Telecommunications – Transmitter – Measuring – testing – or monitoring of transmitter
Reexamination Certificate
1999-09-28
2001-07-03
Nguyen, Lee (Department: 2683)
Telecommunications
Transmitter
Measuring, testing, or monitoring of transmitter
C455S069000, C455S116000, C455S126000, C455S127500, C330S279000
Reexamination Certificate
active
06256483
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates generally to wireless transmitters. More specifically, the invention relates to power control in a wireless transmitter.
II. Description of the Related Art
The use of wireless communication systems for the transmission of digital data is becoming more and more pervasive. In a wireless system, the most precious resource in terms of cost and availability is typically the wireless link itself. Therefore, one major design goal in designing a communication system comprising a wireless link is to efficiently use the wireless link.
In a system in which multiple units compete for finite system resources, in order for multiple remote units to access common system resources, the wireless link is divided into a series of channels. Channelization can be achieved by one of a variety of well-known techniques such as time division multiple access (TDMA), code division multiple access (CDMA), frequency division multiple access (FDMA) or a combination of these. Each of these channelization techniques, to some extent, limits the frequency bandwidth of the signal transmitted from the remote unit.
In addition, each of these channelization techniques requires the use of power control to some extent to determine the power level at which the remote unit transmits. If the remote unit signal arrives at the hub station at a signal level that is too low, the system performance level may be inadequate to support communication due to excessive errors caused by thermal noise and interference. If the remote unit signal arrives at the hub station at a signal level that is too high, the remote unit generates unnecessary interference to other system users.
FIG. 1
is a schematic diagram illustrating a wireless satellite communication system. A hub station
10
provides digital data transfer capabilities to a plurality of remote units such as a remote unit
14
. The hub station sends signals over an uplink forward channel
20
to a satellite
12
. The satellite
12
repeats the signal and transmits it over a downlink forward channel
22
. The remote unit
14
receives the signal and processes it. The remote unit
14
sends a signal over an uplink reverse channel
24
to the satellite
12
. The satellite
12
repeats the signal and forwards it over a downlink reverse channel
26
.
A link budget is a design tool used to determine the level at which signals are transmitted offer the system. For example, a link budget is used to determine a nominal level at which the remote unit
14
transmits the reverse link signal over the uplink reverse channel
24
based upon the expected path loss experienced over the uplink reverse channel
24
. The satellite
12
may amplify the signal before forwarding it over the downlink reverse channel
26
. The link budget estimates the expected path loss of the uplink and downlink reverse channels
24
and
26
. In addition, the link budget estimates the expected interference and noise levels introduced by the uplink and downlink reverse channels
24
and
26
as well as noise introduced by the satellite
12
and the hub station
10
such as due to the noise figure of these units. In addition, the link budget estimates the expected variations of these parameters. Using the link budget, a system designer determines a nominal and worst case power level at which the remote unit transmits.
In a communication system which comprises fixed location remote units and which uses a geosynchronous satellite, the path loss of the wireless link channel is fairly consistent overt time. However, weather conditions may vary the path loss to some extent. Depending on the frequency at which the system operates, adverse weather conditions, such as heavy fog, snow, rain or hail, may increase the path loss by several decibels (dB) or more. Therefore, in order to operate efficiently, most wireless systems include a power control loop to control the level at which signals are transmitted and received in the system. For example, the hub station
10
monitors the signal-to-noise ratio of a signal received from the remote unit
14
over the reverse link channels
24
and
26
and notifies the remote unit
14
if the signal-to-noise ratio of the signal falls below a predetermined level. In response, the remote unit
14
increases the power level at which it is transmitting. If the hub station
10
determines that the path loss has decreased, the hub station notifies the remote unit
14
over the forward link channels
20
and
22
and the remote unit
14
decreases the level at which it is transmitting.
In order to reduce the distortion of the reverse link signal, the remote unit
14
is typically designed to comprise a class A power amplifier. Class A power amplifiers provide a high degree of linearity over a substantial range of output power. In order to operate linearly, class A amplifiers require substantially more supply power than the power level of the radio frequency (RF) signals which they produce. A class A amplifier draws the same supply power regardless of the output power which it is producing. Therefore, the size and heat dissipation capabilities of a class A power amplifier may be significant, as well as the cost of their operation. Typically, the size of a class A amplifier doubles for each 3 decibels (dB) extra of power which it is capable of producing. In addition, the cost of the power amplifier increases significantly for each 3 dB extra of power capability. Therefore, it is advantageous to use a link budget to determine the maximum power output level which the remote is required to transmit and to limit the capability of the power amplifier based upon the determination.
FIG. 2
is a graph showing the characteristics of a typical class A amplifier. The horizontal axis represents the RF input power level in units of decibels referred to 1 milliwatt (dBm). The vertical axis represents the RF output power level of the amplifier in the same units. The gain of the amplifier illustrated by curve
32
is approximately 54 dB. For example, when the input drive level is −30 dBm, the output power is 24 dBm. As the input level is increased in 1 dB steps, the output level also increases in 1 dB steps as is characteristic of a linear amplifier. However, at some point, the output power stops tracking the input power on a one-to-one basis. For example, data point
36
on curve
32
represents the point at room temperature at which the gain of the amplifier has decreased by approximately 1 dB to 53 dB of gain. At this point, the input to the amplifier is approximately −19.5 dBm and the output is approximately 33.5 dBm. As the input drive level is increased further beyond the 1 dB compression point, the output of the amplifier does not increase significantly above 34 dBm.
The non-linearities introduced by use of a class A amplifier close to and beyond the 1 dB compression point cause distortion in the modulated signal which causes increased interference levels in adjacent channels for the non-constant envelop signals generated by many modem communication systems.
FIG. 3
is a spectrum plot showing the distortion caused when a modulated signal is amplified by a power amplifier which has the characteristics shown by curve
32
of FIG.
2
. The horizontal axis represents frequency and the vertical axis represents power level relative to the power level of the modulated signal. The horizontal axis is measured in terms of channels. Channel
1
represents the channel in which the remote unit is operating. Channels
2
-
8
correspond to other channels in the system. The spectrum plot shown in
FIG. 3
is a single side-band plot. However, it may be assumed that the spectrum created is relatively symmetric about the left-most axis as shown in FIG.
3
.
Curve
40
represents the spectrum output by the power amplifier when the modulated drive level is −30 dBm. Referring again to
FIG. 2
, one can see that the power amplifier is quite linear in this region. The modulation bandwidth is limited to less than the bandwidth of the
Becker Donald W.
Moerder Karl E.
Knobbe Martens Olson & Bear LLP
Nguyen Lee
Tachyon, Inc.
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