Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1999-07-26
2004-02-17
Chin, Vivian (Department: 2682)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S127100, C455S127200, C370S337000, C370S347000
Reexamination Certificate
active
06694148
ABSTRACT:
BACKGROUND
The present invention relates generally to cellular communication systems. More precisely, the present invention relates to a transmit power supervision method for a base station equipped with a multi-carrier power amplifier (MCPA).
In conventional cellular systems, a base station is allocated a predetermined number of frequency channels for communication with mobile stations. In the base station a separate transmitter is employed for each frequency channel. However, the use of separate transmitters for each frequency channel results in a duplication of parts and an increase in cost due to the additional hardware required. Thereafter, it was realized that the hardware cost per channel could be reduced by using multicarrier transmitters in place of the plurality of single carrier transmitters to transmit a plurality of frequency channels. Since multicarrier transmitters transmit over a broad range of frequencies, they are also sometimes referred to in the art as wideband transmitters. However, for ease of discussion, the transmitters will be referred to herein as multicarrier transmitters.
FIG. 1
illustrates a conventional multicarrier transmitter
100
which may be used to transmit multiple frequency channels from a base station in a radiocommunication system. The conventional multicarrier transmitter
100
operates as follows. A number N of baseband frequency data signals BB
1
. . . BB
N
are modulated by modulators Mod
1
. . . Mod
N
, respectively, where the bits associated with each data signal are symbol encoded for transmission, i.e., the modulator generates the corresponding baseband waveform. Each of the modulated data signals is forwarded to a corresponding digital power control module DPC
1
. . . DPC
N
, where each DPC adjusts the signal power level of the corresponding modulated data signal based on the commands provided by the Radio Control Unit
150
. More specifically, the power level of each modulated data signal is adjusted such that the absolute power level of each carrier P
k,out
at the transmitter is equal to the amount of power required for the carrier to reach a particular mobile station which is to receive the carrier, where k varies from 1 to N and identifies the corresponding baseband frequency data signals BB
1
. . . BB
N
.
The modulated data signals are then forwarded from the digital power control modules DPC
1
. . . DPC
N
to multipliers Mult
1
. . . Mult
N
, respectively, where each modulated data signal is upconverted to a corresponding carrier frequency. The upconverted signals are then summed by adder
110
. The compound signal produced by adder
110
is then forwarded to the digital-to-analog converter (DAC)
120
. The resulting compound analog signal is then passed from DAC
120
through an analog transmitter chain which includes analog amplifier
160
, upconverter (not shown), and filters (not shown). Analog amplifier
160
then amplifies the compound signal by a fixed gain G
ana
. For ease of discussion G
ana
has been described as the gain of analog amplifier
160
, however, one skilled in the art will recognize that G
ana
represents the total gain of the analog section of the transmitter, including losses due to filters and upconverters. A more detailed discussion of multicarrier transmitters can be found in “Base-Station Technology Takes Software-Definable Approach” by Richard M. Lober, Wireless System Designs, Feb. 1998, which is herein incorporated by reference.
Multicarrier transmitters are designed to handle a maximum number of simultaneous carriers N. In designing a multicarrier transmitter, care must be taken to ensure that the instantaneous in-phase power sum, P
sum
of the N carriers does not exceed the maximum tolerable power of the MCPA. P
sum
can be calculated using equation (1) below, where P
n
represents the power of a specific user, n, in a specific time slot on a specific carrier frequency, and N is the total number of carrier frequencies used by the base station. Normally, P
n
is equal to the peak power within the specific time slot.
The instantaneous in-phase power sum of a single time slot for a system having, for example, a constant envelope (such as a GSM system), is given by:
P
sum
=
(
∑
n
=
1
N
P
n
)
2
(
1
)
For example, if the instantaneous sum of the N carrier frequencies exceeds the full scale range of the DAC, i.e., the value associated with the greatest digital code that can be converted into an analog value, the DAC will clip the analog signal. Clipping, i.e., preventing the analog signal from exceeding the amplitude corresponding to the full scale range of the DAC will have an adverse effect on the quality of the transmitted signal. However, one skilled in the art will recognize that in practical applications, a system might tolerate a power level which exceeds the DAC's full scale range by a small amount for short periods of time without suffering a decrease in system performance.
In a multicarrier transmitter with N carrier frequencies, the abovementioned “clipping” of the analog signal can be avoided by setting the full scale range of the DAC to 20*log(N) dB above the maximum allowed peak power level of any individual carrier
1
. . . N, since the full scale range set 20*log(N) dB above the maximum power level of any individual carrier represents the greatest power level attainable by the sum of the N carriers.
Designing MCPAs with a high output power is a difficult and expensive task. As the MCPA is designed to have a higher maximum output power, design costs become increasingly more expensive. For a base station operating using time division multiple access (TDMA), the maximum total output power of the base station limits the total output power of the frequency carriers at any time slot. TDMA, as one skilled in the art will appreciate, is a communication technique whereby different signals are assigned to different time slots on the same frequencies. One problem associated with MCPAs designed for a particular output power and operating in a TDMA environment is that a MCPA can only serve a predetermined maximum number of users for the respective particular output power. If more than the predetermined maximum number of users were to be allocated to the MCPA, the MCPA would lose linearity resulting in a decrease in link quality.
FIG. 2
illustrates an exemplary time chart which may be associated with a base station. In
FIG. 2
, seven frequencies (
1
-
7
) in use by an exemplary base station are illustrated over eight time slots. The numbers in the time chart indicate the required output power, in watts, for a mobile unit which is operating at a particular frequency and assigned to a particular time slot. For example, at frequency
1
and time slot
1
, the mobile unit requires 4 watts (W). P
sum
for each time slot is depicted below the time chart.
Assuming, for example, that the maximum power level that can be allocated for each mobile unit is set to 8W, then using Equation (1), the serving MCPA must be designed for at least a maximum output power of 392W. That is, the MCPA must be designed to handle the worst-case scenario of each of the mobile units receiving at their maximum allocated power, 8W. As seen in
FIG. 2
, the in-phase power sum per time slot, P
sum
will usually be lower than the maximum output power of 392W. This difference illustrates how the MPCA will not be used efficiently, since it must be designed to handle a worst-case scenario of all users being allocated the maximum output power for a given time slot.
Several techniques have been developed for extending the maximum capacity for which MCPAs have been dimensioned. Load sharing is one such technique. Conventional load sharing is basically a type of load balancing where a user is transferred from one cell which has reached its maximum capacity to another cell which can accommodate the user. This technique avoids overload situations. The following patents illustrate conventional load sharing techniques.
A method of balancing the load among cells which are operating at maximum capacity is described i
Eriksson Patrick
Frodigh Magnus
Hedberg Bo
Johansson Christer
Chin Vivian
Persino Raymond B
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