Multi-radio technology coexistence with 2.4GHz WLAN front-end module

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Wireless local area network (WLAN) technology has become an inevitable standard for mobile computing and data communications in home and industrial appliances. Recently, this technology has been used in the fields of wireless voice over internet (VoIP) telephony, multi-media distribution, gaming, and surveillance systems. The need to integrate WLAN into mobile phones is growing, but mobile phones must also have games, PDAs, digital cameras, general packet radio services (GPRS) for global email and web browsing, and global positioning systems (global positioning systems). Various functions such as positioning system, GPS) and Bluetooth applications. Table 1 lists the various wireless services and their operating bands that are currently integrated into handheld devices.

Future high-end handsets will also have to join 5GHz UMTS cellular wireless technology for voice and data communications. The recent rise of wireless VoIP services provides another low-cost telephone service for mobile phone users. In this case, it seems inevitable that the 2.4GHz WLAN function will be integrated into the UMTS cellular-capable smart phone. The band problem between the UMTS receive band and the 2.4 GHz WLAN band poses a daunting challenge for the wireless front end design developed for synchronous operation.

Architecture

Figure 1 shows an optimized architecture for WLAN wireless technology suitable for embedding in multi-radio technology cellular handsets. This architecture uses a single-pole, single-pole, double-throw (SPDT) T/R switch design that simplifies the structure of a dual-antenna diversity switch, which often lacks sufficient distance in mobile phones to achieve spatial diversity (spatial). Diversity); and the isolation of the antenna is very low, which can not meet the needs of multi-antenna applications. The transmit path of this optimized architecture includes an input matched filter, a 3rd order SiGe power amplifier integrated with an on-chip regulator and power detector, a centralized bandpass matched filter, and an SPDT T/R switch. A high rejection bandpass filter (BPF). Its receive path includes the same BPF, a T/R switch, a bandpass matched filter, and a balun. In some applications, a low noise amplifier (LNA) is needed to ensure that the insertion loss of the receive path is low enough to optimize receiver sensitivity and improve overall coverage of WLAN wireless technology.

Figure 1: High-suppression 2.4GHz WLAN front-end module architecture for multi-standard mobile phones coexisting with UMTS

Power amplifier

The performance of a power amplifier (PA) is the most critical factor in meeting front-end module specifications. High-level IC integration with SiGe BiCMOS technology for all active functions including RF signal amplification, power detection, power regulation, and true CMOS-compatible on/off startup circuitry to meet the rigorous signal for fast setup WLAN applications Timing requirements). In addition, the use of a BiCMOS bias circuit enables a bandgap-based reference voltage and full temperature compensated PA performance. To meet the rigorous linearity requirements of WLANs over a wide operating power range while achieving maximum efficiency, these two points are critical to mobile phones. In general, because most WLAN PAs require 8 to 6 dB of power back-off to achieve low distortion during peak-to-average ratio (PAR) digital modulation, they are often efficient. Very low. Moreover, the post power amplifier losses caused by the high rejection BPF and T/R switches also generate an additional 2.5 to 3.5 dB loss. Therefore, in the worst case, the linear power supply requires an additional 3.5dB power backoff to meet the rejection requirements of up to 2.17GHz. This PA input-output matching network enables low-noise 50 ohm matching, so it can be easily integrated into the wireless front-end module and provides band-pass filtering at the PA input to further improve noise rejection.

SPDT switching filter

This T/R switching filter design includes a novel switching architecture that improves isolation even in the event of a severe antenna mismatch, while achieving a compact form factor that minimizes the overall size of the module. This switch incorporates parallel and series FET devices for the ON and OFF states on each path. To increase isolation, you can do this by connecting the FET in series with the off state; or by paralleling the FET with an AC ground on the idle path of this switch. With this switch design, TX-RX isolation greater than 30 dB can be achieved in the 802.11b/g band with an insertion loss of less than 0.7 dB. Mechanically, this switch can be integrated with a high rejection bandpass filter that specifically provides the out of band blocking required for UMTS compatible handset operation. In our evaluation, if the all-phase anti-missing capability of the receive path is 10:1, the error vector amplitude (EVM) performance degradation is negligible at 54 Mbps and a maximum output power of 15 dBm. Therefore, the idle path can be completely turned off without affecting the performance of the active path. This feature is what handheld devices require because it reduces current consumption and helps save battery power. In the case of the same antenna-end mismatch, the isolation is still greater than 27dB, so it is not only affected by linear TX performance changes, but also can be accurately set by the on-chip monitor; in addition, even in the case of severe antenna mismatch The power amplifier will not be damaged.

Figure 2: Small Signal Gain, Matching, and Reverse Isolation Performance for Antenna Path TX Input

Balun converter
With printed traces and 0201 passive components, a 2:1 balun with integrated low-pass filter can be implemented. The balun converter provides a 50 ohm and 100 ohm match for the switching filter and the receive output, respectively. The use of passive components and printed traces can significantly increase cost and performance. With this circuit, the front-end module achieves a 0.2dB amplitude balance with a phase difference between the balanced ends of less than 1 degree.

performance

The performance of this front-end module is shown in Figures 2, 3 and 4. As can be seen from Figure 2, the overall gain is 28 dB and the input and output return loss is greater than 10 dB. Reverse isolation, which is critical to the stability of tightly shielded handsets, is better than -49 dB over the harmonic band. Figure 3 shows the noise emissions measured at an output power level of 17dBm. At 2.17Ghz, the UMTS receive band is fairly close to the WLAN passband, with a noise power of approximately -170dBm/Hz. Taking into account the 15dB leakage path loss caused by antenna or system board coupling, this will make the front-end module less noisy than the thermal noise floor and compatible with the synchronous operation of the handset UMTS receiver. Even with 1 Mbps modulation, the worst WLAN harmonic at maximum power +17 dBm has a 5 dB margin from the US Federal Communications Commission (FCC) stipulated 0dBi antenna -41.2 dBm/Mhz limit.

Figure 3: Noise emission measurement of the front-end module at an output power of 17dBm

Figure 4 shows the measured module linearity with 54Mbps 802.11g signal modulation over a 17 dB supply range. The linearity is less than 3% EVM at 15 dBm power level with a current consumption of 120 mA. This feature includes a 2.5 to 3.5dB post power amplifier loss for high rejection switching filter circuits.

In the receive mode, the measured impedance below 2 GHz is greater than 45 dB; below 2.17 GHz is greater than 30 dB. The in-band insertion loss is 3.5dB (into a balanced receiver); the return loss is better than -16.5dB. These low in-band losses and high rejection performance ensure the sensitivity of the WLAN receiver during simultaneous operation of multiple wireless technologies.

Figure 4: Measured EVM and current vs. front-end module output power with 54Mbps 802.11g modulation and 3.3V supply

to sum up

SiGe Semiconductor has developed a 2.4GHz WLAN front-end module with high linearity, low power consumption, low out-of-band noise emissions and high out-of-band noise rejection to support the coexistence of multiple wireless technologies in mobile phones. These wireless modes operate on the UMTS band up to 2.17 GHz. The SiGe semiconductor module integrates power detectors and regulators, greatly simplifying the structure of WLAN wireless technology. The compact switching filter design minimizes the overall size of the module, reducing the footprint to only 6 x 5 X 1.4mm. These features make it easy to integrate transceivers or baseband/transceiver chips to build a dual- or single-package 2.4GHz WLAN radio architecture for third-generation multi-radio cellular handsets with UMTS bands.

Table 1: Bands of common embedded wireless applications in multi-radio technology handsets

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