Diversity detection for WCDMA

Details Diversity detection for WCDMA Diversity detection for WCDMA

2000-03-15

2004-10-12

Chin, Stephen (Department: 2634)

Pulse or digital communications

Receivers

Interference or noise reduction

C375S267000

Reexamination Certificate

active

06804311

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to wideband code division multiple access (WCDMA) for a communication system and more particularly to modulation of primary or secondary synchronization codes to indicate space-time transmit diversity for WCDMA signals.
BACKGROUND OF THE INVENTION
Present code division multiple access (CDMA) systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected receiver to determine the proper recipient of a data signal. These different data signals arrive at the receiver via multiple paths due to ground clutter and unpredictable signal reflection. Additive effects of these multiple data signals at the receiver may result in significant fading or variation in received signal strength. In general, this fading due to multiple data paths may be diminished by spreading the transmitted energy over a wide bandwidth. This wide bandwidth results in greatly reduced fading compared to narrow band transmission modes such as frequency division multiple access (FDMA) or time division multiple access (TDMA).
New standards are continually emerging for next generation wideband code division multiple access (WCDMA) communication systems as described in Provisional U.S. patent application Ser. No. 60/082,671,filed Apr. 22, 1998,and incorporated herein by reference. These WCDMA systems are coherent communications systems with pilot symbol assisted channel estimation schemes. These pilot symbols are transmitted as quadrature phase shift keyed (QPSK) known data in predetermined time frames to any receivers within range. The frames may propagate in a discontinuous transmission (DTX) mode. For voice traffic, transmission of user data occurs when the user speaks, but no data symbol transmission occurs when the user is silent. Similarly for packet data, the user data may be transmitted only when packets are ready to be sent. The frames are subdivided into fifteen equal time slots of 0.67 milliseconds each. Each time slot is further subdivided into equal symbol times. At a data rate of 30 KSPS, for example, each time slot includes twenty symbol times. Each frame includes pilot symbols as well as other control symbols such as transmit power control (TPC) symbols and rate information (RI) symbols. These control symbols include multiple bits otherwise known as chips to distinguish them from data bits. The chip transmission time (T
C
), therefore, is equal to the symbol time rate (T) divided by the number of chips in the symbol (N).
Previous studies have shown that multiple transmit antennas may improve reception by increasing transmit diversity for narrow band communication systems. In their paper
New Detection Schemes for Transmit Diversity with no Channel Estimation
, Tarokh et al. describe such a transmit diversity scheme for a TDMA system. The same concept is described in
A Simile Transmitter Diversity Technique for Wireless Communications
by Alamouti. Tarokh et al. and Alamouti, however, fail to teach such a transmit diversity scheme for a WCDMA communication system.
Referring to
FIG. 1
, there is a simplified block diagram of a typical transmitter using Space-Time Transit Diversity (STTD) of the prior art. The transmitter circuit receives pilot symbols, TPC symbols, RI symbols and data symbols on leads
100
,
102
,
104
and
106
, respectively. Each of the symbols is encoded by a respective STTD encoder. Each STTD encoder produces two output signals that are applied to multiplex circuit
120
. The multiplex circuit
120
produces each encoded symbol in a respective symbol time of a frame. Thus, a serial sequence of symbols in each frame is simultaneously applied to each respective multiplier circuit
124
and
126
. A channel orthogonal code C
m
is multiplied by each symbol to provide a unique signal for a designated receiver. The STTD encoded frames are then applied to antennas
128
and
130
for transmission.
Turning now to
FIG. 2
, there is a block diagram showing signal flow in an STTD encoder of the prior art that may be used with the transmitter of
FIG. 1
for pilot symbol encoding. The pilot symbols are predetermined control signals that may be used for channel estimation and other functions. The encoding pattern of STTD encoder
112
is given in TABLE I. The STTD encoder receives pilot symbol
11
at symbol time T, pilot symbol S
1
at symbol time 2T, pilot symbol
11
at symbol time 3T and pilot symbol S
2
at symbol time 4T on lead
100
for each of sixteen time slots of a frame. The encoder has an exemplary data rate of 32 KSPS and produces a sequence of four pilot symbols for each of two antennas corresponding to leads
204
and
206
, respectively, for each of the sixteen time slots of TABLE I. The STTD encoder produces pilot symbols B
1
,S
1
,B
2
and S
2
at symbol times T-4T, respectively, for a first antenna at lead
204
. The STTD encoder simultaneously produces pilot symbols B
1
, −S
2
*, −B
2
and S
1
* at symbol times T-4T, respectively, at lead
206
for a second antenna. Each symbol includes two bits representing a real and imaginary component. An asterisk indicates a complex conjugate operation or sign change of the imaginary part of the symbol. Pilot symbol values for the first time slot for the first antenna at lead
204
, therefore, are 11, 11, 11 and 11. Corresponding pilot symbols for the second antenna at lead
206
are 11, 01, 00 and 10.
The bit signals r
j
(i+&pgr;
j
) of these symbols are transmitted serially along respective paths
208
and
210
. Each bit signal of a respective symbol is subsequently received at a remote mobile antenna
212
after a transmit time &pgr; corresponding to the j
th
path. The signals propagate to a despreader input circuit (not shown) where they are summed over each respective symbol time to produce input signals R
j
1
, R
j
2
, R
j
3
and R
j
4
corresponding to the four pilot symbol time slots and the j
th
of L multiple signal paths as previously described.
TABLE I
ANTENNA 1
ANTENNA 2
SLOT
B
1
S
1
B
2
S
2
B
1
−S
2
*
−B
2
S
1
*
1
11
11
11
11
11
01
00
10
2
11
11
11
01
11
11
00
10
3
11
01
11
01
11
11
00
00
4
11
10
11
01
11
11
00
11
5
11
10
11
11
11
01
00
11
6
11
10
11
11
11
01
00
11
7
11
01
11
00
11
10
00
00
8
11
10
11
01
11
11
00
11
9
11
11
11
00
11
10
00
10
10
11
01
11
01
11
11
00
00
11
11
11
11
10
11
00
00
10
12
11
01
11
01
11
11
00
00
13
11
00
11
01
11
11
00
01
14
11
10
11
00
11
10
00
11
15
11
01
11
00
11
10
00
00
16
11
00
11
00
11
10
00
01
The input singals corresponding to the pilot symbols for each time slot are given in equations [5-8]. Noise terms are omitted for simplicity. Received signal R
j
1
is produced by pilot symbols (B
1
,B
1
) having a constant value (11,11) at symbol time T for all time slots. Thus, the received signal is equal to the sum of respective Rayleigh fading parameters corresponding to the first and second antennas. Likewise, received signal R
j
3
is produced by pilot symbols (B
2
,−B
2
)having a constant value (11,00) at symbol time 3T for all time slots. Channel estimates for the Rayleigh fading parameters corresponding to the first and second antennas, therefore, are readily obtained from input signals R
j
1
and R
j
3
as in equations [9] and [10].
R
j
1
=&agr;
j
1
+&agr;
j
2
  [5]
R
j
2
=&agr;
j
1
S
1
−&agr;
j
2
S
2
*  [6]
R
j
3
=&agr;
j
1
−&agr;
j
2
  [7]
R
j
4
=&agr;
j
1
S
1
+&agr;
j
2
S
1
*  [8]
&agr;
j
1
=(
R
j
1
+R
j
3
)/2  [9]
&agr;
j
2
=(
R
j
1
−R
j
3
)/2  [10]
Referring now to
FIG. 3
, there is a schematic diagram of a phase correction circuit of the prior art that may be used with a remote mobile receiver. This phase correction circuit receives input signals R
j
1
and R
j
2
on leads
324
and
326
at symbol times 2T and 4T, respectively. Each input sig

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