Note: Descriptions are shown in the official language in which they were submitted.
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DEVICE AND METHOD FOR REDUCING THE PEAK-TO-AVERAGE
POWER RATIO OF A MOBILE STATION'S TRANSMIT POWER
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to mobile communication systems,
and in particular, to a device and method for reducing the peak-to-average
power
1 o ratio of a mobile station's transmit power in a mobile communication
system.
2. Description of the Related Art
A typical CDMA mobile communication system focuses on voice service,
whereas a 3G mobile communication system provides the additional services of
high quality voice, high speed data, moving pictures, and Internet browsing.
In such
a mobile communication system, a radio link consists of a forward link
directed
from a base station (BS) to a mobile station (MS) and a reverse link directed
from
the MS to the BS.
When zero-crossing occurs during spreading and modulation in a reverse link
transmission (a phase variation is n), the peak-to-average power ratio of the
mobile
2 o station's transmit power (mobile transmit power) increases, thereby
producing
regrowth. Regrowth adversely affects the communication quality of calls being
made by other subscribers. Hence, the peak-to-average power ratio is a
significant
factor in the design and performance of a power amplifier in an MS.
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Regrowth occurs due the existence of a linear portion and a non-linear
portion in a characteristic curve of the mobile station's power amplifier. As
the
mobile transmit power increases, a transmit signal of the MS due to the non-
linear
characteristics generates interference in the frequency area of a different
user
s causing the regrowth phenomenon.
Regrowth can be prevented by shrinking down a cell and sending a signal
from an MS in the cell to a corresponding BS at a Iow power level. Thus,
mobile
transmit power can be flexibly controlled if the mobile station's peak-to-
average
power ratio can be limited to a specific range. However, it is not economical
to
1 o physically shrink a cell, since more cells are then needed for a given
area and each
cell requires its own communications equipment.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide device and
method for reducing the peak-to-average power ratio of a mobile station's
transmit
15 power in a mobile communication system.
Another object of the present invention is to provide a method of flexibly
controlling mobile transmit power by limiting its peak-to-average power ratio
to a
specific range.
A further object of the present invention is to provide a method of flexibly
2 o varying a cell size in a mobile communication system to counteract
regrowth.
A still further object of the present invention is to provide a method of
enhancing auto-correlation characteristics of a multipath signal and cross-
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correlation characteristics relative to other users.
To achieve these and other objects, a device and method for reducing the
peak-to-average power ratio of mobile transmit power in a mobile communication
system are provided. The device and method spread mobile transmission data by
s a complex spreading sequence.. The complex spreading sequence includes a
plurality of chips and is generated to have a phase difference of 90 °
between every
two successive complex chips in response to each chip of a PN (Pseudo Noise)
sequence.
BRIEF DESCRIPTION OF THE DRAWINGS
1 o FIG. 1 is a block diagram of a mobile station for performing the spreading
and modulating method according to an embodiment of the present invention;
FIG. 2 is a block diagram of a first embodiment of a n/2 DPSK (Differential
Phase Shift Keying) shown in FIG. 1;
FIGS. 3A and 3B illustrate signal constellation and phase transition of
1 s complex spreading sequences according to the structure of the a~/2 DPSK
generator
shown in FIG. 2;
FIG. 4 is a block diagram of a second embodiment of a n/2 DPSK generator
shown in FIG. I ;
FIGS. 5A and SB illustrate signal constellation and phase transition of
2 o complex spreading sequences according to the structure of the n/2 DPSK
generator
shown in FIG. 4;
FIG. 6 is a block diagram of a mobile station in a 3G IS-95 system to which
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the spreading and modulating method according to the present invention is
applied;
and
FIG. 7 is a block diagram of a mobile station in a W-CDMA (Wideband
Code Division Multiple Access) system to which the spreading and modulating
method according to the present invention is applied.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will be described herein
below with reference to the accompanying drawings. In the following
description,
well known constructions or functions are not described in detail so as not to
obscure the present invention.
The present invention includes the following novel and inventive features:
( 1 ) mobile transmit power can be flexibly controlled by limiting its peak-to-
average power ratio to a specific range and thus confining the mobile transmit
power to a linear characteristic portion in a characteristic curve of a power
amplifier;
(2) the phase of a complex spreading sequence is prevented from shifting by
180° (i.e., n) to maintain the mobile transmit power in the linear
portion of the
power amplifier characteristic curve;
(3) the phase difference between every two successive complex chips of a
2 o complex spreading sequence (PNI and PNQ) is 90° (i.e., n/2) to
limit the output
power range of baseband filters and thereby, reduce the peak-to-average power
ratio
of mobile transmit power; and
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(4) auto-correlation characteristics of a multipath signal and cross-
correlation
characteristics relative to other users are improved by respreading a signal
which
has passed through a complex spreader by a spreading sequence PNZ generated
from
a PN code generator.
It is to be appreciated in the embodiment of the present invention that "n/2
DPSK (Differential Phase Shift Keying)" does not refer to a typical DPSK and
is
so named because the complex spreading sequence PN,+~PN~ generated in the n/2
DPSK generator has a phase variation of n/2 for one chip duration.
With reference to FIG. 1, there is shown a schematic block diagram of a
1 o mobile station (MS) which will be referred to for describing methods of
spreading
and modulating mobile transmission data for reducing the peak-to-average power
ratio of mobile transmit power according to embodiments of the present
invention.
A complex signal including in-phase data I-data and quadrature-phase data Q-
data
is applied as a first input signal to a complex spreader 2. A PN, generator 4
1 s generates a sequence PN,, and a nl2 DPSK generator 6 generates complex
spreading sequences PN, and PNQ with the sequence PN, received from the PN,
generator 4. The complex spreading sequences PNl and PNQ are fed as a second
input signal to the complex spreader 2. The embodiment of the present
invention is
characterised in that there is no zero-crossing since the phase difference
between
2 o every two successive complex chips of the complex spreading sequence (PNl
and
PNQ) is n/2. The structure and operation of the n/2 DPSK generator 6 is
described
in detail below with reference to FIGS. 2 to SB.
In FIG. 1, the complex spreader 2 includes multipliers 8, 10, 12, and 14 and
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adders 16 and 18 for complex-spreading the complex signal by the complex
spreading sequences PN, and PNQ. A detailed description of the operation of
the
complex spreader 2 can be found in Korean Patent Application No. 98-7667
having
a common assignee.
Multipliers 20-1 and 20-2.multiply the resulting in-phase spread signal XI
and quadrature-phase spread signal XQ received from the complex spreader 2 by
a sequence PN2 generated from a PNZ generator 21 for additional spreading. In
the
embodiment of the present invention, the sequences PN, and PN2 are
independent.
It is contemplated that sequences PN, and PN2 may entail a PN sequence
generated
1 o by user identification code. And in this invention, multiplying the output
of
complex spreader 2 by PN 2 could be optional feature.
The outputs of the multipliers 20-1 and 20-2 are subjected to baseband
filtering by baseband filters 22-l and 22-2, and to gain (Gp) control by gain
controllers 24-1 and 24-2, respectively. Then, mixers 26-1 and 26-2 multiply
the
1s outputs of the gain controllers 24-1 and 24-2 by their respective
corresponding
carriers, cos(2nf~t) and sin(2nf~t), for frequency up-conversion, and an adder
28
sums the outputs of the mixers 26-1 and 26-2.
In accordance with the present invention, auto-correlation characteristic of
a multipath signal and cross-correlation characteristics relative to other
users are
2 o improved by spreading an input complex signal two times: once time by the
sequence PN, and another time by the sequence PN2. Here, the sequences PN,,
PN2,
PN,, and PNQ have the same chip rate.
If the phase of a complex spreading sequence PNI+~PNQ output from a
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spreading sequence generator drastically changes, (for example, from 0
° to 180 °)
it causes increase of the peak-to-average power ratio of mobile transmit power
leads
to regrowth and degrades the communication quality of a different user.
However, the spreading sequence generator is configured that no zero-
crossing (no phase variation of n) is produced in generating the complex
spreading
sequence PNI+jPNQ in the embodiment of the present invention.
FIG. 2 is a block diagram of the n/2 DPSK generator 6 provided as the
spreading sequence generator according to the present invention. The feature
of the
~/2 DPSK generator 6 is that a maximum phase difference between every two
1 o successive complex chips of the complex spreading sequence PNI+jPNQ is
n/2.
The n/2 DPSK generator 6 includes a complex function calculator 32, a
complex multiplier 34, and delay registers 36 and 38. A multiplier 30
multiplies the
PN chips of the sequence PN, by (+-)n/2 or {+-)3n/2. It is contemplated that
the
multiplier 30 multiplies every one PN chip of the sequence PN, by any phase in
the
range of (+-)n/2 or (+-)3n12.
The complex function calculator 32 produces complex data Re+jlm by
operating every one phase shifted PN chip output of the multiplier 30 in a
complex
function exp{jj~]). The complex multiplier 34 complex-multiplies the complex
data
Re+jlm by values (complex data) received from the delay registers 36 and 38
and
2 0 outputs the complex spreading sequence PNI+jPNQ chip unit. The delay
register 36
stores the value PNI for one chip duration and the delay register 38 stores
the value
PNQ for one chip duration. The initial values (complex data) of the delay
registers
36 and 38 are determined by
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(Equation 1)
delay register 36 = Re[exp(j ~]
delay register 38 = Im[exp(j~],
where B may be any value, preferably n/4.
Assuming that the consecutive chips of the sequences PN, and PN2 are { 1,
-1, l, - l, ...} and {-1, 1, - l, l, ...}, respectively, and the initial
values ofthe delay
registers 36 and 38 are 1, the consecutive chips of the complex spreading
sequence
PN,+jPNQ generated from the n/2 DPSK generator 6 are {(-1+j), (1+j), (-1+j),
( 1~j), ... }, and the consecutive chips of a complex spreading sequence input
to the
to baseband filters 22-1 and 22-2 are {(1 j), (1+j), (1-j), (1+j), ...}. The
sequences
PN, and PNZ can be long codes for user identification in the 3G CDMA system.
FIGS. 3A and 3B illustrate signal constellations and phase transitions of the
complex spreading sequence PNI+jPNQ output from the n/2 DPSK generator 6 and
the complex spreading sequence input to the baseband filters 22-l and 22-2,
respectively. Referring to FIGS. 1 to 3B, for the first PN chip 1 of the
sequence
PN,, the output of the multiplier 30 in the n/2 DPSK generator 6 is n/2 since
the
other input to the multiplier 30 is n/2, and the complex data output from the
complex function calculator is e'z expressed as (0+1j) in the complex numeral
form (Re+jlm). Therefore, the complex multiplier 34 produces complex data (-
1+j)
= (0+j)x(1+j). Here, (0+j) is the complex data output from the complex
function
calculator 32 and (1+j) is the initial values of the delay registers 36 and
38.
In FIG. 3A, the complex data (-1+j) exists in the second quadrant of an
orthogonal coordinates graph defined by real components (Re) and imaginary
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components (Im) of a complex signal. The real part -1 of the complex data (-
1+j)
is stored in the delay register 36 for one chip duration, and the imaginary
part 1 is
stored in the delay register 38 for one chip duration.
For the second PN chip -1 of the sequence PN,, the output of the multiplier
30 in the n/2 DPSK generator 6 is -n/2, and the complex data. output from the
_n
complex function calculator 32 is e' 2 expressed as (0 j) in the complex
numeral
form (Re+jlm). Therefore, the complex multiplier 34 produces complex data
(I+j)
_ (0-, j)x(- I~j). Here, (0 j) is the complex data output from the complex
function
calculator 32 and (-1+j) is the previous values of the delay registers 36 and
38.
1 o In FIG. 3A, the complex data ( 1+j) exists in the first quadrant of the
orthogonal coordinates graph. The real part I of the complex data (1+j) is
stored in
the delay register 36 for one chip duration, and the imaginary part 1 is
stored in the
delay register 38 for one chip duration. In this manner, the complex data
output
from the complex multiplier 34 is (-1+j) for the third PN chip 1 of the
sequence
~5 PNi, and (1+j) for the fourth PN chip -1 of the sequence PN,.
With continued reference to FIG. 3A, the complex spreading sequence
PNr+jPNQ exists in the second and first quadrants of the orthogonal
coordinates
graph defined by the real components (Re) and the imaginary components (Im) of
a complex signal, with a phase difference of n/2 between every two successive
2 o complex chips.
The n/2 phase difference between every two successive complex chips is
maintained in a complex spreading sequence obtained by respreading the
sequence
PN2. Referring to FIG. 1, a complex spreading sequence {(I j), (1+j), (1-j),
(1+j),
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... } is produced by multiplying the chips {(-1+j), (1+j), (-1+j), (1+j), ...}
of the
complex spreading sequence PNI+jPNQ by the chips {-1, 1, -1, 1, ...{ of the
sequence PNZ. As shown in FIG. 3B, the complex spreading sequence input to the
baseband filters 22-1 and 22-2 has the phase difference of n/2 between every
two
s successive complex chips like the complex spreading sequence PNI+jPNQ.
Since the phase difference between every two successive complex chips of
the complex spreading sequences is small, that is, n/2 as noted by FIGS. 3A
and 3B,
the peak-to-average power ratio of mobile transmit power after processing in
the
baseband filters 22-l and 22-2 is reduced, decreasing the influence of
regrowth. As
1 o a result, communication quality and performance are improved.
If a predetermined radian value input to the multiplier 30 of the ~/2 DPSK
generator 6 is -3n/2, the complex spreading sequence PNI+jpNQ also shows the
signal constellation of FIG. 3A. If the radian value is - n/2 or 3 n/2, the
chips of the
complex spreading sequence PNl+jpNQ are successively shown at the same
1 s positions in the alternating first and second quadrants, starting from the
first
quadrant in FIG. 3A.
FIG. 4 is a block diagram of a second embodiment of the nl2 DPSK
generator 6 shown in FIG. 1. As with the first embodiment, the maximum phase
difference between every two successive complex chips of the complex spreading
2 o sequence PNI+jPNQ is (+-) n/2. The n/2 DPSK generator 6 of the second
embodiment includes an adder 40, a delay register 42, and a complex function
calculator 44. The adder 40 adds a PN chip of the sequence PNlwith the
previous
output of the adder 40 stored in the delay register 42. It is preferable to
set the initial
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value of the delay register 42 to 1/2. The complex function calculator 44
produces
the complex spreading sequence PN,+jPNQ by operating the output of the adder
40
in a complex function exp[(j(n/2(~))].
The phase variation of the complex spreading sequence PNl+jPNQ is given
by
(Equation 2)
~ (PN,~~ + jpNQ~x~)= e~~
9(k) = e(k-1)+ 2 PN,.
It is noted from equation (2) that the phase in the current chip of the
complex
1 o spreading sequence PN,+jPNQ is the sum of the phase in the previous chip
thereof
and the product of the current chip of the sequence PN, by n/2.
Assuming that the consecutive chips of the sequences PN, and PN2 are { 1,
-1, l, -1, ...} and {- l, 1, -1, l, ...}, respectively and the initial value
of the delay
register 42 is 1/2, then the consecutive chips of the complex spreading
sequence
PN1+jPNQ generated from the n/2 DPSK generator 6 are {(-1+j), (1+j), (-1+j),
( 1 +j), . , , }, ~d the consecutive chips of the complex spreading sequence
input to the
baseband filters 22-1 and 22-2 are { ( 1 j), ( 1+j), ( 1- j), ( 1+j), ... } .
The sequences
PN, and PNZ can be long codes for user identification in the 3G CDMA system.
FIGS. SA and SB are views illustrating the signal constellations and phase
2 o transitions of the complex spreading sequence PN,+jPNQ output from the n/2
DPSK
generator 6 and the complex spreading sequence input to the baseband filters
22-1
and 22-2, respectively.
Referring to FIGS. 1 to 5B, for the first PN chip 1 of the sequence PN,, the
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output of the adder 40 is 3/2 (=1+1/2), which is stored in the delay register
42 for
one chip duration, and the complex data output from the complex function
3s
calculator 44 is e' 4 expressed as (-1+j) in the complex numeral form
(Re+jlm),
and a chip of the complex spreading sequence PNI+jPNQ. Here, (-1+j) exists in
the
second quadrant of an orthogonal coordinates graph shown in FIG. SA.
For the second PN chip -1 of the sequence PN,, the output of the adder 40
is I /2 (_-1+3/2}, which is stored in the delay register 42 for one chip
duration, and
the complex data output from the complex function calculator 44 is e' a
expressed
as (1+lj) in the complex numeral form (Re+jlm). Here, (1+lj) is present in the
first
1 o quadrant of the orthogonal coordinates graph shown in FIG. SA. In this
manner, the
complex data output from the complex function calculator 44 is (-1+j) for the
third
PN chip 1 of the sequence PN,, and ( 1+j) for the fourth PN chip -1 of the
sequence
PN, .
With continued reference to FIG. 5A, the complex spreading sequence
15 PN,+jPNQ exists in the second and first quadrants of the orthogonal
coordinates
plane defined by the real components (Re) and the imaginary components (Im) of
a complex signal, with a phase difference n/2 between every two successive
complex chips.
The phase difference of n/2 between every two successive complex chips is
2 o maintained in a complex spreading sequence obtained by respreading the
complex
spreading sequence PNI+jPNQ by the sequence PN2 (It is noted that the complex
spreading sequence can also be respread by the original sequence PN or
different
PN sequence.}. Referring to FIG. 1, a complex spreading sequence {(1 j),
(1+j),
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( I j), (1+j), ... } is produced by multiplying the chips {(-1+j), (1+j), (-
1+j), (I+j),
... } of the complex spreading sequence PNI+jPNQ by the chips {-1, 1, -1, 1,
... } of
the sequence PN2. As shown in FIG. SB, the complex spreading sequence input to
the baseband filters 22-1 and 22-2 has the phase difference n/2 between every
two
successive complex chips like the complex spreading sequence PNl+jPNQ.
Since the phase difference between every two successive complex chips of
the complex spreading sequence is small, that is, n/2 as noted from FIGS. SA
and
SB, the peak-to-average power ratio of mobile transmit power after processing
in
the baseband filters 22-I and 22-2 is reduced, thereby, counteracting the
regrowth
1 o phenomenon. As a result, communication quality and performance are
improved.
FIG. 6 is a block diagram of an MS in a 3G IS-95 system to which the
spreading and modulation method according to the embodiment of the present
invention is applied. Reverse communication channels are comprised of a pilot
channel which is always activated, a control channel, a fundamental channel
1 s deactivated in a specific frame, and a supplemental channel. The pilot
channel is
unmodulated and used for performing initial acquisition, time tracking, and
synchronization of a rake receiver. This allows reverse-link closed-loop power
control. A dedicated control channel transmits an uncoded fast power control
bit
and coded control information. The two types of information are multiplexed
and
2 o sent on one control channel. The fundamental channel is used to send RLP
(Radio
Link Protocol) frames and packet data.
The channels are spread by Walsh codes for orthogonal channelization.
Control, supplemental, and fundamental channel signals are multiplied by
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corresponding Walsh codes in multipliers 50, 52, and 54, respectively.
Relative gain
controllers 56, 58, and 60 control the relative gains Go of the outputs of the
multipliers 50, 52, and 54, respectively. An adder 62 adds a pilot channel
signal
with a control channel signal received from the relative gain controller 56.
The
added information of the adder 62 is applied as an I-channel signal. An adder
64
adds a supplemental channel signal output from the relative gain controller 58
with
a fundamental channel signal output from the relative gain controller 60. The
added
information of the adder 64 is assigned as a Q-channel signal.
A signal sent on the pilot, dedicated control, fundamental, and supplemental
1 o channel signals is a complex signal as shown in FIG. 1. The sum of the
pilot
channel and the control channel is assigned as the I-channel, and the sum of
the
fundamental channel and the supplemental channel is assigned as the Q-channel.
The complex signal of the I and Q channels is complex-spread by the complex
spreading sequence PNI~jPNQ in the complex spreader 2 of FIG. 6. The complex-
15 spread signal is multiplied by the sequence PN2, that is, a long code for
user
identification. The resulting complex spreading sequence is subjected to
baseband
filtering in the baseband filters 22-1 and 22-2, and sent through the gain
controllers
24-1 and 24-2, the mixers 26-1 and 26-2, and the adder 28 with a low peak-to-
average power ratio.
2 o FIG. 7 is a block diagram of an MS in a W-CDMA system to which the
spreading and modulating method of the present invention is applied. In FIG.
7, a
traffic signal is sent on a dedicated physical data channel (DPDCH), and a
control
signal is sent on a dedicated physical control channel (DPCCH). The DPDCH is
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multiplied by a channelization code CD at a chip rate in a multiplier 70 and
assigned
as an I channel. The DPCCH is multiplied by a channelization code C~ at a chip
rate
in a multiplier 72, converted to an imaginary numeral form by an
imaginary~operand
(~j) 74, and assigned as a Q channel. Here, CD and C~ are mutually orthogonal
codes. The I and Q channels form a complex signal. The complex signal is
complex-spread by the complex spreading sequence PNI+~PNQ in the complex
spreader 2 of FIG. 7, and multiplied by the sequence PNz; that is, a long code
for
user identification generated in the PN2 generator 21. The resulting complex
spreading sequence is subject to baseband filtering in the baseband filters 22-
1 and
22-2, and sent through the gain controllers 24- l and 24-2, the mixers 26-1
and 26-2,
and the adder 28 with a low peak-to-average power ratio.
According to the present invention as described above, the peak-to-average
power ratio of mobile transmit power is limited to a specific range by
ensuring a
phase difference of 90 ° between every two successive complex chips of
a complex
1 s spreading sequence. Therefore, the mobile transmit power appears only in a
linear
portion of a characteristic curve of a power amplifier, thereby allowing the
mobile
transmit power and the cell size to be flexibly controlled. In addition, auto-
correlation characteristics of a multipath signal and cross-correlation
characteristics
relative to other users can be improved by respreading a signal which has
passed
2 o through a complex spreader by another PN sequence generated from a PN code
generator.
While the invention has been shown and described with reference to certain
preferred embodiments thereof, it will be understood by those skilled in the
art that
___-T . __-____~ __
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various changes in form and details may be made therein without departing from
the
spirit and scope of the invention as defined by the appended claims.
