Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-03-10
2004-02-03
Yao, Kwang Bin (Department: 2664)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S342000
Reexamination Certificate
active
06687238
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to Code Division Multiple Access (CDMA) systems. More particularly, the present invention includes, but is not limited to, a novel and improved CDMA base station that performs various combinations of the following: 1) decresting CDMA signal peaks, 2) shaping the in-band frequency spectrum of CDMA signals, 3) generating a ratio of in-band to out-of-band signal strength, and/or 4) controlling transmit power based on quadrature signal calculations.
II. Description of the Related Art
Code Division Multiple Access (CDMA) technology is commonly used in communications systems. In a typical CDMA system, a CDMA base station transmits a CDMA signal to numerous CDMA communications devices, such as wireless telephones. The CDMA signal is comprised of numerous individual user signals. The CDMA base station generates the CDMA signal by encoding each individual user signal with a unique spreading sequence, such as a pseudo random sequence. The CDMA base station then adds the encoded user signals together to form the CDMA signal.
In a CDMA system, individual user signals are not separated based on frequency or time, but are spread across the entire frequency band. Each CDMA communications device derives its particular user signal based on the unique spreading sequence. Due to this combination of multiple signals encoded with random sequences, the CDMA signal has random signal peaks that cause problems when the CDMA signal is amplified. In contrast, non-CDMA signals do not typically have such random characteristics. For example, a frequency modulated signal fits within a constant signal envelope because individual user signals are placed within discreet frequency bands and are not combined or encoded with random sequences.
CDMA signal transmission has special power concerns because the CDMA signals are spread across the frequency band. Since the CDMA signals share the frequency band, each signal represents noise to the other signals. Thus, CDMA transmission systems must carefully track the power of each signal.
Baseband CDMA signals are typically generated in a well-known quadrature format comprised of quadrature CDMA signals I and Q. Quadrature CDMA signals I and Q are transmitted using carriers of the same frequency, but in phase quadrature. In other words, an RF CDMA signal can be constructed by modulating I by cosine (2×pi×frequency×time) and by modulating Q by sine (2×pi×frequency×time). In IS-95A, quadrature signals carry the same data with different pseudo-random sequence codes.
FIG. 1
illustrates an ideal frequency spectrum of a typical CDMA signal. The vertical axis represents signal power, and the horizontal axis represents frequency. The desired in-band signal power is contained within the bandwidth defined by corner frequencies around a center frequency. A typical example is a 1.25 MHz bandwidth centered about a 1.96 GHz center frequency with corner frequencies at (1.96 GHz−625 KHz) and (1.96 GHz+625 KHz). The signal power drops significantly outside of the bandwidth, but some undesired out-of-band signal power is still present and is shaded on FIG.
1
. Out-of band signal power is undesirable because it represents wasted power that interferes with other signals in neighboring frequency bands.
FIG. 2
illustrates a time domain plot of a typical CDMA signal. The vertical axis represents CDMA signal amplitude in volts, and the horizontal axis represents time. The dashed lines represent a maximum positive signal voltage (+Vmax) above the zero voltage point, and a negative maximum signal voltage (−Vmax) below the zero voltage point. The CDMA signal has “peaks” above and below the Vmax voltages. The peaks are shaded on FIG.
2
.
FIG. 3
illustrates the operating characteristics of a typical power amplifier used to amplify a CDMA signal. The horizontal axis represents the input signal power (Pin), and the vertical axis represents the output signal power (Pout). If Pin is below a maximum power level (Pmax), then the power amplifier operates in a linear manner where an increase in Pin is matched by a proportional increase in Pout. If Pin is above Pmax, then the power amplifier operates in a nonlinear manner where an increase in Pin is not matched by a proportional increase in Pout. Pout is less than ideal in the nonlinear operating range.
It should be noted that the Vmax voltage levels on
FIG. 2
correspond to the Pmax on FIG.
3
. Thus, the random signal peaks above +Vmax and below and −Vmax drive the power amplifier above Pmax into the nonlinear operating range. When operated in the nonlinear range, the power amplifier exhibits undesirable performance in the form of decreased fidelity and increased noise. In contrast, the typical Frequency Modulated (FM) signal does not have random signal peaks, so the power amplifier is able to continuously operate below the maximum power level.
The power amplifier generates additional out-of-band signal power when operated in the nonlinear range. Out-of-band signal power is a problem because it interferes with other signals in the neighboring frequency bands. Government agencies, such as the Federal Communications Commission in the United States, strictly regulate the interference caused. by out-of-band signal power.
An existing solution to the problem is implemented during base station testing. Test equipment is used to calculate a ratio for a test CDMA signal transmitted by the base station. The ratio represents the in-band signal power versus the out-of-band signal power. The base station transmit power is adjusted during the testing so the ratio is below a maximum value with a margin for some ratio increase under the maximum value. This usually Unfortunately, the ratio is not calculated and is not used during normal base station operation in the field. Test equipment is used to calculate the ratio, and base stations are not equipped to calculate the ratio in the field. Thus, the ratio is not automatically generated and used to control operation in the field where changes in temperature and load alter base station operation.
Another existing solution to this problem is to operate the CDMA base station so a ratio of the power out to the pilot signal does not exceed a value, such as five. This solution is lacking because a maximum power level based on the pilot signal is not an optimal estimate of the point where out-of-band signal power becomes a problem. As a result, the range and capacity of the base station is not optimized.
FIG. 4
depicts a multi-sector base station
1100
that is currently known in the art. The base station
1100
is divided into geographic sectors with callers A-F in sector
1
and callers G-L in sector
2
. For the sake of illustration, caller F will move from sector
1
to sector
2
as indicated by the dashed lines, but the operation of the base station
1100
is first discussed prior to the caller F move from sector
1
to sector
2
. Those skilled in the art will appreciate that the diagram of the base station
1100
has been simplified for clarity.
The sector
1
portion of the base station
1100
includes cell site modems
1102
and
1104
, gain control
1106
, summing circuit
1108
, CDMA signal processor
1110
including gain
1112
, and antenna
1114
. The sector
2
portion of the base station
1100
includes cell site modems
1122
and
1124
, gain control
1126
, summing circuit
1128
, CDMA signal processor
1130
including gain
1132
, and antenna
1134
.
In operation, the cell site modem
1102
receives signals for callers A, B, C and applies conventional CDMA processing to generate CDMA quadrature signals I and Q. The cell site modem
1102
provides the CDMA quadrature signals I and Q to the summing circuit
1108
. The cell site modem
1104
receives signals for callers D, E, F and applies conventional CDMA processing to generate CDMA quadrature signals I and Q. The cell site modem
1104
provides the CDMA quadrature signals I and Q to the summing circuit
Flowers Larry D.
Funk Thomas J.
Harms Brian K.
Night Robin
Pressley Todd A.
Edwards Christopher
Miller Russell B.
Qualcomm Incorporated
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