Pulse or digital communications – Spread spectrum
Reexamination Certificate
1998-09-16
2003-10-21
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
C370S479000
Reexamination Certificate
active
06636549
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for generating a pseudorandom noise sequence with an arbitrarily designated phase, which is applied to a communication system using a spread-modulation method.
2. Description of the Related Art
In a communications system using spread spectrum modulation, the spectra of many signals can be spread over a broadband, multiplexed, and transmitted by using a Code Division Multiple Access (CDMA).
FIG. 1
shows one principle of the configuration of a CDMA communication system.
On a CDMA transmitter
901
side, a spread-modulation unit
905
spread-spectrum modulates the transmission signal which is output from a transmission signal source
903
, for example, which is frequency-modulated or phase-modulated by using the spread code generated by a spread code generating unit
904
, and the resultant transmission signal is transmitted to a transmission line
906
.
On a CDMA receiver
902
side, a despread-demodulation unit
908
must despread (demodulate) a reception signal by using the despread code which has the same sequence and phase as those of the spread code on the transmitter side and which is output from a despread code generating unit
907
in synchronization with the timing of the transmitter side.
Accordingly, the despread code generating unit
907
must have the capability for generating a sequence code having an arbitrary phase according to a timing synchronization signal (normally, this signal is autonomously generated from a reception signal within the CDMA receiver
902
).
In a CDMA communication, the spread code (and the despread code) for spreading a spectrum must satisfy the following conditions in addition to the condition that the spread code must be a broadband signal: (1) the number of types of spread codes must be large in order to allow codes to be assigned to many users; (2) a cross-correlation must be small in order to allow the spread code to be identified from a different user code; (3) a self-correlation must be strictly identified in order to ensure the synchronization with the signal addressed to a local station; and (4) the spread code must be as random as possible, have a long cycle, and be difficult to be decoded in order to improve the confidentiality of a communication signal.
Conventionally, a PN (Pseudorandom Noise) sequence is known as the code for satisfying such conditions.
Since the PN sequence can be generated by using a shift register, its generation process is not really random but deterministic. However, the PN sequence is a code having the following properties of randomness. Therefore, this is suitable as the spread code of the CDMA communication, which requires the above described conditions.
Property 1: Balance Property
The numbers of times that “1” and “0” respectively appear in one cycle of the sequence are, different only by 1.
Property 2: Run Property
In the runs of “1s” and runs of “0s” which are included in one cycle, the length of each run is “1” when a classification number of that run is ½, “2” when a classification number of that run is ¼, “3” when a classification number of that run is ⅛, . . . . That is, there is a number of {(classification number of run)×(½ k)} of the runs which have the run number k. Note that, for the runs of which this number is less than 1, these runs become meaningless runs.
Property 3: Correlation Property
If sequences are made cyclic and a comparison is made between the respective code values of the corresponding digits of the two sequences in every state, the number of the digits whose code values match and that of the digits whose code values do not match are different only by 1.
An M sequence (maximum length sequence) is known as a typical PN sequence satisfying such properties. The M sequence is generated using the circuit including an n-stage shift register, which is shown in FIG.
2
.
In
FIG. 2
, the respective outputs of the stages of the n-stage shift register are multiplied with a coefficient f
i
(0 or 1), and the multiplication results are fed back to the input side of the shift register via exclusive-OR circuits (+signs encircled in this figure).
If the coefficient f
i
satisfies a particular condition when the values of all of the stages of the shift register are not “0” in an initial state, the cycle of the sequence a
i
output from the shift register will become the maximum cycle (2
n
−1) that the n-stage shift register can generate. Such a sequence is referred to as an M sequence.
The circuit shown in
FIG. 2
can be represented by the following equation.
∑
j
=
0
n
f
j
a
i
+
j
=
0
(
1
)
If f
n
=1 is assigned to this equation, the following equation can be obtained.
a
i
+
n
=
∑
j
=
0
n
-
1
f
j
a
i
+
j
(
2
)
The above described equations (1) and (2) are referred to as linear recurring equations. Here, if a delay operator x which satisfies a
i+1
=x
j
a
i
is assigned, the equation (2) becomes as follows.
(
∑
j
=
0
n
f
j
x
j
)
a
i
=
0
(
3
)
The polynomial f(x) of the following equation, which is represented by the term on the left side of the above equation (3), is referred to as a characteristic polynomial.
f
(
x
)
=
∑
j
=
0
n
f
j
x
j
(
f
0
≠
0
,
f
n
=
1
)
(
4
)
If the coefficient f
j
included in this equation (4) belongs to a Galois field GF (2), and if f(x) is the minimal polynomial possessed by a primitive element &agr; of the Galois field GF (2
n
), it is known that the circuit shown in
FIG. 2
, which includes the n-stage shift register, can generate the M sequence having the maximum cycle (2
n
−1). This minimal polynomial is referred to as a primitive polynomial of degree k. Its details are referred to, for example, in the document “Sensing/Recognition Series Vol. 8, M Sequence and its Application, pp. 16-”, written by J. Kashiwagi and published by Shokodo.
The primitive polynomial can be calculated as described on pp. 171 to 191 of this document, and many types of primitive polynomials were previously obtained in some of the papers cited in this document.
For example, the coefficient f
j
, which is included in the equation (4) and corresponds to a primitive polynomial f(x)=x
4
+x+1 of the Galois field GF (2
4
), becomes f
0
=1, f
1
=1, f
2
=f
3
=0, and f
4
=1. As a result, the M sequence generating circuit shown in
FIG. 3
can be configured based on the circuit shown in FIG.
2
.
Here, an M sequence x
d
a
i
whose phase is shifted by d bits from the output a
i
of the M sequence will be obtained. If a predetermined initial state of the n-stage (4 stages in
FIG. 3
) shift register is provided, all of the states of the M sequence, which succeed the initial state, are determined. Therefore, the M sequence having an arbitrary phase is proved to be obtained with the linear combination of the outputs of the respective stages of the shift register, as represented by the following equation (5).
x
d
a
i
=b
0
x
0
a
i
+b
1
x
1
a
i
+b
2
x
2
a
i
+ . . . +b
n−1
x
n−1
a
i
(5)
Consequently, the circuit for generating the M sequence having an arbitrary phase can be configured from the M sequence generating circuit shown in
FIG. 3
, which includes the 4-stage shift register, as shown in FIG.
4
.
In
FIG. 4
, an initial value is assigned to each of the stages of the 4-stage shift register (SR)
1203
in a PN generator (PNG)
1201
. The feedback equivalent to that shown in
FIG. 3
is provided by a TAP
1204
. TAP information (TAPINFO)
1205
, which corresponds to the respective coefficients b
0
through b
3
included in the equation (5), is provided to 4 AND circuits (ANDs)
1206
in a variable tap (ATAP)
1202
. Consequently, the output selected according to the TAPINFO
1205
from among the outputs of the respective stages of the SR
1203
is added to another output by the corresponding AND circuit
Kawabata Kazuo
Nakamura Takaharu
Ohbuchi Kazuhisa
Bayard Emmanuel
Pham Chi
LandOfFree
Method for calculating phase shift coefficients of an M... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for calculating phase shift coefficients of an M..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for calculating phase shift coefficients of an M... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3130315