Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via code word channels
Reexamination Certificate
1998-01-24
2001-06-12
Vu, Huy D. (Department: 2739)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via code word channels
C375S213000
Reexamination Certificate
active
06246697
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related in general to wireless communication systems, and more particularly to a method and system for processing code division multiple access signals with a complex pseudonoise sequence.
BACKGROUND OF THE INVENTION
In power amplifiers used to transmit modulated radio frequency signals, is desirable to operate with an input signal having a low peak-to-average ratio. Signals with high peak-to-average ratios are undesirable because the power amplifier produces extraneous side bands when a peaking signal causes it to operated in a nonlinear portion of its operating range. These extraneous side bands are produced by a mechanisms called AM-to-PM conversion and AM-to-AM conversion when passing a signal with large amplitude fluctuations. Furthermore, these side bands deprive the information signals of some of their portion of the transponder power, and also can interfere with nearby channels (adjacent channel interference).
In a communications system using quartenary phase shift keying (QPSK) the signal phase can be any of one of four phases for the duration of each phase shift interval. This is shown in the signal space diagram in
FIG. 1
, wherein phase
30
illustrates the phase of constellation point
32
, which is one of the constellation points
32
-
38
. Transitions
40
-
46
illustrate the permitted phase changes between phase shift intervals. A zero degree transition is shown at reference numeral
40
. Examples of &pgr;/2 radians or 90° transitions are shown at reference numerals
42
and
44
, and a 180° or &pgr; radian transition is shown at reference numeral
46
.
In a code division multiple access (CDMA) system, such as a CDMA system implemented according to American National Standards Institute (ANSI) J-STD-008, user data is spread and modulated by a pseudorandom noise (PN) sequence, which is periodic and has noise-like properties. For example, with reference to
FIG. 2
, in direct sequence QPSK transmitter
60
, real-valued user data
62
is split and multiplied by 2 PN sequences: a PN
I
sequence
64
and a PN
Q
sequence
66
, using multipliers
68
and
70
, respectively. The PN sequences are generated by PN
I
and PN
Q
sequence generators
72
and
74
, respectively. The duration of the output of these PN sequence generators may be referred to as a chip time or chip interval, which is the duration of a single pulse in a direct sequence modulated signal.
After in-phase (I) and quadrature (Q) components of user data
62
have been multiplied by PN
I
sequence
64
and PN
Q
sequence
66
, the signals output by multipliers
68
and
70
are each separately filtered by pulse shaping filters
76
. Pulse shaping filters
76
may be implemented with finite impulse response filters that filter higher frequency components from the signal.
Next, the filtered I and Q signal components are multiplied by quadrature carrier components
78
and
80
using multipliers
82
to produce I and Q radio frequency (RF) signals
84
and
86
. Signals
84
and
86
are then added together in summer
88
. The output of summer
88
is RF modulated signal
90
, which is then amplified by power amplifier
92
. The output of power amplifier
92
is then coupled to antenna
94
for transmitting the signal to a receiving unit.
As shown in
FIG. 2
, PN sequence generators
72
and
74
are typically implemented with a maximal-length linear feedback N-bit shift register, wherein selected stages are tapped and exclusive ORed with the shift register output to form a signal that is fedback to the shift register input. Other ways of implementing PN sequence generators may be used. For example, nonlinear feedback shift registers may be used to generate the PN sequences.
A combination of the outputs of PN
I
and PN
Q
generators
72
and
74
may be referred to as having a complex value that corresponds to a phase. For example, referring again to
FIG. 1
, if PN
I
equals 1 and PN
Q
equals 1 the complex PN value of (1, 1) corresponds to phase
30
, which is &pgr;/4 radians. Other values output by the complex PN generator correspond to constellation points
34
-
38
. Transitions
40
-
46
from one constellation point to another are determined by the difference between a previous complex PN chip and a next complex PN chip generated by the complex PN sequence generator in the next chip time.
When RF modulated signal
90
peaks and causes power amplifier
92
to operate in a non-linear region, extraneous side bands are created in the transmitted signal. These side band signals may be eliminated by reducing the occurrence of peaks in RF modulated signal
90
, hence the desirability of reducing the peak-to-average ratio.
Peaks in RF modulated signal
90
occur as a result of receiving a sequence of chip values in pulse shaping filter
76
that highly correlates with the impulse response of pulse shaping filter
76
. Furthermore, the peaking of signal
90
is greater when peaks are formed in pulse shaping filters
76
in both the I and Q channels at the same time.
In the prior art, &pgr;/2 BPSK modulation has been used to reduce the peak-to-average in signals sent to the power amplifier. However, &pgr;/2 BPSK modulation produces BPSK spreading, which is inferior because signals from other users are not easily rejected.
QPSK spreading, on the other hand, provides superior rejection between user's signals, but produces a signal with an inferior peak-to-average ratio. For a more detailed discussion regarding spreading methods, see the book “CDMA, Principles of Spread Spectrum Communications,” by Andrew J. Viterbi, published by Addison Wesley in 1995, pages 26-32.
Thus, it should be apparent that a need exists for an improved method and system for generating a complex pseudonoise sequence for processing a code division multiple access signal wherein the complex pseudonoise sequence helps reduce the peak-to-average ratio of a modulated communications signal.
REFERENCES:
patent: 4774715 (1988-09-01), Messenger
patent: 5020075 (1991-05-01), Tachika
patent: 5440597 (1995-08-01), Chung et al.
patent: 5687166 (1997-11-01), Natali et al.
Laird Kevin Michael
Whinnett Nicholas William
Motorola Inc.
Terry L. Bruce
Vu Huy D.
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