Communication systems, such as satellite communication systems, typically include one or more control loops for power transmission. FIG. 1 illustrates, in block format, a typical power detect and control scheme for a satellite communication system. System 100 includes an indoor unit (IDU) 102 which provides signals to an outdoor unit (ODU) 104 over a cable 103. Generally, IDU 102 is configured to control the level of the RF (radio frequency) signal to be transmitted to a satellite. For example, by increasing or decreasing the signals provided to ODU 104, IDU 102 can vary the output power of the RF signal transmitted to the satellite.
Collectively, IDU 102, ODU 104, and cable 103 are generally termed “ground station”. The ground station may be located on, for example, a stationary structure (e.g., building) or a moving structure (e.g., vehicle) such that communication with the satellite is permissible. In many applications, the IDU is located near or within a computer, e.g., a card that fits inside the computer processor or a box in proximity to the computer. The cable or multiple cables interconnect the IDU with the ODU, which may be located outside, e.g., on the roof of a building or a vehicle. Another ground station, generally illustrated in FIG. 1 as a dish antenna, provides similar functionality for the opposite end of the communication link in order to connect the user of ground station 100 to a telecommunications or computer network.
As is often the case in wireless transmission, obstructions, e.g., clouds, rain, structures, and the like, can decrease the transmission reception of the RF signal received at the satellite, i.e., the signal-to-noise (SNR) of the RF signal decreases. The satellite may transmit a signal back to the ground station indicating that the RF signal received at the satellite is too weak, for example, the strength (i.e., power level) of the received signal is too low for optimum detection. In response, the ground station typically has two options; reduce the data bit rate (which is generally undesirable) by either a method of bandwidth reduction or increased error correction coding, or increase the RF signal strength. To increase the RF signal strength, the IDU increases the signal power to the ODU, thereby increasing the RF signal strength transmitted to the satellite.
ODU 104 includes a power amplifier (not shown) to increase or boost the RF signal in preparation for transmission to the satellite. Power amplifiers are often characterized by the maximum power capability of the device. For instance, as increasing RF power is supplied to an amplification device, the output power of the device increases accordingly, until a “saturation level” of the device is reached. At saturation, or the maximum RF power capability, the amplifier output no longer behaves linearly to an increase in power, regardless of the amount of input power. When in saturation the RF waveform is “clipped” and the maximum amount of energy is contained in the primary signal. As the input power is further increased, the excess signal energy creates additional signal distortion and signal harmonics.
Signal harmonics can mix together to form a spectrum image of the primary signal in an adjacent communication channel. The adjacent channel interference caused by the power amplifier distortion begins to decrease the SNR of the signals from other users in adjacent transmission channels. To compensate for the lower SNR, the IDU of those users, operating in adjacent channels, typically increases the power to the ODU. Increasing the power continues to increase the harmonics and in turn decrease the SNR; thus the cycle continues. In addition to causing disruptive interference, governing regulatory bodies such as the FCC generally place limits on the levels of acceptable adjacent channel interference levels.
To reduce the interference caused by harmonics, it is desirable to control the level of power supplied to the ODU. When the input signal is amplified by about 1 db (decibel) less than the small signal gain, it is commonly called the 1 db compression point (P1 db). As the input signal to the amplifier is increased past P1 db, the output signal is no longer in a linear relationship with the input signal and a rapid decrease in gain is experienced; thereby causing signal harmonics. It is desirable to detect and limit the amount of power to a component, such as a power amplifier, to approximately P1 db of the component to avoid creating signal harmonics which can adversely affect signal transmission.
Referring to FIG. 2, a ground station 200 of a prior art uplink power detect and control system is illustrated. Ground station 200 includes an IDU 202, an ODU 204, and signal transfer means 203. Signal transfer means 203 includes, for example, one or more cables suitable for signal transmission between IDU 202 and ODU 204, e.g., industry standard RG-6 type cable (coax) such as Belden 9114.
Generally, IDU 202 includes a modem 210 to receive and transmit IF (intermediate frequency) signals to ODU 204 and a DC (direct current) power supply 212 to transmit DC power to ODU 204. A typical modem 210 has a demodulator 214, a modulator 216, an automatic gain control 218, and an automatic level control 220. Demodulator 214 converts the received IF signals into digital data which can be coupled to a computer or other digital appliance via a serial or parallel digital interface. Automatic gain control 218 is used to adjust the input IF signal level up or down to provide an approximately constant signal level to demodulator 214. Modulator 216 converts digital data from the serial or parallel digital interface to an IF signal. Automatic level control 220 is used to increase or decrease the output signal level provided to the ODU based on link conditions (e.g., weather, temperature, interference, etc.).
Generally, ODU 204 includes a receiver 222, an antenna 224, a transmitter and power amplifier 226, an RF power detector 228, and interface circuitry 230. The receiver 222 comprises a low noise amplifier to amplify the input RF signal from the antenna and a down conversion mixer to convert the RF signal to an IF signal. Additionally, receiver 222 may contain one or more RF, IF or local oscillator (LO) amplifiers as well. Antenna 224 may be any antenna suitable for receiving and transmitting the proper frequencies; such as a dish, dipole, phased array, or any other suitable antenna. Transmitter 226 receives the IF signal from IDU 202. A signal mixer (not shown) within transmitter 226 generates an RF signal from the received IF and DC signals. Transmitter 226 also comprises a power amplifier, e.g., a high power amplifier (HPA), which boosts the RF signal in preparation for transmission to the satellite.
Inclusion of RF power detector 228 is one prior art technique for detecting and limiting the power to transmitter 226 by means of the automatic level control 220 for reducing the effects of signal interference. RF power detector 228 (also called a “forward power detector”) generally includes a coupler, detector diode, comparator, and signal transmission means (e.g., interface circuitry 230) for transmitting signals back to IDU 202.
In operation, RF power detector 228 samples the output signal from the power amplifier (prior to antenna transmission) and sends the signal back to the IDU. Interface circuitry 230 is required in order to use automatic level control 220. An additional signal transfer means 203, e.g., an additional cable or multiplexing circuitry, is required to transmit the output RF signal from ODU 204 to IDU 202 (generally illustrated in FIG. 2 as “Forward Power Telemetry”). Using the sampled power output at the ODU, the IDU can gauge how much more power station 200 can transmit without causing the amplifier to go into compression.
There are significant disadvantages to the forward power detector system of the prior art. For example, each of the components in the forward power detector system, e.g., diode, coupler, op amp, operate differently under environmental variations, in particular to temperature changes. To accommodate for the variations, each system using a forward power detector unit must be calibrated individually under various conditions. This data must then be stored in an accessible database for each unit for subsequent reference. Such data collection is labor intensive, requires memory storage, and is susceptible to erroneous calculations.
Also, the forward power detector system of the prior art draws power from the output of the power amplifier. Any power loss at the output of the amplifier must be compensated for by increasing the overall output of the amplifier. This is not always trivial. In fact, the cost of the power amplifier unit is often dependent upon the power output capabilities of the unit. Therefore, as more output power is required to account for loss, a more expensive power amplifier is generally needed.
In addition, interfacing the signals from the forward power detector system to the IDU can require additional cables or multiplexing circuitry, complex interface components and circuitry, or a combination of both.
Another technique for detecting power levels includes detecting the input power to a FET-based amplifier by gate current sensing. As RF power is increased to the amplifier, the gate-source junction of the FET mimics the behavior of a schottky diode. The input RF signal becomes rectified, resulting in a net DC current into the device. A relationship between the input power level and the gate current can then be established through careful monitoring of the gate-source junction.
While the gate current sensing technique overcomes some of the problems associated with the forward power detector system, e.g., eliminates the hardware at the power amplifier output and reduces system loss, disadvantages to the gate current sensing technique are readily identified. Due to system variations that are inherent to the component and the environment of the unit, this technique is extremely susceptible to error. For example, gate current sensing measures the input drive power to the amplifier, not the output power which is the relevant measurement. While it is feasible to determine the output power mathematically from the input power and the gain, this calculation can vary substantially due to gain variations caused by environmental changes, unit variations, and the like. Therefore, as the gain of the amplifier changes, the calculated output power will change.
Moreover, the diode characteristics of the FET amplifier vary from device to device; thereby increasing the risk of erroneous readings. Under high RF input drive, the normally reverse biased gate-drain junction can begin to breakdown and cause a net current flow out of the gate. Accurate gate current readings may be difficult due to some or all of the current flow into the gate being cancelled.
Accordingly, an improved system and method for power control in a communication system is needed. In particular, a power control and detection system, that is substantially independent from environmental variations, for providing improved detection accuracy without substantially increasing cost, components, and system interconnect requirements is needed.