Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1999-06-15
2001-02-06
Bocure, Tesfaldet (Department: 2731)
Pulse or digital communications
Receivers
Interference or noise reduction
C455S135000
Reexamination Certificate
active
06185266
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related in general to wireless communication systems, and more particularly to an improved method and system for generating a power control metric in an orthogonal transmit diversity communications system.
BACKGROUND OF THE INVENTION
In many wireless communications systems, and especially in cellular communication systems, it is important to control the transmitted power of a traffic channel in order to reduce cochannel interference. Cochannel interference is generated by other transmitters assigned to the same frequency band as the desired signal. And because all users transmit traffic on the same carrier frequency in a code division multiple access (CDMA) cellular system, reducing cochannel interference in CDMA systems is especially important because it directly impacts system capacity. If the cochannel interference is reduced, the CDMA system capacity may be increased. Therefore, it is a design goal to transmit a traffic signal with only an amount of power necessary to provide acceptable signal quality at the receiver, after it passes through the channel.
In this document, a “channel” may be defined as a path or paths of communication through a medium between a transmitter and a receiver. If the medium is air and communication takes place with radio frequency (RF) signals, such a channel is typically affected by fading. A “traffic channel” may be defined as a channel that carries data, whether representing voice or other information generated by the user, which the user intends to transmit via the channel. The traffic channel may be distinguished from other channels used by the communication system, such as channels that may be used to transmit timing, control, or other information supporting system operation.
Power control systems in cellular communication systems should compensate not only for signal strength variations due to the varying distance between the base station transceiver and the subscriber unit, but should also attempt to compensate for channel quality fluctuations typical of a wireless channel. These fluctuations are due to the changing propagation environment between the transmitter, or base station, and the receiver, or subscriber unit, as the user moves in the service area.
Existing power control systems used in CDMA cellular systems that operate according to J-STD-008, published by the Joint Technical Committee on Wireless Access, use the measurement and reporting of cyclic redundancy check (CRC) errors at the subscriber unit to control the power of the traffic channel at the base unit. This method of power control in response to CRC errors is used to implement a slow “ramping” power control scheme. The “ramping” occurs because the traffic channel power is increased by a relatively large amount when the subscriber unit reports CRC errors. After the large power increase, which often eliminates the CRC errors for some subsequent period, the power is reduced by a relatively small amount for each subsequent frame transmitted. Eventually, the power is reduced to a point where another CRC error occurs, and the power is once again increased by a relatively large amount. If channel quality remains constant, a graph of power transmitted in the traffic channel resembles a saw tooth, with large power increases followed by a series of small power decreases.
One problem with this method of power control is the delay encountered between the degradation of channel quality and the request for a power increase and the subsequent actual increase in power. The delay in requesting a power increase is caused by waiting for a frame to be received, and then waiting for frame decoding and the detection of a cyclic redundancy check error. Once the CRC error is detected, it must be reported to the base station, and the base station must respond by increasing traffic channel power. In current CDMA systems, it takes 20 milliseconds (mS) to receive a frame. Thus, the rate at which CRC reports or power control commands are sent to the transmitter is 50 Hz. This delay in the power control loop periodically causes the base to transmit too much power on the traffic channel, such as when a relatively large increase in power is requested and granted just as the channel quality has reached a minimum and starts to improve. If the traffic channel has too much power, cochannel interference increases and system capacity decreases.
With reference now to
FIG. 1
, there are depicted relevant portions of transceiver
20
that uses orthogonal transmit diversity (OTD). As illustrated, traffic channel data source
22
provides a stream of symbols, which may represent voice or data traffic of a plurality of users or channels. The rate that symbols are output from data source
22
is controlled by symbol clock
24
.
Symbols from traffic channel data source
22
are convolutionally encoded by convolutional encoder
26
. Convolutional encoder
26
encodes at a rate of one divided by “n”. This means that for every symbol entering convolutional encoder
26
, n encoded symbols are output. Clock multiplier
28
provides a clock for convolutional encoder
26
that is n times the rate of symbol clock
24
.
After traffic channel data symbols have been encoded, power control encoder
30
places uplink power control information into the stream of encoded symbols. In one proposed system, this is accomplished by inserting a power control bits in a predetermined bit locations in power control groups of a frame in the data stream. Thus, some traffic channel data bits are replaced, or punctured, by bits intended to direct the subscriber unit to raise or lower its transmit power level. The frequency at which power control bits are punctured remains at a predetermined frequency, which in a preferred system is 800 Hz. Additionally, the power level of the punctured power control bits are set at the full vocoder rate traffic power level. The power control bits are preferably evenly distributed among the transmit antennas.
After the power control bits are inserted in the data stream, commutator
32
distributes symbols among diversity branches of the orthogonal transmit diversity transmitter. As shown in transceiver
20
, there are two diversity branches defined by paths through spreaders
34
and
36
, which paths use different spreading codes to spread the symbols in each branch.
After the symbols are spread with multiple orthogonal spreading codes, the spread data outputs are amplified by amplifiers
38
and
40
. Amplifiers
38
and
40
are coupled to power controller
42
which controls the gain of amplifiers
38
and
40
.
The outputs of amplifiers
38
and
40
are each coupled to separate antennas
44
and
46
, which provide different signals that propagate through different paths r
1
and r
2
, before they may be received by a subscriber unit. Also note that one or both antennas, such as antenna
46
, may be used to receive power control information PC transmitted from a subscriber unit. This power control information is coupled to power controller
42
so that power controller
42
may set the gain of amplifiers
38
and
40
.
With reference now to
FIG. 2
there is depicted selected portions of a subscriber unit
50
according to the prior art. As shown, antenna
52
receives signals through paths r
1
and r
2
, which carry traffic channel data and other control data. Antenna
52
is also used to transmit power control information PC to transceiver
20
shown in FIG.
1
.
Antenna
52
is coupled to down converter and demodulator
54
, which down converts and demodulates the received signals.
The output of downconverter and demodulator
54
is split to form diversity branches within subscriber unit
50
. These diversity branches correspond to the antennas and diversity branches within transceiver
20
. Thus, transceiver
20
in
FIG. 1
is shown with two diversity branches, and subscriber unit
50
is also shown with two corresponding diversity branches.
The paths along diversity branches pass through despreaders
56
and
58
, respectively. These despreaders use
Ahmed Mansoor
Kuchi Kiran
Bocure Tesfaldet
Motorola Inc.
Terry L. Bruce
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