Pulse or digital communications – Systems using alternating or pulsating current
Reexamination Certificate
1998-09-17
2002-10-01
Pham, Chi (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
C375S347000
Reexamination Certificate
active
06459740
ABSTRACT:
BACKGROUND OF THE INVENTION
Wireless communications services are provided in different forms. For example, in satellite mobile communications, communications links are provided by satellite to mobile users. In land mobile communications, communications channels are provided by base stations to the mobile users. In PCS, communications are carried out in microcell or picocell environments, including outdoors and indoors. Regardless the forms they are in, wireless telecommunication services are provided through radio links, where information such as voice and data is transmitted via modulated electromagnetic waves. That is, regardless of their forms, all wireless communications services are subjected to vagaries of the propagation environments.
The most adverse propagation effect from which wireless communications systems suffer is the multipath fading. Multipath fading, which is usually caused by the destructive superposition of multipath signals reflected from various types of objects in the propagation environments, creates errors in digital transmission. One of the common methods used by wireless communications engineers to combat multipath fading is the antenna diversity technique, where two or more antennas at the receiver and/or transmitter are so separated in space or polarization that their fading envelopes are de-correlated. If the probability of the signal at one antenna being below a certain level is p (the outage probability), then the probability of the signals from L identical antennas all being below that level is p
L
. Thus, since p<1, combining the signals from several antennas reduces the outage probability of the system. The essential condition for antenna diversity schemes to be effective is that sufficient de-correlation of the fading envelopes be attained.
A classical combining technique is the maximum-ratio combining (MRC) where the signals from received antenna elements are weighted such that the signal-to-noise ratio (SNR) of their sum is maximized. The MRC technique has been shown to be optimum if diversity branch signals are mutually uncorrelated and follow a Rayleigh distribution. However, the MRC technique has so far been used exclusively for receiving applications. As there are more and more emerging wireless services, more and more applications may require diversity at the transmitter or at both transmitter and receiver to combat severe fading effects. As a result, the interest in transmit diversity has gradually been intensified. Various transmit diversity techniques have been proposed but these transmit diversity techniques were built on objectives other than to maximize the SNR. Consequently, they are sub-optimum in terms of SNR performance.
SUMMARY
Improved performance is achieved with an arrangement where the transmitter has a plurality of transmitting antennas that concurrently transmit the same symbol, and where the signal delivered to each transmitting antenna is weighted by a factor that is related to the channel transmission coefficients found between the transmitting antenna and receiving antenna(s). In the case of a plurality of transmit antennas and one receive antenna, where the channel coefficient between the receive antenna and a transmit antenna i is h
i
, the weighting factor is h
i
* divided by a normalizing factor, a, which is
(
∑
k
=
1
K
&LeftBracketingBar;
h
k
&RightBracketingBar;
2
)
1
/
2
,
where K is the number of transmitting antennas. When more than one receiving antenna is employed, the weighting factor is {fraction (1/a)}(gH)
H
, where g=[g
1
. . . g
L
], H is a matrix of channel coefficients, and a is a normalizing factor
(
∑
p
=
1
L
∑
q
=
1
L
&LeftBracketingBar;
∑
k
=
1
K
h
p
k
h
q
k
*
&RightBracketingBar;
)
1
/
2
.
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A. Wittneben, Basestation modulation diversity for digital simulcast, IEEE 1991, pp. 848-853.*
A. Wittneben, A New Bandwidth Efficient Transmit Antenna Modulation Diversity Schem for Linear Digital Modulation, IEEE 1993, pp. 1630-1634.*
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N. Seshadri and Jack H. Winters, “Two Signaling Schemes for Improving the Error Performance for Frequency-Division-Duplex (FDD) Transmission Systems Using Transmitter Antenna Diversity,” 43rdIEEE Vehicular Technology Conference, May 18-20, 1993, Secaucus, NJ.*
Armin Wittneben, “A New Bandwidth Efficient Transmit Antenna Modulation Diversity Scheme for Linear Digital Modulation,” IEEE International Conference on Communications, May 23-26, 1993, Geneva, Switzerland.*
Jack J. Winters, “The Diversity Gain of Transmit Diversity in Wireless Systems with Rayleigh Fading,” IEEE International Conference on Communications, May 1-5, 1994, New Orleans, Louisiana.*
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Al-Beshrawi Tony
AT&T Wireless Services Inc.
Perkins Coie LLP
Pham Chi
LandOfFree
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