Adaptive RF amplifier prelimiter

Details Adaptive RF amplifier prelimiter Adaptive RF amplifier prelimiter

1999-08-31

2002-08-13

Hunter, Daniel (Department: 2684)

Multiplex communications

Communication over free space

Combining or distributing information via code word channels...

C370S318000, C375S295000, C375S296000, C375S297000, C375S298000, C375S299000, C375S300000, C375S301000, C375S302000, C375S303000, C375S304000, C375S305000, C375S306000, C375S307000, C375S308000, C375S309000, C375S310000, C375S311000, C375S312000, C375S313000, C375S314000, C375S315000, C455S522000

Reexamination Certificate

active

06434135

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to spread spectrum code division multiple access (CDMA) communication systems. More particularly, the present invention relates to a system and method for adaptively limiting forward and reverse link transmission power within CDMA communication systems.
2. Description of the Prior Art
Wireless communication systems using spread spectrum modulation techniques represent the state of the art in digital communications and are increasing in popularity. In code division multiple access (CDMA) systems, data is transmitted using a wide bandwidth (spread spectrum) by modulating the data with a pseudo random chip code sequence. The advantage gained is that CDMA systems are more resistant to signal distortion and interfering frequencies in the transmission channel than communication systems using other multiple access techniques such as time division multiple access (TDMA) or frequency division multiple access (FDMA).
One indicator used to measure the performance of a communication system is the signal-to-noise ratio (SNR). At the receiver, the magnitude of the desired received signal is compared to the magnitude of the received noise. The data within a transmitted signal received with a high SNR is readily recovered at the receiver. A low SNR leads to loss of data.
A prior art CDMA communication system is shown in FIG.
1
. The communication system has a plurality of base stations
20
1
,
20
2
. . .
20
N
connected together through a local Public Switched Telephone Network (PSTN) exchange. Each base station
20
1
,
20
2
. . .
20
N
communicates using spread spectrum CDMA with mobile and fixed subscriber units
22
1
,
22
2
. . .
22
N
within its cellular area.
Shown in
FIG. 2
is a simplified CDMA transmitter
24
and receiver
26
. A data signal having a given bandwidth is mixed with a spreading code generated by a pseudo random chip code sequence generator producing a digital spread spectrum signal for transmission. Upon reception, the data is reproduced after correlation with the same pseudo random chip code sequence used to transmit the data. By using different pseudo random chip code sequences, many data signals or subchannels can share the same channel bandwidth. In particular, a base station
20
, can communicate with a group of subscriber units
22
1
,
22
2
. . .
22
N
using the same bandwidth. Forward link communications are from the base station
20
1
to the subscriber unit
22
1
,
22
2
. . .
22
N
, and reverse link communications are from the subscriber unit
22
1
,
22
2
. . .
22
N
to the base station
20
1
.
For timing synchronization with a receiver
26
, an unmodulated pilot signal is used. The pilot signal allows respective receivers
26
to synchronize with a given transmitter
24
, allowing despreading of a traffic signal at the receiver
26
. In a typical CDMA system, each base station
20
1
,
20
2
. . .
20
N
sends a unique global pilot signal received by all subscriber units
22
1
,
22
2
. . .
22
N
within communicating range to synchronize forward link transmissions. Conversely, in some CDMA systems for example in the B-CDMA™ air interface each subscriber unit
22
1
,
22
2
. . .
22
N
transmits a unique assigned pilot signal to synchronize reverse link transmissions.
FIG. 3
is an example of a prior art transmitter
24
. Data signals
28
1
,
28
2
. . .
28
N
including traffic, pilot and maintenance signals are spread using respective mixers
30
1
,
30
2
. . .
30
N
with unique chip code sequences
32
1
,
32
2
. . .
32
N
, respectively. Each mixers' output is coupled to a combiner
34
which adds the individual mixed signals as a combined signal
44
. The combined signal
44
is modulated up to radio frequency (RF) by a mixer
36
mixing the combined signal
44
with an RF carrier, shown in
FIG. 3
as COS &ohgr;t. The modulated signal is amplified to a predetermined transmission power level (TLP) by an amplifier
38
and radiated by an antenna
40
.
Most CDMA systems use some form of adaptive power control. In a CDMA system, many signals share the same bandwidth. When a subscriber unit
22
1
,
22
2
. . .
22
N
or base station
20
1
,
20
2
. . .
20
N
is receiving a specific signal, all the other signals within the same bandwidth are noise like in relation to the specific signal. Increasing the power level of one signal degrades all other signals within the same bandwidth. However, reducing TLP too far results in undesirable SNRs at the receivers
26
. To maintain a desired SNR at the minimum transmission power level, adaptive power control is used.
Typically, a transmitter
24
will send a signal to a particular receiver
26
. Upon reception, the SNR is determined. The determined SNR is compared to a desired SNR. Based on the comparison, a signal is sent in the reverse link to the transmitter
24
, either increasing or decreasing transmit power. This is known as forward channel power control. Conversely, power control from the subscriber unit
22
, to the base station
20
, is known as reverse channel power control.
Amplifiers
64
1
,
64
2
. . .
64
n
are used for adaptive power control in FIG.
3
. The amplifiers
64
1
,
64
2
. . .
64
n
are coupled to the inputs of the combiner
34
to individually control each signal's power level.
FIG. 4
a
,
4
b
,
4
c
and
4
d
show a simplified illustration of three spread spectrum signals
42
1
,
42
2
,
42
3
and a resultant combined signal
44
. Although each signal
42
1
,
42
2
,
42
3
is spread with a different pseudo random chip code sequence, each signal
42
1
,
42
2
,
42
3
is synchronous at the chipping rate. When the individual chips within the sequences are summed, the combined signal may have extreme transients
46
,
48
where the chip energies combine or low transients
47
where they subtract.
High transient peaks are undesirable. For every 3 dB peak increase, twice the base amplification power in Watts is required. Not only does the transient burden the amplifier, but the power sourcing the amplifier must have a capacity greater than the maximum transient that may be expected. This is particularly undesirable in hand-held battery operated devices. Additionally, to design for higher power levels resulting from high transients, more complex amplifier circuitry is required or compromises between amplifier gain, battery life and communication time result. High valued transients force the amplifier
38
into the nonlinear region of its dynamic range resulting in increased out-of-band emissions and reduced amplifier efficiency. Accordingly, there exists a need for an adaptive RF transmitter system that addresses the problems associated with the prior art.
SUMMARY OF THE INVENTION
The invention reduces transient peaks in signals transmitted in CDMA communication systems. A plurality of spread spectrum data signals are combined into a combined signal having fluctuating power level corresponding to the combination of the data signals. The combined signal is modulated to produce an RF signal for transmission. The average power of the combined signal is measured over a selected time period. The combined signal power level is adaptively limited to a calculated power level based at least in part on the measured power.


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patent: 6301485 (2001-10-01), Lee
patent: 0751630 (1997-01-01), None
patent: WO9918686 (1999-04-01), None
Fatih M. Ozluturk and Gary Lomp, “Effect of Limiting the Downlink Power in CDMA Systems with or without Forward Power Control”, InterDigital Communications Corporation, (1995).
Fatih M. Ozluturk, Ariela Zeira and Steve Dick, “Eff

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