Method of providing downlink transmit diversity

Details Method of providing downlink transmit diversity Method of providing downlink transmit diversity

1999-08-17

2004-04-13

Nguyen, Steven H. D (Department: 2665)

Multiplex communications

Communication techniques for information carried in plural...

Combining or distributing information via time channels

C370S310000, C370S329000, C370S342000, C455S101000, C455S115200

Reexamination Certificate

active

06721339

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to wireless communications; more specifically, a method for providing transmit diversity.
2. Description of the Prior Art
Both transmit and receive diversity are used to counter channel fading. In the case of a receiver, diversity is provided by using two antennas that are spaced a sufficient distance apart so that only one antenna experiences a fading signal at one time. Similarly, transmit diversity is provided using two or more antennas that are spaced a sufficient distance apart so that a receiver is unlikely to have the signals from all of the antennas simultaneously fade.
FIG. 1
illustrates a prior art CDMA (Code Division Multiple Access) transmitter providing transmit diversity. Encoder
10
receives data to be transmitted and adds coding such as error correction and detecting coding. The data is then passed to interleaver
12
that reorders the data so that the loss of consecutive bits can be spread out in time when the bits are reordered at a receiver. The output of interleaver
12
is provided to demux
14
which splits the data into two parallel paths that are provided to multipliers
16
and
18
. Multipliers
16
and
18
encode the data using orthogonal codes such as Walsh codes W
n1
and W
n2
. It should be noted that by passing through demux
14
the data rate is decreased by one half. It should also be noted that one CDMA channel typically uses a single Walsh code such as Walsh code W
n
. Since the data rate is decreased by one half, the Walsh code W
n
may be broken into two longer orthogonal Walsh Codes W
n1
and W
n2
. Equations 1 and 2 illustrate the relationship between longer Walsh Codes W
n1
and W
n2
and shorter Walsh Code W
n
.
W
n1
=[W
n
, W
n
]  (Eq. 1)
W
n2
=[W
n
, −W
n
]  (Eq. 2)
An example of generating two longer Walsh Codes from a single Walsh code is illustrated by Equations 3, 4 and 5.
W
n
=1 1 −1 −1  (Eq. 3)
W
n1
=1 1 −1 −1 1 1 −1 −1   (Eq. 4)
W
n2
=1 1 −1 −1 −1 −1 1 1   (Eq. 5)
Equation 3 illustrates a simple four bit Walsh code and Equations 4 and 5 illustrate longer Walsh codes W
n1
and W
n2
, respectively. It can be seen that Walsh code W
n1
is simply two repetitions of Walsh code W
n
and that Walsh code W
n2
is Walsh code W
n
followed by −1 times Walsh code W
n
.
Returning the
FIG. 1
, multipliers
20
and
22
apply a pseudo-random code to each of the data paths and then the data is passed to RF sections
24
and
26
. The RF sections perform functions such as modulating a carrier signal having a carrier frequency f
1
with the encoded data and providing sufficient amplification before transmission over antennas
28
and
30
. It should be noted that the system of
FIG. 1
provides transmit diversity by dividing the data into two paths that transmit at the same frequency over two antennas; however, the two paths maintain an orthogonal relationship by using different Walsh codes to encode the data.
FIG. 2
illustrates a second CDMA transmitter providing transmit diversity. As in
FIG. 1
, the data is processed by encoder
10
and interleaver
12
before being passed to a demux. Demux
40
divides the data into three parallel paths that are provided to multipliers
42
,
44
and
46
. Each of the multipliers encodes the data using Walsh code W
n
. The data from multipliers
42
,
44
and
46
is then passed to multipliers
48
,
50
and
52
, respectively, where the data is further encoded with a pseudo-random code. The data from multiplier
48
is provided to RF section
54
which modulates the data onto a carrier having frequency f
1
. The data from multiplier
50
is provided to RF section
56
which modulates the data onto carrier having frequency f
2
. The data from multiplier
52
is provided to RF section
58
which modulates the data onto a carrier having frequency f
3
. The output of the RF sections is provided to antennas
60
,
62
and
64
. In this case, transmit diversity is provided using three antennas where the orthogonality of the three channels is provided by the use of different carrier frequencies.
SUMMARY OF THE INVENTION
The present invention provides a transmitter for wireless communications that has multiple types of orthogonality to improve transmit diversity. Transmit diversity is improved by using both coding and carrier frequency orthogonality. Data to be transmitted is broken into four parallel channels. Two of the channels are transmitted on a first carrier signal and the other two channels are transmitted on a second carrier signal. Channels transmitted on the same carrier signal are provided with orthogonal codes so that they may be separated by a receiver. Channels transmitted on different carrier signals may be encoded with identical orthogonal codes. The modulated carrier signals are then transmitted using at least two antennas, where one antenna is used for each carrier. It should be noted it is also possible to transmit both carriers on each of the antennas.


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