Effectively Designing a WiMAX Receiver Chain Using Low Noise Amplifiers with an Integrated Bypass Switch
By Asrul Sani Abdullah and Tan Chew Ean, Avago Technologies

Notebooks and handheld devices with integrated WiMAX technology are expected to appear in the market later this year. The 802.16e standard is a mobile version of WiMAX which addresses non-line of sight (NLOS) transmission to mobile devices using 2 to 6 GHz frequencies. For designers of WiMAX receiver chains, the challenge is to tackle the wide dynamic range of received signal levels due to a highly variable transmission path.

The WiMAX receiver’s ability to effectively detect signals from a variable transmission path is critical for system efficiency and data accuracy. Due to WiMAX’s unique requirements, a discrete solution design approach will be more complicated and expensive. An alternative to implementing discretes in the RF front end of a WiMAX receiver is to use an LNA with integrated bypass switch. An LNA with an integrated bypass switch is a simpler, space saving and more efficient solution which allows the receiver chain to have variable gain, low current consumption and excellent linearity.

LNA Performance
The following system analysis illustrates the performance of an LNA with integrated bypass. In this example, the LNA is housed in a low profile (0.4 mm) small footprint (2.0x1.3 mm2) package. With a 3 volts dc supply and 7 mA current, the LNA has 15 dB gain, 1.1 dB noise figure at LNA mode and 4.5 dB insertion loss at bypass mode. The current consumption at bypass mode is <80 uA. The input and output return loss for both the LNA and bypass mode is >10 dB. Additionally, the LNA is designed so the mismatch change is minimal when the LNA mode is switched to bypass mode or vice versa. Therefore, the same matching circuit can be used for both modes. This approach will simplify the external circuitry, yield smaller board space and reduce the manufacturing cost compared to a discrete solution. Figure 1 shows the implementation circuit and the simplified internal structure of the LNA.

System Analysis
Figure 3 shows the simplified block diagram of the WiMAX receiver’s RF front end. Each LNA can be switched into either amplifier mode or bypass mode to allow the receiver chain to have optimized performance. When both the first and second stage LNAs are in amplifier mode, the gain is high with a low noise figure to detect a weak incoming signal. The low noise figure at the first stage LNA is important for a weak incoming signal. A 2 dB noise figure at the first stage LNA can cause the system’s noise figure to degrade to 3.61 dB.

For higher input power, the second stage LNA can be switched into bypass mode to maintain the signal linearity without much sacrifice to the system noise figure since the system’s noise figure is mainly contributed by the front stage. Because the bypass switch can improve the system’s signal linearity and attenuate some gain at higher power, the designer can set a lower linearity requirement for the backend stage since a very high linearity device may result in higher costs and require more current consumption.

When the receiver is close to the base station, the received signal could be too strong and bring the backend of the receiver to a saturation state which can produce a nonlinear signal. Under this condition, both of the LNAs can be switched into bypass mode to attenuate the strong received signal and thus protect the backend stage from being overloaded.

Table 1 shows how the LNAs with bypass switch helps to take care input signal with different power levels. The system analysis is based on the parameter shown in Figure 3. By switching the first and second stage LNA to either LNA or bypass mode, the receiver is able to ensure that the incoming RF signal from variable transmission paths will not force the backend stage of the receiver to go into nonlinear mode.

In addition to supporting a dynamic range for a receiver, modern mobile devices equipped with multiple functions always require more power consumption but its limited battery life imposes a constraint. Thus, another important design requirement for a mobile device is having efficient power consumption while maintaining good performance. An LNA with integrated bypass switch helps to improve power consumption and extend battery life of the mobile device. When high gain is not needed, the LNA can be switched into bypass mode. As a result, the current consumption in bypass mode can be in the low uA range.

Conclusion
Ensuring system efficiency and data accuracy will be critical as WiMAX systems are deployed. The difficult task to design a WiMAX receiver which provides sufficient dynamic range becomes simpler and easier by using an LNA with an integrated bypass switch. This approach extends the dynamic range of the receiver and delivers more reliable performance – a key requirement when introducing a new technology to the market.

Avago Technologies
www.avagotech.com
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