TD-SCDMA RF Power Amplifier
TD-SCDMA (Time Division Synchronous Code Division Multiple Access) is the third generation mobile communication standard of the three mainstream, one of China's communications standards with independent intellectual property, which indicates that China has entered the mobile communications world advanced level, the current , TD-SCDMA's commercialization process is smoothly underway [1]. TD-SCDMA system uses the QPSK/8PSK modulation, high-speed data transmission applications, it is used such as 16QAM modulation. The modulation is a non-constant envelope modulation. As the modulation signal amplitude and phase errors are present, with a simple phase error and frequency error are not sufficient to reflect the precision of the signal modulation, so the introduction of the error vector magnitude (EVM) indicators to measure the quality of transmission signals. In modern mobile communication systems, EVM is a measure of RF power amplifier performance of one of the important indicators [2-3]. In frequency division duplex mode mobile communication system, due to the frequency transceiver is different, so the RF power amplifier and receiver in working condition also affect the EVM performance of RF power amplifiers are the main factors of non-linear characteristics of power amplifier and transmission signals such as PAR. In the TD-SCDMA mobile communication system, due to adopt time division duplex mode, transceiver can not simultaneously work, that the RF power amplifier for transmitting signals according to system requirements hourly work [4]. In addition to these factors will affect the RF power amplifier EVM indicators, Based on the production-based Freescale LDMOS RF power amplifier consisting of transistors MW6IC2240 Yanjiu, and the establishment of corresponding circuit 模型, a major study of the transient response of RF power amplifiers up Shijianduiqi EVM performance impact, according to the simulation and test results obtained in the TDD mode, the performance impact of radio frequency power amplifier EVM circuit parameters, proposed an improved TD-SCDMA RF power amplifier circuit design, the EVM performance close to the Frequency Division Duplex mode performance.
TD-SCDMA RF Power Amplifier
TD-SCDMA is different from the WCDMA, CDMA2000 and other third-generation mobile communication system, which uses a TDD mode, which receive and fired at the same time-frequency conducted, which need to switch to ensure the normal communication systems send and receive signals switch. Therefore, the time division duplex mode TD-SCDMA and WCDMA RF power amplifier is also different from the CDMA2000 system, the working status of RF power amplifiers, but work in the time division duplex mode, which only emission signals in the system within a time slot work must be closed within a time slot in the other, in order to avoid self-excited system. This not only ensure the orderly operation of the system, but also improves system efficiency and spectral efficiency.
RF power amplifier working status is determined by its bias. If a power amplifier plus a fixed bias voltage, its has been in a conduction state, here defined as the normal open mode; and make the amplifier work in time division duplex mode, you can control the power amplifier gate bias voltage to achieve, the control signal according to TD-SCDMA physical channel characteristics to generate the signal.
Here with Freescale's LDMOS power amplifier transistor MW6IC2240 designed a three-output power 2W carrier TD-SCDMA power amplifier. MW6IC2240 functional block diagram shown in Figure 1, which includes two zoom, the saturated output power greater than 40W.
TD-SCDMA RF Power Amplifier
Figure 1 VDS1 and VDS2 is the power amplifier power supply drain, where additional 28V fixed voltage; VGS1 and VGS2 is the power amplifier supply-side gate were added to the fixed voltage and bias by the system control voltage can make it working at normal open mode and time division duplex mode. By actual test, the normally open mode and TDD mode EVM indicators shown in Figure 2.
TD-SCDMA RF Power Amplifier
As can be seen from Figure 2, with the increase of output power, EVM indicators deteriorating, this is because the power amplifier with output power close to the 1dB compression point, are significantly increased nonlinear distortion, nonlinear distortion is would seriously affect the EVM indicators, which are reported in many other articles; major study here in time division duplex mode, power amplifier (ie the normal operating mode) of the EVM value is always higher than normal open state of the EVM value of large, namely, PAs work in time division duplex mode, the signal deterioration can be seen from Figure 2, power amplifier in a time division duplex mode, the EVM value is higher than the normal open mode, about 0.5% (time division duplex mode power amplifier transient response rise time is 1.5us). Analysis produced the following main reasons for this difference.
Power amplifier transient response to the impact of EVM
Time division duplex mode, power amplifier and the characteristics of TD-SCDMA signal frames are closely related. TD-SCDMA a sub-frame of the length of 5ms, the seven regular time slots and 3 special slots composition, shown in Figure 3. Here the main consideration of conventional time slot: a slot in the TDMA channel information in the format known as sudden, TD-SCDMA system uses the burst structure shown in Figure 3, burst length from the two data blocks were 352chip , an intermediate length 144chip code and a slot length 16chip protection (GP) composition [5].
TD-SCDMA RF Power Amplifier
Can see from Figure 3, TD-SCDMA's regular time slot of the front is a 352chip data blocks, including a number of TD-SCDMA signal system information. The RF power amplifier on the gate input voltage square wave pulse bias always have a transient response, particularly the impact of the rise time. Then produced a pair of TD-SCDMA signal clipping phenomenon will cause some loss of data symbols, the resulting signals on the TD-SCDMA EVM indicators of deterioration. Figure 2 in the time division duplex mode, EVM indicator is the rise time is 1.5us bias voltage under the test data.
Power amplifier transient response is not only the device itself, but also with the closely related bias circuit design. In order to better analyze the transient response of power amplifiers, where the model of the transistor with a second-order RC network equivalent of the power amplifier transient response, shown in Figure 4. Where, C1, R1, R2 on behalf of the equivalent parameters of power transistors; and C2, R3, R4 is the power amplifier power supply circuit parameters. When the PA opened, the control switches J1 to 3 feet and a foot connected to the capacitor charging power supply V1, showing not only the rise time circuit and power transistor capacitance C1, but also with the power supply circuit of the filter capacitor C2 and resistor R4 about. In practical applications, R4 generally choose 10? Redundant, but not too much because the rise time, filtering can only choose pF capacitance magnitude. But the power amplifier off, the switch J1 3 feet and 2 feet connected to the circuit at this time through a small resistor R3 resistance to discharge, to ensure the transient response of power amplifier fall time short enough.
Power amplifier transient response rise time and in Figure 4 C1, R1 and R4 was closely related to C1 and R1 is a tube within the parameters used by the power transistor type decisions; and R4 and the bias circuit, you can change R4 to change the size of the power amplifier transient response. Figure 5 is the resistance in R4 under different transient response of power amplifier circuit. Can be seen from the figure, when R4 = 10Ω, the amplifier gate bias voltage rise time 0.6us; when R4 = 20Ω, the rise time becomes 1.1us; when R4 = 30Ω, the rise time of 1.6 us. In other words, with the resistor R4 resistance increases, the power amplifier gate bias voltage rise time also increased.
TD-SCDMA RF Power Amplifier
TD-SCDMA RF Power Amplifier
Figure 6 shows the bias circuit in the power amplifier when different values of R4 EVM test value. As can be seen from Figure 6, the bias voltage increase faster, the smaller the deterioration of EVM; the other hand, the greater the deterioration of the EVM. Testing also found that if the rise time is too long, and may even lead to demodulate the signal can not. This shows that the power amplifier transient response rise time and the EVM is indeed a logical connection.
According to TD-SCDMA-related specifications, requirements collection, made up time when switching switch must be less than 2? Zi s, which is transmitted from the protection signal integrity and to avoid deterioration of EVM indicators to consider in this regard. And by choosing appropriate transistor and power amplifier design suitable amplifier bias circuit and switching control signals, can fully meet the standards proposed by the state, and even makes the switch rise time of less than 1us.
Power amplifier bias voltage control signal design
Shown in Figure 6, even if the power amplifier transient response rise time as small as 1us, amplifiers working in TDD mode, the EVM is still greater than 1.2%, still higher than normal open mode power amplifier in the EVM indicators, namely, power amplifier transient response is still causing a deterioration of signal quality. Clearly, power amplifier bias circuit itself and the impact of the power amplifier transient response rise time would not be zero, it will inevitably produce clipping phenomenon, which worsened the EVM indicators.
In order to avoid the transient response of power amplifier rise time on the impact of EVM, we must ensure that TD-SCDMA signal arrives, the power amplifier transient response is over, that is, switching amplifier has been fully opened. Therefore, the amplifier must be open ahead of time. As TD-SCDMA system is a synchronous system with uniform clock reference and synchronization control, the realization of earlier open control switch is not difficult, no discussion on this article. As for the switch to open the amount set in advance how much more appropriate, they have specific power amplifier circuit according to the transient response speed to decide. Experiment, when the amplifier switches 1.5us rise time, the change switch to open the advance amount under the EVM corresponding values shown in Figure 7.
TD-SCDMA RF Power Amplifier
Can be seen from Figure 7, when the power amplifier when the switch is not opened in advance, EVM> 1.5%; and with the increasing amount of open ahead of schedule, EVM and the value decreases; when the switch is turned on in advance with the amplifier volume to open considerable rise time (for this case 1.5us), EVM values decreased to normal open mode with the EVM values exactly the same level; continue to increase if the volume switch turned on in advance, EVM remains unchanged. It can be seen, when the amplifier switches to open the advance is not less than PA itself to open up time, TD-SCDMA power amplifier in the signal comes already in a fully open state, transient response has ended, there could not generate signal clipping phenomenon, naturally there will be no additional deterioration of the EVM.
Can be seen from Figure 3, the TD-SCDMA time slot between the conventional, protected only 12.5us interval (GP), which is in the downlink switching variable switching point, only 12.5us's on the downside protection time. Taking into account the need to ensure, on down to have a good isolation between to ensure the stable operation of the system, the state provides up (or down) switch completely off and the downlink (or uplink) switch turns on the link must be greater than 3us protection time; and TD-SCDMA transceiver device itself, there are 3? Zi s ~ 5? Zi s delay. So even if the switch can be opened in advance to reduce the deterioration of EVM, the switch to open the advance amount is strictly limited. For example: As the switch down to open up ahead of time may cause excessive has not completely shut off, up on the case has been opened, this time, the downstream work simultaneously, it is prone to instability, such as the consequences of self-excited, resulting in system failure . Therefore, the state protection time between the upper and lower rows, top to bottom-line power switch and the switching speed downlink power switch to open and close the lag ahead of volume has a clear and stringent requirements, where no specific description. Can be seen from the above analysis, the switch to open fast enough premise (less than 2? Zi s), through the switch ahead to open (the switch to open ahead of schedule is not less than the rise time switch is turned on) can make amplifier In the TDD mode EVM indicators have reached levels normally open mode, that switch at this time does not make the transient response of the signal quality deterioration, amplifiers can run well.
This paper analyzes the TD-SCDMA Power Amplifier EVM indicators in time division duplex mode, and the difference between normally open mode. By explaining the transient response of amplifier power amplifier in the time division duplex mode, the deterioration of the EVM main power switch is turned on from time constraints, that is, the longer the switch is turned on, the greater the deterioration of the EVM result. For in-depth analysis of the power amplifier transient response, this paper, a second-order RC model, introduced the amplifier transient response constraints related factors. Finally, to improve TD-SCDMA time division duplex mode power amplifier EVM indicators of the program: to improve the speed and the power amplifier switch to open ahead of schedule to achieve amplifier switch open. Specific recommendations are given: the rising power amplifier switching time of less than 2? Zi s; amplifier switch to open the advance is not less than the rise time of switching power amplifier. The test show that the realization of this theory in the TD-SCDMA power amplifier in wireless devices and the TD-SCDMA system, the network can work and good performance.