Multiplex communications – Generalized orthogonal or special mathematical techniques – Plural diverse modulation techniques
Reexamination Certificate
1999-02-22
2001-11-13
Kizou, Hassan (Department: 2662)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Plural diverse modulation techniques
C370S335000, C375S267000
Reexamination Certificate
active
06317411
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related in general to wireless communications systems, and more particularly to an improved method and system for transmitting and demodulating a communications signal that has been transmitted from an antenna array using a new combination of transmit diversity techniques.
BACKGROUND OF THE INVENTION
An important goal in designing a wireless communication system is to increase the number of users that may be simultaneously served by the communication system. This goal may be referred to as increasing system capacity. In an interference limited system, such as a code division multiple access (CDMA) wireless communications system, one way to increase capacity is by lowering the transmit power allocated to each user. By lowering the allocated transmit power, interference for all users is lowered, which provides additional capacity which may be used to add new users.
One way to lower the transmit power for each user is to increase the efficiency of the wireless link or channel between the user or subscriber unit and the base station that serves that user. One phenomena that reduces the efficiency of the communications link is fading. Fading may take several forms, one of which is referred to as multi-path fading. Multipath fading is caused by two or more copies of a transmitted signal combining at the receiver in a way that reduces the overall received signal level.
In the prior art, several diversity techniques have been proposed for reducing the effects of fading. These techniques include orthogonal transmit diversity (OTD) and space-time transmit diversity (STTD).
With reference now to
FIG. 1
, there is depicted a high-level block diagram of a transmitter and receiver for implementing an orthogonal transmit diversity system. As illustrated, data source
20
provides a stream of symbols, which may be encoded and interleaved. Such symbols may represent data in one or more traffic channels which are to be transmitted to the subscriber unit. Data in the traffic channels may represent voice, data, video, or other data a user desires to transport via the communication system.
The rate that symbols are output from data source
20
is controlled by symbol clock
22
. Symbols S
1
and S
2
are shown coming from data source
20
wherein each symbol is output for 1 period of symbol clock
22
, or a symbol period which may be described as the duration from T
0
to T
1
.
The serial stream of symbols from data source
20
is coupled to commutator
24
, which switches at the rate of symbol clock
22
. Commutator
24
sends the first symbol to spreader
26
, then switches to send the second symbol, S
2
, to spreader
28
. Subsequent symbols alternate each symbol period between spreader
26
and
28
.
Spreaders
26
and
28
spread the symbols by multiplying them by a spreading code, such as a Walsh code. Because the symbol rate at spreaders
26
and
28
is half the rate that symbols are sourced from data source
20
, a single Walsh code may be concatenated to form a new Walsh code at spreader
26
, and concatenated with an inverted copy to form the spreading code at spreader
28
. With these double-length Walsh codes used to spread half-rate symbols, the chip rate output by spreaders
26
and
28
remains the same as a transmission without OTD.
The outputs of spreaders
26
and
28
are coupled to radio frequency transmitters
30
and
32
. These radio frequency transmitters may include a modulator, followed by an up converter for up converting the modulated signal to a selected carrier frequency, and an amplifier for providing suitable power for transmitting the radio frequency signal.
The outputs of radio frequency transmitters
30
and
32
are coupled to antennas
34
and
36
for simultaneously transmitting symbols S
1
and S
2
. Because antennas
34
and
36
are spaced apart, the characteristics of the various paths or rays that the signals follow from each antenna to the subscriber unit may be measured separately, and described by coefficients shown as r
1
, and r
2
, where r
1
and r
2
are complex numbers that represent the gain and phase of the channel. Although r
1
and r
2
are treated here as single values, they may be vectors which describe the gain and phase of a plurality of resolvable multipath rays.
Antenna
38
is used by the subscriber unit to receive signals transmitted from antennas
34
and
36
. The received signal is down converted an demodulated in down converter and demodulator
40
and decoded in OTD decoder
42
.
The output of OTD decoder
42
is recovered symbols multiplied by the square of the magnitude of the channel coefficients r
1
and r
2
, respectively. Further details of the operation of OTD decoder
42
are shown in
FIG. 2
, which is discussed below.
The OTD decoder outputs are coupled to deinterleaver and decoder
44
for the deinterleaving and decoding processes that corresponds to the encoding and interleaving processes performed in data source
20
. The output of deinterleaver and decoder
44
is the traffic channel data. Transmit power is reduced for the same quality of service with the OTD diversity technique because different symbols experience different channel gains. This lowers the likelihood that both symbols will simultaneously experience a deep fade. This statistical unlikelihood that both symbols will be faded improves the decoder performance.
With reference now to
FIG. 2
there is depicted a schematic representation of OTD decoder
42
, which is used in FIG.
1
. The input to OTD decoder
42
is a down-converted received signal, which was received from antenna
38
. This signal contains traffic channels for all users along with pilot signals that may be used to estimate the channels from each transmit antenna. Channel estimator
50
evaluates the pilot signals and calculates channel coefficients r
1
and r
2
.
In a preferred embodiment, despreaders
52
and
54
despread the received signal using a single Walsh code that has been concatenated, as in the transmitter, in order to recover symbols S
1
and S
2
. Multipliers
56
and
58
multiply these recovered symbols by the conjugate of the channel estimates in order to compensate for gain and phase changes that occurred in the channel. Decommutator
59
is used to restore the symbol order and thereby double the symbol rate of the outputs from multipliers
56
and
58
. The outputs of OTD decoder
42
are the symbols multiplied by the magnitude of the respective channel estimate squared.
With reference now to
FIG. 3
, there is depicted another method and system for providing transmit diversity.
FIG. 3
illustrates a space-time transmit diversity transmitter and receiver. As illustrated, data source
20
and symbol clock
22
provide symbols S
1
and S
2
to space-time coder
60
. At the input, S
1
is received by space-time coder
60
during the period from T
0
to T
1
. Symbol S
2
is received at the input of space-time coder
60
during the period from T
1
to T
2
. Space-time coder
60
, which is a special type of transform operation, has two outputs that provide transform signals to two branches of the transmitter.
At the first output of space-time coder
60
, symbol S
1
is output during the symbol time from T
0
to T
1
, followed by symbol S
2
from symbol time T
1
to T
2
. The second output of space-time coder
60
outputs the negative complex conjugate of symbol S
2
during time T
0
to T
1
, followed by the complex conjugate of symbol S
1
from the period T
1
to T
2
.
The first and second space-time encoded data streams output by space-time coder
60
are then input into spreaders
62
and
64
. As shown, spreaders
62
and
64
use Walsh code W
1
. Note that the chip rate per symbol remains the same as in the OTD diversity transmitter.
Following the spreading function at spreaders
62
and
64
, the spread data streams are modulated, up converted, and amplified by radio frequency transmitters
30
and
32
.
The outputs of radio frequency transmitters
30
and
32
are coupled to antennas
34
and
36
, which transmit the sig
Boixadera Francesc
Clop Oscar
Kuchi Kiran Kumar
Rohani Kamyar
Whinnett Nicholas William
Elallam Ahmed
Haas Kenneth A.
Kizou Hassan
Motorola Inc.
Terry L. Bruce
LandOfFree
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