Optimized Transceiver Aids Broadband Wireless Access

Broadband-wireless-access (BWA) technology promises high-speed Internet connections by means of reliable wireless devices. But, in order to achieve cost-effective products in a competitive marketplace, the RF hardware in broadband-wireless-access products must be compact and reproducible. Fortunately, the authors have developed an RF/microwave module for broadband wireless access applications that provides excellent performance in a compact size. The module consists of an antenna switch, an RF transmitter, and an RF receiver. An optimized transceiver architecture was developed to obtain more deliverable power and higher sensitivity than existing designs. In order to get the best RF performance and to reduce the size and cost of the RF module, an embedded parallel-coupled-transmission-line band rejection filter (BRF) was designed. By integrating the antenna with the other system blocks, it was possible to develop a miniature RF transceiver module with high-performance test results.

Growth in demand for high-speed data and multimedia services drives the need for BWA technology. Modern BWA systems can provide higher data rates, higher spectrum efficiencies, and greater system capacities than traditional wireless access systems and other mobile data services. Compared to traditional wireless access systems, BWA systems offer many advantages, including flexibility, low cost, and short time to market (refs. 1-3).

Because of the available spectrum in the 5-GHz band, BWA technology represents tremendous market potential due to the high data rates possible and the availability of spectrum for multimedia services. But the success of a practical BWA system hinges on developing an RF transceiver capable of high performance in a small, low-cost design. With this goal in mind, the authors developed an RF transceiver targeting high output power, high linearity, and low noise figure in a compact module for 5-GHz operation.

The RF transceiver developed by the authors includes all the circuitry required for the signal processing between the baseband inphase/quadrature (I/Q) signals and the RF signals in the BWA air-interface system. A direct-conversion transceiver architecture was adopted for the design to achieve a low-cost solution with a high level of integration compared to radio architectures based on the use of intermediate-frequency (IF) translation. Figure 1 shows a block diagram of the transceiver module. The module consists of an RF transmitter, an RF receiver, and an antenna switch.

The module's transmitter section is composed of a quadrature modulator, an automatic-power-controlled (APC) amplifier, a spurious rejection filter, and a power amplifier. Generally, in a transmitter, a filter is needed after the power amplifier for spurious suppression. However, the insertion loss of the filter may reduce the maximum deliverable power at the antenna ports significantly. In order to avoid this reduction in transmit power, the spurious rejection filter is inserted instead between the modulator output and the power-amplifier input. With this improved configuration, the output power is greatly increased in comparison to the traditional filtering approach, without degrading the transmitter's spurious suppression performance.

The module's receiver section is composed of a low-noise amplifier (LNA), a bandpass filter, an automatic-gain-control (AGC) amplifier, and a quadrature demodulator. As with the approach in the transmitter section, the image-rejection bandpass filter is inserted between the LNA output and demodulator input to maximize sensitivity while reducing the effects of image signals. To support time-division duplexing and antenna-diversity techniques, the module also employs a high-isolation, low-loss, double-pole, double-throw (DPDT) switch.

The RF module is fabricated on a four-layer printed-circuit board (PCB) using TLX-8 laminated substrate material from Taconic (www.taconic-add.com) is used for its low-loss and high-performance capabilities. The PCB material features a dielectric constant of 2.55 and stable dielectric constant with frequency. It exhibits extremely low moisture absorption and has a dissipation factor of only 0.0019 at 10 GHz. The material is stable with temperature, with a coefficient of thermal expansion (CTE) of 9 to 12 ppm/°C in the xy direction and 130 to 145 ppm/°C in the z direction. Manufacturing processes based on this material are very similar those used with low-cost FR4 material. To maintain a low profile, all of the module's components are mounted on the top layer of the TXL-8.

To achieve a module with small size, multifunction RF integrated circuits (RF ICs) are used wherever possible. For example, the main RF ICs used in the module are a model LM5506M power amplifier from Microsemi Corp. (www.microsemi.com), a model RF2472 LNA from RF Micro Devices (www.rfmd.com), a model MASWSS0094 antenna switch from M/A-COM (www.macom.com), and a model MAX2828 integrated transceiver from MAXIM Integrated Products (www.maxim-ic.com). In addition to the selection of the RF ICs, the matching networks and spurious rejection filters are carefully designed for the best performance. The matching networks are used to achieve the highest output power and linearity, the lowest noise figure, and good VSWR.

In the modulator design, the spurious rejection filter enables the transmitter to meet the spectral mask requirements for the 5-GHz frequency band. Output signals from the MAX2828 have strong RF spurious emissions at (4/5)f_RF (approximately 4 GHz) which must be filtered prior to the PA. But rather than performing the filtering function with a bandpass filter, a band-reject filter (BRF) was selected. A BRF has many merits for this application, including the fact that it is easily integrated into the module design, achieves lower passband insertion loss, and higher stopband rejection. Using the BRF instead of a bandpass filter can also improve the effective efficiency of the PA as well.

The BRF was designed with parallel coupled transmission lines. It was simulated and analyzed with the help of the Advanced Design System (ADS) suite of computer-aided-engineering (CAE) software tools from Agilent Technologies (www.agilent.com)(ref. 4). Figures 2 and 3 show the circuit layout and CAE schematic diagram for the 4-GHz BRF, respectively. Simulated performance is shown in Fig. 4. By using this embedded BRF in the transceiver, the RFspurious emissions from the MAX2828at 4 GHz are greatly reduced.

After elaborate tuning, it was possible to achieve good RF performance. Figures 5 and 6 show the output spectrum of the transmitter obtained with a sinusoidal baseband I/Q signal. The third-order intermodulation-distortion (IMD3) output products were less than –34 dBc when the output power is about +20 dBm. Due to the improved transmitter configuration, the output power can be increased by 2 dB without compromising linearity.

The baseband output spectrum of the receiver was obtained with an RF signal at the antenna port, as shown in Figs. 7 and 8. The level of the input RF signal used in the measurements was –87 dBm. According to these results, the maximum conversion gain and noise figure of the receiver can be calculated: the maximum conversion gain is about 100 dB and the noise figure is about 5 dB. Again, due to the improved receiver configuration, it is possible to reduce the noise figure by an additional 2 dB.

In short, the transceiver module represents an improved RF front-end design for BWA systems. The transceiver was optimized for high transmit power and low receiver sensitivity. Spurious rejection filters achieve spectral mask requirements. The RF module is for wireless terminals requiring high output power and high receiver sensitivity.

REFERENCES

1. Walter Honcharenko, Jan P, Kruys, David Y, Lee, and Nitin J, Shah, "Broadband Wireless Access," IEEE Communication Magazine, January 1997, pp. 21-27.
2. Keith Warble, John Locke, and Bomber Bishop, "Wireless Aspects of Broadband Access," 2001 IEEE MTT-S Digest, pp. 993-996.
3. Terje Tjelta, Lars Erling Braten, Chris Howson, and Thierry Montalant, "Developments in Broadband Wireless Access," Broadband Europe Conference, December 8-10, 2004.
4. Zachary M. Thomas, Tomasz M. Grzegorczyk, Bae-Ian Wu, and Jin Au Kong, "Enhanced Microstrip Stopband Filter Using A Metamaterial Substrate," Microwave and Optical Technology Letters, August 2006, Vol. 48.
5. Walid Y. Ali-Ahmad, "Radio Transceiver Architectures and Design Issues for Wideband Cellular Systems," RFIT2005-IEEE, November 30 to Dec 2, 2005, pp. 21-25.