Using RF Power Detector to Control Power of CDMA Access Terminal

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Using RF Power Detector to Control Power of CDMA Access Terminal [Copy link]

Each mobile station is an interference signal source for users in the same cell and adjacent cells, so good transmission power control is very important for improving the communication performance and capacity of the CDMA system. The closed-loop power control method in the CDMA system introduced in this article can effectively control the power of the mobile station. The article also introduces the practical application of high-precision RF power detector LMV225 to achieve power control.

Since the commercialization of CDMAIS-95 cellular networks in 1996, CDMA technology has proven to be the best wireless technology to advance the cellular personal communications industry. According to the CDMA Development Group (CDG), CDMA users increased by 24 million in the first six months of 2004, reaching a total of 212.5 million users worldwide. All major 3rd generation standards, such as CDMA2000, W-CDMA and TD-SCDMA, use CDMA as the access method. CDMA is based on spread spectrum modulation technology, which is based on Shannon's information theory. Shannon's capacity law states that the channel capacity in additive white Gaussian noise is:

Where: Csh is the channel transmission capacity, the unit is bps; BRF is the channel bandwidth, the unit is Hz; SNR is the signal-to-noise ratio.

Compared with narrowband systems (i.e., smaller BRF), wideband systems (i.e., larger BRF) require a smaller signal-to-noise ratio (SNR) to achieve the same capacity Csh. On the other hand, in a channel with a given bandwidth BRF, a higher SNR has a greater transmission capacity. This means that if all users transmit the same amount of data, the same channel can have more users.

IS-95 and CDMA2000 Power Control for Mobile Stations or Access Terminals

The power control feature is to control the interference signal level in the reverse link by estimating the optimum transmit energy level and responding to power control instructions sent by the network or base station.

In CDMA IS-95 and CDMA2000 1X, the base station determines the power control, while in CDMA2000 EV-DO, the access terminal performs the power control. The power control schemes of the two standards are similar, and they use two power control methods, namely open-loop control and closed-loop control.

1. Open-loop power control

The open-loop method uses the power level PRX of the mobile station receiver to estimate the forward link loss, and then specifies the initial transmit power PTX of the mobile station, so that based on different user terminal selections (such as cellular, PCS or 3G), the sum of the forward and reverse link powers remains a constant, that is, PTX+PRX is a constant. PRX is calculated by Eb/Io, which is measured by the mobile station's digital signal processor (DSP).

After obtaining the initial PTX, both the mobile station and the base station start closed-loop control. According to the CDMA standard implemented, the base station sends an error signal to the mobile station, instructing the mobile station to increase or decrease one unit of energy.

Closed loop power control

Closed-loop power control consists of two steps: an outer loop (performed only by the base station) and an inner loop (performed simultaneously by the mobile station and the base station). In IS-95 and CDMA 1X, closed-loop control can achieve a power control rate of 800 Hz.

The main purpose of closed-loop power control is to minimize the rapid attenuation effect caused by signal multipath propagation loss based on the measurement results of the base station. Combining the outer loop and inner loop closed-loop power control processes, 20-35dB attenuation compensation can be achieved in a 20ms inter-frame interval, with a dynamic range of up to 80dB.

a. Outer loop closed loop power control

In the outer loop, the base station sets a target Eb/Io for each frame of the receiver every 20 milliseconds (from the mobile station to the base station). When a frame error occurs, the Eb/Io value is automatically reduced by 0.2 to 0.3 or increased to 3 to 5 dB.

The entire outer loop closed-loop control step is only related to the base station and has nothing to do with the mobile station.

b. Inner loop closed loop power control

In the inner loop, the base station compares the Eb/Io of the reverse channel with the target Eb/Io every 1.25 milliseconds, and then instructs the mobile station to reduce or increase the transmission power so that the target Eb/Io can be achieved. For CDMA2000, the power change range is between ±0.25dB and ±0.5dB, while for CDMA IS-95, the power change range is ±1.0dB. The correction rate is 800bps.

Hardware Implementation of Power Control in CDMA Mobile Station

In summary, CDMA IS-95 requires the mobile station to adjust the transmit power every 1.25 milliseconds by ±1.0dB, while CDMA2000 can be ±0.25dB to ±0.5dB. Figure 1 shows the general output power control of the linear power amplifier signal chain of a handheld device. Since CDMA requires high linearity, the output power amplifier is usually biased at a fixed gain, and then the output power level must be adjusted by the gain control linear driver amplifier, which is usually called an automatic gain control (AGC) amplifier in a CDMA mobile station.

It has been found that the RF transmit architecture in Figure 2 can reduce the DC power consumption of the power amplifier by using an isolator (such as Murata CE04 and CES30) and a high-precision RF power detector LMV225. The isolator provides a nearly perfect 50-ohm load to the output of the power amplifier, while the LMV225 detects the precise transmit power level. The mobile station's DSP then sets the output power to the level required by the base station. In this application circuit, a resistor is used to divert the RF signal of the main signal channel to the input of the LMV225. In addition, a capacitor of about 100pF is required for DC blocking to prevent the enable control signal from entering the main signal channel. This DC blocking capacitor is necessary because it is undesirable to have a DC voltage enter the output of the power amplifier or the isolator. Since there is already an isolator, most of the RF energy that is diverted comes from the transmit power amplifier. The reflected energy from the antenna will be diverted to the built-in 50-ohm load of the isolator, and little will reach the output of the power amplifier or the LMV225. Therefore, the power coupled to the LMV225 can be estimated by 20log[R1/(R1+50)].

The actual test results show that the distortion performance of a power amplifier with a supply current of 500mA and an adjacent channel power rejection (ACPR) of -40dBc can be improved to a supply current of 450mA and an adjacent channel power rejection of -50dBc. In this case, the current is reduced by 10% and the distortion is improved by about 10dB.

We have now demonstrated that the LMV225 and a CES30 isolator can provide good performance in terms of power consumption and RF distortion in linear CDMA power amplifier applications for IS-95, W-CDMA, CDMA2000 and TD-SCDMA air interfaces. In fact, due to the uncertainty and variation of components in the transmit signal path (such as AGC, power amplifier gain and passive component losses, etc.), it is necessary to use the LMV225 as a transmit power detector in a CDMA2000 mobile station or access terminal in order to achieve strict inner loop power control.

Mobile station transmission signal channel

The RF transmit circuit architecture in Figure 2 can be used in many different CDMA chipsets. Figure 3 is a simplified diagram of the LMV225 application recommended for CDMA2000 1X or EV-DO single-band handheld device transmit power detection.

In this transmitter configuration, the power delivered to the antenna is:

RFout=PRFT-LSAW+GPA-LISOLATOR-LDUPLEXER

Where: RFout is the RF power to the antenna (assuming a 50 (impedance load) has been achieved); PRFT is the output power of the RF transmitter chip; LSAW is the insertion loss of the surface acoustic wave filter; GPA is the fixed gain of the CDMA power amplifier; LISOLATOR is the insertion loss of the isolator; LDUPLEXER is the insertion loss of the duplexer.

Since R1 and LMV225 have formed a high impedance parallel load for the signal channel, the insertion loss of the resistor power divider formed by R1 and LMV225 here can be ignored. At room temperature, LSAW, GPA, LISOLATOR and LDUPLEXER can be regarded as unchanged. Then, the RF power RFout to the antenna can be adjusted by PRFT, and PRFT is controlled by the AGC in the transmitter chip. In fact, the AGC amplifier usually supports the 80dB dynamic range required by IS-95 or CDMA2000. We also found that the output power of CDMA mobile stations is medium for most of the working time, so the accuracy of power control is very important during the change from medium output power to high output power. Inappropriate high power level will reduce the talk time of the mobile station and cause more interference to other network users.

Advantages of LMV225

The LMV225 design is optimized to provide the best power detection range in CDMA handsets. As mentioned above, accurate power control from medium to high power is particularly important, and the coupling resistor R1 is used to allow the LMV225 to know the critical range of RF signal presence. Assume that the AGC is set to high/highest gain to achieve the maximum output of the CDMA power amplifier, such as 28dBm. If the crest factor of the RF transmit signal is 3dB at this time, then the instantaneous peak power of the CDMA power amplifier will be 28+3=31dBm. If we choose this as the maximum reference point that the LMV225 should be able to detect, that is, when the instantaneous output power of the power amplifier is 31dBm, the input to the RFin/enable pin of the MV225 is 0dBm, and a coupling factor of 31dBm should be used. We found that using a 1.8K(R1) can produce a coupling factor of 31dBm in this circuit.

Linearity characteristics of LMV225

The LMV225 has a 30dB linear detection range, which reduces the complexity of the production calibration process. The calibration process is an important part of the CDMA mobile station production process. Automatic test equipment is used to collect the mobile station output power information during the process of the control code/signal from weak to strong, and the information is stored in the mobile station's memory for field use. When the base station requests output power, the mobile station's DSP finds the control code/signal from the memory to achieve the requested output power level.

Some AGCs on the market may have an exponential characteristic between the control signal and the output gain. If this type of AGC is used with the LMV225, the linear characteristic in dB will not make the original control curve more complex than the original AGC characteristic curve compared to other detection methods (such as diode detection). However, if the AGC has a linear control range, the linear characteristic in dB will reduce the calibration points from more than 20 to about 2. The two-point calibration process is based on the following principle: In a two-dimensional plane, only two different points are needed to represent a first-order linear equation. If the equation is y=mx+b, the slope m and intercept b can be calculated using two test coordinates (x1, y1) and (x2, y2).

LMV225 in Dual-Band CDMA2000 Mobile Station

Figure 4 is a recommended block diagram for a CDMA2000 handset. Although resistors R1 and R1'' may not be the same, the user can optimize the performance of the two bands by making R1 and R1'' the same value. On the other hand, since only one power amplifier is active in actual applications and resistors R1 or R1'' usually provide 30+dB isolation, the isolation between the low and high bands should be within an acceptable range.

By Barry Yuen
RF Systems Engineer
National Semiconductor