Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2001-05-01
2004-10-26
Chin, Stephen (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S140000, C375S150000, C375S148000, C375S149000, C375S134000, C370S320000
Reexamination Certificate
active
06810071
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) code timing tracking in a CDMA type mobile communication system (i.e., a cellular communication system, a personal communication system or an IMT(International Mobile Telecommunication)-2000). More particularly, the present invention relates to a CDMA code timing tracking apparatus capable of exact code timing synchronization without regard to signal gain change by normalizing signal gain to synchronize code timing.
2. Description of the Related Art
In general, a CDMA (Code Division Multiple Access) is a method to enlarge frequency band of data signal with diffusion code, and then restores to original signal using the same diffusion signal. The CDMA method has several advantages that it is good for communication security and can effectively deal with multiple path signal attenuation.
In case of a CDMA type receiver, the receiver uses the same diffusion code as a transmitter and can restore to exact signal only when code phase of the received diffusion code and phase of diffusion code generated inside the receiver must correspond with each other.
As the algorithm for corresponding such code phase or timing synchronization, there are DLLs (Delay Locked Loops) using correlation value between early code, which is faster than on-time code, and late code, which is later than the on-time code, and TDLs (Tau-Dither Loops) using only one correlator.
Conventional synchronous DLLs, asynchronous DLLs, synchronous TDLs and asynchronous TDLs change speed for convergence of code phase synchronization according to the size change of signal.
FIG. 1
shows spreading code (PN Sequence) used for restoring CDMA code when the receiver of the conventional CDMA type mobile communication system applies the DLL.
Here,
FIG. 1
a
shows on-time spreading code,
FIG. 1
b
shows early code, which is /2 as fast as the on-time, and
FIG. 1
c
shows late code which, is /2 as late as the on-time.
The on time means timing in the case of corresponding with the PN code phase of received signal.
The difference subtracting correlation value of the early code and received signal from correlation value of the late code and received signal is error signal being in proportion to code timing error.
The code timing synchronization using the DLL is to adjust the phase of spreading code to minimize error signal. The DLL is classified into two groups: one being a synchronous DLL performing in the state of restoring phase with carrier and the other being an asynchronous DLL performing without regard to phase restoration with carrier frequency.
FIG. 2
shows a structure of the synchronous DLL in the conventional CDMA type mobile communication system.
The synchronous DLL includes a PN code generator
11
for generating early code, which is faster than on-time code, and late code, which is later than the on-time code, a first correlator
12
for correlating the early code generated from the PN code generator
11
with baseband CDMA signal, which is input, a second correlator
13
the late code generated from the PN code generator
11
with the baseband CDMA signal, first and second low-pass filters for low-pass filtering signal output from the first and second correlators
12
and
13
respectively, a subtracter
16
for subtracting the output signal of the second low-pass filter
15
from output signal of the first low-pass filter
14
and generating difference signal, a third low-pass filter
17
for low-pass filtering the output signal of the subtracter
16
, and a voltage-to-current converter
18
for converting voltage output from the third low-pass filter
17
into current and transmitting it to the PN code generator
11
.
The synchronous DLL having the above structure generates early code, which is faster than on-time code, and late code, which is later than on-time code, and the first correlator
12
takes the correlation between the early code and the input baseband CDMA signal (r(t)).
Furthermore, the second correlator
13
takes the correlation between the late code generated from the PN code generator
11
and the baseband CDMA signal (r(t)). The first low-pass filter
14
low-pass filters output signal of the second correlator
13
and removes noise and high frequency element.
Next, the subtracter
16
subtracts the output signal of the second low-pas filter
15
from the output signal of the first low-pass filter
14
and generates difference signal, i.e., error signal in proportion to phase difference of spreading codes of the receiver and transmitter. The final role of the loop is to minimize error signal.
The output signal of the subtracter
16
is formularized as follows:
e
(
τ
)
=
p
(
τ
)
R
(
τ
-
1
2
Δ
)
-
R
(
τ
+
1
2
Δ
)
,
Formula
1
wherein p(&tgr;) is electric power of input signal, R(&tgr;−½&Dgr;) is the output signal of the first correlator
12
, &tgr; is chip timing error and &Dgr; is phase difference between early spreading code and late spreading code of the receiver. Here, &Dgr; uses the length of one chip.
The output signal of the subtracter
16
is low-pass filtered in the third low-pass filter
17
, converted into current in the voltage-to-current converter
18
and transmitted to the PN code generator
11
.
Here, when the size of the input signal is not uniform and changed by noise and multiple path, also the size of error signal is changed according to the change in the size of electric power P(&tgr;) of input signal. When error signal is changed, not only it has an influence on convergence time of the loop but also there is every probability that jitter is induced after the convergence.
FIG. 3
shows a structure of an embodiment of the asynchronous DLL in the conventional CDMA type mobile communication system.
The asynchronous DLL includes a PN code generator
20
for generating early code and late code, a first correlator
21
for correlating the early code generated from the PN code generator
20
with CDMA intermediate frequency signal which is input, a second correlator
22
for correlating the late code generated from the PN code generator
20
with the CDMA intermediate frequency signal, first and second band-pass filter
23
and
24
for band-pass filtering output signal of the first and second correlators
21
and
22
respectively, a first square unit
25
for squaring output signal of the first band-pass filter
23
, a second square unit
26
for squaring output signal of the second band-pass filter
24
, a subtracter
27
for subtracting output signal of the second square unit
26
from output signal of the first square unit
25
and generating difference signal, a low-pass filter
28
for low-pass filtering output signal of the subtracter
27
, and a voltage-to-current converter
29
for converting voltage output from the low-pass filter
28
and transmitting it to the PN code generator
20
.
The asynchronous DLL having the above structure generates early code, which is faster than on-time code, and late code, which is later than on-time code, in the PN code generator
20
. The first correlator
21
takes the correlation between the early code and the CDMA intermediate frequency signal (r(t)).
Moreover, the second correlator
22
takes the correlation between the late code generated from the PN code generator
20
and the CDMA intermediate frequency signal (r(t)), the first band-pass filter
23
band-pass filters the output signal of the first correlator
21
, and also the second band-pass filter
24
band-pass filters output signal of the second correlator
22
.
Next, the first square unit
25
squares the output signal of the first band-pass filter
23
, and the second square unit
26
squares the output signal of the second band-pass filter
24
.
The subtracter
27
subtracts the output signal of the second multiplier
26
from the output signal of the first multiplier
25
and generates difference signal, i.e., baseband error signal. The final role of the loop is
Baker & Botts LLP
Hyundai Electronics Ind. Co. Ltd.
Pathar Sudhanshu C.
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