Assuring Sufficient Receiver Dynamic Range for WiMAX Transceivers

By: BY TAN CHEW EAN, AVAGO TECHNOLOGIES
( 1 Oct 2006 )

WiMAX is a form of BWAthat is based on the IEEE 802.16 standard for wireless MANs. It can be used for a variety of applications including a "last-mile" broadband connection, hotspot and cellular backhaul and high-speed enterprise connectivity for businesses.

One fixed version of WiMAX, IEEE 802.16d, provides NLOS transmission to stationary devices using 2 to 11GHz frequencies over distances of approximately four to six miles. The mobile version of the standard, 802.16e, is an extension of 802.16d for mobile use in the 2 to 6GHz band. It allows WiMAX technology to be built into notebook computers and other mobile devices. These NLOS and mobile applications, in which received signal levels can vary over an extremely wide range, call for receivers combining low noise figure for sensitivity with wide dynamic range to assure reliable service at data rates that users/subscribers anticipate.



The front-end LNA establishes both the baseline noise figure and, to a large extent, the overall dynamic range of a receiver. Using today's GaAs pHEMT MMICs can provide noise figures of 1dB, but providing sufficient dynamic range can be a difficult design task. One approach to substantially extend dynamic range performance is to add a switching arrangement that will automatically bypass the amplifier when its low noise figure and gain are not required because of the high level of the received signal.

Avago Technologies developed a GaAs pHEMT MMIC amplifier (MGA-71543) incorporating an integral bypass switch, with external circuitry optimized for low-noise operation. For the receiver designer, this represents a simpler solution than designs using discrete transistors and separate switching components.



When operating in bypass mode (automatically selected when the device current is reduced sufficiently), both input and output are internally matched through the mitigative circuit, which duplicates the input and output impedance of the LNA. The result is minimum mismatch change from LNA to bypass mode, which is very important when the amplifier is used between duplexers, filters or both. Using the bypass switch when high gain is not needed helps preserve the input intercept performance and lowers the power consumption by reducing the total current drawn to zero. The input and output are partially matched and only a simple series/shunt inductor match is required to achieve low noise figure and VSWR to 50Ω.

GOOD PERFORMANCE

An amplifier demonstration board using this device was built on 10mm thickness RO4350B material from Rogers Corporation, which has a low loss dielectric constant (∈r= 3.48) and offers good performance at high frequency. This material was laminated on top of conventional FR4 glassreinforced epoxy PC board material to provide additional mechanical strength. Figure 1 shows the schematic of the single-stage amplifier.



Table 1 shows the board-level performance of the amplifier operating from a 3.3V source, drawing 10mA in operation. One of the benefits of the integrated mitigated bypass amplifier design is that it permits the easy cascading in two stages to provide even greater flexibility (Figure 2). When both the first and second amplifiers are in use, they provide greater than 21dB gains with a noise figure of about 1.2dB at 3.5GHz. The input and output return loss is greater than 10dB and the operating current is about 21mA.

Bypassing the second stage first reduces gain to approximately 6.3dB, while tending to preserve the input intercept performance and reduces total current consumption to approximately 10mA. Since the noise figure of the first stage dominates the overall noise figure of an amplifier, the resulting noise figure is approximately 1.8dB.

Under the strongest signal conditions both stages can be bypassed, resulting in a total attenuation of approximately 11.4dB, while reducing the total current consumption to almost zero.

With the detector and following circuitry optimized, the use of one or two stages of mitigated bypass front-end LNA should represent an effective approach to providing WiMAX receivers capable of providing reliable performance over difficult and variable transmission paths.

About the Author

Chew Ean is an application engineer at Avago Technologies, Wireless Semiconductor Division. She can be reached at chew-ean.tan@avagotech.com