Adaptive beam-time coding method and apparatus

Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...

Reexamination Certificate

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Details

C455S127200, C455S129000, C455S272000, C455S522000, C455S562100, C370S320000, C342S359000, C342S360000, C342S373000

Reexamination Certificate

active

06788661

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to down link signal transmission from a base station of a cellular radio system to a remote station. In particular, the invention relates to adaptive control of multiple down link beams, beam powers and beam widths.
2. Description of Related Art
Cellular telephone systems are operated in environments that give rise to multi-path or reflections of the their signals, particularly in urban environments. In
FIG. 1
, base station transmitter
1
broadcasts its signal to remote station
2
(often mobile) along direct path
3
. However, owing to the presence of tall building
4
, transmitter
1
also broadcasts its signal to remote station
2
along indirect path
5
, thus, giving rise to angular spread AS between the direction of arrival of direct path
3
at remote station
2
and the direction of arrival of indirect path
5
at remote station
2
. Direct path
3
and indirect path
5
are recombined at remote station
2
where constructive and destructive superimposed signals cause random or what appears to be random fading and black out zones.
To reduce the effects of multi-path, known systems employ space time transmit diversity techniques. In
FIG. 2
, a known transmitter includes space time transmit diversity encoder
10
, complex multipliers
12
and
14
, and antennas
16
and
18
. Space time transmit diversity encoder
10
processes input signal S
IN
into two channel signals CH
1
and CH
2
. Multipliers
12
and
14
may impart a same orthogonalizing code OC on the two channel signals CH
1
and CH
2
to identify the two channels as containing information about input signal S
IN
; however, different orthogonal identifiers (e.g., pilot sequences or training sequences) are applied to the different antenna signals so that the remote station can separately identify the signals from the two antennas. The multiplied channel signals are transmitted on respective antennas
16
and
18
substantially spaced apart by a distance (e.g., 20 wavelengths). Such spaced apart antennas are referred to as diversity antennas. In multi-path environments severe fading results when different propagation paths sum destructively at the receiving antenna. Using diversity antennas, the probability that both signals CH
1
and CH
2
will be in deep fade is low since the two signals are likely to propagate over different paths such as the multi-paths
3
and
5
. Diversity antennas may be omni-directional antennas or antennas directed at antenna sectors with overlayed sectors. When diversity antennas are sufficiently separated in space, they can be regarded as orthogonal since they propagate signals in non-correlated channels (i.e., paths).
Input signal S
IN
carries two symbols, S
1
and S
2
, in time succession, the first symbol in symbol slot between 0 and T, and the second symbol in symbol slot between T and 2T. In
FIG. 3
, exemplary encoder
10
uses a QPSK modulation technique and includes time align register
20
and hold registers
22
to hold the two symbols. Base band carrier signal SBBC is inverted in inverter
24
to produce negative base band carrier -SBBC. QPSK modulator
26
encodes symbol S
1
onto base band carrier signal SBBC to produce a modulated first symbol, and QPSK modulator
28
encodes symbol S
1
onto negative base band carrier signal −SBBC to produce a modulated conjugate of the first symbol. QPSK modulator
30
encodes symbol S
2
onto base band carrier signal SBBC to produce a modulated second symbol, and QPSK modulator
32
encodes symbol S
2
onto negative base band carrier signal −SBBC to produce a modulated conjugate of the second symbol. The modulated conjugate of the second symbol is inverted in inverter
34
to produce a negative modulated conjugate of the second symbol. Analog multiplexer
36
switches the modulated first symbol into the first channel signal during the first symbol time slot (i.e., 0 to T,
FIG. 2
) and switches the negative modulated conjugate of the second symbol into the first channel signal during the second symbol time slot (i.e., T to 2T,
FIG. 2
) so that the signal on CH
1
is [S
1
, - S
2
*]. Analog multiplexer
38
switches the modulated second symbol into the second channel signal during the first symbol time slot (i.e., 0 to T,
FIG. 2
) and switches the modulated conjugate of the first symbol into the second channel signal during the second symbol time slot (i.e., T to 2T,
FIG. 2
) so that the signal on CH
2
is [S
2
, S
1
*].
In
FIG. 2
, code OC consists of one code applied to both multipliers
12
,
14
that is used as a CDMA spreading function to isolate the two signals transmitted from antennas
16
and
18
from other signals that may generate co-channel interference. Multipliers
12
and
14
, multiply the first and second channel signals before being transmitted through antennas
16
and
18
. RF up converters are not shown for simplicity.
At remote station
2
, a receiver receives signals from both antennas
16
and
18
on a single antenna, down-converts the signals, despreads the signals using code OC, and recovers a composite of channels CH
1
and CH
2
as transmitted from antennas
16
and
18
, respectively. In the first symbol time slot between 0 and T, the composite QPSK modulated signal R
1
is received (where R
1
=k
11
S
1
+k
12
S
2
), and in the second symbol time slot between T and 2T, the composite QPSK modulated signal R
2
is received (where R
2
=−k
21
S
2
*+k
22
S
1
* and the asterisk refers to a complex conjugate). Constant k
11
is a transmission path constant from first antenna
16
to remote station
2
during the first time slot, constant k
12
is a transmission path constant from second antenna
18
to remote station
2
during the first time slot, constant k
21
is a transmission path constant from first antenna
16
to remote station
2
during the second time slot, and constant k
22
is a transmission path constant from second antenna
18
to remote station
2
during the second time slot. The receiver derotates the channel to recover soft symbols S
1
′ and S
2
′, where
S
1
′=k
11
R
1
+k
12
R
2
and
S
2
′=k
21
R
2
*+k
22
R
1
*.
In this time space encoder technique, the first and second symbols are redundantly transmitted from separate antennas. The first symbol is encoded to be transmitted in both the first and second symbol time slots, and the second symbol is also encoded to be transmitted in both the first and second symbol time slots. The effect of this symbol recovery technique is that fading or drop out regions that may appear during one symbol time slot are less likely to appear during both symbol time slots when interleaving is also exploited. Interleaving is used before space-time coding to make adjacent bits less correlated in time. Since the received symbols are recovered from received signals during both time slots, R
1
and R
2
, the effect of fading is diminished.
However, the prior art does not exploit advantages provided by independent power management of individual beams transmitted by different diversity type antennas to achieve greater spectral efficiency at the base station while minimizing co-channel interference. The prior art does not exploit advantages provided by spatial power or beam width management of independently directed beams to achieve greater spectral efficiency at the base station while minimizing co-channel interference. The prior art does not exploit advantages provided by angle of arrival diversity to achieve greater spectral efficiency at the base station while minimizing co-channel interference.
SUMMARY OF THE INVENTION
It is an object to the present invention to improve the spectral efficiency of transmissions from the base station. It is another object of the present invention to minimize co-channel interference. It is a further object to minimize undesired effects of fading and drop out.
These and other objects are achieved in a system that includes a base station

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