame having an advance/retard
bit inserted therein in accordance with the invention.
FIG. 3 generally depicts, in block diagram form, an alternate embodiment of
a transmitter which may be employed in accordance with the invention.
FIG. 4 generally depicts, a typical microcellular system.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
A coherent reverse channel, a per-chip spreading function, orthogonal
spreading functions and a time alignment of all traffic channels are
implemented such that the main signal of each channel arrives at a
base-station within a fraction of a chip of one another in accordance with
the invention. With this, the orthogonality among all channels is
maintained, and, when demodulated, all channels except the channel of
interest provides a cross-correlation of substantially zero with respect
to the remaining signals. In this way, at least 3 dB of extra sensitivity
can be achieved in typical CDMA communication systems, and as much as 10
dB in a personal communication system (PCS) environment where delay spread
effects are less likely to be encountered.
FIG. 1 generally depicts, in block diagram form, a communication system in
accordance with the invention. In the preferred embodiment, the
communication system of FIG. 1 is the coherent communication system
substantially shown and described in patent application Ser. No.
08/031,258, entitled "Method and Apparatus for Coherent Communication in a
Spread Spectrum Communication System," filed Mar. 11, 1993, invented by
Fuyun Ling and herein incorporated by reference. Additionally shown in
FIG. 1 is comparator 169, modulator 170, demodulator 178 and clock 151.
Comparator 169 has as input a clock signal 149 output from clock 151 and a
power estimate signal 148 output from power estimator 146. In the
preferred embodiment, power estimator 146 utilizes a sum and square
technique to perform power estimation, which is well known in the art.
During operation, demodulator 122 outputs signals 148 and 149 (as does,
inter alia, demodulator 123 with it's corresponding clock signal 153 and
power estimate signal 155) to comparator 169 where the signal with the
strongest estimated power is determined. In the preferred embodiment, four
demodulators are utilized to receive a single transmission from a
subscriber unit. Each demodulator demodulates a delayed ray of the
subscriber unit's transmission based on an assignment determined by the
energy of the delayed ray. While not separately shown, each of the four
demodulators may be switched between any of a plurality of antennas
coupled to a base-station. Continuing, the clock signal from the
demodulator having the strongest estimated power is then utilized to
compare, in comparator 169, the time of arrival of that delayed ray
received with a reference value so as to determine a time position for
each signal. Two rays having substantially the same delay will be combined
in comparator 169 before comparison to the reference value. In addition, a
mean of the times of arrival of the plurality (four) delayed rays which
are representative of the signal may likewise be utilized for comparison
to the reference value in comparator 169.
In the preferred embodiment, the reference value may be an (absolute)
expected time of arrival for the signal (i.e., a single subscriber unit's
transmission) or a value related to the time of arrivals of signals with
respect to one another (i.e., a group of subscriber unit transmissions).
For example, a value related to the time of arrival of signals with
respect to one another could be the mean of the time of arrival of all
signals (and/or their delayed rays), the latest of the time of arrival of
all signals, etc. As one of ordinary skill in the art will appreciate,
many characteristics/combinations of the signals/rays exist to determine
the reference value.
Once the time position of the signal is determined, comparator 169 produces
an alignment signal 171 which is transmitted to a subscriber unit, such as
a subscriber unit comprised of, inter alia, blocks 100 and 116. The
subscriber unit receives the alignment signal, adjusts a clock based on
the alignment signal, and transmits information to a base-station
utilizing the adjusted clock. In this manner, the technique in accordance
with the invention is utilized to either advance/retard a subscriber
unit's transmission as seen by the base-station.
In the preferred embodiment, operation of modulator 170 is substantially
equivalent to the operation of modulator 117. Alignment signal 171 is
input into modulator 170, as is signal 172 (substantially equivalent to
signal 114). Modulator 170 then transmits, inter alia, alignment signal
171 to the corresponding subscriber unit (and other alignment signals to
other corresponding subscriber units) such that any subsequent signals
transmitted by the plurality of subscriber units are received by the
base-station substantially in time alignment.
In the preferred embodiment, alignment signal 171 may be produced in
several ways. First, it may be produced as a message. In IS 95, published
by Telecommunications Industry Association, 2001 Pennsylvania Avenue NW,
Washington, D.C., there is a provision titled "Ordered Messages" which
allows an equipment manufacturer to create custom messages. For alignment
signal 171 in accordance with the invention, the message may look like the
message found in Table 1 below:
TABLE 1
______________________________________
For- Order
ward Code, Order ACTION
Paging
Traffic OR- Qual. TIME Add-
Chan- Chan- DER Code can be tional
nel nel (bi- ORDQ specified
Fields
Name/
Order Order nary) (binary)
ORDQ other Func.
______________________________________
No Yes 011111 snnnnnnn
No No Time
Align.
Order
______________________________________
where nnnnnnn is a timing value field having the value of the timing change
(in nanoseconds) and s is a polarity change field having the polarity of
the change (advance/retard). The message would be transmitted on the
forward traffic channel with an order code (for example) of "011111", and
be titled "Time Alignment Order."
A second embodiment would incorporate an advance/retard bit inserted into a
forward channel frame as shown in FIG. 2. The frame would be a modified
version of the forward channel frame defined in the IS 95 standard. At bit
position 1, when message mode (MM) is a "0", a subscriber unit would know
that an advance/retard is required. At bit position 2, advance/retard (AR)
bit would indicate which action is to be taken; if AR="1", then advance
timing and if AR="0", then retard timing. Typically, the timing change is
a small fraction of a chip (for example, less than 1/10.sup.th of the
period of the chip).
For each subscriber unit within the communication system, there is a unique
spreading code associated therewith. When a base-station transmits an
alignment signal 171 to each particular subscriber unit, each subscriber
unit will advance/retard its transmission so that any subsequent
transmission will be time aligned when received and demodulated by the
base-station.
FIG. 3 generally depicts, in block diagram form, an alternate embodiment of
a transmitter which may be employed in accordance with the invention. In
this embodiment, transmitting portion 116 comprises a quadrature phase
shift keyed (QPSK) modulator, as is well known in the art, coupled to a
linear power amplifier (LPA) 318). Transmitter portion 116 of FIG. 3 can
likewise be implemented utilizing a .pi./4 QPSK modulator if circuit 306
rotates the phase by .pi./4 radians every chip. One skilled in the art
would also recognize that transmitter portion 116 can be readily adapted
to implement offset QPSK modulation.
As is well known to one of ordinary skill in the art, CDMA communication
systems employ soft hand-off. In a soft hand-off situation, a subscriber
unit receives (synchronized) transmissions from more than one
base-station. As such, a subscriber unit receiving the alignment signal
transmitted in accordance with the invention, in a preferred embodiment,
would receive multiple transmissions but only follow the advance/retard
command contained within one of the transmissions (for example, the
transmission having it the strongest power as seen by the subscriber
unit). In an alternate embodiment, a subscriber unit could be instructed
by a base-station to follow the timing commands of a particular one of the
base-stations received.
Time alignment in accordance with the invention is applicable to a typical
microcellular system as depicted in FIG. 4 where a controller 400 couples
base-stations 409, 412 in microcells 403, 406, respectively, to one
another. Also depicted in FIG. 4 is a cell 415 of a cellular system having
a base-station 418 likewise coupled to controller 400 (the coupling link
is not shown). In typical microcellular systems, the cell 403, 406 is
substantially smaller than the cell 415 of a traditional cellular system
as can be seen in FIG. 4.
As a consequence, timing advance/retard is not usually a concern in
microcellular systems since, due to the small size of the cell,
transmissions of individual subscriber units within a microcell 403 or 406
arrive substantially in time alignment. However, adjacent microcells 403,
406 still present interference between one another as occurs in adjacent
cells of cellular systems. Consequently, the value related to the time of
arrivals of signals with respect to one another may be used to adjust an
entire group of subscriber unit transmissions (within a particular
microcell) by a common time, so that the principal of orthogonality may be
beneficially utilized between adjacent microcells. For example, if
orthogonal spreading codes are implemented between microcells (as compared
to orthogonal spreading codes implemented between subscriber units), an
adjustment to substantially time align an entire group of subscriber unit
transmissions (within a particular microcell) with an entire group of
subscriber unit transmissions (within an adjacent microcell) would provide
the orthogonality benefits between microcells as is seen between
subscriber units in the above preferred embodiment. In this alternate
embodiment, comparator 169, for each receiver demodulating a single
subscriber unit's transmission, would have an output (not shown) which
would be input into controller 400. Controller 400 would determine a
"group" time of alignment signal, and command base-stations 409, 412 to
have each subscriber unit in the group adjust accordingly. By adjusting
the entire group within a microcell 403, 406, the principal of
orthogonality may be beneficially utilized between adjacent microcells so
that any cross-correlation between microcells is substantially zero. One
example, inter alia, of an application of group time advance/retard would
be to time advance/retard a group of subscribers in a lightly loaded
microcell to the benefit of a heavily loaded microcell.
While the invention has been particularly shown and described with
reference to a particular embodiment, it will be understood by those
skilled in the art that various changes in form and details may be made
therein without departing from the spirit and scope of the invention.
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Description |
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