Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1998-06-17
2002-07-30
Nguyen, Chau (Department: 2733)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S350000, C375S140000
Reexamination Certificate
active
06426949
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a signal reception apparatus for DS-CDMA communication system, particularly to a take receiver for combining multi-path signals with phase compensation.
2. Prior Art
A spread spectrum communication system absorbs attention due to its high frequency efficiency as the users of the land mobile communication steeply increases. Among various types of spread spectrum communication, a direct sequence code division multiple access (DS-CDMA) communication system is going to be standardized by an international committee of communication, mainly in the field of the mobile cellular radio and wireless LAN.
Usually, one signal transmitted causes a plurality of propagation signals passing through different paths with different path lengths. Since these signals cannot be coherently added, a multi-path fading occurs. In the DS-CDMA system, the multi-path signal are resolved and utilized by combining them.
FIG.
7
(
a
) shows an example of the frame format in the DS-CDMA system. Each frame consists of a plurality of slots, for example, 6 slots. Each slot consists of a pilot symbol block and a information symbol block. Each of the pilot symbol blocks P
1
, P
2
, . . . , Pn has a predetermined number of symbols, for example, 4 symbols, and includes a predetermined symbol sequence. Each of the information symbol blocks I
1
, I
2
, . . . , In has a predetermined number of symbols, for example, 36 symbols. The pilot symbol blocks and information symbol blocks are arranged one after another so that each information symbol block follows one pilot symbol block.
Symbol blocks are modulated by QPSK information modulation, and modulated by BPSK spreading modulation or QPSK spreading modulation, then transmitted.
A composite code is formed by composing a short code with a length equal to the symbol duration and a long code with length equal to multiple symbol duration.
FIG.
7
(
b
) shows a conventional rake receiver. The signal received by an antenna
101
is converted into a intermediate frequency signal by a high frequency receiver portion
102
. An output of the portion
103
is divided by a divider
103
into two components of in-phase component (I-component) and quadrature component (Q-component) to be input to multipliers
106
and
107
, respectively. A wave of a local frequency is generated by an oscillator
104
. The wave is input directly to the multiplier
106
, and is input through a phase shifter
105
for shifting the wave in phase by &pgr;/2 to the multiplier
107
. The multiplier
106
multiplies the intermediate frequency signal from the divider
103
by the wave from the oscillator
104
. An output of the multiplier
106
is processed by a low-pass filter
108
so that the I-component base band signal Ri is generated. The multiplier
107
multiplies the intermediate frequency signal from the divider
104
by the wave from the phase shifter
105
. An output of the multiplier
107
is processed by a low-pass filter
109
so that the Q-component base band signal Rq is generated. The quadrature detection is performed.
The base band signal Ri and Rq are input to a complex matched filter
110
for multiplying the base band signal by I- and Q-components of PN code sequence supplied from a PN code generator
111
. This is despreading. The I- and Q-components Di and Dq of the output of the matched filter
110
are input to a signal level detector
112
, frame synchronization circuit
114
and a phase compensation portion
115
.
The signal level detector
112
calculates the power of received signal Di and Dq. The signal power level is input to a multi-path selection portion
113
for selecting N paths, for example 4 paths, which are with higher power level of than others.
The frame synchronization circuit
114
receives an information of the path with maximal power from the multi-path selection portion
113
for detecting the head of the frame according to the symbol pattern of the pilot symbol block.
An output of the multi-path selection portion
113
is input to the phase compensation portion
115
which compensates the phase of the selected paths for example up to 4 paths. The outputs are synchronized and combined by a rake combiner
116
. Using the output of rake combinent
116
, a decision is made by data decision portion
117
and the information symbol is recovered.
As mentioned above, the phase of the despread received complex signal are compensated by the portion
115
according to the phase rotation of the known pilot symbol in the received signal. This is necessary for the coherent detection because the absolute phase is needed in the coherent detection.
FIG. 8
shows the phase compensation portion
115
. The despread pilot symbol Di and Dq output from the complex matched filter
110
is input to a means
120
for extracting and averaging the phase error in Di and Dq.
A compensation signal is output from the means
120
to a phase compensation means
130
. The means
130
multiplies the despread information symbol block by the compensation signal so as to compensate the phase of Di and Dq.
When a pilot symbol transmitted is expressed as a complex a=a
i
+j a
q
and the pilot symbol received is P=P
i
+j P
q
, “a” and “P” are different only in phase &thgr; by ignoring the difference between the amplitudes, as shown in the formula (1).
P=P
i
+j·P
q
=(a
i
+j·a
q
)·e
j&thgr;
(1)
As shown in the formula (2), the phase of the pilot symbol P is extracted by multiplying “P” by a conjugate complex of “a”. (Pi, Pq) of the pilot symbol is called “phase vector”, hereinafter.
ev
=
(
P
i
+
j
·
P
q
)
(
a
i
+
j
·
a
q
)
=
(
P
i
·
a
i
+
P
q
·
a
q
)
+
j
·
(
P
q
·
a
i
-
P
i
·
a
q
)
=
ⅇ
j0
(
2
)
The average phase error of the pilot symbol is expressed by the formula (3).
E
=
1
L
∑
k
=
1
L
(
P
i
k
+
j
·
P
q
k
)
·
(
a
i
k
-
j
·
a
q
k
)
=
P
q
+
j
·
P
q
(
3
)
Here, “L” is the total number of symbols included within one pilot symbol block. L=4 for example. The upper letter “k” is an ordinal number of the pilot symbol. (Ei, Ej) is called “error vector”, hereinafter.
Usually, the pilot symbol a=a
i
+j a
q
is with a
i
=(−1, +1) and a
q
=(−1, +1). So the multiplication in the formula (2) can be implemented by controlling positive and negative sign of the received pilot symbol. The phase error E in the pilot symbol block can be calculated by an adder. Therefore, the calculation is executed by a simple circuit.
There are two methods for phase compensation using the average phase error of the pilot symbol.
FIG.
9
(
a
) shows the first method of phase compensation. The information symbols I
1
, I
2
and I
3
are compensated by the phase error vectors E
(1)
, E
(2)
and E
(3)
just before the information symbols, respectively. This method can be called extrapolating compensation. The first pilot symbol blocks P
1
and the first information symbol block I
1
are representatively described.
A vector for compensating the phase error of the pilot symbol block in each path can be calculated by the formulae (4) to (6). The vector (Mi, Mq) for the compensation is called “compensation vector”, hereinafter.
M=M
i
+j·M
q
(4)
M
i
=E
i
(5)
M
q
=E
q
(6)
The received symbol D=D
i
+j D
q
is multiplied by a conjugate complex of the compensation vector M as shown in the formula (7) so that the information symbol block in the slot is compensated in phase. A compensated received signal is expressed by Dhat (D with a symbol like a hat).
D
^
=
(
D
i
+
j
·
D
q
)
·
(
M
i
-
j
·
M
q
)
=
(
D
i
M
i
+
D
q
M
q
)
+
j
·
(
D
q
M
i
-
D
i
M
q
)
(
7
)
Other paths selected by the portion
113
of the multi-path signal is similarly processed.
The rake combiner synchronizes
Adachi Fumiyuki
Sawahashi Mamoru
Zhou Changming
Zhou Xuping
Armstrong Westerman & Hattori, LLP
Lee Andy
Nguyen Chau
Yozan Inc.
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