Mobile phones are evolving into a data-oriented device, and functions such as color screens, digital cameras, and on-board memory have gradually become mainstream, used to support a variety of data-oriented applications, including multimedia messaging, mobile games, Internet access, e-mail and Mobile commerce. To promote this development trend, mobile phone operators have begun to add high-speed E-GPRS (EDGE) and WCDMA (UMTS) data functions to existing GSM networks, and are backward compatible with existing services. This approach requires mobile phones to support all GSM / GPRS / EDGE / WCDMA standards. However, supporting multiple standards will bring many design challenges to mobile phones, especially the design of mobile phone radios.
Multi-standards bring radio design challenges Multi-standards will bring a variety of different specifications, so that mobile radio designers must face harsh, sometimes conflicting performance requirements. The GSM / GPRS and GSM / EDGE standards use Time Division Duplex (TDD) technology, so mobile radios must switch between transmit and receive modes through an antenna switch module, allowing multiple users to share the same channel at different times. On the other hand, WCDMA is based on frequency division duplex (FDD) technology. The transmitter and receiver switch simultaneously through a duplexer, allowing multiple users to share the same channel at the same time, and then use orthogonal codes to identify each other. Of the content. Both EDGE and WCDMA use linear modulation. This modulation mechanism must use a linear transmission architecture, while GSM / GPRS uses a non-linear GMSK modulation technology. In order to meet the linearity requirement, the channel spacing of WCDMA must reach 5 MHz, and EDGE requires only 200 kHz bandwidth. The channel bit rate of GSM / GPRS / EDGE is based on a 13 MHz reference clock [t1], and the chip rate of WCDMA is based on a 19.2 MHz reference clock (refer to Table 1).
Three important trends in radio performance optimization are very likely to make multimode mobile phone radios have the best performance: front-end modules, highly integrated transceivers, and system optimization that balances the needs of RF transceivers and baseband processors technology.
Front-end modules There are two integration trends for front-end modules: integrating switches / duplexers and SAW filters in one package, or integrating power amplifiers and switches together. If you want to integrate switches, duplexers and SAW filters, the main challenge is to minimize the insertion loss while maintaining good rejection characteristics of the blocker, thereby improving the receiving sensitivity while maintaining linear operation. The duplexer is particularly important for WCDMA because it must provide good transmission signal isolation in the receive band. If the power amplifier and the switch are integrated together, the designer can optimize and adjust the harmonic filter of the switch of the power amplifier, thereby improving the harmonic suppression capability of the transmitter. In this integration method, all SAW filters of the receiver can be integrated into one filter website.
Figure 1 Integration trend of front-end modules
Figure 2 Important components integrated in a single-chip GSM / GPRS transceiver
Figure 3 Using digital low-IF receiver architecture to prevent DC offset voltage
Highly Integrated Transceivers Today's single-chip transceivers are highly integrated components that integrate all voltage-controlled oscillators (VCOs), frequency synthesizers, and receiver and transmitter circuits. The integration of most or all voltage-controlled oscillators is gradually becoming a standard feature, but the integration of loop filters is often difficult, because the cut-off frequency of about 1MHz may require a large on-chip capacitor. If an external implementation is used, loop filters usually also require high-precision film capacitors, which may cause a significant increase in material costs. The integration of tuning components is also difficult because their losses must be low to minimize the phase noise of the voltage-controlled oscillator. The integrated loop filter and tuning element can shield the local oscillator from external interference sources, thereby improving the reception line and the transmitter's phase error.
The integration of DCXO transformers is also often difficult because it requires extremely accurate linear tuning capabilities to support automatic frequency control loops under the control of the baseband processor. The integration of DCXO makes it easier for multimode systems to provide a system clock based on the 13MHz reference signal of GSM / GPRS / EDGE and the 19.2MHz reference signal of WCDMA. Silicon Labs' Aero I + transceiver is an excellent example of a highly integrated transceiver. It is the smallest GSM / GPRS multi-frequency mobile phone transceiver in the industry. Aero I + is a transceiver that completely uses CMOS technology. The integrated functions include DCXO, phase-locked loop, voltage-controlled oscillator, voltage-controlled oscillator tuning element, and loop filter.
Digital baseband interfaces, such as those based on the DigRF standard, are expected to eliminate the need for analog functions such as analog-to-digital converters in the baseband processor. After the baseband unit does not have any analog functions, the remaining digital functions can use the latest generation of process technology.
System optimization to eliminate DC offset voltage As the data rate continues to increase, system optimization of RF transceivers and baseband processors has become the most important method to improve performance. For example, DC offset voltage has many in mobile radios Possible sources include the inherent DC offset of the circuit, the self-mixing of the local oscillator or reference clock harmonics, and the second-order nonlinearity caused by the blocker. Some offset voltage sources are easy to detect and offset in the calibration procedure, and their impact on performance is negligible. However, the DC offset voltage generated by the blocker is difficult to detect, and may cause the receiving sensitivity or the performance of the automatic gain control circuit to decrease significantly. The blockers used in EDGE and WCDMA are both amplitude modulation, which makes software calibration very difficult.
A more reliable method to prevent the DC offset voltage from entering the baseband processor is to use the receiver architecture of FIG. 3, which mixes and converts the unwanted DC offset voltage to the intermediate frequency, and then eliminates it by the filter; since direct conversion is no longer required The receiver's common baseband DC offset voltage correction algorithm, so the receiving sensitivity will be improved. From the measurement of the product mobile phone, it can be found that the receiving sensitivity may be increased by 0.5dB or more, and the impact on the barrier performance is negligible. Silicon Labs' Aero I + transceiver uses this low-IF receiver architecture.
Data-oriented mobile phones are in a stage of rapid growth, and new high-speed data services bring extremely challenging performance requirements to all levels of mobile phones, especially the design of mobile phone radios. In order to have the highest performance of multi-mode mobile phones, the integration of the radio front-end module and the transceiver is continuously improved, and the functional division between the transceiver and the baseband should be very cautious. These methods will eventually bring a highly integrated multi-mode radio. Once this integration is implemented, the radio design that can provide the best RF performance in the existing single-mode environment will naturally become the best choice for multi-mode design.
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